Mid-wavelength high operating temperature barrier infrared detector and focal plane array Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-10 David Z. Ting, Alexander Soibel, Arezou Khoshakhlagh, Sir B. Rafol, Sam A. Keo, Linda Höglund, Anita M. Fisher, Edward M. Luong, Sarath D. Gunapala
We analyze and compare different aspects of InAs/InAsSb and InAs/GaSb type-II superlattices for infrared detector applications and argue that the former is the most effective when implemented for mid-wavelength infrared detectors. We then report results on an InAs/InAsSb superlattice based mid-wavelength high operating temperature barrier infrared detector. At 150 K, the 50% cutoff wavelength is 5.37 μm, the quantum efficiency at 4.5 μm is ∼52% without anti-reflection coating, the dark current density under −0.2 V bias is 4.5 × 10−5 A/cm2, and the dark-current-limited and the f/2 black-body (300 K background in 3–5 μm band) specific detectivities are 4.6 × 1011 and 3.0 × 1011 cm-Hz1/2/W, respectively. A focal plane array made from the same material exhibits a mean noise equivalent differential temperature of 18.7 mK at 160 K operating temperature with an f/2 optics and a 300 K background, demonstrating significantly higher operating temperature than InSb.
Resonance fluorescence from an atomic-quantum-memory compatible single photon source based on GaAs droplet quantum dots Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-10 Laxmi Narayan Tripathi, Yu-Ming He, Łukasz Dusanowski, Piotr Andrzej Wroński, Chao-Yang Lu, Christian Schneider, Sven Höfling
Single photon sources, which are compatible with quantum memories, are important components of quantum networks. In this article, we show optical investigations on isolated GaAs/Al0.25Ga0.75As quantum dots grown via droplet epitaxy, which emit single photons on resonance with the Rb-87-D2 line (780 nm). Under continuous wave resonant excitation conditions, we observe bright, clean, and narrowband resonance fluorescence emission from such a droplet quantum dot. Furthermore, the second-order correlation measurement clearly demonstrates the single photon emission from this resonantly driven transition. Spectrally resolved resonance fluorescence of a similar quantum dot yields a linewidth as narrow as 660 MHz (2.7 μeV), which corresponds to a coherence time of 0.482 ns. The observed linewidth is the smallest reported so far for strain free GaAs quantum dots grown via the droplet method. We believe that this single photon source can be a prime candidate for applications in optical quantum networks.
Switching Purcell effect with nonlinear epsilon-near-zero media Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 Saman Jahani, Hangqi Zhao, Zubin Jacob
An optical topological transition is defined as the change in the photonic iso-frequency surface around epsilon-near-zero (ENZ) frequencies which can considerably change the spontaneous emission of a quantum emitter placed near a metamaterial slab. Here, we show that due to the strong Kerr nonlinearity at ENZ frequencies, a high-power pulse can induce a sudden transition in the topology of the iso-frequency dispersion curve, leading to a significant change in the transmission of propagating as well as evanescent waves through the metamaterial slab. This evanescent wave switch effect allows for the control of spontaneous emission through modulation of the Purcell effect. We develop a theory of the enhanced nonlinear response of ENZ media to s and p polarized inputs and show that this nonlinear effect is stronger for p polarization and is almost independent of the incident angle. We perform finite-difference time-domain simulations to demonstrate the transient response of the metamaterial slab to an ultrafast pulse and fast switching of the Purcell effect at the sub-picosecond scale. The Purcell factor changes at ENZ by almost a factor of three which is an order of magnitude stronger than that away from ENZ. We also show that due to the inhomogeneous spatial field distribution inside the multilayer metal-dielectric super-lattice, a unique spatial topological transition metamaterial can be achieved by the control pulse induced nonlinearity. Our work can lead to ultra-fast control of quantum phenomena in ENZ metamaterials.
Nonlinear frequency conversion in one dimensional lithium niobate photonic crystal nanocavities Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 Haowei Jiang, Hanxiao Liang, Rui Luo, Xianfeng Chen, Yuping Chen, Qiang Lin
We demonstrate flexible nonlinear frequency up-conversion in high-Q lithium niobate photonic crystal nanobeam resonators. The high optical Q together with strong optical mode confinement allows us to observe clear second harmonic generation and sum frequency generation with an optical power around only tens of microWatts. These demonstrations show that high-Q lithium niobate photonic crystal nanoresonators are of great promise for nonlinear photonic applications.
Optical bistability in shape-memory nanowire metamaterial array Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-13 Yusuke Nagasaki, Behrad Gholipour, Jun-Yu Ou, Masanori Tsuruta, Eric Plum, Kevin F. MacDonald, Junichi Takahara, Nikolay I. Zheludev
Non-volatile temperature-induced structural phase transitions such as those found in chalcogenide glasses are known to lead to strong changes in optical properties and are widely used in rewritable optical disk technology. Herein, we demonstrate that thermally activated optical memory can be achieved via the nanostructural reconfiguration of a metallic nanowire metamaterial array made from a shape-memory alloy: A nickel-titanium film of nanoscale thickness structured on the subwavelength scale exhibits bistability of its optical properties upon temperature cycling between 30 °C and 210 °C. The structure, comprising an array of NiTi nanowires coated with a thin film of gold to enhance its plasmonic properties, can exist in two non-volatile states presenting an optical reflectivity differential of 12% via nanoscale mutual displacements of alternating nanowires in the structure. Such all-metal shape-memory photonic gratings and metamaterials may find applications in bistable optical devices.
Generation of vector beam with tandem helical undulators Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-13 Shunya Matsuba, Keigo Kawase, Atsushi Miyamoto, Shigemi Sasaki, Masaki Fujimoto, Taro Konomi, Naoto Yamamoto, Masahito Hosaka, Masahiro Katoh
We propose a scheme to produce structured light in synchrotron light sources. In this scheme, light beams from two undulators are superposed by using a technique akin to the “cross undulator.” We demonstrate that a vector beam, in which the polarization direction varies with the azimuthal angle about the beam axis, is produced by superposing harmonic radiation from two helical undulators in tandem. Although this scheme is demonstrated in the ultraviolet range at the low-energy synchrotron UVSOR-III, it can be applied to high-energy synchrotrons to produce vector x-ray beams, which would open a new field in the application of synchrotron radiation.
Graphdiyne under pressure: A Raman study Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 Yan Wang, Mingguang Yao, Yanhuan Chen, Jiajun Dong, Xigui Yang, Mingrun Du, Ran Liu, Huibiao Liu, Yuliang Li, Bingbing Liu
High pressure Raman spectra of graphdiyne (GDY) have been studied up to 34.63 GPa. We found that sp-hybridized carbons in GDY are highly active and start to undergo a bonding change at around 5.2 GPa. Such a bonding change affects the C-C stretching vibration of sp2 hexagon rings in GDY, leading to an anomaly in the corresponding G-band pressure coefficient. A three-dimensional sp2 structure is proposed to form via pressure-induced interlayer cross-linking of sp carbons in GDY and is stable up to at least 34.63 GPa. Our study presents an important example in the study of graphyne family under pressure.
Bubble dimer dynamics induced by dual laser beam ablation in liquid Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-11 Kai Liu, Jun Chen, Huasong Qu, Yuhang Dong, Yujie Gao, Jiaxin Liu, Xuhai Liu, Yousheng Zou, Haibo Zeng
An approach based on dual laser beam ablation in liquid is presented for the preparation of bimetal hydroxide. The dynamics of a dual laser-induced bubble dimer is studied by the combination of an inviscid Rayleigh-Plesset model, an ultrafast camera, and time-resolved shadowgraphy experiments. It indicates that the dual laser-induced bubble pair will rapidly merge into a dimer and then undergo evolution as a single bubble, which can promote the agglomeration of two kinds of metal particles in this thermodynamic environment. Moreover, the atomic ratio of bimetal hydroxide with high crystallinity can be precisely adjusted by the dual laser energy ratio without any additives.
Thermal property engineering of InSe layer by a thin Al2O3 stress liner Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 Kuilong Li, Yuehua Hong, Zhiwen Li, Xinke Liu
We investigate the thermal properties of thin InSe layers with high-κ oxide Al2O3 stress liners. Temperature-dependent Raman spectroscopy demonstrated that the Al2O3 passivation layer significantly reduced the thermal variation coefficients of the in-plane E2g1 phonon mode of the InSe layer from −0.03284 cm−1/K to −0.0212 cm−1/K in comparison with the InSe sample without the Al2O3 capping layer. Combined with power-dependent Raman spectroscopy, the in-plane thermal conductivity of InSe reaches about 53.1 W/mK, ∼40% greater than that without the Al2O3 capping layer, 38.2 W/mK, which is attributed to the large thermal conductivity of Al2O3 and the electron-phonon interactions at the interface. Generally, this work will contribute to improving the performances of the InSe-based nano-devices and extending their applications profoundly.
Contributions of the lead-bromine weighted bands to the occupied density of states of the hybrid tri-bromide perovskites Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 A. R. Kirmani, A. E. Mansour, M. I. Saidaminov, X. Cui, D. Shi, A. Alofi, Ya. B. Losovyj, G. Gurung, T. R. Paudel, A. J. Yost, P. A. Dowben, E. Y. Tsymbal, A. Amassian, K. Katsiev
The electronic structure of methylammonium lead bromide (CH3NH3PbBr3) single crystals has been investigated through a combination of resonant photoemission and theoretical modeling. There are Pb spectral contributions throughout the valence band. Importantly, the electronic structure at the top of the valence band is found to be dominated by the hybridized Pb-Br bands, not methylammonium bromide. The results line up with the partial density of states obtained from density functional theory and confirm that much of the valence band has some Pb spectral weight.
Electrical properties of extended defects in strain relaxed GeSn Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 Somya Gupta, Eddy Simoen, Roger Loo, Yosuke Shimura, Clement Porret, Federica Gencarelli, Kristof Paredis, Hugo Bender, Johan Lauwaert, Henk Vrielinck, Marc Heyns
We report the electrical properties of 60° dislocations originating from the +1.2% lattice mismatch between an unintentionally doped, 315 nm thick Ge0.922Sn0.078 layer (58% relaxed) and the underlying Ge substrate, using deep level transient spectroscopy. The 60° dislocations are found to be split into Shockley partials, binding a stacking fault. The dislocations exhibit a band-like distribution of electronic states in the bandgap, with the highest occupied defect state at ∼EV + 0.15 eV, indicating no interaction with point defects in the dislocation's strain field. A small capture cross-section of 1.5 × 10−19 cm2 with a capture barrier of 60 meV is observed, indicating a donor-like nature of the defect-states. Thus, these dislocation-states are not the source of unintentional p-type doping in the Ge0.922Sn0.078 layer. Importantly, we show that the resolved 60° dislocation-states act as a source of leakage current by thermally generating minority electrons via the Shockley-Read-Hall mechanism.
Compensated thermal conductivity of metallically conductive Ta-doped TiO2 Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-10 Joonki Suh, Tarapada Sarkar, Hwan Sung Choe, Joonsuk Park, T. Venkatesan, Junqiao Wu
Electrical and thermal conductivities of epitaxial, high-quality Ta-doped TiO2 (Ta:TiO2) thin films were experimentally investigated in the temperature range of 35–375 K. Structurally identified as the anatase phase, degenerate Ta doping leads to high electrical conductivity in TiO2, reaching >105 (Ω-m)−1 at 5 at. % of Ta, making it a potential candidate for indium-free transparent conducting oxides. In stark contrast, Ta doping suppresses the thermal conductivity of TiO2 via strong phonon-impurity scattering imposed by the Ta dopant which has a high mass contrast with Ti that it substitutes. For instance, the near-peak value shows a >50% reduction, from 9.0 down to 4.4 W/m-K, at just 2 at. % doping at 100 K. Interestingly, further Ta doping beyond 2 at. % no longer reduces the measured total thermal conductivity, which is attributed to a high electronic contribution to thermal conduction that compensates the alloy-scattering loss, as well as possibly the renormalization of phonon dispersion relation in the heavy doping regime originating from doping-induced lattice stiffening. As a result, at high Ta doping, TiO2 exhibits high electrical conductivity without much degradation of thermal conductivity. For example, near room temperature, 5 at. % Ta doped TiO2 shows over 3 orders of magnitude enhancement in electrical conductivity from undoped TiO2, but with only less than 10% reduction in thermal conductivity. The metallic Ta:TiO2 maintaining reasonable good thermal conductivity might find application in energy devices where good conduction to both charge and heat is needed.
Influence of effective channel length in self-aligned coplanar amorphous-indium-gallium-zinc-oxide thin-film transistors with different annealing temperatures Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-11 Hyeong Wook Kim, Eok Su Kim, Joon Seok Park, Jun Hyung Lim, Bo Sung Kim
Electrical characteristics of self-aligned coplanar amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) were investigated for different annealing temperatures. The field-effect mobility of the a-IGZO TFTs increased with the annealing temperature, in particular, for a small channel length. The effective channel length (Leff) of the a-IGZO TFTs was extracted using the transmission line method. The decrease in Leff significantly depended on the annealing temperature, due to the hydrogen diffusion into the a-IGZO channel region. The intrinsic mobility calculated from the channel resistance of the a-IGZO TFTs was in good agreement with the mobility corrected by Leff.
Phonon anharmonicity in thermoelectric palladium sulfide by Raman spectroscopy Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-13 Liu-Cheng Chen, Zi-Yu Cao, Hao Yu, Bin-Bin Jiang, Lei Su, Xun Shi, Li-Dong Chen, Xiao-Jia Chen
Recent advances in the study of thermoelectric materials mainly focus on the developments or designs of methods to reduce thermal conductivities. The information of phonon scattering processes is the key to the understanding of the thermal transfer and transport of a material. Such information is essential for the understanding of the thermal conductivity of a material itself and for the further improvement to demand the requirements for technological applications. Recently, palladium sulfide has been examined as a potential thermoelectric material. However, the high thermal conductivity limits its thermoelectric performance and technological applications. Here, Raman scattering spectroscopy is used to investigate the thermal transport properties of this material over a wide range of temperatures. The nonlinear temperature-dependent frequencies and linewidths of the Raman modes illustrate the anharmonicity of phonon scattering for thermal transport in this material. Three-phonon scattering processes are found to account for the thermal transport in the temperature range of 10–620 K. The high-energy bands of the Bg mode related to the light atom (S) contribute most to the thermal transport properties. More phonon scattering processes including higher orders are seemingly needed to further reduce the thermal conductivity of this material.
Terahertz magnon and crystal-field transition manipulated by R3+-Fe3+ interaction in Sm0.5Pr0.5FeO3 Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 Xiumei Liu, Tao Xie, Jiajia Guo, Senmiao Yang, Yuna Song, Xian Lin, Shixun Cao, Zhenxiang Cheng, Zuanming Jin, Anhua Wu, Guohong Ma, Jianquan Yao
We use terahertz (THz) magnetic and electric fields to investigate the magnetic and optoelectronic responses of the Sm0.5Pr0.5FeO3 (SPFO) crystal, respectively, by THz time-domain spectroscopy. It is found that the spin reorientation transition (SRT) in SPFO occurs in the temperature range of 175–210 K. The SRT is not observed in PrFeO3. The quasi-antiferromagnetic magnon frequency has a blue-shift from 0.42 THz (PrFeO3) to 0.46 THz (SPFO) at room temperature, due to the enhanced anisotropy constant. The refractive index of SPFO in the THz frequency decreases around 3% compared with that of the PrFeO3 crystal. In addition, it can be found that the energy scale of crystal-field transitions has a red-shift for the doped single crystal. We expect our results to make rare-earth orthoferrites accessible to potential applications in THz spintronic devices.
Engineering magnetic heterostructures to obtain large spin Hall efficiency for spin-orbit torque devices Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-10 Lisen Huang, Shikun He, Qi Jia Yap, Sze Ter Lim
By sandwiching a CoFeB ferromagnetic layer between Ta and Pt heavy metals with an opposite spin Hall angle, spin currents of the same polarity are transmitted from both interfaces of the Ta/CoFeB/Pt trilayer to the CoFeB layer simultaneously. Here, we investigated the spin-orbit torque, magnetization dynamics, and interface spin transmission efficiency of the trilayer heterostructure by spin-torque ferromagnetic resonance. A large effective spin Hall angle, substantially larger than both Ta and Pt, was obtained in the Ta/CoFeB/Pt stack. The thickness-dependence study showed that with the reducing of CoFeB thickness, Gilbert damping enhances by spin pumping and spin Hall angle increases by the spin Hall effect and the Rashba effect. Furthermore, the spin transparency derived from effective spin mixing conductance was 0.63 ± 0.07 and 0.48 ± 0.02 at the CoFeB/Pt and Ta/CoFeB interfaces, respectively. Hence, the spin Hall angle could be further enhanced by improving the spin transmission efficiency at the interface. Our method of increasing spin-orbit torque through stack engineering would have potential applications in domain wall racetrack memory, logic gates, and magnetic tunnel junctions.
A non-collinear double MgO based perpendicular magnetic tunnel junction Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-10 James Lourembam, Bingjin Chen, Aihong Huang, Salauddeen Allauddin, Sze Ter Lim
Double MgO based magnetic free layers are state-of-the-art solutions for providing high performance perpendicular spin-transfer torque-magnetic random access memory devices. We provide device measurements showing reduction of switching current in perpendicular magnetic tunnel junctions (p-MTJs) using non-collinear ferromagnets on the double MgO template. This structure is engineered by introducing an in-plane ferromagnetic cap, which produces in-plane stray field effects on the free layer. The non-collinear structure delivers ∼53% reduction in critical current density in STT switching without weakening the thermal stability of the devices. The advantages in device performance using non-collinearity in magnetization is sustained down to ∼20 nm MTJs. Micromagnetic simulations suggest inherent differences in the magnetization reversal process between our proposed non-collinear p-MTJ and a well-known double MgO based p-MTJ.
Electromagnetic proximity effect in planar superconductor-ferromagnet structures Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 S. Mironov, A. S. Mel'nikov, A. Buzdin
The spread of Cooper pairs in a ferromagnet in proximity coupled superconductor-ferromagnet structures is shown to cause a strong inverse electromagnetic phenomenon, namely, the long-range transfer of the magnetic field from the ferromagnet to the superconductor. Contrary to the previously investigated inverse proximity effect resulting from the spin polarization of a superconducting surface layer, the characteristic length of the above inverse electrodynamic effect is of the order of the London penetration depth, which usually is much larger than the superconducting coherence length. The corresponding spontaneous currents appear even in the absence of the stray field of the ferromagnet and are generated by the vector-potential of magnetization near the S/F interface, and they should be taken into account in the design of nanoscale S/F devices. Similarly to the well-known Aharonov-Bohm effect, the discussed phenomenon can be viewed as a manifestation of the role of vector potential in quantum physics.
Period multiplication in a parametrically driven superconducting resonator Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 Ida-Maria Svensson, Andreas Bengtsson, Jonas Bylander, Vitaly Shumeiko, Per Delsing
We report on the experimental observation of period multiplication in parametrically driven tunable superconducting resonators. We modulate the magnetic flux through a superconducting quantum interference device, attached to a quarter-wavelength resonator, with frequencies nω close to multiples, n = 2, 3, 4, and 5, of the resonator fundamental mode and observe intense output radiation at ω. The output field manifests n-fold degeneracy with respect to the phase, and the n states are phase shifted by 2π/n with respect to each other. Our demonstration verifies the theoretical prediction by Guo et al. [Phys. Rev. Lett. 111, 205303 (2013)] and paves the way for engineering complex macroscopic quantum cat states with microwave photons.
Thickness dependence of superconductivity in single-crystal Ta4Pd3Te16 nanoribbons Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 Lin Bao, Yiqing Bi, Xiaotong Liu, Xiaohui Yang, Tingting Hao, Shibing Tian, Zongli Wang, Junjie Li, Changzhi Gu
We present the thickness-dependent electrical properties of mechanically exfoliated single crystal Ta4Pd3Te16 nanoribbons. By decreasing the nanoribbon thickness in the range of 500–20 nm, we observed a suppression of superconductivity driven by both the thickness and the external magnetic field. In particular, for the thinner nanoribbons with the thickness less than 40 nm, there is a non-zero resistance state extending down to low temperature, followed by the loss of superconductivity when the thickness is decreased to the order of the coherence length. We found that the theory of a thermally activated phase slip can well describe the temperature dependence of the resistance below Tc. The disorder-induced enhanced Coulomb interaction with the decrease in the thickness is expected to be dominant in the gradual crossover behavior from superconducting to normal or very weakly insulating behavior in the low-dimensional system.
Superconducting nano Josephson junctions patterned with a focused helium ion beam Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-10 Ethan Y. Cho, Yuchao W. Zhou, Jennifer Y. Cho, Shane A. Cybart
We report the fabrication of nanoscale wires and Josephson junctions in 25 nm thick YBa2Cu3O7–δ thin films with wire widths as narrow as 50 nm. Our approach utilizes a finely focused gas field ion source from a helium ion microscope to directly modify the material on the nanometer scale to convert irradiated regions of the film into insulators. In this manner, the film remains intact and no material is milled or removed. Transport data show that the electrical properties, critical current and conductance, scale linearly with the lithographically defined width ensuring that the actual and lithographically defined dimensions are commensurate with each other. Unlike in typical ion damage proximity effect Josephson junctions, we observe a low temperature saturation of the critical current and near temperature interdependent resistance which we attribute to a narrower and more resistive barrier. Furthermore, we also demonstrate the ability to fabricate devices exhibiting high resistance and capacitance with hysteretic underdamped Josephson junction properties. This patterning technique allows for a broad range of electrical properties for Josephson devices that will expand potential applications.
Coherently strained epitaxial YBa2Cu3O7−δ films grown on NdGaO3 (110) Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 Sogol Khanof, Jochen Mannhart, Hans Boschker
YBa2Cu3O7−δ is a good candidate to systematically study high-temperature superconductivity by nanoengineering using advanced epitaxy. An essential prerequisite for these studies is coherently strained YBa2Cu3O7−δ thin films, which we present here using NdGaO3 (110) as a substrate. The films are coherent up to at least 100 nm thickness and have a critical temperature of 89 ± 1 K. The a and b lattice parameters of YBa2Cu3O7−δ are matched to the in-plane lattice parameters of NdGaO3 (110), resulting in a large reduction of the orthorhombicity of YBa2Cu3O7−δ. These results imply that a large amount of structural disorder in the chain layers of YBa2Cu3O7−δ is not detrimental to superconductivity.
Evidence for a surface anomaly during the cubic-tetragonal phase transition in BaTiO3(001) Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 N. Barrett, J. Dionot, D. Martinotti, E. K. H. Salje, C. Mathieu
We have used low energy electron microscopy (LEEM) to study the evolution of the surface structure of BaTiO3(001) during the ferroelectric-paraelectric phase transition (120 °C, P4mm-Pm3m). Transient surface structures appear under heating at temperatures slightly above TC. Intersections between polar domain walls and the sample surface persist in the surface layer at 126.0 °C while the bulk has already transformed into the cubic phase. The wall signals are criss-crossed by a second set of stripe patterns with roughly perpendicular orientation at 126.3 °C. These surface patterns coarsen under further heating to 126.9 °C. The LEEM image is dominated in the ferroelectric state by intersections of ferroelastic/ferroelectric 90° walls and the surface. The intersection lines are charged.
Direct observation of ferroelectricity in Ca3Mn2O7 and its prominent light absorption Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-10 Meifeng Liu, Yang Zhang, Ling-Fang Lin, Lin Lin, Shengwei Yang, Xiang Li, Yu Wang, Shaozhen Li, Zhibo Yan, Xiuzhang Wang, Xiao-Guang Li, Shuai Dong, Jun-Ming Liu
Layered perovskites A3M2O7 are known to exhibit the so-called hybrid improper ferroelectricity. Despite experimentally confirmed cases (e.g., nonmagnetic M = Ti and Sn), the ferroelectricity in magnetic Ca3Mn2O7 remains a puzzle. Here, the structural, ferroelectric, magnetoelectric, and optical properties of Ca3Mn2O7 are systematically investigated. Switchable polarization is directly measured, demonstrating its ferroelectricity. In addition, magnetoelectric response is also evidenced, implying the coupling between magnetism and ferroelectricity. Furthermore, strong visible light absorption is observed, which can be understood from its electronic structure. Its direct and appropriate bandgap, as well as wide conducting bands, makes Ca3Mn2O7 a potential candidate for ferroelectric photoelectric applications.
Bistability in a multiferroic composite resonator Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-11 Yuri K. Fetisov, Dmitri A. Burdin, Nikolai A. Ekonomov, Leonid Y. Fetisov, Alexey A. Berzin, Patrick Hayes, Eckhard Quandt
Bistable characteristics of a nonlinear multiferroic composite resonator containing ferromagnetic and piezoelectric layers are investigated. The resonator was a borosilicate glass substrate of 25 mm × 2 mm dimensions and 150 μm thickness with a 2 μm thick amorphous ferromagnetic FeCoSiB layer and a 2 μm thick piezoelectric AlN layer deposited on its sides by magnetron sputtering. The resonator was excited by ac voltage at a frequency of 156 kHz, matching its longitudinal acoustic resonance frequency. The bistability loops were observed with increasing and decreasing frequency at constant excitation voltage and with increasing and decreasing voltage at constant frequency. With increasing excitation voltage, the resonator frequency first decreases by ∼0.7 kHz and then increases again to the initial value. A bistability model is suggested that uses Lorentzian shape resonance line and measured dependences of the resonance frequency and transmission coefficient on the output signal, which quantitatively describes experimental data. It is shown that bistability in a multiferroic resonator arises due to the nonlinearity of the ferromagnetic layer.
Experimental demonstration of deep traps in silica-based polyethylene nanocomposites by combined isothermal surface potential decay and pulsed electro-acoustic measurements Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-13 Y. Wang, M. Hao, Z. Xu, D. Qiang, G. Chen, A. Vaughan
The ability to suppress space charge accumulation at high electric fields makes nanocomposites attract significant research interest as potential insulation materials in high-voltage direct current cable development. At present, the deep trap introduced by nanoparticles is frequently applied to be responsible for the observed space charge suppression in nanocomposites. However, the experimental results that support deep-trap formation have not been rigorously examined. We therefore propose herein a simple and more direct approach based on isothermal surface potential decay combined with pulsed electro-acoustic measurements to verify the presence of deep traps in silica-based blend polyethylene nanocomposites. The results indicate that the deep traps are indeed introduced by filling nanosilica and the space charge suppression observed in the nanocomposite with a low loading ratio is caused by deeply trapped charges in the sub-surface region of specimens.
Enhanced piezoelectricity of thin film hafnia-zirconia (HZO) by inorganic flexible substrates Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-13 H. Alex Hsain, Pankaj Sharma, Hyeonggeun Yu, Jacob L. Jones, Franky So, Jan Seidel
Hf0.5Zr0.5O2 (HZO) films are grown on rigid glass and flexible polyimide substrates using non-rapid thermal annealing. Films are comparatively investigated using macroscopic and local probe-based approaches to characterize their ferroelectric and piezoelectric properties. The polarization-electric field (P-E) measurements reveal that the ferroelectric characteristics of these thin films agree with the observed switchable piezoresponse hysteresis loops as well as electrically written, oppositely oriented domains. Moreover, the HZO thin films grown on flexible polyimide substrates display significantly enhanced piezoelectric response in comparison to the films grown on rigid substrates. This effect is likely due to improved domain wall motion caused by the mechanical release of the film-substrate couple. These findings suggest that inherently lead-free HZO thin films on flexible substrates are potential candidate materials for improved piezoelectric applications in wearable devices.
Tailored tunneling magneto-dielectric effects in Co–MgF2 granular nanostructures by in-situ insertion of thin MgF2 layers Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-13 Yang Cao, Nobukiyo Kobayashi, Shigehiro Ohnuma, Hiroshi Masumoto
We report a Co–MgF/MgF heterostructure that comprises periodic layers of super-paramagnetic Co0.23–(MgF)0.77 and thin crystalline MgF, to tailor the frequency response of tunneling magneto-dielectric (TMD) effect. The results indicate that increasing MgF interlayer thickness (t) from 0 to 4 nm causes the position of peak dielectric change (Δε′/ε′0) at a specific frequency fTMD, to shift from 300 to 3 kHz, while also retaining a slight decrease in Δε′/ε′0 from 2.9% to 2.4%. The magnitude of Δε′/ε′0 can be controlled by varying the Co content in the granular layers. Theoretical curve fittings predict that the TMD effect in the heterostructure arises from both the granular layers and interlayers, and a change in inter-granular distance within the interlayers leads to a shift in the position of fTMD. This study may prove helpful for tailoring the magneto-dielectric response of granular nanocomposites to a particular frequency, with potential magnetoelectric applications over a wide frequency range.
On Raman scattering cross section ratio of crystalline and microcrystalline to amorphous silicon Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 D. M. Zhigunov, G. N. Kamaev, P. K. Kashkarov, V. A. Volodin
In this letter, we report on accurate comparative measurements of Raman scattering from bulk crystalline Si and from hydrogenated amorphous Si thin films before and after their pulse laser annealing, performed for the purpose of Si crystalline grain formation. Being accompanied by the respective optical transmittance/reflectance measurements, these data allowed us to estimate the integrated Raman scattering cross section ratios of crystalline and microcrystalline Si to hydrogenated amorphous Si and to compare the results with those known from the literature. For crystalline Si, the obtained ratio is equal to 0.75, while for microcrystalline Si, it is equal to at least 2. Our results are found to contradict the proposed earlier exponential decay dependence of the integrated Raman scattering cross section ratio of microcrystalline to amorphous Si on the crystalline grain size. The physical reasons, which support our findings, are discussed.
Yagi-Uda nanoantenna enhanced metal-semiconductor-metal photodetector Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 W. Rieger, J. J. Heremans, H. Ruan, Y. Kang, R. Claus
An array of 400 nanolithographic Yagi-Uda antennas on a metal-semiconductor-metal rectifier photodetector demonstrates control of wavelength selectivity and directivity. The nanoantenna array response is obtained using a direct electrical measurement approach. Resonances in rectified photocurrent are detected at the incident electromagnetic radiation of free-space wavelengths 1110 nm and 1690 nm, corresponding to scaled effective wavelengths of 388 nm and 776 nm, respectively. The scaling is consistent with a theory based on plasmonic effects in nanoscale devices at optical frequencies, and the two resonant wavelength modes are found to match at, respectively, full-wavelength and half-wavelength operation of the detector dipole element. Quantum efficiencies are estimated as 5.1% and 3.1% at 1110 nm and 1690 nm wavelengths, respectively, representing a fourfold increase over a device lacking the antenna array.
Off-resonance intermittent contact mode multi-harmonic scanning force microscopy Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 M. Penedo, H. J. Hug
A robust off-resonance intermittent contact mode scanning force microscopy technique suitable for operation under different environmental conditions is presented. The technique relies on a multi-channel lock-in amplifier to measure multiple high harmonic magnitudes and phases. For distance control, the fundamental harmonic magnitude is used. With this, high intermittent contact frequencies become feasible even with older atomic force microscope data acquisition systems with limited measurement bandwidths, provided high frequency tip-sample distance actuation techniques are used. Suitable higher harmonic magnitude images provide a qualitative materials' contrast. If a sufficiently high number of high harmonic magnitudes and phases are recorded, force-distance curves at all imaged points can be reconstructed. From fitting models of the contact mechanics to force versus tip-sample penetration data, the elastic modulus of the sample can be obtained.
Interdot spin transfer dynamics in laterally coupled excited spin ensemble of high-density InGaAs quantum dots Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 Satoshi Hiura, Kazuki Takeishi, Masayuki Urabe, Kodai Itabashi, Junichi Takayama, Takayuki Kiba, Kazuhisa Sueoka, Akihiro Murayama
Interdot spin transfer dynamics is studied in a laterally coupled excited spin ensemble of high-density InGaAs quantum dots (QDs). We observe a rise time of the photoluminescence intensity of ∼100 ps and a simultaneous increase in the spin polarization of the excited spin ensemble, indicating spin injection from higher-energy levels in smaller QDs. Moreover, this coupled ensemble exhibits decay properties of the spin polarization that vary with the excited spin density. This phenomenon can be quantitatively understood by considering interdot spin transfer into lower-energy levels of the surrounding QDs, where the transfer rate depends on the degree of state filling of each QD level.
Improvement of durability and switching speed by incorporating nanocrystals in the HfOx based resistive random access memory devices Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 Quantan Wu, Writam Banerjee, Jingchen Cao, Zhuoyu Ji, Ling Li, Ming Liu
Resistive random access memory (RRAM) has attracted significant interest for next-generation nonvolatile memory applications. However, it is somehow difficult to design a high speed RRAM device with enhanced data reliability. This paper deals with the improvement of high speed durable switching in nanocrystals based RRAM (NC-RRAM) devices. The high performance RRAM devices were prepared by incorporating the NCs into the HfOx oxide layer. As compared to the without (w/o) NC devices, the NC-RRAM devices are capable to execute uniform switching with higher set speed of 100 ns and reset speed of 150 ns, longer retention time and higher endurance of 108 cycles at 85 °C. The possible switching mechanism is due to the formation and rupture of the conductive filaments (CFs) inside the oxide film. The improvement of the NC-RRAM devices is due to the enhanced electric field intensity on the surface of the NCs, which can effectively facilitate the formation and rupture of the CFs.
Impact of UV spot position on forward and reverse photocurrent symmetry in a gold-diamond-gold detector Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 Kang Liu, Weihua Wang, Bing Dai, Lei Yang, Jiwen Zhao, Jingjing Xue, Benjian Liu, Zhijun Lv, Minghao Bi, Guoyang Shu, Mingqi Sun, Kaili Yao, Ge Gao, Jiecai Han, Jiaqi Zhu
A UV detector based on a planar quadrant electrode structure with a 100-μm isolating cross-gap deposited on chemical vapor deposition diamond was fabricated. An asymmetrical behavior was observed for the amplitude of photocurrent when in forward and reverse bias. A linear relationship between the sum of the photocurrents at biases U and −U, and the light spot position was also observed. The aim of this study is to clarify the mechanism underlying this asymmetrical behavior and exhibit its potential application based on this behavior. A mechanism involving lowering of the contact barrier height between gold and i-diamond by carrier generation upon UV illumination was adopted to account for the photocurrent behavior and confirmed when an opposite asymmetrical photocurrent behavior was observed in other samples. In addition, position-sensitive detection performance, based on this asymmetrical photocurrent behavior, was preliminarily studied. The results exhibited the potential for application as a position-sensitive detector.
High voltage, high current GaN-on-GaN p-n diodes with partially compensated edge termination Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-10 Jingshan Wang, Lina Cao, Jinqiao Xie, Edward Beam, Robert McCarthy, Chris Youtsey, Patrick Fay
An approach to realizing high-voltage, high-current vertical GaN-on-GaN power diodes is reported. We show that by combining a partially compensated ion-implanted edge termination (ET) with sputtered SiNx passivation and optimized ohmic contacts, devices approaching the fundamental material limits of GaN can be achieved. Devices with breakdown voltages (Vbr) of 1.68 kV and differential specific on resistances (Ron) of 0.15 mΩ cm2, corresponding to a Baliga figure of merit of 18.8 GW/cm2, are demonstrated experimentally. The ion-implantation-based ET has been analyzed through numerical simulation and validated by experiment. The use of a partially compensated ET layer, with approximately 40 nm of the p-type anode layer remaining uncompensated by the implant, is found to be optimal for maximizing Vbr. The implant-based ET enhances the breakdown voltage without compromising the forward characteristics. Devices exhibit near-ideal scaling with area, enabling currents as high as 12 A for a 1 mm diameter device.
High-efficiency virtual cathode oscillator with photonic crystal Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-12 Nikita S. Frolov, Semen A. Kurkin, Alexey A. Koronovskii, Alexander E. Hramov, Alexey O. Rak
We study the properties of microwave generation in a virtual cathode oscillator with a photonic crystal composed of metal grids. Our simulation results show the high efficiency of photonic crystal structure utilization in comparison with the standard scheme of an axial virtual cathode oscillator: operation efficiency reaches 20% at the optimal parameters. The obtained results demonstrate that the virtual cathode oscillator with a photonic crystal can be considered as a prospective high-power microwave source where the vircator operation mechanism and photonic crystal properties complement each other to produce high-power electromagnetic radiation.
Contact electrification efficiency dependence on surface energy at the water-solid interface Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-09 Amir Shahzad, K. Rohana Wijewardhana, Jang-Kun Song
Liquid-solid contact electrification is a useful mechanism to harvest wasted micromechanical energy. In this study, we investigate how the surface properties of a solid substrate affect contact electrification efficiency. Substrate surfaces were modified from hydrophilic to hydrophobic by changing the density of self-assembled monolayers (SAMs) on a SiO2 surface. A substrate with a partially-covered SAM exhibited superior performance. The partially-covered SAM substrate is hydrophobic enough to induce quick dewetting of water from the surface and sufficiently electronegative to induce a high charge density on the surface. The quick dewetting results from the aliphatic tail groups of the SAM and -OH groups make the SiO2 surface electronegative; these two competing properties can be simultaneously obtained by optimizing the SAM density. Our findings contribute to the understanding of contact electrification in liquid-solid-type energy-harvesting devices and advance the strategies to maximize the electrification efficiency by optimizing surface geometries and properties.
Enhanced stability of perovskite solar cells using hydrophobic organic fluoropolymer Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-13 Hongzhu Liu, Minhuan Wang, Jiming Bian, Yulin Feng, Zefeng Wang, Bingye Zhang, Yantao Shi
Hydrophobic organic fluoropolymers (HOFPs) with excellent hydrophobic, heat-resistant, and sunlight-transparent properties were synthesized by emulsion polymerization. The HOFP layer was inserted between a (FAMA)Pb(IBr)3 active layer and a hole transport layer in perovskite solar cells (PSCs). The performance of the resulting PSC devices depends highly on the thickness of the HOFP layer. Under optimized HOFP thickness, a moderate steady power conversion efficiency (PCE) of 16.9% was achieved. Remarkably, the optimized PSCs without any encapsulation exhibit outstanding shelf stability under ambient conditions, and the PCE could maintain 80% of its initial value after 2400 h (100 days), which was among the ever reported best stability whereas, the reference device without HOFP shows rapid severe degradation after only a few days. The significantly improved stability of PSCs was mainly ascribed to the impermeable barrier properties of the HOFP layer, which protect the perovskite active layer against moisture and oxygen from the ambient atmosphere.
Thermal tuning of silicon terahertz whispering-gallery mode resonators Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-02 Dominik Walter Vogt, Angus Harvey Jones, Rainer Leonhardt
We investigate thermal tuning of an ultra-high quality (Q) terahertz (THz) whispering-gallery mode resonator (WGMR) made of low loss silicon. The Si THz WGMR can be continuously tuned with a rate of about 29 MHz/K in the frequency range from 650 GHz to 666 GHz. Furthermore, we utilize the Si WGMR to extract the thermo-optic coefficient of Si at 459 GHz and 659 GHz in the temperature range from 295 K to 363 K.
Deep and fast free-space electro-absorption modulation in a mobility-independent graphene-loaded Bragg resonator Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-02 Spyros Doukas, Alma Chatzilari, Alva Dagkli, Andreas Papagiannopoulos, Elefterios Lidorikis
Deep and fast electro-optic modulation is critical for high-speed near infrared signal processing. We combine the electro-absorption tunability of graphene with the high-Q resonance of a Bragg-based Fabry-Perot resonator at λ = 1550 nm and show that ∼100% free-space signal modulation at small insertion loss and GHz speed can always be achieved independently of graphene quality (mobility), provided that the device operates in the reflection mode and is tuned in critical coupling with graphene. Remarkably, the critical coupling mechanism produces a higher extinction ratio for lower graphene mobility. We use practical considerations to optimize the device architecture and operation as a function of graphene mobility. With a small modification, this scheme can be turned into a very sensitive acousto-absorption modulator with an extinction ratio of ∼30 dB/ Å or an index sensor with a sensitivity of 10 7 % / RIU. These designs can be extended throughout the IR spectrum by appropriate material selection and scaling of layer dimensions.
Optical near-field mapping with a superconducting nanowire detector Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-02 Karol Luszcz, Eric Bonvin, Lukas Novotny
Optical near-fields are interesting from a theoretical perspective and of importance for practical applications, such as high-resolution imaging, sensing, and antenna-coupled quantum light sources. In this work, we use a custom-designed superconducting nanowire single-photon detector to directly read out the near-field interaction between a source and a detector. We use a subwavelength-sized aperture at the end of an optical fiber to record spatial near-field maps and to measure the distance dependence of the optical near-field interaction. Our measurements can be well described by a superposition of evanescent source fields with no noticeable probe-sample coupling. Our approach is a first step towards the development of near-field imaging techniques with single quantum sensitivity.
Terahertz quantum cascade VECSEL with watt-level output power Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-02 Christopher A. Curwen, John L. Reno, Benjamin S. Williams
We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) whose output power is scaled up to watt-level by using an amplifying metasurface designed for increased power density. The metasurface is composed of a subwavelength array of metal-metal waveguide antenna-coupled sub-cavities loaded with a terahertz quantum-cascade gain material. Unlike previously demonstrated THz QC-VECSELs, the sub-cavities operate on their third-order lateral modal resonance (TM03), instead of their first-order (TM01) resonance. This results in a metasurface with a higher spatial density of the gain material, leading to an increased output power per metasurface area. In pulsed mode operation, peak THz output powers up to 830 mW at 77 K and 1.35 W at 6 K are observed, while a single-mode spectrum and a low divergence beam pattern are maintained. In addition, piezoelectric control of the cavity length allows approximately 50 GHz of continuous, single-mode tuning without a significant effect on output power or beam quality.
Improved performance of 1.3-μm InAs/GaAs quantum dot lasers by direct Si doping Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-03 Zun-Ren Lv, Zhong-Kai Zhang, Xiao-Guang Yang, Tao Yang
We demonstrate significantly enhanced performances of 1.3-μm InAs/GaAs quantum dot (QD) lasers by directly Si-doped QDs. The lasers were grown by molecular beam epitaxy. Following Si doping, the ridge waveguide laser, with uncoated facets, showed a remarkably reduced continuous-wave threshold current density of 71.6 A/cm2 (14.3 A/cm2 per QD layer), compared with 167.3 A/cm2 (33.5 A/cm2 per QD layer) for an undoped device with an identical structure, measured at 20 °C. Moreover, doping improved the single-side slope efficiency from 0.28 to 0.42 W/A. In addition, the Si-doped QD laser exhibited a higher lasing temperature of up to 140 °C compared with 120 °C for the undoped QD laser.
Deep imaging in highly scattering media by combining reflection matrix measurement with Bessel-like beam based optical coherence tomography Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-03 Qiang Yang, Yusi Miao, Tiancheng Huo, Yan Li, Emon Heidari, Jiang Zhu, Zhongping Chen
Multiple scattering in biomedical tissue limits the imaging depth within a range of 1–2 mm for conventional optical imaging techniques. To extend the imaging depth into the scattering medium, a computational method based on the reflection matrix measurement has been developed to retrieve the singly back-scattered signal light from the dominant detrimental multiple-scattered background. After applying singular value decomposition on the measured matrix in the post-process, the target image underneath the turbid media is clearly recovered. To increase the depth of focus of the incident light by elongating the focal spot along the optical axis, a digital grating pattern is specially designed and displayed on a phase-only spatial light modulator to generate the Bessel-like beam for lateral point scanning. According to the results, the depth of focus is increased up to 2.4 mm which is much longer than the value of ∼50 μm obtained by using the conventional focused Gaussian beam, leading to a deeper penetration depth due to the self-healing feature of the Bessel-like beam. In addition, generation of the Bessel-like beam simplifies the axial scanning process by getting rid of the need to mechanically translate the focal zone along the optical axis of an objective with a high numerical aperture. By combining this method with an optical coherence tomography system with a low coherence light source, a depth-resolved optical image is obtained underneath a highly turbid medium.
Whispering gallery mode lasing in lead halide perovskite crystals grown in microcapillary Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-05 Naho Kurahashi, Van-Cao Nguyen, Fumio Sasaki, Hisao Yanagi
Microcapillaries enable us to simply prepare crystalline perovskites in a microcavity. A precursor solution of methyl ammonium lead bromide (CH3NH3PbBr3) is injected by capillary action into quartz microcapillaries with an inner diameter (ϕ) of 2–40 μm. After drying at 70 °C in the atmosphere, cylindrical crystals are precipitated to fill up the inner cavity of the microcapillary. Under optical pumping, whispering gallery mode lasing depending on ϕ is observed. With reducing ϕ, the mode number and the lasing threshold fluence are found to be decreased. This suggests that the stimulated emission can be enhanced by cavity quantum electrodynamics in CH3NH3PbBr3 crystals which are densely confined in the microcavity. Consequently, the microcapillary with ϕ = 2 μm realizes single-mode lasing at a threshold fluence of 4.7 μJ/cm2.
An approach to the measurement of the nonlinear refractive index of very short lengths of optical fibers Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-05 E. Rivera–Pérez, A. Carrascosa, A. Díez, E. P. Alcusa-Sáez, M. V. Andrés
A method for the measurement of the nonlinear-refractive index coefficient in single-mode optical fibers is presented. It takes advantage of the high sensitivity of the acousto-optic interaction effect in optical fibers to the fiber properties. Direct measurement of the nonlinear-refractive index change resulting from cross-phase modulation between a probe and a pump signal is obtained from the fiber's acousto-optic interaction performance. It is a non-interferometric method in which a very short length of fiber (<0.25 m) is required.
Electrically activated spin-controlled orbital angular momentum multiplexer Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-05 Gianluca Ruffato, Etienne Brasselet, Michele Massari, Filippo Romanato
We present and test the integration of a static orbital angular momentum mode multiplexer with a dynamical geometric-phase optical element, enabling on-demand spin-controlled angular momentum multiplexing. A diffractive optic multiplexer fabricated by 3D high-resolution electron beam lithography performs a conformal mapping for the conversion from linear to azimuthal phase gradients. The latter is functionalized by a dynamic spin-orbit add-on that consists of a self-engineered electrically activated liquid crystal optical vortex generator having large clear-aperture and high-resolution. By combining several functionalities based on the optical angular momentum of light in a compact manner, the proposed hybrid device could find applications in next-generation high-dimensional mode switchers and routers based on orbital angular momentum.
Room temperature plasmon-enhanced InAs0.91Sb0.09-based heterojunction n-i-p mid-wave infrared photodetector Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-05 Jinchao Tong, Landobasa Y. M. Tobing, Shupeng Qiu, Dao Hua Zhang, A. G. Unil Perera
Middle wavelength infrared (MWIR) photodetectors have a wide range of applications, but almost all of them operate at low temperature due to the limit of materials and device structures. The capability of plasmonic structures to localize electromagnetic wave on the deep subwavelength scale provides the possibility for MWIR photodetectors operating at room temperature. Here, we report a high sensitivity room temperature MWIR photodetector which is an InAs0.91Sb0.09-based heterojunction n-i-p photodiode integrated with a Au-based two-dimensional subwavelength hole array (2DSHA). A room temperature detectivity of 0.8 × 1010 cm Hz1/2 W−1 and a response time of 600 ns are achieved. The non-cooling high performance of 2DSHA-InAs0.91Sb0.09 based heterojunction photodetectors will make their applications easier, broader, and economic.
226 nm AlGaN/AlN UV LEDs using p-type Si for hole injection and UV reflection Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-05 Dong Liu, Sang June Cho, Jeongpil Park, Jiarui Gong, Jung-Hun Seo, Rafael Dalmau, Deyin Zhao, Kwangeun Kim, Munho Kim, Akhil R. K. Kalapala, John D. Albrecht, Weidong Zhou, Baxter Moody, Zhenqiang Ma
Deep ultraviolet (UV) light-emitting diodes (LEDs) at a wavelength of 226 nm based on AlGaN/AlN multiple quantum wells using p-type Si as both the hole supplier and the reflective layer are demonstrated. In addition to the description of the hole transport mechanism that allows hole injection from p-type Si into the wide bandgap device, the details of the LED structure which take advantage of the p-type Si layer as a reflective layer to enhance light extraction efficiency (LEE) are elaborated. Fabricated LEDs were characterized both electrically and optically. Owing to the efficient hole injection and enhanced LEE using the p-type Si nanomembranes (NMs), an optical output power of 225 μW was observed at 20 mA continuous current operation (equivalent current density of 15 A/cm2) without external thermal management. The corresponding external quantum efficiency is 0.2%, higher than any UV LEDs with emission wavelength below 230 nm in the continuous current drive mode. The study demonstrates that adopting p-type Si NMs as both the hole injector and the reflective mirror can enable high-performance UV LEDs with emission wavelengths, output power levels, and efficiencies that were previously inaccessible using conventional p-i-n structures.
Quantum stereomagnetometry with a dual-core photonic-crystal fiber Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-05 S. M. Blakley, I. V. Fedotov, J. Becker, A. M. Zheltikov
A monolithic dual-core photonic crystal fiber (PCF) probe employing an integrated nitrogen-vacancy diamond microcrystal and microwave transmission line is shown to enable highly sensitive dual-channel vectorial magnetic field measurements by means of optical detection of magnetic resonances with both fiber cores. Reliable detection of microscale spatial magnetic-field variations on the order of 10 μT is demonstrated with a PCF probe possessing a core-to-core separation of 6 μm. Such a fiber probe is shown to provide a powerful tool for three-dimensional vectorial mapping of weak magnetic fields emanating from spatially localized objects.
Proof of concept for an optogalvanic gas sensor for NO based on Rydberg excitations Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-06 J. Schmidt, M. Fiedler, R. Albrecht, D. Djekic, P. Schalberger, H. Baur, R. Löw, N. Fruehauf, T. Pfau, J. Anders, E. R. Grant, H. Kübler
We demonstrate the applicability of 2-photon Rydberg excitations of nitric oxide (NO) at room temperature in a gas mixture with helium (He) as an optogalvanic gas sensor. The charges created initially from preceding collisions of excited NO Rydberg molecules with free electrons are measured as a current on metallic electrodes inside a glass cell and amplified using a custom-designed high-bandwidth transimpedance amplifier attached to the cell. We find that this gas sensing method is capable of detecting NO concentrations lower than 10 ppm even at atmospheric pressures, currently only limited by the way we prepare gas dilutions.
Effect of the surface wettability changes on nanostructured polymer film for heat exchanger applications Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-03 Cheonji Lee, Muhammad Salman Abbasi, Seungchul Park, Hyuneui Lim, Jinkee Lee
Polymer heat exchangers have drawn attention due to their special characteristics such as flexibility, low weight, corrosion, and bio-fouling resistance, as well as their ease of manufacturing. However, since their thermal conductivity is low, they require a way to increase their heat transfer rate. We investigated the effect of modifying the surface wettability of polymer films on the heat transfer rate during the condensation process, both theoretically and experimentally. Condensed water formation on the film surface depends on the surface wettability control, and thus, hydrophilic or hydrophobic surfaces can induce film-wise or drop-wise condensation, respectively. Generally, the polymer surface shows a hydrophobic property due to the intrinsic C-C and C-H bonding of polymer frames. Here, we used four different types of polymer films, which have different thermal conductivity and controlled wettability properties, including polyimide film, polyimide film containing aluminum nanoparticles, and both films with super-hydrophobic treatment. The results show that nanostructured polymers with hydrophobic treatment have a 25% lower wetted area fraction than those without hydrophobic treatment, independent of the thermal conductivity of the polymer films. To compare the heat transfer rates, we designed a thermal resistance model considering vapor convection, vapor-water interfaces and curvatures, and conductions of the droplet, nanostructure, and polymer. We found that the convective thermal resistance dominantly affects the heat transfer rate and treated polymers showed over 200% higher total heat transfer than bare polymer.
Enhanced n-doping of epitaxial graphene on SiC by bismuth Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-03 Tingwei Hu, Qinglong Fang, Xiaohe Zhang, Xiangtai Liu, Dayan Ma, Ran Wei, Kewei Xu, Fei Ma
Doping in epitaxial graphene (EG) is challenging because of the high-temperature process and the ultra-thin nature of graphene. In this work, a facile one-step method is demonstrated to generate doping in EG with bismuth (Bi) during thermal decomposition of SiC, in which Bi atom flux acts as the doping source. Raman spectroscopy and scanning tunneling microscopy/spectroscopy are employed to characterize the quality, morphology and electronic properties of Bi doped EG. Both the intercalated and incorporated Bi atoms can be considered as dopants. It was found that the Dirac point shifts away from the Fermi level as a result of electron transfer from Bi to EG, and thus enhances the n-doping behavior of EG significantly. First principles calculations were done to address the enhanced n-doping of EG by Bi. This in-situ doping procedure can be extended to other metals, showing great potential applications in tailoring the performance of EG and significance to electronics in the future.
Investigation of multipactor-induced surface plasma discharge and temporal mode transition Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-05 Guang-Yu Sun, Bai-Peng Song, Guan-Jun Zhang
Multipactor over a dielectric in vacuum inclines to engender interfacial gas desorption or evaporation, precipitating surface flashover and insulator failure. However, no consensus has been achieved regarding the exact mechanism during final breakdown stage, an expatiation of which therefore serves as our major motivation for this letter. By implementing the particle-in-cell simulation code, we investigate the microscopic evolution of the discharge development process and confirm the major component escalating the explosive space charge accumulation. The obtained current waveform validates the balance of charged particles between electrodes, corroborated by experimental results. A theoretical discharge model is then constructed to elucidate the physical reasoning of the previous phenomenon. Two distinct discharge modes are defined correspondingly, and the transition therein is found to be induced by rapid plasma density build-up.
GaAs/GaNAs core-multishell nanowires with nitrogen composition exceeding 2% Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-02 M. Yukimune, R. Fujiwara, H. Ikeda, K. Yano, K. Takada, M. Jansson, W. M. Chen, I. A. Buyanova, F. Ishikawa
We report the growth of GaAs/GaNAs/GaAs core-multishell nanowires having N compositions exceeding 2%. The structures were grown by plasma-assisted molecular beam epitaxy using constituent Ga-induced vapor-liquid-solid growth on Si(111) substrates. The GaNAs shell nominally contains 0%, 2%, and 3% nitrogen. The axial cross-sectional scanning transmission electron microscopy measurements confirm the existence of core-multishell structure. The room temperature micro-photoluminescence measurements reveal a red-shift of the detected emission with increasing N content in the nanowires, consistent with the expected changes in the GaNAs bandgap energy due to the bowing effect.
Oscillating planar Hall response in bulk crystal of topological insulator Sn doped Bi1.1Sb0.9Te2S Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-02 Bin Wu, Xing-Chen Pan, Wenkai Wu, Fucong Fei, Bo Chen, Qianqian Liu, Haijun Bu, Lu Cao, Fengqi Song, Baigeng Wang
We report the low-temperature magneto-transport in the bulk-insulating single crystal of topological insulator Sn doped Bi1.1Sb0.9Te2S. Shubnikov-de Haas oscillations appear with their reciprocal frequency proportional to cos θ, demonstrating the dominant transport of topological surface states. While the magnetic field rotates on the sample surface, the planar Hall effect arises with sizeable oscillations following a relation of cos θ sin θ. Its amplitude reaches the maximum at the lowest temperature and drops to nearly zero at temperature higher than 100 K. All these evidences consolidate such planar Hall oscillations as another golden criterion on the topological surface transport.
Deformation induced frequency shifts of oscillating droplets during molten metal surface tension measurement Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-05 Xiao Xiao, Robert W. Hyers, Rainer K. Wunderlich, Hans-Jörg Fecht, Douglas M. Matson
Surface tension is an essential thermophysical property of liquids, and the oscillating droplet method is particularly effective for investigations involving reactive molten alloys. The Rayleigh equation is commonly used to evaluate surface tension from measurements of the damping frequency response of an oscillating droplet with small deformation, but non-linear effects are expected to arise for larger deformation. This work describes an improved method for interpreting frequency analysis and validates previous numerical simulation and theoretical analyses which predict a decrease in observed frequency at moderate deformation amplitude. Experimental results from microgravity tests are used to determine a correction of the Rayleigh equation to eliminate the influence of finite deformation.
Direct observation of stacking fault shrinkage in 4H-SiC at high temperatures by in-situ X-ray topography using monochromatic synchrotron radiation Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-02 Fumihiro Fujie, Shunta Harada, Haruhiko Koizumi, Kenta Murayama, Kenji Hanada, Miho Tagawa, Toru Ujihara
An in-situ X-ray topography system using monochromatic synchrotron radiation for the observation of the stacking faults in 4H-SiC during a high-temperature annealing process was developed. We demonstrated that the stacking faults in nitrogen-doped 4H-SiC not only expand but also shrink at high temperatures. Furthermore, it was confirmed that the types of the core structure of partial dislocations enclosing the stacking fault can be determined at high temperatures.
Entanglement control and magic angles for acceptor qubits in Si Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-03 J. C. Abadillo-Uriel, Joe Salfi, Xuedong Hu, Sven Rogge, M. J. Calderón, Dimitrie Culcer
Full electrical control of quantum bits could facilitate fast, low-power, scalable quantum computation. Although electric dipoles are highly attractive to couple spin qubits electrically over long distances, mechanisms identified to control two-qubit couplings do not permit single-qubit operations while two-qubit couplings are off. Here, we identify a mechanism to modulate electrical coupling of spin qubits which overcomes this drawback for hole spin qubits in acceptors which is based on the electrical tuning of the direction of the spin-dependent electric dipole by a gate. This allows the inter-qubit coupling to be turned off electrically by tuning to a “magic angle” of vanishing electric dipole-dipole interactions, while retaining the ability to manipulate the individual qubits. This effect stems from the interplay of the Td symmetry of the acceptor state in the Si lattice with the magnetic field orientation and the spin-3/2 characteristic of hole systems. The magnetic field direction also allows us to greatly suppress spin relaxation by phonons that limit single qubit performance, while retaining sweet spots where the qubits are insensitive to charge noise. We propose suitable protocols to practically achieve full electrical tunability of entanglement and the isolation of the qubit.
Control of dipole properties in high-k and SiO2 stacks on Si substrates with tricolor superstructure Appl. Phys. Lett. (IF 3.495) Pub Date : 2018-07-03 Yasushi Hotta, Iwao Kawayama, Shozo Miyake, Ikuya Saiki, Shintaro Nishi, Kota Yamahara, Koji Arafune, Haruhiko Yoshida, Shin-ichi Satoh, Naomi Sawamoto, Atsushi Ogura, Akira Ito, Hidetoshi Nakanishi, Masayoshi Tonouchi, Hitoshi Tabata
The concept of the tricolor superstructure (TCS), which is a triple-layer stack structure containing two types of high dielectric constant (high-k) layers (designated HK1 and HK2) and a SiO2 layer, is proposed to control the moment and the polarity of the interface dipole layer that are induced at the high-k/SiO2 interfaces. The interface dipole layer is formed by oxygen ion migration from the layer with higher oxygen areal density (σ) to that with lower σ. When the two high-k materials are selected with the order of σHK1 > σSiO2 > σHK2 in a SiO2/HK2/HK1/SiO2 TCS, the dipole directions of the interface dipole layers at the SiO2/HK2 and the HK1/SiO2 interfaces are aligned. Additionally, in the transposed SiO2/HK1/HK2/SiO2 TCS, the total polarity is reversed. The concept is demonstrated using Al2O3 and Y2O3 layers because they offer the order of σAl2O3 > σSiO2 > σY2O3. The two stacking sequence samples composed of SiO2/Y2O3/Al2O3/SiO2 and SiO2/Al2O3/Y2O3/SiO2 that were fabricated using superlattice technique by pulsed laser deposition obviously show opposite dipole polarities. Increasing repetition of the deposited TCS unit also causes the dipole moments to increase systematically. The TCS technique enables control of the properties of the interface dipole layer at high-k/SiO2 interfaces in amorphous systems.
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