THz spintronic emitters: a review on achievements and future challenges

4.1 Material dependence

A variety of materials combination of ferromagnetic (FM)/nonmagnetic (NM) layer systems have been so far explored as spintronic emitters. We initially focus on the choice of the magnetic layer. Direct comparison between the 3d metals like Fe, Co and Ni, in structures like FM (3 nm)/Pt (3 nm) [5] and in FM (2 nm)/Pt (5 nm) [23] have revealed that Ni yields the lowest signal with Fe having a slightly higher signal than Co. The comparison of ferromagnetic alloys like Ni₈₉Fe₁₉, Co₇₀Fe₃₀, Co₄₀Fe₄₀B₂₀ and Co₂₀Fe₆₀B₂₀/Pt (3 nm) [5] showed the prominent role of the CoFeB alloy. Sasaki et al. [24] varied the composition of Co and Fe in the CoFeB alloy. They found that the THz emission in Ta/(Co_xFe_1−x)₈₀B₂₀ is enhanced at compositions of approximately x = 0.1–0.3 which show a maximum saturation magnetization. Modification of the magnetic layer with ferrimagnetic gadolinium and terbium-iron alloys was achieved by Schneider et al. [25], [26]. Both systems Gd_xFe_1−x/Pt and Tb_xFe_1−x/Pt present the highest THz emission for small rare-earth content. However, the Gd_xFe_1−x exhibit up to 17 times higher amplitudes. A strong THz output was observed from compensated ferrimagnetic Co_1−xGd_x (7 nm)/Pt (6 nm) bilayers. The THz signal decreases as the Gd fraction increases. At the compensation point for x = 26, the almost zero net magnetization was not strongly correlated to the emitted THz signal [20]. The replacement of the ferromagnetic layer with DyCo₅, Gd₂₄Fe₇₆, Fe₃O₄ and FeRh showed that they exhibit lower THz emission with respect to CoFeB layer [27]. Furthermore, Co-based Heusler alloys and in particular Co₂MnSi (CMS) was investigated as spin injector in CMS/Pt bilayers [28]. The achievement of the highly ordered B2 structural phase after annealing led to up to two times higher THz emission compared to CoFe/Pt bilayers. The effect was attributed to the small conductivity and the comparable to FeCo spin current injection of the CMS samples.

The efficiency of the conversion of spin-to-charge current is quantified by the spin Hall angle and the spin current relaxation length of the nonmagnetic layer. Different nonmagnetic materials have been studied so far for THz emission. The THz signal amplitude seems to scale with the intrinsic spin Hall conductivity of the used NM layer [5], see Figure 4. The THz emission was even used to estimate the relative spin Hall angle [21], [29]. The prominent choice at the moment is Pt since it has provided so far the highest signal compared to W [21], Cu₈₀Ir₂₀ [21], Ta [29], Ru [29], [30], Ir [29], Pd [30], Pt₃₈Mn₆₂ [5]. Interestingly, NM layers with opposite sign of the spin Hall angle compared to Pt give rise to THz signals with opposite polarities and subsequently confirm the ISHE origin of the THz emission.

Figure 4:

Spin Hall angle and spin Hall conductivity for the nonmagnetic (NM) layer. Pt has so far the highest efficiency for THz emission.

Reprinted by permission from Springer Nature, Nature Photonics, T. Seifert et al. [5] Copyright 2016.

The spin-to-charge-conversion mechanism driven by the spin Seebeck effect was additionally probed in insulating magnetic/NM interfaces like YIG/Cu_1−xIr_x [31] and YIG/Pt [32] showing, however, much lower efficiency of the THz emission compared to the metallic magnetic layers.

4.2 Thickness dependence

Various thickness dependence studies on the efficiency of the THz emission have revealed the critical role of the thicknesses of the individual layers. Studies on the variation of the thickness of the NM-layer and by keeping constant the thickness of the FM layer have shown the existence of an optimum thickness around 2–3 nm [13]. This range of thicknesses holds for different NM-layers like Pt [21], [29], [13], W [21], [29] where the maximum signal is obtained, see Figure 5. By fitting the NM-thickness dependence of the THz amplitude, the spin current diffusion length was extracted [21], [13]. Surprisingly, the latter was found to agree with values extracted from GHz spin pumping experiments although the excitation of the spin current is performed in different time- and energy scales. The physical mechanism behind this similarity is currently under on-going research [36].

In the same way, for a constant NM-layer thickness, there is an optimum FM layer thickness in the same range of about 2–3 nm [21], [29], [13]. When the ferromagnetic layer is too thin, then either the loss of magnetic order due to heating effects or the reorientation of the magnetization due to monolayer thickness cause a drastic reduction of the THz signal. The theory of the effect of changing layer thickness was first developed in the study by Seifert et al. [5] for detection of THz-signal from NM-layer side. A further expansion of the model was achieved in the study Torosyan et al. [13], and it is expressed by Eq. (2). The equation holds when the fs-laser illuminates first the NM-layer side and the detection is performed after the THz-signal has travelled through the substrate (see also the study by Torosyan et al. [13]):

where n_air, n_Sub and Z₀ are the index of refraction of air, the index of refraction of the substrate at THz frequencies and the impedance of vacuum, respectively. Equation (2) takes into account all successive effects that take place, after the laser pulse impinges on the bilayer. In particular, the first term contains the spin Hall angle, the second term accounts for the absorption of the femtosecond laser pulse in the metal layers. As only spin-polarized electrons within a certain distance from the boundary between FM and NM will reach the NM layer, only a fraction of the measured total absorbed power contributes to the generated THz signal. This fraction scales with the inverse of the total metal layer thickness 1/(d_Fe + d_Pt). The third term describes generation and diffusion of the generated spin current flowing in FM toward the interface with NM. The possibility that extra thin FM layers can lose their ferromagnetic properties below a certain thickness can be captured by introducing the term d₀ in Eq. (2). Below this critical thickness, it is considered that the flow (if any) of spin current in FM does not reach the NM layer. Above this critical thickness, the generated spin polarization saturates with a characteristic constant λ_pol. The fourth and fifth terms, the tangent hyberbolic function divided by the total impedance, refer to the spin accumulation in Pt, which is responsible for the strength of the THz radiation and it depends on the finite diffusion length λ_Pt of the spin current in NM layer. The symbols σ_Fe and σ_Pt correspond to the electrical conductivities of the layers. This part of the equation also includes the shunting effect of the parallel connection of the resistances of the individual FM and NM layers. The last term describes the attenuation of the THz radiation during propagation through the metal layers (with s_THz as an effective inverse attenuation coefficient of THz radiation in the two metal layers. The last term is required since as aforementioned, the sample is excited from the outer Pt side, whereas the THz radiation is collected behind the substrate, thus propagating through both metal layers [13]. In the case of small losses, the attenuation can be taken into account by this single exponential factor and the fourth factor, which accounts for the multiple reflections of the THz pulse at the metal/dielectric interfaces. All terms together describe the layer thickness dependence of the measured THz amplitudes.

Furthermore, additional factors have been proposed to influence the thickness dependence like the factor of interfacial spin memory loss [37] in Co/Pt bilayers.

Figure 5:

(Upper panel) Nonmagnetic layer thickness dependence of THz emission for different NM layers like Pt, W, CuIr in CoFeB/NM bilayers [21], and the geometrical arrangement of the experiment. Reproduced under the terms of a Creative Commons Attribution 3.0 Licence [21], 2018 IOP Publishing Ltd.

(Lower panel) (a) Pt thickness dependence of the THz field amplitude for a constant Fe thickness of 12 nm. (b) Fe thickness dependence of the THz field amplitude for a constant Pt thickness of 3 nm [13]. The geometrical arrangement is also depicted to the right of the graph. Reproduced under a Creative Commons Attribution 4.0 International License [13], The Authors 2018, published by Springer Nature.

4.3 Wavelength dependence

The majority of the experiments up to now with spintronic THz emitters are performed at 800 nm excitation wavelength using femtosecond Ti:sapphire lasers. Would different excitation energies of the incoming fs-laser pulses influence the spin current dynamics and subsequently the THz-emission? Recent investigations have tried to answer this question using different excitation wavelengths. Papaioannou et al. [38], used an optimized spintronic bilayer structure of 2 nm Fe and 3 nm Pt grown on 500 μm MgO substrate to show that the emitter is just as effective as a THz radiation source when excited either at λ = 800 nm or at λ = 1550 nm by ultrafast laser pulses from a fs fiber laser (pulse width 100 fs, repetition rate 100 MHz). Even with low incident power levels, the Fe/Pt spintronic emitter exhibits efficient generation of THz radiation at both excitation wavelengths. It should be mentioned that there is a linear dependence of the generated THz amplitude on pump power in the low-power excitation regime for both pump wavelengths (see Figure 6). At higher pump powers, there will be a deviation from the linear behavior, as also shown in the study by Yang et al. [23]. Herapath et al. [39] also found that the efficiency of THz generation is essentially flat for excitation by 150 fs pulses with center wavelengths ranging from 900 to 1500 nm, using a CoFeB ferromagnetic layer between adjacent nonmagnetic W and Pt layers. From both experiments, it seems that the crucial factor is the amount of energy that is deposited by the pump pulse in the electronic system and not the details of the involved optical transitions. By further probing the THz emission of Fe/Pt bilayers at λ = 400 nm [40], the spintronic THz emission efficiency of an optimized spintronic emitter was equal strong as with probing at 800 and 1550 nm. So it still remains independent of the optical pump wavelength. In addition, the efficiency is highly tunable with optical pump power [40]. The reason behind the observed effect is a consequence of the fact that the out of equilibrium transport is done not only by the electrons directly excited by the laser, but also by all the electrons that are excited at intermediate energies due to the scattering of the first-generation electrons. Considering that the electron–electron scattering lifetimes sharply decrease with the energy of the excited electron, the direct impact of high energetic electrons on the transport is expected to be minor at high energy excitation. Instead, the most important impact is that, by scattering with another electron, the electrons will lose energy, they will go down to an intermediate energy and transfer that energy to secondary electrons. This results in an intermediate energy electrons multiplication, which is very similar to a direct excitation by a laser at lower frequency. Even with lower energies now, they will have longer lifetimes and they will contribute to the transport more importantly. If the system is excited with very low-frequency photons, other effects can become important, changing the qualitative scenario mentioned. In this case, one has to take into account that the spin diffusion requires not only asymmetry between the two spin channels, but also between electrons and holes.

Figure 6:

Excitation wavelength dependence of the THz emission.

(Left panel) Maximum THz-E-field as a function of pump power of the laser recorded at 1550 nm (black squares) and 800 nm pump wavelength, λ_excitation. Figure modified from the study by Papaioannou et al. [38], Copyright 2018, IEEE. (Right panel) THz-E-field, emitted from a W/CoFeB/Pt sample as a function of the pump wavelength while the energy, focus diameter and duration of the pump pulses were kept constant. Reprinted from the study by Herapath et al. [39] with the permission of AIP Publishing, Copyright 2019.

4.4 Interface dependence

4.4.1 Interface dependence—structural properties

Even though the transfer of a spin current from a FM to a NM layer (that is, the source of THz emission) is a highly interface-sensitive effect, only few investigations have tried to correlate the structural quality of the interface with the THz signal strength and spectrum. A direct comparison of an epitaxial Fe(3 nm)/Pt(3 nm) bilayer with a signal-optimized polycrystalline CoFeB/Pt structure with the same layer thicknesses revealed a comparable THz signal strength [5]. In contrast, a significant increase in signal amplitude between Fe/Pt emitters grown epitaxially on MgO (100) substrates compared to polycrystalline emitters grown on sapphire substrates was reported by Torosyan et al. [13]. Similarly, the better crystal quality of a CoFeB layer, controlled by the annealing temperature, has significantly enhanced THz emission intensity [41]. Nenno et al. [22] have investigated in detail the performance of spintronic terahertz emitters by modifying the interface quality and its defect density. In particular, the presence of defect density was correlated with the elastic electron-defect scattering lifetime τ_el in the FM and NM layers and the interface transmission T for spin-polarized, nonequilibrium electrons. A decreased defect density increases the electron-defect scattering lifetime and this results in a longer-lasting and stronger spin current pulse. Accordingly, a significant enhancement of the THz-signal amplitude and a shift of the spectrum toward lower-THz frequencies [22] were observed, see Figure 7. Furthermore, besides the defect density, the presence of the parameter of the interface transmission T plays an important role. The latter is correlated to the ability of the interface to transfer hot carriers into the NM layer. It was shown [22] that the interface transmission influences the spectral amplitude of the emitted THz field but conserves the composition of the spectrum, see Figure 7.

Figure 7:

Dependence of the THz emission on the structural quality of the interface.

(Upper panel-Left) Numerical simulations of the Boltzmann transport equation for an Fe/Pt spintronic emitter for different elastic scattering lifetimes τ_el. The temporal evolution of the laser pulse intensity with a length of 70 fs is also shown on top. (Upper panel-Right) Spectral amplitude of the simulated spin current (and consequently of the charge current) inside Pt layer obtained by the Fourier transform of the pulses on the left. (Lower panel—Left) Experimental THz-E-field amplitudes for Fe/Pt samples with different τ_el as detected after traveling the optical beam path of a THz-TDS setup. (Lower panel—Right) Experimental spectra amplitudes of Fe/Pt heterostructures with different interface transmissions T. Figure modified from the study by Nenno et al. [22].

Similarly, Li et al. [42] showed the decisive role of the microstructural properties of a Co–Pt interface in the THz emission. High interfacial roughness between Co–Pt interface led to a decrease of the THz emission. The introduction of an Co_xPt_1−x alloy as a spacer at the Co–Pt interface showed that the intermixing amplified the THz emission. Maximum amplification by a factor of 4.2 was achieved for a Co₂₅Pt₇₅(1 nm thick) interlayer. Possible explanation could be the higher flux of spin currents into Pt due to reduction of spin decoherence at the interface caused by the presence of the alloy.

4.5 Stack geometry dependence

The geometrical arrangement of the layers can play a decisive role for the THz emission (see Figure 8). Seifert et al. [5] used instead of a FM/NM bilayer a trilayer in the form of NM1/FM/NM2 layers. The special feature of the trilayers is that the NM1 and NM2 layers have opposite spin Hall angles, like W and Pt. In such a way, the spin Hall currents in W and Pt flow in the same direction and radiate in phase and thus enhance the THz amplitude. Large improvement of the signal was achieved in multilayers made of Fe/Pt/MgO [23]. The MgO layer as an insulating layer hinders the flow of the spin current. The spin current can flow from each Fe layer to its neighboring Pt layer and generate THz radiation. Since the transverse charge currents are almost in phase, the THz signal from each layer is added constructively and boost the emission. Maximum signal was observed for 3 repetitions of Fe/Pt/MgO layers while above 3 the detected THz signal dropped, due to the absorption of light. Hibberd et al. [44] followed another route to manipulate the THz radiation. They altered not the layer stack but the magnetic field pattern that is applied to the spintronic source. The goal was to manipulate the magnetic state of the ferromagnetic layer and therefore the polarization profile of the THz radiation. When the spintronic source was placed between two magnets of opposing polarity, THz radiation with a quadrupole-like polarization was generated. When focused, it resulted in longitudinal THz electric field amplitudes twice as much as that of the transverse signals achieved with linearly polarized THz radiation. Ogasawara et al. [45] used synthetic magnets to manipulate the THz radiation. In particular, they prepared NM1(Ta)/CoFeB(FM)/Ir(NM2)/CoFeB(FM)/Ta(NM2) synthetic magnetic multilayers where the Ir spacer had different thicknesses. In such stacking, THz emission is expected when the magnetizations of the CoFeB layers are antiparallel. The authors observed THz emission under no applied magnetic field for samples with antiferromagnetic coupling and oscillations of the signal with respect to Ir thicknesses. In addition, Li et al. [46] showed as a proof-of-concept, the use a magnetic tunnel junction (MTJ) as a THz source. They used a MTJ multistack of Ta/Ru/pinned FM stack/MgO/free FM stack/Ta/Ru. The source of the THz radiation due to the ISHE was the free FM layer/Ta interface while the pinned FM layer controlled the magnetic state of the free FM layer and so the THz signal strength and polarization. A different kind of multilayered structure was examined by Fix et al. [47] composed of ferrimagnetic (FIM) rare-earth-3d transition metal alloys. In particular, the structure had the form of Pt(3 nm)/Gd₁₀Fe₉₀(3 nm)/W(3 nm)/Gd₃₀Fe₇₀ (3 nm)/Pt(3 nm). The combination of NM-layers with opposite spin Hall angle (Pt, W) with FIM-layers of GdFe with suitable film thickness and composition was utilized to control the terahertz high and low emission state by temperature. Temperature was able to switch the relative alignment of the Fe moments in the different GdFe layers and thus the strength of the THz emission. Another approach was presented by Feng et al. [48] where the performance of the spintronic THz emitter was improved by utilizing optics. In particular, the work utilized metal (NM1/FM/NM2)-dielectric photonic crystal structure where the metal Pt (1.8 nm)/Fe(1.8 nm)/W (1.8 nm) served as the spintronic emitter and the dielectric interlayers was SiO₂, while the maximum number of layer repeats in the photonic crystal was 3. The goal was to suppress the reflection and transmission of laser light simultaneously aiming to maximize the laser field strength in the metal layers. As a result, the maximization of the laser energy absorption in the metallic emitter improved the conversion efficiency of about 1.7 times compared to a NM1/FM/NM2 single THz emitter. Similarly, one year later, Herapath et al. [39] utilized a dielectric cavity composed of TiO₂ and SiO₂, which was attached on a W/CoFeB/Pt emitter resulting in an enhancement factor of 2 in the THz emission. Chen et al. [49] used a cascade of two spintronic emitters to produce circularly polarized terahertz waves. They used the residual transmitted optical pump power after the first emitter to excite the second-stage emitter. Equal amplitudes were achieved by adjusting the losses of the pump laser in the first emitter. The arrangement of the applied magnetic field directions on both emitters was such that perpendicular electric-field directions were achieved for the terahertz beams generated in the two stages. By adjusting the refractive index difference of low-pressure air between optical and terahertz-frequency ranges, a phase difference between the terahertz waves was introduced in such a way that the mixing of the waves could lead to arbitrary control of the terahertz polarization-shaping, including also the chirality.

Figure 8:

Graphical representation of geometrical stacking order that can influence the intensity, bandwidth and the polarization state of the emitted THz signals. FM denotes the ferromagnetic layer, NM the nonmagnetic layer, IL the isolation layer, STE is the spintronic emitter, D1,2 the dielectric layers and INT the interlayer in synthetic magnetic structures.

5 Future perspectives of THz spintronic emitters

THz spintronic emitters are certainly a new direction in physics with a huge potential for technological applications. They define the new field of ultrafast terahertz optospintronics. The emitters have many advantages: they are of low cost, ultrabroadband, easy to use and robust. The performance of the emitters is comparable to state-of-the-art terahertz sources like the ones routinely used to cover the range from 0.3 to 8 THz, the nonlinear optical crystals like ZnTe (110), GaP and high-performance photoconductive switches [5]. The challenge nowadays is to generate THz broadband radiation with sufficient power. In this direction, Seifert et al. [50] explored the up-scaling capabilities of metallic spintronic stacks in order to achieve intense THz sources. They used large area W/CoFeB/Pt stack of 5.6 nm thickness.

The emitted THz pulse was found to reach energies of 5 nJ with peak field values of up to 300 kVcm⁻¹ when excited with a laser pulse energy of 5.5 mJ. This corresponds to a conversion efficiency of 10⁻⁶. Similar conversion efficiencies were reported for high repetition rate systems with repetition rates of 100 MHz and moderate average powers (up to 500 mW) using Fe/Pt bilayers [38]. The estimated absolute average power of the generated THz pulses was almost 50 μW for a pump power of 500 mW using 800 nm excitation wavelength. Taking into account a THz pulse length of approximately 1 ps, this corresponds to a comparable conversion efficiency.

Currently, the down-scaling through nanopatterning of the emitters is under investigation. In one of the first investigations on patterned emitters by Yang et al. [23], Fe/Pt heterostructures were patterned in stripes of 5 μm width and a spacing of 5 μm and measurements were performed with the magnetic field along and perpendicular to the stripes. Anisotropic emission was observed with higher intensity for the perpendicular configuration and a blue shift in frequency compared to the parallel configuration. Further, modulation of THz radiation in terms of magnitude, bandwidth and center frequency was achieved based on W/CoFeB/Pt stripe patterns [51]. The patterns had dimensions of 5 μm width and 5 μm spacing (see Figure 9, upper panel). By changing the angle between the applied magnetic field and the stripe direction, modulation of the THz emission was measured. Interestingly, Jin et al. [51] attributed this effect to nanoconfinement of the charge current due to nanopatterning. For a specific configuration of stripe and magnetic field at 90°, the spatial separation of the photo-induced positive and negative charges accumulate and then create a nonequilibrium electric potential U(t) at the edge of the stripe. The transient potential creates an opposite built-in electric field. The latter induces a back flowing current, changes the effective length of the charge flowing and so alters the THz emission. Song et al. [52] fabricated CoFeB/Pt patterned as rectangles and orthogonal structures of 20 μm width and various lengths L, of 20, 40, 80, 160 and 320 μm, while the separation between the elements was 10 μm (see Figure 9, lower panel). They observed a reshaping of the THz wavefront, which indicates that the pattern structure influences the temporal distribution of the transient dynamics of the charge current. The FWHM decreased by decreasing L pointing to the backflow effect as mentioned before, while the center frequency increases by decreasing L as a result of the antenna effect of the microstructures, where it is expected that shorter antennas will lead to higher oscillation frequencies. Microfabricated spintronic emitters were also studied by Wu et al. [53]. Fe (3 nm)/Pt (3 nm) bilayers were patterned by optical lithography in the form of stripes of constant separation of 510 μm and a variable width of 240, 300, 400 and 490 μm. The THz waveform was modulated by the different stripe widths. As the stripe width is reduced, a second peak in the spectra appears, showing that the bandwidth can be increased in such microstructures. Wu et al. interpreted the results in terms of a simplified multislit interference model, which can be a useful tool for designing future patterned emitters. Another approach in patterning was followed by Nandi et al. [54] where the spintronic trilayer of W (2 nm)/CoFeB (1.8 nm)/Pt (2 nm) was inserted in the gap of two antenna types: a slotline (with spacing of 25 μm) and an H-type dipole antenna (with length of 200 μm and gap size of 10 μm). Such antennas are frequently used to improve the radiation efficiency in the state-of-the-art semiconductor-based photoconductive THz emitters and receivers. The coupling between the spintronic emitter and the antennas led to a significant increase in the THz signal compared to plain continuous film emitters. The incorporation of spintronic emitters with already existing THz technologies can drive the on-chip application of this new class of THz emitters based on ultrafast spin and charge currents. In conclusion, the first studies in nanopatterned emitters revealed the ability to modify the THz emission, rendering nanostructuring as a key ingredient for future applications of THz emitters. Meanwhile the first implementation of spintronic emitters in real applications has been realized. Arrays of W/Fe/Pt were proposed to form a THz near-field microscope capable of illuminating an object at an extreme near field [55]. Guo et al. [56] examined the possibility to integrate W/CoFeB/Pt heterostructures in laser terahertz emission microscopy (LTEM) technique. Müller et al. [57] have demonstrated efficient coupling of THz-pulses emitted from W/CoFeB/Pt trilayer to the junction of a scanning tunneling microscope, that has the potential to enable spatiotemporal imaging with femtosecond temporal and sub-nm spatial resolution.

Certainly, future investigations will deal with THz emission from giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) structures, where the potential for THz manipulation by defining the magnetic states of the layers is large. Open challenges for the coming years are the use of spintronic structures as detectors of THz radiation, the exploration of appropriate materials, interfaces and nanopatterns, the electrical detection of the ultrashort current pulses, and the use of emission for microscopy and imaging in the near- and far-field.

Figure 9:

(Upper panel) Illustration of the patterned W/CoFeB/Pt film in stripes of 5 μm width and length. Shift of the central frequency and changes in the FWHM are observed for different orientations between stripes and external magnetic field. Reproduced from the study by Jin et al. [51] with permission of Wiley-VCH, Copyright 2019. (Lower panel) Blocky patterned CoFeB/Pt with lengths ranging from 20 to 160 μm. Shaping of the THz spectral amplitude is observed for different patterned dimensions. Reproduced from the study by Song et al. [52] with permission of The Japan Society of Applied Physics, Copyright 2019.