Titanium nitride as a plasmonic material for excitation of Tamm plasmon states in visible and near-infrared region
Graphical Abstract
Introduction
In recent years, many studies have aimed to understand a special type of electromagnetic surface state called Tamm plasmon polaritons (TPPs) [1], [2], [3]. TPPs are confined electromagnetic states that form at the interface of metals and photonic crystals (PhCs) [4], [5]. The excitation of TPPs has been explained based on Zak phases. The Zak phases are due to the topological properties of a one-dimensional PhC (1D PhC) [6], [7]. The surface impedance of 1D PhCs is related to the Zak phase of the photonic bands [6]. Zero reflectivity or complete absorption of incident light due to the excitation of TPPs at the interface of a thin metal film and PhC can be achieved under conditions where the surface impedance of the terminating layer of the PhC facing the metal (ZPhC) is equal to the conjugate impedance of the metal (). Thus, the necessary condition for the perfect absorption of light due to the excitation of TPPs is ZPhC = . These TPP modes represent standing surface states that cannot transfer energy [8]. Due to the interesting features of TPPs, many studies have investigated them both theoretically and experimentally [9], [10], [11], [12], [13], [14]. Experimental observations have detected TPPs as a narrow peak in the transmittance or absorption spectra and a narrow dip in the reflectance spectra. In contrast to traditional surface states such as surface plasmons, TPPs can be excited at normal incidence as well as for both transverse electric and transverse magnetic polarizations [4]. In addition, the spectral properties of TPPs can be controlled by the thickness of the dielectric material, thickness of the spacer layer, and the plasmonic metal [15]. The unique dispersion properties exhibited by TPP modes are suitable for various applications, such as optical filters [16], polariton lasers [14], optical switching [17], sensors [12], [18], [19], absorbers [20], harmonic generation [21], enhanced emission, and nonlinear optical effects [22], [23]. Recently, Want et al. demonstrated the use of Tamm plasmon states in thermal emission and photodetection [24], [25], [26]. It is important to note that the direct optical excitation of TPPs makes them highly suitable for applications in optical integrated circuits. In addition, the TPP modes can be effectively coupled to exciton polaritons in hybrid Tamm microcavities. However, the fields of TPP modes are strongly confined at the interface of the metal and PhCs, and thus external dynamic control over their wavelength is not possible [13], [27]. As a consequence, it is very difficult to manipulate these surface modes from the outside. Therefore, recent studies have investigated TPP modes using alternative materials, such as transparent conducting oxides (AZO, GZO, and ITO) [6], a nano-patterned metal film acting as a non-diffracting optical metasurface [28], an anisotropic nanocomposite layer [29], and grapheme [30]. It is important to note that the use of alternative materials for TPP excitation broadens our understanding and applications of these surface states because problems in the field of photonics cannot be solved with limited conventional plasmonic materials. Therefore, alternative plasmonic materials are required in order to solve various problems, such as operating a device at a specific wavelength or functioning under different ambient conditions. Metal nitrides such as titanium nitride (TiN), zirconium nitride (ZrN), tantalum nitride (TaN), and hafnium nitride (HfN) exhibit metallic properties in the visible and infrared (IR) spectral range [31]. These nitride materials are attractive because of their refractory nature. They are also stable and hard, and it is possible to control their optical properties by modifying their compositions. The optical properties of transition metal nitrides depend on the deposition conditions due to their non-stoichiometric nature. It is well known that metal-rich films are metallic and that nitrogen-rich films are dielectric. However, TiN is metallic under both conditions, i.e., when metal-rich or nitrogen-rich. It should be noted that these metal nitrides are already in use as gate metals in n-type and p-type transistors, and as barrier layers in silicon CMOS technology [32]. These metal nitrides also have advantages in terms of fabrication and integration, and they could be useful for integrating plasmonics with nanoelectronics [33]. A TiN-based narrowband thermal emitter was recently demonstrated with a TPP structure in the mid-wavelength IR region [34]. By utilizing the aforementioned properties in an optimal manner, I demonstrated the excitation of TPPs using TiN in the present investigation.
In the present study, I demonstrated the use of TiN as a plasmonic material for the excitation of TPP modes in the visible and near-infrared (NIR) region in a metal nitride-1D PhC structure at normal incidence [35]. TiN is an alternative plasmonic material to the conventional plasmonic materials such as Ag, Au, Al, and Cr. I also investigated the dependence of critical coupling in the TPP modes on the TiN thickness, distributed Bragg reflector (DBR) bilayers, and PhC stop band region. In particular, I demonstrated how variations in the TiN thickness and DBR bilayers led to changes in the reflectance and absorption of the TPP structure. It is important to note that TiN was selected because of its important properties, such as low optical loss, chemical inertness, and high thermal stability in various applications including interconnects, photocatalysis, and energy harvesting. TiN also has less metallic characteristics compared with conventional plasmonic materials such as Au and Ag, which may result in far field penetration into TiN. Due to its high thermal stability and high field enhancements, a TiN-based TPP structure could be utilized for designing devices with high temperature applications [36]. Moreover, the light concentration and trapping properties of TiN could be useful in the design of devices for light harvesting. As mentioned above, the inertness properties of TiN could be applied in the field of photocatalysis [36]. Furthermore, a TiN-based TPP structure may allow access to a different spectral region compared with conventional plasmonic materials. Therefore, the results obtained in the present study may be useful for the design of metal nitride-based TPP devices with various photonic applications.
Section snippets
Structure and theoretical method
Fig. 1 shows a schematic diagram of the proposed structure for supporting TPP modes. The 1D PhC is in the form of a multilayer terminated by a metallic nitride TiN thin film of finite thickness. The 1D PhC comprises the two dielectric materials GaAs and AlAs with layer thicknesses of dGaAs and dAlAs, respectively, and refractive indices of nA and nB. The 1D PhC structure comprises 14 bilayers of GaAs and AlAs. The 1D PhC is considered to satisfy the quarter-wave-conditions as nB dAlAs = nA dGaAs
Results and discussion
The reflectance, transmittance, and absorption spectra for the multilayer configuration were theoretically obtained using the standard TMM [38]. The structure investigated comprised 14 units of (GaAs/AlAs) covered with a thin metal nitride (TiN) film. The forbidden band is a prerequisite for the formation of Tamm plasmons, so I selected a quarter-wavelength distributed Bragg reflector such that the quarter-wave-conditions were satisfied and stop bands formed in the visible and NIR region. Tamm
Conclusions
In this study, I investigated the existence of TPP modes at the interface between a PhC and metal nitride (TiN) thin film. Alternative plasmonic materials such as TiN have many advantages compared with conventional plasmonic metals. The possible existence of TPP modes was predicted based on the metallic properties of the metal nitride in the visible and IR regions. Using the TMM, I demonstrated the existence of TPP modes in the visible and IR regions. Changing the thickness of the TiN layer was
Notes
The author has no competing financial interests to declare.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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