1. Introduction

Plasmonics are of great interest for various applications over a broad range of optical frequencies. Up to date, the main interest of plasmonic research has been in the shorter wavelengths of the visible spectrum, since nano-plasmonic structures are considered to be the missing connection between electronics and optics [1]. Nevertheless, the plasmonic research in the mid-infrared (MIR) region has been growing rapidly in recent years owing to its great potential for photonic devices with applications in the infrared regime (IR), ranging from waveguides [2,3] and photonic integrated circuits [4,5] to surface-enhanced infrared absorption spectroscopy (SERIA) [6,7], selective thermal emitters [8,9] and IR detectors [10,11]. However, the performance and reliability of these IR plasmonic devices strongly depend on the intrinsic properties of the underlying materials and the fabrication compatibility, restricting the capability of merging plasmonics into advanced IR opto-electronic devices.

Even though a lot of effort has been put in finding alternative plasmonic materials, including metallic alloys, transition metal nitrides, carbides, borides, silicides, as well as metal oxides and doped semiconductors, still the majority of applications is based on noble metals like gold (Au) and silver (Ag) [1,12]. However, the choice of the ideal plasmonic material is highly dependent on the desired application. In addition to the optical properties of the materials, which depend on the material itself, but also on the fabrication process, other factors such as the availability, price, compatibility with other materials and fabrication processes, the fabrication practicality and the assigned operational environment must be considered. Moreover, specific applications might have additional requirements like chemical stability in harsh environments or at high temperatures and tunability of the optical response.

In this work, plasmonic aluminium metal (Al), gold-tin (AuSn) and titanium-tungsten (TiW) metallic alloys were investigated by means of surface plasmon polaritons (SPPs) in the MIR region using a grating configuration. Al is very promising in supporting SPPs and very cheap. In addition, a thin layer of native oxide (typically 2.5 nm - 3 nm [13]) serves as protection from chemical reactions with the ambient air (including further oxidation [14]), but at the same time makes the fabrication more challenging [12]. TiW, on the other hand, shows high temperature resistance [15] and hence is very suitable as thermal emitter [16] or for other thermal devices. It is commonly used in electronics as electrode material and therefore has the advantage of prospective standard fabrication and integration. AuSn, a material often used for soldering [17], is cheaper than pure gold, very soft and has a low melting point [17]. This can be an advantage in the fabrication, for example when using molding-nanoimprint lithography or other mechanical deformation and thermal de-wetting techniques and 3D printing.

Al proved to be a good plasmonic material for a broad spectral range from ultraviolet to the visible region [18], and it has been also successfully applied for IR plasmonic devices including perfect absorbers [19], SEIRA [20,21] and IR detectors [22]. However, the SPP excitation of Al in the MIR regime using a grating configuration has not been demonstrated so far. Furthermore, to our best knowledge AuSn and TiW are investigated as plasmonic materials in the MIR region for the first time.

Within this work, first spectroscopic ellipsometry studies and an analytical investigation of the optical properties (quality factor, propagation length) of the three proposed materials were carried out, leading to some first predictions. This is presented in the second section of this paper. For experimental investigations a grating configuration was chosen. Thus, gratings with three different depths for all three materials were fabricated. A setup for free-beam reflection measurements [23] was used to detect the excitation of SPPs in the MIR region, particularly around an absorption band of CO₂. This wavelength regime is well fitted for possible applications of CO₂ sensors. Details about the fabrication process and about the experimental setup are stated in the third section. The measurement results are compared to simulations and discussed in the fourth section of this work. All three materials exhibit strong and narrow resonances, which are in good agreement with the simulations. At the end, in the fifth section, a summary is given.

2. Basics

2.1 Excitation of surface plasmon polaritons

The SPP dispersion relation of a planar interface is given by Eq. (1) [24].

Here $k_{\mathrm {SPP}}$ is the wave number of the SPP mode, $k_{\mathrm {0}}$ is the free space wave number, $\epsilon _{\mathrm {d}}$ and $\epsilon _{\mathrm {m}}$ are the relative permittivities of the dielectric and the metal, respectively. For non-ideal metals $\epsilon _{\mathrm {m}}$ is complex. Since for a given frequency, the SPP wave number is always greater than the wave number in the dielectric, a momentum mismatch between the electromagnetic wave in the dielectric and the SPP modes has to be overcome [24–27]. In this regard, several possibilities have been investigated in the past. One of the most commonly used configurations is grating coupling [24]. Equation (1) also holds for non-planar surfaces as long as the disturbance of the planar interface is small, hence for relatively shallow gratings (groove depth $<$ period) [28,29].

Within this work, a one-dimensional grating was used in order to excite SPPs at the interface between air and different metals and alloys as schematically represented in Fig. 1(a). If the plane of incidence is normal to the grooves of the grating, the grating equation is given by Eq. (2) [28].

 

Fig. 1. a) For the reflection at a grating different wave vectors play a role: the incoming wave vector $\mathbf {k}_{\mathrm {0}}$, the parallel component $\mathbf {k}_{||}$ of $\mathbf {k}_{\mathrm {0}}$, several orders of reflection (+1, −1^st , 0) and the reciprocal lattice vector of the grating $\mathbf {G}_{\mathrm {x}}$. b) Comparison of the contribution of the simulated −1^st and the 0^th order to the total reflectance for an Al grating (2.8 µm period, 1.6 µm groove width) with 150 nm depth at an incident angle of 27$^{\circ }$ and c) simulation of the 0^th order reflectance of this Al grating over a wide incident angle and wavelength regime.

Download Full Size | PDF

Here $\mathbf {k}_{\mathrm {SPP}}$ is the wave vector of the SPP, $\mathbf {k}_{||} = \mathbf {k}_{\mathrm {0}} \, \sin (\theta )$ is the component parallel to the interface of the incident wave vector $\mathbf {k}_{\mathrm {0}}$, $\theta$ is the incident angle, $m$ is an integer number and $\mathbf {G}_{\mathrm {x}}$ is the reciprocal lattice vector of the grating ($\mathbf {G}_{\mathrm {x}} = \frac {2\pi }{P}$ with the grating period $P$). As indicated in Fig. 1(a), the reflection is split in different orders. The total reflectance shows one dip due to the excitation of SPPs, but the specular reflectance (0^th order) can show more than one dip, as is depicted in the simulation result of a 150 nm deep Al grating with a period of 2.8 µm (groove width 1.6 µm) at an incident angle of 27$^{\circ }$, which is shown in Fig. 1(b). Figure 1(c) illustrates the simulated 0^th order reflectance of the same grating for several incident angle and wavelength combinations. For long wavelengths and small incident angles, the reflectance is close to unity. Nevertheless, the darker lines in Fig. 1(c) show for which combinations Eq. (2) is fulfilled, leading to absorption due to the excitation of SPPs. For a more extensive presentation of SPP excitation at gratings the reader is referred to other literature as for example [30].

2.2 Optical properties of proposed plasmonic materials

Figure 2(a) and (b) compare the real and imaginary parts of the retrieved relative permittivity of the investigated materials Al, AuSn and TiW with literature values of widely used plasmonic materials Au and Ag (Rakić et al. [31]) within the near and MIR range. To retrieve the data for Al, AuSn and TiW an ellipsometry measurement at Semilab (SE-2000 combined with IRSE extension for full UV-MIR spectroscopic ellipsometry) was performed and modeled with a Drude-Lorentz model. The raw data and the corresponding fit are given in Fig. 9 in the appendix.

 

Fig. 2. Comparison of a) real part of complex permittivity $\epsilon$ and b) imaginary part of $\epsilon$ of Al, AuSn and TiW, retrieved from ellipsometry measurement, with literature data of Au and Ag, taken from [31]. Comparison of the corresponding c) surface plasmon polariton quality factor $Q$_SPP (Eq. (3)) and d) propagation length $L$_SPP (Eq. (4)) of SPPs at a metal (or alloy) air interface. The values of TiW are multiplied by 10 in c) and d).

Download Full Size | PDF

All materials exhibit a negative real part in the investigated spectral region (wavelength of 0.25 µm - 20 µm), indicating them as promising candidates for plasmonics. Al and Ag have the most negative real part of their permittivity and thus promise stronger plasmonic resonances. On the other hand, the imaginary part of the relative permittivity is an indicator for the expected losses. Since Al has a higher imaginary part than the other materials across the shown wavelength range, the strongest losses are expected for this material. In general, the imaginary part increases with increasing wavelength for all investigated materials.

The quality factor or Q-factor ($Q$_SPP) is a combination of the real and imaginary part of the relative permittivity and often is used as a figure-of-merit (FoM) to compare different materials’ SPP-properties [12]:

It is depicted in Fig. 2(c). The Q-factor is an indicator for the propagation length of the SPP at a specific wavelength, measured in wavelengths (high $Q$_SPP signifies a propagation length of many wavelengths) and is highest for Ag and Al. The propagation length ($L$_SPP) itself given by [32]

3. Experimental

3.1 Fabrication of plasmonic gratings

The proposed plasmonic grating structures were fabricated on 8-inch silicon (Si) substrates. Figure 3(a) shows the layer stack of the structures. The oxide layer with a thickness of about 2 µm was deposited by repeating several cycles of tetraethyl orthosilicate (TEOS) deposition, each followed by an annealing step. Then a doped polycrystalline Si layer (PolySi) of 600 nm thickness was deposited via low pressure chemical vapor deposition (LPCVD) on top. For lithography a standard deep ultraviolet (UV) lithography stepper (with a wavelength of 248 nm) was used with a positive photoresist as mask for deep reactive ion etch (DRIE). Afterwards the Si grating structure was etched using a standard Bosch etch process, which is a DRIE (see e.g. [33]). By varying the etching times, gratings with three different depths (150 nm, 225 nm and 375 nm) with a period of 2.8 µm (1.6 µm groove width) were fabricated. The period of 2.8 µm was chosen in order to observe the resonance at an incident angle around 30$^{\circ }$ for a wavelength close to 4.2 µm, which is within the wavelength tunability of the laser

 

Fig. 3. a) The layer stack of the plasmonic gratings is as follows: Si substrate, oxide, doped polycrystalline Si, sputtered metal/alloy. b) The thickness of the plasmonic layer at the side walls decreases from top to bottom. This is illustrated schematically and by a close-up cross-sectional SEM image of an Al-coated grating with 225 nm etching depth. c) SEM images of Al-coated grating cross-section with 150 nm, 225 nm and 375 nm depth (from top to bottom). d) and e) SEM image in top view of a polycrystalline Si grating before metal sputtering and an Al coated grating with a depth of 375 nm, respectively.

Download Full Size | PDF

Finally, the plasmonic material with a thickness of about 100 nm was added by a sputtering process to ensure good sidewall coverage. For Al an Evatec Clusterline tool with 3 kW DC sputtering was used, for AuSn an Evatec Clusterline tool with 480 W DC sputtering and for TiW a tool from Tricon with a power of 2 kW. This leads to a decreasing thickness of the metal or alloy at the sidewall from top to bottom, as shown in Fig. 3(b). Due to this varying thickness at the sidewalls the shape of the resonance dip is changed slightly. The formation of surface morphology of the fabricated gratings was also characterized using a scanning electron microscope (SEM, Thermo Scientific Helios G4). Figure 3(c) shows the cross-sectional view of SEM images of 150 nm, 225 nm and 375 nm (from top to bottom) deep gratings with a 100 nm Al layer sputtered on top. Figure 3(d) and (e) depict SEM images in top view of a polycrystalline Si grating before metal deposition and a 375 nm deep Al grating, repectively. A comparison of them reveals, that the large grains in the grooves (a groove is between the two arrows in Fig. 3(d) and (e)) have their origin in the structure of the polycrystalline Si layer after the etching process.

3.2 SPP excitation using plasmonic gratings

The plasmonic gratings were characterized with a free-beam reflection measurement. The setup is shown in Fig. 4. The red arrows in Fig. 4(a) indicate the path of the laser beam. A quantum cascade laser (QCL, MIRcat, DRS Daylight Solutions) with tunable wavelength (3.96 µm - 4.43 µm) in the MIR region was the light source (beam diameter: 1.5 mm). The laser light was guided to the sample, passing a periscope to adjust the height of the beam as well as its polarization. The laser polarization was perpendicular to the grating grooves (p-polarization) for efficient coupling to SPPs, as indicated in Fig. 4(b). The specular reflection (0^th order) of the plasmonic grating was then guided to a mercury cadmium telluride (MCT, PVI-4TE-6, VIGO System) detector. The measured signal of the detector was acquired using a lock-in amplifier (SR830 DSP, Stanford Research Systems) combined with an oscilloscope (PicoScope, 5000 Series).

 

Fig. 4. Measurement setup with a quantum cascade laser (QCL) as light source and a mercury cadmium telluride (MCT) detector. a) The red arrows in the photograph of the setup indicate the path of the laser light. b) Schematic of the setup: the incident laser light is p-polarized and the grating can be rotated to adjust the incident angle.

Download Full Size | PDF

The measurements were done by performing a frequency sweep of the laser for different fixed angles of incidence. A reference measurement was performed on a flat Au surface for calibration of the spectra. In addition, measurements with lower resolution over a large spectral range were performed using an FTIR ellipsometer (IR-VASE, J.A. Woollam Co.).

For analysis the reflectance spectra of the gratings were also simulated. Thereby, the −3^rd, −2^nd, −1^st, 0^th, 1^st, 2^nd and 3^rd order were considered. All simulations were carried out using a rigorous coupled-wave analysis software (RCWA) (DiffractMOD, Synopsys’ RSoft package).

4. Results and discussion

Figure 5 shows the angle dependence of the specular reflectance spectrum for an Al grating with a depth of 150 nm. The incident angle increases from top to bottom and hence also the resonance wavelength increases, shifting the resonance dip to longer wavelengths. The measurement covers a wavelength range between 3.96 µm and 4.43 µm, which is part of the MIR regime. The measurement results in the left panel are in good agreement with the corresponding simulations in the right panel. The shift in the position of the dip can be explained by an experimental shift of the zero-angel, which is estimated to be $\pm$1$^{\circ }$. Above a wavelength of about 4.2 µm (marked by the gray shaded rectangle in the uppermost left panel of Fig. 5), the absorption due to CO₂ in the ambient air can be observed in the measured data. Although the resonance dip coincides with the CO₂ absorption for incident angles above 31$^{\circ }$ it can still be observed clearly. Nevertheless, the shape of the dip is distorted in this region of the spectrum, which leads to significant deviations with regard to the simulated curves.

 

Fig. 5. Reflectance at a 150 nm deep grating with a coating of 100 nm Al. The incident angle increases from top to bottom (26$^{\circ }$ −33$^{\circ }$). The left panel shows measured data of the specular reflectance, the right one corresponding simulations of the specular reflectance (solid line) and the −1^st order reflectance (dashed line). The gray shaded rectangle in the uppermost left panel indicates the CO₂ absorption band.

Download Full Size | PDF

The specular reflectance approaches unity towards longer wavelengths. For wavelengths shorter than the SPP resonant wavelength, the specular reflectance is diminished slightly. A comparison with the simulation indicates that for these wavelengths the −1^st order reflectance (dashed line) cannot be neglected. At the resonance dip itself the measured specular reflectance reduces to about 0.24, which is less than predicted by simulation. There the specular reflectance approaches 0 at the resonance dip minimum. For short wavelengths, the simulated specular reflectance stays at a constant level followed by a steep decrease towards the minimum of the resonant dip. The measured data, on the other hand, show a steady decrease of the specular reflectance to about half height of the dip, where it starts to fall quickly. This might be caused by the roughness of the Al layer within the grating grooves and in particular for higher angles it might be an artefact due to the overlap with the CO₂ absorption from the free beam passage through ambient air. Towards longer wavelengths the reflectance increases instantly, leading to an asymmetry in the resonance dip (Fano-like), which results from the coupling between the surface wave (bound plasmon) and diffraction (leaky wave) from the periodic grating [34]. To estimate the width (full width at half maximum - FWHM) of the resonances, the data with 26$^{\circ }$ incident angle were fitted with a Fano resonance (see e.g. [35]). The FWHM of the measured resonance dip of Al is 4.7 nm according to the fit, whereas it is 5.5 nm for the simulated data.

Similar results were obtained for AuSn and TiW gratings, which are depicted in Fig. 6(a) and Fig. 6(b), respectively. For the AuSn grating the measured specular reflectance at the SPP resonance is about 0.10, which is even better than the result of the Al grating (for the simulation the specular reflectance approaches 0 again). The FWHM of the measured resonance dip for the AuSn grating is 9.7 nm (again for 26$^{\circ }$ incident angle), whereas the fit of the simulated data yields a FWHM of 11.5 nm. Hence, the measured SPP resonance at the AuSn grating is broader than the one of the Al grating at 26$^{\circ }$. The minimum specular reflectance at the SPP resonance of the TiW grating is about 0.17, which is slightly higher than the one of the corresponding simulation which is about 0.12. The width of the resonance is 11.6 nm for an incident angle of 26$^{\circ }$ in the experiment and 16.9 nm in the simulation. Compared to AuSn and Al the resonance dip of TiW is slightly broader. This trend is in good agreement with the $Q$_SPP shown in Fig. 2(c).

 

Fig. 6. Reflectance at 150 nm deep gratings with different coatings. The incident angle increases from top to bottom (26$^{\circ }$−33$^{\circ }$). In a) and b) the left panel shows measured data of the specular reflectance, the right one corresponding simulations of the specular reflectance (solid line) and the −1^st order reflectance (dashed line) of a grating with a) 100 nm AuSn coating and b) 100 nm TiW coating.

Download Full Size | PDF

The angle dependent specular reflectance of Al sputtered gratings with a depth of 225 nm and 375 nm are depicted in Fig. 7(a) and (b), respectively. A comparison of Fig. 5 and Fig. 7 reveals that the shape of the resonance dip changes a lot for different grating depths. With an increase in grating depth, the resonant dip broadens. Additionally, a second dip can be clearly observed for the grating with a depth of 225 nm (Fig. 7(a)) next to the first one in the shorter wavelength side. The total reflection is mainly composed of the −1^st diffraction order and the specular reflection and would only show one reflection dip within this wavelength regime as explained in Fig. 1. However, only the specular reflectance (0^th order) was measured here. When the grating depth increases from 150 nm to 225 nm and to 375 nm, the −1^st order diffraction (dashed line) increases significantly, leading to the second dip observable in the specular reflectance. This behaviour can also be observed for the 375 nm deep grating in the simulated data in Fig. 7(b).

 

Fig. 7. Reflectance at Al coated gratings with different depths. The incident angle increases from top to bottom (27$^{\circ }$−32$^{\circ }$). The left panel shows the measured specular reflectance, the right one corresponding simulations of the specular reflectance (solid line) and the −1^st order reflectance (dashed line) of a grating with a) a depth of 225 nm and b) 375 nm.

Download Full Size | PDF

AuSn and TiW coated gratings with depths of 225 nm and 375 nm show a similar trend: the −1^st order reflectance increases with increasing grating depth, resulting in a pronounced second reflection dip. The angle dependent specular reflectance for those gratings are depicted in the appendix, Fig. 10.

In Fig. 8 the specular reflectance of a 225 nm deep Al grating for a wider range of incident angles and wavelengths is depicted. The wavelength regime for this measurement was between 2 µm and 6 µm and the incident angle was changed from 25$^{\circ }$ to 85$^{\circ }$ with a step-size of 5$^{\circ }$. Even though the absolute reflectance does not coincide, the measurement result (Fig. 8(a)) is confirmed by the simulated one (Fig. 8(b)). Towards longer wavelengths and smaller incident angles the specular reflectance approaches unity.

 

Fig. 8. Comparing measured values of the specular reflectance at an Al grating with a depth of 225 nm in a) with the corresponding simulations in b).

Download Full Size | PDF

Overall the experimental data coincide with the simulations very well, indicating that the material parameters are well understood. A small discrepancy in the dip position is explained by a shift in the zero-angle of the measurement (about $\pm$1$^{\circ }$). When comparing the attenuation of measurement and simulation, one should note, that the surface roughness is not included in the simulations and the optical material parameters might differ slightly. In addition, within the measurement the laser light is not perfectly p-polarized, as is the case in simulations. A different peak shape might come due to surface roughness (in particular in the grating grooves), losses in the material or poor film quality. Furthermore, the grating parameters (width of grooves, depth) and shape of the grating might differ slightly from the ideal ones used for simulations due to variations in the fabrication processes.

5. Summary

We have numerically and experimentally investigated three promising alternatives (Al, AuSn, TiW) to Ag and Au for plasmonic applications in the MIR region. The optical properties of Al, AuSn and TiW were received from ellipsometry measurments. The angle dependence of the SPPs resonance as well as the influence of different grating depths on the resonance were also systematically studied. The measured results were in good agreement with the corresponding simulations, wherein plasmonic structures exhibited strong and narrow resonances (e.g. FWHM at around 4.2 µm resonance: 4.7 nm for Al, 9.7 nm for AuSn and 11.6 nm for TiW alloys) in shallow gratings (150 nm depth), which proves good plasmonic properties of Al, AuSn and TiW in the MIR wavelength region. The best plasmonic response (narrowest resonance dip) was found for Al, nevertheless as discussed above, when choosing a plasmonic material also many other properties should be considered and the ideal plasmonic material is highly dependent on the application as well as the fabrication compatibility. Since AuSn and TiW also showed a good plasmonic response and they are already commonly used in electronics, they are good candidates to bridge the gap between plasmonics and electronics. While AuSn can be used in general IR plasmonic devices, refractory TiW can be applied for thermal IR plasmonic applications working at high temperature.

Appendix

 

Fig. 9. Raw data (solid line) and corresponding fit (dashed line) of the ellipsometry measurement (75$^{\circ }$ angle) of the investigated Al, AuSn and TiW (from left to right) layers within a wavelength range of 0.25 µm - 20 µm.

Download Full Size | PDF

 

Fig. 10. Reflectance at gratings with different coatings and depths. The incident angle increases from top to bottom (27$^{\circ }$ - 29$^{\circ }$). The left panel shows measured data, the right one corresponding simulations (solid line: 0^th order, dashed line: −1^st order) of a grating with a) AuSn coating and 225 nm depth, b) AuSn coating and 375 nm depth, c) TiW coating and 225 nm depth and d) TiW coating and 375 nm depth.

Download Full Size | PDF

Funding

Austrian Federal and Local Governments (K2 COMET Center Symbiotic Mechatronics); Österreichische Forschungsförderungsgesellschaft (871417); Bundesministerium für Klimaschutz, Umwelt, Energie, Mobilität, Innovation und Technologie (BMK).

Acknowledgments

This work was performed within the PICASSO-project funded by the Bundesministerium für Klimaschutz, Umwelt, Energie, Mobilität, Innovation und Technologie (BMK) in the framework of the program "Produktion der Zukunft" (Prj. Nr. 871417). Furthermore, the authors want to thank the involved staff working at the cleanroom of Infineon Technologies Austria AG, Villach, for their kind support with the fabrication of the structures and T. Ostermann for the mask layouts. In addition, the authors would like to thank L. Makai and A. Ertl at Semilab for their kind support in spectroscopic ellipsometry measurement. Jasmin Spettel wants to thank P. Hadley (Graz University of Technology) for his kind advice and fruitful discussions.

Disclosures

The authors declare no conflicts of interest.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

References

1. Y. Zhong, S. D. Malagari, T. Hamilton, and D. Wasserman, “Review of mid-infrared plasmonic materials,” J. Nanophotonics 9(1), 093791 (2015). [CrossRef]  

2. J.-P. Tetienne, A. Bousseksou, D. Costantini, R. Colombelli, A. Babuty, I. Moldovan-Doyen, Y. De Wilde, C. Sirtori, G. Beaudoin, L. Largeau, O. Mauguin, and I. Sagnes, “Injection of midinfrared surface plasmon polaritons with an integrated device,” Appl. Phys. Lett. 97(21), 211110 (2010). [CrossRef]  

3. B. Schwarz, P. Reininger, D. Ristanić, H. Detz, A. M. Andrews, W. Schrenk, and G. Strasser, “Monolithically integrated mid-infrared lab-on-a-chip using plasmonics and quantum cascade structures,” Nat. Commun. 5(1), 4085 (2014). [CrossRef]  

4. M. Ono, H. Taniyama, H. Xu, M. Tsunekawa, E. Kuramochi, K. Nozaki, and M. Notomi, “Deep-subwavelength plasmonic mode converter with large size reduction for Si-wire waveguide,” Optica 3(9), 999 (2016). [CrossRef]  

5. C. Haffner, A. Joerg, M. Doderer, F. Mayor, D. Chelladurai, Y. Fedoryshyn, C. I. Roman, M. Mazur, M. Burla, H. J. Lezec, V. A. Aksyuk, and J. Leuthold, “Nano–opto-electro-mechanical switches operated at CMOS-level voltages,” Science 366(6467), 860–864 (2019). [CrossRef]  

6. M. Osawa, K.-I. Ataka, K. Yoshii, and Y. Nishikawa, “Surface-enhanced infrared spectroscopy: The origin of the absorption enhancement and band selection rule in the infrared spectra of molecules adsorbed on fine metal particles,” Appl. Spectrosc. 47(9), 1497–1502 (1993). [CrossRef]  

7. F. Neubrech, A. Pucci, T. W. Cornelius, S. Karim, A. García-Etxarri, and J. Aizpurua, “Resonant plasmonic and vibrational coupling in a tailored nanoantenna for infrared detection,” Phys. Rev. Lett. 101(15), 157403 (2008). [CrossRef]  

8. H. T. Miyazaki, K. Ikeda, T. Kasaya, K. Yamamoto, Y. Inoue, K. Fujimura, T. Kanakugi, M. Okada, K. Hatade, and S. Kitagawa, “Thermal emission of two-color polarized infrared waves from integrated plasmon cavities,” Appl. Phys. Lett. 92(14), 141114 (2008). [CrossRef]  

9. X. Liu, T. Tyler, T. Starr, A. F. Starr, N. M. Jokerst, and W. J. Padilla, “Taming the blackbody with infrared metamaterials as selective thermal emitters,” Phys. Rev. Lett. 107(4), 045901 (2011). [CrossRef]  

10. C.-C. Chang, Y. D. Sharma, Y.-S. Kim, J. A. Bur, R. V. Shenoi, S. Krishna, D. Huang, and S.-Y. Lin, “A Surface Plasmon Enhanced Infrared Photodetector Based on InAs Quantum Dots,” Nano Lett. 10(5), 1704–1709 (2010). [CrossRef]  

11. T. D. Dao, S. Ishii, A. T. Doan, Y. Wada, A. Ohi, T. Nabatame, and T. Nagao, “An on-chip quad-wavelength pyroelectric sensor for spectroscopic infrared sensing,” Adv. Sci. 6(20), 1900579 (2019). [CrossRef]  

12. P. West, S. Ishii, G. Naik, N. Emani, V. Shalaev, and A. Boltasseva, “Searching for better plasmonic materials,” Laser Photonics Rev. 4(6), 795–808 (2010). [CrossRef]  

13. C. Langhammer, M. Schwind, B. Kasemo, and I. Zoric, “Localized surface plasmon resonances in aluminum nanodisks,” Nano Lett. 8(5), 1461–1471 (2008). [CrossRef]  

14. D. Gerard and S. K. Gray, “Aluminium plasmonics,” J. Phys. D: Appl. Phys. 48(18), 184001 (2015). [CrossRef]  

15. D. R. Marx, J. C. Turn, and J. Shi, “Tungsten titanium targets for VLSI device fabrication,” Technical Note 202, Materials Research Corporation (1999).

16. M. Laroche, C. Arnold, F. Marquier, R. Carminati, J.-J. Greffet, S. Collin, N. Bardou, and J.-L. Pelouard, “Highly directional radiation generated by a tungsten thermal source,” Opt. Lett. 30(19), 2623 (2005). [CrossRef]  

17. C. H. Lee, Y. M. Wong, C. Doherty, K. L. Tai, E. Lane, D. D. Bacon, F. Baiocchi, and A. Katz, “Study of Ni as a barrier metal in AuSn soldering application for laser chip/submount assembly,” J. Appl. Phys. 72(8), 3808–3815 (1992). [CrossRef]  

18. M. W. Knight, N. S. King, L. Liu, H. O. Everitt, P. Nordlander, and N. J. Halas, “Aluminum for plasmonics,” ACS Nano 8(1), 834–840 (2014). [CrossRef]  

19. T. D. Dao, K. Chen, S. Ishii, A. Ohi, T. Nabatame, M. Kitajima, and T. Nagao, “Infrared perfect absorbers fabricated by colloidal mask etching of Al-Al₂O₃-Al trilayers,” ACS Photonics 2(7), 964–970 (2015). [CrossRef]  

20. K. Chen, T. D. Dao, S. Ishii, M. Aono, and T. Nagao, “Infrared aluminum metamaterial perfect absorbers for plasmon-enhanced infrared spectroscopy,” Adv. Funct. Mater. 25(42), 6637–6643 (2015). [CrossRef]  

21. B. Cerjan, X. Yang, P. Nordlander, and N. J. Halas, “Asymmetric aluminum antennas for self-calibrating surface-enhanced infrared absorption spectroscopy,” ACS Photonics 3(3), 354–360 (2016). [CrossRef]  

22. B. Zheng, H. Zhao, B. Cerjan, S. Yazdi, E. Ringe, P. Nordlander, and N. J. Halas, “A room-temperature mid-infrared photodetector for on-chip molecular vibrational spectroscopy,” Appl. Phys. Lett. 113(10), 101105 (2018). [CrossRef]  

23. M. Shahzad, G. Medhi, R. E. Peale, W. R. Buchwald, J. W. Cleary, R. Soref, G. D. Boreman, and O. Edwards, “Infrared surface plasmons on heavily doped silicon,” J. Appl. Phys. 110(12), 123105 (2011). [CrossRef]  

24. S. A. Maier, Plasmonics: Fundamentals and Applications (Springer, 2007).

25. E. Kretschmann and H. Raether, “Radiative decay of non radiative surface plasmons excited by light,” Z. Naturfaroschung 23s(12), 2135–2136 (1968). [CrossRef]  

26. A. Otto, “Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection,” Z. Phys. A: Hadrons Nucl. 216(4), 398–410 (1968). [CrossRef]  

27. J. Zhang, L. Zhang, and W. Xu, “Surface plasmon polaritons: Physics and applications,” J. Phys. D: Appl. Phys. 45(11), 113001 (2012). [CrossRef]  

28. J. R. Sambles, G. W. Bradbery, and F. Yang, “Optical Excitation of Surface Plasmons: An introduction,” Contemp. Phys. 32(3), 173–183 (1991). [CrossRef]  

29. L. Meng, D. Zhao, Z. Ruan, Q. Li, Y. Yang, and M. Qiu, “Optimized grating as an ultra-narrow band absorber or plasmonic sensor,” Opt. Lett. 39(5), 1137 (2014). [CrossRef]  

30. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, vol. 111 of Springer Tracts in Modern Physics (Springer Berlin Heidelberg, 1988).

31. A. D. Rakić, A. B. Djurišić, J. M. Elazar, and M. L. Majewski, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt. 37(22), 5271–5283 (1998). [CrossRef]  

32. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef]  

33. J. X. J. Zahng and K. Hoshino, Molecular Sensors and Nanodevices (Elsevier, 2014).

34. A. E. Miroshnichenko, S. Flach, and Y. S. Kivshar, “Fano resonances in nanoscale structures,” Rev. Mod. Phys. 82(3), 2257–2298 (2010). [CrossRef]  

35. B. Luk’yanchuk, N. I. Zheludev, S. A. Maier, N. J. Halas, P. Nordlander, H. Giessen, and C. T. Chong, “The Fano resonance in plasmonic nanostructures and metamaterials,” Nat. Mater. 9(9), 707–715 (2010). [CrossRef]