1. Introduction

Plasmonics deals with the coupling of the incoming electromagnetic radiation to the collective coherent oscillation of the free electrons on the surface of a metal [1]. This provides a pathway to manipulate light at scales smaller than the diffraction limit and drastically enhances light-matter interaction. Despite the optical losses incurred in plasmonics, these benefits have been utilized for a wide range of applications, including on-chip photonics [2], biosensors [3], and plasmonic colors [4–7]. Moreover, the sub diffractive nature of plasmonic resonances has served as a foundation for a plethora of plasmonic color applications such as transmissive color filters and reflective and subtractive color printing [4,6]. These have the potential to be used as security features, product branding as well as data storage and imaging applications. However, these applications are largely limited by practical considerations of cost and robustness stemming from the use of most common plasmonic metals, silver, gold, or aluminum. Despite benefits such as providing a wide color gamut due to the near UV plasma frequency (Ag and Al), low loss in the visible (Ag and Au), and the self-passivation of Al, the key challenge of robustness remains to be addressed. Moreover, the tendency of gold and silver to grow in a 3-dimensional mode, their poor adhesion to commonly used substrates, and non-tunable optical properties make them a less than ideal choice for a range of practical applications.

Transition metal nitrides on the other hand are attracting interest, as alternative plasmonic materials, due to their tunable optical properties, CMOS compatibility, as well as high thermal and chemical stability. Among them, TiN has been the most widely explored because of its gold-like properties and refractory nature along with other added benefits such as low cost, biocompatibility, and self-passivating nature [8–14]. These benefits have led to the use of TiN in devices demonstrating broadband absorbance in the visible and IR spectral region [15], hyperbolic metamaterials with high photonic density of states [16], and high-efficiency local heating [17].

For security applications though, the durability of TiN along with its refractory nature is of paramount importance. With a Young modulus of over 5 times that of silver (400 GPa) [18], and a 9 on the Moh’s hardness scale (0 to 10, while silver is 2.5 and oxides such as TiO₂ and SiO₂ are 5 to 7), which denotes the scratch resistance of a material, TiN is an attractive choice for security devices on heavily handled objects such as bills, identity cards, and legal documents. However, high-quality TiN has usually been grown via sputtering at temperatures exceeding 650°C, which renders the process incompatible with most standard lithographic processes, as it could lead to unwanted material degradation, oxidation or dopant diffusion [19,20].

In this work, we design and fabricate structures exhibiting robust plasmonic colors which are based on nanohole arrays patterned onto TiN thin films deposited via plasma-enhanced atomic layer deposition (PEALD), a scalable low-temperature method [10]. The resulting nanohole array exhibits polarization-independent and angle insensitive (+/- 10°) plasmonic colors in the visible and short-wave IR spectral regions, accessible by inexpensive Si-based CCD detectors. More importantly, the TiN nanohole array exhibits vastly improved durability and scratch resistance, demonstrating promise for use in security applications such as identity cards and legal documents.

2. Results

The metasurface consists of a hexagonal array of nanoholes patterned into a 41 nm thick TiN film with varying periodicity and radii, as shown in Fig. 1(a). The radii vary from 75 nm to 150 nm while the separation between the nanoholes varies from 75 nm to 200 nm. The hexagonal pattern enables a higher packing efficiency of nanoholes, while also resulting in polarization-independent response, a critical feature for enhancing the practicality of the design.

 

Fig. 1. (a) Schematic of the hexagonal array metasurface; (b) XRD ω − 2θ scan of TiN thin films; Simulated (dashed) and measured (solid) transmission spectra of three representative metasurfaces with separation of 200 nm and radii of (c) 75 nm, (d) 125 nm and (e) 150 nm; The kinks at wavelengths below 650 nm are due to diffraction effects, while the smooth peaks between 800 nm and 1000 nm, are due to extraordinary transmission; (f) Permittivity of the TiN thin film; (g) Simulated transmission spectra for metasurface based on Au, Ag and TiN. Simulations for Au and Ag metasurfaces have been performed assuming the presence of a 4 nm Cr adhesion layer, which limits their efficiency, making them on par with that of TiN.

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The thin film of TiN was deposited on a c-plane sapphire substrate. The single crystal growth and permittivity of the film is shown in Fig. 1(b) and (d). The deposition and fabrication details can be found in the supplemental section [10]. Figure 1(c), d, and e show the simulated and experimentally measured transmission spectra of three representative structures, with peaks positioned at 800 nm, 1000 nm, and 1040 nm for structures with a separation between the holes of 200 nm and a hole radius of 75 nm, 125 nm, 150 nm and, respectively. These peaks are caused by the formation of plasmonic Bloch modes on the surface of the metallic film (TiN/ Air interface), due to the interference of propagating surface plasmons, and is referred to as extraordinary transmission [21]. The spectral position of the peaks along with the efficiency of transmittance, match well with the simulations. This is partly due to the epitaxial growth of TiN on c-plane sapphire which ensures single crystalline growth, thus enabling a smooth sidewall profile, unlike gold and silver which exhibit rough edges due to grain formation. Furthermore, Fig. 1(g) compares the responses of similar structures simulated for gold and silver films with a 4 nm Cr adhesion layer. While these noble metals exhibit lower losses than TiN, practical constraints such as the required adhesion layers limits their performance. Furthermore, inherent side-wall roughness would lead to higher scattering/radiation losses, further limiting their performance [22].

Figure 2(a) shows the color palette achievable, imperative to the design of security features, by varying the periodicity and radii of the nanohole array. The transmission peak is continuously tunable between 750 nm to 1040 nm, as shown in Fig. 2(b). Figure 2(c) shows the appearance of the designed metasurfaces in the visible spectrum, which have distinct colors as their spectral shape is modified by the broad EOT resonance and enhanced by diffractive effects. As seen in Fig. 1(b)-(d), the sharp diffractive features present in the simulated transmission spectra, observed around 600 nm, manifest themselves as broad bends in the measured spectral response, which in conjunction with EOT peaks on the edge and into the near IR spectrum result in distinct colors of the metasurfaces (Fig. 2(a) and (c)). As a proof-of-concept, these structural colors are used to create an example logo security feature (Fig. 2(c)), whereby two structures with different radii and periodicity are combined as shown in the SEM inset. Furthermore, as the spectral response of all the designed metasurfaces can be easily captured using inexpensive silicon-based sensors, these structures can be utilized to design covert security features, using arrays that have similar appearances in the visible spectrum but a different spectral response in the short-wave IR region.

 

Fig. 2. (a) Transmission spectra of metasurfaces by varying the separation (75 nm to 200 nm) and radius (75 nm to 150 nm) in 25-nm steps, (b) Surface plot of EOT peak positions, as the radius (R/2) and period (R + S) are varied. This results in EOT peaks covering the wavelength range from 750 nm 1040 nm, which is accessible by common Si CCD detectors. (c) Optical images of the 24 metasurfaces, normalized to white background. These colors are a result of EOT peaks and diffraction effects and can be used to design security features such as QR codes or logos. The zoomed in image of the VCU logo shows structures with different radii and periodicity used to create the logo (black scale bar is 100 nm, white scale bar 90 µm).

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As stated earlier, the transmission peaks are attributed to extraordinary transmission resonances, often observed in patterned thin metallic films [21,23–25]. Figure 3(a) plots the simulated transmission intensity in the energy-momentum plane for a structure with a radius of 125 nm and periodicity of 150 nm, at different angles of incidence which affects the momentum being imparted by the metasurface. Briefly, the incoming electromagnetic waves couple to the propagating surface plasmon modes, due to the momentum imparted by the periodic structure. These surface plasmons, launched from the edge of each nanohole, interfere to form plasmonic Bloch modes which result in a band structure as shown in Fig. 3(a) [25]. In an ideal case, additional bright bands would exist at higher energies, but due to the relatively large size of the holes (∼ λ/2) along with dielectric nature of TiN, diffraction effects take over.

 

Fig. 3. (a) Transmission intensity as a function of photon energy and momentum of the grating, for a metasurface with a radius of 125 nm and a separation of 150 nm. As the angle of incidence increases, so does the momentum imparted by the grating. This results in a shift of EOT peak, as manifested by the bright white band, (inset shows the normalised E-Field plot on resonance, scaling from 0, blue to 1, red) (b) Transmission spectra plotted as a function of increasing period for structures with a radius of 125 nm. White dashed line is the peak position calculated using by the first order approximation of the above equation. As expected, the observed peaks are red shifted, as the first order approximation does not consider the depth of holes and associated scattering losses.

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While full-field simulations using a commercially available finite element solver software (COMSOL Multiphysics) aids in accurately describing the EOT effect, a quick first-order approximation of the transmission peak can be found by using the momentum-conservation principle. As the coupling of the incoming energy to surface plasmons is enabled by the grating momentum, conservation of momentum between the single interface plasmon and grating momentum results in the first-order approximate equation, given by Genet and Ebbesen [23]:

To investigate the durability and scratch-resistance of the metasurface, the film was rubbed against a lint-free cloth using a mechanical polishing machine, cyclically for 10 minutes, to simulate the movement of the sample in a pocket. With half a kilogram of load, evenly distributed over the area of the sample (one-sixth of a 2-inch sapphire wafer), the pressure on the sample was 14.5kPa. For comparison, an identical test is performed with a structure made on a 55 nm thick gold film, with a 5 nm thick Cr adhesion layer. It is observed that the TiN sample remains unaffected through the process, whereas samples fabricated with gold are destroyed within the first 10 minutes, as shown in Fig. 4(a)-(f). Consequently, the spectral response of TiN remains the same, while gold films no longer exhibit any EOT peaks. To further test the limits of the TiN structures, we subjected them to an additional 30 minutes of polishing against the same lint free cloth. The TiN film survived and showed no signs of deterioration, thus implying a long-lasting lifetime due to the robust nature of TiN. However, as shown in Fig. 4(c), the nanoholes tend to accumulate dust particles over time. Nonetheless, as shown in Fig. 4(d), the nanoparticles result in less than a 30 nm peak shift. While day to day circumstances would not subject these samples to such extreme conditions, this test demonstrates the high durability of TiN.

 

Fig. 4. SEM image of TiN metasurface (a) Before the wear test, (b) After subjecting it to 10 minutes of rubbing against a lint-free cloth, (c) After subjecting it to 30 minutes of rubbing, (d) Transmission spectra of TiN before and after the wear test, SEM image of Au metasurface (e) Before the wear test, (f) After subjecting the film to 10 minutes of rubbing. TiN metasurface survives more than 30 minutes of rubbing and reproducibly shows the transmission spectra, while Au metasurface is destroyed within the first 10 minutes of rubbing. The scale bar is 500 nm.

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3. Conclusion

In summary, we designed, fabricated, and characterized nanohole arrays exhibiting EOT peaks using a low-cost, scratch-resistant refractory material, TiN. In addition to mechanical robustness of TiN, the structures further benefit from smooth sidewalls (due to single crystal nature) and good adhesion to substrates (negates the use of lossy adhesion layers), unlike commonly used noble metals such as gold and silver. As a result, despite nominally lower optical loss in noble metals, they offer only marginal performance improvement compared to TiN because of high-loss adhesion layers and rough surfaces. More importantly, while TiN nanohole arrays survive under extreme shear force, noble metal structures do not, rendering them unsuitable for a range of security-driven practical applications. While protective coatings of oxides could potentially alleviate some of the durability concerns, it is important to note that TiN is harder than commonly used oxides, such as SiO₂, TiO₂ and HfO₂. Furthermore, the high refractive index oxide coating would further deteriorate the performance of the metasurfaces, because the higher index leads to better confinement which in turn results in plasmons more sensitive to the surface roughness. In addition to the benefits of TiN, the current metasurface design has some of the EOT peaks positioned just outside the visible spectrum, which could potentially be used for covert security applications, wherein the metasurface would look uniformly colored in the visible spectrum, however, when viewed under a Si-based CCD camera (with access to 700 nm to 1000 nm spectrum) would reveal the security code. Moreover, depending on the application, the EOT spectrum can be extended down to the permittivity cross-over point of TiN with appropriate design, as shown in recent works [13,14]. Lastly, the proposed design is not only limited to TiN, but can be extended to other transition metal nitrides, such as Zirconium and Tantalum Nitride, which along with robustness, provide tunable optical properties [28]. Thus, adding another layer of complexity, which would make the replication of the security features more challenging.