Optical properties of single crystal silver thin films on mica for high performance plasmonic devices

We systematically investigate the optical properties of silver films to clear up the inconsistency in the published values of the dielectric function of silver. The silver films were deposited on mica by using a facing target sputtering system, which yielded a large area single crystal of silver suitable for the fabrication of high-finesse plasmonic devices and metamaterials. We confirmed that wide variations in the optical properties of silver were associated with the overall quality of the silver films including crystal structure, thickness, and surface roughness. The quality factor of the surface plasmon polaritons calculated for the obtained single crystal is 5 × 10, which is about five times higher than that for polycrystalline films. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (160.3918) Metamaterials; (250.5403) Plasmonics; (310.6860) Thin films, optical properties; (310.6628) Subwavelength structures, nanostructures. References and links 1. V. G. Veselago and E. E. Narimanov, “The left hand of brightness: past, present and future of negative index materials,” Nat. Mater. 5(10), 759–762 (2006). 2. N. I. Zheludev, “The road ahead for metamaterials,” Science 328(5978), 582–583 (2010). 3. J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85(18), 3966–3969 (2000). 4. W. Rechberger, A. Hohenau, A. Leitner, J. R. Krenn, B. Lamprecht, and F. R. Aussenegg, “Optical properties of two interacting gold nanoparticles,” Opt. Commun. 220(1–3), 137–141 (2003). 5. N. I. Zheludev, S. Prosvirnin, N. Papasimakis, and V. Fedotov, “Lasing spaser,” Nat. Photonics 2(6), 351–354 (2008). 6. S. A. Maier and H. A. Atwater, “Plasmonics: localization and guiding of electromagnetic energy in metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). 7. D. A. Bobb, G. Zhu, M. Mayy, A. V. Gavrilenko, P. Mead, V. I. Gavrilenko, and M. A. Noginov, “Engineering of low-loss metal for nanoplasmonic and metamaterials,” Appl. Phys. Lett. 95(15), 151102 (2009). 8. M. G. Blaber, M. D. Arnold, and M. J. Ford, “Optical properties of intermetallic compounds from first principles calculations: a search for the ideal plasmonic material,” J. Phys. Condens. Matter 21(14), 144211 (2009). 9. P. R. West, S. Ishii, G. V. Naik, N. K. Emani, V. M. Shalaev, and A. Boltasseva, “Searching for Better Plasmonic Materials,” Laser Photonics Rev. 4(6), 795–808 (2010). 10. K. Tanaka, E. Plum, J. Y. Ou, T. Uchino, and N. I. Zheludev, “Multifold enhancement of quantum dot luminescence in plasmonic metamaterials,” Phys. Rev. Lett. 105(22), 227403 (2010). 11. M. Kuttge, E. J. R. Vesseur, J. Verhoeven, H. J. Lezec, H. A. Atwater, and A. Polman, “Loss mechanisms of surface plasmon polaritons on gold probed by cathodoluminescence imaging spectroscopy,” Appl. Phys. Lett. 93(11), 113110 (2008). 12. V. A. Fedotov, T. Uchino, and J. Y. Ou, “Low-loss plasmonic metamaterial based on epitaxial gold monocrystal film,” Opt. Express 20(9), 9545–9550 (2012). 13. Y. Wu, C. Zhang, N. M. Estakhri, Y. Zhao, J. Kim, M. Zhang, X. X. Liu, G. K. Pribil, A. Alù, C. K. Shih, and X. Li, “Intrinsic Optical Properties and Enhanced Plasmonic Response of Epitaxial Silver,” Adv. Mater. 26(35), 6106–6110 (2014). 14. C. Y. Wang, H. Y. Chen, L. Sun, W. L. Chen, Y. M. Chang, H. Ahn, X. Li, and S. Gwo, “Giant colloidal silver crystals for low-loss linear and nonlinear plasmonics,” Nat. Commun. 6(1), 7734 (2015). 15. A. A. Baski and H. Fuchs, “Epitaxial growth of silver on mica as studied by AFM and STM,” Surf. Sci. 313(3), 275–288 (1994). 16. T. Mori, T. Mori, Y. Tanaka, Y. Suzaki, and K. Yamaguchi, “Fabrication of single-crystalline plasmonic nanostructures on transparent and flexible amorphous substrates,” Sci. Rep. 7, 42859 (2017). Vol. 8, No. 6 | 1 Jun 2018 | OPTICAL MATERIALS EXPRESS 1642 #320678 https://doi.org/10.1364/OME.8.001642 Journal © 2018 Received 25 Jan 2018; revised 28 Apr 2018; accepted 16 May 2018; published 30 May 2018 17. S. Buchholz, H. Fuchs, and J. P. Rabe, “Surface structure of thin metallic films on mica as seen by scanning tunneling microscopy, scanning electron microscopy, and low-energy electron diffraction,” J. Vac. Sci. Technol. B 9(2), 857 (1991). 18. M. J. Hall and M. W. Thompson, “Epitaxy and twinning in foils of some noble metals condensed upon lithium fluoride and mica,” Br. J. Appl. Phys. 12(9), 495–498 (1961). 19. N. P. Logeeswaran VJ, M. S. Kobayashi, W. Islam, P. Wu, N. X. Chaturvedi, S. Y. Fang, Wang, and R. S. Williams, “Ultrasmooth silver thin films deposited with a germanium nucleation layer,” Nano Lett. 9(1), 178– 182 (2009). 20. Y. Jiang, S. Pillai, and M. A. Green, “Re-evaluation of literature values of silver optical constants,” Opt. Express 23(3), 2133–2144 (2015). 21. H. U. Yang, J. D’Archangel, M. L. Sundheimer, E. Tucker, G. D. Boreman, and M. B. Raschke, “Optical dielectric function of silver,” Phys. Rev. B 91(23), 235137 (2015). 22. Y. Jiang, S. Pillai, and M. A. Green, “Realistic Silver Optical Constants for Plasmonics,” Sci. Rep. 6(1), 30605 (2016). 23. P. B. Johnson and R. W. Christy, “Optical Constants of the Noble Metals,” Phys. Rev. B 6(12), 4370–4379 (1972). 24. P. Nagpal, N. C. Lindquist, S. H. Oh, and D. J. Norris, “Ultrasmooth patterned metals for plasmonics and metamaterials,” Science 325(5940), 594–597 (2009). 25. W. L. Barnes, “Surface plasmon–polariton length scales: a route to sub-wavelength optics,” J. Opt. A, Pure Appl. Opt. 8(4), S87–S93 (2006). 26. S. Kadokura, M. Naoe, S. Nakagawa, and Y. Maeda, “Nano-size magnetic crystallite formation in Co-Cr thin films for perpendicular recording media,” IEEE Trans. Magn. 34(4), 1642–1644 (1998). 27. J. Moon and H. Kim, “Sputtering of aluminum cathodes on OLEDs using linear facing target sputtering with ladder-type magnet arrays,” J. Electrochem. Soc. 155(7), J187–J192 (2008). 28. V. A. Fedotov, M. Rose, S. L. Prosvirnin, N. Papasimakis, and N. I. Zheludev, “Sharp trapped-mode resonances in planar metamaterials with a broken structural symmetry,” Phys. Rev. Lett. 99(14), 147401 (2007). 29. W. P. Davey, “Precision Measurements of the Lattice Constants of Twelve Common Metals,” Phys. Rev. 25(6), 753–761 (1925). 30. M. Higo, K. Fujita, M. Mitsushio, T. Yoshidome, and T. Kakoi, “Epitaxial growth and surface morphology of aluminum films deposited on mica studied by transmission electron microscopy and atomic force microscopy,” Thin Solid Films 516(1), 17–24 (2007). 31. E. Palik, Handbook of Optical Constants of Solids (Academic Press 1998). 32. D. J. Nash and J. R. Sambles, “Surface plasmon-polariton study of the optical dielectric function of silver,” J. Mod. Opt. 43(1), 81–91 (1996). 33. J. M. Bennett, J. L. Stanford, and E. J. Ashley, “Optical constants of silver sulfide tarnish films,” J. Opt. Soc. Am. 60(2), 224–232 (1970).


Introduction
Plasmonic structures and metamaterials exploiting surface plasmons have attracted great attention over the last decade [1,2] since they gave rise to some innovative concepts and novel devices such as superlenses, nano-antennas, spasers, and subwavelength waveguides [3][4][5][6].Research on metamaterials operating in the infrared and visible spectral regions is often carried out using metasurfaces rather than their three-dimensional counterparts because of the ease of manufacture.However, the response of the metasurfaces is very sensitive to the presence of dissipative losses in the subwavelength resonators making it difficult to obtain the optimum performance.Several approaches to overcome the losses were investigated, including the search for better plasmonic materials among metallic alloys, heavily doped semiconductors, graphene, and conductive oxides [7][8][9] in addition to direct compensation of the losses by integrating metamaterials with optical gain media [10].Although these approaches aim to minimize Joule losses, the actual dissipation rates are often much higher than those expected from Ohm's law.The additional drawback associated with surface roughness and grain boundary scattering due to polycrystalline nature of thin metal films was reported [11].As a result, employing single crystals of noble metals could become a major step towards the reduction of plasmonic losses.We demonstrated that metamaterials fabricated using epitaxial gold thin films with a surface roughness of less than 0.2 nm showed a strong resonant response in the near-infrared spectral region [12].Silver is far less expensive than gold and has the lowest intrinsic loss in the visible and near-infrared regions among all metals.Thus, to improve the performance of plasmonic devices and metamaterials, the use of single crystal films of silver is desirable [13][14][15][16].However, silver has low cohesive energy as compared with other metals, while silver films on dielectric substrates are easily agglomerated by heating.In fact, it is difficult to obtain continuous silver films with a thickness of 100 nm or less using conventional methods [17][18][19].To overcome the difficulty, chemical methods and molecular beam epitaxy techniques have been developed.They enabled the growth of high-quality single-crystal films at temperatures lower than room temperature [13,14].Another problem related to the use of silver films stems from an inconsistency in the measured values of the optical dielectric function [20][21][22][23][24].The dielectric function is important to understand electronic and optical properties of noble metals, especially for transmission and reflection of light.The propagation length of surface plasmon polaritons, plasmon lifetime, and non-radiative loss are directly related to the dielectric function [25].However, silver has wide variations in the dielectric function associated with sample preparation, measurement techniques, and surface texture.While it is well known that optical properties are affected by surface roughness, grain boundary, and film thickness [11,12], there have been only a few systematic studies on how film structure affects the optical dielectric function of silver.
In this work, we present a systematic investigation of optical properties of silver thin films deposited by a facing target sputtering system, which yielded large area single crystal thin films.We investigated the effects of film thickness, surface texture, and crystal structure on the optical properties of silver thin films.The deposition conditions were optimized by evaluating the optical characteristics of silver films.We found that the inconsistency in the measured values of the optical dielectric function of silver resulted from the overall quality of the films including crystal structure, thickness, and surface roughness.The obtained single crystal silver thin films allowed us to reduce plasmonic losses and, in contrast to single crystal gold films, extend the useful spectral range to the near ultraviolet wavelengths.The obtained films were also used to fabricate a nanostructured metasurface with a structurally complex pattern, which showed high-Q resonance in the near infrared region.We believe that the silver growth technique, described here, makes it possible to realize inexpensive and low-loss plasmonic systems and devices for various practical applications.

Experimental details
Silver thin films were deposited on freshly cleaved mica substrates (Nilaco) with the help of a facing target sputtering system (Biemtron LS-420R), which enabled to avoid plasma damage.The parallel facing target direction was perpendicular to the substrate holder in this system [26,27].The substrates were heated during deposition from the back side of the substrate holder.The deposition temperature ranged from room temperature to 500 °C, and the film thickness ranged from 44 to 150 nm.The sputtering was performed at a deposition rate of 2 nm/s and a base pressure of less than 3 × 10 −5 Pa.For comparison, we also prepared samples deposited at room temperature on both mica and glass substrates.The mica sheets were cut into pieces of approximately 1 × 1 cm 2 and freshly cleaved to expose clean and atomically flat surface just before loading into the sputtering system.Mica is a highly transparent dielectric with an exceptionally broad transmission window spanning from UV to mid-IR (0.2 to 10 μm) which makes it an ideal substrate for hosting metamaterial-based optical devices.The glass substrates were cleaned with acetone, isopropanol, and distilled water before the deposition.The thin films were characterized by using various analytical techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM).The surface morphology of the samples was examined by using atomic force microscope (AFM) operating in a tapping mode under ambient conditions.Spectroscopic ellipsometry measurements using a spectroscopic ellipsometer (J. A. Woollam M-2000) were carried out to extract the optical constants of silver in the wavelength range of 200-1700 nm and at an incidence angle of 60°.The complex dielectric constant ε = ε 1 -iε 2 was obtained from the measured ellipsometric angles Ψ and Δ th.The quality aluated as the r (ε 1 ) 2 /ε 2 [9].ll potential of smonic metasu tured asymmetr Fano-type reso les was 23 111) and ( 20shows the ful er (111) reflec ubstrate is abo l crystallite siz s than one de y.
silver films wa ates using adhe nm using ion s typical TEM action pattern t en in the diffra ystal.Figure 4 in the metal, are smaller than those in the case of polycrystalline silver film.This result is consistent with the fact that the electron scattering due to surface roughness and grain boundaries in polycrystalline metal films causes losses [11] and, hence single crystal silver film should have a lower loss in the visible and near-infrared region.The obtained values of ε 2 are comparable to those recently reported for silver films grown by molecular beam epitaxy (MBE) [13] and smaller than previously published values including widely used data by Johnson and Christy [23,31,32].The quality factors of SPP for all samples were estimated using the measured dielectric permittivity.Figure 6(a) shows the quality factors of SPP at the wavelength of 1 µm as a function of the thickness of silver films deposited at room temperature.The quality factors corresponding to silver films on mica are seen to increase with increasing film thickness, and their values are about two times larger than those in the case of silver films on the glass when the film thickness exceeds 100 nm.On the other hand, silver films on glass are characterized by only a small increase in the quality factors for thicker samples.These results are consistent with the previous results [11] and suggest that the penetration of SPPs into single crystal films is deeper than into polycrystalline ones.Correspondingly, the presence of grain boundaries and surface roughness lead to the reduction of the quality factors of SPP. Figure 6(b) shows the quality factors of SPP for the silver films on mica as a function of deposition temperature.The quality factor corresponding to the single crystal silver film with 110-nm-thick is around 5 × 10 3 , which is about five times higher than the value of the commonly used silver film on glass.The drastic increase in the quality factors is seen around 350 °C in the case of 110-nmthick silver films, which is attributed to the transition from polycrystalline to the single crystal structure.As for 70-nm-thick silver films, the deposition at temperatures above 350 °C causes the formation of voids on the film surface, and consequently, the quality factor was reduced.
As an additional quality test for the obtained single crystal silver films, we investigated plasmonic metasurfaces exhibiting sharp Fano resonances.Figure 7(a) shows an SEM image of a high-finesse plasmonic metasurface fabricated in a 110-nm-thick single crystal silver film deposited at 500 °C.The metasurface is formed by an array of 40 nm wide slits shaped in the form of asymmetrically-split rings (ASR).The radius of the rings is 120 nm and the period of the array is 320 nm.The long and short arcs have the length of 377 and 272 nm and are separated by narrow gaps of about 53 nm. Figure 7(b) shows the reflection spectra of the fabricated metasurface.The spectra feature a Fano resonance at around 1.4 µm, which are similar results of the previously demonstrated ASR metasurface using epitaxial gold films on LiF substrates [12].Remarkably, the resonances from both 110-nm-thick silver films and 80nm-thick gold films have the same Q-factors even though the volume of ASR metamolecules in the silver metasurface is larger by about 40%.This result indicates that intrinsic loss in the obtained silver films was substantially lower than that in the epitaxial gold films, suggesting high quality.The endurance of the silver films is great concerns in practical use.The dot-anddash line in Fig. 7(b) shows the reflection spectrum of the silver ASR metasurface after leaving the sample in air for nine months.The aged silver ASR metasurface exhibits an almost identical reflection spectrum with only a slight red-shift.Exposing the sample to high vacuum did not affect the spectrum, which suggested that the red shift could not have resulted from the accumulation of moisture.The other possible explanation was that over time a certain fraction of silver in the film had been transformed into silver sulfide.To test the conjecture, we performed an energy dispersive X-ray spectroscopy (EDX) analysis of the sample.It revealed the presence of 0.5% of sulfur on the background of 67% of silver (with the remaining 32.5% taken mostly by aluminum and oxygen, which came from the mica substrate).Given that the thickness of the silver film was 110 nm, the detected amount of sulfur would be equivalent to a 0.
Fig. 1 110-n depos Fig. 5 500 °C the di 8-nm-thick surface layer of silver sulfide, which is a