Abstract
Phase change materials exhibit tunable electrical and optical response, providing rich potential to build active devices with tunable properties. Here, we propose and demonstrate a tunable infrared absorber based on vanadium dioxide () thin films. Compared with conventional absorbers relying on either nanostructures or Fabry-Perot cavities, our proposed device shows near perfect absorption while having a subwavelength thick absorbing film. Moreover, the absorption intensity can be controlled dynamically around the phase transition temperature of VO2. We model the optical response of the intermediate states with an effective medium theory to help fitting and understanding the phase change behavior during the phase transition. The calculated electric field distribution as well as the absorption maps are presented to show how the light is absorbed in the thin film platform. The proposed device has the potential for many applications including thin photodetectors, modulators and tunable emitters.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Resonant light absorption has been investigated extensively in recent years with various materials and different resonance mechanisms. Conventional approach is to enhance the absorption by structuring either the absorbing material or using resonant elements such as optical antennas to enhance the local electromagnetic field inside the absorbing media [1–4]. Alternatively, planar devices composed of thin-film materials have been recently investigated as an optical absorber platform [5–8], that are used for applications such as modulators [9, 10], photodetectors [11, 12], and thermal emitters [13–15]. Most of these planar lithography-free devices rely on the interference of the reflected light with the incident light to enhance the absorption. In particular, multilayer thin films with Fabry-Perot resonance response have been investigated thoroughly [13, 16, 17]. However, those platforms require at least wavelength-scale dielectric cavities to form the resonances, making the whole devices quite thick. Thus, it is favorable to design a platform, which could mimic the resonant behavior of the lithography cavity while still remains compact as the devices incorporating nanostructures. One possible solution is to use highly absorbing materials [15–17], which could drastically decrease the thickness to fulfill the effective interference.
In this letter, we demonstrate a tunable multilayer absorber, operating at near infrared (near-IR) wavelengths, composed of vanadium dioxide () thin film on a gold reflecting substrate. is a phase transition material that undergoes phase transition at 68 °C. Phase transition occurs through formation domain switching during which nanoscale metallic islands are formed inside an insulating film [18–25]. If the temperature is well above the transition temperature, metallic domains will merge together and form completely metallic thin film. The optical losses in gradually increases as well. The absorption thus could be tuned by controlling the environmental temperature. Moreover, when the imaginary part of the refractive index increases to the order of the real part, the losses cannot be considered as a perturbation anymore and the light is quickly attenuated due to the resonance behavior in the thin film [7]. Previously, thin-film absorber on sapphire substrate has been proposed for mid-IR wavelength region [26]. Tunable absorption near visible wavelength region is also investigated with Pt back reflector [27]. A comprehensive study for choosing epsilon-near-zero substrate has been showed and verified on tunable aluminum-doped zinc oxide substrate [28]. Here, we quantitatively utilize the effective medium theory to model the tunable optical response of in the intermediate states, which could serve as a useful tool to understand this ultra-thin film absorption platform. Particularly, the metallic percentage factors in the material model are fitted for selected temperatures. We propose using gold substrate as a broadband reflective back mirror and obtaining the near-IR absorption resonances. This indicates that the absorption of the intermediate state is not necessarily due to the metamaterial structure. And the gold substrate could serve as an epsilon-near-zero material to fulfill the requirement for near-perfect absorption [28] and make the absorption almost only occur in in the near-infrared region. The performances for film in 100 nm and 200 nm thickness are compared to both verify the effective medium theory model and show the possibility to shift the target resonances by changing the layer thickness. The mechanism of the thin film absorption is also elaborated with the electric field distribution and absorbed power density map.
2. Results and discussions
The tunable absorber design is schematically shown in Fig. 1. The thin film was first grown on a double side polished sapphire substrate. Then the sample was flipped over and an optically thick gold layer (150 nm) was deposited just on top of the film. The near-IR light source was incident from the sapphire side and the sample was placed on a metal ceramic heater (below the gold layer) to control the temperature of the film. Fourier transform infrared (FTIR) spectrometer was used to measure the reflection (R) from the sapphire side. Since the back gold layer served as a mirror in this case, no transmission could be observed. Thus, the absorption (A) in could be deducted as A = 1 – R.
To obtain the theoretical absorption of the proposed devices, finite-difference time-domain (FDTD) modeling was used to calculate the optical response. The refractive index of the sapphire was chosen as 1.7 and the complex dispersive refractive index of gold was fitted from the Palik database [29]. The refractive indices of in both dielectric (room temperature) case and metallic (hot temperature) case were taken from previous experimental study [30]. To calculate the optical response of the in the intermediate states, we employ a Lorentz-Lorenz model to evaluate the binary mixture of two different permittivity. With this effective medium model, we could describe total response from both the surrounding dielectric and the metallic nanoscale islands [31]. For the intermediate state, the polarization density could be written as the mixture of the polarizations in dielectric () and metallic states () [32]:
where is the percentage of the metallic state. In particular, indicated the room temperature case and indicated the hot temperature case. The polarization density could be expressed with the Clausius-Mossotti relation. Thus, we obtained the corresponding permittivity with different metallic percentage factor :Comparing with the previously employed Bruggeman model [18, 23], our effective medium model could also describe all the intermediate states but only one fitting parameter is needed. After obtaining the permittivity for dielectric and metallic cases, the model could predict the optical collective response for states in between. As compared later with the experimental results, our effective medium model works quite well.The films were put in x-y planes with periodic boundary conditions in the simulation, as shown in Fig. 1. Broadband near-IR plane waves were incident from z directions. Along the z directions, perfect matched layers were used to absorb all the electromagnetic power coming out to the boundaries. Electric field distribution was gathered by the field profile monitors around the structures.
Two samples with 100 nm and 200 nm thickness of were prepared and measured with identical procedures. The measurements were taken by controlling the ceramic heater from room temperature 23 °C to hot temperature 120 °C with every 2 °C increments. To observe the hysteresis behavior [33], the reverse measurements were also taken by 2 °C decrements by cooling down the ceramic heater. The detailed measurement results including both the temperature-increasing and decreasing cases are presented in the Appendix. In Fig. 2(a) and Fig. 2(c), we plot the selected normalized spectra taken from 100 nm and 200 nm thickness samples in the temperature-increasing measurement. The phase transition happens around 68 °C. For both samples, we observed a red shift in the absorption peak position when transforming from the insulator state to the intermediate state. During the transition, the intensity of the resonance becomes higher and leads to almost perfect absorption around 1.5 for 100 nm and 3 for 200 nm layers. The absorbing film thickness is in the order of , which is very thin compared with the common Fabry-Perot cavities. While increasing the temperature, the gradually becomes metallic beyond which no resonant behavior could be observed. In this case, the electromagnetic field partially penetrates the lossy film and approximately 60% of the light was still absorbed over a broad range of wavelengths. Fig. 2(b) and Fig. 2(d) present the corresponding simulation results. The metallic percentage factors were fitted for various temperatures to match the peak positions in the 100 nm thickness film measurements. Good agreement is shown between the calculated (Fig. 2(b) and Fig. 2(d)) and measured (Fig. 2(a) and Fig. 2(c)) optical responses, both in magnitude and in spectral profile. The calculated and measured absorption peak positions slightly deviate, which could be attributed to the differences in the growth conditions of our sample and the measured in Ref 22. Nevertheless, it seems that the effective medium theory for models the intermediate states quite well. The near perfect absorption performance is also well presented in the simulation results.
The expected hysteresis properties of the based absorber have been observed in both cases as well. The absorption spectra at resonant wavelengths (1.5 for 100 nm and 3 for 200 nm ) as a function of temperature are plotted in Fig. 3(a) and Fig. 3(b), respectively. For both cases, ~15 °C hysteresis range was observed in the measurements. Note that the absorption for 200 nm is substantially more sensitive to temperature change and that high absorption occurs only at narrow range of the temperature (~2 °C). On the other hand, in 100 nm , the absorption remains quite high over a range of about 20 °C.
To better understand the absorption mechanism, we calculate the absorption maps and the electric field distribution (as a function of wavelength and vertical position) of an intermediate state with metallic percentage . The absorbed power density is calculated spatially using the equation , where is the angular optical frequency, is the imaginary part of the dielectric permittivity and is the intensity of the electric field. The maps for the 100 nm layer cases are depicted in Fig. 4(a) and Fig. 4(c). It is clearly seen that near the interface of the layer and back gold, the absorbed power is weak. The highest absorbed power is obtained around 1.5 and 3 for 100 nm and 200 nm respectively, which match the peak positions in the absorption spectra. The intensities of the electric field distributions (Fig. 4(c) and Fig. 4(d)) indicate that at the mentioned resonance peaks, the interference leads to weak electric field density at the interface of layer and back gold. This explains why the light is absorbed mostly in the top part of film, near the sapphire side. As mentioned before, the enhanced electric field intensity near the top part stems from the reflection phase shift due to the high losses in the film. The near-perfect absorption occurs at the wavelengths where the destructive interference largely suppresses the reflection. Note that at wavelengths far from the resonances, the electric field inside the films is relatively weak.
3. Conclusions
In conclusion, we investigated theoretically and experimentally a tunable thin absorber structure based on phase transition. By enhancing the electromagnetic field above the reflector, the near-IR absorption in thin films can be largely enhanced. The absorption intensity can be tuned by controlling the ambient temperature around the phase transition temperature. We utilized the effective medium model to fit the spectral properties of the intermediate states at selected temperatures. Good agreements were obtained between the calculated and measured optical responses. We also compare the hysteresis properties of layers with different thicknesses. The absorption mechanism was explained in detail by the intensity of calculated electric field distribution and absorption maps. We believe our study will both enhance the fundamental understanding of the material properties of and motivate new device design implementations including thin photodetectors, modulators and tunable emitters.
Appendix detailed experimental spectra
In Fig. 2(a) and Fig. 2(c), we plot the measured absorption of six chosen temperature data points for the temperature-increasing case. In Fig. 5(a) and Fig. 5(b), detailed experimental spectra with more temperature data points are plotted. Fig. 5(c) and Fig. 5(d) show the spectra for temperature-decreasing case. The top two figures are for 100 nm thickness, and absorption of 200 nm thickness is shown in bottom two figures.
Funding
Office of Naval Research Young Investigator Program (ONR-YIP) Award (N00014-17-1-2425); Binational Science Foundation (BSF) (2016388); Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy (DE-AC02-05CH11231).
Acknowledgments
This material is based upon work supported by the Office of Naval Research Young Investigator Program (ONR-YIP) Award (N00014-17-1-2425). K.A. and J.S. acknowledge partial support from the Binational Science Foundation (BSF) under grant 2016388. The materials growth was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
References and links
1. K. Aydin, V. E. Ferry, R. M. Briggs, and H. A. Atwater, “Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers,” Nat. Commun. 2(1), 517 (2011). [CrossRef] [PubMed]
2. Y. Cui, Y. He, Y. Jin, F. Ding, L. Yang, Y. Ye, S. Zhong, Y. Lin, and S. He, “Plasmonic and metamaterial structures as electromagnetic absorbers,” Laser Photonics Rev. 8(4), 495–520 (2014). [CrossRef]
3. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef] [PubMed]
4. Z. Li, S. Butun, and K. Aydin, “Ultranarrow band absorbers based on surface lattice resonances in nanostructured metal surfaces,” ACS Nano 8(8), 8242–8248 (2014). [CrossRef] [PubMed]
5. Z. Li, S. Butun, and K. Aydin, “Large-area, lithography-free super absorbers and color filters at visible frequencies using ultrathin metallic films,” ACS Photonics 2(2), 183–188 (2015). [CrossRef]
6. S. Shu, Z. Li, and Y. Y. Li, “Triple-layer Fabry-Perot absorber with near-perfect absorption in visible and near-infrared regime,” Opt. Express 21(21), 25307–25315 (2013). [CrossRef] [PubMed]
7. M. A. Kats, R. Blanchard, P. Genevet, and F. Capasso, “Nanometre optical coatings based on strong interference effects in highly absorbing media,” Nat. Mater. 12(1), 20–24 (2013). [CrossRef] [PubMed]
8. Z. Li, E. Palacios, S. Butun, H. Kocer, and K. Aydin, “Omnidirectional, broadband light absorption using large-area, ultrathin lossy metallic film coatings,” Sci. Rep. 5(1), 15137 (2015). [CrossRef] [PubMed]
9. R. Yan, R. Simes, and L. Coldren, “Electroabsorptive fabry-perot reflection modulators with asymmetric mirrors,” IEEE Photonics Technol. Lett. 1(9), 273–275 (1989). [CrossRef]
10. K. K. Law, R. Yan, L. Coldren, and J. Merz, “Self-electro-optic device based on a superlattice asymmetric Fabry–Perot modulator with an on/off ratio≳ 100: 1,” Appl. Phys. Lett. 57(13), 1345–1347 (1990). [CrossRef]
11. M. S. Ünlü and S. Strite, “Resonant cavity enhanced photonic devices,” J. Appl. Phys. 78(2), 607–639 (1995). [CrossRef]
12. A. Chin and T. Chang, “Multilayer reflectors by molecular-beam epitaxy for resonance enhanced absorption in thin high-speed detectors,” J. Vac. Sci. Technol. B 8, 339–342 (1990).
13. E. Nefzaoui, J. Drevillon, Y. Ezzahri, and K. Joulain, “Simple far-field radiative thermal rectifier using Fabry-Perot cavities based infrared selective emitters,” Appl. Opt. 53(16), 3479–3485 (2014). [CrossRef] [PubMed]
14. W. Streyer, S. Law, G. Rooney, T. Jacobs, and D. Wasserman, “Strong absorption and selective emission from engineered metals with dielectric coatings,” Opt. Express 21(7), 9113–9122 (2013). [CrossRef] [PubMed]
15. H. Kocer, S. Butun, B. Banar, K. Wang, S. Tongay, J. Wu, and K. Aydin, “Thermal tuning of infrared resonant absorbers based on hybrid gold- VO2 nanostructures,” Appl. Phys. Lett. 106(16), 161104 (2015). [CrossRef]
16. K. Kishino, M. S. Unlu, J.-I. Chyi, J. Reed, L. Arsenault, and H. Morkoc, “Resonant cavity-enhanced (RCE) photodetectors,” IEEE J. Quantum Electron. 27(8), 2025–2034 (1991). [CrossRef]
17. H. Kocer, S. Butun, E. Palacios, Z. Liu, S. Tongay, D. Fu, K. Wang, J. Wu, and K. Aydin, “Intensity tunable infrared broadband absorbers based on VO2 phase transition using planar layered thin films,” Sci. Rep. 5(1), 13384 (2015). [CrossRef] [PubMed]
18. M. M. Qazilbash, M. Brehm, B. G. Chae, P. C. Ho, G. O. Andreev, B. J. Kim, S. J. Yun, A. V. Balatsky, M. B. Maple, F. Keilmann, H. T. Kim, and D. N. Basov, “Mott transition in VO2 revealed by infrared spectroscopy and nano-imaging,” Science 318(5857), 1750–1753 (2007). [CrossRef] [PubMed]
19. H. S. Choi, J. S. Ahn, J. H. Jung, T. W. Noh, and D. H. Kim, “Mid-infrared properties of a VO2 film near the metal-insulator transition,” Phys. Rev. B Condens. Matter 54(7), 4621–4628 (1996). [CrossRef] [PubMed]
20. G. Zhang, H. Ma, C. Lan, R. Gao, and J. Zhou, “Microwave tunable metamaterial based on semiconductor-to-metal phase transition,” Sci. Rep. 7(1), 5773 (2017). [CrossRef] [PubMed]
21. G. Wehmeyer, T. Yabuki, C. Monachon, J. Wu, and C. Dames, “Thermal diodes, regulators, and switches: physical mechanisms and potential applications,” Appl. Phys. Rev. 4(4), 041304 (2017). [CrossRef]
22. S. Wang, L. Kang, and D. H. Werner, “Hybrid resonators and highly tunable terahertz metamaterials enabled by vanadium dioxide VO2,” Sci. Rep. 7(1), 4326 (2017). [CrossRef] [PubMed]
23. J. Rensberg, S. Zhang, Y. Zhou, A. S. McLeod, C. Schwarz, M. Goldflam, M. Liu, J. Kerbusch, R. Nawrodt, S. Ramanathan, D. N. Basov, F. Capasso, C. Ronning, and M. A. Kats, “Active optical metasurfaces based on defect-engineered phase-transition materials,” Nano Lett. 16(2), 1050–1055 (2016). [CrossRef] [PubMed]
24. F. Menges, M. Dittberner, L. Novotny, D. Passarello, S. Parkin, M. Spieser, H. Riel, and B. Gotsmann, “Thermal radiative near field transport between vanadium dioxide and silicon oxide across the metal insulator transition,” Appl. Phys. Lett. 108(17), 171904 (2016). [CrossRef]
25. M. Currie, M. A. Mastro, and V. D. Wheeler, “Characterizing the tunable refractive index of vanadium dioxide,” Opt. Mater. Express 7(5), 1697–1707 (2017). [CrossRef]
26. M. A. Kats, D. Sharma, J. Lin, P. Genevet, R. Blanchard, Z. Yang, M. M. Qazilbash, D. N. Basov, S. Ramanathan, and F. Capasso, “Ultra-thin perfect absorber employing a tunable phase change material,” Appl. Phys. Lett. 101(22), 221101 (2012). [CrossRef]
27. J. Liang, L. Hou, and J. Li, “Frequency tunable perfect absorber in visible and near-infrared regimes based on VO2 phase transition using planar layered thin films,” J. Opt. Soc. Am. B 33(6), 1075–1080 (2016). [CrossRef]
28. J. Rensberg, Y. Zhou, S. Richter, C. Wan, S. Zhang, P. Schöppe, R. Schmidt-Grund, S. Ramanathan, F. Capasso, M. A. Kats, and C. Ronning, “Epsilon-Near-Zero Substrate Engineering for Ultrathin-Film Perfect Absorbers,” Phys. Rev. Appl. 8(1), 014009 (2017). [CrossRef]
29. E. D. Palik, Handbook of Optical Constants of Solids (Academic press, 1998).
30. M. J. Dicken, K. Aydin, I. M. Pryce, L. A. Sweatlock, E. M. Boyd, S. Walavalkar, J. Ma, and H. A. Atwater, “Frequency tunable near-infrared metamaterials based on VO2 phase transition,” Opt. Express 17(20), 18330–18339 (2009). [CrossRef] [PubMed]
31. I. Webman, J. Jortner, and M. H. Cohen, “Theory of optical and microwave properties of microscopically inhomogeneous materials,” Phys. Rev. B 15(12), 5712–5723 (1977). [CrossRef]
32. G. Kaplan, K. Aydin, and J. Scheuer, “Dynamically controlled plasmonic nano-antenna phased array utilizing vanadium dioxide [Invited],” Opt. Mater. Express 5(11), 2513–2524 (2015). [CrossRef]
33. F. Beteille and J. Livage, “Optical switching in VO2 thin films,” J. Sol-Gel Sci. Technol. 13(1/3), 915–921 (1998). [CrossRef]