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 (${\text{VO}}_2$) thin film on a gold reflecting substrate. ${\text{VO}}_2$ 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 ${\text{VO}}_2$ gradually increases as well. The absorption thus could be tuned by controlling the environmental temperature. Moreover, when the imaginary part of the ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ in the near-infrared region. The performances for ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ could be deducted as A = 1 – R.

 

Fig. 1 Schematics of ${\text{VO}}_{\text{2}}$ film grown on double polished sapphire with deposited gold. The near-IR light source was incident from the top side and the ceramic heater was placed below the gold.

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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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ and the metallic nanoscale islands [31]. For the intermediate state, the polarization density $P_i$ could be written as the mixture of the polarizations in dielectric ($P_d$) and metallic states ($P_{m }$) [32]:

The ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 $\text{μm}$ for 100 nm ${\text{VO}}_2$ and 3 $\text{μm}$ for 200 nm ${\text{VO}}_2$ layers. The absorbing film thickness is in the order of $\text{λ}/15$, which is very thin compared with the common Fabry-Perot cavities. While increasing the temperature, the ${\text{VO}}_2$ gradually becomes metallic beyond which no resonant behavior could be observed. In this case, the electromagnetic field partially penetrates the lossy ${\text{VO}}_2$ 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 ${\text{VO}}_2$ measured in Ref 22. Nevertheless, it seems that the effective medium theory for ${\text{VO}}_2$ models the intermediate states quite well. The near perfect absorption performance is also well presented in the simulation results.

 

Fig. 2 Absorption spectra with various material states. To compare the trend, the scale of the absorption is offset by additional translation. The amplitude of the absorption for each material state is still in the range of 0 and 1 but with additional translation of 1,2,3,4 and 5 from states 2 to 6. (a, c) Measured absorption at selected temperatures for 100 nm thickness ${\text{VO}}_{\text{2}}$ and 200 nm thickness ${\text{VO}}_{\text{2}}$ respectively. (b, d) Calculated absorption with fitted metallic percentage factors for 100 nm thickness ${\text{VO}}_{\text{2}}$ and 200 nm thickness ${\text{VO}}_{\text{2}}$ respectively. The fitted dash lines track the shifts of resonance positions.

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The expected hysteresis properties of the ${\text{VO}}_2$ based absorber have been observed in both cases as well. The absorption spectra at resonant wavelengths (1.5 $\text{μm}$ for 100 nm ${\text{VO}}_2$ and 3 $\text{μm}$ for 200 nm ${\text{VO}}_2$) 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$, the absorption remains quite high over a range of about 20 °C.

 

Fig. 3 Measured absorption intensity as a function of increasing temperature and decreasing temperature at 1.5 µm wavelength for (a) 100 nm thickness ${\text{VO}}_{\text{2}}$ and at 3 µm wavelength for (b) 200 nm thickness ${\text{VO}}_{\text{2}}$.

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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 $\text{α}=10\text{\%}$. The absorbed power density is calculated spatially using the equation $\text{P}=-\frac{1}{2}\omega {\left|E\right|}^2{ϵ}^{\text{'}\text{'}}$, where $\omega$ is the angular optical frequency, ${ϵ}^{\text{'}\text{'}}$ is the imaginary part of the dielectric permittivity and ${\left|E\right|}^2$ is the intensity of the electric field. The maps for the 100 nm ${\text{VO}}_2$ layer cases are depicted in Fig. 4(a) and Fig. 4(c). It is clearly seen that near the interface of the ${\text{VO}}_2$ layer and back gold, the absorbed power is weak. The highest absorbed power is obtained around 1.5 $\text{μm}$ and 3 $\text{μm}$ for 100 nm ${\text{VO}}_2$ and 200 nm ${\text{VO}}_2$ 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 ${\text{VO}}_2$ layer and back gold. This explains why the light is absorbed mostly in the top part of ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ films is relatively weak.

 

Fig. 4 Calculated electrical field intensity distribution and absorption maps for (a, c) 100 nm thickness ${\text{VO}}_{\text{2}}$ and (b, d) 200 nm thickness ${\text{VO}}_{\text{2}}$. The left two plots show the absorbed power density maps. The positions of the VO₂ films are indicated by the dashed line.

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3. Conclusions

In conclusion, we investigated theoretically and experimentally a tunable thin absorber structure based on ${\text{VO}}_2$ phase transition. By enhancing the electromagnetic field above the reflector, the near-IR absorption in ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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 ${\text{VO}}_2$ 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.

 

Fig. 5 Measured absorption with more temperature data points for 100 nm thickness case (a and c) and 200 nm thickness case (b and d).

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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.

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