Phase change material-based metasurface structure and related method
A metasurface structure including an array of sub-wavelength structures including a phase change material (PCM), encoded with different holographic images based on different phases of the PCM, the different phases including a first phase and a second phase. Phase transition between the first phase and the second phase occurs when the metasurface structure is thermally tuned. Each sub-wavelength structure in the array has a distinctive phase difference between the first phase and the second phase of the PCM.
1 . A metasurface structure comprising:
an array of sub-wavelength structures including a phase change material (PCM), encoded with different holographic images based on different phases of the PCM, the different phases including a first phase and a second phase,
wherein phase transition between the first phase and the second phase occurs when the metasurface structure is thermally tuned between different temperatures, and
wherein each sub-wavelength structure in the array has a distinctive phase difference between the first phase and the second phase of the PCM, and
wherein the different holographic images comprise a first holographic image displayed in the first phase at a first temperature and a second holographic image displayed in the second phase at a second temperature.
2 . The metasurface structure of claim 1 , wherein the phase change material comprises vanadium dioxide (VO 2 ).
3 . The metasurface structure of claim 1 , wherein the metasurface structure is optically excited by visible radiation.
4 . The metasurface structure of claim 3 , wherein the visible radiation has a wavelength which ranges between 600 nm to 800 nm.
5 . The metasurface structure of claim 1 , wherein the array of sub-wavelength structures comprises a plurality of sub-wavelength micro-structures or nano-structures.
6 . The metasurface structure of claim 1 , wherein the array of sub-wavelength structures is in the form of nano-blocks having a height, a length and a width.
7 . The metasurface structure of claim 6 , wherein dimensions and/or rotation states of the array of the nano-blocks are optimized based on a meta-atom library.
8 . The metasurface structure of claim 7 , wherein the array of the nano-blocks comprises four types of nano-blocks which are selected to have distinctive phase differences between two phases of the PCM and high cross-polarized light transmittance.
9 . The metasurface structure of claim 8 , wherein the four nano-blocks are different in terms of at least one of their rotation states and dimensions.
10 . A method for encoding information on a metasurface structure including an array of sub-wavelength structures, comprising:
selecting the array of sub-wavelength structures including a phase change material (PCM) such that each sub-wavelength structure in the array has a distinctive phase difference between a first phase and a second phase of the PCM; and
encoding at least two different holographic images into the array of sub-wavelength structures based on the first phase and the second phase of the PCM,
wherein the first phase and the second phase are different, and phase transition between the first phase and the second phase occurs when the metasurface structure is thermally tuned between different temperatures,
wherein the at least two different holographic images comprise a first holographic image displayed in the first phase at a first temperature and a second holographic image displayed in the second phase at a second temperature.
11 . The method of claim 10 , wherein selecting the array of sub-wavelength structures comprises:
selecting dimensions and/or rotation states of respective sub-wavelength structures in the array.
12 . The method of claim 10 , further comprising:
constructing a meta-atom library to show cross-polarized light transmittance and phase shift at different temperatures as a function of dimensions of the array of sub-wavelength structures.
13 . The method of claim 12 , wherein selecting the array of sub-wavelength structures comprises selecting, from the meta-atom library, four nano-blocks with distinctive phase differences and high cross-polarized light transmittance.
14 . The method of claim 10 , wherein the array of sub-wavelength structures is in the form of nano-blocks having a height, a length and a width.
15 . The method of claim 14 , wherein selecting the array of sub-wavelength structures comprises:
selecting all nano-blocks with cross-polarized light transmittance higher than an allowed minimum of transmittance T min and not exceeding an allowed maximum of transmittance T max at an arbitrary wavelength in the range of 600 nm to 800 nm;
comparing every two nano-blocks selected from the previous selecting step, and finding pairs with phase differences between −Δφ max and Δφ max at the first temperature and π−Δφ max and π+Δφ max at the second temperature at one wavelength where Δφ max is an allowed maximum error in phase differences;
obtaining pairs of nano-blocks satisfying the state transitions of 0 to 0 and 0 to π; and
exchanging the nano-block's length and width of the selected pair of nano-blocks to obtain another pair satisfying π to π and π to 0 state transitions.
16 . The method of claim 15 , wherein the first temperature is a room temperature (RT) and the second temperature is a temperature higher than the room temperature (HT).
17 . The method of claim 10 , wherein encoding the at least two different holographic images is based on a gradient descent-based iterative approach.
18 . The method of claim 17 , wherein the gradient descent-based iterative approach is based on a machine learning model comprising three layers of an input layer, a hidden layer, and an output layer, corresponding to an incident light, a diffraction plane, and an image plane, respectively.
19 . The method of claim 17 , wherein encoding the at least two different holographic images based on the gradient descent-based iterative approach comprises:
calculating two binary-phase holographic images based on the gradient descent-based iterative approach; and
encoding the two holographic images into the two different phases of the PCM at the two temperatures including the first temperature and the second temperature.
20 . The method of claim 10 , wherein encoding the at least two different holographic images comprises applying additional work conditions including wavelength, polarization, and/or observation distance.
21 . The method of claim 20 , wherein different observation distances and/or different polarizations are assigned for respective holographic images in addition to the different temperatures.
22 . The method of claim 10 , wherein the phase change material (PCM) comprises vanadium dioxide (VO 2 ).
23 . The method of claim 10 , wherein the at least two different holographic images are generated when the metasurface structure is optically excited by visible radiation.