Design and fabrication of a vanadium dioxide-based actively switchable wire grid polarizer for near-infrared applications

. This study introduces an actively switchable wire grid polarizer exploiting the semiconductor-metal transition of vanadium dioxide. Operating at a near-infrared wavelength, the device features a SiO 2 substrate with VO 2 deposited by atomic layer deposition. We demonstrate the design using rigorous coupled wave analysis and show a viable fabrication route. Polarisation-resolved spectral transmission measurements show switching of the extinction ratio from 37.5 (on-state) to 1.6 (off-state). Despite observed deviations between measured and theoretical transmission values, the device shows potential in miniaturized imaging processes, polarization measurements, and ellipsometry.


Introduction
Vanadium dioxide (VO2) has been extensively investigated [1,2] due to its semiconductor-to-metal transition (SMT), first observed by Morin [3].The structural phase change occurs at a transition temperature of 68 °C, from the low-temperature monoclinic VO2(M) to the hightemperature rutile VO2(R) crystal structure, influencing many physical properties such as the refractive index and extinction coefficient [4].The accessible transition temperature renders VO2 suitable for various switchable devices, including smart windows [5,6].
Wire grid polarizers (WGP) can achieve high extinction ratios for linearly polarized light, defined as the ratio of TM-to TE-polarized transmission (TTM and TTE), denoted as ER = TTM/TTE [7].By incorporating phase change materials, it becomes possible to create actively switchable devices with tunable extinction ratios [8], or the possibility to alternate between an on-and off-state.This change can be expressed as a switching factor ERhigh/ERlow.
In this work, we present the design and fabrication of such devices operating in the near-infrared wavelength range, specifically around 1550 nm, using SiO2 substrate and VO2 deposited by an atomic layer deposition (ALD) process.We verify the optical response through spectral transmission measurements and compare the results to simulations by rigorous coupled wave analysis (RCWA).

Manufacturing
We propose a design and fabrication method for an actively switchable WGP based on a high aspect ratio 400 nm period grating etched into a fused silica substrate.The line width is 168 nm, and the grating depth is 1000 nm.A layer of 42 nm VO2 conformally covers the grating, whereas the material on the upper horizontal surface is removed, resulting in U-like VO2-structures within the grating trenches, as illustrated in fig. 1 d).
The fabrication process begins by patterning a chromium hard mask via e-beam lithography and subsequent reactive ion etching, as shown in fig. 1 a), followed by the transfer into the substrate by inductively coupled plasma etching process with fluoroform (CHF3).This method creates deep and straight sidewalls for the ridges, see fig. 1 b).We established an ALD process to deposit a uniform VO2 layer, as shown in fig. 1 c).The deposited material exhibits an SMT after 30-minute annealing at 520 °C in a nitrogen and oxygen atmosphere (100:1 ratio), at a total pressure of 10 mbar.Tetra-kis(ethylmethylamino)vanadium was utilized as the precursor and H2O as the oxidant for the ALD process, with a constant growth per cycle of 0.086 nm and a deposition temperature of 150 °C.To planarize the surface, the trenches were filled with a thick resist, and isotropically removed by ion beam sputter etching to remove the VO2 and Cr layer from the top of the ridges.Lastly, the polymer was removed from the trenches through oxygen plasma ashing to achieve the final structure depicted in fig. 1 d).

Complex refractive index of VO2
Using variable-angle spectroscopic ellipsometry we measured the complex refractive index of VO2 films deposited on silicon substrates by means of ALD and subsequent thermal annealing.During measurement the temperature is controlled via the sample stage.The refractive index and extinction coefficient were recorded at 25 °C and 90 °C.The measurements displayed in fig. 2 reveal significant changes for near-infrared wavelengths in both the refractive index and, especially, the extinction coefficient.

Polarization dependent transmission
By adjusting the substrate temperature the polarization properties of the fabricated grating structure can be effectively altered.The measured TE-and TM-transmission for λ = 1550 nm at 25 °C is 50.1 % and 80.4 %, respectively.Hence the extinction ratio (ER) is 1.6.For temperatures above the transition temperature (90 °C), the TE-transmission is 1.1 %, and TM-transmission is 42.2 %.Thus, an ER of over 37.5 can be achieved (fig.3 a)).This results in a switching factor of 46 for the transmission of TE polarized light.

RCWA simulation
Using the design parameters and measured refractive indices obtained by ellipsometry, we calculated transmission values using RCWA.At 25 °C, the TE-and TMtransmissions are as high as 47.3 % and 92.6 %, respectively, with an ER of 2.0.For temperatures above the SMT, the TE-transmission is 0.2 %, and TM-transmission is 63.4 %.This results in a theoretical ER of over 330 (fig.3 b)), with a switching factor of 240 for TEpolarization.

Conclusion
We have successfully fabricated an actively switchable wire grid polarizer utilizing the unique properties of vanadium dioxide.Our device demonstrates the switching of the extinction ratio.The deviation between simulated and measured optical parameters imply slight structural deviations.In a future work we will optimize the fabrication process to unlock the full potential of the semiconductor to metal transition of VO2 for optical applications.
Potential areas of application for such a device include miniaturized imaging processes requiring the capture of local polarization states of light.As the structure also influences the phase, the optical function can be switched between retarder and polarizer.This technology could be particularly useful in polarization measurements with a Stokes polarimeter without moving parts or in ellipsometry for layer characterization.

Fig. 1 .
Fig. 1.(a) Chromium hard mask with period of 400 nm, (b) etched fused silica grating 1 µm deep, (c) conformal 42 nm ALD layer of VO2, (d) removed top surfaces of the grating and final design of actively switchable WGP.

Fig. 2 .Fig. 3 .
Fig. 2. a) refractive index and b) extinction coefficient of VO2 films deposited by ALD below and above the SMT.