Sina Abedini Dereshgi1 Xianglian Song1 Akshay Murthy1 Vinayak Dravid1 Koray Aydin1

1, Northwestern University, Evanston, Illinois, United States

Since successful demonstration of graphene as a monolayer 2D material, scientific community significantly expanded layered van der Waals materials family with different electronic, optical, mechanical and thermal properties which are pivotal in the quest for miniaturized photonics. Here, we investigate the optical properties of α-MoO3 both theoretically and experimentally.
Due to strong anisotropy of α-MoO3 in all 3 dimensions, it is expected to show three distinct Reststrahlen (RS) bands in mid-IR wavelengths which can give rise to the excitation of phonon polaritons within these RS bands. Phonon polaritons have significant implications for optical device design due to their unique properties such as wavelength shrinking and low loss. The optical properties of α-MoO3 can adequately be described using the phenomenological Lorentz oscillator model with parameters estimated in literature. Applying this relation, the complex dielectric constant of α-MoO3 is calculated which is in turn used to obtain the dispersion relation of α-MoO3 in x,y and z directions. After confirming our optical model, we propose a thin film multi-layer structure in order to experimentally examine the traces of the mentioned three RS bands through enhanced absorption as a result of phonon polariton excitation. The proposed structure, from bottom to top, is composed of thick Au, Ge and transferred α-MoO3. In particular, two samples with two different thicknesses of Ge (400 nm and 800 nm) are fabricated. The reflectance (R) versus wavenumber measurements are carried out using FTIR to obtain the total absorption (A). Since the thick Au suppresses any transmission, the total absorption can easily be calculated using the R data. The FTIR results illustrate absorption peaks around 800 cm-1 (12 μm) and 550 cm-1 (18 μm) which represent the x and y direction (in-plane) phonons respectively. Furthermore, polarized incident light measurements in FTIR emphasize polarization-dependent absorption in α-MoO3 which is an explicit outcome of perpendicular x and y phonons, where one of the in-plane resonance peaks is maximized only when the other one disappears.
In order to explain the observed results in further depth, simulations are carried out using TMM and FDTD methods which are in agreement with the experiments. In order to have accurate simulation results, the thickness of α-MoO3 flakes are measured by AFM and introduced to simulation. Our simulation results show Fabry-Perot (FP) effect in the Au/Ge/α-MoO3 structure which enhances the absorption. Specifically, for the sample with 800 nm thick Ge, the FP resonance is intentionally designed to occur in the vicinity of the x-phonon peak. Therefore, the x-phonon peak is intensified which further supports our discussion.
Since our measurement method relies on normal incidence measurement, there is no electric field component in z-direction. As a result, excitation of z-phonons is not viable. In order to circumvent this issue, we have designed and fabricated a new structure. A patterned layer of periodic Au nanodisks is added to the top of α-MoO3 in Au/Ge/α-MoO3 structure. When illuminated, the nanodisks diffract light and facilitate the coupling of radiation to the z-phonons of α-MoO3. The absorption peak near 10 cm-1 (10 μm) can be spotted in both experiments and simulations which justifies the observation of z-phonon polariton excitation. The electric field simulations are in line with the expected enhancement of z-component of the electric field.
The outstanding anisotropy of α-MoO3 in three directions can open new paths for mid-IR optics and it can provide unprecedented opportunities to engineer low-loss optical devices for the crucial mid-IR atmospheric window (8 – 12 μm).