High Temperature Mid‐IR Polarizer via Natural In‐Plane Hyperbolic Van der Waals Crystals

Abstract

anisotropy is invoked by lithographic patterning, natural hyperbolicity of these vdW materials is attributed to structural anisotropy of crystal unit cell. In particular, hexagonal boron nitride (h-BN), [3] α-phase molybdenum trioxide (α-MoO 3 ) [1,6] and α-phase vanadium pentoxide (α-V 2 O 5 ) [2] are natural hyperbolic materials (NHMs) which exhibit Reststrahlen Bands (RBs)the spectral region between longitudinal optical (LO) and transverse optical (TO) phonons-in the mid-IR spectral region and show hyperbolicity due to interaction of optical phonons with photons (lightmatter interaction). Phonons have a relatively long lifetime compared to plasmons resulting in lower optical losses than their analogous plasmonic-based metamaterials [7][8][9] in which photons are coupled with plasmons. Many NHMs [1][2][3] exhibit hyperbolic anisotropy in mid-IR spectral region (3-30 µm) which has diverse applications like polarized IR imaging, [10] molecular sensing, [11,12] free space communication, [13] and quantum interference. [14] Unlike h-BN, which possesses uniaxial hyperbolic anisotropy (i.e., ε xx = ε yy ≠ ε zz ), α-MoO 3 exhibits in-plane hyperbolic anisotropy (i.e., ε xx ≠ ε yy ≠ ε zz ) which is particularly beneficial for planar mid-IR optical devices. [15,16] With this motivation, there has been recent interest in developing flat optics based on vdW layered materials which can be integrated with chip-scale platforms using vdW integration, operational at room temperature, and does not involve complex lithographic fabrication techniques. [17][18][19][20] We present an application of α-MoO 3 in the mid-IR spectral region, that is, from 545-1000 cm −1 (around 10-18 µm), as a thin film polarizer that reflects the light with one state of polarization while transmitting the light with its orthogonal state of polarization. Here, single-crystal α-MoO 3 thin films are synthesized using physical vapor deposition (schematically shown in Figure 1a) and are transferred on top of potassium bromide (KBr) window, purchased from Edmund Optics, using mechanical exfoliation technique. In-plane anisotropy of the synthesized α-MoO 3 thin film is confirmed using polarizationresolved Raman spectroscopy. We optimize the mid-IR optical responses of α-MoO 3 , mainly transmittance and reflectance, as a function of the thickness. Optimum thickness of α −MoO 3 based IR polarizer is found to lie in the range of 2.5-3.5 µm for which the extinction ratio (ER) is obtained more than 7.5Integration of conventional mid to long-wavelength infrared (IR) polarizers with chip-scale platforms is restricted by their bulky size and complex fabrication. Van der Waals materials based polarizer can address these challenges due to its nonlithographic fabrication, ease of integration with chip-scale platforms, and room temperature operation. In the present work, mid-IR optical response of the sub-wavelength thin films of α-phase molybdenum trioxide (α-MoO 3 ) is investigated for application toward high temperature mid-IR transmission and reflection type thin film...

Planar hyperbolic polaritons in 2D van der Waals materials

“…Planar hyperbolic media are most relevant in the development of ultrathin polarization controllers as shown in Fig. 6f [122][123][124] . Size is a bottleneck in traditional polarization controllers, for instance, a state-ofthe-art quarter-wave plate uses bulky linear birefringent crystals, since it requires a significant propagation distance to establish the phase difference between orthogonal polarizations.…”

Optical properties and polaritons of low symmetry 2D materials

“…Due to the variation of phase difference from in-plane hyperbolic birefringence, the ellipticity of transmitted light exists almost throughout the band. Sahoo et al also investigated the optical response of high temperature α-MoO3 and the tolerance for high temperature in MIR transmission and reflection type thin film polarizers [286] . The optimal thickness in the range of 2.5 to 3.5 µm for a large bandwidth and ER was explored, where 7.5 and 10 dB of ER can be achieved in RB-1 and RB-2 spectral regions, respectively.…”

Two-dimensional natural hyperbolic materials: from polaritons modulation to applications

“…In 2022, N. R. Sahoo et al firstly proposed the transmission and reflection type polarizer based on α-MoO 3 above RT. 52 As illustrated in Fig. 14(a), the single-crystal α-MoO 3 film, which is synthesized via using physical vapor deposition, is transferred to on top of a potassium bromide (KBr) window.…”

“…50,51 A comprehensive comparison of hyperbolic phonon, plasmon, exciton, and shear polaritons remains lacking. Moreover, various novel and improved applications for natural HMs (e.g., valley quantum interference and high temperature polarizers 45,52 ) have been also presented during the last two years. Thereby, it is meaningful to provide a thorough overview of these four types of hyperbolic polariton, as well as the most recent developments in the applications of 2D natural HMs.…”