Anisotropic van derWaals Materials for Polarization-Sensitive Photonics

Abedini Dereshgi, Sina

doi:https://doi.org/10.21985/n2-4n9n-2r56

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Anisotropic van derWaals Materials for Polarization-Sensitive Photonics

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The on-going demand for miniaturized optical and on-chip photonic systems of the future has led to a few potential solutions in the literature. Recent advances in van der Waals and 2-dimensional materials signal a bright future for the next generation, compact electronic and photonic devices. With reduced dimensionality and material thicknesses reaching down to atomic and molecular levels, 2-dimensional materials provide access to unique electrical, optical and mechanical properties that can enable compact, sub-diffraction limit and yet efficient devices. van der Waals materials are also compatible with all substrates and usually do not require restrictive crystal-matching requirements of conventional birefringent materials and can make up high-crystalline complex heterostructures. In this thesis several emerging van der Waals materials with anisotropic crystals are investigated and reintroduced as potential polarizationmodulation candidates for the future on-chip photonics in visible, near-infrared and infrared, in the context of van der Waals metamaterials and low-symmetry vdW multilayers. The mentioned bands of electromagnetic spectrum house critical technologies such as display systems, optical communication systems and atmospheric sensing systems.For visible and near-infrared range, 2-dimensional metal borophene and transparent α-MoO3 are investigated as strong candidates for polarization-dependent on-chip photonics. A detailed theoretical study is conducted on the anisotropic plasmonic response of borophene monolayer (a monolayer 2-dimensional metal) and patterned nanoribbons and nanopatches of borophene monolayer where polarization-sensitive absorption values in the order of 50% is obtained. It is demonstrated that by adding a metal layer, this absorption can be enhanced to 100%. We also examine giant dichroism in monolayer borophene which can be tuned passively (patterning) and actively (electrostatic gating) and our simulations yield 20% reflected light with significant polarization rotation. In another attempt to identify a vdW material that does not require patterning and metamaterial approach and is birefringent by nature, α-MoO3 is investigated which exhibits a polarization-dependent refractive index due to its anisotropic crystal structure. Using polarizers and analyzers, we demonstrate that α-MoO3 has negligible loss and birefringence values as high as 0.15 and 0.12 at 532 nm and 633 nm, respectively, is achievable. With such a high birefringence, we demonstrate quarterand half-wave plate actions for a 1400 nm α-MoO3 flake at green (532 nm) and red (633 nm) wavelengths and we report polarizability as high as 90%. Furthermore, we investigate a system of double α-MoO3 heterostructure layer that provides the possibility of tuning polarization as a function of rotation angle between the α-MoO3 layers. As a proof of concept, polarization-sensitive photonic devices including polarization reflectors and polarization color filters are designed and realized by constructing metal–insulator–metal FP cavities. It is observed that resonance frequencies for designed transmission and reflection filters change up to 25 nm with incident polarization which stems from the birefringence of α-MoO3. In infrared, emerging anisotropic vdW materials, hexagonal boron nitride (hBN) and α-MoO3 are studied and their rich phononic properties are tailored to polarizationsensitive photonics. For the van der Waals metamaterial approach, we investigate a structure composed of Au grating arrays fabricated onto a Fabry-Perot cavity composed of hBN, Ge, and Au back reflector layers. The plasmonic Fabry-Perot cavity reduces the required device thickness by enhancing modal interactions and introduces in-plane polarization sensitivity due to the Au array lattice. Our experiments show multiple absorption peaks of over 90% in the mid-infrared region and band stop filters with 80% efficiency using only a 15 nm hBN slab. Moreover, mode interaction with experimental coupling strengths as high as 10.8 meV in the mid-infrared region is investigated. Anticrossing splitting ascribed to the coupling of optical phonons to plasmonic modes can be tuned by the designed geometry which can be tailored to efficient response band engineering for infrared photonics. hBN is also analyzed by highlighting birefringence introduced by grating design on top of it. For the heterostructure van der Waals birefringent solution to polarization-sensitive photonics, α-MoO3 has been identified as a birefringent van der Waals material capable of sustaining naturally orthogonal in-plane phonon modes in infrared. We investigate the polarizationdependent optical characteristics of cavities formed using α-MoO3 on Ge-Au stacks to extend the degrees of freedom in the design of infrared photonic components exploiting the in-plane anisotropy of this material. Polarization-dependent absorption over 80% in a multilayer Fabry- Perot structure with α-MoO3 is reported without the need for nanoscale fabrication on the α-MoO3. We observe coupling between the α-MoO3 optical phonons and the Fabry-Perot cavity resonances. Using cross-polarized reflectance spectroscopy we show that the strong birefringence results in 15% of the total power converted into the orthogonal polarization with respect to incident wave. Infrared wave plate action is also demonstrated relying on the anisotropic optical phonons of α-MoO3. In this thesis, a further step is taken to tailor the cross-polarization spectrosopy of phonons to identifying crystal quality of materials with low crystal symmetry. We investigate the far-field characteristics of MOCVD-grown Ga2O3 thin films. With a combination of cross-polarization Fourier Transform Infrared Spectroscopy and Atomic Force Microscopy characterization techniques, we propose an easy and non-invasive route to distinguish κand βphases of Ga2O3 and study the quality of these crystals. Using numerical methods and cross-polarization spectroscopy, the depolarization characteristics of β-Ga2O3 is examined and depolarization strength values as high as 0.95 and 3.3 are measured respectively for 400 and 800 nm-thick β-Ga2O3. The strong birefringence near optical phonon modes of an 800 nm β-Ga2O3 on sapphire substrate is used to obtain several polarization states for the reflected light in the second atmospheri window (8-14 μm). As a future path toward realizing tunable polarization modulation in infrared, a phase change material, VO2, is combined with anisotropic van der Waals materials. We investigate the tunability of optical phonons of α-MoO3 in a multilayer structure with VO2 sandwiched between α-MoO3 layer on top and a bottom reflector. Our experiments show the frequency and intensity tuning of 2 cm−1 and 11% for optical phonons in the [100] direction and 2 cm−1 and 28% for optical phonons in the [010] crystal direction of α-MoO3. Using the effective medium theory and dielectric models of each layer, we verify these findings with simulations. We also report preliminary results for actively tunable wave plate action in the same multilayer configuration. Our simulations reveal tunability of the response of the proposed multilayer system with heat that can toggle between quarter- and half-wave plate action in infrared. These findings reveal the possibility to manipulate phase, amplitude and polarization of light in visible, near-infrared and infrared and provides insight into tunable manipulation of the properties of light using emerging van der Waals materials. W envisage that our findings can open new avenues in the quest for tunable polarization filters and low-loss, integrated planar photonics and in dictating polarization control, as well as camouflage and radiative cooling devices of the next generation.

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