Adventures in lower dimensional optomechanics, electromechanics and non-linear optics
Creators: name: Sripathy, Kabilan
Publisher: name: The Australian National University
Year: 2024

description: In the past decades there have been numerous advances in the field of optics. UV LEDs, metamaterials, optical quantum computing, optical tweezers and much more. It is even possible to couple the electromagnetic force to the gravitational force through advanced optomechanical systems. This was the first area of study of this thesis. Wherein the dynamics of an optical cavity levitated mirror was modeled and optimized. An optomechanical spring, scattering free, sensitive enough to detect the smallest bits of gold and uranium beneath our feet, and perhaps even gravitational waves or quantum gravitational states. Beyond optomechanics, photon interactions with two dimensional (2D) materials have drawn great interest by physicists the world over. Straight away, it is possible to tell that 2D materials are special. These atomically thin crystals, even one atom thick can be seen with a simple optical microscope. 2D materials have exceptional optical properties such as high refractive indexes and non-linear coefficients. They can form unique systems such as defects within insulator h-BN which can act as single photon emitters. These and other such 2D material devices such as transistors and resonators which are crucially light and compact. This is a favorable property for space devices especially. However it is unclear whether such delicate devices could work in space primarily due to the radiation. The second study of this thesis concerns this exactly and concluded that raw transition metal dicalcogenide (TMD) based 2D monolayers and devices are resistant to well above typical low earth orbital levels of gamma, p+ and e- radiation. The next study of this thesis deviated further from the optics field, into the condensed matter physics of 2D materials. Particularly looking into the strong piezoelectric and flexoelectric effect exhibited by TMDs. They exhibit strong flexoelectric properties due to their lack of inversion symmetry and this effect has been scarcely studied. Moreover, the aim of this study was to create an opto-electromechanically coupled monolayer resonator. The drum resonator being optically coupled to an interferometer field and an optical pump field, which excites the mechanical mode of the resonator, which simultaneously creates a coupled piezo and flexoelectric response. Unfortunately, a working system like this proved difficult to fabricate and could not be done. Thus, instead the final study of this thesis pivoted back to purely optical studies of 2D materials, particularly looking into their non-linear properties. A particular polytype of MoS2, 3R-MoS2 has in very recent times been reported to have exceptionally high non-linearities unmatched by any 3D crystals due to their 2D geometry. The nonlinear 2D crystal layers in this polytype stack in phase with one another, thereby creating a perfectly atomically phase matched crystal only limited by chromatic dispersion. The main objective of this final study was to observe and characterize atomic phase matching in 3R-MoS2. Furthermore, this study endeavored to create a quasi-phase matched heterostructure stack of 3R-MoS2 by stacking two thin layer crystals at an appropriate twist angle. Initially, only the atomic phase matching phenomenon of 3R-MoS2 could be observed by twisting two insufficiently thin layers of 3R-MoS2. This is because the initial experiments were done at 860nm, which is an absorption dominated regime. Thicknesses corresponding to the coherence length of the 2D crystal could not be used in this regime as the signal is quickly depleted by absorption. However as just mentioned, atomic phase matching was observed via a threefold symmetry in the angle depended SHG of a twisting hetrostructure. Then, the study was repeated at 1550nm, which itself a far more ubiquitous wavelength and not absorption dominated. In this regime, quasi phase matching could be observed and SHG could be enhanced to nearly perfectly phase matched levels.