Abstract

Optoelectronic applications and the exploration of 2D light-matter interactions often require an increase on the bare light absorption of ultra-thin 2D materials (2DM)s, such as graphene. Light absorption by these 2DMs can be enhanced by their incorporation in planar heterostructures which can act as optical interferometric cavities. Furthermore, by fabricating an optical cavity with a suspended 2DM, we can tune its light absorption by tuning the thickness of the air spacer (h_air).
We report the development of an all-dry deterministic transfer technique that is capable of suspending ultra-thin 2DMs in order to fabricate suspended optical cavities, and also the ability to tune the light absorption of said cavities in-situ by the electrostatic tuning of h_air. We studied the absorption of visible light in suspended bilayer graphene (BLG) through Raman spectroscopy, which can distinguish the light scattering and absorption of the graphene layer from the rest of the heterostructure.
We first adapted models for the exclusive light absorption and the Raman scattering enhancement (Raman factor) of arbitrary planar 2DM heterostructures using Fresnel equations. In order to fabricate pristine, suspended heterostructures, we developed a transfer method which capitalizes on the softness and adhesion of a nitrocellulose micro-stamp to dry-pickup and deterministically transfer 2DMs. This transfer method proved capable of fabricating on-substrate (95% success) and suspended (93%) optical cavities. Raman measurements on a partially-suspended BLG optical cavity agreed with our Raman factor model and demonstrated our ability to tune Raman factor (3.8x) and light absorption (6.3x) over a cavity thickness varying from 0 - 150 nm.
We then sought to further enhance and tune Raman factor in-situ through the addition of an aluminium (Al) back-plane mirror, which can also act as a gate electrode to electrostatically tune h_air for suspended devices. On-substrate BLG optical cavity devices with an Al back-plane mirror demonstrated that Raman scattering can be tuned by a factor of 19 with cavity thickness differences on the order of 75 nm. We next assembled an optical cavity by suspending BLG over Al, whose Raman measurements at V_g = 0 V demonstrated that Raman factor can be tuned 1.8x over a 10 nm change of h_air in the same device. Once applying a gate voltage, our final measurements demonstrated an ability to electrostatically tune Raman factor by 20% and h_air by 3 nm with the application of 500 mV. These results pave the way for further research into the in-situ tunability of light-matter interactions for enhanced light absorption at higher gate voltages.