Functionalized nano-optics for studying optical and mechanical properties of low-dimensional materials

Abstract

A central challenge in nanoscience is the development of new tools to non-invasively probe the rich physics of low-dimensional materials whose properties are dominated by quantum confinement and surface effects. Conventional characterization techniques often lack the required sensitivity or can perturb the fragile systems they aim to measure. To address this metrological gap, we introduce and develop "functionalized nano-optics" where we transform static photonic crystal cavities into dynamic, reconfigurable instruments by endowing them with active degrees of freedom. Two principal functionalities are explored: in-situ strain tuning for precise spectral control, and spatial mobility, which recasts the nanocavity as a scanning probe tool. The first functionality is demonstrated through the development of a cryo-compatible, in-situ strain-tuning platform for hybrid photonic systems. By integrating a Gallium Phosphide (GaP) photonic crystal cavity with a monolayer of the 2D semiconductor WSe2 on a strain cell, we overcome the common problem of spectral mismatch between emitters and cavities. This system achieves a continuous and reversible tuning of the cavity resonance by 5.5 nm at 5 K, an order of magnitude larger than the cavity linewidth, without degrading the Q-factor. This spectral control is used to modulate the cavity-enhanced exciton photoluminescence, establishing a robust method for systematically studying light-matter interactions in 2D materials. As a complementary approach and followup to hybrid integration, we also explore the use of the van der Waals material itself as the core photonic component. By fabricating waveguides and integrated devices directly from bulk MoS2, we demonstrate deeply subwavelength light confinement, with guided modes in structures as thin as $\lambda$/16. These devices are characterized using a combination of far-field spectroscopy and scattering-type near-field optical microscopy (SNOM) to reveal the properties of highly confined exciton-polaritons. The second core functionality of spatial mobility is realized through the development of a novel scanning optomechanical probe. A high-Q Silicon Nitride nanobeam cavity is integrated onto the tip of a tapered optical fiber, creating a versatile instrument for studying nanomechanical motion. This platform is applied to perform the first direct, high-bandwidth measurements of the thermally-driven mechanical vibrations of suspended DNA bundles, which are prepared via self-assembly on super-hydrophobic micropillar arrays. Additionally, we observe significant optomechanical back-action, first of its kind in a DNA resonator, which manifests as a symmetric frequency softening characteristic of a dissipative-like coupling mechanism. Together, these results establish functionalized nano-optics as a powerful and versatile platform for exploring the complex optical and mechanical properties of diverse low-dimensional material systems.