DISSERTATION DEFENSE
Ryan Kowalski, Interdisciplinary Materials Science
*Under the supervision of Josh Caldwell and Richard Haglund
“Infrared Light at the Nanoscale: Confinement and Spectroscopy”
04.25.25 | 10:30AM | Buttrick Hall 206 | Zoom: 942 8904 4051 Passcode: 450959
Conventional optical microscopy and spectroscopy are limited by the diffraction limit, which restricts spatial resolution to approximately half the free-space wavelength of light. This constraint poses a significant barrier for the nanoscale characterization of materials and quantum systems. Nanophotonics offers a powerful solution to this challenge by leveraging sub-wavelength optical modes supported by materials that exhibit strong electromagnetic responses, including plasmonic and phononic media. In the infrared spectral regime, the photon energy is resonant with lattice vibrational modes, enabling direct coupling to fundamental excitations in solids. The combination of nanophotonics and infrared spectroscopy thus enables a new class of nanoscale imaging and sensing tools. This dissertation presents a systematic investigation into the confinement and manipulation of infrared light at the nanoscale, with particular emphasis on phonon polaritons in van der Waals materials. It is shown that the confinement of polaritonic modes is governed by the dielectric environment, anisotropy of the optical response, and strength of light–matter interaction. Using materials such as HfS2 and HfSe2, the work demonstrates the ability to support ultraconfined polariton modes with spatial compression exceeding λ0/250. Experimental measurements of the dielectric functions are combined with analytical and numerical modeling to reveal the dispersion relations and confinement limits of these modes. Infrared scattering-type scanning near-field optical microscopy is employed to achieve spatially resolved nanospectroscopy of individual quantum point defects in hexagonal boron nitride. By correlating near-field infrared response with far-field photoluminescence spectra and photon autocorrelation measurements, this work provides insight into the local dielectric environment, strain, and structural inhomogeneity that modulate quantum emission properties. These findings demonstrate how infrared nanophotonics can be applied as a sensitive probe of optically active point defects and strongly confined excitations in quantum materials. The results presented here establish a platform for nanoscale optical characterization, opening avenues for nanoscale quantum sensing, optoelectronic device engineering, and atomically resolved spectroscopy in low-dimensional materials.