Sum-Frequency Generation Spectro-Microscopy in the Reststrahlen Band of Wurtzite-type Aluminum Nitride

TL;DR

This paper demonstrates a novel SFG spectro-microscopy technique combining nonlinear optical imaging and spectroscopy to spatially resolve atomic and vibrational details in wurtzite aluminum nitride, enhancing analysis of complex materials.

Contribution

It introduces the first application of SFG spectro-microscopy with an infrared free-electron laser to study in-plane anisotropic materials like aluminum nitride.

Findings

Experimental spectra match theoretical calculations.

Potential to image domains in ferroic materials.

Shows feasibility of spatially resolved vibrational spectroscopy.

Abstract

Nonlinear-optical microscopy and spectroscopy provide detailed spatial and spectroscopic contrast, specifically sensitive to structural symmetry and order. Ferroics, in particular, have been widely studied using second harmonic generation imaging, which provides detailed information on domain structures but typically lacks spectroscopic detail. In contrast, infrared-visible sum-frequency generation (SFG) spectroscopy reveals details of the atomic structure and bonding via vibrational resonances, but conventionally lacks spatial information. In this work, we combine the benefits of nonlinear optical imaging and SFG spectroscopy by employing SFG spectro-microscopy using an infrared free-electron laser. Specifically, we demonstrate the feasibility of SFG spectro-microscopy for spectroscopy using in-plane anisotropic wurtzite-type aluminum nitride as a model system. We find the experimental spectra to agree well with our theoretical calculations and we show the potential of our microscope to provide spatially resolved spectroscopic information in inhomogeneous systems such as ferroics and their domains in the near future.