A Low Temperature Nonlinear Optical Rotational Anisotropy Spectrometer for the Determination of Crystallographic and Electronic Symmetries

TL;DR

This paper introduces a low-temperature nonlinear optical rotational anisotropy spectrometer capable of probing the symmetries of complex electronic phases in small bulk crystals, overcoming previous technical limitations.

Contribution

The paper presents a novel spectrometer design that enables symmetry measurements of small crystals under extreme conditions, expanding the applicability of nonlinear optical techniques.

Findings

Successfully measured symmetries of Sr₂IrO₄ crystal

Demonstrated sensitivity to nonlinear optical susceptibility tensor

Operated effectively at low temperatures and micron scale

Abstract

Nonlinear optical generation from a crystalline material can reveal the symmetries of both its lattice structure and underlying ordered electronic phases and can therefore be exploited as a complementary technique to diffraction based scattering probes. Although this technique has been successfully used to study the lattice and magnetic structures of systems such as semiconductor surfaces, multiferroic crystals, magnetic thin films and multilayers, challenging technical requirements have prevented its application to the plethora of complex electronic phases found in strongly correlated electron systems. These requirements include an ability to probe small bulk single crystals at the micron length scale, a need for sensitivity to the entire nonlinear optical susceptibility tensor, oblique light incidence reflection geometry and incident light frequency tunability among others. These measurements are further complicated by the need for extreme sample environments such as ultra low temperatures, high magnetic fields or high pressures. In this review we present a novel experimental construction using a rotating light scattering plane that meets all the aforementioned requirements. We demonstrate the efficacy of our scheme by making symmetry measurements on a micron scale facet of a small bulk single crystal of Sr$_2$IrO$_4$ using optical second and third harmonic generation.