Optical Characterization of Freestanding Oxide Membranes and Their Heterostructures

Optical characterizations have played a pivotal role in investigating the structures and properties of two-dimensional (2D) materials. The emergence of freestanding complex oxide membranes, fabricated through the epitaxial growth and selective chemical etching of sacrificial layers, introduces a novel class of 2D material systems. The advancement enables the synthesis of a wide range of oxide membranes with unique physical properties that are unattainable in bulk or substrate-supported thin films. Despite the rapid progress in the field, optical spectroscopy of freestanding oxide membranes has been limited.

In this study, we present the synthesis and optical characterization of freestanding titanate membranes, focusing on SrTiO₃ and BaTiO₃, along with their heterostructures. The Raman spectra from suspended membranes shows several first-order optical phonon modes which are not present in bulk crystals. We also explore the second-harmonic generation of oxide membranes, which is attributed to the ferroelectric order in oxide membranes. Understanding the optical response of oxide membranes would provide key information of the symmetry and ferroic orders of oxide membranes and their heterostructures.