Unzipping hBN with ultrashort mid-infrared pulses

TL;DR

This paper introduces a novel room-temperature optical method to create atomically sharp line defects in hexagonal boron nitride using resonant mid-infrared pulses, enabling precise nanostructuring without residue.

Contribution

It demonstrates a new unzipping technique for hBN that relies on resonant phonon excitation, offering a residue-free, highly directional nanostructuring approach.

Findings

Creates <30 nm wide cracks in hBN via mid-IR pulses.

Requires resonant phonon excitation for defect formation.

Features are highly sensitive to crystal orientation and laser polarization.

Abstract

Manipulating the nanostructure of materials is critical for numerous applications in electronics, magnetics, and photonics. However, conventional methods such as lithography and laser-writing require cleanroom facilities or leave residue. Here, we describe a new approach to create atomically sharp line defects in hexagonal boron nitride (hBN) at room temperature by direct optical phonon excitation in the mid-infrared (mid-IR). We term this phenomenon "unzipping" to describe the rapid formation and growth of a <30-nm-wide crack from a point within the laser-driven region. The formation of these features is attributed to large atomic displacements and high local bond strain from driving the crystal at a natural resonance. This process is distinguished by (i) occurring only under resonant phonon excitation, (ii) producing highly sub-wavelength features, and (iii) sensitivity to crystal orientation and pump laser polarization. Its cleanliness, directionality, and sharpness enable applications in in-situ flake cleaving and phonon-wave-coupling via free space optical excitation.