Cecilia Y. Chen

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. We describe an approach to creating atomically sharp line defects in hexagonal boron nitride (hBN) at room temperature by direct optical phonon excitation with a mid-infrared pulsed laser from free space. We term this phenomenon “unzipping” to describe the rapid formation and growth of a crack tens of nanometers wide from a point within the laser-driven region. Formation of these features is attributed to the large atomic displacement and high local bond strain produced by strongly driving the crystal at a natural resonance. This process occurs only via coherent phonon excitation and is highly sensitive to the relative orientation of the crystal axes and the laser polarization. Its cleanliness, directionality, and sharpness enable applications such as polariton cavities, phonon-wave coupling, and in situ flake cleaving.

We fabricate polariton cavities in hBN with Q-factors > 70. Our approach is based on strong, resonant phonon excitation at 7.3 µm with femtosecond pulses causing the sample to “unzip” and produce a phonon reflection.

We report the first measurements of the nonlinear response of phonon polaritons near 7.3 µm in atomically thin layers of hexagonal boron nitride. We observe the phonon-enhanced third harmonic and the transient phonon-induced second harmonic.