Unzipping hBN with ultrashort mid-infrared pulses

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. 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.

Fig. 1.. A phonon-resonant effect.

“Unzipping” occurs only when hBN is strongly driven at its TO phonon resonance and yields ablation-free line defects. (A) Top: In this experiment, a pulsed mid-IR laser is focused onto an hBN flake, producing a localized edge or “zip.” Bottom: The zip is oriented along the armchair axis. (B) Fourier transform infrared (FTIR) spectroscopy linear reflectance spectrum about the hBN TO(E_1u) phonon resonance on pristine hBN relative to the SiO₂/Si substrate, centered at 1367 cm⁻¹ (λ = 7.3 μm). Inset: AA′-stacked hBN illustrating the mode of interest. We directly drive this mode with a laser tuned to 7.3 μm to subject the crystal to high in-plane lattice-scale strain. (C) Comparison of off- and on-resonant ultrafast irradiation of a 70-nm flake at λ = 4.5 and 7.3 μm, respectively. The latter results in a highly sub-wavelength line defect (zip); burning is absent, and the line is oriented roughly perpendicular to the laser polarization. Off-resonant irradiation at 4.5 μm generates a wavelength-scale burned spot and lacks polarization dependence. (D) A series of clean, parallel zips produced by a perpendicularly polarized laser on a single flake with height of 38 nm. The width here measures <30 nm (fig. S6).

Fig. 2.. Polarization dependence.

Unzipping is sensitive to the pump laser polarization. (A to C) Atomic-scale lateral force microscopy (LFM) determines the crystal orientation of an hBN flake and hence the unzipping direction. The scan region is marked in (C). (A) LFM friction channel image after filtering, with zigzag and armchair directions marked. Inset: 2D fast Fourier transform (FFT) of unfiltered friction channel. (B) Linecuts along the zigzag and armchair directions yield periodicities of 29 and 50 Å, respectively, confirming the measurement. The y axes are offset for clarity. a.u., arbitrary units. (C) The 70-nm-thick flake imaged with LFM. The zips measure 66° from the armchair-oriented flake edges, making them nearly parallel to an armchair axis of the crystal. (D and E) The unzipping phenomenon occurs independently of the choice of substrate. (D) Unzipped lines on a 60-nm-thick hBN flake on SiO₂/Si. The sensitivity of unzipping to pump laser polarization is evident in the transition from an X-shaped line defect to a parallel single line under a 10° shift in polarization. (E) Unzipped lines on an 84-nm-thick hBN sample on sapphire. Zips generated by the same polarization are parallel.

Fig. 3.. Applications in flake patterning and nanostructuring.

All-optical in situ patterning of hBN flakes using the mid-IR phonon-resonant technique. (A and B) Unzipping can be controllably and boundlessly extended once initiated, achieving an ultrahigh aspect ratio. (A) AFM topography: A 24-nm-thick flake is cleaved in two by extending an initial unzipped line in both directions. A slight edge offset at the left and right ends of the cleavage line reveal that the bottom section has rotated and shifted along the substrate. Inset: Height profile along the dashed linecut confirms full separation of the cleaved sections. (B) Micrograph: The initial unzipped line is localized within the slight discolored spot. (C and D) Quasi-periodic gratings generated by the unzipping technique on a 50-nm-thick flake, represented in a micrograph (C) and topographic AFM image (D). (E) Linear trend between grating period and flake height. Error bars indicate the range of periodicities displayed by the quasi-periodic gratings.

Fig. 4.. Atomically sharp edges.

Unzipped lines are atomically sharp and exhibit highly efficient coupling to phonon-polaritons. This flake is 38 nm in height. (A) AFM topography of a kinked (suboptimal) zip. (B and C) Near-field phase images under 1494.5 cm⁻¹ excitation, as probed by scattering-type scanning near-field optical microscopy (s-SNOM). (B) Phonon-polariton coupling via an unzipped line. (C) Coupling via a natural flake edge formed by mechanical exfoliation. (D) Unzipped lines are atomically sharp and can outperform flake edges in coupling to phonon-polaritons, as evidenced by a comparison of modulation depth and decay rate along the dashed linecuts in (B) and (C).

Fig. 5.. Polariton cavity and defect seeding.

(A) AFM topography of a 50-nm-thick flake subject to nanoindentation patterning. The flake was then irradiated on resonance until an unzipped line sprouted from a defect-seeded location. Inset: A unique nanoindented pattern on the flake, before irradiation at λ = 7.3 μm, written for localized identification. (B to E) A Fabry-Perot polariton cavity in hBN fabricated by the unzipping technique, imaged by near-field techniques. This flake is 40 nm in height and features a 630-nm-wide cavity. (B) AFM topography of the unzipped hBN cavity. (C) s-SNOM amplitude image of the cavity probed near resonance at 1453 cm⁻¹ shows field confinement. The cavity boundaries are marked by dotted white lines. (D) Cavity resonance lineshape with an estimated Q ≈ 70 extracted from a Lorentzian fit to the data points. Its corresponding linecut in the hyperspectral image (E) is marked by the dashed white line. (E) A hyperspectral line scan across the cavity obtained via nano-FTIR. The cavity boundaries are marked by dotted white lines.