The structure of a material, whether at the atomic scale or patterned at the nanoscale, is the basis of many of its physical properties—color, emission wavelength, optical nonlinearity, electrical conductivity, thermal conductivity, brittleness, and more. Therefore, one of the most important developments in photonics, electronics, and magnetics is the ability to manipulate the nanostructure of materials as a way to augment their natural qualities and adapt them to greater applications. The cleanroom debuted in the mid-20th century, alongside and followed by an assortment of precision nanofabrication instruments performing photolithography, electron-beam lithography, ion implantation, femtosecond laser machining, etc. While these techniques have demonstrated breakthroughs such as fabricating ever-smaller transistors keeping pace with the famous Moore’s Law, they require cleanroom facilities, multi-step processing, or leave behind debris or residue. Such impurities have an outsize effect on a burgeoning class of materials with desirable optical and electronic properties—two-dimensional (2D) layered van der Waals materials—as their dimensions approach the single-atom limit, leading a desire for additional approaches to material nanostructuring.
In this thesis, we describe a novel approach to generating atomically sharp linear nanostructures in hexagonal boron nitride (hBN) via resonant optical phonon pumping with a pulsed mid-infrared laser and detail its development from discovery to a useful technique that complements established approaches to nanopatterning. The femtosecond laser is tuned to the material’s infrared-active transverse optical TO (E1u) phonon, located at ? = 7.3 ?? or 1367 cm-1, and its polarization aligned parallel to the crystal zigzag axis, in the direction of the phonon’s characteristic atomic motion. The optical field coherently drives and amplifies the intrinsic ionic motion toward bond breakage, resulting in a gentle tearing of the hBN flake along the crystal axis at the material damage threshold. All processing is performed in situ at room temperature under ambient conditions, free from cryogenics and vacuum setups, unlike in the conventional nanofabrication methods confined to the cleanroom.
This phenomenon is termed “unzipping” to depict the rapid formation and emanation of a crack tens of nanometers wide from a point within the laser-excited area. The generation of these fea- tures is ascribed to the large atomic displacements and localized bond strain produced by strongly driving the crystal at an intrinsic resonance, which is absent under non-resonant irradiation and is greatly sensitive to the relative angle between the crystal orientation and the linear laser polarization.
We perform detailed characterization of the unzipped features and their host hBN flakes us- ing atomic force microscopy (AFM) topographic imaging, scanning electron microscopy (SEM), atomic-scale lateral force microscopy (LFM), nanoindentation in the plastic deformation regime, and near-field optical probing (scattering-type scanning near-field optical microscopy, s-SNOM) to reveal their atomically sharp, six-fold symmetric, orientation-selective, defect-seeded nature. Then, we fabricated several nanostructures—gratings, Fabry-Perot resonators, and cleaved and shaped flakes—to demonstrate the technique in useful nanophotonics applications. The preliminary Fabry-Perot resonator, examined in the near-field with nanoscale Fourier-transform infrared spectroscopy (nano-FTIR), exhibited performance that is competitive with similar structures fabricated by cleanroom etching. Our initial approach achieved a quality factor of ? ≈ 70, already on par with ? = 50 to 100 achieved by conventional nanofabrication methods.
The cleanliness, sharpness, and directionality of nanostructures fabricated in situ via unzipping, along with the ability to deterministically seed the location of its constituent line defects using nanoindentation, enable vast future applications in patterning hBN and other polar crystals that possess optically-addressable, high-energy optical phonon modes in the mid-infrared.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/ksv5-1h69 |
| Date | January 2024 |
| Creators | Chen, Cecilia |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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