THz polaritonics : optical THz generation, detection, and control on a chip

Abstract

The THz polaritonics system is an on-chip platform for THz generation, detection, and control. THz-frequency electromagnetic waves are generated directly in a thin slab of lithium niobate crystal where they can be amplified and guided. Time-resolved phase-sensitive imaging lets us capture movies of THz waves as they propagate at the light-like speeds. I developed polaritonics methodologies and used the platform to study various microstructures interacting with THz waves. I began technique development by deriving a quantitative model explaining THz wave propagation in an anisotropic slab waveguide. From this model, I extracted the frequency-dependent wave velocity and used this knowledge to design an optical pumping geometry that phase-matches and coherently amplifies a selected THz frequency. This geometry can generate high-amplitude THz waves with a tunable center frequency and bandwidth. Much like the generation, the detection was also revamped. New optical designs, acquisition procedures, and hardware let us quantitatively measure THz field strengths. The image resolution was improved from ~50 [mu]m to 1.5 [mu]m, and measurement noise was reduced by 50-fold. Using the improved generation and detection methods, we studied two classes of microstructures: laser-machined air gaps and deposited metal antennas. Air gaps cut into the lithium niobate slab effectively reflect, waveguide, and scatter THz waves. We fabricated structures that demonstrate wave phenomena such as diffraction and interference and captured movies of THz waves interacting with these structures. The movies can be useful tools in lectures on electromagnetism because they beautifully illustrate the fundamental effects and bring cutting-edge research into the classroom. In addition to air structures, we studied metal antennas, which are interesting because of their ability to enhance optical fields and localize electromagnetic waves well below the diffraction limit. The polaritonics platform enabled incisive study of fundamental antenna behavior and scaling because we could map the antenna's near-field with [lambda]/100 spatial resolution and we could quantify large THz electric field amplitudes and enhancements in a deeply sub-wavelength gap between antennas. Antenna field enhancement is already facilitating nonlinear THz research, and the polaritonics platform will enable improved study of photonic systems such as metamaterials and photonic crystals.