Polaritonics : an intermediate regime between electronics and photonics

Abstract

This thesis contains the foundational work behind the field of polaritonics. Corresponding to a frequency range from roughly 100 gigahertz up to 10 terahertz, polaritonics bridges the gap between electronics and photonics. In this regime, signals are carried by an admixture of electromagnetic and lattice vibrational waves known as phonon-polaritons, rather than currents or photons. Impulsive stimulated Raman scattering (ISRS) is employed for phonon-polariton generation, whereby lattice vibrations are driven by optical femtosecond laser pulses directed into ferroelectric LiNbO3 or LiTaO3. The vibrational amplitude is proportional to the intensity of the excitation pulses. Due to the high dielectric constants of these crystals, phonon-polaritons travel in a predominantly lateral direction away from the excitation region. Lateral propagation is further facilitated by employing crystals whose thickness is on the order of the phonon-polariton wavelength, such that propagation occurs within one or more of the slab waveguide modes of the crystal. Direct observation of phonon-polaritons is achieved using real-space imaging, which monitors and records the spatiotemporal evolution of phonon-polaritons within a ferroelectric crystal. The details of both broadband and narrowband phonon-polariton generation and propagation in bulk and thin film crystals are presented. Additionally, robust polaritonic waveform generation is illustrated that relies on temporal or spatial shaping of the optical excitation pulses. Guidance, control, and other types of signal processing are demonstrated by patterning of the host crystal using femtosecond laser micromachining.

(cont.) Waveguides that direct propagation, resonators that confine polaritonic signals, reflectors that direct, shape, and focus polaritonic waveforms, and periodic photonic crystal structures that restrict phonon-polaritons to a narrow band of frequencies are fabricated and their functionality demonstrated. The details of the laser micromachining employed for fabrication of these structures in a variety of crystal thicknesses are also presented here. Experimental measurements are supported by a novel implementation of finite-difference- time-domain (FDTD) simulations that accurately model both phonon-polariton generation and propagation in bulk, thin film, and patterned crystals. Additionally, numerical experiments are performed to predict functionality that will enable advanced polaritonic bistable devices for use in digital polaritonics and negative refractive polaritonic materials for unique waveform generation, signal processing, and sub-diffraction terahertz imaging. Polaritonics offers lower signal-to-noise than photonics and higher bandwidth signals than electronics, with generation, propagation, guidance, and control integrated into a single all- optical platform. Direct visualization of signal propagation makes device design and testing substantially easier than in either electronics or photonics. With continued development, fabrication of polaritonic materials should prove less demanding than traditional photonic structures, as it requires feature sizes on the order of micrometers rather than nanometers. Due to the high terahertz electric field strengths associated with ISRS phonon-polariton generation and the robust signal processing tool chest presented here, polaritonics promises to be useful in various spectroscopic applications including, but not limited to, linear and nonlinear terahertz spectroscopy and terahertz near field microscopy.