Developments in THz Polaritonics: Towards Integrated Nonlinear THz Spectroscopy

Abstract

The terahertz (THz) polaritonics platform is a compact, waveguide-based platform for the generation, manipulation, and detection of THz waves. The platform uses thin (<100 μm) lithium niobate (LiNbO₃, LN) and lithium tantalate (LiTaO₃, LT) slabs, which can be patterned to control THz propagation. One of the unique features of the platform is that the THz fields can be imaged directly within the slab with subwavelength spatial resolution and subcycle temporal resolution. Both the amplitude and phase of the fields are recorded, which allows the full spatiotemporal evolution of the fields to be visualized. This makes the platform appealing for compact, waveguide-based THz experiments. The work in the thesis aims to develop tools to enable robust, compact THz spectroscopy using the polaritonics platform.

The first phase of my research aims to develop methods for enhanced THz generation in the waveguides. In a typical polaritonics experiment, the optical pump light is focused to a single line which launches THz fields with electric field strengths of approximately 10 kV/cm. Although the fields are sufficiently strong for THz imaging, any nonlinear spectroscopic applications would require the use of much larger THz fields so that the much weaker THz transients that result from multiple interactions with the sample could be reliably detected. To this end, I developed two methods. The first method uses thin LN waveguides with a beveled edge for enhanced narrowband THz generation. The optical pump light is focused onto the bevel, after which it refracts and becomes confined within the waveguide by total internal reflection. This allows the pump beam to repeatedly drive the generated THz field during its multiple back-and-forth traversals within the LN slab. Using this method, we observe a 10-fold enhancement of the THz spectral amplitude at the velocity-matched frequency. The second method combines the tilted pulse front geometry with THz focusing to generate a strong THz field in the time domain. A circular stair-step "echelon" mirror is used to shape the pump pulse into a conical tilted pulse front composed of a series of concentric rings of pump light. When the pump rings are imaged onto a thin LT waveguide, coherent superposition of the focusing THz fields excited individually by each pump ring results in a dramatically enhanced THz field at the focus. When optimized, the method generates THz fields with electric field strengths up to 175 kV/cm, which is roughly 20x larger than what is generated by a single line of pump light.

The second phase of my research focuses on methods for expanding the polaritonics toolset for spectroscopic applications. Previous experiments coupling the THz phonon-polaritons in a LN waveguide to the quasi-antiferromagnetic magnon mode in an adjacent slab of ErFeO₃ took advantage of the fact that both materials have similar refractive indices. Furthermore, the ErFeO₃ layer complicates THz imaging because it strongly absorbs the optical probe light. I investigated two experimental geometries to address these concerns. The first geometry uses a high-reflecting coating sandwiched between the LN slab and the sample material. The coating is designed to reflect the optical probe light, which enables THz imaging in LN by preventing the probe light from entering the sample and greatly expands the range of possible samples. The second geometry uses a slot waveguide to localize the THz field within a low-index slot region, which results in much stronger interactions between the THz fields and a sample inserted into the slot. Using this geometry, the linear THz absorption spectrum of a test sample was measured with good sensitivity and the complex dielectric function was recovered.

The work presented here describes methods for enabling robust integrated THz spectroscopy in the polaritonics platform. The methods, when combined, should also form the basis for future polaritonics experiments that interrogate the nonlinear THz responses of materials.