Terahertz time-domain ellipsometry based on spintronic phenomena

Abstract

Ellipsometry is a foundational technique in materials science, enabling precise characterization of optical properties, thickness, and composition. Extending ellipsometry into the terahertz (THz) range unlocks new opportunities to probe fundamental material characteristics such as conductivity and carrier dynamics—key for advancements in semiconductors, energy storage, and ultra-fast telecommunications. However, existing THz ellipsometry methods often adapt traditional optical ellipsometry without fully exploiting the unique capabilities of time-domain spectroscopy. These systems result in incomplete polarization descriptions and face challenges in achieving high-quality polarization control due to the limitations of THz components. Consequently, they are insufficient for analyzing general anisotropic materials, lacking complete amplitude and phase information essential for comprehensive characterization.

This dissertation overcomes these limitations by developing a novel terahertz complete time-domain spectroscopic ellipsometry (THz-cTDSE). Our system captures both amplitude and phase information, determining the full Jones and Mueller matrices of anisotropic samples. Ellipsometry often requires advanced expertise in interpreting complex matrix data; therefore, we introduced Pauli exponential coefficients, which intuitively represent polarization responses such as retardation and diattenuation across all linear, diagonal, and circular polarizations—until now experimentally unattainable due to the absence of complete coherent ellipsometry. Furthermore, we developed a robust calibration technique compensating for misalignments and eliminating the need for precise component alignment, enhancing measurement accuracy and reliability applicable to many ellipsometric systems.

To address the requirement for refined polarization, we employ spintronic terahertz emitters (STEs), which provide broadband, gapless emission of linear polarization that is easily controllable via a magnetic field and offers high extinction ratios. We present the first experimental application of STEs for polarization control in ellipsometry. To fully leverage the potential of STEs, we developed emitters capable of generating robust signals with advanced methods for polarization control: (i) By integrating STEs with photonic cavities, we maximized pump absorption within the STE layers, achieving an 8dB increase—one of the highest reported enhancements—facilitating faster measurements and improved data quality. Additionally, we incorporated THz resonant cavities, enabling constructive interference that doubles the THz extraction, leading to an overall estimated improvement of 15dB. (ii) We engineered uniaxial magnetic anisotropy in STEs using a ferromagnetic trilayer for advanced polarization control, enabling precise 360° polarization rotation essential for accurate ellipsometry. Additionally, we developed voltage-controlled polarization mechanisms using magnetostrictive emitters, marking a crucial step toward fully integrable STE systems.

Recognizing the potential of STEs' ultrafast and uniform spectral response for frequency-domain applications, we extended our work beyond the time domain. We demonstrated a proof-of-concept for highly tunable spintronic photomixers with frequency-stable efficiency, overcoming limitations of conventional photomixers that suffer efficiency loss at higher frequencies.