Session Index

Spintronic terahertz (THz) emitters with ferromagnetic (FM) and non-ferromagnetic (NM) metallic heterostructure have several advantages over conventional THz emitters, such as photoconductive antennas (PCAs). The emission bandwidth is much broader than that of a typical PCA. The spintronic THz emitters are robust, and a broad range of optical pump wavelengths from UV to mid-IR region is usable. However, the efficiency of spintronic emitters is still low compared to PCAs. Therefore, improvement of the THz emission efficiency of spintronic emitters is required for practical applications. Fe(FM-layer)/Pt(NM-layer) heterostructure is one of the most efficient spintronic THz emitters among those reported so far. In this paper, the optimization of the Fe/Pt hetero-structures for efficient THz emission is reported considering the following points: the FM and NM layer thickness, the optical pump wavelength, the choice of the substrate, and the out-coupling efficiency to the free-space. To improve the out-coupling efficiency, we introduce antenna structures with various shapes. It has been demonstrated that a well-designed antenna structure can enhance the THz emission efficiency by 5-6 times. In addition, the authors will report a magnetic-field bias modulation, with which the signal detection efficiency can be almost doubled.

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THz photoconductive antennas (PCAs) have found widespread use in THz generation, detection, and various applications such as sensing, imaging, and communication. For achieving ultrafast operation, most commercially available THz PCAs rely on III-V epitaxial materials due to their high mobility and ultrafast response. However, launching the entire device fabrication process through IC foundries presents significant challenges, thereby limiting the capability of device mass production. In this study, we propose the use of GeSn alloys as the photoconductive material for THz generation. Furthermore, the use of GeSn alloys can offer additional advantages such as cost-effective, scalable, and improved performance.

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We present an optoelectronic transmitter at 300 GHz, composed of a uni-traveling-carrier photodiode module, a fundamental mixer, a filter, and a horn antenna. The designs of those waveguide-based components will be shown.

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The challenges of THz electronics in CMOS technologies for THz sensing and communication applications are addressed in this work, focusing on device-level design to system integration approaches. State-of-the-art THz circuits, components, design methodologies, and system demonstration were proposed, including a 40-nm-CMOS transistor layout design using an electromagnetic modeling approach, a 340-GHz higher-order-mode high-gain dielectric resonator antenna, a 340-GHz CMOS heterodyne receiver, a 340-GHz heterogeneously-integrated THz transmitter with 2×25 antennas, a THz heterogeneously-integrated platform, and a THz transmissive imaging system with a spatial resolution of 1.4 mm at 336 GHz.

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We demonstrate the emission and detection of orthogonally polarized terahertz pulses simultaneously in a fiber-coupled time-domain system. Here, a multi-pixel emitter multiplexes the s- and p- states with different modulation frequencies and the detected signal is demultiplexed with post-processing. A commercial photoconductive detector is used and the the photoconductive emitter is made using standard lithographic techniques. This work can significantly improve the speed of polarization-resolved THz spectroscopy and imaging, so we demonstrated its efficiency by imaging an anisotropic metamaterial and characterizing birefringent crystal.

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Shift and injection currents unveils the pivotal role of the topological band structures and is a prerequisite to development of advanced photovoltaics. This is especially interesting for chiral Weyl semimetals that were proposed to exhibit giant quantized circular photogalvanic effect (CPGE). Here we report a comprehensive THz emission spectroscopic analysis for quantifying nonlinear optical conductivity spectrum of undoped CoSi. Results confirm photoexcitation of giant circular injection currents but provide limited evidence of the quantized CPGE. Moreover, the linear shift current is found significantly weaker. This work consolidates the THz spectroscopy as a contact-free in situ ammeter for probing ultrafast photocurrents.

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We present a theory on refractive index tuning for symmetry-protected bound states in the continuum (SP-BICs) in high-contrast gratings (HCGs). Our study introduces a compact analytical formula for tuning sensitivity, verified through numerical analysis. Additionally, a novel type of SP-BIC in HCGs is discovered, characterized by an accidental nature with a spectral singularity. This behavior is explained by hybridization and strong coupling among the odd- and even-symmetric waveguide-array modes. Our research provides valuable insights into the physics of tuning SP-BICs in HCGs, offering simplified design and optimization for dynamic applications in light modulation, tunable filtering, and sensing.

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We applied metal-dielectric interactions to induce Localized Surface Plasmon Resonance (LSPR) in terahertz (THz) metamaterials. Two studies introduced a graphene-based filter with adjustable resonance peaks and a graphene-metamaterial ultrabroadband absorber. By tuning graphene's Fermi energy, we controlled conductivity and resonance, impacting future THz communications. Another study integrated perovskite ZnTiO3 into a reduced graphene oxide aerogel for a sensitive THz metamaterial gas sensor. A high-surface-area ZTrGOA variant facilitated precise NOx detection. This compact, energy-efficient innovation holds potential for optical gas sensors in biomedicine and wearables, and the absorber's potential in silicon photonics for high-sensitivity 6G and THz communication is explored.

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