Quasi-cyclic Hermitian construction of binary quantum codes

In this paper, we propose a sufficient condition for a family of 2-generator self-orthogonal quasi-cyclic codes with respect to Hermitian inner product. Supported in the Hermitian construction, we show algebraic constructions of good quantum codes. 30 new binary quantum codes with good parameters improving the best-known lower bounds on minimum distance in Grassl's code tables \cite{Grassl:codetables} are constructed

Terahertz Ultrafast Spectroscopy : A Paradigm for Material Characterization and Light Interaction

Recent advancements in terahertz (THz) technology have heralded the development of cutting-edge sources, detectors, and modulators, catalyzing progress in fields such as molecular spectroscopy, 6G wireless communication, and biomedical imaging. Central to these innovations is terahertz time-domain spectroscopy (THz-TDS), which has garnered significant attention in both foundational and applied research. THz-TDS's unique capability to concurrently obtain time-domain and frequency-domain spectral data, along with its non-destructive nature, positions it as an indispensable tool for elucidating the physical and chemical properties of various materials without causing damage. This technique provides profound insights into diverse fields, such as molecular dynamics and vibrational modes of crystal lattices. Leveraging the state-of-the-art measurements enabled by THz-TDS, this thesis delves into the characterization of advanced materials and the examination of light interactions within artificial photonic structures. It showcases a variety of innovative THz-TDS applications aimed at propelling research in diverse areas: Initially, the thesis presents material characterization applications utilizing a standard THz-TDS system configured for both transmission and reflection measurements. Through the analysis of different transmission characteristics of samples, it highlights two exemplary methods: evaluating the surface roughness of 3D-printed metals to within micrometer accuracy, and determining the shielding effectiveness of bio-degradable THz absorbers up to 99.99999%. These methodologies underscore the pivotal role of THz-TDS in advancing next-generation manufacturing technologies. Furthermore, the thesis explores the investigation of artificial photonic structures using a general THz-TDS system. It details the initial measurements of two types of metamaterials—each designed for high Q-factor enhancement and increased light absorption—using both standard and cryogenic THz-TDS. The photonic samples, meticulously engineered through finite element simulations and crafted with cutting-edge microfabrication techniques, demonstrate a seamless correlation between THz-TDS results and simulation predictions, establishing a robust framework for future THz photonic development. In the concluding chapters, specialized THz-TDS techniques, such as aperture near-field scanning microscope (a-SNOM) THz-TDS and high-field THz-TDS, are introduced for detailed analysis of localized light interactions in high Q-factor resonators and high harmonic generation in integrated graphene metamaterials, respectively. The a-SNOM THz-TDS, with its sub-micron spatial resolution and comprehensive time-domain investigation capabilities, facilitates the mapping and quantitative analysis of tightly confined modes, including weakly radiative modes through a cross-polarized configuration. This insight is invaluable for advancing metamaterial-based optoelectronic platforms in THz photonics. High-field THz-TDS is employed to generate tunable, high-power pulses, intensifying the nonlinear effects in materials. Graphene, with its massless electron dynamics and linear band characteristics, emerges as the most effective material for generating high-order harmonic generation in the THz range. The coherent measurement capabilities of high-field THz-TDS unveil distinct high-harmonic trends in graphene-based opto-electronics, showcasing the potential for the development of future THz components

Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators

Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light–matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics.

Theoretical and Experimental Analysis of the Directional RI Sensing Property of Tilted Fiber Grating

In this article, we have theoretically and experimentally investigated the unique vector refractive index (RI) sensing property of tilted fiber grating (TFG). Due to the orthogonal symmetric grating structure, TFGs would mainly achieve the coupling between the fiber core mode and the two orthogonal polarization LP1m of cladding mode. And the numerical simulation results showed that the coupling coefficient between fundamental core mode to the LP1m cladding mode is higher than the others. In the experiment, we have furthermore observed the cladding mode field distribution of excessively TFG (Ex-TFG) and long period fiber grating (LPFG), which indicated that the evanescent field distribution of cladding mode of TFG shows an asymmetric near field distribution with two lobes oriented along the fast axis of TFG, and the one of LPFG has a circularly symmetric cladding mode field distribution. In addition, by employing side-immersion method, we have measured the azimuth RI sensitivities of Ex-TFG, tilted fiber Bragg grating (TFBG) and LPFG, which exhibited that both Ex-TFG and TFBG have shown a direction-dependency RI sensitivity, and the RI sensitivity with side-immersion along fast axis is almost half of the one along slow axis, and the RI sensitivity of LPFG is azimuth independent. Overall, the experiment results show that the TFGs inherently show unique directional RI sensing property, which could be potentially applied in vector sensing area

Contactless 3D surface characterization of additive manufactured metallic components using terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy has experienced significant progress in imaging, spectroscopy, and quality inspection, e.g., for semiconductor packaging or the automotive industry. Additive manufacturing alloys (also known as alloys for use in 3D printing) have risen in popularity in aerospace and biomedical industries due to the ability to fabricate intricate designs and shapes with high precision using materials with customized mechanical properties. However, these 3D-printed elements need to be polished thereafter, where the surface roughness is inspected using techniques such as the laser scanning microscope. In this study, we demonstrate the use of terahertz time-domain spectroscopy to assess the average roughness profile and height leveling of stainless steel for comparisons against the same parameters acquired using laser scanning microscopy. Our results highlight the potential of the proposed technique to rapidly inspect 3D-printed alloys over large areas, thus providing an attractive modality for assessing surface profiles of AM-manufactured terahertz components in the future

Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators

Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light–matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics

Active Polarization Modulation of Terahertz Radiation Using Metamaterial/Graphene-Based Optoelectronic Devices

We investigate the modulation performance of a metamaterial/graphene optoelectronic device in the THz range. Operating characteristics such as amplitude, phase and polarization modulations of broadband THz radiation are reported. The accomplishments of modulation depth include >75% in spectral amplitude, >15∘ in spectral phase, >0.2 active modulation of ellipticity ratio, and active rotational angle changes of 20°. These achievements are the key elements towards efficient manipulations of THz radiation for applications such as next-generation wireless communications, spectroscopy, and imaging

Graphene-based External Optoelectronic Terahertz Modulators for High Speed Wireless Communications

The realization of terahertz external amplitude modulators with a carrier frequency of 0.8 THz is presented for application in the next generation near-field wireless communications

Optimal Quaternary Hermitian LCD Codes

Linear complementary dual (LCD) codes, which are a class of linear codes introduced by Massey, have been extensively studied in the literature recently. It has been shown that LCD codes play a role in measures to counter passive and active side-channel analyses on embedded cryptosystems. In this paper, tables are presented of good quaternary Hermitian LCD codes and they are used in the construction of puncturing, shortening and combination codes. The results of this, including three tables of the best-known quaternary Hermitian LCD codes of any length n≤25 with corresponding dimension k, are presented. In addition, many of these quaternary Hermitian LCD codes given in this paper are optimal and saturate the lower or upper bound of Grassl’s code table, and some of them are nearly optimal