Advanced Terahertz Refractive Sensing And Fingerprint Recognition Through Metasurface-Excited Surface Waves

Interface Dominated Spin-to-Charge Conversion in Terahertz Emission by Band Structure Engineering of Topological Surface States

The rapid advancement of future information technologies necessitates the development of high-efficiency and cost-effective solutions for terahertz emitters, which hold significant practical value in next-generation communication, terahertz sensing, and quantum computing applications. Distinguished from trivial materials, three-dimensional topological insulators exhibit spin-momentum locking in helical Dirac surface states, making them highly efficient spin-to-charge converters that have the potential to revolutionize electronics. However, the efficiency of utilizing topological insulators for spin terahertz emission has not yet matched that of spin manipulation in other spintronic devices. Here, we investigate the spin terahertz emission properties of high crystalline quality (Bi1-xSbx)2Te3/Fe heterostructures through band structure engineering. Notably, contrary to expectations, the strongest terahertz radiation is not achieved at the charge neutrality point. Through an analysis of influencing factors and a temperature-independent investigation, we identify interface transparency as the primary factor affecting emission efficiency. To optimize interfaces and enhance spin-to-charge conversion efficiency, a Rashba-mediated Dirac surface state is constructed by attaching a Bi layer. Furthermore, with doping concentrations of 0, 0.5, and 1, respectively, we observe enhancements in intensity by 35.1, 50.3, and 44.3%. These results provide a detailed assessment of interfacial and doping effects in topological-insulator-based terahertz emitters and contribute to the understanding of spin-to-charge dynamics in topological materials.

Enabling beam-scanning antenna technologies for terahertz wireless systems: A review

Due to the exponentially growing global mobile data of wireless communications evolving from 5 G to 6 G in recent years, research activities of leveraging terahertz (THz) waves to obtain larger channel capacities have shown an ever-increasing pace and reached an unprecedented height than before. Historically, the past few decades have already witnessed much progress in THz generation and detection technologies, which have been recognized for a long time as the bottleneck preventing the THz waves from being tamed by human beings. However, the importance of developing advanced components such as antennas, transmission lines, filters, power amplimers, etc., which constitute the basic building blocks of a THz wireless system, should not be overlooked for the sake of exploiting the THz spectra for future advanced wireless communications, sensing and imaging applications. While producing a scannable highly-directive antenna beam proves to be indispensable in the period of microwaves, the significance of such functionality is more critical in the THz era, considering that THz waves have more intractable challenges such as the severity of free-space propagation losses, the susceptibility to atmospheric environments, and the unavailability of efficient signal sources. This article is structured under this background, which is dedicated to reviewing several enabling beam-scanning antenna concepts, structures, and architectures that have been developed for THz wireless systems. Specifically, we divide these THz beam-scanning solutions into four basic groups based on different mechanisms, i.e., mechanical motion, phased array, frequency beam-scanning, and reconfigurable metasurfaces.

Retime-mapping terahertz vernier biosensor for boosting sensitivity based on self-reference waveguide interferometers

The optical vernier effect serves as a potent mechanism for boosting sensitivity and accuracy in the communication band, which is a prominent hotspot in coherent detection. Extending vernier gain to the terahertz window exhibits significant appeal in next-generation wireless communication and high-resolution sensing. Here, a terahertz vernier biosensor is constructed utilizing two overlapping Mach-Zehnder interferometers within a three-channel metallic waveguide. The self-reference feature of the vernier biosensor facilitates a sensitive envelope, and the vernier gain significantly amplifies the detection sensitivity and accuracy from the superposition of slightly detuned terahertz interference spectra mapping within the time-frequency-time domain. An exalting sensitivity of 22.54 THz/RIU is demonstrated at operating frequencies near 0.9 THz and experimentally shows immense sensing performance in detection sensitivity and accuracy of biochemical sample areic mass are 107 GHz/(g/mm2) and 10-8 g/mm2, respectively, presenting an enhancement of > 3000% compared to a single interferometer. Moreover, the sensor is employed to assess the amino acid oxidation characteristic curve analysis in the terahertz range, which assists in identifying specific amino acids. The validation of the vernier effect operating in the terahertz regime demonstrates the development of a rapid and label-free assistance tool for the identification of biochemical samples.

The theory, technology, and application of terahertz metamaterial biosensors: A review

Terahertz metamaterial biosensors combine terahertz time-domain spectroscopy with metamaterial sensing to provide a sensitive detection platform for a variety of targets, including biological molecules, proteins, cells, and viruses. These biosensors are characterized by their rapid response, sensitivity, non-destructive, label-free operation, minimal sample requirement, and user-friendly design, which also allows for integration with various technical approaches. Advancing beyond traditional biosensors, terahertz metamaterial biosensors facilitate rapid and non-destructive trace detection in biomedical applications, contributing to timely diagnosis and early screening of diseases. In this paper, the theoretical basis and advanced progress of these biosensors are discussed in depth, focusing on three key areas: improving the sensitivity and specificity, and reducing the influence of water absorption in biological samples. This paper also analyzes the potential and future development of these biosensors for expanded applications. It highlights their potential for multi-band tuning, intelligent operations, and flexible, wearable biosensor applications. This review provides a valuable reference for the follow-up research and application of terahertz metamaterial biosensors in the field of biomedical detection.

Terahertz Metamaterials Inspired by Quantum Phenomena

The study of many phenomena in the terahertz (THz) frequency spectral range has emerged as a promising playground in modern science and technology, with extensive applications in high-speed communication, imaging, sensing, and biosensing. Many THz metamaterial designs explore quantum physics phenomena embedded into a classical framework and exhibiting various unexpected behaviors. For spatial THz waves, the effects inspired by quantum phenomena include electromagnetically induced transparency (EIT), Fano resonance, bound states in the continuum (BICs), and exceptional points (EPs) in non-Hermitian systems. They facilitate the realization of extensive functional metadevices and applications. For on-chip THz waves, quantum physics-inspired topological metamaterials, as photonic analogs of topological insulators, can ensure robust, low-loss propagation with suppressed backscattering. These trends open new pathways for high-speed on-chip data transmission and THz photonic integrated circuits, being crucial for the upcoming 6G and 7G wireless communication technologies. Here, we summarize the underlying principles of quantum physics-inspired metamaterials and highlight the latest advances in their application in the THz frequency band, encompassing both spatial and on-chip metadevice realizations.

Photon-Induced Ultrafast Multitemporal Programming of Terahertz Metadevices

Dynamic terahertz (THz) metasurface can feature modulated and multiplexed electromagnetic functionalities, important for wave-based computation, six-generation communications, and other applications. The versatile dynamic switching typically relies on a series of complex or incompatible multifield activations, with excessive system complexity, additional loss, slow modulation speed, and inertial time-varying properties, limiting more widespread applications. Here, a photon-induced ultrafast programmable THz metadevice is reoprted in time-frequency dimensions with polarization-decoupled temporal responses. By the pixelated design with multi-materials and triggering switches, the multimodal modulation transcends the constraints inherent in materials, enabling the ultrafast programmable temporal evolution. All the resonances can be independently programmed at the working band from 0.6 to 2 THz. The tri-temporal (with switching time of 1.25, 1, and 4.75 ps) and bi-temporal (with switching time of 2.25 and 4.75 ps) dynamic manipulations are performed by all-optical driven molecularization process of hybrid metasurfaces loaded with silicon (Si) and germanium (Ge) under different polarizations. Combining these features, the temporally programmed THz logic gates are last experimentally demonstrated, possessing basic operation of XNOR, NOR, and OR, as a proof-of-concept application. This reported light-driven programmable THz flat-optics allows ultrafast hybrid molecularization processes and new possibilities for miniaturized, flexible, multifunctional, and temporally programmable integrated devices.

Terahertz Science and Technology in Astronomy, Telecommunications, and Biophysics

This paper reviews recent developments and key advances in terahertz (THz) science, technology, and applications, focusing on 3 core areas: astronomy, telecommunications, and biophysics. In THz astronomy, it highlights major discoveries and ongoing projects, emphasizing the role of advanced superconducting technologies, including superconductor-insulator-superconductor (SIS) mixers, hot electron boundedness spectroscopy (HEB), transition-edge sensors (TESs), and kinetic inductance detectors (KIDs), while exploring prospects in the field. For THz telecommunication, it discusses progress in solid-state sources, new communication technologies operating within the THz band, and diverse modulation methods that enhance transmission capabilities. In THz biophysics, the focus shifts to the physical modulation of THz waves and their impact across biological systems, from whole organisms to cellular and molecular levels, emphasizing nonthermal effects and fundamental mechanisms. This review concludes with an analysis of the challenges and perspectives shaping the future of THz technology.

Advances in Nanoengineered Terahertz Technology: Generation, Modulation, and Bio-Applications

Recent advancements in nanotechnology have revolutionized terahertz (THz) technology. By enabling the creation of compact, efficient devices through nanoscale structures, such as nano-thick heterostructures, metasurfaces, and hybrid systems, these innovations offer unprecedented control over THz wave generation and modulation. This has led to substantial enhancements in THz spectroscopy, imaging, and especially bio-applications, providing higher resolution and sensitivity. This review comprehensively examines the latest advancements in nanoengineered THz technology, beginning with state-of-the-art THz generation methods based on heterostructures, metasurfaces, and hybrid systems, followed by THz modulation techniques, including both homogeneous and individual modulation. Subsequently, it explores bio-applications such as novel biosensing and biofunction techniques. Finally, it summarizes findings and reflects on future trends and challenges in the field. Each section focuses on the physical mechanisms, structural designs, and performances, aiming to provide a thorough understanding of the advancements and potential of this rapidly evolving technology domain. This review aims to provide insights into the creation of next-generation nanoscale THz devices and applications while establishing a comprehensive foundation for addressing key issues that limit the full implementation of these promising technologies in real-world scenarios.

Ultrasensitive Terahertz Label-Free Metasensors Enabled by Quasi-Bound States in the Continuum

Advanced sensing devices based on metasurfaces have emerged as a revolutionary platform for innovative label-free biosensors, holding promise for early diagnostics and the detection of low-concentration analytes. Here, we developed a chip-based ultrasensitive terahertz (THz) metasensor, leveraging a quasi-bound state in the continuum (q-BIC) to address the challenges associated with intricate operations in trace biochemical detection. The metasensor design features an open-ring resonator metasurface, which supports magnetic dipole q-BIC combining functionalized gold nanoparticles (AuNPs) bound with a specific antibody. The substantial enhancement in THz-analyte interactions, facilitated by the potent near-field enhancement enabled by the q-BICs, results in a substantial boost in biosensor sensitivity by up to 560 GHz/refractive index units. This methodology allows for the detection of conjugated antibody-AuNPs for cardiac troponin I at concentrations as low as 0.5 pg/ml. These discoveries deliver valuable insight for AuNP-based trace biomolecule sensing and pave the path for the development of chip-scale biosensors with profound light-matter interactions.

Reconfigurable terahertz multifunctional wave plates with VO2/Ge hybrid metasurfaces

Active control of polarization using metasurfaces is crucial in terahertz optics, offering promising advancements in sensing, imaging, and telecommunications. Here, we developed reconfigurable terahertz multifunctional wave plates by leveraging vanadium dioxide/germanium hybrid metasurfaces. This approach allows for mutual role changing of metasurface among quarter-wave plate, half-wave plate, and full-wave plate, facilitated by the introduction of continuous-wave and pulse lasers. The photoinduced phase change of vanadium dioxide, along with the bridging control of germanium, plays a key role in the transition of multifunctional wave plates. Also, the analysis of polarization conversion ratio, ellipticity, and underlying physics demonstrates the ability of multifunctional wave plates. These discoveries deliver valuable insight into advanced polarization control and demonstrate the potential for innovative active-control devices.

Ultrafast light-driven metasurfaces with an ultra-broadband frequency agile channel for sensing

Active terahertz metasurface devices have been widely used in communication technology, optical computing and biosensing. However, numerous dynamically tunable metasurfaces are only operating at a single frequency point or in a narrow range, limiting the further possibility of the devices to meet contemporary broad-spectrum biosensing requirements. In this paper, a novel compact biosensor is proposed with an ultrawide resonance frequency agile channel shifted from 0.82 to 1.85 THz, with a tuning functionality up to 55.7%. In addition, under optical pumping irradiation, the modulator with ultra-fast response is able to complete the ultra-wideband resonant mode conversion from the Fano mode to the electromagnetically induced transparency (EIT) mode within 4 ps, and achieves a frequency shift sensitivity of 118 GHz RIU-1 and 247 GHz RIU-1 at 0.82 and 1.85 THz, respectively. This mechanism implements both refractive index and conductivity sensing functions, which provide a wealth of sensing information. Thus, this work presents the possibility of realising the detection of ultra-wide fingerprint spectra and can be extended to a wider range of optical fields.