Search Results (85)

The unique molecular fingerprint spectral characteristics in the terahertz (THz) band provide distinct advantages for non-destructive and rapid biomolecular detection. However, conventional THz metasurface biosensors still face significant challenges in achieving highly sensitive and precise detection. This study proposes a sensing platform based on quasi-bound states in the continuum (Quasi-BIC), which enhances molecular fingerprint recognition through resonance amplification. We designed a symmetric graphene double-split square ring metasurface structure. By modulating the Fermi level of graphene, this system generated continuously tunable Quasi-BIC resonance peaks across a broad THz spectral range, achieving precise spectral overlap with the characteristic absorption lines of lactose (1.19 THz and 1.37 THz) and tyrosine (0.958 THz). The results demonstrated a remarkable 763-fold enhancement in absorption peak intensity through envelope analysis for analytes with 0.1 μm thickness, compared to conventional bare substrate detection. This terahertz BIC metasurface sensor demonstrates high detection sensitivity, holding significant application value in fields such as biomedical diagnosis, food safety, and pharmaceutical testing. Full article

In recent years, micro-combs, due to their compact structure and high efficiency, have proven to be a practical solution for optical sources. In this paper, an approach to flexibly modulating micro-combs is proposed, and a simulation platform based on ${\mathrm{Si}}_3{\mathrm{N}}_4$ micro-combs with highly integrated, tunable, and reconfigurable features is built. By means of the Lugiato–Lefever equation model, the dynamic evolution process of micro-combs is analyzed, and a micro-ring resonator is designed with a free spectral range of 7.24 nm, an effective mode area of $1.0829{\mu \mathrm{m}}^2$, and coherent comb lines spanning over 125 THz. Cascaded silicon nitride micro-ring filters are utilized to obtain reconfigurable modulation effects for Kerr-frequency micro-combs. Due to the significance of flexibly controlled optical sources with high-repetition rates and multiple channels for system-on-chip, our proposal has potential in photonic integrated circuit systems, such as high-density photonic computing and large-capacity optical communications, in the future. Full article

The rapid advancements in optical communication and sensing technologies have significantly increased the demand for advanced tunable spectral systems. This study presents a dual-band terahertz transmission and manipulation approach by leveraging the topologically protected properties of valley-topological photonic crystal edge states. The designed structure facilitates the excitation of the K valley within the range of 0.851–0.934 THz and the K′ valley from 1.604 to 1.686 THz, while also demonstrating anomalous refraction and birefringence. The calculated emission angles, derived through momentum matching, enable transitions between single-wave and dual-wave emissions and allow for precise angle control. The introduction of the liquid crystal material NJU-LDn-4 enables continuous tuning of the dual-band spectral range under a varying electric field, broadening the operating frequency bands to the ranges of 0.757–0.996 THz and 1.426–1.798 THz, respectively. These findings suggest promising applications in tunable filter design, optical communication, photonic computing, optical sensing, and high-resolution imaging, particularly in novel optical devices requiring precise control over spectral characteristics and light propagation. Full article

The present study explores a multi-analytical non-invasive approach based on the application of terahertz continuous wave (THz-CW) spectroscopy for the non-invasive characterization of historically produced synthetic copper silicate pigments. For the first time, Han Blue, Han Purple and Egyptian Blue were examined within the THz spectral region using a compact and portable THz-CW spectrometer. The three pigments exhibit distinct absorption features, which facilitate the differentiation of molecular structures within the same chemical and mineralogical category. Moreover, the same compound was analyzed using Energy Dispersive X-Ray Fluorescence (ED-XRF) to determine its elemental composition, alongside Fiber Optics Reflectance Spectroscopy (FORS) in the range 350–2500 nm, providing crucial insights into its optical properties and molecular structure. To the best of the authors’ knowledge, the present study presents the first spectra for these copper silicates at these wavelengths, thereby expanding the shortwave infrared spectral database of Cultural Heritage materials. This synergistic approach enables a comprehensive characterization, offering a deeper understanding of the compounds’ chemical nature and paving the way for potential applications in the Cultural Heritage domain. Furthermore, the findings underscore the potential of THz-CW spectroscopy as an innovative and effective tool for Cultural Heritage research, providing a non-destructive method to investigate artistic materials. Full article

Pharmaceutical quality control plays a critical role in safeguarding patient safety and ensuring therapeutic efficacy. However, conventional analytical methods are often hindered by laborious procedures and complex chemical preparation requirements. This study presents a rapid, non-destructive pharmaceutical analysis approach by introducing terahertz spectroscopy for the dual-parametric detection of the anticoagulant warfarin. Characteristic absorption peaks of warfarin within the 4–10 THz range were experimentally identified and theoretically resolved through density functional theory calculations, employing both single-molecule and unit cell models. Furthermore, three strong absorption peaks were selected to construct multivariate regression models correlating spectral parameters (peak intensity and area) with warfarin weight, achieving a detection limit of 0.641 mg within a 5 min analytical workflow. This approach enables simultaneous molecular fingerprint identification and quantitative determination without chemical modification, meeting the requirements for the rapid screening of active pharmaceutical ingredients. Full article

The terahertz (THz) band, ranging from 0.1 to 10 THz, offers substantial bandwidths that are essential for meeting the ever-increasing demands for high data rates in future wireless communication systems. This paper presents a comprehensive comparative analysis of various multicarrier waveforms suitable for THz-band communications. We explore the performance, advantages, and limitations of several waveforms, including Orthogonal Frequency Division Multiplexing (OFDM), Filter Bank Multicarrier (FBMC), Universal Filtered Multicarrier (UFMC), and Generalized Frequency Division Multiplexing (GFDM). The analysis covers key parameters such as spectral efficiency, the peak-to-average power ratio (PAPR), robustness to phase noise, and computational complexity. The simulation results demonstrate that while OFDM offers simplicity and robustness to multipath fading, it suffers from high PAPR and phase noise sensitivity. FBMC and UFMC, with their enhanced spectral efficiency and reduced out-of-band emissions, show promise for THz-band applications but come at the cost of increased computational complexity. GFDM presents a flexible framework with a trade-off between complexity and performance, making it a potential candidate for diverse THz communication scenarios. Our study concludes that no single waveform universally outperforms the others across all metrics. Therefore, the choice of multicarrier waveform for THz communications should be tailored to the specific requirements of the application, balancing performance criteria and implementation feasibility. Future research directions include the development of hybrid waveforms and adaptive techniques to dynamically optimize performance in varying THz communication environments. Full article

The conjoined split-ring resonator (Co-SRR) is proposed as the unit cell to construct terahertz (THz) metamaterial. The size and position of the gaps on both sides of the structure were adjusted, and the impact on the electromagnetic response to the incident THz wave was investigated via simulation. Results show that by properly controlling the structural asymmetry, the resonances can be tuned simultaneously or independently. The devices exhibit frequency shifts of up to 510 GHz, a tuning range of free spectral range (FSR) as wide as 613 GHz, and a high modulation depth (MD) of 93.4%. Additionally, a wide range of amplitude modulation can occur across multiple frequencies. Incorporating spatial asymmetry further enhances the performance, resulting in a high quality factor (Q) of 44.8 and a figure of merit (FOM) of 40.1. The impressive characteristics prove that Co-SRR-based metamaterial is a great candidate for applications in optical sensing, switching, filtering and programming devices. Full article

The year 2024 marks the 30-year anniversary of the quantum cascade laser (QCL), which is becoming the leading laser source in the mid-infrared (mid-IR) range. Since the first demonstration, QCL has undergone tremendous development in terms of the output power, wall plug efficiency, spectral coverage, wavelength tunability, and beam quality. Owing to its unique intersubband transition and fast gain features, QCL possesses strong nonlinearities that makes it an ideal platform for nonlinear photonics like terahertz (THz) difference frequency generation and direct frequency comb generation via four-wave mixing when group velocity dispersion is engineered. The feature of broadband, high-power, and low-phase noise of QCL combs is revolutionizing mid-IR spectroscopy and sensing by offering a new tool measuring multi-channel molecules simultaneously in the μs time scale. While THz QCL difference frequency generation is becoming the only semiconductor light source covering 1–5 THz at room temperature. In this paper, we will introduce the latest research from the Center for Quantum Devices at Northwestern University and briefly discuss the history of QCL, recent progress, and future perspective of QCL research, especially for QCL frequency combs, room temperature THz QCL difference frequency generation, and major challenges facing QCL in the future. Full article

The tunability of spectral lineshapes, ranging from Lorentzian to Fano profiles, is essential for advancing nanoscale photonic technologies. Conventional far-field techniques are insufficient for studying nanoscale phenomena, particularly within the terahertz (THz) range. In this work, we use a U-shaped resonant ring on a waveguide substrate to achieve precise modulation of Lorentzian, Fano, and antiresonance profiles. THz scattering scanning near-field optical microscopy (s-SNOM) reveals the underlying physical mechanism of these transitions, driven by time-domain phase shifts between the background excitation from the waveguide and the resonance of the U-shaped ring. Our approach reveals a pronounced asymmetry in the near-field response, which remains undetectable in far-field systems. The ability to control spectral lineshapes at the nanoscale presents promising applications in characterizing composite nanoresonators and developing nanoscale phase sensors. Full article

Spectral tunable technology has to meet the requirements of strong robustness and wide spectral range. We propose a method for the transmission and manipulation of infrared topological photonic crystal valley states based on tunable refractive index method that exhibits broad-spectrum and multi-band characteristics, along with a tunable emission angle. With this structure, different rotational directions of vortex light sources can independently excite the K valley and K′ valley within the frequency band ranging from 75.64 THz to 99.61 THz. At frequencies from 142.60 THz to 171.12 THz, it is possible to simultaneously excite both the K valley and K′ valley. The dual refractive index tunable design allows for the adjustment of the emission angle at a fixed frequency, enabling control over the independent excitation of either a single K valley or K′ valley, as well as their simultaneous excitation. This capability has significant implications for photonic computation and tunable filtering, offering enhanced operational flexibility and expanded functionality for future optical communications and integrated optical circuits. Full article

This study demonstrates that diffusion doping of ZGP single crystals with impurity atoms (Mg, Se, Sn, Pb) leads to a decrease in the specific conductivity of the samples. Consequently, this results in reduced absorption in the terahertz frequency range (150–1000 μm). It has been shown that doping ZGP samples with selenium (Se) and lead (Pb) atoms reduces absorption in the infrared region from 0.3–0.6 cm⁻¹ to 0.06–0.09 cm⁻¹. Doping with tin (Sn) leads to a decrease in absorption only in the wavelength region near 2.1 μm from 0.2 cm⁻¹ to 0.05 cm⁻¹. The proposed mechanism for the decrease in infrared absorption is a reduction in zinc vacancies due to doping with impurity atoms. This research lays the groundwork for a technology that produces ZGP crystals with minimal absorption within the 2–8 μm wavelength range, eliminating the need for fast electron beam irradiation technology. This advancement will facilitate the fabrication of ZGP crystals with arbitrary apertures. Full article

Terahertz spectroscopy systems, which integrate terahertz sources and detectors, have important applications in many fields such as materials science and security checking. Based on highly sensitive frequency up-conversion detection, large dynamic range spectral measurements in a terahertz region are reported. Our system realized the detection sensitivity at a 10 aJ level with a 2-(3-(4-hydroxystyryl)-5,5-dime-thylcyclohex-2-enylidene) malononitrile (OH1) crystal and a dynamic range up to seven orders. Based on this system, we verified the validity of the spectral measurement with tests which were conducted on monohydrate glucose, anhydrous glucose and mixed tablet samples with a thickness of 0.8 mm in 1~3 THz, respectively. Also, a mini coppery elbow tube with an inner diameter of 1 mm was used for the transmission of a terahertz wave to simulate some strip biological tissue samples. By allowing terahertz to transmit through this tube filled with 0.5 g glucose powder, we successfully obtained the absorption spectrum with a minimum transmittance at the absorption peak in the order of 10⁻⁴. Full article

Unmanned aerial vehicles (UAVs) are extensively employed in pursuit, rescue missions, and agricultural applications. These operations require substantial data and video transmission, demanding significant spectral resources. The ultra-broad bandwidth of 0.1–10 THz in the Terahertz (THz) frequency range is essential for future UAV-based wireless communications. However, the available bandwidth in the THz frequency spectrum varies with transmission distance. To enhance spectral efficiency over this variable bandwidth, we propose using hierarchical modulation (HM) in the overlapped spectrum and traditional quadrature amplitude modulation (QAM) in the non-overlapped spectrum for closer users. Furthermore, we analyze the single-user case and utilize the block-coordinated descent (BCD) method to jointly optimize the modulation order, subcarrier bandwidth, and sub-band power to improve the system sum rate. Finally, considering the mobility and randomness of UAV users, we design a modulation switching rule to dynamically adjust to changes in distance as users move, thereby enhancing data rates. Simulation results demonstrate superior performance in data rate and design complexity compared to existing methods such as hierarchical bandwidth modulation (HBM) and HM schemes. Full article

The performance of 5G/6G cellular systems operating in millimeter wave (mmWave, 30–100 GHz) and sub-terahertz (sub-THz, 100–300 GHz) bands is conventionally assessed by utilizing the static distributions of user locations. The rationale is that the use of the beam tracking procedure allows for keeping the beams of a base station (BS) and user equipment (UE) aligned at all times. However, by introducing 3GPP Reduced Capability (RedCap) UEs utilizing the Radio Resource Management (RRM) Relaxation procedure, this may no longer be the case, as UEs are allowed to skip synchronization signal blocks (SSB) to improve energy efficiency. Thus, to characterize the performance of such UEs, methods explicitly accounting for UE mobility are needed. In this paper, we will utilize the tools of the stochastic geometry and random walk theory to derive signal-to-noise ratio (SNR), spectral efficiency, and rate as an explicit function of time by accounting for mmWave/sub-THZ specifics, including realistic directional antenna radiation patterns and micro- and macro-mobilities causing dynamic antenna misalignment. Different from other studies in the field that consider time-averaged performance measures, these metrics are obtained as an explicit function of time. Our numerical results illustrate that the macro-mobility specifies the overall trend of the time-dependent spectral efficiency, while local dynamics at 1–3 s scales are mainly governed by micro-mobility. The difference between spectral efficiency corresponding to perfectly synchronized UE and BS antennas and time-dependent spectral efficiency in a completely desynchronized system is rather negligible for realistic cell coverages and stays within approximately 5–10% for a wide range of system parameters. These conclusions are not affected by the utilized antenna array at the BS side. However, accounting for realistic radiation patterns is critical for a time-dependent performance analysis of 5G/6G mmWave/sub-THz systems. Full article

The application of terahertz (THz) radiation in scientific research as well as in applied and commercial technology has expanded rapidly in recent years. One example is the progress in high-resolution THz spectroscopy based on quantum cascade lasers, which has enabled new observations in astronomy, atmospheric research, and plasma diagnostics. However, the lack of easy-to-use and miniaturised detectors has hampered the development of compact THz spectroscopy systems out of the laboratory environment. In this paper, we introduce a new high-speed pyroelectric receiver as a cryogen-free detector for THz absorption spectroscopy. Its performance is characterised by absorption spectroscopy measurements on a reference gas cell (RGC) with ammonia using a tunable THz quantum cascade laser at approximately 4.75 THz as the light source. It is shown that the receiver can record spectra up to 281 Hz without any artefacts to the observed spectral absorption profile, and the results reproduce the known pressure of ammonia in the RGC. This demonstrates that the pyroelectric receiver can be reliably used as an alternative to helium-cooled bolometers for absorption spectroscopy measurements in the THz range, with its main advantages being the high bandwidth, compactness, relatively low cost, and room-temperature operation. Its simplicity and high sensitivity make this receiver a key component for compact THz spectroscopy systems. Full article