Terahertz (THz) radiation, with unique properties and wide-ranging applications, hinges on efficient sources and detectors for further development. Research on THz array detectors for low-repetition-frequency, high-pulse-energy sources is in its infancy. This study presents a THz optoacoustic array detector. It has high response speed and sensitivity, enabling 3D THz spot scanning and imaging. Size reduction of the piezoelectric probe crystal improves resolution, parallel scanning boosts efficiency, and it is highly scalable for real-time imaging.

Aiming at low-loss terahertz (THz) fiber fabrication, we propose a negative curvature terahertz tube fiber (NC-TF). Simulation results show that the NC-TF has similar transmission losses (TLs) to the recognized half-ring fiber but with a significantly simpler fabrication structure. NC-TF samples are fabricated by extruding polymers from a specially designed mold, presenting a novel approach for obtaining fibers with shaped boundaries. Experimental data demonstrate that the NC-TF exhibits TLs below 3 dB/m in transmission bands, with a minimum TL of 0.2 dB/m at 0.6 THz. The simplicity and practicality of the NC-TF enable its application in various THz transmission or sensing scenarios.

The optoelectronic performance of quantum cascade detectors (QCDs) is highly sensitive to the design of the energy level structure, leading to the inability of a single structure to achieve broad wavelength tuning. To address this issue, we propose and demonstrate a modular concept for very long wave infrared (VLWIR) QCDs based on a miniband diagonal transition scheme. The modular design makes the wavelength tuning only need to be adjusted for the absorption quantum well module rather than for the whole active region. Theoretical simulation shows that the wavelength tuning range is 39.6 meV (∼14–30 μm). To prove the feasibility of the scheme, three samples with different absorption well widths were fabricated and characterized. At 10 K, the response wavelengths of the three QCDs are 14, 16, and 18 μm, respectively, corresponding to responsivities and detectivities exceeding 2 mA/W and 1 × 1010 Jones.

In this Letter, we employ fused silica and two types of optical glass as examples to investigate the coherent terahertz (THz) wave emission from laser-ionized isotropic transparent dielectrics. Based on the laser energy and incident angle dependences, we ascribe the THz emission to the ponderomotive force-induced dipole oscillation. Additionally, our investigation on the dependence of THz amplitude on the laser pulse duration confirms the dominant role of avalanche ionization in solid dielectrics. The THz emission can be utilized to indirectly monitor the ultrafast dynamics of carrier generation and motion during the laser ionization process of solid dielectrics.

Free manipulation of electromagnetic waves in the terahertz (THz) band based on metasurface functional devices has been the focus of research in recent years. Among these devices, active metasurfaces have generated extensive research interest due to their reconfigurability. In this work, we demonstrate a mechanically reconfigurable THz polarization converter that consists of two parallel transmissive metasurfaces with a tunable spacing. By mechanically adjusting the coupling strength between the metasurfaces, the orthogonal polarization conversion of the incident linearly polarized THz waves can be tuned. Specifically, the device can be tuned from efficient dual-frequency orthogonal polarization conversion to efficient single-frequency orthogonal polarization conversion. After a gradual decrease in efficiency, it is finally changed to a low transmission state as the gap distance increases from 150 to 800 µm. We theoretically analyze the tuning process under different spacings and experimentally verify it using a vector network analyzer. Our proposed design is straightforward and robust, with the potential to find wide applications in THz science and technology.

Coherent control of terahertz (THz) wave radiation with two-color laser excitation requires good temporal overlap with good dispersion control of both the fundamental (ω) and the second harmonic (2ω). Herein, we experimentally determined the temporal overlap of the ω and 2ω pulses in the time-domain, which was corroborated by theoretical calculations. Furthermore, the coherent control of THz radiation of ZnSe also proves the good temporal overlap of two-color femtosecond lasers. This work provides an experimental tool for finding temporal overlap and realizing the dispersion control of two femtosecond lasers.

Topological valley photonics has recently gained widespread interest owing to its robustness and backscattering immunity against disorders. Previous topological valley transport based on kink states required an interface between two topologically distinct domains, while recent studies have reported that chiral edge states (CESs) can be realized at the external boundary of topological insulators by changing the on-site edge potentials. However, current research on CESs is predominantly focused on the microwave frequency range, leaving challenges for emerging terahertz communications. Here, cladding-free CESs are demonstrated at the external boundary of terahertz all-silicon topological valley photonic crystals with gapless, single-mode, and linear dispersion. We show that CESs are immune to backscattering against sharp corners and support unidirectional propagation of chiral excitations. We also achieved smooth transition between kink states and CESs supported by an all-silicon platform, which could be used as the terahertz inner-chip connection. Finally, a terahertz wireless link between two disconnected CESs is verified for the near-field information interconnection between distinct mobile phones. Our work indicates CESs can improve the compactness of terahertz circuits and inspire advanced terahertz interchip communications.

Metalenses are essential components in terahertz imaging systems. However, without careful design, they show limited field of view and their practical applications are hindered. Here, a wide-angle metalens is proposed whose structure is optimized for focusing within the incident angles of ±25°. Simulation and experiment results show that the focusing efficiency, spot size, and modulation transfer function of this lens are not sensitive to the incident angle. More importantly, this wide-angle metalens follows the ideal Gaussian formula for the object-image relation, which ensures a wider field of view and better contrast in the imaging experiment.

Terahertz (THz) NH3 lasing with optical pumping by electron-beam-sustained discharge “long” (∼100 µs) CO2 laser pulses was obtained. The NH3 laser emission pulses and the “long” pulses of the CO2 pump laser were simultaneously measured with nanosecond response time. The NH3 lasing duration and its delay with respect to the pump pulse were measured for various CO2 laser pulse energies. For the CO2 laser pump line 9R(30), three wavelengths of 67.2, 83.8, and 88.9 µm were recorded. For the CO2 laser pump line 9R(16), only a single NH3 laser line with a wavelength of 90.4 µm was detected.

Bilayer graphene, which is highly promising for electronic and optoelectronic applications because of its strong coupling of the Dirac–Fermions, has been studied extensively for the emergent correlated phenomena with magic-angle manipulation. Due to the low energy linear type band gap dispersion relationship, graphene has drawn an amount of optoelectronic devices applications in the terahertz region. However, the strong interlayer interactions modulated electron-electron and electron-phonon coupling, and their dynamics in bilayer graphene have been rarely studied by terahertz spectroscopy. In this study, the interlayer interaction influence on the electron-electron and the electron-phonon coupling has been assigned with the interaction between the two graphene layers. In the ultrafast cooling process in bilayer graphene, the interlayer interaction could boost the electron-phonon coupling process and oppositely reduce the electron-electron coupling process, which led to the less efficient thermalization process. Furthermore, the electron-electron coupling process is shown to be related with the electron momentum scattering time, which increased vividly in bilayer graphene. Our work could provide new insights into the ultrafast dynamics in bilayer graphene, which is of crucial importance for designing multi-layer graphene-based optoelectronic devices.

Terahertz (THz) absorbers for imaging, sensing, and detection are in high demand. However, such devices suffer from high manufacturing costs and limited absorption bandwidths. In this study, we presented a low-cost broadband tunable THz absorber based on one-step laser-induced graphene (LIG). The laser-machining-parameter-dependent morphology and performance of the absorbers were investigated. Coarse tuning of THz absorption was realized by changing the laser power, while it was fine-tuned by changing the scanning speed. The proposed structure can achieve over 90% absorption from 0.5 THz to 2 THz with optimized parameters. The LIG method can help in the development of various THz apparatuses.

With the framework of exterior product, we investigate the relationship between composite multiscale entropy (CMSE) and refractive index and absorption coefficient by reanalyzing six concentrations of bovine serum albumin aqueous solutions from the published work. Two bivectors are constructed by CMSE and its square by the refractive index and absorption coefficient under vectorization. The desirable linear behaviors can be captured, not only between the defined two bivectors in normalized magnitudes, but also between the normalized magnitude of bivectors pertinent to CMSE and the magnitude of a single vector on the refractive index or absorption coefficient, with the processing of optimum selection. Besides that, the relationship between the coefficients of two bivectors is also considered. The results reveal that plenty of sound linear behaviors can be found and also suggest the scale of 15, 16 and frequency of 0.2, 0.21 THz are prominent for those linear behaviors. This work provides a new insight into the correlation between terahertz (THz) time and frequency domain information.

Terahertz metasurfaces have great applications for efficient terahertz modulation, but there are still problems in designing terahertz metadevices in terms of complexity and inefficiency. Herein, we demonstrate an inversely-designed terahertz metasurface with double electromagnetically induced transparency (EIT)-like windows by incorporating a particle swarm optimization (PSO) algorithm with the finite-difference time-domain method. We prepared and tested the metadevices, and the experimental terahertz signals are close to the designed results. By hybridizing amorphous germanium film with the inversely-designed metasurface, two EIT-like windows, including transmission and slow-light effect, exhibit ultrafast modulation behavior in 25 ps excited by a femtosecond laser. The modulation depths of transmission in two transparency windows are 74% and 65%, respectively. The numerical simulations also illustrate the ultrafast dynamic process and modulation mechanism, which match well with the experiment results. Our work thus offers opportunities for designing other objective functions of the terahertz metadevice.

An active ultrafast formation and modulation of dual-band plasmon-induced transparency (PIT) effect is theoretically and experimentally studied in a novel metaphotonic device operating in the terahertz regime, for the first time, to the best of our knowledge. Specifically, we designed and fabricated a triatomic metamaterial hybridized with silicon islands following a newly proposed modulating mechanism. In this mechanism, a localized surface plasmon resonance is induced by the broken symmetry of a C2 structure, acting as the quasi-dark mode. Excited by exterior laser pumps, the photo-induced carriers in silicon promote the quasi-dark mode, which shields the near-field coupling between the dark mode and bright mode supported by the triatomic metamaterial, leading to the dynamical modulation of terahertz waves from individual-band into dual-band PIT effects, with a decay constant of 493 ps. Moreover, a remarkable slow light effect occurs in the modulating process, accompanied by the dual-transparent windows. The dynamical switching technique of the dual-band PIT effect introduced in this work highlights the potential usefulness of this metaphotonic device in optical information processing and communication, including multi-frequency filtering, tunable sensors, and optical storage.

We experimentally investigate the linear polarization conversion for terahertz (THz) waves in liquid crystal (LC) integrated metamaterials, which consist of an LC layer sandwiched by two orthogonally arranged sub-wavelength metal gratings. A Fabry–Perot-like cavity is well constructed by the front and rear gratings, and it shows a strong local resonance mechanism, which greatly enhances the polarization conversion efficiency. Most importantly, the Fabry–Perot-like resonance can be actively tuned by modulating the refractive index of the middle LC layer under the external field. As a result, the integrated metamaterial achieves multi-band tunable linear polarization conversion.

Three-dimensional (3D) refractive index (RI) distribution is important to reveal the object’s inner structure. We implemented terahertz (THz) diffraction tomography with a continuous-wave single-frequency THz source for measuring 3D RI maps. The off-axis holographic interference configuration was employed to obtain the quantitative scattered field of the object under each rotation angle. The 3D reconstruction algorithm adopted the filtered backpropagation method, which can theoretically calculate the exact scattering potential from the measured scattered field. Based on the Rytov approximation, the 3D RI distribution of polystyrene foam spheres was achieved with high fidelity, which verified the feasibility of the proposed method.

This Letter proposes a novel method for enhancing terahertz (THz) radiation from microstructure photoconductive antennas (MSPCA). We present two types of MSPCA, which contain split-ring resonators (SRRs) and dipole photoconductive antennas (D-PCAs). The experimental results reveal that when the femtosecond laser is pumping onto the split position of the SRR, the maximum THz radiation power is enhanced by 92 times compared to pumping at the electrode edge of the D-PCA. Two π phase shifts occur as the pumping laser propagates from the negative electrode to the positive electrode. Analysis shows that photoinduced carrier charges move within the split position of the SRR.