Terahertz Sources Research Articles

Terahertz emission from diamond nitrogen-vacancy centers.

Coherent light sources emitting in the terahertz range are highly sought after for fundamental research and applications. Terahertz lasers rely on achieving population inversion. We demonstrate the generation of terahertz radiation using nitrogen-vacancy centers in a diamond single crystal. Population inversion is achieved through the Zeeman splitting of the S = 1 state in 15 tesla, resulting in a splitting of 0.42 terahertz, where the middle Sz = 0 sublevel is selectively pumped by visible light. To detect the terahertz radiation, we use a phase-sensitive terahertz setup, optimized for electron spin resonance (ESR) measurements. We determine the spin-lattice relaxation time up to 15 tesla using the light-induced ESR measurement, which shows the dominance of phonon-mediated relaxation and the high efficacy of the population inversion. The terahertz radiation is tunable by the magnetic field, thus these findings may lead to the next generation of tunable coherent terahertz sources.

Off-plane quartz-enhanced photoacoustic spectroscopy.

In this work, we developed off-plane quartz-enhanced photoacoustic spectroscopy (OP-QEPAS). In the OP-QEPAS the light beam went neither through the prong spacing of the quartz tuning fork (QTF) nor in the QTF plane. The light beam is in parallel with the QTF with an optimal distance, resulting in low background noise. A radial-cavity (RC) resonator was coupled with the QTF to enhance the photoacoustic signal by the radial resonance mode. By offsetting both the QTF and the laser position from the central axis, we enhance the effect of the acoustic radial resonance and prevent the noise generated by direct laser irradiation of the QTF. Compared to IP-QEPAS based on a bare QTF, the developed OP-QEPAS with a RC resonator showed a >10× signal-to-noise ratio (SNR) enhancement. The OP-QEPAS system has great advantages in the use of light emitting devices (LEDs), long-wavelength laser sources such as mid-infrared quantum cascade lasers, and terahertz sources. When employing a LED as the excitation source, the noise level was suppressed by ∼2 orders of magnitude. Furthermore, the radial and longitudinal resonance modes can be combined to further improve the sensor performance.

Edge-Terminated AlGaN/GaN/AlGaN Multi-Quantum Well Impact Avalanche Transit Time Sources for Terahertz Wave Generation

Edge-Terminated AlGaN/GaN/AlGaN Multi-Quantum Well Impact Avalanche Transit Time Sources for Terahertz Wave Generation

Easily variable and scalable terahertz pulse source based on tilted-pulse-front pumped semiconductors

A new type of terahertz source containing only two optical elements - a volume phase holographic grating, and a semiconductor nonlinear slab - is proposed. The setup does not require any microstructuring, has only one diffraction order, and can be scaled to large pump sizes without any principal limitations. Furthermore, it can be easily adapted to different pump wavelengths and THz phase-matching frequencies. The Fresnel loss at the boundary of the materials can be significant at conventional pump polarizations (s-pol), but a single-layer anti-reflection (AR) coating can reduce it. Pumping such a setup with polarization in the dispersion plane (p-pol, TM mode) can reduce the effective nonlinear polarization and consequently the terahertz generation efficiency. However, in the absence of AR coating, this reduction is overcompensated by the reduced Fresnel loss.

Terahertz wave radiation simulation in the Fe thin film*

Femtosecond laser (FL) induced terahertz (THz) source is a new type of THz source based on injecting FL beams into ferromagnetic thin films by nonlinear effects to generate THz wave. It has a wider bandwidth compared to the traditional THz source, which provides higher flexibility and tunability in the application. In this paper, the three-temperature model and the stochastic Landau Lifshitz Gilbert equation at the atomic level are applied to simulate THz wave generation in Fe thin film induced by FL. Simulation results show that under a FL irradiance of 2 J m−2, the maximum demagnetization of the Fe thin film reaches 8.7%. The electromagnetic waves generated completely cover the THz band (0.1–10 THz), which fully satisfied the application requirements of the THz technology, verifying the feasibility of FL inducing the Fe thin film as a THz source. However, when the Fe thin film is overheated, it will be difficult for FL to excite valuable THz waves. Therefore, additional cooling devices are needed to keep the THz source in a workable temperature state, or to use ferromagnetic materials with magnetic moments that can quickly recover to saturation.

Temperature-Dependent Spin-to-Charge Conversion and Efficient Manipulation of Elliptical THz Waves in Bi2Te3/TbFeCo Heterostructures.

Topological insulators (TIs) with spin-momentum-locked surface states and considerable spin-to-charge conversion (SCC) efficiency are ideal substitutes for the nonmagnetic layer in the traditional ferromagnetic/nonmagnetic (FM/NM) spintronic terahertz (THz) emitters. Here, the TI/ferrimagnetic structure as an effective polarization tunable THz source is verified by terahertz emission spectroscopy. The emitted THz electric field can be separated into two THz components utilizing their opposite symmetry on pump polarization and the magnetic field. TI not only emits a THz electric field via the linear photogalvanic effect (LPGE) but also serves as the medium of SCC via the inverse Edelstein effect (IEE) in the heterostructure. In addition, the amplitude and polarity of the SCC component can be efficiently manipulated by temperature in our ferrimagnetic TbFeCo layer compared with Co or Fe. Once these two THz components are delicately set orthogonally, an elliptical THz wave is generated by the intrinsic phase difference at the THz frequency range. The feasible control of its polarization and chirality is demonstrated by three means: pump polarization, magnetic field, and temperature. These appealing observations may pave the way for the development of elliptical THz wave emitters and polarization-sensitive THz spectroscopy.

Optical to terahertz frequency conversion using near zero-index nonlinear hybrid waveguides

Terahertz communications has emerged as an excellent candidate for the next generation of wireless communication networks. Despite of significant advances, one still looks for a high-efficiency terahertz source using nonlinear optics, such as optical refraction and difference-frequency generation. Concurrently, recent years have witnessed significant progress in metamaterials-assisted nonlinear optical processes. In this study, we propose a theoretical approach using near zero-index nonlinear hybrid waveguides to realize a high generation efficiency of terahertz waves via on-chip difference-frequency generation. Remarkably, thanks to the simultaneous realization of near zero-index both at terahertz and telecom frequencies, such a nonlinear hybrid waveguide can overcome two major bottlenecks in the conversion from optical wave to terahertz waves: phase mismatch and weak nonlinear optical-terahertz waves interactions, resulting in a high conversion as 2.2 × 10−3W−1 just within the propagation length of several terahertz wavelengths. Our results demonstrate the feasibility of an on-chip terahertz source, offering unique properties such as high integration and ultra-low power consumption for terahertz communication systems.

Millijoule Terahertz Radiation from Laser Wakefields in Nonuniform Plasmas.

We report the experimental measurement of millijoule terahertz (THz) radiation emitted in the backward direction from laser wakefields driven by a femtosecond laser pulse of few joules interacting with a gas target. By utilizing frequency-resolved energy measurement, it is found that the THz spectrum exhibits two peaks located at about 4.5 and 9.0THz, respectively. In particular, the high frequency component emerges when the drive laser energy exceeds 1.26J, at which electron acceleration in the forward direction is detected simultaneously. Theoretical analysis and particle-in-cell simulations indicate that the THz radiation is generated via mode conversion from the laser wakefields excited in plasma with an up-ramp profile, where radiations both at the local electron plasma frequency and its harmonics are produced. Such intense THz sources may find many applications in ultrafast science, e.g., manipulating the transient states of matter.

Terahertz nanoscopy: Advances, challenges, and the road ahead

Exploring nanoscale material properties through light-matter interactions is essential to unveil new phenomena and manipulate materials at the atomic level, paving the way for ground-breaking advancements in nanotechnology and materials science. Various elementary excitations and low-energy modes of materials reside in the terahertz (THz) range of the electromagnetic spectrum (0.1–10 THz) and occur over various spatial and temporal scales. However, due to the diffraction limit, a slew of THz studies are restricted to drawing conclusions from the spatially varying THz responses around half of the probing wavelengths, i.e., from tens to a couple of hundred micrometers. To address this fundamental challenge, scanning near-field optical microscopy (SNOM), notably scattering-type SNOM (s-SNOM), combined with THz sources has been employed and is fueling growing interest in this technique across multiple disciplines. This review (1) provides an overview of the system developments of SNOM, (2) evaluates current approaches to understand and quantify light-matter interactions, (3) explores advances in THz SNOM applications, especially studies with THz nano-scale spatial responses employing an s-SNOM, and (4) envisions future challenges and potential development avenues for the practical use of THz s-SNOM.

Terahertz reflective imaging systems with improved angle compatibility realized by the designed quasi-scattering screen

Active imaging can provide significantly larger signal margins in the terahertz (THz) spectral region than passive imaging. However, the imaging effect of THz active imaging is severely constrained by the orientation requirement of target reflection, which limits the application scenarios of this technique. In view of the shortcomings of existing THz reflective imaging systems, this paper proposes a new system optimization scheme from the aspect of hardware - a compact quasi-scattering screen (QSS) is proposed and designed as a beam-shaping element to eliminate the need for the best direction of specular reflection to the greatest extent. The corresponding relationship between the divergent Gaussian beam and target light field is directly established by using the equal-energy mapping method in non-imaging optics, which rejects the collimation process of the THz source. By customizing the optimization function in ZEMAX® to automatically optimize the surface of the QSS, the direction of incident light propagation can be adjusted. A non-coaxial reflective THz imaging system using a 0.1 THz continuous wave source demonstrates the ability of the QSS to improve the angle compatibility of the traditional THz reflective imaging system. The results show that the imaging angle compatibility range of the system can be increased to more than 5 times without changing the original imaging optical path and its resolution. In addition, it can also improve the imaging quality of the original imaging optical path for the object with a smooth surface or certain structure.

THz radiation distribution for the identification of infiltrating ductal carcinoma in human breast model: a computational study

With numerous biomedical applications of terahertz (THz) imaging, spectroscopy, and sensing, the THz regime of the spectrum is quickly emerging as an important area of research with the potential to usher in a new era in the healthcare industry. In the medical field, THz radiation has been explored to diagnose and monitor several ailments, including foot diabetes, skin dryness, wounds, and burns. Compared to X-rays, THz waves' phase sensitivity to bodily fluid levels and water results in superior contrast and stronger absorption. This feature makes THz potentially useful for the diagnosis of various malignant tumor forms because cancerous tumors have higher water molecule contents than healthy tissues. The current study presents a computational analysis of a CW-THz point-by-point scanning technique for breast cancer detection. COMSOL multiphysics platform was utilized to create a multilayer three-dimensional breast model containing an irregular-shaped infiltrating ductal carcinoma in the glandular layer. The propagation of the THz radiation in the constructed breast model was simulated based on the radio frequency module and electromagnetic waves in the frequency domain interface. Furthermore, different volumes and locations of the induced tumor were examined. The results demonstrated significant disparities in the acquired electric distribution at different tumor sizes and sites between normal and diseased breasts. The resultant electric field for healthy breast ranged from 1.1 × 10–5 to 0.61 V/m, however, the electric field ranges following tumor addition fluctuate based on its volume and location. Additionally, the position of the THz source and detector array influences the reflected THz signal.

Terahertz plasmonic resonances in coplanar graphene nanoribbon structures

We analyze plasmonic oscillations in the coplanar graphene nanoribbon (GNR) structures induced by the applied terahertz (THz) signals and calculate the GNR impedance. The plasmonic oscillations in the GNR structures are associated with the electron and hole inductances and the lateral inter-GNR capacitance. A relatively low inter-GNR capacitance enables the resonant excitation of the THz plasmonic oscillations in the GNR structures with long GNRs. The GNR structures under consideration can be used in different THz devices as the resonant structures incorporated in THz detectors, THz sources using resonant-tunneling diodes, photomixers, and surface acoustic wave sensors.

Efficient strong-field THz generation from DSTMS crystal pumped by 1030 nm Yb-laser

We report collinear strong-field terahertz (THz) generation from the organic crystal 4-N, N-dimethlamino-4-N-methyl-stilbazolium 2,4,6-trimethyl benzenesulfonate (DSTMS), driven by an industrial-grade ytterbium (Yb) laser operating at 1030 nm. The laser pulse duration is compressed from 800 to 90 fs by using a multi-pass-cell compressor. The output THz energy reaches 0.4 μJ, with a conversion efficiency of 0.22%. The spectrum covers over 0.1–6 THz with a peak electric field of 236 kV/cm. The phase matching condition and material absorption are illustrated. We demonstrate that a 1030 nm Yb-laser is used to pump organic crystal DSTMS for efficient THz generation in a simple collinear geometry, which provides a promising THz source based on an industrial-grade ultrafast laser.

Terahertz spectroscopy of thick and diluted water solutions.

While bright terahertz sources are used to perform nonlinear experiments, they can be advantageous for high-precision linear measurements of opaque samples. By placing the sample away from the focus, nonlinearities can be suppressed, and sizeable amounts of transmitted radiation detected. Here, this approach is demonstrated for a 0.5 mm thick layer of liquid water in a static sample holder. Variations of the index of refraction as small as (7 ± 2) · 10-4 were detected at 0.58 THz for an aqueous salt solution containing ten millimoles of sodium chloride. To my knowledge, this precision is unprecedented in time-domain spectroscopy studies of diluted aqueous systems or other optically thick and opaque materials.

Multifield-Modulated Spintronic Terahertz Emitter Based on a Vanadium Dioxide Phase Transition.

The efficient generation and active modulation of terahertz (THz) waves are strongly required for the development of various THz applications such as THz imaging/spectroscopy and THz communication. In addition, due to the increasing degree of integration for the THz optoelectronic devices, miniaturizing the complex THz system into a compact unit is also important and necessary. Today, integrating the THz source with the modulator to develop a powerful, easy-to-adjust, and scalable or on-chip THz emitter is still a challenge. As a new type of THz emitter, a spintronic THz emitter has attracted a great deal of attention due to its advantages of high efficiency, ultrawide band, low cost, and easy integration. In this study, we have proposed a multifield-modulated spintronic THz emitter based on the VO2/Ni/Pt multilayer film structure with a wide band region of 0-3 THz. Because of the pronounced phase transition of the integrated VO2 layer, the fabricated THz emitter can be efficiently modulated via thermal or electric stimuli with a modulation depth of about one order of magnitude; the modulation depths under thermal stimulation and electrical stimulation were 91.8% and 97.3%, respectively. It is believed that this multifield modulated spintronic THz emitter will provide various possibilities for the integration of next-generation on-chip THz sources and THz modulators.

Parasitic mixing in photomixers as continuous wave terahertz sources

We present observations of parasitic frequency components in the emission spectrum of typical photomixer sources for continuous wave (CW) terahertz generation. Broadband tunable photomixer systems are often used in combination with direct power detectors, e.g., for source and/or detector characterization. Here, spectral components besides the intended terahertz emission at the difference frequency of the two excitation lasers can significantly distort the measurement results. In this work, the appearance of parasitic mixing signals is observed in broadband measurements with a broadband antenna-coupled field-effect transistor as terahertz detector (TeraFET). The measurements reveal weaker spectral absorption features than expected and also a signal plateau towards higher frequencies, both strongly indicating a background in the detection signals. The photomixer emission is investigated in detail with a terahertz Fourier-transform infrared spectrometer (FTIR). We relate the observed parasitic frequency components with good quantitative agreement with the mode spectra of the semiconductor lasers. We also present one possible approach to overcome some of the issues, and we emphasize the importance of our findings to avoid distorted measurement results. To our knowledge, the essential aspect of parasitic mixing has so far been largely ignored in the literature where terahertz CW photomixer emitters are widely used for spectrally resolved measurements.

High-efficiency and frequency-controllable terahertz pulses driven by two-color relativistic laser pulses

The two-color laser filamentation technique serves as a high-quality, laser-based strong-field terahertz source. However, the terahertz energy obtained from this method tends to saturate at high laser intensity, limiting its ability to fully exploit the advantage of the extremely strong field offered by relativistic lasers. Here, we proposed a novel approach based on photon deceleration in plasma to obtain controllable terahertz radiations with two-color relativistic femtosecond laser pulses. In our method, an 800-nm laser drives the plasma wave and controls the plasma density distribution, while the other 10.6-μm laser experiences a strong frequency downshift in the plasma wave and converts to a multi-cycle terahertz pulse, whose central frequency is adjustable by altering the parameters of laser and plasma. The conversion efficiency from the 10.6-μm laser to terahertz radiation can be over 12%, potentially unlocking novel applications in the terahertz domain.

Actively switchable spintronic terahertz emission with arbitrary polarization states

Flexible manipulation of the polarization state is essential for the practical application of terahertz (THz) waves in many fields. However, the lack of effective, high-quality polarization-tunable THz sources hinders the further development of THz technology. Here, we demonstrate an actively switchable spintronic THz source with polarization states among linear, circular, and elliptical states in the CoFeB/Pt/SiO2(sub)/Ta/Co/IrMn structure by using the antiferromagnet/ferromagnet exchange bias effect, as well as a temporary magnetic field to combine the magnetization directions, and designing the thickness of the substrate to achieve a suitable phase difference. In addition, the chirality, ellipticity, and azimuth of the THz waves can be manipulated arbitrarily by controlling the magnetization combination. More importantly, using rotating motors and electromagnets enables fully automated operations. This highly efficient, polarization-tunable THz source meets most of the existing needs, and its low cost and small size make it more suitable for integration into various devices. It paves the way for accelerating THz spintronic devices and unveiling mechanisms in condensed matter physics.

Generation of THz radiation through molecular modulation in hydrogen-filled hybrid anti-resonant fibers.

We study the generation of narrowband terahertz (THz) pulses by stimulated Raman scattering and molecular modulation in hydrogen-filled hybrid hollow-core fibers. Using a judicious combination of materials and transverse structures, this waveguide design enables simultaneous confinement of optical and THz signals with reasonably low attenuation, as well as high nonlinear overlap. The THz pulses are then generated as the second Stokes band of a ns-long near-infrared pump pulse, aided by Raman coherence waves excited in the gaseous core by the beat-note created by the pump and its first Stokes band. Optimization of the fiber characteristics facilitates phase matching between the corresponding transitions and coherence waves while avoiding coherent gain suppression, resulting in potential optical-to-THz conversion efficiencies up to 60%, as confirmed by rigorous numerical modelling under ideal zero-loss conditions. When the current optical material constraints are considered, however, the attainable efficiencies relax to 0.2%, a still competitive value compared to other systems. The approach is in principle power and energy scalable, as well as tunable in the 1-10 THz range without any spectral gaps, thereby opening new pathways to the development of fiber-based THz sources complementary to other mature technologies such as quantum cascade lasers.

Linear and phase controllable terahertz frequency conversion via ultrafast breaking the bond of a meta-molecule

The metasurface platform with time-varying characteristics has emerged as a promising avenue for exploring exotic physics associated with Floquet materials and for designing photonic devices like linear frequency converters. However, the limited availability of materials with ultrafast responses hinders their applications in the terahertz range. Here we present a time-varying metasurface comprising an array of superconductor-metal hybrid meta-molecules. Each meta-molecule consists of two meta-atoms that are “bonded” together by double superconducting microbridges. Through experimental investigations, we demonstrate high-efficiency linear terahertz frequency conversion by rapidly breaking the bond using a coherent ultrashort terahertz pump pulse. The frequency and relative phase of the converted wave exhibit strong dependence on the pump-probe delay, indicating phase controllable wave conversion. The dynamics of the meta-molecules during the frequency conversion process are comprehensively understood using a time-varying coupled mode model. This research not only opens up new possibilities for developing innovative terahertz sources but also provides opportunities for exploring topological dynamics and Floquet physics within metasurfaces.