Broadband Terahertz Source Research Articles

A Chirped Characteristic-Tunable Terahertz Source for Terahertz Sensing.

In broadband terahertz waves generated by femtosecond lasers, spatial chirp will be simultaneously produced with the introduction of angular dispersion. The chirp characteristics of the terahertz wave will directly affect the frequency response, bandwidth response, and intensity response of the terahertz sensor. To enhance the capability of terahertz sensors, it is necessary to control and improve the chirped characteristics of broadband terahertz sources. We generate a chirped terahertz wave via optical rectification in a LiNbO3 prism using the technique of pulse front tilt. The effect of the pump-beam spot size on THz generation is systematically studied. The pump's spot size is manipulated using a telescope system. With a pump spot diameter of 1.8 mm, the scanning spectrum of the THz pulse is narrower and is divided into multiple distinct peaks. In contrast, using a pump spot diameter of 3.7 mm leads to increased efficiency in the generation of THz pulses. Also, we investigate the underlying properties governing the generation of chirped terahertz pulses using varying pump pulse spot diameters.

Backward THz Emission from Two-Color Laser Field-Induced Air Plasma Filament.

Two-color laser field-induced plasma filaments are efficient broadband terahertz (THz) sources with intense THz waves emitted mainly in the forward direction, and they have been investigated intensively. However, investigations on the backward emission from such THz sources are rather rare. In this paper, we theoretically and experimentally investigate the backward THz wave radiation from a two-color laser field-induced plasma filament. In theory, a linear dipole array model predicts that the proportion of the backward emitted THz wave decreases with the length of the plasma filament. In our experiment, we obtain the typical waveform and spectrum of the backward THz radiation from a plasma with a length of about 5 mm. The dependence of the peak THz electric field on the pump laser pulse energy indicates that the THz generation processes of the forward and backward THz waves are essentially the same. As the laser pulse energy changes, there is a peak timing shift in the THz waveform, implying a plasma position change caused by the nonlinear-focusing effect. Our demonstration may find applications in THz imaging and remote sensing. This work also contributes to a better understanding of the THz emission process from two-color laser-induced plasma filaments.

Tunable High‐Field Terahertz Radiation from Plasma Channels

AbstractTunable broadband terahertz (THz) sources with power at the gigawatt level are desirable for many applications. A scheme to generate such THz emission by off‐axially injecting a weakly relativistic ultrashort laser into a parabolic plasma channel is presented. By utilizing two‐dimensional particle‐in‐cell simulation, it is demonstrated that there are two major physical mechanisms involved, i.e., linear mode conversion from laser wakefields and the electromagnetic waveguide mode excitation inside the plasma channel. The two radiation modes can lead to linearly polarized and radially polarized THz emissions, respectively, with distinct frequency spectra and spatial distributions. It is found that they predominate alternatively with the change of the plasma channel length. For a given plasma channel, one can switch the radiation modes by adjusting the injection position and the injection angle of the laser pulse. In particular, the radiation mode of the linear mode conversion can produce THz pulses with the peak amplitude of sub‐GV cm−1 with the energy conversion efficiency ≈10−3, even though the peak power of the incident laser is just at the terawatt level. The scheme provides a powerful THz source with tunable intensity, spatial distributions, spectra, and polarizations by simply adjusting the injection conditions of incident laser pulses.

Broadband Spintronic Terahertz Source with Peak Electric Fields Exceeding 1.5 MV/cm

Spintronic terahertz emitters (STEs) are desirable broadband terahertz sources, but their limited signal strength has hindered practical application. By optimizing the photonic and thermal environment, the authors present an STE that could overcome this obstacle. Benchmarking against the state-of-the-art terahertz emitters based on optical rectification, this STE delivers strong terahertz pulses with comparable peak electric field and fluence, and offers additional features such as broadband radiation, easy alignment, and rotation of the terahertz polarization plane without power loss. This work will open up a promising pathway to nonlinear terahertz spectroscopy with spintronic sources.

A Biomedical Perspective in Terahertz Nano-Communications—A Review

Terahertz (THz)-band (0.1~10 THz) communications are celebrated to be a crucial enabling technology for sixth generation (6G) wireless systems that fulfil the stringent requirements of future healthcare scenarios. Broadband THz sources have drawn the attention of researchers by virtue of their prominent detectability and their noninvasive and non-ionization properties. More importantly, the most advanced wideband THz sources enable THz communication in vivo with many appealing properties, including potential link capacities (terabit-per-second), miniature transceivers, and high energy efficiency. Compared to conventional devices, nano-scale devices will be potentially pivotal in subsequent medical diagnostics and treatment technologies, by virtue of the non-ionizing property of THz light and its high sensitivity in conveniently reaching delicate body sites. Thereby, real-time, label-free detection methods are expected to perform a crucial effect in clinical practice; however, difficulties such as unknown biological safety still need to be overcome. With channel modeling progress of the THz frequencies, we have considered both the radiation of the medium and the molecular absorption from the transmitted signal, and envisoned the possibility to solve the challenges such as the spectrum scarcity and capacity limitation.

Quasi‐BIC Enhanced Broadband Terahertz Generation in All‐Dielectric Metasurface

AbstractTerahertz waves have widespread applications in various fields ranging from wireless communication to fundamental physics. Achieving bright and broadband terahertz sources still remains a challenge. Recent research has revealed that all‐dielectric metasurfaces with strong electromagnetic resonances and negligible Ohmic loss can enhance nonlinear optical processes and weaken the influences of phase‐matching conditions, providing new opportunities for developing terahertz sources. Here, an all‐dielectric metasurface supporting quasi‐BIC (bound states in the continuum), which enhances terahertz emission in a nonlinear optical film under the excitation of femtosecond laser pulses, is proposed. It is observed that the terahertz emission in lithium niobate (LiNbO3) films is enhanced up to 17 times by a silicon dioxide metasurface and that the terahertz emission enhancement spans the frequency range in 0.1–4.5 THz. This mechanism for the terahertz emission enhancement is attributed to the tight confinement of the pump field in the LNibO3 which is caused by quasi‐BIC resonances. The all‐dielectric metasurface is patterned on the LiNbO3 film instead of etching it, providing a simple way for engineering nonlinear optical processes in LiNbO3. This work paves the way for broadband terahertz emitters.

Air-photonics terahertz platform with versatile micro-controller based interface and data acquisition.

We present a terahertz (THz) platform employing air plasma produced by an ultrashort two-color laser pulse as a broadband THz source and air biased coherent detection (ABCD) of the THz field. In contrast to previous studies, a simple peak detector connected to a micro-controller board acquires the ABCD-signal coming from the avalanche photodiode. Numerical simulations of the whole setup yield temporal and spectral profiles of the terahertz electric field in both source and detection area. The latter ones are in excellent agreement with our measurements, confirming THz electric fields with peak amplitude in the MV/cm range. We further illustrate the capabilities of the platform by performing THz spectroscopy of water vapor and a polystyrene reference sample.

Lateral terahertz wave emission from laser induced plasma in liquid water line

Laser ionized liquid has been considered as a promising broadband terahertz (THz) source recently. The ponderomotive force induced dipole model can be used to describe the mechanism of THz generation, implying that the lateral THz emission is stronger than the forward emission. Thus, we systemically investigate the lateral THz emission from laser induced plasma in liquid water line. THz energy about 4.4-times of that from the forward emission is achieved from the lateral emission scheme. Results on the angular distribution of coherent THz radiation indicate that the maximal THz radiation falls between 120° and 150° with respect to the pump laser propagation direction. Moreover, dependence of collected THz energy on the relative position between laser focus and the water line shows a counterintuitive “two peaks” trend, which can be explained by the impact of cylindrical water surface on the divergence angle of the generated THz wave. Therefore, the enhancement in THz energy can be attributed to the high-efficiency lateral collection scheme and the tradeoff between the THz wave converging effect of the cylindrical surface and the absorption in water. Our work helps to further optimize the coupling efficiency of generated THz wave out of liquid samples.

Enabling high-power, broadband THz generation with 800-nm pump wavelength.

The organic terahertz (THz) generation crystal BNA has recently gained traction as a source for producing broadband THz pulses. When pumped with 100 fs pulses, the thin BNA crystals can produce relatively high electric fields with frequency components out to 5 THz. However, the THz output with 800-nm pump wavelength is limited by the damage threshold of the material, particularly when using a 1 kHz or higher repetition rate laser. Here, we report that the damage threshold of BNA THz generation crystals can be significantly improved by bonding BNA to a high-thermal conductivity sapphire window. When pumped with 800-nm light from an amplified Ti:sapphire laser system, this higher damage threshold enables generation of 2.5× higher electric field strengths compared to bare BNA crystals. We characterize the average damage threshold for bare BNA and BNA-sapphire, measure peak-to-peak electric field strengths and THz waveforms, and determine the nonlinear transmission in BNA. Pumping BNA bonded to sapphire with 3 mJ 800-nm pulses results in peak-to-peak electric fields exceeding 1 MV/cm, with broadband frequency components >3 THz. This high-field, broadband THz source is a promising alternative to tilted pulse front LiNbO3 THz sources, enabling many research groups without optical parametric amplifiers to perform high-field, broadband THz spectroscopy.

Intra-oscillator broadband THz generation in a compact ultrafast diode-pumped solid-state laser.

We demonstrate broadband and powerful terahertz (THz) generation at megahertz repetition rate based on intra-oscillator optical rectification (OR) in gallium phosphide (GaP). By placing the nonlinear crystal directly inside the cavity of a Kerr-lens mode-locked ultrafast diode-pumped solid-state laser (DPSSL) oscillator, we demonstrate a compact and single-stage THz source. Using only 7 W of diode-pump power, we drive OR in a GaP crystal with 22 W of average power at ∼80 MHz repetition rate. In a first configuration, using a 0.3-mm-thick GaP and 105 fs driving pulses, we generate up to 150 µW of THz radiation with a spectrum extending to 5.5 THz. In a second configuration allowing for sub-50-fs pulse duration, we generate up to 7 THz inside a 0.1-mm-thick GaP crystal. This performance is well suited for THz time-domain spectroscopy and THz imaging. Intra-oscillator THz generation in sub-100-fs DPSSLs is a promising way to scale down footprint, complexity and cost of powerful broadband THz sources.

Temperature-Insensitive Imaging Properties of a Broadband Terahertz Nonlinear Quantum Cascade Laser

Terahertz (THz) quantum cascade laser sources based on optical nonlinearity are the only electrically pumped monolithic semiconductor sources operable at room temperature in the 0.6–6 THz range. We investigated the temperature dependence of the imaging characteristics of a broadband THz nonlinear quantum cascade laser and evaluated several important properties: the spectrum, far-field pattern and THz imaging results. Consequently, we found that the far-field patterns were single-lobed Gaussian-like, and THz images were well-resolved despite the lower operating temperature of the device. The stable temperature-performance indicates that this broadband THz source is promising for THz imaging applications.

Terahertz wave propagation and imaging detection characteristics in plasma

In this paper, the theoretical model of ununiform plasma sheath is established based on scattering matrix method and the transmission characteristics of 0.1 THz~10 THz wave are simulated. A kind of plasma jet is produced in laboratory environment according to the principle of dielectric barrier discharge. Then the measurement of transmission spectrum of terahertz time-domain spectroscopy (THz-TDS), broadband terahertz source, and the terahertz wave reflective imaging of target under plasma shelter are carried out, respectively. Both theory and experiment results show that terahertz wave has good penetration in plasma, which provides a new way for communication and radar detection in blackout area.

MHz-repetition-rate, sub-mW, multi-octave THz wave generation in HMQ-TMS

We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10-4 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm-2 at 10 MHz with a 1/e2 pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm-1 and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications.

Terahertz emitters based on ultrafast spin-to-charge conversion

Terahertz technology shows great potential applications in imaging, sensing and security. As is well known, the conventional solid-state broadband terahertz sources rely primarily on the nonlinear optical crystals and photoconductive antennas. Therefore, one major challenge for the next generation of terahertz technology is to develop the high-efficient, ultra-broadband and low-cost terahertz sources. In recent years, much attention has been paid to the spintronic terahertz emitters made of the metallic magnetic heterostructures on a nanometer scale. In this paper, the underlying physical mechanisms associated with this type of terahertz emitter is discussed. They mainly include the ultrafast demagnetization and the spin-charge interconversion processes. In order to further improve the terahertz emission efficiency, three main aspects are considered: appropriate choice of the materials (including conditions of the sample growing), film thickness, and new structure design. In the end, a short conclusion and future perspective for this research direction are given briefly.

Generation and manipulation of chiral broadband terahertz waves from cascade spintronic terahertz emitters

Polarization shaped terahertz sources play a key factor in terahertz wireless communications, biological sensing, imaging, coherent control in fundamental sciences, and so on. Recently developed spintronic terahertz emitters have been considered as one of the next-generation promising high performance broadband terahertz sources. However, until now, polarization control, especially for twisting the circularly polarized terahertz waves at the spintronic terahertz source, has not yet been systematically explored and experimentally achieved. In this work, we not only demonstrate the generation of circularly polarized terahertz waves in cascade spintronic terahertz emitters via delicately engineering the amplitudes, applied magnetic field directions, and phase differences in two-stage terahertz beams but also implement the manipulation of the chirality, azimuthal angle, and ellipticity of the radiated broadband terahertz waves. We believe our work can help with further understanding of the ultrafast optical magnetic physics and may have valuable contributions for developing advance terahertz sources and optospintronic devices.

Broadband and narrowband laser-based terahertz source and its application for resonant and non-resonant excitation of antiferromagnetic modes in NiO.

A versatile table-top high-intense source of terahertz radiation, enabling to generate pulses of both broadband and narrowband spectra with a tunable frequency up to 3 THz is presented. The terahertz radiation pulses are generated by optical rectification of femtosecond pulses of Cr:forsterite laser setup in nonlinear organic crystal OH1. Electric field strengths of broadband and narrowband terahertz pulses were achieved close to 20 MV/cm and more than 2 MV/cm, correspondingly. Experiments on excitation of spin subsystem oscillations of an antiferromagnetic NiO were carried out. Selective excitation of 0.42 THz mode was observed for the first time at room temperature by a narrowband terahertz pulses tuned close to mode frequency.

2D Materials for Terahertz Modulation

AbstractTerahertz waves spanning over the 0.1 to 10 THz region of the electromagnetic spectrum have attracted significant attention owing to a variety of potential applications such as short‐range high‐speed data transmission, noninvasive screening and detection, materials characterization, spectroscopy, etc. This has resulted in massive strides in the development of essential system components such as broadband terahertz sources, detector arrays with high responsivity, as well as modulators. In parallel to this, spurred by the isolation of graphene in 2004, a tremendous interest in 2D systems has led to the rapid exploration and development of a library of atomically thin materials. These can exhibit a myriad of electrical and optical functionalities stemming from semiconducting, insulating, semi‐metallic, or superconducting behavior. In this context, since the early 2010s, 2D materials have been actively explored for active control of terahertz electromagnetic radiation. This paper aims to provide a concise overview of the pioneering efforts as well as the latest progress in these two overlapping research areas. In particular, the discussion is focused on the application of graphene and transition metal dichalcogenides in optically and electrically actuated terahertz amplitude and phase modulators. Furthermore, it provides an outlook on the technological prospects and challenges in these devices.

Broadband Spintronic Terahertz Emitter with Magnetic‐Field Manipulated Polarizations

AbstractFlexible manipulation of terahertz wave polarizations during the generation process is very important for terahertz applications, especially for the next‐generation on‐chip functional terahertz sources. However, current terahertz emitters cannot satisfy such demand, hence calling for new mechanisms and conceptually new terahertz sources. Here an efficient and broadband terahertz source with magnetic‐field‐controlled flexible switching for the polarizations between linear and elliptical states in ferromagnetic heterostructures driven by femtosecond laser pulses is demonstrated. More importantly, the chirality, azimuthal angle, and ellipticity of the generated elliptical terahertz wave can be precisely manipulated by harnessing external magnetic fields via effectively tailoring the photoinduced spin currents. Such an ultrafast photomagnetic interaction‐based, magnetic‐field‐controlled, and broadband tunable solid‐state terahertz source integrated with polarization tunability functions not only provides the capability to reveal physical mechanisms of femtosecond spin dynamics, but also demonstrates the feasibility to realize novel on‐chip terahertz functional devices, boosting the potential applications for controlling elementary molecular rotations, phonon vibrations, spin precessions, high‐speed communications, and accelerating the development of ultrafast terahertz optospintronics.

Effect of terahertz pulse on gene expression in human eye cells**Project supported by the National Natural Science Foundation of China (Grant Nos. 61675151 and 81570872), the Tianjin Municipal Science and Technology Commission Grants, China (Grant No. 15JCYBJC24900), and the Clinical Research Foundation of Tianjin Medical University Eye Institute, China (Grant No. 16YKJS002).

In recent years, the advances in terahertz applications have stimulated interest in the biological effects associated with this frequency range. We study the gene expression profile in three types of cells exposed to terahertz radiation, i.e., human ARPE-19 retinal pigment epithelial cells, simian virus 40-transformed human corneal epithelial cells, and human MIO-M1 Müller cells. We find that the gene expression in response to heat shock is unaffected, indicating that the minimum temperature increases under controlled environment. The transcriptome sequencing survey demonstrates that 6-hour irradiation with a broadband terahertz source results in specific change in gene expression and also the biological functions that are closely related to these genes. Our results imply that the effect of terahertz radiation on gene expression can last over 15 hours and depends on the type of cell.

Spectral Characterization of a Microbolometer Focal Plane Array at Terahertz Frequencies

We have developed a method to characterize the spectral response of an uncooled microbolometer focal plane array at a broad range of terahertz (THz) frequencies (4–50 THz). This is achieved by using a spectrum-shaped blackbody radiator as a broadband THz source and measuring its spectral power with a Fourier transform infrared interferometer. With an additional measurement with a pyroelectric detector as a reference, the spectral response of the microbolometer relative to the pyroelectric reference is obtained with a signal-to-noise ratio of 100 over a $>$ 50-THz bandwidth.