Narrowband Terahertz Radiation Research Articles

Wavelength measurement of narrowband terahertz radiation using a diffraction grating

We propose a reflective binary amplitude grating-based setup for determining the frequency of narrowband terahertz radiation. Theoretical analysis was conducted on transmissive and reflective configurations utilizing binary and phase gratings. Simulations were performed to determine the parameters of metallic diffraction gratings with millimeter-scale constants, which were produced using laser cutting techniques. Our implementation consists of a cost effective and easy-to-implement setup that operates within the 0.15–1 THz range. The grating is installed on a rotary stage. The experimental results obtained align closely with the theoretical calculations. By understanding the setup parameters and the angle of the 1st diffraction order, we could calculate the frequency of the unknown radiation. The relative error in determining the frequency was less than 2%.

Design and Beam Dynamic Studies of an Injector for a Compact THz Coherent Radiation Source

An intense narrow-band terahertz (THz) radiation source has been designed to generate a broad tuning range of radiation frequencies between 0.5 THz and 5.0 THz. The THz radiation is produced when a short-bunch electron beam propagates through an undulator. To achieve high-power peak radiation, the source requires high-brightness electron beams with low beam emittance and short bunch length. A proposed design for the photocathode RF gun used as the electron source is presented. The gun with high mode separation and high Q-factor can be achieved for producing a good beam quality. The beam dynamics of the injector have been preliminarily optimized using the software ASTRA and Elegant, investigating the impact of laser pulse shape on electron beam quality. The results of the beam dynamics studies are comprehensively discussed in this paper.

Coherent generation and control of tunable narrowband THz radiation from a laser-induced air-plasma filament.

We report on the proof-of-principle experiment of generating carrier-envelope phase (CEP)-controllable and frequency-tunable narrowband terahertz (THz) radiation from an air-plasma filament prescribed by the beat of a temporally stretched two-color laser pulse sequence. The pulse sequence was prepared by propagating the fundamental ultrafast laser pulse through a grating stretcher and Michelson interferometer with variable inter-arm delay. By partially frequency-doubling and focusing the pulse sequence, an air-plasma filament riding a beat note was created to radiate a THz wave with primary pulse characteristics (center frequency and CEP) under coherent control. To reproduce experimental results and elucidate complex nonlinear light-matter interaction, numerical simulation has been performed. This work demonstrates the feasibility of generating coherently controlled narrowband THz wave with high tunability in laser-induced air plasma.

Observation of a massive phason in a charge-density-wave insulator.

The lowest-lying fundamental excitation of an incommensurate charge-density-wave material is believed to be a massless phason-a collective modulation of the phase of the charge-density-wave order parameter. However, long-range Coulomb interactions should push the phason energy up to the plasma energy of the charge-density-wave condensate, resulting in a massive phason and fully gapped spectrum1. Using time-domain terahertz emission spectroscopy, we investigate this issue in (TaSe4)2I, a quasi-one-dimensional charge-density-wave insulator. On transient photoexcitation at low temperatures, we find the material strikingly emits coherent, narrowband terahertz radiation. The frequency, polarization and temperature dependences of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interactions. Our observations underscore the role of long-range interactions in determining the nature of collective excitations in materials with modulated charge or spin order.

Generating High-Power, Frequency Tunable Coherent THz Pulse in an X-ray Free-Electron Laser for THz Pump and X-ray Probe Experiments

Precisely synchronized X-ray and strong-field coherent terahertz (THz) enable the coherent THz excitation of many fundamental modes (THz pump) and the capturing of X-ray dynamic images of matter (X-ray probe), while the generation of such a light source is still a challenge for most existing techniques. In this paper, a novel X-ray free-electron laser based light source is proposed to produce a synchronized high-powered X-ray pulse and strong field, widely frequency tunable coherent THz pulse simultaneously. The technique adopts a frequency beating laser modulated electron bunch with a Giga-electron-volt beam energy to generate an X-ray pulse and a THz pulse sequentially by passing two individual undulator sections with different magnetic periods. Theoretical analysis and numerical simulations are carried out using the beam parameters of the Shanghai soft X-ray free-electron laser facility. The results show that the technique can generate synchronized 4 nm X-ray radiation with a peak power of 1.89 GW, and narrow-band THz radiation with a pulse energy of 1.62 mJ, and the frequency of THz radiation can be continuously tuned from 0.1 to 40 THz. The proposed technique can be used for THz pump and X-ray probe experiments for dynamic research on the interaction between THz pulse and matter at a femtosecond time scale.

Widely tunable electron bunch trains for the generation of high-power narrowband 1–10 THz radiation

Terahertz radiation enables the coherent excitation of many fundamental modes, such as molecular rotation, lattice vibration and spin precession. To excite the transient state of matter far out of equilibrium, high-power and tunable narrowband terahertz radiation sources have been in great demand for years. However, the terahertz source available at present cannot meet these tunability and power demands, leaving a large scientific gap that has yet to be fully explored. Here we convert the energy modulation induced by the nonlinear longitudinal space charge force to a density modulation and experimentally demonstrate the generation of electron bunch trains with modulation frequencies that are adjustable between 1 and 10 THz. The electron bunch trains can be directly used to produce tunable high-power narrowband terahertz radiation that fully covers the long-standing ‘terahertz gap’, which will open up many more possibilities in terahertz science. A tunable terahertz radiation pulse is demonstrated based on a linear accelerator. The emission frequency of this terahertz radiation is tunable from 1 to 10 THz by changing the bunching frequency of a 34 MeV electron beam. The pulse energy is at the submillijoule level.

Perspective on Terahertz Applications of Molecular Crystals

In this review, we present a survey on the use of molecular nonlinear crystals in the context of terahertz (THz) photonics. The fundamentals of nonlinear optics for converting optical and infrared radiation into THz radiation with the basic theory of femtosecond optical rectification and difference frequency generation are described. Various types of phase-matching conditions that can be achieved in molecular crystals are discussed. It is shown that one of the unique features of molecular crystals is the ability to generate tunable narrowband terahertz radiation using femtosecond lasers. We also provide a detailed description of the most commonly used and promising molecular crystals such as DAST, DSTMS, OH1, HMQ-TMS, DCMBI, and GUHP. This review also presents a description of recent publications which show the prospects of using molecular nonlinear optical crystals in THz photonics.

High power terahertz radiation generated by beam-plasma system in multi-filament regime

In the particle-in-cell simulation study of this paper, it is found that the high-power terahertz radiation at twice the plasma frequency can be generated by the interaction between a weakly relativistic, large-radius, focused electron beam and an overdense plasma. The terahertz radiation is emitted from a multi-filament structure in the electron beam, which is caused by the coupling of the longitudinal two-stream instability and the transverse filamentation instability. The analysis of the k space of the plasma waves indicates that the radiation can be explained by the mode coupling among a forward oblique mode, a backward oblique mode, and a second harmonic radiation mode. The influences of the beam Lorentz factor and plasma density on the radiation power, efficiency, and coherence are also investigated. The present results not only contribute to the development of novel megawatt-level narrowband terahertz radiation sources, but also propose a potential terahertz radiation mechanism for the fundamental research of the beam-plasma system.

Highly efficient generation of narrowband terahertz radiation driven by a two-spectral-line laser in PPLN.

We demonstrate record ∼0.9% efficiencies for optical conversion to narrowband (<1% relative bandwidth) terahertz (THz) radiation by strongly cascaded difference frequency generation. These results are achieved using a novel, to the best of our knowledge, laser source, customized for high efficiencies, with two narrow spectral lines of variable separation and pulse duration (≥250 ps). THz radiation generation in 5% MgO-doped periodically poled lithium niobate (PPLN) crystals of varying poling period was explored at cryogenic and room temperature operation as well as with different crystal lengths. This work addresses an increasing demand for high-field THz radiation pulses which has, up to now, been largely limited by low optical-to-THz radiation conversion efficiencies.

A monoclinic semiorganic molecular crystal GUHP for terahertz photonics and optoelectronics

In this paper we describe the properties of the crystal of guanylurea hydrogen phosphite (NH_2)_2CNHCO(NH_2)H_2PO_3 (GUHP) and propose its application in terahertz photonics and optoelectronics. GUHP crystal has a wide window of transparency and a high optical threshold in the visible and NIR spectral regions and narrow absorption bands in the terahertz frequency range. The spectral characteristics of absorption and refraction in the THz range were found to be strongly dependent on crystal temperature and orientation. Computer simulations made it possible to link the nature of the resonant response of the medium at THz frequencies with the molecular structure of the crystal, in particular, with intermolecular hydrogen bonds and the layered structure of the lattice. The possibility of application of the crystal under study for the conversion of femtosecond laser radiation from visible an NIR to terahertz range was demonstrated. It was shown that dispersion properties of the crystal allow the generation of narrow band terahertz radiation, whose spectral properties are determined by conditions close to phase matching. The properties of the generated terahertz radiation under various temperatures suggest the possibility of phonon mechanism of enhancement for nonlinear susceptibility of the second order.

A Narrowband Spintronic Terahertz Emitter Based on Magnetoelastic Heterostructures.

Narrowband terahertz (THz) radiation is crucial for high-resolution spectral identification, but a narrowband THz source driven by a femtosecond (fs) laser has remained scarce. Here, it is computationally predicted that a metal/dielectric/magnetoelastic heterostructure enables converting a fs laser pulse into a multicycle THz pulse with a narrow linewidth down to ∼1.5 GHz, which is in contrast to the single-cycle, broadband THz pulse from the existing fs-laser-excited emitters. It is shown that such narrowband THz pulse originates from the excitation and long-distance transport of THz spin waves in the magnetoelastic film, which can be enabled by a short strain pulse obtained from fs laser irradiation of the metal film when the thicknesses of the metal and magnetoelastic films both fall into a specific range. These results therefore reveal an approach to achieving optical generation of narrowband THz pulse based on heterostructure design, which also has implications in the design of THz magnonic devices.

Particle-in-Cell Simulations of High-Power THz Generator Based on the Collision of Strongly Focused Relativistic Electron Beams in Plasma

Based on particle-in-cell simulations, we propose to generate sub-nanosecond pulses of narrowband terahertz radiation with tens of MW power using unique properties of kiloampere relativistic (2 MeV) electron beams produced by linear induction accelerators. Due to small emittance of such beams, they can be focused into millimeter and sub-millimeter spots comparable in sizes with the wavelength of THz radiation. If such a beam is injected into a plasma, it becomes unstable against the two-stream instability and excites plasma oscillations that can be converted to electromagnetic waves at the plasma frequency and its harmonics. It is shown that several radiation mechanisms with high efficiency of power conversion (∼1%) come into play when the radial size of the beam–plasma system becomes comparable with the wavelength of the emitted waves.

Generation of Tunable 10-mJ-Level Terahertz Pulses through Nonlinear Plasma Wakefield Modulation

High-density electron-bunch train--based tunable coherent terahertz radiation with a narrow spectral bandwidth is highly essential for many scientific applications. Here we propose a scheme to produce ultrahigh-peak-current (more than 10 kA) electron-bunch trains with tunable picosecond spacing for the generation of such terahertz sources. Passing through a plasma section and interacting with the self-excited nonlinear plasma wake, the beam gains a ``sawtooth'' energy modulation. By means of magnetic optics, it is then effectively converted into a beam density modulation forming microbunches with a high charge of up to a few nanocoulombs and a bunching factor as high as approximately $0.8$. Tunable narrowband terahertz radiation with energy on the order of 10 mJ can be generated, which we show in both theoretical analyses and simulations via this process. This scheme can be realized in accelerator facilities with repetition rates on the order of kilohertz or even submegahertz, which provides great potential in modern science and related technologies.

Directional THz generation in hot Rb vapor excited to a Rydberg state.

We optically excite 85Rb atoms in a heated vapor cell to a low-lying Rydberg state 10D5/2 and observe directional terahertz (THz) beams at 3.3 THz and 7.8 THz. These THz fields are generated by amplified spontaneous emission from the 10D5/2 state to the 11P3/2 and 8F7/2 states, respectively. In addition, we observe ultraviolet (UV) light produced by four-wave mixing of optical pump lasers and the 3.3 THz field. We characterize the generated THz power over the detuning and power of pump lasers, and identify experimental conditions favoring THz and UV generation, respectively. Our scheme paves a new pathway towards generating high-power narrowband THz radiation.

Generation of narrowband terahertz radiation via phonon mode enhanced nonlinearities in a BaGa4Se7 crystal.

We report on narrowband terahertz (THz) radiation generation via optical rectification from a BaGa4Se7 crystal. The dense phonon mode distribution of the BaGa4Se7 crystal causes narrow transmission bands in the THz frequency range with enhanced nonlinear susceptibility magnitudes, thus permitting strong narrowband THz radiation generation at the frequencies of 1.97 and 2.34 THz. In comparison to THz radiation generated from a ZnTe crystal, the narrowband THz radiation produced by the BaGa4Se7 crystal is 4.5 times higher at 1.97 THz and 63% higher at 2.34 THz, thus making BaGa4Se7 a viable crystal for use in such areas as security and medicine.

Strong and narrowband terahertz radiation from GaAs based pHEMT and terahertz imaging

AbstractStrong and narrowband terahertz (THz) radiation is obtained from a GaAs‐based pseudomorphic high‐electron‐mobility transistor (pHEMT). The radiation properties with respect to the bias conditions and operating temperature are extensively characterized. The maximum emission power is measured to be 63 and 278 μW at room temperature and 77 K, respectively. By using the THz radiation from the pHEMT, a metal object hidden in cloth was successfully imaged.

Backward terahertz difference frequency generation via modal phase-matching in a planar LiNbO3 waveguide.

The backward difference frequency generation process is used to produce narrowband terahertz radiation via modal phase-matching in a SiO2-LiNbO3-air planar waveguide. The TM0 pump mode, TE0 signal mode, and TE0 or TE2 idler modes are selected to satisfy the backward difference frequency generation phase-matching condition, thus allowing narrowband (i.e., <100GHz linewidth) terahertz radiation generation in the spectral range of 2.4-3.2 THz. To date, this is the first investigation of terahertz radiation generation in a waveguide via the modal phase-matched backward difference frequency generation process.

Conductivity Induced by High-Field Terahertz Waves in Dielectric Material

An intense, subpicosecond, relativistic electron beam traversing a dielectric-lined waveguide generates very large amplitude electric fields at terahertz (THz) frequencies through the wakefield mechanism. In recent work employing this technique to accelerate charged particles, the generation of high-power, narrow-band THz radiation was demonstrated. The radiated waves contain fields with measured amplitude exceeding 2 GV/m, orders of magnitude greater than those available by other THz generation techniques at a narrow bandwidth. For fields approaching the GV/m level, a strong damping has been observed in SiO_{2}. This wave attenuation with an onset near 850 MV/m is consistent with changes to the conductivity of the dielectric lining and is characterized by a distinctive latching mechanism that is reversible on longer timescales. We describe the detailed measurements that serve to clarify the underlying physical mechanisms leading to strong field-induced damping of THz radiation (hω=1.59 meV, f=0.38 THz) in SiO_{2}, a bulk, wide band-gap (8.9eV) dielectric.

Spectral phase control of interfering chirped pulses for high-energy narrowband terahertz generation

Highly-efficient optical generation of narrowband terahertz radiation enables unexplored technologies and sciences from compact electron acceleration to charge manipulation in solids. State-of-the-art conversion efficiencies are currently achieved using difference-frequency generation driven by temporal beating of chirped pulses but remain, however, far lower than desired or predicted. Here we show that high-order spectral phase fundamentally limits the efficiency of narrowband difference-frequency generation using chirped-pulse beating and resolve this limitation by introducing a novel technique based on tuning the relative spectral phase of the pulses. For optical terahertz generation, we demonstrate a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz pulses, yielding a record energy of 0.6 mJ and exceeding previous optically-generated energies by over an order of magnitude. Our results prove the feasibility of millijoule-scale applications like terahertz-based electron accelerators and light sources and solve the long-standing problem of temporal irregularities in the pulse trains generated by interfering chirped pulses.

Terahertz magnetic field enhancement in an asymmetric spiral metamaterial

We use finite element simulations in both the frequency and the time-domain to study the terahertz resonance characteristics of a metamaterial (MM) comprising a spiral connected to a straight arm. The MM acts as a RLC circuit whose resonance frequency can be precisely tuned by varying the characteristic geometrical parameters of the spiral: inner and outer radius, width and number of turns. We provide a simple analytical model that uses these geometrical parameters as input to give accurate estimates of the resonance frequency. Finite element simulations show that linearly polarized terahertz radiation efficiently couples to the MM thanks to the straight arm, inducing a current in the spiral, which in turn induces a resonant magnetic field enhancement at the center of the spiral. We observe a large (approximately 40 times) and uniform (over an area of ∼10 μm2) enhancement of the magnetic field for narrowband terahertz radiation with frequency matching the resonance frequency of the MM. When a broadband, single-cycle terahertz pulse propagates towards the MM, the peak magnetic field of the resulting band-passed waveform still maintains a six-fold enhancement compared to the peak impinging field. Using existing laser-based terahertz sources, our MM design allows to generate magnetic fields of the order of 2 T over a time scale of several picoseconds, enabling the investigation of nonlinear ultrafast spin dynamics in table-top experiments. Furthermore, our MM can be implemented to generate intense near-field narrowband, multi-cycle electromagnetic fields to study generic ultrafast resonant terahertz dynamics in condensed matter.