THz Source Research Articles

Formation of gigahertz pulse train by chirped terahertz pulses interference

The state-of-art broadband THz sources can contribute to the development of short-range 6G communications. This paper has demonstrated the feasibility of forming the controllable sequence of THz subpulses in the temporal domain and the corresponding quasidiscrete spectrum by the interference of two THz pulses with an exponential chirp. Moreover, due to small time delay between these pulses the temporal and spectral structures are similar to each other (so-called “linkage relation”). This will benefit information encoding in the THz range. The calculated metrics for the prototype communication channel based on the proposed method are competitive with existing short-range THz CW channels.

A Versatile THz Source from High-Brightness Electron Beams: Generation and Characterization

Ultra-short electron bunches, such as those delivered by a high-brightness photo-injector, are suitable to produce high peak power THz radiation, both broad and narrow band, with sub-picosecond down to femtosecond pulse shaping. The features of this kind of source in the THz range of the electromagnetic spectrum are extremely appealing for frequency- and time-domain experiments in a wide variety of fields. The present manuscript will overview the method of generation and characterization of THz radiation produced by high-brightness electron beams, as those available at the SPARC_LAB test facility.

Photonic integrated circuits for terahertz source generation

The authors introduce four kinds of terahertz photonics components based on photonic integrated circuits (PICs). A PIC-based integrated optoelectronic synthesiser for THz communications is described, which can be tuned continuously over the range 0.254-2.723 THz using photomixing. A laterally-coupled dual-grating distributed feedback laser (DFB) diode integrated with an electroabsorption modulator is used to generate an 820 GHz beat signal. THz signal production is reported using a dual-wavelength DFB diode laser with a two-section phase-shifted sampled Bragg grating. Finally, a THz source at 640 GHz, based on a sampled grating distributed Bragg reflector semiconductor mode-locked laser diode, is reported offering superior reproducibility, controllability, and a wider operation range than other reported mode-locked THz laser diodes. Each of these sources is a monolithic construction emitting light at 1.5 μm. The light can be amplified in an erbium-doped fibre amplifier, delivered over silica optical fibre and used to generate THz radiation via a photodiode antenna or photoconductive antenna in a remote location.

A superconducting flux-flow oscillator of terahertz range

We have elaborated, fabricated and tested a THz source radiating to open space based on the superconducting flux-flow oscillator (FFO). In this concept, the oscillator is integrated with the transmitting lens antenna based on a slot structure in Nb film with a thickness of ~200 nm located on the same chip. The slot planar antenna is matched to the oscillator (by input) and to the semielliptical Si lens with a diameter of 10 mm (by output) providing a narrow output beam of THz emission. A harmonic mixer based on the superconductor-insulator-superconductor junction embedded in the “FFO and antenna” integrated structure has been used for the phase locking of the oscillator. Several designs of antenna coupled with the oscillator by microstrip lines have been numerically simulated, and the batches of experimental samples based on Nb-AlN-NbN superconducting trilayers with Rn ·A ~ 20 Ω·μm2 (jc ~ 10 kA/cm2) have been fabricated and tested. Two different setups were used for experimental study: a THz spectrometer based on the SIS receiver with a high spectral resolution (better than 0.1 MHz) and a Si bolometer. The overall operating range of 250 to 700 GHz is covered by all the developed designs.

Excitation and coupling of volume and surface fields on complex electrodynamic surfaces at mm‐wave and THz frequencies

The analytical theory describing the resonant excitation and coupling of volume and surface fields on the surface of two-dimensional complex electrodynamic structures is presented. The theoretical analysis is valid over a broad frequency spectrum from mm-wave frequencies through THz and even optical frequencies. An experimental study of planar periodic structures has been carried out using a vector network analyser calibrated to operate in the 140–220 GHz frequency range. Experimental results compare resonant eigenmode formation in two periodic surface lattice structures designed to operate within the 140–220 GHz frequency band; one periodic surface lattice etched onto a metal-backed substrate and the other arranged to have an equivalent air separation. Dispersion diagrams derived from the analytical theory are presented. The results and theory are fundamental to some of the routes to the innovation of high-power, mm-wave and THz sources, solar cells, and novel sub-wavelength absorbers.

Observation of grating diffraction radiation at the KEK LUCX facility

The development of linac–based narrow–band THz sources with sub–picosecond, mu J-level radiation pulses is in demand from the scientific community. Intrinsically monochromatic emitters such as coherent Smith–Purcell radiation sources appear as natural candidates. However, the lack of broad spectral tunability continues to stimulate active research in this field. We hereby present the first experimental investigation of coherent grating diffraction radiation (GDR), for which comparable radiation intensity with central frequency fine–tuning in a much wider spectral range has been confirmed. Additionally, the approach allows for bandwidth selection at the same central frequency. The experimental validation of performance included the basic spectral, spatial and polarization properties. The discussion of the comparison between GDR intensity and other coherent radiation sources is also presented. These results further strengthen the foundation for the design of a tabletop wide–range tunable quasi–monochromatic or multi–colour radiation source in the GHz–THz frequency range.

Observation of extremely efficient terahertz generation from mid-infrared two-color laser filaments

Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here, we experimentally demonstrate that two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm allows one to generate ultrashort sub-cycle THz pulses with sub-milijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm−1. Our numerical simulations predict that the observed THz yield can be significantly upscaled by further optimizing the experimental setup. Finally, in order to demonstrate the strength of our THz source, we show that the generated THz pulses are powerful enough to induce nonlinear cross-phase modulation in electro-optic crystals. Our work paves the way toward free space extreme nonlinear THz optics using affordable table-top laser systems.

Compact and powerful THz source investigation on laser plasma wakefield injector and dielectric lined structure

The advancement of the techniques of producing electromagnetic radiation in the terahertz range (1--20 THz) attracts a wide range of potential applications and the quest for powerful THz sources is ever-increasing. The means to generate THz is an active research field and the energy per pulse is limited due to breakdown of the medium used. In this paper, we propose a compact yet powerful narrowband THz source based on two pillars of active research in the field of novel accelerators: laser plasma wakefield acceleration and high gradient dielectric lined structures. The laser plasma wakefield (LPW) injector is used to provide the relativistic electron bunches to an interaction tube, a dielectric lined waveguide (DLW), in order to generate THz. Due to the high gradient nature of both devices, the combination ends up with a very compact solution for mJ scale THz pulse generation. Potentially these pulses can be used as drivers for nonlinear phenomena in condensed matter, which can be probed using ultrashort x-ray pulses at the same free electron laser facility.

Influence of Beam Distribution on the Quality of Compressed Sensing-Based THz Imaging

This paper investigates the impact of incident beam inhomogeneity on the quality of THz imaging based on the compressed sensing (CS) method. Image sampling and reconstructions under point and Gaussian beams with various geometric parameters are compared with the standard one. The simulation results show that the geometric parameters of beams strongly affect the peak signal-to-noise ratio (PSNR) of the reconstructed images. Especially for the Gaussian one, expanding the beam size at the position of the mask (BZPM) dramatically increases the PSNR. To achieve high sensitivity and resolution, new measurement matrices correlating to the incident beam distribution are proposed and the simulation results are demonstrated. Experiments under VDI THz source reveal that by using the new matrices, the PSNR of CS-based imaging at 100 GHz is evidently improved from 6 dB to 13 dB, informing the new measurement matrices are highly efficient and accurate in removing the beam effect on CS-based THz imaging. Our results may provide a new way for the high-quality CS-based THz imaging for target recognition.

Efficient THz radiation source from spoof surface plasmons modes on a dielectric-filled metallic grating by electron beam

Abstract Spoof surface plasmons(SSP) is attractive to develop efficient THz radiation source owing to its strong localized electric field. In this work, a THz radiation source based on SSP modes on a layered metal-dielectric plasmonic metamaterial is proposed and investigated. The dispersion characteristics of SSP modes on the structure are solved analytically and numerically. Besides, the propagation of SSP mode which is tuned by the filled dielectric is also demonstrated. Following the studies, THz radiation source which is induced by injected electron beam in the system is studied by a FDTD simulation method. The outputs of THz source based on the interaction between SSP mode and electron beam with various kinds of filled dielectric are given and compared with the same injected beam energy. It is shown that maximum power is decreased in the structure compared with no dielectric situation and the frequency spectrum is lowered. The presented studies about the excitation of SSP modes by electron beam on the structure may provide a good candidate of enhanced and tunable THz radiation source.

Oscillations up to terahertz frequency in resonant tunneling diodes

AbstractWe investigate the THz current oscillations in resonant tunneling diodes (RTD) oscillators with the transient drift‐diffusion model. The oscillation frequencies have a strong dependence on the biased voltage and doping concentration. With proper biased voltage and doping concentration, the high frequency up to THz can be obtained. The simulations are helpful to deepen the understanding of physical mechanism and guide to improve the performances of RTDs and RTD oscillators. The investigations make RTDs promising candidates as THz sources at room temperature.

Fiber laser-driven gas plasma-based generation of THz radiation with 50-mW average power

We present on THz generation in the two-color gas plasma scheme driven by a high-power, ultrafast fiber laser system. The applied scheme is a promising approach for scaling the THz average power but it has been limited so far by the driving lasers to repetition rates up to 1 kHz. Here, we demonstrate recent results of THz generation operating at a two orders of magnitude higher repetition rate. This results in a unprecedented THz average power of 50 mW. The development of compact, table-top THz sources with high repetition rate and high field strength is crucial for studying nonlinear responses of materials, particle acceleration or faster data acquisition in imaging and spectroscopy.

Novel Schemes for Compact FELs in the THz Region

The rapid advance of terahertz technologies in terms of radiation generators, systems, and scientific or industrial applications has put a particular focus on compact sources with challenging performances in terms of generated power (peak and/or average), radiation time structure, and frequency band tunability. Free electron laser (FEL)-based sources are probably the best candidates to express such a versatility; there are a number of schemes that have been investigated over the years to generate coherent radiation from free electrons in the mm-wave and terahertz regions of the spectrum, covering a wide frequency range from approximately 100 GHz to 10 THz. This paper proposes novel schemes for exploring the limits in the performance of radio frequency-driven free-electron devices in terms of ultrashort pulse duration, wide bandwidth operation, and energy recovery for near continuous wave (CW) operation. The aim of the present work is to demonstrate the feasibility of an FEL achieving performance comparable to a conventional photoconductive THz source, which is commonly used for time-domain spectroscopy (TDS), in terms of bandwidth and pulse duration. We will also demonstrate that a THz FEL could be very powerful and flexible in terms of tailoring its spectral features.

Design studies of a proof-of-principle experiment on THz SASE FEL at PITZ

A free-electron laser based THz source is undergoing design studies at the Photo Injector Test facility at DESY in Zeuthen (PITZ). It is considered as a prototype for pump-probe experiments at the European XFEL, benefiting from the fact that the electron beam from the PITZ facility has an identical pulse train structure as the XFEL pulses. In the proposed proof-of-principle experiment, the electron beam (up to 4 nC bunch charge and 200 A peak current) will be accelerated to 16-22 MeV/c to generate SASE radiations in an LCLS-I undulator in the THz range between 60 and 100 pm with an expected energy of up to ~1 mJ/pulse. In this paper, we report our simulations on the optimization of the photo-injector and the design of the transport and matching beamline. Experimental investigations on the generation, characterization and matching of the high charge beam in the existing 22-m-long beamline will also be presented.

Circular-Photon-Drag-Effect-Induced Elliptically Polarized Terahertz Emission from Vertically Grown Graphene

The circular photon drag effect (CPDE) is important for helicity-dependent optoelectronic emitters and detectors, but in graphene the relatively weak light-matter interaction gets drowned out by other nonlinear optical effects. Vertically grown graphene (VGG), though, with its particular symmetry point group, can exclude the other effects and enhance the light-graphene interaction. This study of the CPDE from VGG shows that the emitted THz states can be tuned to have linear or left- or right-handed elliptical polarization, by changing the helicity of the pump laser. This observation points to graphene-based polarization-sensitive THz sources for chiral analysis.

Double-pump technique - one step closer towards efficient liquid-based THz sources.

By irradiating a water jet with double pulses, we demonstrate 4-fold higher THz wave generation than for a single pump pulse. The dependence of the enhanced THz signal on the temporal delay between two collinear pulses reveals the optimal time for launching signal pulse is near 2-4 ps, which corresponds to the time needed to create the complete pre-ionization state when sufficient electron density is already induced, and there is no plasma reflection of the pump pulse radiation. The increase in THz waves generation efficiency corresponds to the case of water jet excitation by the pulses with an optimal duration for a certain jet thickness, which is determined by the spatial pulse size. Using a theoretical model of the interaction of a high-intensity sub-picosecond pulse with an isotropic medium, we held a numerical simulation, which well describes the experimental results when using 3 ps value of population relaxation time. Thus, in this work, double pump method allows not only to increase the energy of the generated THz waves, but also to determine the characteristic excited state lifetime of liquid water. The optical-to-terahertz conversion efficiency in case of double pulse excitation of water column is of the order of 0.5⋅10 -3, which exceeds the typical values for THz waves generation during two-color filamentation in air and comparable with the achievable values due to the optical rectification in some crystals.

Pulse- and field-resolved THz-diagnostics at 4 t h generation lightsources.

Multi-color pump-probe techniques utilizing modern accelerator-based 4th generation light sources such as X-ray free electron lasers or superradiant THz facilities have become important science drivers over the past 10 years. In this type of experiments the precise knowledge of the properties of the involved accelerator-based light pulses crucially determines the achievable sensitivity and temporal resolution. In this work we demonstrate and discuss the powerful role pulse- and field-resolved- detection of superradiant THz pulses can play for improving the precision of THz pump - femtosecond laser probe experiments at superradiant THz facilities in particular and at 4th generation light sources in general. The developed diagnostic scheme provides real-time information on the properties of individual pulses from multiple accelerator based THz sources and opens a robust way for sub femtosecond timing. Correlations between amplitude and phase of the pulses emitted from different superradiant THz sources furthermore provide insides into the properties of the driving electron bunches and is of general interest for the ultra-fast diagnostics at 4th generation light sources.

Milliwatt-class broadband THz source driven by a 112 W, sub-100 fs thin-disk laser.

We demonstrate a high repetition-rate, single-cycle THz source with a maximum average power of 1.35 mW, operating at a center frequency of 2 THz. This result was obtained by optical rectification (OR) in GaP using an amplifier-free, nonlinearly compressed modelocked thin-disk oscillator based on Yb:YAG, delivering 8.4 µJ pulses with 88 fs duration at a repetition rate of 13.4 MHz, resulting in driving pulses for OR with 112 W average power and 80 MW peak power. To the best of our knowledge, our result represents the highest average power so far achieved with OR in GaP. The demonstrated performance is very attractive for improving current linear THz time-domain spectroscopy experiments, which are currently restricted by low signal-to-noise ratio and long measurement times.

On the Magnetic Designing of Compact Alpha-Magnets

Alpha-magnets present unique achromatic properties for beam instrumentation and phase-space manipulation. Nowadays they are acquiring a substantial interest for application in a rapidly growing number of facilities around the world: Free Electron Lasers, far infrared and THz sources, inverse Compton sources of ultra-bright hard X-rays. However, classic design of an alpha-magnet based on hyperbolic poles makes it dimensions large, resulting in significant weight and cost. Several magnetic designs of alpha magnets with reduced dimensions especially for low energy electron beams is considered here including very compact alpha magnets based on permanent magnets with about flat poles as well as compact electromagnetic designs with co-axis coils and substantially non-hyperbolic or truncated poles. These novel design approaches can be applied in relatively small facilities where weight and/or space are limited.

Tunable plasmonic force switch based on graphene nano-ring resonator for nanomanipulation.

Using a plasmonic graphene ring resonator of resonant frequency 10.38 THz coupled to a plasmonic graphene waveguide, we design a lab-on-a-chip optophoresis system that can function as an efficient plasmonic force switch. Finite difference time domain numerical simulations reveal that an appropriate choice of chemical potentials of the waveguide and ring resonator keeps the proposed structure in on-resonance condition, enabling the system to selectively trap a nanoparticle. Moreover, a change of 250 meV in the ring chemical potential (i.e., equivalent to 2.029 V change in the corresponding applied bias) switches the structure to a nearly perfect off-resonance condition, releasing the trapped particle. The equivalent plasmonic switch ON/OFF ratio at the waveguide output is -15.519 dB. The designed system has the capability of trapping, sorting, controlling, and separating PS nanoparticles of diameters ≥30 nm with a THz source intensity of 14.78 mW/µm2 and ≥22 nm with 29.33 mW/µm2.