THz Radiation Source Research Articles

High-Power Terahertz Free Electron Laser via Tapering-Enhanced Superradiance

A superradiant FEL in the THz (3 THz) region is currently operating at Ariel University. It is based on the novel ORGAD accelerator, which is a hybrid linear RF photo-cathode 6 MeV electron gun. The hybrid term stands for its unique standing wave (SW)—traveling wave (TW) structure. The undulator generates spontaneous superradiance, which corresponds to spontaneous emission when the electron bunch duration is shorter than the radiated frequency, resulting in a much higher photon yield. However, the efficiency of this scheme is still quite low. In order to achieve higher emission (by improved efficiency), we intend to implement a new and promising radiative interaction scheme: tapering-enhanced superradiance (TES). This particular undulator design employs a tapered (amplitude) undulator in the zero-slippage condition to obtain a significantly more powerful and efficient THz radiation source. At the current stage, the scheme is designed for emission at approximately 0.5 THz. The design and start-to-end simulations demonstrate significant enhancement of superradiant energy and extraction efficiency using this method compared to a reference uniform case.

Joule-class THz pulses from microchannel targets.

Inference of joule-class THz radiation sources from microchannel targets driven with hundreds of joule, picosecond lasers is reported. THz sources of this magnitude are useful for nonlinear pumping of matter and for charged-particle acceleration and manipulation. Microchannel targets demonstrate increased laser-THz conversion efficiency compared to planar foil targets, with laser energy to THz energy conversion up to ∼0.9% in the best cases.

Excitation of Terahertz Spoof Surface Plasmons on a Roofed Metallic Grating by an Electron Beam.

In this paper, both fundamental SSP modes on a roofed metallic grating and its effective excitation of the bounded SSP mode by an injected electron beam on the structure are numerically examined and investigated in the THz regime. Apart from the bounded SSP mode on the metallic grating with open space, the introduced roofed metallic grating can generate a closed waveguide mode that occupies the dispersion region outside the light line. The closed waveguide mode shifts gradually to a higher frequency band with a decreased gap size, while the bounded SSP mode line becomes lower. The effective excitation of the bounded SSP mode on this roofed metallic grating is also implemented and studied by using a particle-in-cell simulation studio. The output SSP power spectrums with various gap sizes by the same electron beam on this roofed metallic grating are obtained and analyzed. The simulation results reveal that the generated SSP spectra show a slight red shift with a decreased gap size. This work on the excitation of the SSP mode using an electron beam can benefit the development of high-power compact THz radiation sources by utilizing the strong near-field confinement of SSPs on metallic gratings.

Fine characterization of MBE-grown super-multiperiod AlGaAs/GaAs superlattices designed for THz radiation sources

Fine characterization of MBE-grown super-multiperiod AlGaAs/GaAs superlattices designed for THz radiation sources

Terahertz imaging through emissivity control

Electromagnetic radiation in the terahertz (THz) frequency band has unique potential for future communication and imaging applications. However, the adoption of THz technologies is hindered by the lack of cost-effective THz sources. Here we demonstrate a way to generate and control THz radiation, via spatio-temporal emissivity modulation. By patterning the optical photoexcitation of a surface-passivated silicon wafer, we locally control the free-electron density, and thereby pattern the wafer’s emissivity in the THz part of the electromagnetic spectrum. We show how this unconventional source of controllable THz radiation enables a form of incoherent computational THz imaging. We use it to image various concealed objects, demonstrating that this scheme has the penetrating capability of other THz imaging approaches, without the requirement of femtosecond pulsed laser sources. Furthermore, the incoherent nature of thermal radiation also ensures the obtained images are free of interference artifacts. Our spatio-temporal emissivity control could enable a family of long-wavelength structured illumination, imaging, and spectroscopy systems.

Terahercų emisijos spektroskopija pavidalų panašumui tirti: Ukrainos ir Lietuvos simbolių nagrinėjimas

In this paper, the thermal emission from the n-GaAs/GaAs structure equipped with the Ti/Au metasurface is being investigated in the terahertz region. The metacell shape was chosen to be of heraldic origin, that is either the Ukrainian Trident or the Lithuanian Columns of Gediminids. The experiments were performed using far-infrared Fourier spectroscopy, where the heated sample served as the source of THz radiation. The optical properties of such structures were simulated using the rigorous coupled wave analysis method in order to explain the origin of experimentally observed spectral features. The deconstruction of the simulated spectra was also performed by simulating the properties of simplified metacells which constitute the metacells of the heraldic symbols investigated here. The spectral analysis suggests the similarity between the investigated symbols, which is also reviewed from the historical point of view.

Ultrafast helicity-dependent photocurrents in Weyl Magnet Mn3Sn

Mn3Sn is a material that has attracted a lot of attention lately for its topologically non-trivial band structure, which leads to very promising spintronic properties. In this work we experimentally demonstrate that an epitaxially grown thin film of Mn3Sn acts as a source of THz radiation at room temperature when irradiated by a femtosecond laser pulse. By combining various experimental measurements as a function of pump polarisation, magnetic field, and sample orientation we are able to explain the origin of the THz emission with the photocurrents generated via the photon drag effect. A thorough symmetry analysis combined with electronic band structure calculations using density-functional theory (DFT) are used to support our conclusions and provide a guide towards the important features that lead to photocurrent generation, which is useful for designing ultra-fast current pulses emitters.

Terahertz wave rectification in a ferroelectric triglycine sulfate single crystal.

The effect of optical rectification (OR) in the terahertz range (THz rectification, TR) is experimentally demonstrated. The effect consists of generating a DC voltage on the faces of a ferroelectric triglycine sulfate (TGS) single crystal under the action of pulsed radiation with a frequency of 1.57 and 1.96 THz and an electric field strength per pulse of 1.3 and 1.5 MV/m, respectively. The FLARE FELIX free-electron laser system (Radboud University, The Netherlands) was used as a THz radiation source. The TR effect makes it possible to directly determine the nonlinear susceptibilities of media (including those under conditions of strong absorption) without any reference or optical channel calibration and also without the need of Fourier transform.

Study of terahertz reflection spectra of optically thin RDX samples by terahertz imaging with spectral resolution

The THz reflection spectra of optically thin hexogen (RDX) samples were studied by terahertz imaging with spectral resolution. A photoconductive antenna excited by femtosecond laser radiation was used as a source of broadband THz radiation. The presence of a Fourier spectrometer (as well as band-pass THz filters) and a microbolometric THz video camera in the experimental setup made it possible to use the terahertz imaging method to study reflection spectra taking into account scattering in the range of 0.6 to 1 THz. The influence of the optical characteristics of the substrate on the THz reflection spectra was studied. In particular, the conditions for observing the effect of anomalous dispersion for RDX samples with different dispersion in the frequency region of the RDX absorption band 0.8 THz were studied. The obtained results demonstrate the application of the method of terahertz imaging with spectral resolution based on THz video camera for the identification of explosives with concentrations 0.75 to 50 mg / cm2 on various surfaces.

Терагерцовые излучатели с активной областью на основе сверхмногопериодных решеток AlGaAs/GaAs

In this article, several designs of the active region of the THz radiation source are considered, taking into account grown super-multiperiod AlGaAs/GaAs superlattices. For the proposed designs, the principal device characteristics are computed: energy band diagram, gain spectrum, and transport characteristics. Based on the calculation results, the authors proposed an optimal design of the active region of a tunable THz radiation source.

Towards Watt-Level THz Sources for High-Resolution Spectroscopy Based on 5th-Harmonic Multiplication in Gyrotrons

We propose the concept of high-power THz radiation sources based on five-fold frequency multiplication in gyrotrons intended for plasma applications. The efficient excitation at the 5th cyclotron harmonic is due to the specific property of the eigenmodes of cylindrical waveguides, as a result of which, the conditions of simultaneous electrodynamic resonance at two selected TE modes are satisfied asymptotically with very high accuracy. Previously, we have verified this principle in experiments with a low-frequency kilowatt-level gyrotron in which, due to the low-density spectrum, the operating mode is excited with no competition from parasitic oscillations. The novel concept is a development of this idea applied to the systems with a denser spectrum, which is inevitable in higher frequency and power devices. Simulations within the averaged time-domain model demonstrate that, despite the mode competition, it is possible to excite Watt-level 1.25 THz 5th cyclotron harmonic in a recently developed sub-MW 0.25 THz gyrotron with TE19,8 operating mode. The obtained results open a possibility for implementation of radiation sources with output power/frequency combination, practically inaccessible using other THz generation methods and highly sought for a number of applications, including high-resolution molecular spectroscopy.

The Transformation from Surface Wave into Radiation Wave Using Composite Subwavelength Holes Array

Smith–Purcell radiation (SPR) is a phenomenon in that free electrons moving above a periodic structure deliver a part of their kinetic energies to electromagnetic waves. It plays an important role in the development of radiation sources and beam diagnosis. With the development of nanofabrication technology, various complex micro-nano-metallic periodic or aperiodic structures have been adopted to improve the efficiency and coherence of SPR. In general, when free electrons move above the metallic periodic structure, the localized surface wave and Smith–Purcell radiation wave are excited simultaneously. As the frequency of the surface wave is always lower than the threshold of Smith–Purcell radiation, the surface wave cannot be radiated. Here, the surface wave is transformed into SPR by using a novel composite subwavelength hole array, thus the SPR is enhanced and it becomes coherent and easy to tune. This work provides a competitive approach to achieve a coherent and high-efficiency THz radiation source.

Theoretical and Simulation Studies of Transverse Beam Size Effects on Optical Transition Radiation in Prewave Zone

Optical transition radiation (OTR) has been studied and applied on the beam diagnostics for decades. The potential implication of OTR also includes THz radiation sources. Therefore, the theoretical analysis and simulation tools have become indispensable for the OTR study. Moreover, the OTR theory, for the wave zone (far-field approximation), has been widely used in the literature. However, these theories for the prewave zone have been proposed on the basis of single electron approximation. In this study, we developed a theory with consideration of the electron beam structure based on Kirchhoff’s method for studying the effect of beam transverse size on the angular distribution of the OTR in prewave zone. The proposed formalism involves complicated convolution integral of functions and the dimensions of integrand are not low. To perform such integral, we developed a Fortran program for quasi-monte Carlo method, which is robust and suitable for high dimensional integration. The disadvantage of this method is that a large amount of samplings may need to be employed to achieve good convergence. Therefore, in order to get the radiation angular profile, we need to perform such integration for different observation angles that might be computationally intensive. Therefore, we parallelize the program with the Message Passing Interface (MPI) programming concepts. To verify the theoretical calculations, few two-dimensional FDTD simulations were carried out that show the validity of the proposed model. The proposed theory and numerical tool would be used to predict radiation properties of the NSRRC THz coherent transition radiation (CTR) in the prewave zone.

Terahertz radiation generated by electron-beam-driven plasma waves in a transverse external magnetic field

It is found that the plasma waves driven by an electron beam rotating in a magnetized overdense plasma can be converted into terahertz electromagnetic radiation by new mechanisms. According to the particle-in-cell simulation results, the radiation modes include a dominant extraordinary (X) mode at twice the plasma frequency 2fp and a subordinate X mode at fp. The 2fp radiation can be generated by the coupling of a beam mode and a scattered upper-hybrid (UH) mode, or by the coupling of a beam mode and a right-handed X mode. Here, the beam mode can be a UH mode or a left-handed X mode driven by the beam under the Cherenkov condition, and the right-handed X mode can be induced by high-order electron cyclotron maser instability. The fp radiation is the right- or left-handed fundamental X mode escaping from the plasma boundary. This study also shows that the breakdown of beam modulation is responsible for the radiation attenuation. The scheme proposed in this paper can be applied in high-power THz radiation sources and diagnosis of magnetized plasmas.

Particle-in-cell simulations of THz emission from plasma by oblique collision of two-electron beams

We studied the THz radiation generated by a beam-plasma system using two-dimensional (2D) particle-in-cell (PIC) simulations. The Langmuir waves excited by two counterpropagating electron beams, via two-stream instability, collide with each other at an oblique angle, which forms a high beam-density modulation near the collision region, where both beam electrons become trapped. As a result, spatially localized Langmuir wave packets with large longitudinal-electric field amplitudes are formed, which give rise to bursts of electromagnetic radiation. Our 2D PIC simulations of the two thin, low-density, asymmetric, electron beams colliding obliquely show that a strong THz emission is obtained at the second harmonic of the plasma frequency (f = 1.0 THz), with a narrow spectral width (∼0.80%) in vacuum and significantly higher efficiency than the head-on-collision case. The efficiency of power conversion from electron beams to THz radiation measured in vacuum reaches around ∼0.0289, for a continuous injection of beams into the plasma, making it suitable for applications requiring high-power narrow-band THz radiation sources.

Smith–Purcell radiation improved by multi-grating structure

The photonic crystal structure has attracted much attention due to its ability to confine light. In this paper, we present our study on an improved Smith–Purcell radiation from a simple metal photonic crystal excited by moving electrons. Different from the wide-band Smith–Purcell radiation from a single metal grating, the results show that the injected electrons could induce more dipole oscillations inside the multi-grating structure, and it leads to the enhancement of the radiation intensity. In addition, there are strong resonances in metal multi-grating structure, and the resonance characteristics may narrow the radiation band, which leads to a radiation with an obvious peak in the spectrum. Therefore, the multi-grating structure has the ability to enhance the radiation intensity and shape the radiation frequency band. By optimizing the structure parameters, coherent and tunable Smith–Purcell radiation can be realized, and it provides a potential way to develop band-controllable light or THz radiation source.

Correction to: Exploiting hyperbolic metamaterial as a substrate for graphene surface plasmonic Cherenkov THz radiation source

Correction to: Exploiting hyperbolic metamaterial as a substrate for graphene surface plasmonic Cherenkov THz radiation source

Experimental Demonstration of Frequency-Chirped Density Modulation in the Laser Pulse for Advanced THz Radiation Sources

Experimental Demonstration of Frequency-Chirped Density Modulation in the Laser Pulse for Advanced THz Radiation Sources

Application of Wigner Distribution Function for THz Propagation Analysis.

The construction of a transmission line (TL) for a wide tunable broad-spectrum THz radiation source is not a simple task. We present here a platform for the future use of designs of the TL through our homemade simulations. The TL is designed to be a component of the construction of an innovative accelerator at the Schlesinger Family Center for Compact Accelerators, Radiation Sources and Applications (FEL). We developed a three-dimensional space-frequency tool for the analysis of a radiation pulse. The total electromagnetic (EM) field on the edge of the source is represented in the frequency domain in terms of cavity eigenmodes. However, any pulse can be used regardless of its mathematical function, which is the key point of this work. The only requirement is the existence of the original pulse. This EM field is converted to geometric-optical ray representation through the Wigner transform at any desired resolution. Wigner’s representation allows us to describe the dynamics of field evolution in future propagation, which allows us to determine an initial design of the TL. Representation of the EM field by rays gives access to the ray tracing method and future processing, operating in the linear and non-linear regimes. This allows for fast work with graphics cards and parallel processing, providing great flexibility and serving as future preparation that enables us to apply advanced libraries such as machine learning. The platform is used to study the phase-amplitude and spectral characteristics of multimode radiation generation in a free-electron laser (FEL) operating in various operational parameters.

Plasma reflectivity behavior under strong subpicosecond excitation of liquids

Emerging laser plasma-based technologies require new knowledge on the optical pump optimization and underlying physics of laser–matter interaction. Plasma in liquid jets is still largely unexplored, although this configuration is actively used as a part of optical devices. Hereby, in this work, we report on the reflectivity-based approach to study the optical properties of the induced plasma and quasi-free electron temporal dynamics. The single-shot experiments provide plasma characteristics relative to the laser pump parameters. A strong angular dependence of the plasma absorptivity, which affects rigorously the reflectivity curve, is demonstrated. Furthermore, twofold higher plasma reflectivity in ethanol is revealed and explained through the theoretical model of ultrashort laser pulse interaction with a thin dielectric surface layer. Finally, the time-resolved fundamental and third harmonic reflection measurements and associated analytical study provide the new insight into the electron density dynamics in the case of optical breakdown in water and ethanol. We believe that these findings are of particular significance for the development of highly efficient plasma-based x rays and THz radiation sources.