Coherent Terahertz Radiation Sources Research Articles

The Coming Age of Pnictide and Chalcogenide Ternary Crystals in the Terahertz Frequency Regime

The last decade has witnessed explosive growth in terahertz radiation fields, ranging from fundamental research to applications. This progress has been marked by the introduction of various terahertz radiation sources and detectors. Here, we review the progress of nonlinear pnictide and chalcogenide ternary crystals for use in the terahertz frequency regime. At the center of this topical review are the various physical and optical properties of numerous emerging pnictide and chalcogenide nonlinear crystals, which are anchored to state-of-the-art coherent terahertz radiation sources based on optical rectification and difference frequency generation, as well as terahertz radiation phase-resolved detectors based on electro-optic sampling. This review aims to provide the foundation necessary to continue advancing pnictide and chalcogenide ternary crystals for the generation, detection, and manipulation of terahertz radiation.

Single Free-Falling Droplet of Liquid Metal as a Source of Directional Terahertz Radiation

We show that an individual droplet of liquid metal can be a source of coherent terahertz radiation when it is excited by two femtosecond laser pulses of the same frequency. Under certain delays between these pulses, the intensity of terahertz radiation increases by more than 3 orders of magnitude. We describe the experimental results with the model of dynamic gain control, which considers the interaction of both laser pulses with the droplet and explains the terahertz-generation process by taking into account the dynamics of electrons and ions after photoionization of the metal droplet. The spatial distribution of terahertz radiation has a forward-directed contribution, whose polarization properties are well described by a nonlinear susceptibility of the second order. Our theoretical estimations based on the experimental data show that under the dynamic gain control the observed terahertz output can be considerably increased. Joint generation of x-ray, ultraviolet, and, as shown in the present work, terahertz radiation allows one to forecast that a free-falling photoexcited droplet of liquid metal is a promising source of multifrequency electromagnetic radiation for extreme nonlinear laser science.

Novel radiation sources using relativistic electrons - Applications

Novel sources of radiation using relativistic electrons in periodic structures are being briefly discussed based on different mechanisms and producing radiation of very different characteristic.In this article we will focus among others on the Smith-Purcell effect and the many efforts made since their discovery 55 years ago. Especially Smith-Purcell (SP) free- electron lasers (FELs) using low energy electron beam are being seen as attractive option for a compact source of coherent terahertz radiation. The latter refers to the production of radiation in the THz gap region associated with many applications including very promi- sing and interesting results on imaging techniques in Medicine as well as applications in Biology, Molecular Physics etc. We will present calculations of Radiation Factors based on the Smith-Purcell radiation production with very interesting characteristics and discuss the possibility of calculating and designing a complete system of coherent THz radiation production based on a Smith-Purcell FEL.

A tunable source of coherent terahertz radiation driven by the microbunched electron beam

The studies of coherent terahertz (THz) tunable Smith–Purcell radiation source driven by the microbunched electron beam are presented. We discuss the conceptual design of a high power and broadly tunable THz radiation source capable of bridging the THz technological gap. The results of the numerical and analytical studies using different software packages, models and approaches are shown and good agreement between the results is demonstrated. The spectra of the coherent Smith–Purcell radiation (cSPr) excited by a single bunch and a train of microbunches are compared and the excitation of the harmonics of cSPr is discussed. It is shown that varying the distance between microbunches the power and the frequency of the output cSPr can be controlled.

Filling the terahertz gap

Laser Physics Compared with other wavelengths, coherent sources of electromagnetic radiation in the terahertz regime are relatively scarce. Despite a number of applications in security imaging, spectroscopy, and chemical analysis, it has been experimentally challenging to produce such light. Chevalier et al. demonstrate an approach involving the excitation of a molecular gas with a quantum cascade laser. They show that they can tune into a broad range of desired wavelengths by carefully selecting the required molecular transition. A compact platform—the size of a shoe box—and widely tunable source of coherent terahertz radiation should find immediate application across a number of fields. Science , this issue p. [856][1] [1]: /lookup/doi/10.1126/science.aay8683

Electron rf Oscillator Based on Self-Excitation of a Talbot-Type Supermode in an Oversized Cavity

Powerful terahertz masers are required for various important applications, such as heating and current drive in controlled nuclear fusion. Traditional relativistic electron masers are based on emission of a fixed transverse mode of the operating cavity; at terahertz frequencies, oversized cavities are obviously needed, and it is difficult to provide single-mode generation. This study proposes a fresh strategy for solving this problem, based on excitation of a supermode formed by a fixed set of partial transverse cavity modes. The approach is interesting as a means to realize high-power sources of coherent terahertz radiation.

Resonant Cavity Modes in Bi2Sr2CaCu2O8+x Intrinsic Josephson Junction Stacks

Stacked intrinsic Josephson junctions in the cuprate superconductor Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+x}$ are eagerly investigated as a compact source of coherent terahertz radiation. The authors study the mechanisms of frequency tuning of this source at high and low bias. At high bias, in the presence of a hot spot, the emission frequency ${f}_{e}$ is continuously tunable via changing the bias current and the bath temperature, but at low bias ${f}_{e}$ remains discrete, indicating phase locking to cavity resonances, though some groups of junctions seem to remain unlocked. Though some differences appear, these phenomena can be reproduced in simulations combining electrodynamics and heat diffusion.

Enhanced broadband terahertz radiation generation near the reststrahlen band in sub-wavelength leaky-mode LiNbO3 waveguides.

The generation of terahertz (THz) radiation in a compact geometry is crucial for the implementation of on-chip, coherent THz radiation sources. Here, via numerical time-domain simulations, we show that LiNbO3 (LN) waveguides having sub-wavelength core widths can provide highly efficient optical-to-THz radiation frequency conversion over short lengths. By exploiting the nonlinear susceptibility, χ33(2), enhancement of LN near its phonon reststrahlen band and utilizing THz leaky-mode guidance to minimize reststrahlen band absorption and improve phase matching, we show that broadband (0.2-11.6THz) electric field pulses of 3.4kV/cm can be generated at an optical-to-THz conversion efficiency of 2.5×10-4. These sub-wavelength leaky-mode waveguides provide a compact platform for wideband, coherent THz radiation sources.

Concept of a tunable source of coherent THz radiation driven by a plasma modulated electron beam

We have carried out numerical studies which consider the modulation of a picosecond long relativistic electron beam in a plasma channel and the generation of a micro-bunched train. The subsequent propagation of the micro-bunched beam in the vacuum area was also investigated. The same numerical model was then used to simulate the radiation arising from the interaction of the micro-bunched beam with a metallic grating. The dependence of the radiation spectrum on the parameters of the micro-bunched beam has been studied and the tunability of the radiation by the variation of the micro-bunch spacing has been demonstrated. The micro-bunch spacing can be changed easily by altering the plasma density without changing the beam energy or current. Using the results of these studies, we develop a conceptual design of a tunable source of coherent terahertz (THz) radiation driven by a plasma modulated beam. Such a source would be a potential and useful alternative to conventional vacuum THz tubes and THz free-electron laser sources.

Current Filamentation in LargeBi2Sr2CaCu2O8+δMesa Devices Observed via Luminescent and Scanning Laser Thermal Microscopy

Stacked intrinsic Josephson junctions in the well studied superconductor Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8}$ are promising as a compact source of coherent terahertz radiation, which would have applications ranging from bioimaging to security screening. Understanding localized heating in such stacks is essential to optimizing their performance, and to this end the authors directly image the complex self-heating behaviors of these devices under bias conditions typical for THz emission. For good heat removal, narrow (compared to device size) filaments of current nucleate hot spots in asymmetric locations, which is consistent with theoretical predictions and suggests a means to enhance emission power.

Coherent and tunable terahertz radiation from graphene surface plasmon polaritons excited by an electron beam

Although surface plasmon polaritons (SPPs) resonance in graphene can be tuned in the terahertz regime, transforming such SPPs into coherent terahertz radiation has not been achieved. Here, we propose a graphene-based coherent terahertz radiation source with greatly enhanced intensity. The radiation works at room temperature, it is tunable and can cover the whole terahertz regime. The radiation intensity generated with this method is 400 times stronger than that from SPPs at a conventional dielectric or semiconducting surface and is comparable to that from the most advanced photonics source such as a quantum cascade laser. The physical mechanism for this strong radiation is presented. The phase diagrams defining the parameters range for the occurrence of radiation is also shown.

Seeding, Controlling, and Benefiting from the Microbunching Instability

Advanced light sources using relativistic electrons rely on coherent emission from high-density (compressed) beams. These beams, typically produced by photoinjected linear accelerators, can suffer from uncontrolled microbunching instabilities that are difficult to manage, since a complete understanding of their growth due to space charge and other wakefields is lacking. Here we present the first systematic measurements of microbunching instability using electron beams premodulated in a controlled fashion. By comparing beams having various modulation depths and wavelengths with unmodulated beams, we are able to benchmark, for the first time, the analytical calculations for the microbunching instability. In addition, our results give a proof of principle demonstration of a longitudinal space charge amplifier (LSCA), where a specific beam density pattern develops and grows. A potential application of this particular LSCA scheme is for controlling waveforms and enhancing the spectral content of linac-based sources of coherent terahertz radiation.

Powerful terahertz emission from Bi2Sr2CaCu2O8+δ mesa arrays

Stacks of intrinsic Josephson junctions in high-temperature superconductors enable the fabrication of compact sources of coherent terahertz radiation. Here, we demonstrate that multiple stacks patterned on the same Bi2Sr2CaCu2O8+δ crystal can—under optimized conditions—be synchronized to emit high-power THz-radiation. For three synchronized stacks, we achieved 610 μW of continuous-wave coherent radiation power at 0.51 THz. We suggest that synchronization is promoted by THz-waves in the base crystal. We note that synchronization cannot be achieved in all samples. However even in these cases, powers on the 100-μW scale can be generated.

Nonlinear optical conductance in a graphene pn junction in the terahertz regime

We show that the optical responses of a graphene pn junction is dominated by nonlinear intraband and interband processes. At the experimentally relevant electric field intensity, nonlinear conductance is an order of magnitude larger than the linear conductance. Furthermore, the total conductance is negative in the terahertz to far infrared regime. The negative conductance provides a unique mechanism for photon generation in graphene and could be used for developing coherent terahertz radiation sources.

Development of a coherent THz radiation source based on the ultra-short electron beam and its applications

At the National Institute of Advanced Industrial Science and Technology (AIST), a coherent terahertz (THz) radiation source has been developed based on an ultra-short electron beam using an S-band compact electron linac. The designed THz pulse has a high peak power of more than 1 kW in the frequency range 0.1–2 THz. The entire system is located in one research room of about 10 m square. The linac consists of a laser photocathode rf gun (BNL type) with a Cs 2Te photocathode load-lock system and two 1.5-m-long S-band accelerator tubes. The electron beam can be accelerated up to approximately 42 MeV. The electron bunch was compressed to less than 1 ps (rms) with a magnetic bunch compressor. The coherent synchrotron radiation (CSR) of the THz region was generated from the ultra-short electron bunch at the 90° bending magnet, and it was extracted from a z-cut quartz window for THz applications. In this work, the THz scanning transmission imaging was successfully demonstrated for measuring the freshness of a vegetable leaf over a period of time.

The importance of electron temperature in silicon-based terahertz quantum cascade lasers

Quantum cascade lasers (QCLs) are compact sources of coherent terahertz radiation. Although all existing QCLs use III-V compound semiconductors, silicon-based devices are highly desirable due to the high thermal conductivity and mature processing technology. We use a semiclassical rate-equation model to show that Ge/SiGe THz QCL active region gain is strongly enhanced by reducing the electron temperature. We present a bound-to-continuum QCL design employing L-valley intersubband transitions, using high Ge fraction barriers to reduce interface roughness scattering, and a low electric field to reduce the electron temperature. We predict a gain of ∼50 cm−1, which exceeds the calculated waveguide losses.

Numerical Study of a Coherent Terahertz Radiation Source Based on a SASE FEL

Numerical Study of a Coherent Terahertz Radiation Source Based on a SASE FEL

Effect of transverse nonuniformity of the rf field on the efficiency of microwave sources driven by linear electron beams

This paper contains a simple analytical theory that allows one to evaluate the effect of transverse nonuniformity of the rf field on the interaction efficiency in various microwave sources driven by linear electron beams. The theory is, first, applied to the systems where the beams of cylindrical symmetry interact with rf fields of microwave circuits having Cartesian geometry. Also, various kinds of microwave devices driven by sheet electron beams (orotrons, clinotrons) are considered. The theory can be used for evaluating the efficiency of novel sources of coherent terahertz radiation.

Imaging with a Terahertz quantum cascade laser

We demonstrate bio-medical imaging using a Terahertz quantum cascade laser. This new optoelectronic source of coherent Terahertz radiation allows building a compact imaging system with a large dynamic range and high spatial resolution. We obtain images of a rat brain section at 3.4 THz. Distinct regions of brain tissue rich in fat, proteins, and fluid-filled cavities are resolved showing the high contrast of Terahertz radiation for biological tissue. These results suggest that continuous-wave Terahertz imaging with a carefully chosen wavelength can provide valuable data on samples of biological origin; these data appear complementary to those obtained from white-light images.