Pulsed Terahertz Radiation Research Articles

Generation of Extremely Short Pulses of Terahertz Radiation Based on Superradiation of a Three-Level Resonant Medium

Generation of Extremely Short Pulses of Terahertz Radiation Based on Superradiation of a Three-Level Resonant Medium

Cell proliferation under intense pulses of terahertz radiation

Terahertz (THz) waves can influence a diverse spectrum of cellular processes. In the present study, we focus on the effect of high-power broadband THz pulses on cell culture. An experimental setup for long-term irradiation of cells with THz pulses with intensities of about 32 GW/cm2 has been developed. Human skin fibroblasts were exposed to THz pulses with a field strength of about 3.5 MV/cm for 90 min. Preliminary results on the THz-induced apoptosis process in cells are presented.

Extreme Ultraviolet Radiation Source Based on a Discharge Sustained by a Radiation Pulse from a Terahertz Free-Electron Laser

We consider the possibility of sustaining a discharge in xenon efficiently emitting light in a wavelength range of 11.2 nm ± 1% by a subnanosecond terahertz radiation pulse. We have simulated such a discharge for parameters typical of experiments using modern free-electron lasers as a source of a high-intensity terahertz radiation. It is shown that a NovoFEL-3 modernized free-electron laser at the Budker Institute will allow to sustain a point-like plasma discharge with a density of excited Xe10+–Xe11+ ions sufficient for the yield of extreme ultraviolet radiation with an average power of 35 W in a wavelength range of 11.2 nm ± 1%.

Terahertz pulse emission from photoexcited bulk crystals of transition metal dichalcogenides

Multilayer MoS2, MoSe2, and WSe2 crystals were excited with femtosecond optical pulses of various wavelengths. The emitted terahertz (THz) radiation pulses were found to be the most intense at photon energies coinciding with the direct energy gap at the K points of the Brillouin zone of corresponding materials. It was shown that the transition metal dichalcogenides of different dopant type emit THz pulses of opposite polarity, suggesting that the surface field plays a role in THz emission. Nevertheless, the optical pump–THz probe experiments contradicted THz emission observations, demonstrating a decrease in photoconductivity at higher energy quanta. The proposed THz pulse generation mechanism, therefore, assumes the accumulation of excitonic electric dipoles during optical pulse absorption and a fast build-up of the internal electric field inside the samples. This assumption was supported by double optical pulse investigations.

Генерация предельно коротких импульсов терагерцового излучения на основе сверхизлучения трехуровневой резонансной среды

The possibility of generating extremely short pulses of terahertz (THz) radiation due to superradiance - collective spontaneous emission of stopped polarization of a thin layer of a three-level resonant medium excited by a pair of attosecond (or femtosecond) pulses is studied theoretically. The source of a terahertz pulse is a pulse of stopped nonlinear polarization of the medium, which occurs in the interval between its excitation and de-excitation. The case of a three-level medium with equidistant energy levels (as in a quantum harmonic oscillator), the transition frequency of which lies in the THz range, is considered. The influence of the populations of the excited levels of the medium on the shape of the terahertz superradiance pulse is discussed.

Pulsed terahertz radiation from a double-barrier resonant tunneling diode biased into self-oscillation regime

The study of the bolometer response to terahertz (THz) radiation from a double-barrier resonant tunneling diode (RTD) biased into the negative differential conductivity region of the I–V characteristic revealed that the RTD emits two pulses in a period of intrinsic self-oscillations of current. The bolometer pulse repetition rate is a multiple of the fundamental frequency of the intrinsic self-oscillations of current. The bolometer pulses are detected at two critical points with a distance between them being half or one-third of a period of the current self-oscillations. An analysis of the current self-oscillations and the bolometer response has shown that the THz photon emission is excited when the tunneling electrons are trapped in (the first pulse) and then released from (the second pulse) miniband states.

THz radiation generation in semiconductor–metal nanostructure exposed to femtosecond laser pulse of focused radiation

Generation of terahertz (THz) radiation under the ponderomotive action of a femtosecond laser pulse on a doped semiconductor layer deposited on a metal surface is studied. The spectral composition, radiation pattern, total energy, and shape of the generated THz radiation pulse are found. There is a peak in the emission spectrum at the electron plasma frequency in semiconductors transparent to THz radiation. The contribution to radiation from the plasma resonance region leads to a relative increase in the total energy and manifests itself in magnetic field oscillations at the stage of pulse switching off. Optimal generation conditions are realized when the focal spot size is comparable with the pulse length and skin depth. The stronger the focusing, the wider the THz radiation pattern. With weak focusing, THz radiation is mainly concentrated along the directions close to normal to the semiconductor surface. When an odd number of half-wavelengths of high-frequency radiation is stacked at twice the thickness of the semiconductor, the generation efficiency increases sharply. In particular, in GaAs and GaSb, the efficiency increases by more than two orders of magnitude.

In Situ Hilbert-Transform Spectral Analysis of Pulsed Terahertz Radiation of Quantum Cascade Lasers by High-Tc Josephson Junctions

We have studied the applicability of high- Tc Josephson junctions (JJs) and Hilbert-transform spectral analysis (HTSA) for characterization of pulsed terahertz (THz) radiation. We have reconsidered the extension of HTSA into the THz range, including an impact of such emerging factors as nonequilibrium voltage fluctuations and dc Joule heating in high- Tc JJs, and found an unexpected weak temperature dependence of Josephson linewidth δf and a spectral dependence δf(f) with a minimum. Due to this analysis, we have chosen YBa2Cu3O7-x bicrystal JJs for experimental study. The JJs have analyzed pulsed THz radiation from quantum cascade lasers (QCLs) located in the same cryogenic environment at 50 K. The spectra recovered by the JJ with Rn = 43 Ω consist of a line with a central frequency around 2.2 THz, which have a symmetrical form and the linewidth of 90 GHz close to the intrinsic Josephson linewidth of this JJ. The spectra measured by the JJ with Rn = 7 Ω and the intrinsic Josephson linewidth of 16 GHz consist of several lines at the frequencies fi around 2.2 THz, which are due to the excitation of different resonant modes in the active volume of the QCL. The difference between the fitted central frequencies f 2 - f 1 of the strong lines and the ratio of their intensities are in agreement with the spectrum of the same QCL measured by Fourier spectroscopy. The developed technique paves the way for detailed and rapid characterization of pulsed THz sources in the frequency domain.

Relativistic terahertz radiation generated by direct-laser-accelerated electrons from laser-foil interactions

A novel scheme for generating powerful terahertz (THz) radiation based on laser-solid interactions is proposed. When a $p$-polarized femtosecond laser impinges obliquely on a plane solid target and the target partially blocks the laser energy, surface electrons are extracted out and accelerated by the laser fields, forming a low-divergence electron beam. A half-cycle THz radiation pulse is emitted simultaneously as the beam passes by the edge of the target, due to coherent diffraction radiation. Our particle-in-cell simulations show that the relativistic THz pulse can have an energy of a few tens of millijoule and the conversion efficiency can be over 1$\%$ with existing $\sim$J level femtosecond laser sources.

Highly sensitive electro-optic detection of terahertz waves in a prism-coupled thin LiNbO3 layer

Electro-optic sampling of pulsed terahertz radiation by femtosecond laser pulses in noncollinear geometry in a 40 μm thick layer of LiNbO3 attached to a Si prism is experimentally demonstrated. Terahertz radiation is prism-coupled into the layer through its surface and modulates the frequency and angular spectra of the optical radiation propagating along the layer as a fundamental mode of a dielectric slab waveguide. The modulation results in the optical intensity variations in the optical beam transmitted through the layer. By measuring the intensity variations, we achieved 38 and 216 times enhancement in the sensitivity of electro-optic detection as compared to the conventional collinear ellipsometric technique with 1 mm thick ZnTe and 0.2 mm thick GaP crystals, respectively.

The Effect of Short Pulses of Terahertz Radiation on the Destruction of Thin Lead Films

The Effect of Short Pulses of Terahertz Radiation on the Destruction of Thin Lead Films

THz Pulsed Imaging in Biomedical Applications

Recent advances in technology have allowed the production and the coherent detection of sub-ps pulses of terahertz (THz) radiation. Therefore, the potentialities of this technique have been readily recognized for THz spectroscopy and imaging in biomedicine. In particular, THz pulsed imaging (TPI) has rapidly increased its applications in the last decade. In this paper, we present a short review of TPI, discussing its basic principles and performances, and its state-of-the-art applications on biomedical systems.

Angular Dependence of Copper Surface Damage Induced by an Intense Coherent THz Radiation Beam

In this work, we show the damage induced by an intense coherent terahertz (THz) beam on copper surfaces. The metallic surface was irradiated by multiple picosecond THz pulses generated by the Free Electron Laser (FEL) at the ISIR facility of the Osaka University, reaching an electric field on the sample surface up to ~4 GV/m. No damage occurs at normal incidence, while images and spectroscopic analysis of the surface point out a clear dependence of the damage on the incidence angle, the electric field intensity, and polarization of the pulsed THz radiation. Ab initio analysis shows that the damage at high incidence angles could be related to the increase of the absorbance, i.e., to the increase of the temperature around or above 1000 °C. The experimental approach we introduced with multiple fast irradiations represents a new powerful technique useful to test, in a reproducible way, the damage induced by an intense electric gradient on copper and other metallic surfaces in view of future THz-based compact particle accelerators.

Experimental system for studying bioeffects of intense terahertz pulses with electric field strength up to 3.5 MV/cm

Terahertz (THz) waves can influence a diverse range of cellular processes. The use of high-power THz sources in biological studies may lead to major advances in understanding biological systems and help to determine safe exposure levels for existing THz technologies. We are devoted to the development of an experimental system for irradiating cells with intense broadband THz pulses. Subpicosecond pulses of THz radiation with intensities of 32 GW / cm2 and electric field strength up to 3.5 MV/cm are obtained by optical rectification, using an OH1 organic crystal, of near-infrared femtosecond pulses generated by a Cr:forsterite laser. The system has been developed to allow cells to be kept in suitable conditions for long-term exposure and to be irradiated with THz pulses in single-point mode as well as in scanning mode. The transmission in the THz region of various plastic dishes for cell culture is estimated.

The universal character of the breakdown of condensed media by powerful terahertz radiation and the Abbe criterion

This paper analyzes new experimental results on the surface breakdown of semiconductors and thin metallic films by ultrashort pulses of plane-polarized terahertz radiation. It shows that the character of the observed breakdown is satisfactorily explained by the universal polariton model, tested with ultrashort pulses of visible and near-IR radiation interacting with condensed media. By using a nonlinear mathematical model of the formation of the spatial periods of a nanograting that are multiples of the wavelength of the exciting radiation, it is shown that the periods of the gratings thus formed are substantially shorter than the linear diffraction limit in the optical and terahertz regions. It is demonstrated that the diffraction limit can be overcome in nonlinear processes by which interference nanogratings are formed with normal and anomalous orientations.

Review of photomixing continuous-wave terahertz systems and current application trends in terahertz domain

Within the last two decades, terahertz systems have become a well-established tool in scientific laboratories and industrial research and development departments. Unlike the methods of pulsed terahertz radiation that mainly rely on ultrafast optical technology, the technology behind continuous-wave terahertz sources and detectors has a long history involving many different types of technical schemes. The intent is to provide a brief review of the continuous-wave terahertz systems that are based on photomixing and current application trends in the field of terahertz technology.

Generation of two terahertz radiation pulses with continuously tunable frequency and time delay

We propose to generate two narrow band terahertz pulses radiated from two temporally modulated relativistic electron beams, which are generated in a photo-injector. The temporal profile of the drive laser is modulated by means of the paired chirped pulses beating technique, leading to the generation of two pre-bunched electron beams. Coherent transient radiation (CTR) is considered as the mechanism for terahertz radiation generation. The frequencies of the two terahertz pulses can be independently tuned by adjusting the paired beating frequencies, and the interval between the two terahertz pulses can be adjusted by the optical delay line.

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.

Degradation and Destruction of Thin Steel Films under Repeated Exposure to Ultrashort Pulses of THz Radiation

The destruction of thin films of stainless steel with repeated exposure to ultrashort pulses of directional terahertz radiation has been studied. A terahertz pulse with a duration of picoseconds essentially models a nonresonant impact on an object of a quasi-permanent electric field with high intensity (up to 20 MV/cm).

A Study of the Effect of Terahertz Electromagnetic Radiation on Microbial Cell Viability

Abstract—The effect of pulsed terahertz radiation at a wavelength of 66 μm, a pulse duration of 100 ns, and a pulse energy of 200 mJ on a suspension of microbial cells was studied. The effect of terahertz radiation was shown to cause cell death at a value of total energy of ~6 J. Since the temperature of the cell suspension after radiation remained almost unchanged, it implies that pulsed terahertz radiation exhibited a non-thermal effect. Methods of standard microbial plating, electron microscopy, electroacoustic analysis, and spectral analysis were used in order to verify the results. These experiments make it possible to control the viability of bacterial cells using terahertz radiation with certain parameters.