Investigation of parameters of terahertz pulses generated in single-domain LiNbO<Subscript>3</Subscript> crystal by step-wise phase mask

A new scheme for the efficient generation of broadband terahertz radiation via optical rectification of femtosecond laser pulses in the single-domain lithium niobate crystal equipped with the step-wise phase mask (SPM) is investigated. It is shown that using the SPM one can provide the phase matching for all the spectral components of a terahertz pulse by providing the effective conversion of laser radiation in the terahertz region. The angular distribution of spectral components, as well as the temporal shape of terahertz pulses in the wave zone is studied. These results can be applied in the time-domain spectroscopy, the imaging of hidden objects, and etc.     

Generation of terahertz radiation during reflection of femtosecond laser pulses from a metal surface

The features of terahertz radiation generated upon oblique incidence of a femtosecond laser pulse on a metal surface are investigated theoretically. We propose a Cherenkov-type generation mechanism associated with excitation of low-frequency surface currents by a p-polarized optical field. The nonlinear surface currents and corresponding electrodynamic characteristics of low-frequency radiation are calculated analytically and by numerical simulation using the hydrodynamic model. The features of Cherenkov generation of terahertz radiation are analyzed, and techniques are proposed for increasing efficiency.     

Terahertz emission from a metallic surface induced by a femtosecond optic pulse

Results of experimental and theoretical investigations on generation of terahertz radiation at the interaction of femtosecond laser pulses with a metal surface are presented. Investigations are performed with the laser pulse intensities higher compared with that used in papers [Opt. Lett.29, 2674 (2004); Opt. Lett.30, 1402 (2005)]. The most effective generation is observed for p-polarized optical pulses with incidence angles in the range 5°-10° (from the surface), depending on the kind of metal. For the copper, the exponential growth of terahertz pulse energy with the increase of optical pulse energy was registered. Theoretical interpretation for some of the experimental results is proposed based on the model of free electrons in metal.     

Generating terahertz radiation via optical rectification in nonlinear crystals: Theory and experimental results

Key principles of terahertz radiation generation via laser pulse optical rectification are reviewed. The development of theoretical concepts is considered in connection with recent experimental results. The usability of resonant effects and metamaterials for the further development of this technique of generation is analyzed.     

Generation of nanosecond terahertz pulses by the optical rectification method

The possibility of the generation of quasi-cw terahertz radiation by the optical rectification method for broad-band Fourier unlimited nanosecond laser pulses has been experimentally demonstrated. The broadband radiation of a LiF dye-center laser is used as a pump source of a nonlinear optical oscillator. The energy efficiency of terahertz optical frequency conversion in a periodically polarized lithium niobate crystal is 4 × 10⁻⁹ at a pump power density of 7 MW/cm².     

Theoretical model of terahertz radiation generation by laser pulses with tilted wave fronts

A theoretical model is proposed for terahertz radiation generation by femtosecond laser pulses with tilted wave fronts. Self-consistent evolution of the optical and terahertz pulses is investigated. The red shift of the pumping pulse spectrum and the formation of short-long-wave quasi-solitons are examined.     

Generation and detection of ultrashort pulses of electromagnetic field in the terahertz range and their application for spectroscopy

We present experimental results on generation of pulsed terahertz radiation in a ZnTe crystal using nonlinear optical rectification of high-power femtosecond laser pulses. The use of terahertz pulses for spectroscopy is tested in the rotational lines of absorption of water vapors in the atmosphere. Panoramic spectra of absorption of a number of solvents and protein solutions are measured in the range from 0.2 to 2 THz.     

Čerenkov radiation excited by an ultrashort laser pulse with oblique amplitude front

We develop the theory of generation of terahertz ?erenkov radiation in a homogeneous nonlinear optical crystal if the amplitude and phase fronts of the excitation pulse do not coincide. It is shown that in the case of the nonlinear optical interaction, the use of laser pulses with oblique amplitude front makes it possible to increase the efficiency of generation of terahertz radiation. In contrast to the case where the amplitude and wave fronts coincide, radiation propagates in a strictly determined direction specified by the amplitude-front plane. It is established that the maximum generation efficiency is reached if the angle of inclination of the amplitude front coincides with that of the terahertz-radiation wave vector.     

Off-axis metallic diffractive lens for terahertz beams

A diffractive optical element for off-axis focusing of terahertz radiation is presented. It was designed in a nonparaxial regime and manufactured in a metal slab by laser cutting of curved stripes. The optical function of the structure includes focusing and deflecting the illuminating beam of a chosen frequency in a particular place. Therefore, the element acts as both a spatial and a spectral filter; hence it is especially suitable for separating the terahertz signal from a broadband thermal load in passive detection devices. The experimental evaluation of the proposed diffractive lens by means of time-domain spectroscopy is presented and discussed.     

Effects of the evolution of two cross‐focused Gaussian laser beams on the terahertz generation in thermal collisional plasma

Generation of the terahertz (THz) radiation based on the beating of two cross‐focused high intensity Gaussian laser beams in a warm rippled density plasma is numerically investigated, taking into account the ponderomotive force, Ohmic heating, and collisional nonlinearities. The beat ponderomotive force as a result of cross‐focusing of beams induces a vertical velocity component that by coupling with the rippled density gives rise to a nonlinear current deriving THz radiation. The effect of laser beams spot size evolution and plasma parameters on the THz generation is studied. It was found that there exist special electron temperature and laser intensity ranges with “turning points” where the generation of THz radiation reaches its maximum value and outside of these ranges, it disappears. The results also indicated that increasing the background electron density as well as taking into account the collision frequency help THz generation. Moreover, the maximum yield of THz radiation occurs when the beat wave frequency approaches the plasma frequency.     

Characteristic features of generation of few-cycle pulses in infrared and terahertz spectral ranges upon interaction of two femtosecond pulses of different frequencies in air

The results of a theoretical analysis of the generation of broadband radiation in the infrared and terahertz spectral ranges upon the excitation of plasma in air by two femtosecond pulses at the fundamental and second-harmonic frequencies of a Ti-sapphire laser are presented. It is found that the appearance of long-wavelength radiation in a strong field of pulses of different frequencies can be described in terms of strongly anharmonic oscillations of optical electrons, whereby electrons are pulled far away from their atoms; these oscillations are accompanied by cascade transitions of electrons from their ground state to a bound excited state, followed by a transition to the continuum. It is shown that the generated infrared and terahertz radiation appears in the form of pulses containing a few oscillations of the light field. The efficiency of terahertz generation varies periodically with an increase in the interaction length of the femtosecond pulses of different frequencies.     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.     

Experimental study on terahertz radiation

In this letter, we describe a coherent subpicosecond terahertz (THz) spectroscopy system based on nonresonant optical rectification for the generation of THz radiation. We studied the two-photon absorption (TPA) of ZnTe induced by femtosecond laser pulses via THz generation, and its influence on the generation of THz radiation. Experimental results demonstrated that the intensity of pump beam against TPA must be traded off to get an optimum generation of THz radiation. As an example, we measured absorption spectrum of water vapor by time-domain spectroscopy (TDS) in the frequency range from 0.5 to 2.5 THzwith a high overall accuracy.     

Generation of terahertz radiation by optical rectification in presence of resonance impurities

We demonstrate the possibility of an efficient generation of terahertz radiation under optical rectification in a uniaxial crystal containing impurities with resonance two-level quantum transitions. The group velocity of an optical pulse entering the regime of self-induced transparency decreases to the value of phase velocity of the terahertz radiation, which results in phase matching of long- and short-wavelength waves. The terahertz generation is accompanied by strong self-modulation of the optical pulse. Two-component soliton-like structures can be formed at long distances accompanied by spectral red shift of the optical component.     

Generation of terahertz radiation in cubic non-centrosymmetric crystals

The conditions of parametric radiation generation on polaritons in cubic noncentrosymmetric crystals are studied. The possibility of such generation is theoretically justified. The polariton radiation frequencies are calculated for GaP, ZnSe, ZnTe, and GaAs crystals. The obtained generation frequencies are compared to the experimental results on Raman scattering on polaritons. The block diagram of the terahertz radiation generator operation using a GaP crystal and a pulsed laser with high peak power at low energy of laser pump pulses is presented. The lasing frequency shift is analyzed depending on the scattering geometry. The coefficient of exciting radiation conversion to the terahertz range is determined.     

Diffractive optical elements and quasioptical schemes for experiments on a high-power terahertz free-electron laser

We have developed reflective diffraction optical elements (DOE) for focusing monochromatic coherent radiation of 400 W terahertz Novosibirsk free-electron laser (FEL). Operational characteristics of two modifications of the refractive kinoform lenses were studied. Quasioptical Toepler system with terahertz radiation recording by a thermosensitive luminescent screen was fabricated for the examination of film and solid deformation. A system for real-time terahertz radioscopy of objects with image recording with a microbolometer matrix (MBM) was fabricated. Images of objects illuminated with a plane wave or diffuse radiation were studied with the MBM. It was shown that the speckle pattern, which appears in the second case, can be averaged by means of the scatterer rotation.     

Frequency conversion of CO laser radiation in the ZnGeP<Subscript>2</Subscript> nonlinear crystal

The experimental results of the study of CO laser frequency doubling in ZnGeP₂ and GaSe nonlinear crystals of high optical quality are presented. A multifrequency low-pressure Q-switched CO laser and a selective pulsed electroionization mode-locked CO laser with cryogenic cooling were used in the experiments. The highest external factor of radiation conversion into the second harmonic of the pulse-periodic Q-switched CO laser and pulsed mode-locked CO laser was 1% and 3.5%, respectively. Theoretical calculations showed the possibility of obtaining far IR and terahertz radiation using difference frequency generation of CO laser lines in these crystals.     

差频可调谐太赫兹技术的新进展

太赫兹技术在最近30年来得到快速发展, 并在医学、生物、农业、材料、安检、通信、天文等领域得到广泛应用. 从太赫兹源的频谱特性可以分为窄带(单频)太赫兹源和宽带太赫兹源. 从频谱技术方面来说, 相干的宽带和窄带太赫兹谱是一种互补性关系, 具有各自的技术特点和应用范围. 宽带太赫兹谱可以用于快速获取较宽频谱范围的分子振转谱, 实现混合特征谱的快速检测或成像. 窄带太赫兹源具有很好的光谱灵敏度和分辨率, 适用于太赫兹抽运-探测、分子振转能级谱精细结构分辨 以及太赫兹远程探测和成像. 因此研制具有可调谐的高峰值功率的窄带太赫兹源是适用于探测和识别分子振转能级指纹谱的应用需求, 而差频技术是获得高功率和宽调谐窄带太赫兹源最重要的技术之一. 为了突出该技术的最新进展, 本综述引证论文仅仅限于近5 年来基于差频技术产生太赫兹波的研究进展, 分为光学激光差频源和量子级联激光器差频源两大部分. 对于光学激光差频源, 分别对目前文献报道的各种双波长差频源和太赫兹产生用的非线性晶体进行分类介绍, 并给出所采用的技术和实验结果; 对于量子级联激光器差频源, 分别介绍了量子级联激光器中的差频产生技术和波长调谐技术的最新进展. 量子级联激光器差频太赫兹源是目前实现量子级联激光器在太赫兹波段室温运转的惟一技术, 是实现小型化、窄带宽调谐和室温运转太赫兹源的新发展领域, 值得关注.     

Efficient generation of terahertz radiation by the method of optical rectification of terawatt laser pulses

Factors that lead to a decrease in the width and the mean frequency of the spectrum of terahertz (THz) radiation that were observed in recent experiments on optical rectification of powerful (∼0.5 TW) femtosecond laser pulses with a tilted amplitude front in a lithium niobate crystal are considered. A simple method is proposed that allows one to avoid the above transformation of the spectrum and to ensure a high efficiency of generation of THz radiation when laser pulses with a power of several TW are used.