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.     

Generating terahertz radiation by means of optical rectification

Various means of raising the efficiency of generating wideband terahertz radiation on the basis of optical rectification are analyzed.     

基于等离子体紫外辐射的强激光自动寻焦系统

精准定位激光束焦点位置是提高激光雕刻、切割、焊接等加工精度的重要基础,而传统测量方法不适用于自动寻找强激光焦点。基于激光烧蚀金属后等离子发光含大量紫外谱线的原理,以日盲的氮化镓肖特基光电二极管为传感器,设计了以304不锈钢靶材为耗材的红外强激光束自动寻焦方法及其装置。该方法与共聚焦显微镜检测平均烧蚀坑深的方法相比,当以脉冲宽度100 ns、重复工作频率20 kHz、平均功率10 W的1064 nm光纤激光雕刻机为实验对象时,二者定焦位置相差24 μm。     

Generation of terahertz radiation in the regime of resonant-nonresonant optical rectification

The generation of broadband terahertz radiation in quadratically nonlinear crystals containing additional resonant impurities having a permanent dipole moment has been theoretically analyzed. It has been shown that the contributions to the intensity of the generated radiation from resonance and nonresonance mechanisms of optical rectification are nonadditive. In semiconductor nanostructures and systems of organic molecules, the energy efficiency of generation due to the resonance mechanism can reach about 0.001, which is much higher than the corresponding efficiencies of the currently widespread tilted front technique. The spectra of the input optical pulses exhibit a noticeable red shift due to the resonance-nonresonance rectification.     

Active optical storage ring for high-power laser pulses

In an optical storage ring, a dye laser amplifier synchronously pumped by external laser pulses has been implemented and experimentally tested. The test was done at =580 nm, but the optical system can be used without limitations in a broad band of 400–700 nm. It is shown that a selected turn of the stored laser pulse can be amplified by typically a factor of 200. Power consideration gives an increase of the efficiency of the optical storage ring with a dye amplifying cell, as compared to a single passage of the laser pulse through the experimental section, by 23 times and it is shown that it can reach a factor of more than 100. PACS 42.60.Da; 42.60.Lh     

Study of terahertz radiation generated by optical rectification on thin gold films

Emission of terahertz (THz) radiation as a result of optical rectification of intense femtosecond laser pulses on thin gold films has been studied by time-domain THz spectroscopy. The THz amplitude was measured as a function of film thickness and incidence angle. The experiments reveal that the emitted THz field is suppressed for a thickness below 100 nm, which gives evidence of the nonlocal character of the response. The variation of incidence angle allows us to estimate the components of susceptibility tensor chi2ijk. For thicker films and near grazing incidence, the emitted THz field attains a peak value of 4 kV/cm.     

高功率激光透过窗口材料的传输效应分析

在1.315μm波长氧碘激光的透射下,对目前破坏阈值高、透过率好的白宝石窗口材料进行了解析的热应力与热应变分析,求得了吸收激光功率产生热透镜焦距和热场分布的解析式.计算了白宝石晶片在波长1.315μm激光倾斜入射时的折射效应,在5mrad夹角下影响极微,可以满足氧碘激光远场方向角条件下的使用.     

Nonlinear processes in the optical medium of a high-power neodymium laser

The article is devoted to an analysis of the nonlinear processes that arise when powerful beams propagate in the optical medium of an Nd laser and influence its output parameters. A review is presented of the methods of theoretically describing the processes of temporal and spatial self-focusing, and experimental data are cited that illustrate the character of the course of the self-focusing in the generators and amplifiers of a high-power laser. Possible methods of suppressing self-focusing in a laser installation are considered.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 103, pp. 84–117, 1978.     

周期极化铌酸锂中光整流THz波辐射

从理论上详细研究飞秒激光在周期极化铌酸锂(PPLN)晶体中由光整流效应产生的THz波辐射.着重研究THz波辐射场的频域场、时域场和频谱宽度的分布.并详细讨论辐射场脉冲持续时间、幅度、频谱宽度随晶体长度和辐射角的变化. 关键词： PPLN 光整流 THz波     

Hard protective waterproof coating for high-power laser optical elements

We developed a new method for making a waterproof coating by photooxidation of silicone oil. The silicone oil was spin coated onto the surfaces of optical elements, i.e., a plastic lens, a laser mirror, and a nonlinear optical crystal, and then irradiated with a xenon excimer lamp in air, which transformed the organic silicone oil into an amorphous glass film. This technique has enabled an optical thin film to transmit ultraviolet rays of wavelengths below 200 nm and to exhibit the characteristics of homogeneity, high density, and resistance to environmental effects and to corrosion by water, and a Mohs scale value of 5.     

The generation of terahertz radiation via optical rectification in the self-induced transparency regime

The possibility of efficient terahertz generation via optical rectification in electro-optic crystal doped with the two-level resonant impurities is demonstrated. Under conditions of the self-induced transparency regime the laser pulses slows down to achieve the phase matching condition. The terahertz generation is accompanied by periodic modulation and spectral deformation of the laser pulse. The most efficient generation occurs under condition of small negative detuning from the resonant frequency of the two-level system.     

Interaction of the radiation of the high-power ytterbium-fiber laser with inhomogeneous dielectric targets

The action of the radiation of the ytterbium-fiber laser (λ = 1.07 μm) on the Nd³⁺Y₂O₃ target with nonuniform transparency in the course of the nanopowder production is studied. It is demonstrated that the laser irradiation leads to an extremely rough surface with the stalagmite roughness due to a relatively large melting depth. The resulting powder consists of two fractions. The first fraction (99% of the total mass of the powder) consists of nanoparticles with a mean size of 29 nm (BET data). The second fraction consists of micro- and submicroparticles that represent circular drops condensed from the melt and shapeless debris of the target. The peaks on the diameter distribution of the drops at 2, 8, and 80 μm are determined by different effects. The laser heating of the inhomogeneous target with the nonlinear refractive index is numerically analyzed. It is demonstrated that the melting of the target is initiated at a mean laser power of 700 W, a power density of 5.6 × 10⁵ W/cm², and an irradiation time of 150 μs.     

Photophysical processes stimulated in nanoporous silicon by high-power laser radiation

Photoprocesses initiated on the surface of porous silicon irradiated with laser radiation with wavelengths (λ = 266, 337, and 532 nm) in a wide range of intensities (up to 2 × 10⁷W/cm²) were investigated. Laser-induced luminescence and laser mass-spectrometry were used as experimental procedures. X-ray reflection was used to determine the parameters of the porous silicon films. The photoluminescence spectra obtained at different wavelengths and low intensities were analyzed. This analysis showed that for an optically thin layer of porous silicon the luminescence spectrum does not depend on the wavelength of the exciting radiation. This indicates the existence of a separate system of levels in porous silicon that are responsible for the luminescence. The behavior of the photoluminescence spectra as a function of the intensity q of the exciting radiation was investigated. It was shown that the luminescence intensity is a nonlinear function of q. At high intensities of the exciting radiation, the luminescence intensity saturates and a short-wavelength shift of the spectra is observed; this is due to the high concentrations of photoexcited carriers. This increases the probability of the experimentally observed nonequilibrium photodesorption of H₂ and Si from the surface of porous silicon.     

Investigation of nonlinear characteristics of intensity limiters of high-power laser radiation

The problem of determining nonlinear-optical characteristics of working substances of limiters of high-power laser radiation is analyzed. A new method of simultaneous calculation of the beam-waist radius and nonlinear absorption coefficient from the Z-scan data is proposed. The value of nonlinear optical characteristics of ZnSe, porphyrin-graphene (Graphene-TPP), fullerene-graphene (Graphene-60), polyethylene oxide containing multilayer carbon nanotubes (PEO/MWNT), along with polymethine dyes PD-792 and PD-7098 and dicyanomethylene-pyran dyes DCM-627 and DCM-684, are obtained. Different working substances are compared for the first time under the same conditions by plotting output characteristics of laser-radiation limiters. The change of the laser-pulse shape after propagation through the limiters with different working substances is demonstrated.     

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.     

Absorption and scattering of high-power laser radiation in low-density porous media

We have experimentally investigated the interaction of high-power neodymium laser pulses in the intensity range 10¹³–10¹⁴ W/cm² with flat low-density (0.5–10 mg/cm³) agar-agar targets under conditions of interest for problems of inertial nuclear fusion. Optical and x-ray methods with high temporal and spatial resolution were used to examine the dependence of absorption and scattering of the incident beam on the initial mean density and thickness of the irradiated samples. We show that when a porous target is irradiated, a bulk absorption layer of high-temperature plasma is produced inside the target whose dimensions are determined by the initial density of the material. The time dependence and spectral composition of the harmonics 2ω ₀ and 3ω ₀/2 observed in the plasma-scattered radiation are measured. A theoretical model is developed that describes the interaction of high-power laser pulses with a porous medium. Predictions of the model, based on the hypothesis of two stages of homogenization of the target material—a fast stage (0.1–0.3 ns) and a slow stage (1–3 ns), are in good agreement with the experimental data. Zh. éksp. Teor. Fiz. 115, 805–818 (March 1999)     

Quartz optical filter for wavelength selection of frequency-doubled laser based on optical rotatory dispersion effect

Based on the optical rotatory dispersion effect, an optical filter for selecting the second harmonic of a frequency-doubled laser is constructed from quartz in combination with polarizers. The operating principle is analyzed by matrix formulation, and the result indicates that the second harmonic of a frequency-doubled laser will be obtained when the rotation angle has a difference of (2n + 1)π/2 (n = 0, 1, 2, 3,… ) between the two polarizations of the second-harmonic laser and the fundamental laser. The spectrum of the output laser is taken by the AQ-6315A spectrometer, and the experimental results are in good agreement with the theoretical results.     

Specific features of the welding of metals by radiation of high-power fiber laser

The properties of the plasma that results from the laser welding of metals at various powers of the 10-kW ytterbium-doped fiber laser are experimentally studied. A projection system with an image intensifier based on a copper-vapor laser is employed for the direct observation of metal surface in the course of the laser welding in the presence of the high-intensity background illumination. A method for the shadow photographing is proposed for the analysis of the presence of metal vapor in plasma, since such vapor can lead to the reflection of the laser radiation in the course of welding.