激光大气成丝中的非线性光频转换

超快强激光在光学介质（如空气）中传播时由于克尔自聚焦效应和等离体散焦效应动态平 衡会发生一种独特的非线性激光成丝现象。激光成丝过程会诱导一些独特的物理现象,如非线性 光频转换产生超连续光谱、等离子体诱导高压放电、锥形辐射等,在大气传感、天气控制等研究 领域具有重要的应用前景。本文针对飞秒激光大气成丝过程中与传输介质相互作用所诱导的非线 性发光过程,介绍了激光大气成丝所产生的超连续光谱（白光）激光、谐波产生和太赫兹波辐射 三种非线性光频转换现象,并着重探讨了太赫兹波辐射的物理机理、研究现状和应用前景。     

Spontaneous Faraday rotation due to strong laser radiation in a plasma

Theoretical studies on intense laser radiation interacting with plasma reveal an induced nonlinear birefringence which turns out to be spontaneous Faraday rotation. Here, this rotation is called nonlinear Faraday rotation (NFR). The expressions for the nonlinear refractive indices of the laser fields are calculated both in relativistic and non-relativistic limits for a two-component plasma. The NFR angles due to nonlinear induced birefringence are derived in the absence of externally imposed magnetic fields. It is shown that, in the relativistic limit, the electron motion plays an important role in producing NFR and so in the generation of induced magnetic fields.     

A Broadband Infrared Laser Source (2.5–17 μm) for Plasma Diagnostics

This paper presents the results of studies aimed at the creation of a hybrid laser system which is composed of a gas lasers and a nonlinear crystal and appreciably broadens and enriches the radiation spectrum of these lasers. A highly efficient conversion (37%) is attained when generating the second harmonic in a ZnGeP₂ crystal owing to an increase in the peak power of CO laser radiation in the mode locking regime. The two-cascade conversion (generation of both sum and difference frequencies) of radiation of a broadband CO laser in the single sample of such nonlinear crystals as ZnGeP₂ and AgGaSe₂ is demonstrated. In this case, the radiation spectrum is broadened by nearly a factor of two, and the number of detected spectral lines grows by a factor of four. The use of a comparatively simple laser system of gas-discharge CO and CO₂ lasers to conversion in AgGaSe₂ results in laser radiation tunable over a set of narrow spectral lines within a range from 2.5 to 16.6 μm (more than two and a half octaves).     

The use of three-photon absorption for optical limiting of laser radiation

The results of a study of nonlinear refraction and nonlinear absorption at a wavelength of 532 nm in an LGO (Li₂Ge₂O₁₅) crystal by Z-scan methods are presented. It is shown that the nonlinear refraction is caused by self-focusing of the laser radiation, whereas the nonlinear absorption is three-photon in nature. The possibility of using an LGO crystal for producing the optical limiting of laser radiation of picosecond duration is demonstrated.     

Transient-Pulse Nonlinear Spectroscopy with the Radiation of a Multimode THz Gas Laser

We report on transient-pulse nonlinear spectroscopy with the radiation of a multimode THz gas laser. The method is demonstrated for studying the nonlinear response of a current-carrying superlattice to THz radiation; the current through a superlattice can be suppressed by a strong THz field. The method makes use of the pulses of a high power multimode THz gas laser. By splitting the laser beam for selected laser modes into a main beam and a reference beam we monitored with a reference detector the transient power in the main beam. The simultaneous measurement of both the instantaneous response and the instantaneous power allows to obtain the power dependence of the response within a single laser pulse. The method is suitable to study the nonlinear response of matter to THz radiation fields in a large dynamic range.     

Generation of continuous-wave 243-nm radiation by sum-frequency mixing

We have generated tunable cw radiation near 243 nm with a linewidth of less than 4 MHz by sum-frequency mixing the 351 nm radiation from an argon-ion laser with the 789 nm radiation from a ring dye laser in a crystal of ammonium dihydrogen phosphate held at moderate temperature. An external ring cavity, resonant with the dye laser, gives a power enhancement of about 12 in the sum-frequency generated radiation. Thermal lensing due to laser heating of the nonlinear crystal, distorded the 351 nm mode structure. This effect could limit the efficiency of the sum frequency mixing process.     

Plasma mechanisms of pulsed terahertz radiation generation

We present the results on generating terahertz radiation in the plasma of an optical discharge arising in the atmosphere during the focusing of femtosecond laser radiation. Different generation schemes related to focusing of the optical radiation by spherical and axicon lenses, with a constant electric field imposed on the laser spark region, as well as with the use of bichromatic laser radiation, are studied. Directivity patterns and polarization distributions of the terahertz radiation are analyzed in detail for different generation techniques. Comparison with the experimental results obtained by other research groups is given. Possible nonlinear mechanisms of the terahertz radiation generation are discussed.     

Direct four-photon excitation of amplified spontaneous emission in a nonlinear organic chromophore

We report four-photon pumped amplified spontaneous emission in an organic chromophore. The phenomenon is observed when intense infrared laser radiation illuminates a nonlinear chromophore solution. As a result of this illumination, a strong and highly directional radiation in the visible wavelength range is generated in both the forward and the backward directions, with an angular divergence similar to that of the pump beam.     

一种芴衍生物的三光子吸收光稳幅效应及三光子吸收拟合方法研究

由于忽略激光能量分布的不均匀性,当前常用的非线性透过率方法得到的三光子吸收系数的拟合误差比较大.现有材料的三光子吸收截面还比较小,阻碍了人们对三光子吸收特性的应用.本文提出了一种精确的三光子吸收拟合方法,适合于各种能量分布不均匀的激光脉冲.合成了一种具有三光子吸收特性的对称型芴类衍生物2,7-二(4-甲氧基苯乙炔)-9-硫酮芴,三光子吸收截面较大,具有很好的激光脉冲光稳幅特性.     

Nonlinear frequency conversion of copper vapor laser radiation using convergent and parallel laser beams

Results of experiments are presented on the comparison of the efficiency of radiation generation at the sum frequency (λ=0.271 μm) of copper vapor laser (CVL) in a nonlinear DKDP crystal using either convergent or parallel laser beam. An intensity of UV radiation of 0.34 W and a conversion efficiency of 6% obtained under conditions of strong focusing occur to be substantially lower as compared to those obtained using a parallel laser beam of the same power (0.73 W and 12%, respectively). Experimental data on the structure of a CVL beam generated with an unstable resonator are presented, and physical reasons for the limitation of the efficiency of nonlinear frequency conversion are analyzed.     

Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 4.5 μm

A cw mid-IR coherent source based on difference-frequency generation is designed and characterized. For mid-IR generation, a periodically poled MgO:LiNbO(3) crystal is placed inside a compact Ti:sapphire laser cavity. This provides high-power pump radiation for the nonlinear process. Optical injection by an external-cavity diode laser ensures single-frequency operation of the Ti:sapphire laser, while signal radiation is provided by a fiber-amplified Nd:YAG laser. Mid-IR radiation can be generated with 3850-4540 nm tuning range, narrow linewidth, Cs-standard traceability, and TEM(00) spatial mode. 30 mW power is obtained at 4510 nm.     

On the Effective Metric for an Electromagnetic Wave Propagating in the Field of an Intense Laser Radiation

It is shown that in the nonlinear electrodynamics of vacuum, which is a corollary of quantum electrodynamics, a weak electromagnetic wave propagates in the field of an intense laser radiation as if this took place in some effective space-time whose metric tensor depends on the electric field of the laser radiation. The components of the metric vector of the effective space-time along the geodesic lines of which the rays of the weak electromagnetic wave propagate in the given case have been found. It is pointed out that this property may serve as a basis for observing the manifestations of the nonlinearity of the electrodynamics of vacuum.     

On the fundamental possibility of creating a laser reference for a local acoustic pressure in a fluid

The possibility of establishing an unambiguous relation between the amplitude of a local acoustic pressure in a fluid and the parameter laser radiation scattered on microparticles located in this region has been studied. The fundamental difficulty of the problem, which has not been overcome in preceding works, is due to the uncontrollable phase taper of probe laser beams even in the simplest case of a harmonic acoustic wave. It has been shown that the inclusion of nonlinear components of the phase taper makes it possible to unambiguously express the amplitude of the local excess pressure in the region of interference of two crossing laser beams in terms of the parameters of the medium and the parameters of scattered radiation. This indicates the fundamental possibility of creating a laser reference for acoustic pressure in fluids.     

Cascaded carbon monoxide laser frequency conversion into the 4.3-4.9 μm range in a single ZnGeP2 crystal

Collinear cascaded mid-IR frequency conversion in a single nonlinear optical crystal was accomplished. Concurrent collinear generation of the sum frequency of multiline fundamental band carbon monoxide laser radiation as a first frequency conversion cascade resulted in collinear difference frequency generation within the 4.3 to 4.9 μm spectral range when mixing this sum frequency radiation with the fundamental one as the second cascade in the same ZnGeP(2) nonlinear optical crystal.     

Efficient three-wave mixing in a three-level atomic medium with an assisting microwave driven field

The potential for nonlinear conversion between two laser pulses in a three-level V-type medium with assistance of an auxiliary microwave resonant radiation is studied. The results show that microwave driven field can lead to the parametric generation of a new laser pulse with high conversion efficiency when a weak pump laser pulse is applied.     

Violet light generation by frequency doubling of GaAlAs diode laser using a metallo-organic complex crystal ZnCd(SCN)4

Violet 404-nm radiation is generated by type-I single-pass frequency doubling of the output of an GaAlAs diode laser in critically phase-matched ZnCd(SCN)₄ (ZCTC) crystal, a metallo-organic complex crystal. In this paper, its refractive index and second-harmonic generation phase-matching angles are reported. Using a 3-mm-long ZCTC crystal, a diode laser power of 473 mW generated 390 μW of second-harmonic radiation. More recently, 380-nm, 20 μW ultraviolet (UV) radiation is also realized by frequency doubling of the 100 mW output of a continuous-wave Ti:sapphire laser at 760 nm. ZCTC is a promising UV nonlinear optical candidate material for frequency doubling of diode lasers.     

Coherent bisection of 141 THz using sum frequency generation of 1064 and 709 nm radiation

The frequency interval (141 THz) that exists between 1064 nm radiation and the unusual semiconductor wavelength of 709 nm has been coherently divided by using an optical phase-locked loop to control a slave laser lying at the mean frequency of these two wavelengths. The 709 nm radiation has been generated by a combination of wavelength tuning in an extended cavity and temperature tuning of a ridge-waveguide semiconductor laser with a nominal wavelength of 728 nm. Two nonlinear processes have been used to produce the coherent division: the sum frequency mixing of 1064 and 709 nm radiation to produce 425 nm radiation and the second harmonic generation of 851 nm light to produce the same wavelength radiation.     

基于光子晶体光纤和拉锥式单模光纤的超连续光谱产生的实验研究

本文使用重复频率为250 MHz、脉冲宽度为135 fs、最大功率为2.2 W的锁模掺镱光纤激光作为种子源,利用光子晶体光纤和自制的拉锥式单模光纤两种高非线性光纤研究了超连续光谱的产生特性,通过对比两种光纤的结构、色散等特性,分析了拉曼孤子、色散波及其他非线性效应对产生的超连续谱形状的影响,并均得到了大于一个倍频程的超连续光谱,特别是拉锥式单模光纤产生的超连续光谱,耦合效率达到60%,这为众多研究领域,尤其是光学频率梳的建立提供了实用的超连续光源.     

Modeling of systems for broadening of spectrum of petawatt laser pulses and for their nonlinear compression

We numerically model the propagation of high-power laser pulses in a periodic system of nonlinear elements separated by spatial retransmitter filters. We show that, if the accumulated phase difference in an individual nonlinear element is restricted by a value smaller than 1.6 rad, it is possible to obtain the total accumulated phase difference greater than 14 rad while preserving the high radiation brightness. The considered scheme can be used for compressing superpower laser pulses.     

Frequency synthesis of the 0.63 μm radiation by mixing of three IR radiation frequencies in a gas

The present paper offers a scheme of synthesis and measurement of frequency of 0.63 μm laser radiation based on mixing of three frequencies of a cascade He-Ne laser generating simultaneously the three lines: 3.39, 2.39, and 1.15 μm. Reported is an experimental observation of coherent radiation with a wavelength of 0.63 μm arising in mixing of the three frequencies in a gas. The effective frequency transformation was performed at the nonlinear interaction of three fields resonant with coupled transitions in neon. Due to the resonance conditions an efficiency of the frequency transformation in a gas proved to be 8–10 orders higher than that in nonlinear optical crystals.