Optical rectification at metal surfaces

The emission of freely propagating terahertz (THz) radiation coming from optical rectification at metallic surfaces has been detected and characterized for the first time to the authors' knowledge. The observed THz transients are induced through nonlinear electronic processes at gold and silver surfaces on intense pulsed optical photoexcitation and exhibit a peak electric field of as much as 200 V/cm. This finding opens a qualitatively new way to investigate nonlinear phenomena at metal surfaces and also can be exploited for the development of new THz emitters.     

Optical pattern formation in alkali metal vapors: Mechanisms, phenomena and use

The phenomena in optical pattern formation are reviewed using a single mirror feedback experiment as an example. The nonlinear medium is sodium metal vapor. We discuss the mechanism responsible for the spontaneous formation of spatial structures and the selection between simple periodic patterns as hexagons and squares. Introducing a polarization changing element in the feedback loop, quasiperiodic patterns with a twelve-fold rotational symmetry are obtained and their properties are studied by filtering methods in Fourier space. Finally, we discuss the formation of localized states as spatial solitons due to a self-induced lensing effect. Their interaction is studied and the formation of clusters provides a link between localized states and extended patterns. Genuine optical features, such as the polarization degrees of freedom, the experimental accessibility of the Fourier space, and the possibility to control the optical susceptibility of atomic vapors by means of an external magnetic field, provide very powerful tools for investigating the principles of pattern formation. Received: 15 July 2000 / Revised version: 7 November 2000 / Published online: 6 December 2000     

Optical rectification in CdS

Optical rectification non-linear process in CdS single crystals at the Nd-YA1G and CO₂ frequencies is reported. V_OR signal measurements versus θ angle, between radiation E-field and crystal c-axis, give non-zero terms values of the optical rectification tensor. The frequency dispersion effect of this non-linear process is discussed.     

Optical frequency doubling of a single-mode dye laser in an external ring resonator

Optical frequency doubling of a single-mode cw Rhodamin 6G ring dye laser is performed with a thin angle-tuned LiIO₃ Brewster cut crystal in a stabilized passive ring resonator. A conversion efficiency of =5 mW uv/320 mW fundamental input power was achieved at =603 nm.     

准相位匹配倍频系统的带宽性质研究

根据准相位匹配倍频原理,讨论了实现宽带二次谐波转换需要满足的条件.以周期性极化铌酸锂晶体和掺杂氧化镁（7mol%）周期性极化铌酸锂（7mol%MgO-PPLN）晶体为例,分别比较了0型（e+e→e）和Ⅰ型（o+o→e）两种准相位匹配情况下宽带高效二次谐波转换的特性.研究表明,对于7mol%MgO-PPLN晶体,同时满足准相位匹配条件和群速度匹配条件的中心波长和带宽向短波方向移动,且0型（e+e→e）准相位匹配情况下可以获得更大的带宽.     

双KTP晶体倍频过程的实验研究(二)——光学参量振荡腔内串接倍频

按第一部分指出的串接方位,将串接的双KTP晶体置于光学参量振荡腔中,并采用控温方式补偿程差,实现双模共振,使倍频效率较内腔单模起振提高近30倍,理论分析与实验结果基本符合。     

双KTP晶体倍频过程的实验研究腔外串接倍频

双KTP晶体串接倍频可以克服离散效应并补偿相位失配。本文对串接倍频进行了理论分析与实验观测,指出最佳串接方位。理论分析与实验结果符合很好。     

Optical rectification in surface layers of germanium

In this Letter, we have demonstrated significant electric field induced(EFI) optical rectification(OR) effects existing in the surface layers of germanium(Ge) and measured the distributions of EFI OR signals along the normal directions of surface layers of Ge samples. Based on the experimental results, the ratios of the twoe-order susceptibility components χ2 effTzzz∕χe2 effTeffective secondzxxfor Ge(001), Ge(110), and Ge(111) surface layers can be estimated to be about 0.92, 0.91, and 1.07, respectively. The results indicate that the EFI OR can be used for analyzing the properties on surface layers of Ge, which has potential applications in Ge photonics and optoelectronics.     

Optical rectification effect in nanostructured carbon films

Electrically conducting nanostructured carbon films obtained by chemical vapor deposition and composed of nanodimensional graphite crystals exhibit the effect of optical rectification on exposure to nanosecond pulsed laser radiation. Experiments show that the amplitude and polarity of the pulsed voltage strongly depend on the angle of incidence and polarization of the laser radiation and on the spatial orientation of a carbon film with electrodes relative to the laser beam. Under the optimum conditions corresponding to maximum amplitude of the response signal, the factor of conversion of the laser pulse power into electric voltage was about 500 and 650 mV/MW at a laser wavelength of 1064 and 532 nm, respectively.     

Three-wave interaction including frequency doubling effects

A technique is developed in this work to construct asymptotic solutions describing the resonant interaction of shortwaves in weakly nonlinear dispersive media. Also included is the process of generation of multiple harmonics for low dispersion. A calculation of the acoustooptical interaction is performed, including the generation of multiple harmonics of acoustic waves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 3–23, March, 1986.     

A novel quasi-phase-matching frequency doubling technique

A new quasi-phase-matching technique for efficient second-harmonic generation is reported. It is based on the spatial periodic modulation of the light intensity along the propagation direction, rather than the conventional spatial periodic modulation of the nonlinear optical coefficients. It can be realized by using a novel dual-channel waveguide frequency doubler structure for the desired light intensity distribution. This dual-channel waveguide device has major advantages including very small beam size, high light intensity within long nonlinear-waveguide interaction length, highly efficient second-harmonic generation, ease in fabrication of the nonlinear channel waveguides without any spatially periodic poling, and low waveguide propagation losses. The new quasi-phase-matching technique can also be applied to third-harmonic generation and other nonlinear optics processes.     

Resonant generation of even-order harmonics in metal vapors

Generation of the second- and fourth-order harmonics of laser radiation is reported in Mg vapor at the forbidden 3s ²¹ S _o–4s ¹ S _o transition. The conversion efficiency for the second-order harmonics has been measured to be 10^–2% and for the fourth one 10^–8%.     

Noncollinear optical frequency doubling in strontium barium niobate

The observation of a novel noncollinear optical second-harmonic generation mechanism is reported. In strontium barium niobate crystals, a circular cone of second-harmonic light is generated when a fundamental beam of intensive laser light is directed along the crystallographic c axis. It can be shown that the effect is caused by the nonlinear polarization of antiparallel ordered ferroelectric microdomains.     

Reconfigurable dynamic beam shaping in seeded frequency doubling

A new method that allows simultaneous frequency doubling and dynamic shaping of a Gaussian beam into a variety of light patterns is introduced. The novel technique is based on the controllable generation of multiple-vortex patterns in seeded schemes. Applications include the generation of dynamic patterns of frequency-doubled, coherent optical tweezers.     

Optical rectification in tellurium from 10.6 μm

Optical rectification in single crystals of tellurium produced by infra-red radiation at 10.6 μm has been observed. The value of |χ⁰ ₁₁₁| is found to be 0.9×10⁻⁶ esu. Preliminary experiments to test the usefulness of the effect to monitor mode-locked infra-red laser pulses have been inconclusive.     

High-efficiency frequency doubling of continuous-wave laser light

We report on the observation of high-efficiency frequency doubling of 1550?nm continuous-wave laser light in a nonlinear cavity containing a periodically poled potassium titanyl phosphate crystal (PPKTP). The fundamental field had a power of 1.10?W and was converted into 1.05?W at 775?nm, yielding a total external conversion efficiency of 95±1%. The latter value is based on the measured depletion of the fundamental field being consistent with the absolute values derived from numerical simulations. According to our model, the conversion efficiency achieved was limited by the nonperfect mode matching into the nonlinear cavity and by the nonperfect impedance matching for the maximum input power available. Our result shows that cavity-assisted frequency conversion based on PPKTP is well suited for low-decoherence frequency conversion of quantum states of light.     

Linear optical properties of frequency doubling polymers

Measurements of linear optical properties of methylmethacrylate copolymers with non-linear optical 4-alkoxy-4-alkylsulfone stilbene side-chains are presented. From there two methods for phase matching are discussed; one aiming to equalize the refractive indices of the fundamental and the second harmnic wave, the other to suppress the destructive character of the interference. The matter of photo stability of frequency doubling polymers is also discussed.