Energy transfer coupling of two-photon absorption and reverse saturable absorption for enhanced optical power limiting

We introduce a new approach to enhancing the optical power-limiting function at near-IR wavelength (~800 nm) by coupling effective two-photon absorption in one molecule with excited-state absorption in another molecule. We experimentally demonstrate this approach by using a strong two-photon absorbing dye, AF-380, and a strong reverse saturable absorber, C(60) . A nanosecond time-resolved experiment is used to show that energy transfer from AF-380 to C(60) generates triplet excitation in C(60) that further absorbs the pump beam to enhance the power-limiting function.     

一种新型有机染料的宽带双光子吸收和光限幅特性的研究

研究了一种新型双共轭链有机染料分子的双光子吸收（TPA）及其光功率限幅特性，实验研究了该有机染料的双光子吸收谱和光功率限幅曲线. 结果证实在浓度为1×10^-2 mol/L的四氢呋喃（THF）溶液中，该有机染料在近红外区有宽达400nm的宽带非线性双光子吸收及光功率限幅特性. 它的双光子吸收谱存在三个峰，分别位于730，850和980nm，并且在850nm处有最大的TPA截面，为σ′₂＝25.9×10^-47　cm^{4·s 关键词： 宽带 双光子吸收 光限幅 双共轭链有机分子}     

Ultrafast measurement of two-photon absorption by loss modulation

We demonstrate a direct and sensitive technique for measuring two-photon absorption (TPA). An intensity-modulated femtosecond laser beam passes through a sample exhibiting TPA. A TPA signal at twice the modulation frequency is then generated and subsequently measured by a lock-in amplifier. The absolute TPA cross section of Rhodamine 6G at 800nm is found to be (15.3+/-2.0)x10(-50) cm(4) s/photon and agrees well with previously published results obtained with much higher intensity [J.Chem.Phys.112, 9201 (2000)]. Our method may be especially useful in measuring nonlinear absorptions of nonfluorescent materials.     

Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP

The two-photon QWIP comprises three equidistant subbands, namely two bound states localized in the quantum well and an extended state in the continuum. This device is very promising for quadratic autocorrelation measurements of pulsed mid-infrared lasers due to its resonantly enhanced optical nonlinearity and sub-ps time resolution. We report on interferometric autocorrelation measurements of ps optical pulses from a free-electron laser (FEL). The intense FEL radiation further allows us to study the saturation properties of two-photon QWIPs at liquid nitrogen temperature and their detection properties at 300 K. The device is well suited for standard diagnostics of the FEL pulse shape via interferometric autocorrelation.     

Infrared multiple photon decomposition of CHBrF2 induced by a free-electron laser

2 . The laser generates an intense infrared macropulse with a duration of 17 μs; the macropulse consists of a train of 380 micropulses, each of which has a duration of a few picoseconds. The fluence of a macropulse was estimated to be about 16 Jcm^-2 at a beam waist. Peak wavelengths were set in the range of 9–10 μm. The macropulse induced the IRMPD of 1 and 5 Torr CHBrF₂; most of molecules in the focal region seemed to decompose at a wavelength of 9.3 μm. The mechanism is the initial decomposition of CHBrF₂ to CF₂ and HBr, followed by the dimerization of CF₂ to form C₂F₄. The decomposition was found to be isotopically selective at 9.7 μm; the final product C₂F₄ had a ¹³C atomic fraction of 6%. Th e addition of CO₂ to CHBrF₂ significantly decreased the yield of C₂F₄. vibrationally excited CHBrF₂ molecules produced by laser pulses were efficiently deactivated by CO₂ molecules. Received: 7 October 1996     

Infrared laser emission in two-photon pumped sodium vapour

We report infrared laser emission in the region of 3 to 5 μm from sodium vapour optically pumped by a pulsed dye laser with wavelengths ranging from 585 to 610nm. Twophoton excitation processes are believed to be responsible for the primary excitation. Both molecular transitions (4 to 5 μm) between high lying states, and atomic transitions (5² S _1/2−4² P _3/2,1/2 at 3.41 μm) have been identified.     

Time-resolved phase measurement of a self-amplified free-electron laser

We report on the first time-resolved phase measurement on self-amplified spontaneous emission (SASE) free-electron laser (FEL) pulses. We observed that the spikes in the output of such free-electron laser pulses have an intrinsic positive chirp. We also observed that the energy chirp in the electron bunch mapped directly into the FEL output. Under certain conditions, the two chirps cancel each other. The experimental result was compared with simulations and interpreted with SASE theory.     

Single-shot measurement of free-electron laser polarization at SDUV-FEL

In this paper, a division-of-amplitude photopolarimeter(DOAP) for measuring the polarization state of a free-electron laser(FEL) pulse is described. The incident FEL beam is divided into four separate beams, and four Stokes parameters can be measured in a single-shot. In the crossed-planar undulators experiment at the Shanghai deep ultraviolet FEL test facility, this DOAP instrument constructed in house responsed accurately and timely while the polarization-state of fully coherent FEL pulses were switched, which is helpful for confirming the crossed-planar undulators technique for short-wavelength FELs.     

Tunable compact high power far-infrared grating free-electron laser

The linear and nonlinear operation of a multiwave grating free-electron laser is described. The radiation is generated by the passage of an annular electron beam through a coaxial waveguide, the central conductor being in the form of a corrugated cylinder. The model includes the effects of self-field forces, beam emittance and gyromotion of electrons in a guide magnetic field. The efficiency is determined by numerical simulation and compared with analytical estimates. A compact source utilizing, for example, a high brightness beam from a carbon fibre electron gun, that is tunable in the wavelength band 35-55 μm with output in the kilowatt range, is shown to be feasible     

Recoil-ion momentum distributions for two-photon double ionization of He and Ne by 44 eV free-electron laser radiation

Recoil-ion momentum distributions for two-photon double ionization of He and Ne (variant Planck's over omega=44 eV) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of approximately 1 x 10(14) W/cm2 exploring the dynamics of the two fundamental two-photon-two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models.     

Pulse characterization during femtosecond laser filamentation in air by two-photon fluorescence measurement

This work reports a temporal characteristic of a laser pulse undergoing filamentation in air. The diagnostic method is based on two-photon fluorescence measurement. The results show that the pulse duration, chirp rate and beam radius could be simultaneously quantitatively retrieved. This simple technique would be useful in practice to trace the underlying dynamics of filamentation in air.     

含时电离对飞秒脉冲激光在强双光子吸收介质中传播特性和光限幅行为的影响

以具有强双光子吸收特性的4,4′-二甲氨基二苯乙烯分子体系为研究对象,通过采用时域有限差分法和预估矫正法数值求解Maxwell-Bloch方程,模拟了飞秒脉冲在该分子介质中的传播过程,研究了含时电离对双光子吸收过程和光限幅行为的影响.研究结果表明,光电离使介质与光场的非线性相互作用减弱,同时自发辐射降低, 随着入射电场的增强,光电离对主脉冲演化有明显的影响.光电离使介质的光限幅失效行为减弱.随着光电离截面增加,光限幅的动力学窗口变宽,表明了光电离有利于增强介质的光限幅效应.在不同的传播距离处,脉冲传播呈现 关键词： 含时电离 双光子吸收 光限幅效应 超短脉冲激光     

Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation

We microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF₆. These surfaces display strong absorption of infrared radiation at energies below the band gap of crystalline silicon. We report the dependence of this below-band gap absorption on microstructuring conditions (laser fluence, number of laser pulses, and background pressure of SF₆) along with structural and chemical characterization of the material. Significant amounts of sulfur are incorporated into the silicon over a wide range of microstructuring conditions; the sulfur is embedded in a disordered nanocrystalline layer less than 1 m thick that covers the microstructures. The most likely mechanism for the below-band gap absorption is the formation of a band of sulfur impurity states overlapping the silicon band edge, reducing the band gap from 1.1 eV to approximately 0.4 eV. PACS 78.68.+m; 81.15.Fg; 81.40.Tv; 81.65.Cf; 85.60.Gz     

Exciton-polaritons by two-photon absorption in semiconductors

The creation of excitonic polariton states by two-photon absorption is theoretically investigated. A semi-classical approach is adopted to compute the two-photon transition probability to polariton states through an intermediate exciton state. The numerical results in CuCl show two peaks corresponding to the longitudinal exciton and transverse polariton states, respectively. These results are in good agreement with the experiment.     

中物院高功率THz FEL装置的理论分析和优化设计

在1~3 THz频段内设计了11个分点频率,对每一频率的对应的场强和电子束能量进行了选取和计算。模拟结果表明:采用目前参数基本能够达到设计要求;辐射频段在2.6 THz附近时装置输出功率和增益都比较高,可以先锁定这个频段附近进行实验的调试;在长波段即1 THz左右时,滑移效应显著,腔增益和输出功率较低,实验实现比较困难,因此在1 THz频段附近必须积极想办法来提高输出功率。     

中物院高功率THz FEL装置的理论分析和优化设计

在1~3 THz频段内设计了11个分点频率,对每一频率的对应的场强和电子束能量进行了选取和计算。模拟结果表明:采用目前参数基本能够达到设计要求;辐射频段在2.6 THz附近时装置输出功率和增益都比较高,可以先锁定这个频段附近进行实验的调试;在长波段即1 THz左右时,滑移效应显著,腔增益和输出功率较低,实验实现比较困难,因此在1 THz频段附近必须积极想办法来提高输出功率。     

A free-electron laser pumped by the soliton laser

We propose a scheme for building a free-electron laser in the soft X-ray region pumped by the soliton laser. Making use of soliton laser wave evolution shape and single-pass small signal analysis, we find that this laser has two special advantages over the previous electromagnetic wave undulator free-electron lasers. One is a very small mass-shift effect because of the special characteristics of soliton laser; the other is that it has an additional frequency tuning effect based on the conventional free-electron laser's tunability. We also obtain the small signal gain and present some discussion.     

Contrast ratio variation caused by radiation attenuation in pulse width measurement by two-photon absorption fluorescence

In the technique for measuring pulse width by two-photon absorption fluorescence, theoretical calculations indicate that one can hope to obtain a maximum contrast ratio of three between the point at which the two peaks overlap and the point at which there is no superposition. Nevertheless, this calculation has been carried out assuming that the pulse is not attenuated as it traverses the fluorescent medium. In this paper it will be shown that this attenuation gives rise to a decrease in the contrast ratio. As the fluorescence intensity is proportional to the absorption, an increase in the former will be to the detriment of the contrast ratio. We will attempt to give a criterion for the selection of experimental conditions which lead to maximum intensity without an appreciable decrease in the contrast ratio.