A versatile dye-laser generator-amplifier system for intense,tunable picosecond pulse generation

Passively mode-locked ruby-laser pulses are used to generate nearly diffractionlimited picosecond light pulses in a dye cell by longitudinally amplified spontaneous emission. The output pulses are amplified in three longitudinally pumped dye cells, then spectrally filtered with a grating spectrometer and finally reamplified in a fourth dye amplifier in order to generate intense frequency tunable picosecond light pulses.     

原子系统中远失谐脉冲光束对的群速度操控

基于远失谐的四波混频过程, 在实验上得到了放大的探针光脉冲和产生的共轭光脉冲的同时慢光传输, 并通过改变抽运光和探针光之间的双光子失谐实现了群速度的同时操控. 首先在连续光模式下, 研究了入射探针光和新产生共轭光的增益与单光子失谐之间的变化关系. 随着单光子失谐在一定范围内加大, 探针光和共轭光的增益均表现出先增加后减小的变化趋势. 在具有增益特性的基础上, 分别采用6 μs和365 ns探针光脉冲, 研究了慢光的延迟时间和双光子失谐的关系. 对6 μs的探针光, 得到探针和共轭光脉冲的最大延迟分别为2.1 μs 和1.9 μs, 对应的群速度分别约为0.000119 c和0.000132 c, 相应延迟比分别为0.35和0.32. 对365 ns探针光, 探针和共轭光脉冲的最大延迟分别为756 ns和670 ns, 对应的群速度分别约为0.00033 c和0.00037 c, 相应延迟比提高到2.07和1.83.     

Efficient phase-matched third harmonic light generation in hexafluoroisopropanol solutions of a pyrimidonecarbocyanine dye

The phase-matched collinear third harmonic generation of picosecond laser pulses in a 0.0825 molar hexafluoroisopropanol solution of a pyrimidonecarbocyanine dye is studied. The fundamental pulses are generated in a passively mode-locked Nd-phosphate glass laser. The saturation of third harmonic generation at high intensities is investigated. The influences of two-photon absorption, excited-state absorption, and amplified spontaneous emission are discussed. For input peak intensities above 10¹¹ W/cm² a third harmonic energy conversion of about 2×10^–4 is achieved.     

Absorption and Emission Properties of a New Two-photon Absorption Dye

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Absorption and Emission Properties of a New Two-photon Absorption Dye

The emission properties of a newly synthesized organic two-photon absorption dye, trans-4-[p-(N-ethyl-N-ethylamino)-styryl]-N-methyl-pyridinium tetraphenylborate, have been investigated. When pumped by infrared pulses from a picosecond Nd∶YAG laser, the dye exhibits intense upconverted fluorescence and strong superradiance properties. For comparison, the one-photon induced fluorescence and superradiance are also measured. The one- and two-photon excited fluorescent lifetimes are 86 and 64 ps, respectively. The maximum efficiency of the dye is measured to be 1.97% and the upconversion efficiencies at different pump wavelengths have also been investigated by the optical parametric amplifier.     

Amplification of fluorescence using collinear picosecond optical parametric amplification at degeneracy

We demonstrate the output characteristic of broadband parametric amplification of incoherent light pulses in a 355-nm pumped degenerate picosecond optical parametric amplification with either saturated or unsaturated amplification.The optical parametric amplifier is seeded by the fluorescence generated in a solution of pyridine-1 dye in ethanol.With the saturated amplification,we can obtain high energy incoherent light pulses,whose full width at half maximum bandwidth varies from 16 nm to 53 nm for the different phase matching angles near degeneracy.Moreover,the unsaturated bandwidth of the amplified pulses fits well to the calculated result at degeneracy.Selecting s-polarized fluorescence with a Glan-Taylor prism,the maximum bandwidth of the amplified fluorescence is found to be 59 nm for a purely s-polarized seed.The maximum output energy is 0.67 mJ for the optical parametric amplifier.By using an optical filter and compressor,the generated high energy incoherent light has great potential as the incoherent pump,signal or idler wave of a parametric down-conversion process,so that a wave with a high degree of coherence can be generated from an incoherent pump light.     

An alternative approach for microlithography light source

A hybrid master oscillator–power amplifier (hybrid MOPA) scheme is proposed as a microlithography light source. The seed pulses are generated in the visible spectral range—where the necessary spectral purity is more easily achieved—and after frequency conversion are amplified in an excimer amplifier. The new concept enables us to decrease the bandwidth considerably, approaching the theoretical limit posed by the uncertainty relation. The feasibility of the new approach is demonstrated by a dye/excimer MOPA system, generating deep ultraviolet DUV (248-nm) pulses of 0.2-pm bandwidth.     

Theoretical investigation of amplified spontaneous emission with picosecond light pulses in dye solutions

The amplified spontaneous emission in dye solutions excited with intense picosecond light pulses is studied theoretically. A multi-level model is applied to take into account the effects of the various dye parameters such as ground-state absorption, stimulated emission, excited-state absorption, reabsorption and relaxations from populated excited levels. The influence of the pump laser duration and intensity and of the dye concentration and sample length is investigated. Optimum situations are derived for the generation of intense picosecond light pulses at new frequencies by amplified spontaneous emission. On the other hand, conditions are found where amplification of spontaneous emission may be neglected. The analysis allows the determination of unknown dye parameters by comparing the calculations with the experiment.     

Travelling-wave amplified spontaneous emission excited in a prismatic geometry

A new experimental realisation of the travelling-wave amplified spontaneous emission (TWASE) is described utilising a prismatic arrangement to get the pulse front delay in the pump beam. Rhodamine dye solutions were pumped by the amplified 25 ps long 555 nm pulses of a distributed feedback dye laser. The pulse shortening in the TWASE resulted in 12 ps output pulses with 18% energy efficiency. The observed spectrum showed numerous lines, and it contained the amplified Raman line of the pump beam. Several spots were found in the far-field zone of the generated TWASE.     

Electrically switchable reflection holograms formed using two-photon photopolymerization

Two-photon holographic photopolymerization was used to form switchable Bragg gratings composed of layers of phase-separated liquid-crystal (LC) domains interspersed with cured, crosslinked polymer. These holographic polymer-dispersed liquid crystals form a periodic structure which diffracts red light due to nanostructured planes ∼250 nm in spacing. These structures were formed by interfering two 90-fs pulses coherently upon a reactive syrup consisting of acrylate monomer, liquid crystal, and a two-photon dye. The large two-photon cross-section allows excitation of the two-photon dye that results in electron transfer between this dye and the monomer. Diffraction efficiencies of approximately 10% were obtained, which can be modulated using an electric field applied across the film. Switching speeds below 1 ms were observed due in part to the small size of the LC domains. Received: 10 April 2001 / Accepted: 1 July 2001 / Published online: 2 October 2001     

Two-Photon Activated Two-Photon Fluorescence and Binding of Azidocoumarin in a Gelatin Matrix

We study the creation of fluorescence patterns inside a gelatin gel by way of two-photon photoactivation of 7-azido-4-trifluoromethyl-1,2-benzopyrone (azidocomarin 151) contained in the gel matrix. As ultrafast light pulses are focused into the gel, onset of two-photon fluorescence, highly nonlinear in the applied optical power, is observed as azidocoumarin is converted into a fluorescent dye that binds to the gelatin. We fit the time dependence of the fluorescence to a model that incorporates the competition between coumarin photoactivation and photobleaching as well as the gradual degradation of the gel when it is exposed to the high intensity laser light. The model predicts that the initial rate of fluorescence onset should scale as the P (4), where P is laser power, while the signal at long exposure time should scale as P (3/2). The observed exponents are 4.18 and 1.34, respectively. The model allows us to estimate the cross section and quantum yield of two-photon induced photobleaching of azidocoumarin 151. The numerous technical uses of gelatin and the collagen from which it derives in areas ranging from photography to tissue engineering provide possible applications for the techniques described in this paper.     

双光子失谐对慢光和光存储影响的实验研究

利用电磁感应透明(EIT)效应在87Rb热原子气室中进行了慢光和光存储的实验研究,在单光子红失谐650 MHz处测量了双光子失谐对光脉冲延迟和光存储的影响.结果表明:在双光子失谐0—0.5 MHz范围内存在显著的光脉冲延迟和光存储恢复信号,其慢光波形与理论计算结果基本相符;而恢复光脉冲信号随着双光子失谐的变化出现形变,这是由于多个EIT子系统之间的干涉引起的.这一研究结果为连续变量光场在热原子系综中的存储提供了实验参考.     

Fluctuations in the phase-conjugate signal generated via degenerate four-wave mixing

The excess fluctuations of the phase-conjugate (PC) signal generated via pulsed degenerate four-wave mixing (DFWM) in sodium vapor are investigated. Various dye-laser oscillator and amplifier combinations are employed to trace the sources of these fluctuations. For DFWM driven by an externally-stabilized cw dye-laser oscillator amplified with a dye system pumped by the smoothed pulses of a Nd:YAG laser, the PC excess fluctuations are very highly correlated with the energy fluctuations of the input amplified dye-laser pulses. Thus, input-pulse selection on the basis of energy alone permits observation of the quantum-mechanical fluctuations of the PC signal.     

用蓝色光二极管测量掺钛蓝宝石飞秒激光脉冲宽度

用蓝色光二极管双光子跃迁光电流信号测量掺钛蓝玉石飞秒激光脉冲宽度，实验高精度地记录了二阶干涉自相关曲线及其精细结构，由于用蓝色二极管双光子跃迁代替了常用的二次谐波，而且光信号直接转化为电信号，从而使飞秒激光脉冲宽度的测量大为简化。     

Interferences of ultrashort free electron wave packets

Interferences of free electron wave packets generated by a pair of identical, time-delayed, femtosecond laser pulses which ionize excited atomic potassium have been observed. Two different schemes are investigated: threshold electrons produced by one-photon ionization with parallel laser polarization and above threshold ionization electrons produced by a two-photon transition with crossed laser polarization. Our results show that the temporal coherence of light pulses is transferred to free electron wave packets, thus opening the door to a whole variety of exciting experiments.     

Tunable picosecond pulses from a short-cavity dye laser under ultra-high pressure using diamond-anvil cell

It is demonstrated that broadly tunable picosecond pulses are generated from a dye laser of very short cavity utilizing a diamond-anvil cell, which operates at pressures up to 10 GPa. The pulses as short as 5 ps are obtained from the rhodamine 6G dye laser pumped by a frequency-doubled Q-switched Nd: YAG laser. The way of tuning is based on the pressure induced shift of the emission peak of the dye. The laser is tunable over 20 nm by changing the pressure of the cell within 4 GPa.     

Two-photon and two-photon-assisted slow light

We show that light pulses propagating in two-photon absorbing systems may present time delays like slow light produced via coherent population oscillations in one-photon interactions. Two regimes are numerically studied for a simplified two-level system: (a) a light pulse at frequency ω/2 undergoes two-photon absorption (TPA) and is delayed by the absorbing system (two-photon slow light) and (b) a light pulse at frequency ω is delayed in a system prepared by TPA of a light pulse at frequency ω/2 (two-photon-assisted slow light). The study carried out in solutions of dyes and dendrites shows significant delays, low distortion, and good transmission for easily reachable experimental conditions. The working principle can be applied to other media and can be used in telecommunications technology.     

The features of nonlinear absorption during resonance one- and two-photon excitation of the basic exciton transition in colloidal CdSe/ZnS quantum dots

The features of the nonlinear absorption of CdSe/ZnS quantum dots (colloidal solution) in the case of resonant one- and two-photon excitation of the basic exciton transition by powerful ultra-short laser pulses were determined. In one-photon excitation, with an increasing intensity of impulses, a decrease in absorption (bleaching) is relayed by an increase in absorption, which is associated with the process of the filling of the states (saturation) of a two-level system with the lifetime of the excited state depending on the light intensity. The arising Fresnel or Fraunhofer diffraction of the laser ray that pass through a colloidal solution with a high concentration of quantum dots is associated with the formation of the transparency channel and self-diffraction of laser ray on an induced diaphragm. In two-photon excitation, the features of the nonlinear absorption and luminescence tracks (the dependence of luminescence intensity on distance) were explained by the influence, in addition to the two-photon absorption, of the processes that are responsible for the slower growth of nonlinear absorption and luminescence quenching at high intensities of laser pulses.     

Nonlinear optical absorption and NIR to blue conversion in highly stable polymeric dye rod

A simple method for preparing transparent crack-free solid polymeric matrices is presented. Different dopants may be hosted in the polymeric matrix. The method is applied to prepare Glycidyl-methacrylate polymeric rods doped with solutions of coumarin 500 laser dye (ethanol and chloroform). The solvent??s effect has appeared as a shift in the absorption and photoluminous spectral bands of the laser dye in the solid polymeric host. The prepared solid samples showed nonlinear optical response on excitation with 532?nm of nanosecond laser pulses using the open aperture z-scan technique. Each sample shows different nonlinear behavior in response to the irradiating laser beam. The theoretical fitting of the experimental data indicates that the dominant mechanism of nonlinear absorption is the two-photon absorption process. Visible amplified spontaneous emission is reported in both polymeric dye rods by NIR pumping. High photostability is reported for the samples.     

Comment on phase evolution in two-photon amplifier systems

The evolution of the pulse chirp is examined for the case when two distinct ultrashort pulses are simultaneously amplified in a two-photon amplifier system.