CS_2飞秒超快非线性特性的实验研究

采用中心波长800 nm、脉宽30 fs的超短激光脉冲,通过飞秒光开关技术对CS_2的飞秒超快非线性特性进行了实验研究.在探测光强与抽运光强比为1∶10时,得到了较理想的光克尔时间分辨曲线.通过实验测定的光克尔信号强度与激发光和探测光偏振方向夹角的依赖关系表明:30 fs的超短激光脉冲激发CS_2的克尔信号主要是源于光诱导双折射效应,而非用200 fs的超短激光脉冲时来自瞬态栅的自衍射效应.     

PPV衍生物的激发态动力学特性研究

利用共振飞秒光学克尔效应研究了C10－PPV溶液的非线性光学响应。实验结果表明，在共振激发条件下，C10－PPV挑学克尔效应信号表现为快速上升和随后的驰豫过程，其驰豫过程由两部分组成，一个200fs的快过程和一个400ps的慢过程。通过比较C10－PPV和掺杂C60后的C10－PPV荧光光谱，400ps的慢过程可归属为聚合物链内单重态激子在不同共轭段间的迁移过程。     

Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films

Optical Kerr nonlinearity (n₂) in n-type indium tin oxide (ITO) films coated on glass substrates has been measured using Z-scans with 200-fs laser pulses at wavelengths ranging from 720 to 780 nm. The magnitudes of the measured nonlinearity in the ITO films were found to be dependent on the carrier concentration with a maximum n₂-value of 4.1×10^-5 cm²/GW at 720-nm wavelength and an electron density of N_d=5.8×10²⁰ cm^-3. The Kerr nonlinearity was also observed to be varied with the laser wavelength. By employing a femtosecond time-resolved optical Kerr effect (OKE) technique, the relaxation time of OKE in the ITO films is determined to be ∼1 ps. These findings suggest that the Kerr nonlinearity in ITO can be tailored by controlling the carrier concentration, which should be highly desirable in optoelectronic devices for ultrafast all-optical switching. PACS 42.65.An; 42.65.Hw; 78.40.Fy     

Femtosecond Third-Order Optical Nonlinearity of Au：Bi203 Nanocomposite Films

Ultrafast third-order optical nonlinearities of the as-deposited and annealed Au：Bi2O3 nanocomposite films deposited by magnetron cosputtering are investigated by using femtosecond time-resolved optical Kerr effect （OKE） and pump probe techniques. The third-order optical nonlinear susceptibility is estimated to be 2.6Ф×10^- 10 esu and 1.8 × 10.9 esu at wavelength of 800nm, for the as-deposited and the annealed film, respectively. The OKE signal of the as-deposited film is nearly temporally symmetrical with a peak centred at zero delay time, which indicates the dominant contribution from intraband transition of conduction electrons. For the annealed film, the existence of a decay process in OKE signal implies the important contribution of hot electrons. These characteristics are in agreement with the hot electron dynamics observed in pump probe measurement.     

Third-order nonlinear optical response in transparent solids using ultrashort laser pulses

The third-order optical nonlinearity, χ ⁽³⁾, is measured in transparent glasses (BK7 and fused silica) and crystals (BaF₂ and quartz) using 36-fs, 800-nm laser pulses and the optical Kerr gate (OKE) technique; values are found to lie in the range 1.3–1.7×10^-14 esu, in accordance with theoretical estimates. We probe the purely electronic response to the incident ultrashort laser pulse in fused silica and BK7 glass. In BaF₂ and quartz, apart from the electronic response we also observe contribution from the nuclear response to the incident ultrashort pulses. We observe oscillatory modulations that persist for ～400 fs. The response of the media (glasses and crystals) to ultrashort pulses is also measured using two-beam self-diffraction; the diffraction efficiency in the first-order grating is measured to be in the range of 0.06–0.13 %. Third harmonic generation due to self-phase matching in the transient grating geometry is measured as a function of temporal delay between the two incident ultrashort pulses, yielding the autocorrelation signal.     

Photon echo peak shift experiments in the UV: p-terphenyl in different solvents

In this article we present a new photon echo set-up operating in the UV range and our first results of photon echo peak shift (PEPS) experiments of a small non-polar dye molecule, p-terphenyl (pTP), in three different solvents: ethanol, methanol and 2-propanol. The experiments show the feasibility of UV transient grating and photon echo measurements and highlight the sensitivity of this technique for the study of non-polar solvation dynamics. The transient grating experiments indicate a relaxation of the ground state hole of about 6 ps. The photon echo experiments show that while electronic dephasing occurs on timescales shorter than 100 fs, solvation dynamics timescales are in the picosecond regime, in agreement with the literature.     

Ultrafast X-ray experiments on structural changes in single crystals of polar molecules

Intramolecular charge transfer in aminobenzonitrile (DIABN) single crystals is studied by a combination of ultrafast techniques. The intramolecular dynamics is probed by time-resolved spectroscopy in the UV/Vis and mid-IR. The intermolecular structural response is investigated by transient X-ray scattering and transmission experiments. A theoretical analysis shows that diffuse scattering and dipole solvation determine the X-ray data. The observed dynamics results from charge transfer reaction leading to strong local changes of molecular dipole moments and related structural rearrangements in the crystal.     

Femtosecond non-resonance double-pulse spectroscopy of molecular librations in ortho-dichlorobenzene

The first experimental results on selective detection of the collective (libration) response in a liquid using a scheme of non-resonance double-pulse spectroscopy based on detecting the ultrafast optical Kerr effect (OKE) in ortho-dichlorobenzene at room temperature are presented. Nonrelevant vibrational and rotational responses are dumped and the relevant libration molecular responses are amplified by varying the duration, delay, and relative intensities of linearly polarized pump pulses.     

翠绿亚胺碱的超快光克尔和光致吸收效应

报道了在飞秒染料激光脉冲作用下对翠绿亚胺碱（ＥＢ）的二甲基甲酰胺溶液的瞬态共振光克尔和光致吸收效应的研究．时间分辨的光克尔实验表明，翠绿亚胺碱溶液的光克尔信号仅表现一超快响应的成分，可归结为其基态和由于单光子共振吸收和双光子吸收产生的激发态的各振动能级与转动能级之间的弛豫过程．同时通过拟合计算分别给出了翠绿亚胺碱溶液的弛豫时间和三阶非线性极化率．抽运探测实验结果表明，翠绿亚胺碱分子的超快响应的光致吸收效应，主要来自于导致该分子中苯环结构π—π^*跃迁的双光子吸收的贡献． 关键词：     

基于瞬态光克尔效应的超快光学快门技术

系统介绍了几种基于瞬态光克尔效应的超快光学快门技术,包括传统的光克尔双折射快门技术,瞬态光栅快门技术,以及利用瞬态克尔透镜效应的光学快门技术。这些技术都是在泵浦-探测的实验配置基础上,利用门控光对克尔介质折射率的瞬态调制导致的信号光的相位（偏振态）、传播方向或光束发散角等光学特性的改变实现对信号光的超快时间分辨测量。对比讨论了这些超快快门技术的工作原理和实验配置,结果表明瞬态光束偏折快门技术相比其他快门技术具有无偏振配置要求、无相位匹配条件、超宽带光谱响应范围的特点,在光与物质相互作用的超快动力学研究中具有更为广阔的应用前景。     

Direct measurement of carrier spin relaxation times in opaque solids using the specular inverse Faraday effect

Using a spectroscopic technique based on the transient specular inverse Faraday effect (SIFE) we found the electron spin relaxation time in bulk GaAs at room temperature to be 11 ± 1 ps. The hole spin dynamics gives a SIFE signal of the opposite sign and may be resolved in the time-domain.     

Femtosecond selective optical Kerr-effect spectroscopy: Molecular dynamics study of chloroform

The femtosecond selective spectroscopy of vibrational-rotational dynamics of molecules in a liquid was realized using optical Kerr effect registration under two-pulse nonresonant excitation. The object chosen for the study was the chloroform at room temperature. It was shown that control of the separate molecular motions by creating the constructive or destructive interference of corresponding wave packets allows one to determine directly from the experiment such constants of molecular dynamics as the relaxation times of the coherent vibrations (≈1.5 ps) and those of orientational anisotropy (≈1.2 ps).     

Reduction of pulse-to-pulse fluctuation in laser pulse energy using the optical Kerr effect

An all-optical stabilization method of laser pulse energy is proposed using the optical Kerr effect (OKE). The method uses the OKE induced by a portion of the laser pulse as a power controller. The decrease (increase) in the throughput of the optical setup for OKE compensates for the increase (decrease) in pulse energy, thereby stabilizing the pulse-to-pulse fluctuation in pulse energy. The validity of this principle was proven by experiments with a femtosecond laser.     

Control of condensation and evaporation of electron–hole liquid in diamond by femtosecond laser pulses

We demonstrate the ultrafast control of condensation and evaporation of an electron–hole liquid in diamond prepared by chemical vapor deposition. The electron–hole liquid dynamics was measured using a three‐pulse pump and probe experiment. The transient transmission changes in the presence of electron–hole drops were assigned to the increase in the Drude scattering rate in the model of free carrier absorption due to a high carrier density. The fast (～1 ps) liquid evaporation was induced by infrared femtosecond pulses and its dynamics was investigated under different photon energies and fluences. The value of the electron–hole liquid binding energy per electron–hole pair 80 ± 40 meV determined from the measured transient transmission signal agrees well with previously published values. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature and orientation dependence of surface relief gratings based on dye-doped polymer film with the interface of nematic liquid crystals

The formation of surface relief grating on dye-doped polymer film with the interface of nematic liquid crystals has been investigated by means of the holographic technique. The first-order diffraction efficiency of surface relief grating depends on the temperature and the orientation of molecular director in the interface of nematic liquid crystals. The diffraction efficiency is roughly independent of thermal fluctuations of molecular director in the most part of nematic temperature range and apparently drops near the transition temperature. The morphology of surface relief grating demonstrates that the surface modulation is larger for molecular director parallel to the groove direction. The experimental result also shows that the first-order diffraction efficiency is dependent on the surface modulation of surface relief grating.     

Early dynamics of guest-host interaction in dye-doped liquid crystalline materials

We have studied in detail the early dynamics of laser-induced molecular reorientation in a dye-doped liquid crystalline (LC) medium that exhibits a significant enhancement of the optical Kerr nonlinearity due to guest-host interaction. Experimental results agree quantitatively with theory based on a model in which the anisotropic dye excitation helps reorient the LC molecules through a mean-field intermolecular interaction.     

Ultrafast motion of liquids C2H4Cl2 and C2H4Br2 studied with a femtosecond laser

Transient optical Kerr effect of liquids C₂H₄Cl₂ and C₂H₄Br₂ is investigated, for the first time to our knowledge, with a femtosecond (fs) probe laser delayed with respect to a coherent fs pump laser. Coherent coupling and electronic Kerr signals are observed around zero delay when pump and probe overlap. Persisting after the pump-probe overlap are Kerr signals arising from the torsional and other intramolecular vibrations of the trans and gauche conformations; Kerr signals arising from the intermolecular motion are also observed. Vibrational quantum interference is only observed in liquid C₂H₄Br₂ and the related beats data are fitted with the torsional vibrations, 91 cm⁻¹ (gauche) and 132 cm⁻¹ (trans), and the CCBr angle-bending vibrations, 231 cm⁻¹ (gauche) and 190 cm⁻¹ (trans), with dephasing times, 0.45 ps, 0.45 ps, 2 ps, and 1.5 ps, respectively. These vibrational frequencies agree with those obtained in the frequency-domain. That no vibrational mode is observed for C₂H₄Cl₂ might be attributed to ineffective Raman-pumping. Kerr signals observed after the pump-probe overlap are Fourier transformed to give the spectra of the intermolecular motion and the vibrational spectrum, which agrees with the one observed in the infrared absorption and/or Raman scattering heretofore.     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

Sudden Vanishing of Atomic Dipole-Squeezing in a Kerr Nonlinear Blackbody

The dynamics of atomic dipole momentum and atomic dipole-squeezing effect are investigated inside a Kerr nonlinear blackbody. It is found that in a Kerr nonlinear blackbody, the atomic dipole momentum and its squeezing effect are heavily dependent on the Kerr nonlinear coefficient. It is also found that below a transition temperature T _c , the dipole momentum and its squeezing effect in a Kerr nonlinear blackbody can vanish much faster with time than those in a normal blackbody. Above T _c , the Kerr nonlinear blackbody becomes a normal blackbody, and the dynamics of the atomic dipole momentum and its squeezing effect behaves as those in normal blackbody radiation. The physical origin of the sudden-vanishing phenomenon in a Kerr nonlinear blackbody is also discussed.     

N-乙基吡咯S1态超快动力学：飞秒时间分辨光电子成像研究

N-乙基吡咯是吡咯分子的一个乙基取代衍生物,它的激发态衰变动力学目前为止很少被研究. 本文利用飞秒时间分辨光电子成像的实验方法研究N-乙基吡咯分子S₁态的衰变动力学. 实验采用241.9和237.7 nm的泵浦激发波长. 在241.9 nm激发下,得到5.0±0.7 ps,66.4±15.6 ps和1.3±0.1 ns三个寿命常数. 在237.7 nm激发下,得到2.1±0.1 ps和13.1±1.2 ps两个寿命常数. 所有寿命常数都归属为S₁态的振动态. 本文并对不同S₁振动态的弛豫机理进行了讨论.