抽运-检测型非线性磁光旋转铷原子磁力仪的研究

报道了一种抽运-检测型的非线性磁光旋转铷原子磁力仪.其原理是线偏振光通过处于外磁场环境中被极化的原子介质后,由于原子对线偏振光中左、右圆偏成分不同的吸收和色散,导致光的偏振方向会产生与磁场相关的转动.分析了该磁力仪的工作原理,并测试了它对不同磁场大小的响应.测试结果表明,磁力仪测量范围为100—100000 nT,极限灵敏度为0.2 p T/Hz~(1/2),磁场分辨率为0.1 p T.进一步研究了不同磁场下原子系综极化态的横向弛豫时间,讨论了原子磁力仪高磁场采样率的获得方法.本文的原子磁力仪在5000—100000 n T的磁场测量范围内磁场采样率可实现1—1000 Hz范围内可调,能够测量低频的微弱交变磁场.本文的研究内容为大磁场测量范围、高灵敏度、高磁场采样率的原子磁力仪研制提供了重要参考.     

Coherent transients in the femtosecond photoassociation of ultracold molecules

We demonstrate the photoassociation of ultracold rubidium dimers using coherent femtosecond pulses. Starting from a cloud of ultracold rubidium atoms, electronically excited rubidium molecules are formed with shaped photoassociation pump pulses. The excited state molecules are projected with a time-delayed probe pulse onto molecular ion states which are detected in a mass spectrometer. Coherent transient oscillations of the excited state population are observed in the wings of the pump pulse, in agreement with the time-dependent solution of the Schr?dinger equation of the excitation process.     

飞秒光诱导铽镓石榴石晶体中的磁化响应研究

运用时间分辨抽运-探测光谱技术,研究了磁光晶体铽镓石榴石(TGG)在不同椭圆偏振态的飞秒激光脉冲诱导下的极化和磁化响应.研究表明,当仅存在逆法拉第效应时,探测光旋转角信号和椭圆率信号的变化方向与圆偏振抽运光的旋向相关.这是由于圆偏振光在TGG晶体中产生的瞬态有效磁场的方向依赖于圆偏振光的旋向所致.光诱导磁化过程与材料的性质有关,TGG晶体的顺磁特性决定了其自旋弛豫时间为几十飞秒.由于探测光旋转角信号和椭圆率信号的半高全宽均为500 fs左右,加之信号强度随着抽运光脉冲能量密度的增加呈线性增长,表明TGG晶 关键词： 铽镓石榴石晶体 抽运-探测光谱技术 逆法拉第效应     

泵浦-探测实验影响因素的研究

结合泵浦-探测技术在材料非线性光学特性测量中的应用,介绍了影响泵浦-探测实验结果的几个因素,如样品厚度、样品距泵浦光会聚透镜焦点的距离、泵浦光强度等。通过40ps脉冲条件下的C60-甲苯溶液的实验结果证实了在满足薄样品近似条件下,使用厚一点的样品获得的泵浦-探测信号较大;样品放置距离泵浦光会聚透镜焦点较近时,泵浦-探测信号较大;在一定的泵浦光强范围内,泵浦光强度增大,泵浦-探测信号会随之增强。同时,对这几个因素影响实验结果的原因给与了简单解释。这些结论对于应用泵浦-探测技术探索材料非线性光学特性的物理本质具有实际的指导意义。     

Time-resolved thermoreflectivity of thin gold films and its dependence on film thickness

\valunit{10}{nm} to have been measured by femtosecond time-resolved pump-probe experiments. A conspicuous change of the relaxation behavior was found around for pump pulse fluences of . Thicker films show a nearly exponential decay of the transient linear reflectivity, which turns into a linear decay during the first for films with thicknesses of or less. This observation is evidence of a mean free path of about for hot electrons with temperatures around . Received: 17 December 1996     

Transient intraband light absorption by quantum dots: Pump-probe spectroscopy

We have developed a theory of a transient intraband light absorption by semiconductor quantum dots. This absorption plays an important role in the two-pulse pump-probe method, which enables determining the energy relaxation rates of electron-hole excited states. We have considered all possible schemes of this process wherein the carrier frequency of optical pump pulses is close to the resonance with the interband transition of the quantum-dot electronic subsystem, while the carrier frequency of probe pulses is resonant to the intraband transition. For ensembles of identical and size-distributed quantum dots, the probe pulse energy absorption induced by the pump pulse is analyzed in relation to the delay time between the pulses. We have found that, under certain conditions, this dependence can be described by a single, two, or three exponentials. The exponents of the exponentials are proportional to the energy relaxation rates of electron-hole excited states.     

飞秒时间分辨实验中相关函数和时间零点的确定

结合飞秒激光在研究分子激发态弛豫动力学中的应用,介绍了几种飞秒时间分辨实验中确定泵浦激光脉冲与探测激光脉冲的相关函数和时间零点的方法.对于波长在可见波段的泵浦和探测激光脉冲,我们可以利用非线性光学的技术手段,探测泵浦光与探测光的和频光的强度随二者间的时间延迟的变化来确定相关函数和时间零点;对于波长在紫外甚至更短的波段的泵浦和探测激光脉冲,由于单脉冲能量比较低,目前还很难利用技术手段来测定泵浦激光与探测激光的相关函数及时间零点,可以利用某些原子气体(如Xe)或某些具有短寿命态的分子作平行实验进行间接测量.     

Modulational interactions in magnetised piezoelectric semiconductor plasmas

Based on hydrodynamic model of plasmas an analytical investigation of frequency modulational interaction between copropagating high frequency pump and acoustic mode and consequent amplification (steady-state and transient) of the modulated waves is carried out in a magnetised piezoelectric semiconductor medium. The phenomenon of modulational amplification is treated as four wave interaction process involving cubic nonlinearity of the medium. Gain constants, threshold-pump intensities and optimum-pulse duration for the onset of modulational instabilities are estimated. The analysis has been performed in non-dispersive regime of the acoustic mode, which is one of the preconditions for achieving an appreciable initial steady-state growth of the modulated signal wave. It is found that the transient gain constant diminishes very rapidly if one chooses the pump pulse duration beyond the maximum gain point. Moreover, the desired value of the gain can be obtained by adjusting intensity and pulse duration of the pump and doping concentration of the medium concerned. Received 12 August 1999 and Received in final form 17 December 1999     

非临界相位匹配RTP晶体1.6μm光参量振荡器

采用非临界相位匹配切割,尺寸4mm×4mm×20mm的磷酸钛氧铷(RTP)晶体作为非线性光学晶体,进行了半导体激光端面抽运Nd:YAG/RTP的内腔式光参量振荡获得人眼安全激光的实验研究。对比了不同声光调Q重复频率下的信号光输出特性。在10.5W的抽运功率和15kHz的重复频率下,获得了最高900mW的1.62μm人眼安全激光输出,光光转换效率达8.6%。1.62μm信号光和对应的1.06μm基频光的脉冲宽度分别为4.6和8.2ns。信号光中心波长在1618nm,谱线宽度小于0.5nm。     

基于受激辐射信号的谱域光学相干层析分子成像方法

鉴于不同生理病理状态下组织复折射率实部的变化不大,传统光学相干层析(OCT)成像技术在分子特异性识别上存在先天不足.为此,本文提出了基于受激辐射信号的OCT成像方法,可在实现传统散射成像的同时,实现基于受激辐射信号的分子成像.在超高分辨率谱域OCT系统的基础上,通过增设光谱分光与调制抽运光支路,建立了基于单宽谱光源的抽运探测谱域OCT系统,详细推导了调制抽运下受激辐射信号的获取与成像公式.利用搭建的抽运探测谱域OCT系统,实现了瞬态受激辐射信号的相干探测.基于同时获取的受激辐射OCT信号和传统OCT信号,成功重构了氮化物粉末构建样品的基于受激辐射信号的分子对比OCT图像.     

Optimization of polarization spectroscopy for rubidium D lines

Polarization spectroscopy of the D lines of rubidium atoms is investigated experimentally,especially with different pump powers and cell temperatures.We find that there are four candidate transitions suitable for frequency stabilization,and optimal pump powers and cell temperatures are also presented to obtain a perfect signal with maximal amplitude and slope.The optimal signal is insensitive to the fluctuations of laser power and the temperature,which can enhance the performance of frequency locking.     

Monitoring atomic cluster expansion by high-harmonic generation

High-harmonic generation is shown to be capable of providing time-resolved information about the particle density of a complex system. As an example, we study numerically high-harmonic generation from expanding xenon clusters in a pump-probe laser scheme, where the pump laser pulse induces the cluster explosion and the probe pulse generates harmonics in the expanding cluster. We show that the high-harmonic spectra characterize the properties of the expanding cluster. Hence, measuring the dependence of the harmonic signal on the pump-probe delay suggests itself as an experimental tool to monitor many-particle dynamics with unique temporal resolution; based on optical measurements, this technique is naturally free from any spatial charge effects.     

Time-resolved measurements of picosecond optical breakdown

Picosecond optical breakdown was investigated in order to assess its potential for performing highly localized incisions for laser surgery. Measurements of breakdown were performed using single 40-ps Nd: YAG laser pulses in distilled water. Novel optical pump-probe techniques were developed to characterize the transient spatial and temporal dynamics of the plasma, shock wave, and cavitation phenomena which are associated with the breakdown. The maximum cavity radius and the shock wave zone are shown to scale as the cube root of the pump pulse energy over almost three orders of magnitude. For pulse energies close to the threshold energy of 8 J, the shock range was 100–200 m and the cavity radius was 140 m. Complementary experiments were performed with 10-ns pulse durations. Since picosecond pulses have high peak intensities with low pulse energies, a significant enhancement in localizability may be achieved. The implications for ophthalmic microsurgery are discussed.     

Molecular contrast in optical coherence tomography by use of a pump-probe technique

We describe a novel technique for contrast enhancement in optical coherence tomography (OCT) that makes possible molecular-specific imaging for what is believed to be the first time. A pump-probe technique is employed in which a pulsed pump laser is tuned to ground-state absorption in a molecule of interest. The location of the target molecule population is derived from the resulting transient absorption of OCT sample-arm light acting as probe light. A signal processing technique for three-dimensional localization of the transient absorption signal is described, and preliminary results exhibiting OCT contrast from methylene blue dye in multilayer and scattering phantoms are presented.     

Laser Pump-Probe Fiber-Optic Technique for Characterization of Near-Surface Layers of Solids: Development and Application Prospects for Studying Semiconductors and Weyl Semimetals

A fiber-optic setup incorporating the pump-probe thermoreflectance (TR) technique with Fabry–Perot (FP) interferometer is presented. It includes both heat pump and probe lasers, producing wavelengths of 1470 and 1530 nm, respectively, together with a reflected radiation detector. Heat pump pulse duration varies from a few microseconds to tens of microseconds. The potential of the pump-probe TR-FP technique to investigate the subsurface region of semiconductors with a range of electron spectra is demonstrated. A pronounced dip in time dependence of the TR-FP signal is discovered at the liquid nitrogen temperature in the gapless semiconductor compound HgSe—a candidate for the family of Weyl semimetals with broken inversion symmetry. This finding implies the developed pulsed TR-FP method for the detection of Weyl nodes and surface Fermi arcs in solids.     

Impulsive stimulated Raman scattering: comparison between phase-sensitive and spectrally filtered techniques

Impulsive stimulated Raman scattering is used to generate and control coherent phonons and other low-frequency modes. In transparent materials, pump-probe experiments are usually performed by spectrally resolving the probe beam and measuring the spectral shift as a function of pump-probe time delay. By measuring the optical phase of the probe pulse as a function of time delay, we find that the phonon signal can be increased by a factor alpha(omegadelta)(-1), where omega is the phonon frequency and delta is the pulse duration.     

Transient reflectivity and transmission changes during plasma formation and ablation in fused silica induced by femtosecond laser pulses

We have studied the plasma formation and ablation dynamics in fused silica upon irradiation with a single 120 fs laser pulse at 800 nm by using fs-resolved pump-probe microscope. It allows recording images of the laser-excited surface at different time delays after the arrival of the pump pulse. This way, we can extract both the temporal evolution of the surface reflectivity and transmission, at 400 nm, for different spatial positions in the spots (and thus for different local fluences) from single series of images. At fluences well above the visible ablation threshold, a fast and large increase of the reflectivity is induced by the formation of a dense free-electron plasma. The maximum reflectivity value is reached within ≈1.5 ps, while the normalized transmission decreases within ≈400 fs. The subsequent temporal evolution of both transient reflectivity and transmission are consistent with the occurrence of surface ablation. In addition, the time-resolved images reveal the existence of a free-electron plasma distribution surrounding the visible ablation crater and thus formed at local fluences below the ablation threshold. The lifetime of this sub-ablation plasma is ≈50 ps, and its maximum electron density amounts to 5.5×10²² cm⁻³.     

Autlerben Townes splitting in photoelectron spectra of K2 molecule

We investigate Autler-Townes splitting in the photoelectron spectra of K₂ molecule driven by pump-probe pulses via employing the time-dependent wave packet approach. It is found that the magnitude of Aulter-Townes splitting varies with the wavelength of the intense laser pulse. In particular, the phenomenon of Aulter-Townes splitting vanishes for the far-off resonance of the pump pulse. Also, the split peaks of Autler-Townes in the case of resonant pump pulse give us an approach to directly obtaining the transition dipole moment of a molecule.     

Transient gain in photonic crystals

Using the optical response formula for photonic crystals and the Bloch equations, we analyze coherent emission in arbitrary two-dimensional photonic crystals. The transient emission depends not only on the photon density of the states, but also on the intensity and duration of the pump laser pulses. A transient gain exists if the pump pulse area is a certain value and the transition frequency is tuned to the band edge. The sharp structure of the density of states near the band edge leads to a dramatic enhancement of the transient emission intensity and an abrupt change in the oscillation of the emission field. PACS 42.70.Qs; 32.80.-t; 42.25.Bs     

Influence of transient phonon relaxation on the Brillouin loss spectrum of nanosecond pulses

For pump-probe stimulated Brillouin scattering with a probe pulse of a few nanoseconds duration and with a finite DC level, the acoustic wave relaxation time varies with the pump power and the DC level. For a pump power of 1-6 mW, the acoustic wave relaxation changes from approximately 9 to 90 ns for polarization-maintaining fiber at a temperature of -40 degrees C for a 2 ns pulse width. When the pulse DC ratio of the probe varies from 10 to 20 dB, the acoustic relaxation time changes from 24 to 45 ns for single-mode fiber at 25 degrees C. This induces a power-increment spectral feature in the detected AC pump signal in the Brillouin loss spectrum of two temperature or strain sections, where both spectral components appeared at positions far from those related to the natural phonon relaxation time (approximately 10 ns) equivalent length. The theoretical calculations confirm the prolonged phonon relaxation.