较高阶非线性抽运-探测过程中的空间效应

在飞秒抽运一探测光谱技术中,空间分辨的探测光信号反映了在不同空间位置的材料的非线性效应。当抽运光强度增大时,探测光信号中会出现明显的高阶特别是五阶非线性效应。利用劈裂算子方法直接解决了一维非线性传播方程的问题。在数值模拟中,研究了在不同抽运强度和位置下的抽运一探测过程中的五阶非线性效应。在足够高的抽运场下,探测信号出现清晰的振荡,显示了三阶和五阶非线性效应之间的干涉。当空间位置离抽运场中心足够远时,五阶比三阶非线性效应的衰减快得多,对其物理机制和趋势进行了定性的讨论。     

Spatial and frequency manifestations of laser beam self-action effects in different schemes of inversionless amplification

Results of numerical simulation of coaxial propagation of an intense pump beam and a weak probe beam in a three-level medium with different types of energy diagrams in conditions of inversionless amplification are presented. The effect of the beam self-action and diffraction on the spatial and frequency characteristics of the probe wave were studied. These effects, in combination with linear absorption and refraction, give rise to complex spatial profiles of gain along the beam propagation path. The annular structure of the amplified beam and a small asymmetry of its frequency characteristics, resulting from the nontrivial gain profile over the beam cross section, were found and studied.     

Coherent interaction of picosecond pulses in a fluorescent medium and the timing of pulse trains for two-wavelength excite and probe measurements

Coherent interaction between two cw, picosecond pulse trains of the same wavelength absorbed by a fluorescent material and the effect on the emission has been studied. As the path difference between the two beams is varied, the infra-red emission from colour centres in KCl: Na or KCl: Li fluctuates in phase with the transmitted intensity of the weaker of the two beams. The effect may be applied to the accurate relative timing of cw pulse trains of different wavelengths for use in two-wavelength, excite and probe measurements. A small fraction of the pump pulse train is superimposed on the probe beam and passes along precisely the same optical path. Coherent interaction between the two pulse trains at the pump frequency causes modulation of the transmitted probe beam intensity when pump and probe pulses are accurately synchronised.     

Numerical simulation of nonlinear propagation of sound waves in a finite horn

Nonlinear acoustic propagation generated by a piston in a finite horn is numerically studied.A quasi-one-dimensional nonlinear model with varying cross-section uses high-order low-dispersion numerical schemes to solve the governing equation.Because of the nonlinear wave distortion and reflected sound waves at the mouth,broadband time-domain impedance boundary conditions are employed.The impedance approximation can be optimized to identify the complex-conjugate pole-residue pairs of the impedance functions,which can be calculated by fast and efficient recursive convolution.The numerical results agree very well with experimental data in the situations of weak nonlinear wave propagation in an exponential horn,it is shown that the model can describe the broadband characteristics caused by nonlinear distortion.Moreover,finite-amplitude acoustic propagation in types of horns is simulated,including hyperbolic,conical,exponential and sinusoidal horns.It is found that sound pressure levels at the horn mouth are strongly affected by the horn profiles,the driving velocity and frequency of the piston.The paper also discusses the influence of the horn geometry on nonlinear waveform distortion.     

长寿命吸收过程对超快动力学过程测量的影响

使用飞秒时间分辨抽运-探测透射光谱技术,实验研究了GaAs体材料中光激发载流子的超快弛豫动力学的波长依赖.在相同的光激发载流子浓度和抽运/探测比时,发现760 nm和780 nm两中心波长处的瞬态透射变化延迟扫描信号出现负的和振荡的信号.与模拟计算结果对比,判定该实验瞬态信号是错误的.分析探测器输出波形,发现是由于反相波形导致的,而引起反相波形的原因在于样品中存在长寿命的吸收过程.指出通过提高探测器上的抽运/探测比能够矫正反相波形,从而获得正确的瞬态透射变化动力学.提高探测器上的抽运/探测比与目前的应尽量减小抽运光对探测器的散射贡献的观点是对立的.文章的研究结果对应用抽运-探测时间分辨光谱技术正确地测量超快瞬态动力学过程具有重要的参考价值. 关键词： 时间分辨抽运-探测透射光谱 饱和吸收 吸收增强 GaAs体材料     

有限长号筒中非线性声传播的数值模拟研究

研究了大振幅活塞声源经有限长号筒向外辐射声波的非线性声学问题。采用具有频散保持特性的高精度计算格式求解了适用于变截面管道的一维非线性声场模型,并考虑到声波的非线性畸变及管口处的声反射,加入了宽频时域声阻抗边界条件。宽频阻抗模型的共轭复数系数采用优化拟合方法近似求解,并采用递推卷积算法快速求解出时域声阻抗。在弱非线性条件下模拟指数形号筒中的声传播取得了与已有实验相一致的结果,表明模型能够描述声波非线性畸变带来的宽频特性。进而采用本模型数值模拟了大振幅活塞声源在双曲形、锥形、指数形和正弦形号筒中的非线性声传播问题,结果表明号筒出口声压级受活塞振动速度、频率以及号筒形状的影响,并分析讨论了波形畸变与号筒几何形状之间的关系。      

Spatiotemporal mapping of surface acoustic waves in isotropic and anisotropic materials

We demonstrate a new method for a real-time imaging of surface acoustic waves at frequencies up to 1 GHz with picosecond temporal and micron spatial resolutions using an ultrafast optical pump and probe technique combined with a common path interferometer. Using samples with isotropic or anisotropic substrates coated with metallic thin films, we observe the propagation of Rayleigh-like modes and surface-skimming bulk modes as well as resolving surface phonon focusing effects. In addition we image surface acoustic wave propagation in a laterally inhomogeneous sample.     

Modified model for four-wave mixing-based wavelength conversion in silicon micro-ring resonators

A modified model for wavelength conversion based on the four-wave mixing (FWM) in a silicon micro-ring resonator is presented. Unlike previous contributions, the nonlinear phase shifts caused by self-phase modulation and cross-phase modulation are also taken into account in the present theoretical analysis besides the linear propagation loss and the nonlinear losses caused by two-photon absorption and free-carrier absorption. Analysis shows that the nonlinear phase shifts will cause different red shifts for the pump and signal (or converted) resonant wavelengths, and consequently an additional wavelength difference between the signal transmission dip and the efficiency peak, which will increase/decrease the conversion efficiency of the signal channel far from/near the pump. The conversion efficiency and the conversion peak width of each signal channel are both affected by the micro-ring radius and coupling coefficient. A broader conversion peak width can be obtained by using a micro-ring resonator with a smaller Q factor.     

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.     

Thermooptical Mechanism of Sound Generation by High-Power Laser Radiation Propagating in the Atmosphere

In the present paper, the results of experimental and theoretical investigations into the thermooptical mechanism of sound generation by high-power pulsed laser radiation (HPLR) propagating in the atmosphere are given. Experiments were performed with collimated and weakly-focused beams of a pulsed CO₂ laser with a wavelength of 10.6 m. A sound pulse was recorded with omnidirectional microphones placed at distances of 0.2–0.5 m from the laser beam axis and spaced along the laser beam propagation path whose length was a few hundred meters. A sound pressure level was measured with a precision impulse sound level meter, and time histories of acoustic pulses were also displayed on the screen of an oscilloscope. Based on these investigations, the methods of thermoacoustic sounding of the effective HPLR beam radius and divergence angle, the laser energy absorption coefficient, and the total laser energy have been suggested and realized together with the method of the diagnostics of HPLR propagation regime from the waveform or spectrum of the received acoustic signal.     

A novel phase-sensitive scanning near-field optical microscope

Phase is one of the most important parameters of electromagnetic waves. It is the phase distribution that determines the propagation, reflection, refraction, focusing, divergence, and coupling features of light, and further affects the intensity distribution. In recent years, the designs of surface plasmon polariton(SPP) devices have mostly been based on the phase modulation and manipulation. Here we demonstrate a phase sensitive multi-parameter heterodyne scanning near-field optical microscope(SNOM) with an aperture probe in the visible range, with which the near field optical phase and amplitude distributions can be simultaneously obtained. A novel architecture combining a spatial optical path and a fiber optical path is employed for stability and flexibility. Two kinds of typical nano-photonic devices are tested with the system. With the phase-sensitive SNOM, the phase and amplitude distributions of any nano-optical field and localized field generated with any SPP nano-structures and irregular phase modulation surfaces can be investigated. The phase distribution and the interference pattern will help us to gain a better understanding of how light interacts with SPP structures and how SPP waves generate, localize, convert, and propagate on an SPP surface. This will be a significant guidance on SPP nano-structure design and optimization.     

A Swept-Frequency Wideband Channel Sounding System at the 63.4–64.4?GHz Band with 1?ns Time-Resolution

A Versatile channel sounding impulse response identification system using the swept-frequency method is designed at the 63.4–64.4 GHz band. The swept-frequency channel sounder offers high time and frequency resolution of 1 ns and 625 KHz respectively. The high dynamic range (70 dB) and constant power enable non-linearities to be overcome in the measurement areas lying with the coverage range of the order of a picocell.     

A computer model for simulating reflected ultrasound signals

This letter describes a model for simulating an ultrasound signal reflected from a medium composed of randomly distributed scatterers, as typified by soft biological tissue, such as liver. The model is specified in terms of the effective transducer beam radius B and the mean scatterer spacing S. The novel feature of the model is that the transducer field is partitioned by packing cylinders, called microbeams, into concentric annular regions that lie parallel to the transducer axis. The radii of the microbeams and the annuli are related to S. An independent reflector sequence is generated for each microbeam, the microbeam sequences in each annulus are summed and convolved with the impulse response of a point reflector. The reflected waveform sequence is then generated by summing the annular contributions and convolving with a band-limited pulse waveform. Simulated signals were generated for different values of B and S and compared with actual signals reflected from two in vivo livers and a tissue-equivalent phantom. Estimates of the kurtosis for the simulated signals indicate the range of signals that can be generated by varying the values for S and B.     

各向同性介质中任意偏振光的简并四波混频

在各向同性介质的简并四波混频中,就共线(探测光波矢平行于泵浦光的波矢)和非共线装置,线偏振和圆偏振等情况,推导出反射波的复振幅(包括偏振)与探测光和泵浦光的振幅、偏振、介质的三阶非线性张量元之间的关系式。在非共线装置中,泵浦光为平行的线偏振光时,可利用探测波与反射波的偏振方向间的关系求出x₁₁₁₁与x₁₂₂₁的比值;泵浦光为同旋性的圆偏振光时,可获得理想的复共轭波。在共线装置中,在反射波中总含有一定的非共扼的成分,振幅反射系数与样品厚度的关系可能是正切、双曲正切或线性函数关系;泵浦光为平行的线偏振光时,若|x₁₂₂₁|>|x₁₁₂₂|,可产生自振荡现象;在泵浦光为旋性相反的圆偏振光时,对圆偏振的探测光可产生理想的平面镜反射的效果,等等。结果可用来解释一些已知的实验结果,并对新的实验设计具有一定的指导作用。 关键词：     

Polarization beam splitting in media with light-induced anisotropy

The problem of light beam reflection from a refractive-index inhomogeneity induced by a high-power polarized Gaussian pump beam in a resonant medium has been simulated. A nonlinear optical method of spatial splitting of polarization components of a probe pump beam is proposed.     

Polaritonic stop-band transparency via exciton-biexciton coupling in CuCl

Radiation is almost completely reflected within the exciton-polariton stop band of a semiconductor, as in the typical case of CuCl. We predict, however, that a coherently driven exciton-biexciton transition allows for the propagation of a probe light beam within the stop band. The phenomenon is reminiscent of electromagnetically induced transparency effects occurring in three-level atomic systems, except that it here involves delocalized electronic excitations in a crystalline structure via a frequency and wave-vector selective polaritonic mechanism. A well-developed transparency, favored by the narrow linewidth of the biexciton, is established within the stop band where a probe pulse may propagate with significant delays. The transparency window can be controlled via the pump beam detuning and intensity.     

Ultrashort pulse propagation and nonlinear frequency conversion in superconducting and magnetic photonic crystal

We study the propagation and second harmonic generation of ultrashort pulse in nonlinear photonic crystal using a combination of Fourier transform and transfer matrix method. The focus is on the reflected and transmitted output of fundamental pump pulse and second harmonic pulse in frequency and time domains, the temperature dependence of the reflection and transmission spectra where the superconducting transition frequency is close to the magnetic resonance. Interesting features include output pump and second harmonic pulses that can be strongly modulated with the transmitted pulses being delayed by slow light effect.     

Harmonic propagation of finite amplitude sound beams: experimental determination of the nonlinearity parameter B/A

We propose a method to determine the nonlinearity parameter B/A of a liquid from the spatial evolution of harmonic components. We describe an analytical model, in the parabolic and quasi-linear approximations, that predicts the continuous finite amplitude sound beam propagation radiated by a plane piston source. This model takes into consideration attenuation, diffraction and nonlinear effects. The fundamental and second harmonic ultrasonic fields are expressed as the superposition of Gaussian beams. Axial propagation curves are then compared with those obtained by direct numerical solution of the transformed beam equation using the finite difference method, and with experimental results. Accurate measurements of pressure levels for the nonlinearly generated harmonics in water are performed along and across the propagation axis for different pressure values delivered at the piston surface. Experimental results, for water and ethanol, are in agreement with those of our model, which allows us to obtain the expected value of the nonlinearity parameter B/A.     

Rabi-oscillation-enhanced frequency conversion in quantum-dot semiconductor optical amplifiers

We investigate the nonlinear light propagation in InAs/InGaAs quantum-dot-in-a-well semiconductor optical amplifiers in the limit of strong optical excitation where Rabi oscillations are excited in the active medium. The amplifier is analyzed in a degenerate four-wave-mixing setup and characterized by its frequency conversion and creation performance. Our simulations show that the interplay between the nonlinear four-wave-mixing process and the coherent Rabi oscillations greatly influences the frequency conversion process. Rabi oscillations can be resonantly excited by the correct choice of the frequency detuning between pump and probe signals, which greatly enhances the nonlinear frequency conversion efficiency at frequencies up to several THz. We furthermore show that the coherent pulse shaping of ultrashort optical pulses in the quantum-dot medium can greatly enhance their spectral bandwidth, potentially allowing for ultra-broad-band frequency comb generation.     

Digital EPR with an arbitrary waveform generator and direct detection at the carrier frequency

A digital EPR spectrometer was constructed by replacing the traditional bridge with an arbitrary waveform generator (AWG) to produce excitation patterns and a high-speed digitizer for direct detection of the spin system response at the carrier frequency. Digital down-conversion produced baseband signals in quadrature with very precise orthogonality. Real-time resonator tuning was performed by monitoring the Fourier transforms of signals reflected from the resonator during frequency sweeps generated by the AWG. The capabilities of the system were demonstrated by rapid magnetic field scans at 256 MHz carrier frequency, and FID and spin echo experiments at 1 and 10 GHz carrier frequencies. For the rapid scan experiments the leakage through a cross-loop resonator was compensated by adjusting the amplitude and phase of a sinusoid at the carrier frequency that was generated with another AWG channel.