Nonlinear acoustic transparency phenomena in strained paramagnetic crystals

Nonlinear dynamics of longitudinal acoustic pulses propagating in a strained paramagnetic cubic crystal at low temperature is analyzed. The direction of constant uniform strain is parallel to one of the fourth-order symmetry axes. Effective-spin 1 ions are considered as paramagnetic impurities with the strongest spin-lattice interaction. In this medium, normally degenerate magnetic sublevels are dynamically shifted by the quadrupole Stark effect, and the frequencies of the transitions induced by an acoustic pulse change accordingly. The self-consistent system of equations derived in this study without using the slowly varying envelope approximation describes pulse propagation at an arbitrary angle to the direction of the static strain. Exponentially and rationally decaying monopolar and breather-like solutions to the system are obtained. An analysis of the solutions reveals an asymmetry of the pulse polarity depending on the type of strain (extension or compression) and the pulse propagation direction. In particular, it is shown that acoustic transparency associated with monopolar strain pulses exhibits threshold behavior. The sign of the time area (zeroth harmonic) of breather-like strain pulses is such that the transition frequency averaged over an oscillation period dynamically decreases. This behavior determines the efficiency of generation of high-order harmonics of acoustic pulses in strained paramagnetic crystals.     

Influence of thermal self-action on the diffraction of high-power X-ray pulses

The space and time temperature distributions in a crystal exposed to the high-power pulses of an X-ray free-electron laser are analyzed by solving the heat-conduction equation. Using synthetic diamond crystals as an example, we examine how the temperature and strain of a crystal lattice affect the space-time and integrated intensities of diffraction reflection and the transmission of X-ray laser pulses. The conditions making it possible to minimize the thermal loads and spectral broadening of the diffracted pulses are determined.     

Actively mode-locked dual-wavelength fiber laser with close wavelength spacing using a fiber Bragg grating

A simple actively mode-locked fiber ring laser is proposed and successfully demonstrated to generate dual-wavelength picosecond pulses with close wavelength spacing using one Bragg grating in standard single-mode fiber. The proposed laser can be made to operate in stable dual-wavelength at room temperature, due to the birefringence characteristic of the FBG induced by transverse strain. Transverse strain loading on the FBG allows the wavelength spacing to be controlled. Generation of stable dual-wavelength pulses with a pulsewidth of 212–234 ps and a tunable wavelength separation from 0.2 to 0.44 nm at a pulse rate of 1.05 GHz was demonstrated.     

Solitary surface acoustic waves and bulk solitons in nanosecond and picosecond laser ultrasonics

Recent achievements of nonlinear acoustics concerning the realization of solitons and solitary waves in crystals and their surfaces attained by nanosecond and picosecond laser ultrasonics are discussed and compared. The corresponding pump-probe setups are described, which allow an all-optical contact-free excitation and detection of short strain pulses in the broad frequency range between 10 MHz and about 300 GHz. The formation of solitons in the propagating longitudinal strain pulses is investigated for nonlinear media with intrinsic lattice-based dispersion. The excitation of solitary surface acoustic waves is realized by a geometric film-based dispersion effect. Future developments and potential applications of nonlinear nanosecond and picosecond ultrasonics are discussed.     

Optical and photocurrent spectroscopy with picosecond strain pulses

This paper gives an overview of optical experiments using picosecond strain pulses. The strain pulses, which propagate with the sound velocity, are incident on a semiconductor nanostructure and induce an ultrafast shift of the exciton resonance energy by an amount, that exceeds the spectral width of the corresponding optical transition. When the duration of the high-amplitude strain pulse is long enough compared with the coherence time of the optical resonance, modulation of the resonance takes place adiabatically and exciton energy can be accurately defined at each momentary position. If the coherence time exceeds the characteristic time of the strain pulse, a non-adiabatic regime is realized and the exciton cannot be related to an optical transition with a specific photon energy. In more detail, we describe the recent experiments on the gating of photocurrent in a tunneling p-i-n device and the generation of THz polariton sidebands in an optical microcavity strongly coupled to the excitons in an embedded quantum well. These two experiments represent, respectively, examples of adiabatic and non-adiabatic behaivior of excitons in the presence of the high-amplitude picosecond strain pulse.     

Propagation of unipolar strain pulses in media with hysteretic nonlinearity

A theoretical study of the propagation of unipolar strain pulses in a solid medium with an arbitrary power-law hysteretic nonlinearity is carried out. Exact analytical expressions are derived for describing the propagation and evolution of initially sine-shaped and triangular pulses in such media. The solutions found are analyzed both numerically and graphically.     

磁铁式弦振实验仪的研制

介绍了磁铁式弦振实验仪的测量机理和信号处理方法.在激振信号作用下,磁铁式弦振传感器的钢弦因受洛伦兹力而变形并作横向振动,利用电磁感应原理得到同频率的感应电势,根据钢弦振动的频率可算出传感器受力和变形.     

Strength of liquid tin at extremely high strain rates under a femtosecond laser action

The destruction of liquid tin at the extremely high strain rate caused by femtosecond laser pulses has been studied. The ablation and cavitation thresholds, as well as tensile stresses responsible for the destruction of liquid tin at a strain rate of ～10⁹ s^–1, have been experimentally determined.     

Self-induced acoustic transparency of three-component longitudinal-transverse pulses

The features of the nonlinear dynamics of three-component elastic pulses in a low-temperature crystal containing paramagnetic impurities of electron and nuclear spins have been analyzed in the slowly varying envelope approximation. The presence of the electron spin subsystem makes it possible to equate the velocities of longitudinal sound and transverse acoustic waves; as a result, all components of the strain field efficiently interact with each other through the nuclear spin subsystem. The system of equations for envelopes of harmonics of the components of the strain field and the spin variables has been derived. The relations between the amplitudes and phases of the components have been obtained, the spectral composition has been analyzed, and the regimes of acoustic transparency of three-component longitudinal-transverse pulses have been discussed.     

Generation and propagation of a picosecond acoustic pulse at a buried interface: time-resolved x-ray diffraction measurements

We report on the propagation of coherent acoustic wave packets in (001) surface oriented Al0.3Ga0.7As/GaAs heterostructure, generated through localized femtosecond photoexcitation of the GaAs. Transient structural changes in both the substrate and film are measured with picosecond time-resolved x-ray diffraction. The data indicate an elastic response consisting of unipolar compression pulses of a few hundred picosecond duration traveling along [001] and [001] directions that are produced by predominately impulsive stress. The transmission and reflection of the strain pulses are in agreement with an acoustic mismatch model of the heterostructure and free-space interfaces.     

Simultaneous measurement of temperature and strain using a long-period fiber grating with a micro-taper

An all-fiber Mach–Zehnder interferometer (MZI) consisting of a long-period fiber grating with a micro-taper is proposed for simultaneous measurement of temperature and strain. The experimental results demonstrate that the temperature and strain sensitivities of the proposed MZI are 83 pm/°C and ?2.6 pm/με, respectively. The strain sensitivity is 20 times as high as that of a long-period fiber written by CO₂ laser pulses combined with a fiber bitaper. In addition, the interferometer requires only a common single-mode fiber, and it is easy to fabricate and is inexpensive for temperature and strain sensing applications.     

Coherent interactions of terahertz strain solitons and electronic two-level systems in photoexcited ruby

We observe coherent interactions between an ultrashort, longitudinal acoustic soliton train and the 29-cm(-1) electronic transition in photoexcited ruby. Propagation of the strain pulses over millimeter distance through an excited zone reveals striking behavior of the induced electronic population, which has been explained by impulsive excitation of the two-level systems, combined with the nonlinear properties of the solitons in the resonant medium. This opens up new possibilities for coherent manipulation of ultrashort acoustic pulses by local electronic centers.     

低成本长周期光纤光栅传感系统

研制了一种低成本长周期光纤光栅传感系统.利用高频CO2激光脉冲写入长周期光纤光栅边缘滤波效应原理,设计参考光路以补偿进入光电探测器前光路中各种扰动引起的光功率起伏.当输入为单波长信号光时,可以通过测量探测器输出的电信号来获得光纤光栅所承受的温度和应变信息.在温度实验里,温度和探测到的电信号成较好的二次关系,温度分辨率为0.014℃；在应变实验里,施加的应变和探测到的电信号成线性关系,应变分辨率为2.5 με.该传感系统结构简单,成本较低,响应快.     

Effect of femtosecond laser radiation on mammalian oocytes

Nucleolus-like bodies (NLBs) of mammalian oocytes are a convenient object for studying the effects of laser radiation on cellular organelles during laser surgery operations. NLBs in mouse germinal vesicle oocytes (GV oocytes) were subject to nanosurgery by means of tightly focused femtosecond laser pulses at near-infrared wavelength. The damage threshold of the NLB material was one-half orders of magnitude below the threshold for the water breakdown by femtosecond pulses. An incision of NLB material with submicron resolution was obtained at a threshold pulse energy. At the pulse energy one order of magnitude higher than the threshold level, significant deformation and rupture of NLBs were observed. Anisotropic deformations of the NLBs and the dynamics of their relaxation indicate the presence of granular structure in these organelles. Mechanical strain and the formation of cavitation and steam-gas bubbles in the place of impact of laser pulses can be described with the help of a model of the laser breakdown of medium.     

Acoustic activity effect for picosecond soliton-like pulses

A theoretical analysis is made of the acoustic activity for interfering picosecond acoustic soliton-like pulses of down to a single oscillation period. An analysis is made of the case where these pulses propagate parallel to an external magnetic field and one of the acoustic axes in a cubic crystal containing paramagnetic impurities having effective spin S = 1. Allowance is made for natural, magnetic (Faraday), and cross acoustic activity. This cross activity is caused by the significant spatial nonlocality of the spin-phonon interaction for such short pulses in crystals having no center of inversion in the presence of paramagnetic impurities. A system of nonlinear equations is obtained for the transverse and longitudinal components of the strain in the form of a coupling between the “differentiated” nonlinear Schrödinger equation (with nonlinearity after the derivative sign) and the Korteweg-de Vries equation which generalizes the known systems of long-short-wavelength resonance to the case where the slowly varying envelope approximation is not valid. An approximate solution of this system is used to study the structure of an elastic soliton-like pulse whose transverse component has a rotating plane of polarization, which propagates under conditions of nonlinear coupling with the longitudinal strain.     

Heat affected zone in aluminum single crystals submitted to femtosecond laser irradiations

An experimental procedure is developed to quantify the radial dimension of the heat affected zone (HAZ) in metals submitted to laser pulses. A cube oriented aluminum single crystal is highly deformed by plane strain compression, then micro-drilled by 200 fs or 8 ns laser pulses, and finally analyzed by the electron back scattering diffraction technique. Recrystallized and recovered zones are observed as signatures of the HAZ. A typical value of 1.5 μm is found in the femtosecond regime of illumination, whereas for nanosecond pulses a value of 25 μm is measured. These results are in accordance with previous experiments and numerical simulations.     

Potential of two-photon-excited fluorescence for distributed fiber sensing

The design and operating parameters of a new class of continuously distributed optical fiber sensor are described. By use of counterpropagating pulses, two-photon excitation of fluorescence from ions doped into the fiber enables any position to be monitored. By this means temperature and also strain may be sensed with high spatial and temporal resolution. As the doped fiber is transparent for single-photon absorption at the wavelength of the light pulses, attenuation does not set an upper limit to its length.     

Picosecond X-ray diffraction studies of laser-excited acoustic phonons in InSb

We have employed time-resolved X-ray diffraction with picosecond temporal resolution to measure the time-dependent rocking curves of laser-irradiated asymmetrically cut single InSb crystals. Coherent acoustic phonons were excited in the crystals by irradiation with 800-nm, 100-fs laser pulses at irradiances between 0.25 and 12 mJ/cm². The induced time-dependent strain profiles (corresponding to the coherent phonons) were monitored by diffracting collimated, monochromatic pulses of X-rays from the irradiated crystals. Recording of the diffracted radiation with a fast low-jitter X-ray streak camera resulted in an overall temporal resolution of better than 2 ps. The strain associated with the coherent phonons modifies the rocking curve of the crystal in a time-dependent manner, and the rocking curve is recorded by keeping the angle of incidence of the X-rays upon the crystal fixed, but varying the energy of the incident X-rays around a central energy of 8.453 keV (corresponding to the peak of the rocking curve of the unperturbed crystal). The observed time-dependent diffraction from the irradiated crystals is in reasonable agreement with simulations over a wide range of energies from the unperturbed rocking-curve peak. Received: 22 March 2002 / Revised version: 25 March 2002 / Published online: 6 June 2002     

Nanosecond acoustic strain pulses from CdS phonon masers

Observations of nanosecond strain pulses generated by a mode-locked CdS phonon maser are reported. Peak strains exceeding 5 × 10^-5 have been measured. An analogy is drawn between the observed mode-locked operation and a repetitive pulse generator.     

Ultrafast strain-induced current in a GaAs Schottky diode

Picosecond acoustic pulses generated by femtosecond laser excitation of a metal film induce a transient current with subnanosecond rise time in a GaAs/Au Schottky diode. The signal consists of components due to the strain pulse crossing the edge of the depletion layer in the GaAs and also the GaAs/Au interface. A theoretical model is presented for the former and is shown to be in very good agreement with the experiment.