太赫兹瞬态响应光导开关的实验研究

采用飞秒激光激励光导开关能够产生脉宽皮秒甚至亚皮秒级的太赫兹脉冲,近年来,这项技术成为校准宽带示波器上升时间的有效手段。以低温生长砷化镓（LT-GaAs）为光导开关的基底,在飞秒激光激励下产生太赫兹脉冲,经共面波导传输,通过微波探针耦合为1．85mm同轴输出,然后利用带宽70GHz的取样示波器对其半幅度宽度进行测量。实验获得太赫兹脉冲的半幅度宽度（FWHM）约为7．4ps。     

Dynamics of carriers photogenerated in a dot-in-a-well nanostructure

We review our recent results on the investigation of carrier dynamics of semiconductor nanostructures, i.e., dot-in-a-well. We introduced a technique based on the measurement of time-resolved differential photoluminescence spectra induced by subpicosecond pump and probe laser pulses. In this technique, the temporal resolution was achieved by adjusting the temporal delay between the pump and probe pulses. Using such a new technique, we measured the exciton decay times and then studied the dependences of integrated photoluminescence intensity and photoluminescence linewidth on temperature in InAs quantum dots embedded in InGaAs/GaAs quantum wells. We gave consistent interpretations to our experimental results.     

Nonlinear cross-phase modulation with intense single-cycle terahertz pulses

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100 microJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.     

Ultrafast dynamic control of spin and charge density oscillations in a GaAs quantum well

We use subpicosecond laser pulses to generate and monitor in real time collective oscillations of electrons in a modulation-doped GaAs quantum well. The observed frequencies match those of intersubband spin- and charge-density excitations. Light couples to coherent density fluctuations through resonant stimulated Raman scattering. Because the spin- and charge-related modes obey different selection rules and resonant behavior, the amplitudes of the corresponding oscillations can be independently controlled by using shaped pulses of the proper polarization.     

Femtosecond holography in lithium niobate crystals

Spatial gratings are recorded holographically by two femtosecond pump pulses at 388 nm in lithium niobate (LiNbO3) crystals and read out by a Bragg-matched, temporally delayed probe pulse at 776 nm. We claim, to our knowledge, the first holographic pump-probe experiments with subpicosecond temporal resolution for LiNbO3. An instantaneous grating that is due mostly to the Kerr effect as well as a long-lasting grating that results mainly from the absorption caused by photoexcited carriers was observed. The Kerr coefficient of LiNbO3 for our experimental conditions, i.e., pumped and probed at different wavelengths, was approximately 1.0 x 10(-5) cm2/GW.     

Determination of the Subpicosecond Laser Pulse Chirp in the Middle IR Range Based on the Fourth Harmonic Noncollinear Generation

A new method is proposed to determine the subpicosecond laser pulse chirp in the middle IR range at the central wavelength of 10 μm, based on the generation of the second-harmonic pulses both by the bandwidth-limited and frequency-modulated subpicosecond pulses and the subsequent noncollinear generation of the fourth-harmonic radiation by the corresponding second-harmonic pulses. The time dependences are given of the instantaneous frequency of the frequency-modulated second-harmonic pulse at the central wavelength of 5 μm, generated in the field of the frequency-modulated subpicosecond IR pulse, propagating in the negative uniaxial AgGaS₂ crystal along the direction of 61°36′ relative to the optical axis. These results can be used in designing a nonlinear optical phase correlator to determine the phase and time profile of the subpicosecond laser pulse in the middle IR range.     

Coherent excitation of molecular vibrational modes by high-power ultrashort infrared laser pulses

Coherent dynamics of multiphoton excitation of molecular vibrational modes by subpicosecond IR laser pulses differs greatly from that of picosecond pulse excitation. The resonance response of a molecule is primarily determined by the power broadening rather than the laser carrier frequency. Selective excitation of high vibrational levels is possible with the use of subpicosecond pulses.     

High power subpicosecond pulse generation from a synchronously pumped dye laser combined with an external compressor

A new scheme of subpicosecond pulse generation based on a dye laser is described. Output pulses of 15 ps from a rhodamine-6G laser pumped synchronously by a frequency-double mode-locked Nd: YAG laser are compressed into 0.8 ps pulses of 2 MW by two passes of saturable absorber (DODCI) and saturable amplifier (rhodamine-6G). The technique is useful for generating widely tunable, high power repetitive subpicosecond pulses.     

Color center formation in KCl and KBr single crystals with femtosecond laser pulses

Because of their extremely high instantaneous powers, femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long-term behavior of coloration. In this work, we probe the evolution of color centers generated by femtosecond laser radiation in potassium chloride and potassium bromide single crystals on time scales from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we follow the changes in coloration due to individual or multiple femtosecond pulses and the evolution of that coloration for long times after femtosecond laser radiation is terminated.     

Ultrashort pulse compression and delivery in a hollow-core photonic crystal fiber at 540 nm wavelength

We have fabricated a bandgap-guiding hollow-core photonic crystal fiber (PCF) capable of transmitting and compressing ultrashort pulses in the green spectral region around 532 nm. When propagating subpicosecond pulses through 1 m of this fiber, we have observed soliton-effect temporal compression by up to a factor of 3 to around 100 fs. This reduces the wavelength at which soliton effects have been observed in hollow-core PCF by over 200 nm. We have used the pulses delivered at the output of the fiber to machine micrometer-scale features in copper.     

Nonlinear optical generation and detection of ultrashort electrical pulses in transmission lines

The nonlinear optical generation and detection of subpicosecond electrical pulses on coplanar transmission lines is demonstrated. The electrical pulses are generated by optical rectification of ultrashort optical pulses and detected by electro-optic sampling. Both processes are the result of a second-order nonlinear optical response that occurs in the same poled polymer medium. A bipolar temporal waveform with a FWHM duration of 180 fs for the positive lobe that was measured after a propagation distance of 125 mum was observed. Pulse broadening was minimized by careful attention to the device structure.     

Experimental study of relativistic self-focusing andself-channeling of an intense laser pulse in an underdense plasma

This paper reports on experimental investigations on relativistic self-focusing and self-channeling of a terawatt laser pulse (0.7 TW⩽P⩽15 TW) in an underdense plasma. We present results obtained with picosecond (τ=1 ps) and subpicosecond (τ=0.4 ps) pulses. In the “long pulse” regime, modifications in the laser propagation are observed for Pc, the critical power for self-focusing. By contrast, self-guiding of subpicosecond pulses is observed for P≈P_c. Using a paraxial envelope model describing the laser propagation and taking into account the plasma response to the ponderomotive force, it is shown that a maximum laser intensity of 5-15 times that reached in vacuum may be achieved when P is in the (1.25-4)×P_c range. It is also demonstrated that ion motion may significantly reduce the effective P_c     

Generation of subpicosecond electrical pulses by optical rectification

We describe the generation of subpicosecond electrical pulses by optical rectification of ultrashort optical pulses. The electrical pulses are generated by the second-order nonlinear response of a LiTaO(3) crystal bonded to a coplanar transmission line. A bipolar temporal waveform with a width of 875 fs was measured after a propagation distance of 175mum . This pulse width was limited by the response time of the photoconductive sampler. We observed both broadening and amplitude reduction in the temporal waveform owing to propagation.     

Dynamics of frequency-modulated soliton-like pulses in an amplifier-modulator-compressor cascade

Specific features of the formation of frequency-modulated pulses with large peak powers in amplifier-modulator-compressor cascade schemes based on length-inhomogeneous optic fibers are considered. Optimum profiles of fiber parameters ensuring the most effective amplification and modulation of pico- and subpicosecond frequency-modulated pulses are proposed. It is established that the use of a cascade consisting of active (gain) and passive (modulating) fibers with optimized profiles of the normal group-velocity dispersion and mode area can ensure the formation of pico- and subpicosecond pulses with energies higher than 100 nJ and peak powers above 100 kW.     

Nonlinear propagation of subpicosecond ultraviolet laser pulses in air

We report filamentation of subpicosecond UV laser pulses with only millijoule energy in atmosphere. The results are in good agreement with a numerical simulation using a quasi-three-dimensional propagation code.     

Spectral amplitude and phase measurement of ultrafast pulses using all-optical differential tomography

We demonstrate a simple, all-optical, fiber-based method for characterizing the spectral amplitude and phase of ultrafast pulses using a differential tomographic measurement realized via four-wave mixing. The technique is applied to subpicosecond pulses in the C-band of the telecommunication spectrum. Characterization of amplified pulses and propagation through dispersive media is demonstrated and compared with autocorrelation measurements and calculated predictions. We show how our approach can be extended to larger bandwidths in similar systems, extending tomographic reconstruction of coherent fields to nearly an octave of bandwidth while maintaining a robust, waveguide-based geometry.     

用红外光激励电流法研究非晶半导体带隙态分布

本文提出了用红外光激励电流法研究非晶半导体带隙态的新方法。当一本征脉冲光照射样品时,大量非平衡载流子陷落到带隙态上;经过延迟时间t_d,用红外光激励残存的非平衡载流子,能引起红外光电导的过冲,过冲量与延迟时间t_d成幂次关系。基于多次陷落模型分析红外光电导过冲与温度及时间t_d的关系,可以得到带隙态分布的细节。 关键词：     

Ultrafast Faraday effect and the dynamics of the antiferromagnet-paramagnet phase transition in FeBO3

The optical pump-probe technique using ultrashort laser pulses with a photon energy of 1.55 eV was used to study the dynamics of the antiferromagnet-paramagnet phase transition in FeBO₃. The Faraday magneto-optical effect was measured with a time resolution of 100 fs, and signal transients were observed as functions of sample temperature. The rate of photoinduced phase transition was shown to be limited by the phonon-magnon relaxation rate with a characteristic time of 700 ps. The subpicosecond dynamics of Faraday rotation is not associated with the destruction of magnetic order but is caused by electron photoexcitation and recombination.     

X-ray pump optical probe cross-correlation study of GaAs

Ultrafast dynamics in atomic, molecular and condensed-matter systems are increasingly being studied using optical-pump, X-ray probe techniques where subpicosecond laser pulses excite the system and X-rays detect changes in absorption spectra and local atomic structure(1-3). New opportunities are appearing as a result of improved synchrotron capabilities and the advent of X-ray free-electron lasers(4,5). These source improvements also allow for the reverse measurement: X-ray pump followed by optical probe. We describe here how an X-ray pump beam transforms a thin GaAs specimen from a strong absorber into a nearly transparent window in less than 100 ps, for laser photon energies just above the bandgap. We find the opposite effect-X-ray induced optical opacity-for photon energies just below the bandgap. This raises interesting questions about the ultrafast many-body response of semiconductors to X-ray absorption, and provides a new approach for an X-ray/optical cross-correlator for synchrotron and X-ray free-electron laser applications.     

Ultrafast photocurrent measurement of the escape time of electrons and holes from carbon nanotube p-i-n photodiodes

Ultrafast photocurrent measurements are performed on individual carbon nanotube p-i-n photodiodes. The photocurrent response to subpicosecond pulses separated by a variable time delay Δt shows strong photocurrent suppression when two pulses overlap (Δt=0). The picosecond-scale decay time of photocurrent suppression scales inversely with the applied bias V(SD), and is twice as long for photon energy above the second subband E22 as compared to lower energy. The observed photocurrent behavior is well described by an escape time model that accounts for carrier effective mass.