Dynamics of antiferromagnets exposed to ultrashort magnetic field pulses

An ultrashort magnetic field pulse generated by a laser pulse can excite spin oscillations in a “pure” antiferromagnet even when a weak ferromagnetic moment is absent. This mechanism features the spin oscillation amplitude independent of the sublattice structure and parameters of the ferromagnet and is governed only by the field pulse parameters.     

Mode-locked lasers and ultrashort light pulses

This article reviews some aspects of the generation of very short optical pulses using mode-locked lasers, with the emphasis laid on pulsed laser systems. Active and passive mode-locking is discussed, and the problem of measuring ultrashort light pulses is considered. A survey of some commonly used sources and techniques for the generation of ultrashort light pulses follows. The paper concludes with a short account of limitations in the generation and propagation of these pulses.     

Polarization spectroscopy with ultrashort light pulses

We have demonstrated the feasibility of Doppler-free polarization spectroscopy with a train of ultrashort pulses from a synchronously pumped mode-locked dye laser. The sensitivity of the method is compared with saturation spectroscopy with a single-mode laser. Work supported by U.S. Office of Naval Research under contract N00014-78-0403, Lindemann Fellow 1977–78.     

Interaction of ultrashort light pulses with carbon nanotubes

The possibility of existence of electromagnetic solitons in carbon nanotubes is investigated on the basis of the coupled equations for the classical distribution function of electrons in such nanotubes and the Maxwell equations for electromagnetic fields. Solitons arise due to the matched change in the classical distribution function and the electric field formed by nonequilibrium electrons of a carbon nanotube. The data of numerical calculations, indicative of the existence of solitons, are reported.     

Enhanced soliton self-frequency shift of ultrashort light pulses

Photonic-crystal fibers are used to study scenarios of soliton self-frequency shift for laser pulses with initial pulse lengths much less than the Raman-mode period of the fiber material. A typical frequency shift of subnanojoule Ti: sapphire-laser pulses with an initial duration of about 30 fs transmitted through a fiber with a core diameter of about 1.6 μm and a length of about 7 cm exceeds 100 THz. The rate of soliton self-frequency shift is radically increased by reducing the initial pulse width.     

Ripples and grain formation in GaAs surfaces exposed to ultrashort laser pulses

The damage morphology of GaAs1 0 0 single crystal following femtosecond laser (wavelength 806 nm, pulse duration 110 fs, prf 10 Hz) excitation was studied as a function of laser fluence and number of pulses. The threshold value for damage to occur in a GaAs surface in the present experiment was 1.3×10¹⁴ W/cm² for a single pulse. The cooling rate for threshold fluence was calculated as 2.22×10¹⁴ °C/s. The damage occurred in the form of surface removal. Ripples and grains were formed in the removed surface. At higher fluences micron depth pits were also formed. The damage morphology was explained with the help of Boson-condensation hypothesis.     

Characterization of the spectral phase of ultrashort light pulses

The complete characterization of a short light pulse requires the measurement of the value of the electric field as a function of time, or conversely, as a function of the optical frequency. Many different techniques have been demonstrated for this purpose. They fall in two main categories, whether a known reference pulse whose spectrum encompasses that of the unknown pulse is available or not. In the first case, linear techniques such as time-domain or frequency-domain interferometry can be directly used, with the advantage of a high sensitivity. In the latter case, non-stationary filters can be implemented using optical non-linearities, in techniques such as Frequency Resolved Optical Gating (FROG) or Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER).     

Measurement of relaxation times by phase-modulated ultrashort light pulses

In the measurement of relaxation times of nonlinear response of media by means of ultrashort light pulses the resolution time is determined by the pulse duration. Here we show that using phase-modulated pulses the resolution can be as high as the coherence time $\text{τ}{}_{\mathrm{c}}\text{≈}\text{1}\text{Δν}$ (Δν is the width of pulse spectrum in Hz) which is less than the pulse duration.     

Self-organized nanogratings in glass irradiated by ultrashort light pulses

Periodic nanostructures are observed inside silica glass after irradiation by a focused beam of a femtosecond Ti:sapphire laser. Backscattering electron images of the irradiated spot reveal a periodic structure of stripelike regions of approximately 20 nm width with a low oxygen concentration, which are aligned perpendicular to the laser polarization direction. These are the smallest embedded structures ever created by light. The period of self-organized grating structures can be controlled from approximately 140 to 320 nm by the pulse energy and the number of irradiated pulses. The phenomenon is interpreted in terms of interference between the incident light field and the electric field of the bulk electron plasma wave, resulting in the periodic modulation of electron plasma concentration and the structural changes in glass.     

Filamentation of ultrashort light pulses in a liquid scattering medium

We have studied filamentation of 1-ps laser pulses in a scattering medium (aqueous suspension of 2-μm polystyrene microspheres) and compared filamentation dynamics to that in pure water. Our results indicate that light scattering does not alter filamentation dynamics in general, but rather results in farther position of the nonlinear focus, shorter filament length, and the development of speckle structure in the peripheral part of the beam. The experimental observations are qualitatively reproduced by the numerical model which accounts for diffraction, self-focusing, multiphoton absorption, and light scattering introduced through a stochastic diffusion and diffraction term.     

Raman amplification of ultrashort light pulses in inhomogeneously broadened three-level systems

In the limit of pulse durations that are shorter than the characteristic times of atomic decays, the dynamics of the propagation of a probe light pulse under the action of a driving pulse in the form of a self-induced transparency soliton applied to the adjacent transition of the V-scheme is considered. For a three-level absorber, it is shown that, in the absence of two-photon inhomogeneous broadening, the effect of soliton-induced transparency (SOIT) for the probe field takes place, which consists of the probe pulse undergoing forced localization by the driving pulse and propagates further without a change in the energy or shape. This type of transparency has no threshold with respect to both the energy and the area and is observed for arbitrarily weak pulses. The SOIT effect disappears in the presence of a two-photon inhomogeneous broad-ening. The effect of Raman amplification in a three-level amplifier with the Raman population inversion is considered. It is demonstrated that, under the action of the two-photon inhomogeneous broadening, the effect of Raman amplification becomes a threshold effect. The broader the two-photon inhomogeneous contour, the larger the Raman inversion required to initiate generation.     

Sliding pump method for generation and amplification of ultrashort light pulses

A method for the generation of an ultrashort Raman pulse train with peak power considerably in excess of the pumping one is proposed and theoretically developed. It is shown that it can be used for obtaining tunable picosecond pulses in Raman fiber laser.     

Cross-phase-modulation-induced instability and efficient parametric frequency conversion of ultrashort light pulses

We experimentally demonstrate efficient controlled parametric frequency conversion of femtosecond laser pulses through cross-phase-modulation-induced instability in the presence of a copropagating pump field in a fused silica photonic-crystal fiber. The amplitudes and the frequency shifts of sidebands generated in the spectrum of the probe field at the output of the fiber as a result of parametric four-wave mixing are controlled by varying the intensity of the pump field.     

A diffraction grating autocorrelator for measurement of single ultrashort light pulses

A simple autocorrelation scheme for measurement of single ultrashort light pulses in the 1–500 ps range is proposed. Only a diffraction grating and a mirror are utilized to obtain a variable time delay between two collinear beams. The advantages of the proposed method and the effect of the grating dispersion on the autocorrelator performance are discussed.     

Investigation of the spectral kinetics of ultrashort light pulses using time holography

On the basis of principles of time holography we suggest a method for recording changes of a spectrum in ultrashort light pulses with a time resolution of up to 10⁻¹⁴ sec. As a clear way to describe the spectrum kinetics, a function of the spectral shift is introduced. We consider, as an example, the possibilities of investigating the spectral-temporal structure of pico- and femtosecond pulses and their transformations on interaction with a dispersion medium. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 25–29, January–February, 1999.     

Direct linear measurement of ultrashort light pulses with a picosecond streak camera

The durations of ultra-short (4–7 psec) laser pulses have been directly measured with an image tube streak camera designed for time resolution in the picosecond region. The camera was calibrated with simultaneously recorded two photon fluorescence pulse profiles and it is shown that a time resolution limit of ≈1 psec should be possible with this system.     

Resonance excitation of excitons in a semiconductor by ultrashort frequency-tunable light pulses

An investigation was made of the peculiarities of propagation of frequencytunable powerful ultrashort light pulses (USLP) in a sample of CdS_0,4Se_0,6 (77K) under resonance excitation of excitons. The peculiarities of resonance interaction of USLP with the semiconductor may be fairly well explained by the dominant effect of self-induced transparency on excitons.     

Depolarization of white light generated by ultrashort laser pulses in optical media

We report measurements of the extinction ratio (ER) of white light generated upon irradiation of BK7 glass by ultrashort (36 fs) laser pulses with incident power approximately 10(3) times larger than the critical power for self-focusing. At low incident powers, the continuum is symmetric about the incident laser wavelength; at high powers it becomes broader and distinctly asymmetric towards the blue side. We observe that ER degrades by 100-fold after the onset of multiphoton-induced free-electron generation (at incident intensity approximately 2 TW cm-2), which also corresponds to the onset of asymmetry in white-light spectra.