Tunable optimal compression of ultrabroadband pulses by cross-phase modulation

Few-cycle attosecond pulses via periodic resonance interaction with hydrogenlike atoms

We show that it is possible to produce nearly bandwidth-limited few-cycle attosecond pulses based on periodic resonance interaction of a quasi-monochromatic radiation with the bound states of hydrogenlike atoms. A periodic resonance is provided by a far-off-resonant laser field with intensity much below the atomic ionization threshold via periodic tunnel ionization from the excited states and adiabatic Stark splitting of the excited energy levels. Without external synchronization of the spectral components, it is possible to produce 135 as pulses at 13.5 nm in Li²⁺-plasma controlled by radiation of a mode-locked Nd:YAG laser, as well as 1.25 fs pulses at 122 nm in atomic hydrogen controlled by radiation of a CO₂ laser.

Generation and amplification of tunable multicolored femtosecond laser pulses by using cascaded four-wave mixing in transparent bulk media

We have reviewed the generation and amplification of wavelength-tunable multicolored femtosecond laser pulses using cascaded four-wave mixing (CFWM) in transparent bulk media, mainly concentrating on our recent work. Theoretical analysis and calculations based on the phase-matching condition could explain well the process semi-quantitatively. The experimental studies showed: (1) as many as fifteen spectral up-shifted and two spectral down-shifted sidebands were obtained simultaneously with spectral bandwidth broader than 1.8 octaves from near ultraviolet (360 nm) to near infrared (1.2 μm); (2) the obtained sidebands were spatially separated well and had extremely high beam quality with M(2) factor better than 1.1; (3) the wavelengths of the generated multicolor sidebands could be conveniently tuned by changing the crossing angle or simply replacing with different media; (4) as short as 15-fs negatively chirped or nearly transform limited 20-fs multicolored femtosecond pulses were obtained when one of the two input beams was negatively chirped and the other was positively chirped; (5) the pulse energy of the sideband can reach a μJ level with power stability better than 1% RMS; (6) broadband two-dimensional (2-D) multicolored arrays with more than ten periodic columns and more than ten rows were generated in a sapphire plate; (7) the obtained sidebands could be simultaneously spectra broadened and power amplified in another bulk medium by using cross-phase modulation (XPM) in conjunction with four-wave optical parametric amplification (FOPA). The characterization showed that this is interesting and the CFWM sidebands generated by this novel method have good enough qualities in terms of power stability, beam quality, and temporal features suited to various experiments such as ultrafast multicolor time-resolved spectroscopy and multicolor-excitation nonlinear microscopy.

Simultaneous compression and amplification of a laser pulse in a glass plate

We demonstrated a novel method of simultaneous compression and amplification of a weak laser pulse in a glass plate using cross-phase modulation in conjunction with four-wave optical parametric amplification that was pumped by an intense femtosecond pulse. A proof-of-principle experiment succeeded in smooth broadening of the weak pulse spectrum by a factor of about three and simultaneously amplifying the pulse energy by more than three times. By using chirped mirrors to compensate the dispersion, the weak pulse was compressed from 22.6 fs to 12.6 fs. Furthermore, the output spectrum of seed pulse could be tuned by varying the delay of the intense pump pulse with respect to the weak seed pulse. This method can also be used for simultaneous spectral broadening of several weak beams with different wavelength at the same time.

Observation of relativistic cross-phase modulation in high-intensity laser-plasma interactions

A nonlinear optical phenomenon, relativistic cross-phase modulation, is reported. A relativistically intense light beam (I = 1.3 x 10(18) W cm(-2), lambda = 1.05 microm) is experimentally observed to cause phase modulation of a lower intensity, copropagating light beam in a plasma. The latter beam is generated when the former undergoes the stimulated Raman forward scattering instability. The bandwidth of the Raman satellite is found to be broadened from 3.8-100 nm when the pump laser power is increased from 0.45-2.4 TW. A signature of relativistic cross-phase modulation, namely, asymmetric spectral broadening of the Raman signal, is observed at a pump power of 2.4 TW. The experimental cross-phase modulated spectra compared well with theoretical calculations. Applications to generation of high-power single-cycle pulses are also discussed.

Analytical solutions for the electromagnetic fields of tightly focused laser beams of arbitrary pulse length

The analytical solution for a monochromatic focused laser beam was recently published [Opt. Lett.31, 1447 (2006)]. The effect of introducing bandwidth by including a finite-length temporal pulse envelope is included exactly. This is done formally first in the frequency domain for an arbitrary pulse shape, and the specific case of a cosine-squared envelope is then solved analytically for all pulse lengths, thereby decreasing the computation time by 2 orders of magnitude. The inclusion of longer wavelengths reduces the fraction of laser energy in the focus from 86.5% to 83.5% for a 5 fs Ti:sapphire laser and 72.7% in a single-cycle pulse.

Imaging molecular structures by electron diffraction using an intense few-cycle pulse

As an intense few-cycle pulse interacts with an atomic or molecular target, its strong oscillating field may first pull electrons out of the target and subsequently drive them back to scatter on the target. The scattering may occur only a few times or even once during the interaction. This unique property of few-cycle pulses enables one to image ultrafast transient structures of matter by the means of pulse-driven electron diffraction. We demonstrated this phenomenon with K(+)(2) via three-dimensional calculations of the time-dependent Schro dinger equation.