Femtosecond nonlinear fiber optics in the ionization regime.

By using a gas-filled kagome-style photonic crystal fiber, nonlinear fiber optics is studied in the regime of optically induced ionization. The fiber offers low anomalous dispersion over a broad bandwidth and low loss. Sequences of blueshifted pulses are emitted when 65 fs, few-microjoule pulses, corresponding to high-order solitons, are launched into the fiber and undergo self-compression. The experimental results are confirmed by numerical simulations which suggest that free-electron densities of ∼10(17) cm(-3) are achieved at peak intensities of 10(14) W/cm(2) over length scales of several centimeters.

Influence of ionization on ultrafast gas-based nonlinear fiber optics.

We numerically investigate the effect of ionization on ultrashort high-energy pulses propagating in gas-filled kagomé-lattice hollow-core photonic crystal fibers by solving an established uni-directional field equation. We consider the dynamics of two distinct regimes: ionization induced blue-shift and resonant dispersive wave emission in the deep-UV. We illustrate how the system evolves between these regimes and the changing influence of ionization. Finally, we consider the effect of higher ionization stages.

Bright spatially coherent wavelength-tunable deep-UV laser source using an Ar-filled photonic crystal fiber.

We report on the spectral broadening of ~1 µJ 30 fs pulses propagating in an Ar-filled hollow-core photonic crystal fiber. In contrast with supercontinuum generation in a solid-core photonic crystal fiber, the absence of Raman and unique pressure-controlled dispersion results in efficient emission of dispersive waves in the deep-UV region. The UV light emerges in the single-lobed fundamental mode and is tunable from 200 to 320 nm by varying the pulse energy and gas pressure. The setup is extremely simple, involving <1 m of a gas-filled photonic crystal fiber, and the UV signal is stable and bright, with experimental IR to deep-UV conversion efficiencies as high as 8%. The source is of immediate interest in applications demanding high spatial coherence, such as laser lithography or confocal microscopy.

Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber.

We report tunable third-harmonic generation (THG) in an Ar-filled hollow-core photonic crystal fiber, pumped by broadband <2 microJ, 30 fs pulses from an amplified Ti:sapphire laser system. The overall dispersion is precisely controlled by balancing the negative dielectric susceptibility of the waveguide against the positive susceptibility of the gas. We demonstrate THG to a higher-order guided mode and show that the phase-matched UV wavelength is tunable by adjusting the gas pressure.

Direct observation of self-similarity in evolution of transient stimulated Raman scattering in gas-filled photonic crystal fibers.

A unique characteristic of transient stimulated Raman scattering, in which the spatiotemporal evolution of the fields and the molecular excitation follow a universal self-similarity law, is observed in gas-filled photonic crystal fibers. As the input laser power is increased, the coupled system "optical fields + molecular excitation" goes through the same phases of time evolution but at a higher rate. Using the self-similarity law we are able to completely reconstruct the evolution of the pump and Stokes fields from one measurement.

Solitary pulse generation by backward Raman scattering in H2-filled photonic crystal fibers.

Using a hydrogen-filled hollow-core photonic crystal fiber as a nonlinear optical gas cell, we study amplification of ns-laser pulses by backward rotational Raman scattering. We find that the amplification process has two characteristic stages. Initially, the pulse energy grows and its duration shortens due to gain saturation at the trailing edge of the pulse. This phase is followed by formation of a symmetric pulse with a duration significantly shorter than the phase relaxation time of the Raman transition. Stabilization of the Stokes pulse profile to a solitonlike hyperbolic secant shape occurs as a result of nonlinear amplification at its front edge and nonlinear absorption at its trailing edge (caused by energy conversion back to the pump field), leading to a reshaped pulse envelope that travels at superluminal velocity.

Quasi-phase-matched high harmonic generation in hollow core photonic crystal fibers.

The potential of hollow core photonic crystal fiber as a nonlinear gas cell for efficient high harmonic generation is discussed. The feasibility of phase-matching this process by modulating the phase of ionization electrons using a counter-propagating laser field is shown. In this way, harmonics with energies of several hundreds of eV can be produced using fs-laser pump pulses of microJ energy.

High-order Raman scattering of X rays by optical phonons and generation of ultrafast X-ray transients.

We study the modulation of x-ray diffraction in ideal crystals by a copropagating wave of optical vibrations generated by a fs-laser pulse. Our results suggest that in the synchronous interaction regime the output diffracted x-ray field has the form of ultrafast transients with a time structure even shorter than the period of the excited vibrational mode. The behavior is explained in terms of high-order Raman scattering of x rays by optical phonons.

Electromagnetically induced quantum memory.

We discuss the problem of creating coherence in an optically driven quantum system in conditions where decoherence is caused by the laser field itself, due to coupling of the system to a rapidly decaying state or continuum. It is shown that by applying an additional laser field between this state and a bound state the relaxation channel can be suppressed as a result of a "dark state" formation, giving rise to long living Rabi oscillations in the system. It is found that the same mechanism of preserving coherence exists in systems with level splitting or degeneracy, where the driving field interacts with multiple resonant sublevels simultaneously. We also show that specific coherent propagation phenomena assisted by the interference suppression of decoherence can be observed under these conditions.

Observation of selectivity of coherent population transfer induced by optical interference.

A fs time-resolved selective control of multilevel systems using superposition of two identical, frequency-chirped fields is proposed and demonstrated. By adjusting the delay between the pulses, a selected transition of the Rb doublet was brought into the "holes" of the interference pattern and remained nonexcited, thus allowing to manipulate another transition by the laser field as if it were an isolated two-level system. Based on light interference, this technique needs neither strong driving field intensities nor controlling the chirp direction to achieve the selectivity.

Synthesis of periodic femtosecond pulse trains in the ultraviolet by phase-locked Raman sideband generation.

We demonstrate a new technique for femtosecond-pulse generation that employs ultrafast modulation of a laser field phase by impulsively excited molecular rotational or vibrational motion with subsequent temporal compression. An ultrashort pump pulse at 800 nm performs impulsive excitation of a molecular gas in a hollow waveguide, and a weak delayed probe pulse at 400 nm is scattered on the temporal oscillations of its dielectric index. The resultant sinusoidal phase modulation of the probe pulse permits probe pulse temporal compression by use of both positively and negatively dispersive elements. The potential of this new method is demonstrated by the generation of a periodic train of 5.8-fs pulses at 400 nm with positive group-delay dispersion compensation.

fs-pulse synthesis using phase modulation by impulsively excited molecular vibrations

We report the temporal characteristics of laser radiation transmitted through impulsively excited SF6 and exhibiting sideband Raman lines. Even without special dispersion control we observed a sequence of compressed fs pulses following with the period of the excited A(1g) vibrational mode of SF6. The use of both negative and positive group velocity dispersion compensation for the temporal compression was found to be as appropriately efficient. The results prove our new concept of the ultrafast molecular phase modulator.

Generation of broadly tunable femtosecond vacuum-ultraviolet pulses.

Tunable femtosecond vacuum-ultraviolet radiation in the range omega(D) = 102-124 nm has been generated by twophoton-resonant and near-resonant four-wave difference-frequency mixing (omega(D) = 2omega(p) - omega(I)) in krypton and argon by use of intense 250-fs ArF laser pulses (omega(p)) and tunable femtosecond signal and idler pulses (omega(I)) generated by an optical parametric generator.

Soliton self-frequency shift for pulses with a duration less than the period of molecular oscillations.

The effect of stimulated Raman scattering on soliton propagation in fibers is studied for pulse durations comparable with or shorter than the molecular oscillation period. The self-frequency shift of a soliton in this interaction regime is calculated to be inversely proportional to the soliton duration and does not depend on both the characteristic frequency of the molecular oscillations and the Raman gain width of the medium.

Coherent amplification of ultrashort solitons in doped fibers.

We present the results of computer simulation of coherent amplification of solitons in doped fibers and explain the first stage of an evolution of such solitons by means of a perturbation theory applied to the inverse scattering problem in two limiting cases, noncoherent and pure coherent amplification. We show that it is possible to get adiabatic amplification of solitons. In the case of noncoherent amplification the amplitude of the solitons increases exponentially and the pulse duration decreases exponentially. In the opposite case of pure coherent amplification the amplitude of the solitons increases linearly with distance z, whereas the pulse duration displays a z(-1) dependence.