A. M. Zheltikov

Efficient anti-Stokes generation through phase-matched four-wave mixing in higher-order modes of a microstructure fiber

Phase-matched four-wave mixing in higher-order modes of microstructure fibers allows unprecedentedly high efficiencies of anti-Stokes frequency conversion to be achieved for subnanojoule femtosecond laser pulses. 70-fs pulses of 790-nm radiation were used to generate an anti-Stokes component at 520-530 nm in a higher-order mode of a microstructure fiber with a 4.8-microm core. The maximum ratio of the anti-Stokes signal energy to the energy of the pump component in the output spectrum is estimated as 1.7.

Coherent anti‐Stokes Raman scattering: from proof‐of‐the‐principle experiments to femtosecond CARS and higher order wave‐mixing generalizations

A review of CARS evolution from proof-of-the principle experiments to recent CARS developments, including femtosecond CARS and higher order wave-mixing spectroscopy generalizations, is given, with the main emphasis placed on the discussion of physically new ideas and promising directions of CARS studies. Copyright

Enhanced four-wave mixing in a hollow-core photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to substantially enhance four-wave mixing (FWM) of laser pulses in a gas filling the fiber core. Picosecond pulses of Nd:YAG fundamental radiation and its second harmonic are used to generate a signal at the frequency of the third harmonic by the FWM process 3omega = 2omega + 2omega - omega. The efficiency achieved for this process in a 9-cm-long, 13-microm-hollow-core-diameter photonic-crystal fiber, designed to simultaneously transmit a two-color pump and the FWM signal, is shown to be approximately 800 times higher than the maximum FWM efficiency attainable with the same laser pulses in the tight-focusing regime.

Compression of ultrashort light pulses in photonic crystals: when envelopes cease to be slow

A modification of the transfer-matrix method and the nonlinear finite-difference time-domain technique are applied to simulate the propagation of ultrashort laser pulses in linear and nonlinear one-dimensional photonic band gap (PBG) structures. Dispersion properties of photonic crystals allow the chirp of a short light pulse to be compensated in a controlled fashion. It is demonstrated that photonic crystals with embedded optical nonlinearity provide an opportunity to efficiently compress laser pulses to a duration of several optical cycles on a submillimeter spatial scale, providing new opportunities for the miniaturization of femtosecond solid-state laser systems. Physical factors limiting the minimum duration of compressed pulses are studied for such structures and the ways to optimize pulse compression are discussed. Examples of PBG pulse compressors that can be fabricated by means of currently existing technologies are considered.

Laser breakdown with millijoule trains of picosecond pulses transmitted through a hollow-core photonic-crystal fibre

Sequences of picosecond pulses with a total energy in the pulse train of about 1 mJ are transmitted through a hollow-core photonic-crystal fibre with a core diameter of approximately 14 µm. The fluence of laser radiation coupled into the core of the fibre under these conditions exceeds the breakdown threshold of fused silica by nearly an order of magnitude. The laser beam coming out of the fibre is then focused to produce a breakdown on a solid surface. Parameters of laser radiation were chosen in such a way as to avoid effects related to the excitation of higher order waveguide modes and ionization of the gas filling the fibre in order to provide the possibility to focus the output beam into a spot with a minimum diameter, thus ensuring the maximum spatial resolution and the maximum power density in the focal spot.

Saturation of third-harmonic generation in a plasma of self-induced optical breakdown due to the self-action of 80-fs light pulses

Abstract Generation of the second and third harmonics of 80-fs 1-mJ pulses from a Ti:sapphire laser in the plasma of self-induced optical breakdown in atmospheric air is studied. Spatial self-action of fundamental radiation accompanied by the broadening of spectra of fundamental radiation and optical harmonics is revealed. The self-action of light leads to the saturation of the efficiency of third-harmonic generation as a function of the energy of fundamental radiation. An efficiency of third-harmonic generation up to 1.7 × 10 −3 is achieved with 1-kHz laser pulses.

The physical limit for the waveguide enhancement of nonlinear-optical processes

The waveguide enhancement of nonlinear-optical processes is shown to be physically limited because of the competition of diffraction and refractive-index-step radiation confinement. In the case of the limiting refractive-index step values for fused silica fibers, the maximum waveguide enhancement of nonlinear-optical processes is achieved with submicron fiber core diameters. Microstructure fibers with high air-filling fractions of the cladding are shown to enhance nonlinear-optical processes with waveguide enhancement factors close to the physical limit. Approximate asymptotic expressions derived for several important particular cases provide an adequate qualitative description of the waveguide enhancement of nonlinear-optical processes and allow the optimal fiber core diameters for the maximum enhancement of nonlinear-optical interactions to be estimated with a satisfactory accuracy.

Third-harmonic generation in a laser-pre-excited gas: the role of excited-state neutrals

The generation of the third harmonic of 150-fs 4-mJ Ti:sapphire laser pulses in the atmospheric air preliminary excited by 15-ns 100-mJ pulses of the second harmonic of a Nd:YAG laser is investigated. Preliminary laser excitation of a gas is shown to considerably increase the efficiency of frequency tripling of femtosecond pulses. Characteristic times correspond- ing to the maximum intensity of third-harmonic generation are shown to substantially exceed the characteristic plasma decay times, indicating the important role of excited-state neutrals in the enhancement of THG efficiency. q 2000 Elsevier Science

Highly efficient frequency tripling of laser radiation in a low-temperature laser-produced gaseous plasma

Relatively efficient (up to 3%) third-harmonic generation of picosecond Nd:YAG laser pulses for wavelength λ = 1.06 μm in a low-density laser plasma is reported for a coherent (i.e., collinear) geometry. The third-harmonic beam produced is nearly diffraction limited and coherent and can be used in other nonlinear-optical experiments. A simple theoretical model is proposed in order to explain this result. In the model, third-harmonic generation is considered to be a process that arises from the scattering of electrons by ions in the presence of the strong laser field.

Waveguide modes of electromagnetic radiation in hollow-core microstructure and photonic-crystal fibers

The properties of waveguide modes in hollow-core microstructure fibers with two-dimensionally periodic and aperiodic claddings are studied. Hollow fibers with a two-dimensionally periodic cladding support air-guided modes of electromagnetic radiation due to the high reflectivity of the cladding within photonic band gaps. Transmission spectra measured for such modes display isolated maxima, visualizing photonic band gaps of the cladding. The spectrum of modes guided by the fibers of this type can be tuned by changing cladding parameters. The possibility of designing hollow photonic-crystal fibers providing maximum transmission for radiation with a desirable wavelength is demonstrated. Fibers designed to transmit 532-, 633-, and 800-nm radiation have been fabricated and tested. The effect of cladding aperiodicity on the properties of modes guided in the hollow core of a microstructure fiber is examined. Hollow fibers with disordered photonic-crystal claddings are shown to guide localized modes of electromagnetic radiation. Hollow-core photonic-crystal fibers created and investigated in this paper offer new solutions for the transmission of ultrashort pulses of high-power laser radiation, improving the efficiency of nonlinear-optical processes, and fiber-optic delivery of high-fluence laser pulses in technological laser systems.