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Four-wave mixing with cross-polarized pump and cross-polarized sidebands is identified as a powerful resource of quantum entanglement, enabling creation of efficient fiber-optic sources of entangled photon pairs. Generation of cross-polarized photon pairs is demonstrated by coupling the laser pump into orthogonal polarization modes of a highly birefringent, highly nonlinear, anomalously dispersive photonic-crystal fiber. With the pump wavelength set to lie deeply within the anomalous dispersion region of the fiber, the frequency offset of the photon pairs relative to the pump exceeds 45 THz, enhancing the contrast of photon pairs against the Raman noise to yield photon pairs with a coincidence-to-accidental ratio as high as 250: 1.

Four-wave mixing (FWM) of femtosecond near-infrared laser pulses and its second harmonic in the filamentation regime is shown to give rise to ultrashort field waveforms in the mid-infrared with pulse widths as short as a half of the field cycle and produce ultrabroadband supercontinuum spectra stretching from the mid-IR to the terahertz region. Generation of 7-fs pulses centered at 4.35 μm is demonstrated by a two-color filamentation experiment, where the 25-fs, 800-nm fundamental-wavelength output of a Ti: Sapphire laser is mixed with its second harmonic. The spectral and temporal properties of the mid-IR waveforms, as well as their emission pattern, are consistent with the FWM scenario of frequency conversion generalized to include the Kerr effect and ionization-induced refractive-index modulation.

Nonlinear-optical interactions, such as second-harmonic and sum-frequency generation and coherent anti-Stokes Raman spectroscopy (CARS), are investigated in porous GaP for the first time by means of a novel laser source based on mode-locked picosecond Nd3+:YVO4 laser and subsequent continuum generation in an optical fiber. The efficiency of the former two nonlinear optical processes is shown to be strongly dependent on the wavelengths of the interacting waves and tends to increase with the decrease of the excitation wavelength. The power of the generated second-harmonic and sum-frequency increases by a factor of 2 and 30, respectively, compared to the crystalline GaP. In contrast, the CARS signal in porous GaP is found to be less efficient than one in crystalline GaP. The observed results are explained in terms of competition of the phase-matching effects in GaP nanocrystals and the enhanced photon lifetime in scattering porous GaP layers.

Arrays of submicron air holes are shown to modify the properties of guided modes in optical fibers, enabling a fine tuning of fiber dispersion, nonlinearity, and gain. We demonstrate fiber dispersion nanomanagement solutions that provide ultra-flattened group-velocity dispersion profiles and control the fiber nonlinearity and gain.

The available fiber lasers, photonic-crystal fiber (PCF) frequency converters, and hollow-core PCFs are shown to offer realistic practical solutions for an all-fiber format of coherent anti-Stokes Raman scattering (CARS) in the gas phase. In view of the substantial enhancement of CARS in the guided modes of hollow-core PCFs, a broad class of compact ytterbium and erbium fiber laser sources become suitable for the CARS analysis of gas media. The quantum limit of the signal-to-noise ratio, S/N, in all-fiber CARS is shown to scale as λα-1a-2 with radiation wavelength λ, fiber loss α, and the fiber core radius,aleading, in the case of a hollow-PCF gas cell, to a dramatic improvement of S/N relative to CARS in the regime of tight focusing.

Two-color time-ordered dyads of short laser pulses induce a spatially periodic modulation of the refractive index of the atmosphere through resonance-enhanced multiphoton ionization (REMPI), enhancing the back reflection of radio waves. The carrier frequencies and the timing of laser pulses in a dyad provide a unique code for selectively accessing the manifold of energy levels of impurity molecules, inducing REMPI gratings of the refractive index only in the presence of impurity molecules, thus suggesting an attractive strategy for the radar-based stand-off detection of trace impurities in the atmosphere.

Raman soliton phenomena in photonic crystal fibers are shown to allow efficient tunable frequency shifting of sub-10-fs laser pulses. Soliton self-frequency shift in a photonic-crystal fiber with a core diameter less than 2 μm is used to transform the spectrum of a 6-fs 2-nJ Ti: sapphire-laser pulse, dominated by a 670-nm peak, into a spectrum featuring a well-resolved intense spectral component centered at 1064 nm, which is ideally suited as a seed for Nd: YAG- and ytterbium-based laser devices.

We report an order of magnitude enhancement of second-harmonic generation (SHG) from porous gallium phosphide relative to SHG in crystalline gallium phosphide. Optical heterodyning measurements of photon free-path length reveal a correlation between SHG enhancement and disorder of the porous material.

We study pulse propagation across a boundary that separates an ordinary medium from a medium with simultaneously negative dielectric permittivity and magnetic permeability. Solving Maxwell’s equations with two spatial coordinates (one longitudinal, one transverse) and time we find negative refraction as the wave packet undergoes significant and unusual shape distortions. The pulse acquires and maintains a chirp as it traverses the interface, as expected, but with a sign that is opposite to the chirp attained upon traversal into a positive-index material. Both a direct calculation of the spatial derivative of the instantaneous, local phase of the pulse and a Fourier analysis of the signal reveal the same inescapable fact: that inside a negative-index material, a transmitted, forward-moving wave packet is indeed a superposition of purely negative wave vectors. The central findings of this paper are a confirmation that causality is not violated in the short-pulse regime, and that energy and group velocities never exceed the speed of light in vacuum.

We study soliton phenomena accompanying the propagation of femtosecond Cr: forsterite-laser pulses through a microstructure fiber in the regime of efficient anti-Stokes frequency conversion. The dispersion of the fiber is designed in such a way as to minimize the group delay of the 1.25-μm pump and the Stokes pulse within the length of soliton pulse compression in the regime of anomalous dispersion. Spectrally and temporally isolated solitonic features, resulting from soliton self-frequency shift, are detected at the output of such a microstructure fiber by means of cross-correlation frequency-resolved optical gating.

Broadband and supercontinuum pulses with a linear chirp define a linear transform, mapping the difference between the instantaneous frequencies of pump pulses onto the delay time between these pulses. This delay between the pump pulses can be then mapped onto the spatial coordinate with the use of a broad-beam wave-mixing or pump–probe geometry. The new possibilities offered by these mappings for four-wave-mixing techniques are discussed. The spectral and temporal resolution of chirped-pulse wave-mixing and pump–probe techniques are examined. Single-shot multidimensional wave-mixing techniques using broadband and supercontinuum chirped pulses are discussed.

A new design of supercontinuum-generating microstructure fibers is demonstrated, with supercontinuum emission produced in six submicron-diameter cores surrounding a larger central core with a diameter of 2 to 5 μm. Such a multiple-core microstructure-fiber design not only allows the total energy of supercontinuum emission to be increased, but also offers a practical way of fabricating microstructure-integrated bundles of small-core high-index-step fibers with very large lengths. Submicron-core fused-silica microstructure fibers are shown to provide maximum ratios of the fiber-core-confined laser power to the fiber-core diameter, allowing limiting waveguide enhancement factors to be approached for a broad class of nonlinear-optical processes contributing to supercontinuum generation.

Unamplified subnanojoule 30-fs pulses of Cr:forsterite laser radiation are shown to generate the third-harmonic signal in a tapered fiber. Analysis of the dispersion properties of a tapered fiber indicates the possibility of phase matching third-harmonic generation in higher-order waveguide modes. When the pump and third-harmonic pulses are group-velocity-mismatched, optimal phase matching is achieved for frequency components spectrally offset from the central frequency of the pump pulse, which gives rise to an asymmetry in the spectrum of the third harmonic.

The influence of the structure of holey-fiber cladding on the spectral broadening of femtosecond laser pulses is experimentally studied. These experiments demonstrate that the spectral broadening of 70-fs pulses of 800-nm Ti:sapphire laser radiation transmitted through 2- and 3-μm-pitch holey fibers can be enhanced by a factor of about 1.5 by increasing the air-filling fraction of the fiber cladding from 16 up to 65%.

Third-harmonic, difference-frequency, and sum-frequency generation processes in hollow fibers are experimentally studied with 30-ps pulses having an energy of several millijoules. The experimental dependence of the difference-frequency signal on the pressure of the gas filling the fiber agrees well with the results of calculations when the contribution of higher order waveguide modes is taken into consideration, thus indicating the importance of nonlinear-optical processes involving higher order waveguide modes of a hollow fiber. Hollow fibers are also shown to expand the possibilities of nonlinear-optical analysis of gases by allowing the generation of third-harmonic and sum-frequency signals, which vanish in the regime of tight focusing in a medium with normal dispersion.

Summary form only given. Much attention has been recently focused on the investigation of photonic band gap (PBG) structures. In particular, it was demonstrated that unusual dispersion properties of such structures allow compact tunable optical delay lines to be implemented, parameters of short laser pulses to be controlled, and nonlinear-optical interactions to be phase-matched. We employ the finite difference time-domain (FDTD) technique to analyze the propagation of a light beam in a two-dimensional (2D) PBG structure with a defect. We performed FDTD simulations for macroporous-silicon 2D PBG structures consisting of five to ten periods of cylindrical air holes arranged in a triangular lattice in silicon irradiated with a plane light wave.

A blue-shifted output of a photonic-crystal fiber, providing a frequency upconversion of femtosecond Ti:sapphire laser pulses, is used to seed a double-pass optical parametric oscillator (OPA). The OPA is based on a BBO crystal, pumped by 65-mW 150-fs second-harmonic pulses of a Ti:sapphire laser. Gain factors in excess of 103 are demonstrated for such an OPA, yielding tunable light pulses within the range of wavelengths from 420 to 650 nm, a peak power up to 250 kW, and a typical pulse width of about 200 fs at a repetition rate of 100 kHz.

Copropagating fundamental-wavelength and second-harmonic femtosecond pulses of Cr: forsterite laser radiation are used to study cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber. Parametric instability of the second-harmonic probe pulse induced through cross-phase modulation by the fundamental-wavelength pump pulse gives rise to distinct sidebands in the spectrum of the probe field transmitted through the fiber. The wavelength of these sidebands was tuned in our experiments within approximately 100 nm by varying the peak power and the delay time of the pump pulse, suggesting a convenient way of controlled parametric spectral transformation of ultrashort laser pulses.