A. N. Naumov

Enhanced χ (3) interactions of unamplified femtosecond Cr:forsterite laser pulses in photonic-crystal fibers

Enhancement of nonlinear optical interactions in the core of a photonic-crystal fiber allows several χ(3) processes to be simultaneously observed in the field of unamplified 30-fs pulses of a Cr:forsterite laser. Subnanojoule fundamental-radiation pulses of this laser experience spectral broadening arising from self-phase modulation and generate the third harmonic at 410–420 nm. Third-harmonic pulses also appear spectrally broadened at the output of the fiber as a result of the cross-phase-modulation effect. This catalog of enhanced χ(3) processes observed in photonic-crystal fibers opens the way for using such fibers for frequency conversion of low-energy femtosecond pulses with simultaneous chirp control and subsequent pulse compression.

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

Frequency-tunable supercontinuum generation in photonic-crystal fibers by femtosecond pulses of an optical parametric amplifier

Supercontinuum emission is generated by the propagation of frequency-tunable femtosecond pulses of 1.1–1.5-µm radiation of an optical parametric amplifier through a photonic-crystal fiber. Nearly an octave’s spectral broadening was observed when laser pulses with a duration of 80–100 fs and an energy of several nanojoules per pulse were coupled into a photonic-crystal fiber with a core radius of 1.5–3 µm. The spectral broadening of femtosecond pulses at 1.1–1.5 µm is shown to be much more efficient than the spectral broadening of femtosecond pulses of 800-nm Ti:sapphire laser radiation. The role of dispersion in spectral broadening and supercontinuum generation is discussed. In experiments on supercontinuum generation with an optical parametric amplifier, the influence of dispersion effects was reduced by decreasing the size of the fiber core, which allowed the efficiency of supercontinuum generation to be improved without increasing the laser intensity.

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.

Generation of the second optical harmonic in porous-silicon-based structures with a photonic band gap

Efficient generation of the second optical harmonic is observed experimentally in a multilayer periodic structure based on porous silicon. The second-harmonic signal is much stronger than the signal from a uniform porous silicon layer or from the single-crystal silicon substrate. The orientational dependence of the second-harmonic signal is isotropic. The second-harmonic intensity as a function of the reflection angle reaches a maximum in the direction corresponding to the minimum phase detuning in a multilayer periodic structure.

Supercontinuum generation in photonic-molecule modes of microstructure fibers

Supercontinuum generation in microstructure fibers with a core in the form of a cyclic polyatomic photonic molecule is studied. Air holes arranged in a two-dimensional holey structure in the cladding of this fiber and a larger hole at the center of the fiber form a cyclic-molecule-like structure, consisting of an array of small-diameter glass channels linked by narrow bridges, around the central hole. This photonic-molecule microstructure-integrated bundle of fibers can guide the light through total internal reflection, providing a very high light confinement degree due to the large refractive index step. The properties of supercontinuum emission produced in such fibers can be controlled by coupling the energy of femtosecond light pulses into different photonic-molecule modes of the fiber.

Second- and third-harmonic generation as a local probe for nanocrystal-doped polymer materials with a suppressed optical breakdown threshold

Abstract Second- and third-harmonic generation processes are shown to allow the detection of absorptive agglomerates of nanocrystals in transparent materials and the visualization of optical breakdown in nanocomposite materials. Correlations between laser-induced breakdown and the behavior of the second- and third-harmonic signals produced in SiC/PMMA nanocomposite films are studied. The potential of second- and third-harmonic generation for the on-line visualization of laser breakdown in nanocomposite polymer materials is revealed, with the ablative material removal being monitored by the decay of the second- and third-harmonic signals. The second and third harmonics generated around the optical breakdown threshold by 75-fs pulses of 1.25-μm Cr:forsterite laser radiation are respectively more than two and four orders of magnitude more intense than the second and third harmonics produced under identical conditions by 40-ps pulses of a Nd:YAG laser. The breakdown threshold for PMMA films doped with 10–20-nm SiC nanocrystals forming absorptive agglomerates are demonstrated to be more than an order of magnitude lower than the breakdown threshold for crystalline SiC and about an order of magnitude lower than that for nondoped PMMA films.

Third-harmonic generation in focused beams as a method of 3D microscopy of a laser-produced plasma

Analysis of the phase-matching integral for third-harmonic generation in focused laser beams shows that this process can be used as a method of 3D microscopy, allowing one to characterize the location and magnitude of inhomogeneities of linear and nonlinear optical characteristics of a medium. The potentialities of this method are illustrated by the results of experiments on 3D microscopy of an optical breakdown plasma.

ONE-DIMENSIONAL COHERENT FOUR-WAVE MIXING AS A WAY TO IMAGE THE SPATIAL DISTRIBUTION OF ATOMS IN A LASER-PRODUCED PLASMA

An experimental technique based on coherent one-dimensional hyper-Raman-resonant four-wave mixing in broad cylindrically focused light beams has been developed for line-by-line imaging of spatial distribution of excited atoms in a low-temperature plasma of optical breakdown. The technique was applied to map excited lead atoms in a low-temperature laser-produced plasma.

Optimizing Two-Photon Three-Dimensional Data Storage in Photochromic Materials Using the Principles of Nonlinear Optics

In the context of designing a three-dimensional optical-memory device, we investigate the cross section of two-photon absorption of a photochromic material as a function of the relative orientation of polarization vectors of light waves involved in two-photon data writing. It is demonstrated that, with an appropriate choice of polarization erectors of writing laser waves, one can considerably increase the efficiency of two-photon data writing.