Yu. A. Matveets

Fabrication of fiber Bragg gratings with 267 nm femtosecond radiation

Strong high-quality fiber Bragg gratings with photoinduced refractive-index modulation of more than 10-(3) were written in a Corning SMF-28 fiber, a P(2)O5-doped-core fiber and a pure-silica-core fluorine-doped-cladding fiber by third-harmonic radiation (267 nm, 150 fs and 1.2-1.8x1011 W/cm(2)) of a femtosecond Ti:sapphire laser using a phase mask. We compare the 267-nm photosensitivity responses with the results of irradiation by 193-nm ArF and 157-nm F(2) excimer lasers. The dependence of the refractive-index change on the exposure dose and the annealing characteristics of the fabricated gratings are typical for Type-I UV-written fiber gratings.

Two-photon absorption of powerful femtosecond pulses in C60 film

Abstract Irradiance-dependent transmission measurements were conducted for thin C60 film irradiated by 300 fs laser pulse at 612 nm in the intensity range from 108 W/cm2 to the values ≈ 1012 W/cm2 corresponding to the film damage. The upper limit for the two-photon absorption coefficient was derived from these measurements as (3 ± 1.5) × 10−8 cm/W, which corresponds to an Im χ(3) values of (2.6 ± 1.3) × 10−11 esu.

Effect of intense chirped pulses on the coherent phonon generation in Te

The authors have studied the influence of chirped laser pulses on the coherent phonon generation in single crystal Te. They have shown that the pulse chirp affects the amplitude of coherent phonons with A1 symmetry in the case of intense excitation only. By varying the chirp of an intense exciting pulse, the authors demonstrated that negatively chirped pulses are almost twice more effective in the creation of lattice coherence than positively chirped pulses.

Self-focusing limitation of brightness in amplification of ultrashort pulses in neodymium glass and yttrium aluminum garnet

An experimental investigation was made of ultrashort pulses generated in yttrium aluminum garnet (YAG) and amplified in YAG and neodymium glass. When the power density was less than 3-5 GW/cm2, the main mechanism which limited the amplified power was large-scale self-focusing of the amplified radiation in the active media, which altered greatly the laser beam divergence. Conditions were found under which this self-focusing was avoided completely. The investigated self-focusing produced, around the main beam, a scattered-radiation halo of divergence much greater than the beam divergence. The proportion of the laser beam energy lost to the halo increased with the amplification, so that beginning from a certain stage further amplification simply resulted in a reduction in the brightness. In this situation the enrgy density in the amplified beam could still be below the damage threshold of the active medium.

Excitation and dissociation of polyatomic molecules under the action of femtosecond infrared laser pulses

The effect of high-intensity femtosecond laser pulses (100–200 fs) in the near (0.8–1.8 µm) and medium (4.6–5.8 µm) IR ranges on the CF2HCl, CF3H, (CF3)2C=C=O, and C4F9COI molecules is examined. Irradiation of CF2HCl and CF3H molecules by 0.8-to 1.8-µm laser pulses with intensities of >40 TW/cm2 (>4 × 1013 W/cm2) makes them dissociate to yield CF3H and CF4, respectively. The key mechanism of the dissociation of these molecules is field ionization and fragmentation. The excitation of the stretching vibrations of the C=O bond in the (CF3)2C=C=O and C4F9COI molecules by 4.5-to 5.8-µm femtosecond pulses produced no detectable dissociation up to a fluence of ∼0.5 J/cm2 (or a intensity of ∼2.5 TW/cm2). Probable explanations of this observation are discussed.

Nonperturbing observation of optical near field

The spatial distribution of light intensity in the near field is studied by observing the photoelectron projection images of a subwavelength nanoaperture. The imaging electrons are obtained as a result of a two-photon external photoelectric effect induced in the aperture formed at the end of an optical fiber by femtosecond pulses of the second-harmonic radiation (410 nm) of a Ti:sapphire laser. The light-field distribution in the aperture is not distorted by any near-by object (such as a medium containing fluorescent molecules), which allows nonperturbing measurement of such a distribution.

Dissociation of CF2HCl molecules by intense radiation from a femtosecond laser in the near-IR range

It is found that CF 2HCl molecules dissociate under irradiation by high-intensity pulses of femtosecond radiation (λ=1.8 μm) in the vicinity of resonance with the frequency of overtone vibrational transitions in the C-H bond. It is shown that the dissociation products substantially differ from the products of dissociation of CF2HCl molecules from the ground and excited electronic states under the action of IR and UV nanosecond pulses.

Geometric-optical reconstruction of a wavefront

A new method of achromatic wavefront reconstruction by geometric-optical reflection of the reconstructing radiation from surfaces with constant phase difference between the object and reference waves is theoretically described and experimentally realized. Methods distinction from holographic one is discussed. Femtosecond laser pulses are used for recording in the experiment.

Geometric-optical reconstruction of a wavefront experimental realization with femtosecond laser pulses

Abstract The mechanism of wavefront reconstruction by geometric-optical reflection of reconstruction radiation from surfaces with constant phase differences between the object and reference waves has been investigated. The main difference between this mechanism and a holographic one is the absence of diffraction of the reconstructing radiation by the periodic structure and as a consequence the achromatism of the reconstruction process. Incoherent continuous radiation and ultrashort laser pulses were used in the experiments. The effect of achromatic reconstruction has been obtained after recording the interference of counterpropagating 30–40 fs pulses from an Al 2 O 3 :Ti 3+ laser in bulk media.

Ultrahigh-spatial-resolution photoelectron projection microscopy using femtosecond lasers

Ultrahigh spatial resolution of two-photon photoelectron images (as high as 3 nm, which is the best value that has been achieved to date in photoelectron microscopy with spatial resolution) is obtained when silicon nanotips are irradiated by the second harmonic of a pulsed femtosecond Ti: sapphire laser. In addition, the absolute value of the two-photon external photoeffect coefficient is measured.