N. I. Koroteev

Inhomogeneous deformation of silicon surface layers probed by second-harmonic generation in reflection

Semiconductor crystals possessing inversion symmetry (Si, Ge) are known to have a rather weak second-order nonlinearity of the quadrupole type [ Phys. Rev. Lett.51, 1983 ( 1983)], since the electric-dipole contribution is forbidden by symmetry. We report the experimental observation of anomalously highly efficient second-harmonic generation (SGH) in reflection from the surface of Si under inhomogeneous deformation. This effect is believed to be due to an electric-dipole contribution to the second-order susceptibility induced in the near-surface layer by inhomogeneous mechanical stress. This fact is consistent with theoretical calculations based on the molecular sp3-orbital model. Experimentally we observed an increase in the second-harmonic intensity by more than 2 orders of magnitude and a modification of the second-harmonic intensity dependence on crystal orientation with respect to the surface normal in the case of ion-implanted, pulsed-laser-annealed Si (111) samples. A similar effect was observed with thermally oxidized Si wafers and silicide-on-cSi structures. These results demonstrate the sensitivity of SHG in reflection to the presence of inhomogeneous stress in Si surface layers, which enables one to use SHG for nondestructive monitoring of stress in semiconductor structures.

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.

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.

Laser nonlinear-optical probing of silicon/SiO2 interfaces: Surface stress formation and relaxation

A nonlinear optical technique based on optical second harmonic generation in reflection is shown to provide information on the surface layer structure of semiconductor crystals, thin films and layered systems. The second harmonic intensity is sensitive to inhomogeneous stress in centrosymmetric materials via spatial selection rules and the appearance of an electric dipole contribution to the second order nonlinear optical susceptibility. The technique is used to monitor mechanical stress relaxation in the SiO2/Si interface during several annealing procedures.

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.

Femtosecond dynamics of laser-induced phase transition of the GaAs surface layer to a centrosymmetric phase

Abstract Recently we observed ultrafast laser-induced phase transition (on a time scale less than 100 fs) to a centrosymmetric semiconductor-like phase at the GaAs surface [1,2] by using time-resolved second harmonic generation in reflection. A phenomenological model describing this phase transition is developed. The new phase exists during the first 1 ps after laser excitation due to high plasma carrier density and lattice stress, the lattice temperature remaining well below the melting threshold.

Second-harmonic generation in reflection from crystalline metals: the influence of structure disordering

We report theoretical and experimental studies of the influence of crystalline structure on the nonlinear-optical response of monocrystalline metals. We show that the lattice disordering leads to a decrease of both the anisotropy and the absolute value of the quadrupole second-order nonlinear susceptibility, in agreement with experiments on optical second-harmonic generation in reflection from the surfaces of various Al samples.

Coherent Raman and hyper-Raman spectroscopy of excited and autoionizing states of atoms and ions in laser-produced and electric-discharge plasma

SummaryThe experimental results on four-wave Raman and hyper-Raman scattering in a laser-produced and electric-discharge plasma are presented. It has been shown that in spectra of four-wave mixing processes resonances appear due to the Raman and hyper-Raman scattering on atomic and ionic excited states. The temporal behaviour of the scattered-signal intensity has been found to be connected with the population relaxation of atomic and ionic excited states. We have observed for the first time the resonance in the spectrum of coherent hyper-Raman scattering in electric-discharge plasma associated with the electron transition between the excited and autoionizing state of a copper atom.

Laser picosecond microspectrofluorometry of haematoporphyrin in cells and liposomes

The results of a laser picosecond microspectrofluorometric study of the spectral and kinetic characteristics of haematoporphyrin (Hp) fluorescence at various sites in cultured SPEV cells and phosphatidylcholine liposomes are presented. The computer-controlled detection system is based on the single-photon counting method with picosecond time resolution. In aqueous medium, the Hp fluorescence spectrum is characterized by two bands at 615 and 675 nm. In living cells and liposomes, Hp fluorescence is red shifted to 630 and 690 nm. In addition a new band at 665 nm is detected. The dependence of this band on the incubation time and Hp concentration was investigated. The fluorescence decay kinetics of Hp in a culture medium, liposome and a cell nuclear membrane were measured. Possible Hp aggregate formation in the lipid bilayer and its implications are discussed.

Second-harmonic and sum-frequency generation in reflection: probing of GaAs surface structure and laser-induced phase transitions

The dynamics of pulsed-laser annealing of an ion-implanted GaAs surface is studied by second-harmonic generation in reflection. The results are explained in terms of surface melting. Sum-frequency and second-harmonic generation in reflection are shown to be sensitive, convenient techniques having high temporal and spatial resolution for studying the structure of GaAs surfaces that have been laser annealed or randomized by ion implantation and of laser-induced phase transitions. The tensor nature of the second-order nonlinearity provides an opportunity to make polarizational measurements that are sensitive to small structural changes.