Population-induced optical nonlinearities and photo-induced spectral hole-burning phenomena in semiconductor nanocrystallites are described. Absorption saturation in small crystallites results in a genuine bleaching in the nanosecond time range with very small non-saturable contribution. In large crystallites exciton-exciton interactions lead to photo-induced blue shift of exciton resonance. Inhomogeneous broadening of absorption spectrum due to size distribution in a quantum dot ensemble results in several hole-burning effects. Along with the transient hole-burning due to absorption saturation the other processes are possible resulting in persistent or irreversible hole burning. The mechanism is proposed of persistent reversible hole-burning based on the local electric field effect due to photoionization or carrier surface localization in the resonantly excited crystallites. Permanent irreversible hole-burning is outlined for which the term `selective photochemistry' is relevant. All phenomena provide a scope of possible applications and at the same time may be used as an effective tool to evaluate intrinsic properties of quantum dots in the inhomogeneously broadened ensembles.

A travelling wave optical parametric generator based on a ZnGeP₂ crystal, pumped by an actively mode-locked Cr:Er:YSGG laser ((lambda) equals 2.8 micrometers ) has been used for the study of non-linear-optical phenomena in (InGa)As/(AlGa)As and GaAs/(AlGa)As Multi Quantum Wells (MQWs) which had intersubband absorption features in the range (lambda) approximately 4..10 micrometers . Using a two-color pump-probe scheme a dynamic spectral hole burning effect has been observed for the first time, confirming the inhomogeneous nature of MQW absorption line broadening in these samples.

We propose new and promising technique for studying quantum- size structures. We tested this technique based on optical two-photon excitation and probing of renormalized electron spectrum in our investigation of monocrystalline PbTe films. We revealed sharp dependence of spectral position of two- photon resonances versus film thickness L equals 6, 18, and 30 nm. Our theoretical model explained this result by taking into account Pb Te band structure, electron-electron and electron-phonon interactions, and redistribution of charge carriers due to inter- and intraband processes. Estimation T₂ >= 300 fs for decay time of interband polarization was obtained.

Results of resonant four-photon scattering and photobleaching studies of semiconductor (CdSe) `quantum dots' in phosphate glass matrix are presented. Nonlinear susceptibility was measured by frequency nondegenerate and degenerate scattering with a various pumping power. The data are discussed with a model that use different nonlinearity mechanisms.

The excitation profile of both resonant hyper-Raman and second-harmonic scattering in a CdS quantum dot system are reported for the first time. It is found that in the respective two-photon excitation spectra around the fundamental absorption region, two maxima more pronounced than those of one-photon absorption (OPA) spectrum are observed with narrower widths, and that their energy positions shift from the OPA peaks. The present signals may be interpreted as arising from two-photon resonances between quantum-confined valence- and conduction-band states with their angular momenta different by +/- 1, where as the off- diagonal Frochlih interaction controls mainly the RHRS process.

In this paper, we discuss the dynamics of carrier recombination in semiconductor nanoparticles embedded in a glass matrix, forming the so-called semiconductor-doped glasses. We observe that carrier recombination may be made faster by photodarkening and, at high excitation intensity, by Auger processes. We also discuss the mechanisms of the resonant optical Kerr effect in these materials. It is shown to be comprised of a fast component due to free carriers in particles without traps and of a slow one due to trapped carriers in particles with traps. The figure of merit associated with the fast component decreases when the particle size decreases.

The model of spectral broadening involving the composition fluctuations having a normal distribution law in barrier layers of the GaAs-Al_xGa_1-xAs quantum-well system is presented. Dependence of the energy broadening on the distribution law parameters and quantum well width is determined. A method of calculation of the density of states and optical gain in undoped quantum-well heterostructures having quantum layers or quantum wires is also proposed.

The theory of the optical Stark effect is developed for quantum-size structures. The effect is due to the interaction of the electron system with intense light, whose frequency, (omega) , is in resonance between two subbands in the conduction band. The probe light of the frequency (Omega) falls in resonance with the adjacent transition between the ground state and the state with an electron-hole pair. In the quasi-steady-state mode, the singularities in the interband (Omega) -light absorption spectra are related to the critical points of the bands, rearranged by the (omega) - light. The shape of the higher conduction band can be evaluated from the spectral positions of the singularities. In the case of two-photon interband transitions, a sharp dependence of the absorption on the light intensity is predicted. In the non-steady-state mode, the probe beam consists of two femtosecond (Omega) -light pulses following each other with the delay time, (tau) _d. The dependence of energy, absorbed from the second pulse, on the (tau) _d value is calculated. Under the (omega) -pumping, one of the two phenomena should be observed, depending on the type of band structure, namely, the decay of the induced polarization with the characteristic time approximately 1/(omega) _R or oscillations with the frequency approximately (omega) _R, where (omega) _R is the Rabi frequency. The principal effect, controlling the decay of the Rabi oscillations, is the spreading of the packet of states generated by the (Omega) -light pulse.

The theory is presented for the resonant diffuse scattering in multilayer nanostructures. The dynamic diffraction effects are taken into account. The computational simulation was constructed from this theory and results are presented and discussed. The angular spectra of the diffuse scattering intensity have been measured experimentally. These results confirm the decisive influence of the dynamic diffraction effects on the process of the resonant diffuse scattering.

An effect of local illumination on layered semiconductor heterostructures with 2D electron gas is investigated by measuring the spatial distribution of the photoreflectance and photoluminescence intensity varying the distance between the excitation spot and the probing position.

Second harmonic generation from magnetic materials is shown to lead to a nonlinear magneto optical Kerr effect, that can be orders of magnitude larger than its linear equivalent. The origin of this effect can be found in the differences between the linear and nonlinear solutions of the optical wave equations and in the symmetry properties of the corresponding optical tensors. Applications for the study of magnetic thin films and multilayers will be discussed.

We present an experimental study of the second harmonic light generated at a slab of nonlinear molecules embedded in a 1D periodic structure, truncated by the introduction of a defect in the central period. We have observed an enhancement of the nonlinear interaction in the vicinity of the defect, when the second harmonic wave is excited for modes within the forbidden zone or stop band. We have also observed an enhancement near the band edge, where the group velocity approaches zero. Second harmonic generation is completely suppressed for local modes within the forbidden band other than the defect mode.

Experimental and theoretical investigations of the optical second-harmonic generation (SHG) from ultrathin niobium films embedded in a dielectric are presented. The dependence of the intensity of second-harmonic emission on the film thickness (in the range from 6 to 42 angstroms) and the angular dependencies of SHG are investigated for different polarization configurations. The thickness dependencies of the second-harmonic intensity reveal resonant behavior: the p_(omega )-to-p_2(omega ) second-harmonic intensity exhibits a pronounced maximum for a film thickness of approximately 15 angstroms while the s_(omega )-to-p_2(omega ) second-harmonic signal exhibits a step-like increase at the same thickness. By modeling the metal film as a symmetric quantum well, a microscopic local-field calculation of the second-harmonic generation is performed, and numerical results for the thickness and angular dependencies of the second-harmonic energy reflection coefficient are presented. Using simply infinite-barrier wave functions together with a self-field approximation the theoretical calculations qualitatively describe the observed thickness dependence of the second-harmonic generation which can be accounted for by the intersubband transitions in the quantum wells formed by the niobium films.

Nonlinear photoinduced absorption of hydrogenated amorphous silicon (a-Si:H) was studied by excitation spectroscopy. The experimentally obtained excitation spectra are in compliance with the spectra of unperturbed absorption in 1.2 - 3.2 eV spectral range. The applicability of the method for the absorption coefficient measurement in broad range of 1 - 10⁵ cm^-1 (1 micrometers film) is demonstrated. A proposed model of photoinduced absorption from localized states is discussed.

The polarization of second harmonic signal from adsorbed monolayer of dyes was investigated. The orientation of some type of adsorbed dye molecules on surface semiconductor-- insulator structures was determined. It has been found the influence of the local charged centers of the surface on the orientation of molecules in adsorbed monolayer.

Zinc oxide thin films grown by modified spray pyrolysis technique on MgO substrates have been investigated by optical second harmonic generation (SHG). ZnO films were grown at a substrate temperatures from 300 to 600 degree(s)C. It was found to influence the structure of the films. Dependence of second harmonic intensity on angle of fundamental beam incidence revealed both a- and c-axis oriented growths in ZnO/MgO systems. Effective nonlinear optical susceptibility coefficients as high as 0.5 pm/V have been obtained from as-grown films. SHG polarization dependencies was carried out to show in-plane anisotropy in these films.

The present work is concerned with the problem of the reflection and transmission of electromagnetic waves normally incident upon a set of Cantor fractal layers. The equivalent index and the equivalent phase thickness are found for different stages of fractal growth and for various values of the fractal dimension of the structure by using an iterative method of computation. This algorithm provides the reflection and the transmission coefficients of the fractal stack.

Local field enhancement at Wood's anomalies leads to the nonlinear dynamics of two-wave induced dynamic gratings formed in the thin film that supports surface waves. The enhancement and depletion, the multistability, the self- oscillation and the chaotic behavior of the dynamic gratings were established at these anomalies.

Dependence of minimum reflectivity angle on incidence light pulse energy at SEW excitation in Kretschmann's configuration was studied. CaF₂ prism covered by thin layers of Ag and fullerene was used. Optical constants of fullerene thin layer were found: (epsilon) ' equals 3.86, (epsilon) ' equals 0.013 at wavelength 1.055 micrometers .

We investigate the boundary-value problem of nonresonant reflection and transmission of probe light wave passing through excited nonlinear semi-infinite medium of three- level atoms. The two lower levels are excited by intensive resonant stationary or nonstationary radiation to obtain the interference of atomic states. Some properties of near field effect in this problem are investigated taking into account retarded polarizing fields and the density-dependent near dipole-dipole interactions. We separate in our consideration three waves: the inversion wave and two polarization waves with corresponding nonlinear indices of refraction.

The magnitude and the dispersion of the cubic susceptibility of pseudoisocyanine J-aggregates entrapped in polyvinylpyrrolidone thin films have been determined. (chi) ⁽³)((lambda) ) and (chi) ⁽³)((Delta) (lambda) ) have been measured by four-wave mixing in both degenerate and non-degenerate cases. Z-scan technique was used to separate the Re(chi) ⁽³)((lambda) ) and Im(chi) ⁽³)((lambda) ) contributions to the absolute value of (chi) ⁽³) (DOT) (chi) ⁽³) was found to be approximately (2 divided by 6) 10^-6 esu and the relaxation time was 10^-13 - 10^-12 s.

Crystals of solid solutions of K_0.82Tl_0.18TiOPO₄, K_0.59Tl_0.41TiOPO₄, Rb_0.77Tl_0.23TiOPO₄, KGe_0.04Ti_0.96OPO₄, KGe_0.06Ti_0.94OPO₄, KGe_0.18Ti_0.82OPO₄, TlTiOPO₄ and KGeOPO₄ were obtained by crystallization from nonstoichiometric melts. These crystals belong to the ferroelectric family which possesses superionic conductivity of the KTiOPO₄-type. Their crystal structures were established and electrophysical characteristics were studied. The crystals are orthorhombic, sp.gr. Pna2₁. In these crystals, as compared to KTiOPO₄ crystals, TiO₆-octahedra have a more regular structure, that is one of the reasons of the differences in the nonlinear optical characteristics.

Nonlinear refraction of a new polymer structure based upon diglycidylether of bisphenol-A has been studied. The polymer contains 4-aminoazobensen as nonlinear optically active side groups covalently attached to each monomer unit. Thin dynamic grating writing and relaxation processes have been studied using frequency doubled YAG:Nd⁺³ laser ((lambda) equals 0.53 micrometers ). The dynamics of transient gratings was shown to have two different erasure times corresponding two different nonlinearity mechanisms fast one (relaxation time (3) equals 5 (DOT) 10^-8 esu) and slow one (quasi-stationary). The later makes it possible to write and erase phase gratings reversibly by ns pulses. An analysis of possible mechanisms for refractive index nonlinear behavior in new structures is presented. In particular, a role of TRANS-CIS photoisomerization of the azo-compound in nonlinear optical properties of the structure are discussed.

New nonlinear optical crystal--the octahydrate of aluminium iodate Al(IO₃)₃ (DOT) 8H₂O were grown from multicomponent aqueous solution. Linear and nonlinear optical properties of Al(IO₃)₃ (DOT) 8H₂O crystal have been measured and compared to those of the lithium iodate crystals. The nonlinear optical merits of other iodates have been predicted.

We present a technology of a growing of pure and doped sillenite crystals and the results of a study of two-beam mixing in these crystals at wavelength (lambda) equals 633 nm. We have employed the single-carrier model of the photorefractive effect with shallow and deep traps to the explanation of the experimental results.

The present message is devoted to the results of the tests for the second harmonics generation in powders of substituted stilbenes. The 4-aminostilbene single crystals have been grown from a solution, their linear and nonlinear optical characteristics have been measured. The prospects of application of 4-aminostilbene for nonlinear frequency conversion are discussed.

We present the results of steady-state photoinduced and electroabsorption spectroscopy studies in nonoriented films of highly stable soluble trans-polyacetylene. Using Kramers- Kronig analysis of the electroabsorption spectra the nonlinear third-order non-resonant susceptibility (chi) ⁽³) ((omega) ;(omega) ,0,0) of trans-polyacetylene was estimated to be in the range of 10^-9-8 esu.

Laser ablation process of the polycrystalline and foam graphite samples was studied using optoacoustic spectroscopy. High defect density of the foam graphite allows observe extremely high ablation efficiencies and their nonmonotonous dependence on laser fluence with many peaks. It seems photochemical multiphoton evaporation process of the matter in the form of large surface relief fragments takes place besides thermal one.

The threshold-like formation of strongly modified (nonsolvable) layer in PMMA and other nonreciprocal effects of Nd:YAG laser fifth harmonic radiation on PMMA and PI films are discovered and discussed.

Modifications of structural, electrical and recombinational properties of CdTe under nanosecond pulse of ruby laser were studied by techniques of second harmonic generation, time resolved reflectivity, photoreflectance and photoluminescence. The threshold energy density necessary for surface melting W_m equals 40 +/- 5 mJ/cm² was determined. Changes in linear reflection coefficient indicate intensive evaporation of the components at W > 90 mJ/cm². Laser induced defect formation was registered after treatment with W > W_m. Apart from defect formation, the process of their annealing was detected in narrow area of energy near the melting threshold.

Solid phase crystallization process in thin amorphous silicon films on glass substrates was studied with application of excimer laser annealing (ELA) and rapid thermal annealing (RTA) for stimulation of nucleation. Using of ELA allowed to create homogeneous polycrystalline silicon films on glass with grain sizes up to 3 micrometers at temperatures below 550 degree(s)C. Using of RTA reduced the incubation time of nucleation from 100 to 6 hrs. The textured silicon films on glass with predominant orientation (110) and sizes of textured areas up to 30 micrometers were manufactured using the excimer laser stimulation of nucleation. The mechanism of mechanical stresses influence on grain orientation was suggested as well as it was theoretically shown, that internal stresses retard the nucleation process. Deformation addition to chemical potential difference were estimated for nucleation in amorphous silicon as 11.4 meV per nucleated atom. Retardation of crystallization after Ge implantation was observed and it was proposed to be explained within deformation mechanisms.

In 1986 it was reported , that a single beam ofpicosecond, A.=0.5 μm pulses at a high repetition rate of82MHz with intensity just bellow melting threshold can induce permanent periodic surface structures(relief gratings) on silicon with periods as small as an eighth of the light wavelength (~.06 μm). The grating rulings on (11 l)surtace were oriented along one of [110] axes. Later this phenomenon was more thoroughly investigated in Ref. {2]. The structures were produced on silicon and germanium and were studied as functions of laser wavelength, polariz.ati.on, repetition rate, pulse width and crystallographic orientation. Contrary to Ref.[1] it was fotmd that the surface relief gratings were oriented with wave vectors parallel to the polariz.ation vector of the linear polarized laser beam and the structure period in Si was observed to increase Vith laser pulse width 7: P and to decrease with increase ofthe pulse repetitionrateR (for 150 kHz-150 MHz). Besides atthe same conditions tht: structures with pulse independent periods were recorded in Si. In Ge the structures produced with 75 ps pulses had lesser period t.lian those produced with pulses of considerably less duration (5 ps) but with higher repetition rate. The effect of abrupt transition of structure period from 0.325μm to 0.096μm along the scan line was observed in Si. To our knowledge no model was able to explain the above features of ultrashort period laser-induced periodic surface structures formation. In this work we propose and develop such a model, the physical essence of which is as follows. Intense laser pulse excites electrons across the band gap and creates in a subsurface layer with thickness of order of absorption lengt.11. a. -l a hig..11. concentration of electron-hole plasma. In Si at temperatures T ~ Tm ( T,,. is the melting temperature) a -l - 4*10-<i cm [3J. During the action of a train of laser puls..."S with duration 't P and repetition rate R the subsurface plasma-enriched layer of thickness h < cx-1 is formed. We consider this thin plasma enriched layer as a "film" of thickness h tightly bonded to the underlying crystal ("substrate") (Fig. 1). The initial state with tmbended film and laterally wllfonn plasma concentration becomes unstable at a certain critical value of surface plasma concentration nc and due to the surface plasma-strain (SPS) instability [ 4] the state with periodically bended film and periodic piling up of plasma concentration is formed(Fig.1). The period of th.is SPS structure is shown to be d = 2h with the value of h being determined by interplay of three processes of plasma transport nozmal to the! surface: the canier diffusion, the carrier drift. induced by self-consistent strain and plasma renonnaliz.ation of forbidden band gap, and the canier drift induced by nonwllform heating. Auger recombination plays essential role in restricting the value of h. The competition of diffusion and drifts leads to establishing at times ts of quasistati.onary regime. At low repetition rat·~ ( R-1 < 7:0 , 7:0 is the linear recombination time) ts< 'tp• i.e. steady state is established before the end of a pulse and disappears before· the beginning of the next pulse ("'single pulse" regime of plasma layer formation). No surface plasma accumulation from pulse to pulse in a train occms in this regime and h is independent ofR .. At high repetition rate (R-1 < 7: 0) surface plasma accumulation from pulse to pulse takes place and the time of establishing of steady state is t1 < 7: 0 . In limiting cases of regimes controlled by either diffusion and strain, plasma-induced drift (DSPD) or by diffusion and temperature-induced drift (DTD) the obtained general solution of nonlinear plasma dynamics equation yields explicit formulas for has a function of laser intensity and parameters of the medium. The dep...'"Ildency of d = 2.h onR and 7: P in DSPD regime is shown to describe the corresponding experimental dependencies of structure periods in Si and Ge, recorded in Ret:[2]. The abrupt transition from DSPD to DTD regime is the cause of the occurrence of sudden change of structure period to ultrashortvalue(-0.095 μm ), observed in Si (2]. The orientations of SPS gratings are determined by the interplay of two factors reflecting the dual nature of plasma-strain instability: the crystallographic anisotropy of surface shear elastic modulus and strong linear polarized laser beaminduced anisotropy of lateral ambipolar diffusion coefficient Thus depending on irradiation conditions the SPS grating wave vector may be either crystallographically c·rieated or be directed along the el.tric ,,-ect~r E cf the i:Kident laser field. Tiris feature of the SPS instability model reconciles controversial experimental findings of Refs. [ 1] and ( 2 J. The problem of describing the phenomenon of ultrashort period swface structure formation thus cofuists of two parts. The first is the consideration of plasma transport in direction normal to the surface and calculation of the thickness of subsurface plasma layer, and second is the consideration oflateral plasma transport in self-consistent periodic strain field and calculation of the surface structure periods. In this work we formulate and consider both problems. The work is organized as follows. In Sec.2 plasma tra.'1Sport normal to the surface is treated on the base of nonlinear plasma transport equation and the thickness of subsurface plasma layer his found. In Sec.3 the "film on substrate" model of SPS instability is formulated and in Sec.4 a general set of nonlinear equations in mode representation, describing SPS instability, is obtained. In Sec.5 we perform linear stability analysis ofSPS-instability equations and find grmvth rates of surface periodic SPS structures. In Sec.6, using the obtained expressions for the growth rates, we calculate the characteristics of SPS structures and compare the theoretical results with experimental ones ofRefs.[l,2J.

The optical non-linearity based on the non-parabolicity and the intervalley transfer in highly doped n-GaAs has been proposed. A large fraction of the light mass central valley population can be effectively transferred to the heavy mass satellite valleys by optically heating the free carriers with CO₂ laser light, leading to a non-linear refractive index n₂ of the order of 10^-8 cm²/W. Integrating such non-linear layers in a suitable interferometer drastically decreases the non-linear threshold value. Together with the intrinsically small intravalley and intervalley relaxation times (picosecond time scale), this optical heating effect can be used as a versatile tool for improving our understanding of carrier- lattice interactions and hot electron effects in highly doped semiconductors as well as an optical spectroscopic probe for investigating other ultrafast physical, chemical, or biological processes. By introducing feed-back mechanisms, all-optical clocks and delay lines could be conceived with response times of the order of a few ps.