Nonlinear optics of semiconductors is an important field of fundamental and applied research, but surprisingly the role of excitons in the coherent processes leading to harmonics generation has remained essentially unexplored. Here we report on some examples of experimental and theoretical study of the second- (SHG) and third (THG) harmonic generation involving the exciton resonances in the noncentrosymmetric semiconductors ZnO and GaAs. External electric and magnetic fields are used to modify exciton states and their nonlinear optical properties. Depending on the particular symmetry of the exciton states SHG/THG can originate from the electric- and magnetic-field-induced perturbations of the excitons due to the Stark effect, the spin as well as orbital Zeeman effects, or the magneto-Stark effect. A microscopic theory of SHG and THG on excitons is developed, which shows that the nonlinear interaction of coherent light with excitons has to be considered beyond the electric-dipole approximation.

Nanophotonic devices show interesting features for nonlinear response enhancement but numerical tools are mandatory to fully determine their behaviour. To address this need, we present a numerical modal method dedicated to nonlinear optics calculations under the undepleted pump approximation. It is brie y explained in the frame of Second Harmonic Generation for both plane waves and focused beams. The nonlinear behaviour of selected nanostructures is then investigated to show comparison with existing analytical results and study the convergence of the code.

The features of nonlinear and electro-optical processes has been discovered in the case of two-photon resonant excitation of the excitons in colloidal CdSe/ZnS quantum dots. Self-diffraction arises for two laser beams intersecting in the cell with colloidal CdSe/ZnS quantum dots (QDs) due to the dynamic phase grating formatting. The calculated induced change in the refractive is sufficient to form a phase diffraction grating. Such a large value of χ⁽³⁾ as compared to the third-order nonlinear susceptibility for the solvent (hexane) is due to the increase in χ⁽³⁾ occurring when the intermediate resonance is attained in a medium transparent for laser radiation. In order to identify physical processes responsible for the induced grating formation and the diffraction efficiency self-diffracted pulse intensity dependences on the incident pulse intensity were measured for two samples of colloidal QD CdSe/ZnS, which frequency of the fundamental exciton transition is tuned to the high-frequency and low-frequency region from the double laser frequency. The discovered cubic dependence of the self-diffracted pulse intensity on the incident pulse intensity was explained by four-wave mixing process. Discovered above 5-th index of power dependence of the self-diffracted pulse intensity on the excitation pulses intensity we explained by the increasing magnitude of two-photon absorption (due to shifting of two photons energy of laser radiation to the exact exciton absorption resonance by red Stark shift of the exciton absorption), accompanied by the growth absorption by two-photon excited carriers that leads to the induced amplitude grating formation in addition to the phase grating.

Ways to improve beam divergence and energy consumption of quantum dot lasers emitting via the ground-state optical transitions by optimization of the key parameters of laser active region are discussed. It is shown that there exist an optimal cavity length, dispersion of inhomogeneous broadening and number of QD layers in active region allowing to obtain lasing spectrum of a given width at minimum injection current. The planar dielectric waveguide of the laser is optimized by analytical means for a better trade-off between high Γ-factor and low beam divergence.

Here we discuss the second harmonic generation (SHG) signal raised by a sample surface manifesting circular dichroism. The measurement were performed by studying the SHG efficiency in different polarization states of the light. In particular measurement performed with circular polarized light showing a very high sensitivity to the response of the studied metasurfaces.

Nonlinear processes are investigated in plasmonic nanostructures which display Fabry-Perot resonances. This study focuses on the impact of the resonator geometry on the efficiency of second order effects implying 2 or 3 different wavelengths. These resonances heavily concentrate the electric field, allowing significant enhancement of the local nonlinear polarization and subsequently of the nonlinearly generated signals.

We focus on dynamical analysis of nonlinear structures consisting of coupled ring resonators. We formulate a system of difference-differential equations that take into account non-instantaneous Kerr response and the effect of loss. The system is applied to investigation of a double-ring structure in the all-pass filter configuration. We observe rich dynamics, transitions from steady state to Hopf bifurcations and chaos. The system is highly sensitive for the values of detuning from resonance and input power. The influence of loss on oscillatory states is also presented.

We strive to obtain highly fluorescent planar materials that may be used for the development of nanolasers based on localized plasmons. The promissing candidates for this purpose are materials consisting of mixtures of organic molecules, polymer, and silver nanoparticles. Silver nanoparticles were preliminary deposited on the quartz substrates. These samples were characterized by SEM and absorption spectroscopy. Then, they were covered by the polymer/rhodamine and polymer/coumarin layers using either spin-coating or evaporation techniques and characterized by confocal luminescent microscopy and spectroscopy. As a result of the localized surface plasmon excitation, we observed the enhancement of the rhodamine and coumarine absorption in the near fields of silver nanoparticles. The fluorescence of the thin films of polymer activated by dyes molecules with silver nanoparticles was almost 20-fold more intense than that on the bare dielectric surfaces without silver nanoparticles. In the presence Ag nanoparticles and at increased intensities of excitation we found also slight narrowing of the luminescence spectrum of polymer/coumarin layers.

Studies about light propagation have been undertaken for more than a century. It is now well established that any material that has normal or anomalous dispersion generates slow or fast light. In this paper, we demonstrate an experimental technique to rapidly switch between slow and fast light in ruby. The experiment utilizes transient holeburning to create drastic variation in refractive index of ruby to produce slow as well as fast light. Transient hole-burning involves the depletion of the ground state leading to a highly populated excited state by single frequency laser excitation. This leads to a hole in the absorption spectrum when readout by a laser. We observed a delay of 29 ns and advancement of -11 ns in an external magnetic field of B║c = 12 mT corresponding to a group velocity of c/961 and negative group velocity of –c/365 respectively.

In this paper, we describe an experiment for two-photon excitation in laser-cooled ⁸⁷Rb using an optical nanofiber. A brief description of the multilevel atom scheme is followed by experimental results on two-photon absorption and observation of Autler-Townes splitting in cold atoms. These experiments use powers several orders of magnitude lower than those used in free space experiments.

We demonstrate the fabrication of a highly nonlinear sub-micron silicon nitride trench waveguide coated with gold nanoparticles for plasmonic enhancement. The average enhancement effect is evaluated by measuring the spectral broadening effect caused by self-phase-modulation. The nonlinear refractive index n₂ was measured to be 7.0917×10^-19 m²/W for a waveguide whose W_open is 5 μm. Several waveguides at different locations on one wafer were measured in order to take the randomness of the nanoparticle distribution into consideration. The largest enhancement is measured to be as high as 10 times. Fabrication of this waveguide started with a MEMS grade photomask. By using conventional optical lithography, the wide linewidth was transferred to a <100> wafer. Then the wafer was etched anisotropically by potassium hydroxide (KOH) to engrave trapezoidal trenches with an angle of 54.7º. Side wall roughness was mitigated by KOH etching and thermal oxidation that was used to generate a buffer layer for silicon nitride waveguide. The guiding material silicon nitride was then deposited by low pressure chemical vapor deposition. The waveguide was then patterned with a chemical template, with 20 nm gold particles being chemically attached to the functionalized poly(methyl methacrylate) domains. Since the particles attached only to the PMMA domains, they were confined to localized regions, therefore forcing the nanoparticles into clusters of various numbers and geometries. Experiments reveal that the waveguide has negligible nonlinear absorption loss, and its nonlinear refractive index can be greatly enhanced by gold nano clusters. The silicon nitride trench waveguide has large nonlinear refractive index, rendering itself promising for nonlinear applications.

Second-harmonic generation (SHG) is a coherent nonlinear optical process occurring in non-centrosymmetric molecules, including microtubule (MT). MT is a cytoskeleton playing many important roles in a variety of cellular processes depending on the cell type, and the conformation is crucial for the function. Here we present the use of SHG process to probe MT cytoskeleton in living neuronal tissue. Polarization-resolved SHG (p-SHG) imaging and the second-order tensor analysis were performed on the retinal nerve fibers in order to probe the structure of MTs in axon. The polar anisotropy of tubulins was determined at the molecular level. The effect of MT-stabilizing drug Taxol was also examined and the induced changes were not detectable by p-SHG. Our results demonstrate SHG as a novel optical method to measure conformational changes of MTs in the native cellular context. The technique could be employed in conjunction with existing atomic-resolution methods of structural biology to improve our understanding of cytoskeleton dynamics in vivo.

Laser pulse shaping is reported for applications on multiphoton processes in dye molecules. Particularly phase-tailored pulse shapes are employed for two-photon excited fluorescence of dyes in a liquid environment, also at the distal end of an optical fiber, in order to improve the contrast between dye markers having similar excitation spectra. Precompensation of the optical fiber properties is utilized by analytical pulse shaping in order to receive specific parametric pulse forms after the fiber. This will lead to new endoscopic imaging applications with an increased fluorescence contrast. Moreover, selective excitation is also demonstrated for three-photon transitions of the two dyes, p-Terphenyl (PTP) and BM-Terphenyl (BMT), in solution by using shaped pulses without a fiber. A good agreement between experiment and theoretical simulation is obtained. With this approach it is possible to achieve a considerable change of the fluorescence contrast between the two dyes which is relevant for imaging applications of biological molecules.

The potential of conventional semiconductor lasers to generate complex and chaotic dynamics at a bandwidth that extends up to tens of GHz turns them into useful components in applications oriented to sensing and security. Specifically, latest theoretical and experimental works have demonstrated the capability of mutually coupled semiconductor lasers to exhibit a joint behaviour under various conditions. In an uncoupled network consisting of N similar SLs - representing autonomous nodes in the network - each node emits an optical signal of various dynamics depending on its biasing conditions and internal properties. These nodes remain unsynchronized unless appropriate coupling and biasing conditions apply. A synchronized behaviour can be in principle observed in sub-groups of lasers or in the overall laser network. In the present work, experimental topologies that employ eight SLs, under diverse biasing and coupling conditions, are built and investigated. The deployed systems incorporate off-the-shelf fiber-optic communications components operating at the 1550nm spectral window. The role of emission wavelength detuning of each participating node in the network - at GHz level - is evaluated.

The presentation is devoted to the theoretical investigation of nonlinear scattering of ultrashort electromagnetic pulses (USP) on two-level quantum system.

We consider the scattering of several types of USP, namely, so called corrected Gaussian pulse (CGP) and cosine wavelet pulse. Such pulses have no constant component in their spectrum in contrast with traditional Gaussian pulse. It should be noted that the presence of constant component in the limit of ultrashort pulse durations leads to unphysical results.

The main purpose of the present work is the investigation of the change of pulse temporal shape after scattering as a function of initial phase at different distances from the target. Numerical calculations are based on the solution of Bloch equations and expression for scattering field strength via dipole moment of two-level system exposed by the action of incident USP.

In our calculation we also account for the influence of refracting index of the air on electric field strength in the pulse after scattering.

We develop an explicit solution of the problem describing collinear four-waves mixing in medium with cubic nonlinear response. This solution is carried out for a set of Schrödinger equations using plane wave approximation under phase matching of interacting waves. This solution allows to provide full analysis of four-wave interaction modes in dependence of the problem parameters.

We have shown, in particular, an existence of bistable mode for energy conversion from pump waves to signal wave under certain conditions. In general case, there are greater than 10 various modes of four-wave interaction. Knowledge about these modes is very important for spectroscopic experiment results understanding using four-waves mixing because its result depends on them in a strong way. Analytical solution and developed modes can explain complicated regime of four-wave interaction which may appear at high intensity of interacting waves.

We investigate the propagation of laser pulse with self-similar shape in homogeneous medium with various mechanisms of nonlinear absorption: multi-photon absorption or resonant nonlinearity under detuning the frequency, corresponding to energy transition, from the current frequency of wave packet, or nonlinear absorption with its saturation. Both types of sign for frequency detuning are considered. This results in appearance of a refractive index grating which induced a laser pulse self-action. We analyze also the influence of the laser pulse self-modulation due to cubic nonlinearity on existence of the laser pulse propagation mode with self-similar shape.

We develop an analytical solution of the corresponding nonlinear eigenfunction problem for laser pulse propagation in medium with nonlinear absorption. This solution is confirmed by computer simulation of the eigenfunction problem for Schrödinger equation with considered nonlinearity.

This mode of laser pulse propagation is very important for powerful TW laser pulse propagating in glass.

Nonlinear phenomena induced by an electric field inside a nematic liquid crystal were studied. A planar oriented cell filled with 5CB nematic liquid crystal was subjected to an external voltage, higher than the threshold value for the Freedericksz transition, and a laser beam was sent perpendicular to the cell. Circular fringes appeared on the screen. The maximum number of diffraction fringes for different voltage values was measured and the experimental plot were in good agreement with theory. We performed a set of dynamic measurements of the fringes evolution from the moment when the laser beam was applied until the maximum number of fringes was reached, for each voltage, and the results were consistent with theoretical pattern.

Black carbon (BC) particles are a product of incomplete combustion of carbon-based fuels. One of the possibilities of studying the optical properties of BC structures is to use the DDA (Discrete Dipole Approximation) method. The main goal of this work was to investigate its accuracy and to approximate the most reliable simulation parameters. For the light scattering simulations the ADDA code was used and for the reference program the superposition T-Matrix code by Mackowski was selected. The study was divided into three parts. First, DDA simulations for a single particle (sphere) were performed. The results proved that the meshing algorithm can significantly affect the particle shape, and therefore, the extinction diagrams. The volume correction procedure is recommended for sparse or asymmetrical meshes. In the next step large fractal-like aggregates were investigated. When sparse meshes are used, the impact of the volume correction procedure cannot be easily predicted. In some cases it can even lead to more erroneous results. Finally, the optical properties of fractal-like aggregates composed of spheres in point contact were compared to much more realistic structures made up of connected, non-spherical primary particles.

Mid-IR wavelength range offers variety of interesting applications. Down-conversion in the optical parametric devices is promising to generate high average power mid-IR beam due to inherently low thermal load of the nonlinear crystals if a powerful and high quality pump beam is available. We developed 100 kHz pump laser of 100-W level average power. The stretched pulses of Yb-fiber laser oscillator at 1030 nm wavelength are injected into the regenerative amplifier with an Yb:YAG thin-disk. Diode pumping at zero phonon line at wavelength of 969 nm significantly reduces its thermal load and increases conversion efficiency and stability. We obtained the beam with power of 80 W and 2 ps compressed pulsewidth.

We are developing a watt level mid-IR picosecond light source pumped by a beam of the thin disk regenerative amplifier. Part of the beam pumps PPLN, which is seeded by a continuous wave laser diode at 1.94 μm to decrease the generation threshold and determine the amplified spectrum. The 3 W pumping gave output of 30 mW, which is by up to two orders higher compared to unseeded operation. The gain of about 10⁷ was achieved in the PPLN in the temporal window of the pump pulse. The spectrum and beam of the generated idler pulses in the mid-IR was measured. We obtained an amplified signal from the second stage with the KTP crystal. We expect watt level mid-IR output for initial 50-W pumping. The generation of longer wavelengths is discussed.

Single crystals of Guanidinium L-Ascorbate (GuLA) were grown and crystal structure was determined by direct methods. GuLA crystallizes in orthorhombic, non-centrosymmetric space group P2₁2₁2₁. The UV-cutoff was determined as 325 nm. The morphology was generated and the interplanar angles estimated and compared with experimental values. Second harmonic generation conversion efficiency was measured and compared with other salts of L-Ascorbic acid. Surface laser damage threshold was calculated as 11.3GW/cm² for a single shot of laser of 1064 nm wavelength.

Recent developments and results concerning high quality factor optical resonators and some applications to optoelectronics are described in this paper.

In this paper we report the realization of a specific electronics and oven for optical resonator which needs to be temperature controlled during its annealing process to increase its quality factor for optoelectronic oscillator application.

We present results obtained during investigation of synchronously pumped optical parametric oscillator (SPOPO) with broadband complementary chirped mirror pairs (CMP). The SPOPO based on β–BBO nonlinear crystal is pumped by second harmonic of femtosecond Yb:KGW laser and provides signal pulses tunable over spectral range from 625 to 980 nm. More than 500 mW are generated in the signal beam, giving up to 27 % pump power to signal power conversion efficiency. The plane SPOPO cavity mirror pairs were specially designed to provide 99 % reflection in broad spectral range corresponding to signal wavelength tuning (630–1030 nm) and to suppress group delay dispersion (GDD) oscillations down to ±10 fs². Dispersion properties of designed mirrors were tested with white light interferometer (WLI) and attributed to the SPOPO tuning behaviour.

The non-homogeneous multilayer medium was proposed for laser thermal recording. The mathematical model of laser thermal recording based on the phase transition of non-homogeneous medium (such as melting, ablation, evaporation) to simulate and optimize was developed. It was shown that in multilayer recording medium can be performed narrower structures than in monolayer films on similar conditions. It was shown that for getting small structures the thermoconduction of recording medium should be much smaller than the thermoconduction of substrate on which a medium is deposited. It was demonstrated that in the case of strong light absorption in recording layer its thermal conductivity must be sufficiently large to ensure the transfer of heat on a distance that equals to the thickness of the layer during a time interval equals to the difference in time for warming up to the phase transition temperature of the central part of the light spots and area that separated from a center of light spot on a distance equals to the thickness of layer.

Modeling of self-diffraction pattern formation from the induced diaphragm, arising in the case of the transparency channel saturation by one-photon resonant non-stationary excitation of the basic exciton transition in colloidal quantum dots (QDs) is realized. The simulation results allow us to obtain the reference image of self-diffraction pattern and dependence of the intensity transverse distribution of the output beam from the intensity of the excitation beam, forming a transparency channel. A powerful laser pulse creates a transparency channel, so that it self-diffracts on the induced diaphragm. The possibility to apply the obtained simulation results for intensity estimation of the laser radiation and for the possible application in the technique (nonlinear-optical limiters of intense laser radiation in the visible and nearinfrared region, optical switches) are discussed.