We describe the most relevant optical properties of BiB₃O₆ (BIBO), that make this material an attractive candidate for nonlinear frequency conversion of laser light in general, and down-conversion of femtosecond laser sources operating near 800 nm in particular. Experimental results are presented in the high-energy, low-repetition-rate (1 kHz) regime, demonstrating the unique versatility of BIBO for efficient generation of femtosecond pulses in the infrared up to ~3 μm.

A 2D study in focus point of two identical femtosecond laser sources introducing a temporal displacement for ultra-short pulses generation based on Coherent Beam Combining was developed. This study is based on numerical simulation which shows the spatial and temporal properties of the electromagnetic field. The piston error is introduced by moving one of these two laser sources in the same direction, while the other laser source is fixed. The interval considered for the laser source displacement (piston error) is between lambda/10 and 40 lambda. The results are relevant for the coherent beam combination at the Extreme Light Infrastructure facility.

We review the impact of major effects taking place in high peak power short pulse CPA lasers, that modify the spectral amplitude and phase of a recompressed pulse and thus produce features that precede the main recompressed pulse. Especially effects associated with clipping of the spectrum, influence of the finite beam aperture on appearance of the pedestal preceding the main pulse (over the dynamic range more than 10¹⁰) as well as steepness of the leading front are considered.

A nanosecond Nd:YAG based high-energy kilohertz Master Oscillator Power Amplifier (MOPA) system has been developed. The Q-switched master oscillator emits 6.5-ns, 1-kHz pulses with energy up to 1.6 mJ. The design of the oscillator ensures TEM₀₀ operation with high stability and insensitivity to cavity misalignment. Two different amplification schemes - slab and rod geometry of the amplifiers have been studied. We have developed a compact, five-pass gain module with slab design, pumped by four 30-W collimated laser diode bars. The amplified output energy is 8 mJ at 1 kHz. The optical-to-optical conversion efficiency is 15 %. The second scheme is based on side-pumped rod preamplifier with consequent end amplifier with depolarization compensation. This system delivers up to 18 mJ linearly polarized TEM₀₀ pulses - corresponding to 5 % optical-to-optical conversion efficiency.

Experimental results on passive mode-locking of Nd:YVO4 laser using intracavity frequency doubling in periodically poled KTP (PPKTP) crystal are reported. Both, negative cascaded chi-2 lensing and frequency doubling nonlinear mirror (FDNLM) are exploited for the laser mode-locking. The FDNLM based on intensity dependent reflection in the laser cavity ensures self-starting and self-sustaining mode-locking while the cascaded chi-2 lens process contributes to substantial pulse shortening. This hybrid technique enables generation of stable trains of pulses at high-average output power with several picoseconds pulse width. The pulse repetition rate of the laser is 117 MHz with average output power ranging from 0.5 W to 3 W and pulse duration from 2.9 to 5.2 ps.

Mode locking with frequency dependent nonlinear mirror of Nd:YAG laser with electro-optical negative feedback has been presented. We use a LBO based nonlinear mirror thermally stabilized at a temperature corresponding to noncritical phase matching of the frequency doubling at 1.064 μm. Employing a resonator polarization output detected by a fast photodiode we control resonator losses through a Pockels cell. By creating a negative feedback we achieved q-switch suppression, thus stabilizing the mode-locking regime. Through a simple theoretical model we derived a relation for the laser parameters for stable laser operation. We achieved a stable train of picosecond pulses grouped in the 200 microsecond pump macropulse. Macropulse output energy is 4 mJ at a pump repetition rate of 400 Hz. The picosecond pulse train frequency determined by the resonator length is 115 MHz.

A fully quantum mechanical treatment is presented for the Cherenkov radiation stimulated by a uniformly moving charge particle in both isotopic medium and anisotropic medium. The relation between Cherenkov radiation intensity and dielectric tensor is analyzed, and the result is compared with previous work.

We demonstrate a four-stage optical parametric chirped-pulse amplification system that delivers carrier-envelope phasestable ~1.5 μm pulses with energies up to 12.5 mJ before recompression. The system is based on a fusion of femtosecond diode-pumped solid-state Yb technology and a picosecond 100 mJ Nd:YAG pump laser. Pulses with 62 nm bandwidth are recompressed to a 74.4 fs duration close to the transform limit. To show the way toward a terawatt-peakpower single-cycle IR source, we demonstrate self-compression of 2.2 mJ pulses down to 19.8 fs duration in a single filament in argon with a 1.5 mJ output energy and 66% energy throughput.

The interaction of an extremely intense laser beam with the dense plasma generated on a solid target can lead to coherent electron dynamics at the vacuum plasma interface which causes the emission of ultra short light bursts within each optical cycle of the driving laser pulse. Therefore, the resulting pulse train consists of high harmonics of the laser fundamental frequency. We report on the, to our knowledge, first observation of surface-harmonics up to the 42nd order without any contrast improvement by a plasma mirror. In the spectral region from the 25th to the 40th order approximately 10⁷ photons were detected in a solid angle of 4 • 10^-4 sr.

We present a two and three-beam pumped optical parametric amplifier seeded by chirped pulses from broadband Ti:sapphire oscillator. The seed source was optically synchronized with Nd:YAG pump laser. The signal was amplified in type I BBO crystal pumped by several intersecting beams and the energy of amplified signal ranged from 0.23 mJ to 0.72 mJ depending on a number of pump beams used as well as pumping configuration employed. The interplay of interacting waves was also observed and explained theoretically.

We present a novel interferometric setup working in the XUV spectral range. The interferometer consists of a multi-pinhole mask and a transmission grating. In the case the light source consists of discrete spectral lines as is the case for laser-generated high harmonics the interferometer is capable of recording interferograms for multiple colors simultaneously. The device presented in this publication is an improvement of our recently published setup in a way which allows, in principle, for single-shot measurements of the temporal coherence length of a set of high harmonics. We present an experiment in which the influence of a spectral chirp of the driving laser on the coherence length of the generated harmonic radiation is studied.

In this paper, we give a brief review on different types of applications relying on tilted-pulse-front pumping or angular dispersion. A short overview of the connection between pulse front tilt and angular dispersion is followed by discussing examples of achromatic phase matching in frequency conversion processes, tilted-pulse-front pumping for phasematched high-field THz pulse generation, as well as pumping of short-wavelength x-ray lasers, where tilting the pump pulse front is utilized for synchronization. The connection between pulse front tilt and angular dispersion is also addressed for some of the applications.

In linear regime the single and few cycle femtosecond (fs) pulses propagates on several diffraction lengths changing their shape in semi-spherical (or parabolic) form, far away from plane wave and paraxial approximation. The experiments with fs pulses in air with power little above the critical for self-focusing (nonlinear regime) demonstrated new nonlinear effects as self-guiding, THz longitudinal emission and superbroad enlarging of the spectrum. In this paper we continue with developing of new theoretical model for quantitative description of the above experimental results.

Z-scan is a well established sensitive and accurate technique used to determine nonlinear absorption and refraction. In this paper a review-like study of the Z-scan applications for the examination of LiNbO₃ crystals with different stoichiometry and with different dopants will be presented. By the extension standard Z-scan setup the cubic, the thermo-optical and the photorefractive nonlinearity can be examined. Theoretical calculations together with the measurements make possible to characterize quantitatively the thermo-optical nonlinearity and to determine the threshold dopant concentration of the photorefractive damage which is very important for nonlinear optical applications. Pure and Mg doped stoichiometric as well as congruent, and In, Hf, Zr or Y/Mg doped congruent LN samples were examined. Furthermore, a Z-scan theory based on the solution of the nonlinear paraxial wave equation, completed by the Huygens-Fresnel principle is introduced. This theory is valid for the general case, i.e. for thick samples and strong nonlinearities including both nonlinear refraction and absorption.

We report numerical simulations supported by experimental observations of self-focusing, fillamentation, and supercontinuum generation by an optical vortex beam in a Kerr nonlinear medium in the regime of dominating nonlinearity. Despite the strong self-focusing resulting in multiple filaments ordered along the vortex ring the optical vortex remains well preserved at the exit of the nonlinear medium and in the far-field. The presented quasi-(3+1)- dimensional numerical simulations under azimuthal initial vortex ring perturbations confirm qualitatively the experimentally observed survival of the optical vortex in the course of the white light generation.

In this work we report observation of ultra-fast switching of vacuum ultra-violet (VUV) light caused by saturable absorption by a solid metal foil. A sub-picosecond VUV pulse from a free-electron laser located in SPring-8 is focused on a metal target and transmission is measured as a function of input energy, thickness of the absorbing layer, and VUV laser wavelength. As is well known, metals have a strong linear free electron response associated with the plasma oscillation and collisional absorption (high-frequency resistivity). Due to the plasma screening and strong absorption, it is difficult to use bulk metals for optical components. However, above the plasma frequency as in our experiments, a metal can transmit light and shows phenomena related to the band gap structure, similar to the optical properties observed in transparent materials for visible and infrared light. We observe a strong gating of Sn transmission at energy fluences above 6J/cm² at wavelength of 51nm. The ratio of the transmission at high intensity to low intensity is typically greater than 100:1. The estimated saturated transmittance is about 0.25. The mechanism of the switching phenomena is partially explained by the shift of Sn N shell band edge, however, more details should be investigated with more exact physical models and precise measurements. We think this is the first observation of such a strong nonlinear phenomena for VUV light and this result will promote the development of new nonlinear photonic devices such as auto-correlator and pulse slicer for the VUV region.

We present the results of numerical modelling of population dynamics in a hydrogen atom under the action of laser pulses of various duration, intensity and frequency. The possibility of formation of wave packets, including the states with high values of orbital and magnetic quantum numbers, via multiple transitions between the states of discrete and continuous spectrum under the action of the laser field is studied. The model is based on the expansion of the electron wave function over a large basis of hydrogen eigenstates of discrete and continuous spectrum. Ionization losses are taken into account by using a realistic model of the continuum. Partial localization of electron density in radial and angular variables is demonstrated. Most suitable conditions for wave packet formation are discussed for attainable lasers parameters.

We describe the fabrication and characterization of partially ordered 2D photonic structures prepared by domain inversion in LiNbO3. Two types of structures have been prepared: (i) decagonal quasi-crystals designed by tiling the plane with different kinds of rhomb shaped tiles and (ii) a short-range ordered structure designed by placing randomly oriented basic units on a square lattice. Both types of structures exhibit rich sets of reciprocal vectors that may be used to satisfy the phase matching conditions for different nonlinear optical processes. As examples we describe collinear second harmonic generation of multiple wavelengths in a single decagonal nonlinear photonic quasi-crystal and cascaded third harmonic generation in a short range order nonlinear photonic crystal.

In this paper, we report about a numerical and experimental study of a high energy, micro-joule range, supercontinuum (SC) spectrum which is generated in a large mode area (LMA) photonic crystal fiber (PCF). Our experiment consists of launching a train of 100 femtosecond pulses into a 20 cm-long span of a PCF delivered from a Ti:Sapphire pump parametric amplifier. The optical properties of the PCF were accurately calculated using a finite element method mode solver and the real cross section of the fiber. The PCF exhibits a zero dispersion wavelength at 1250 nm and has an effective area of 660 μm² at λ=1250 nm. We observed an octave-spanning SC with spectral components propagating in the fundamental mode. The physical processes leading to the construction of the continuum spectrum were studied by monitoring the growth of the SC while increasing the input optical power. The main mechanisms behind the spectral broadening are mainly ruled by the effects of self-phase modulation, the stimulated Raman scattering, and the soliton propagation. Our experimental results are compared with the numerical solution of the nonlinear Schrodinger equation and good agreement between experimental and numerical results is found.

In the present paper we address several aspects of ionization-induced laser-gas interaction. First, we consider the ionization dynamics of an ultrashort laser pulse in the presence of additional electromagnetic perturbations, and show theoretically via dispersion relation analysis and numerically via 2D FDTD simulation that ionizationinduced scattering can occur even in the case of limited spatial and temporal scales and significantly affects pulse dynamics. Second, for the case of tight focusing of laser beam we show on the basis of numerical simulation that for 2D TE- and TM-polarized pulses there is a critical angle which delimits two qualitatively different regimes. For angles exceeding the critical one, the formed plasma distribution may become microstructured, otherwise the plasma structures are smooth. It is also shown than the critical angle and plasma-field dynamics depend significantly on pulse spectrum. Finally, we consider the impact of the electron collisions and Kerr nonlinearity and determine the boundaries within which the role of these effects is crucial.

We show theoretically and prove experimentally that with holographic-cut cubic crystals circularly polarized light can also be nonlinearly modified. Although the efficiency of the process is lower it is comparable with the efficiency of cross-polarized wave generation with linearly polarized input light.

A germanium-doped silica-core fiber with an active region in the form of a thin ring of silica doped with bismuth ions was fabricated. Bismuth doping in the ring surrounding the core allows to stabilize bismuth in silica glass, and it does not impose any restrictions on the composition of the core. The bismuth concentration in the ring is less than 0.2 wt.%. The GeO₂ concentration in the core is more than 15 mol.%. A high germanium concentration in the core allows to shift the zero dispersion wavelength to 1860 nm and to obtain a high nonlinear refractive index (n₂ more than 3,2*10^-20 m²/W). Spectroscopic investigations were carried out in the visible and near infrared (800-1700 nm) spectral range. Despite the small concentration of bismuth, we observed the absorption and luminescence characteristic bands, confirming the presence of bismuth active centers in silica glass. Upon pumping at 1350 nm the on/off gain spectrum was measured on a 20-m fiber. The gain was observed throughout investigated range of 1430-1530 nm. The maximal gain of ~9.5 dB was obtained near 1430 nm. The results of the spectroscopic investigations of the fiber with a thin active Bi-doped ring showed prospects of the creation and application of such fiber type for laser and nonlinear optics.

The luminescence response of a new coordination complex of europium Eu(III) ion, namely tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono]Eu(III) 1,4-diaza-bicyclo[2.2.2]octane, applicable in microbiology, biochemistry and molecular biology, is probed upon ultra-high intensity photo-excitation with high-power femtosecond laser pulses. The use of laser light source with ultrashort and high-power pulses should increase by many orders of magnitude the intensity of photoexcitation, and therefore the efficiency of the bioassays and fluorescent microscopy in the research on molecules and structures of biomedical and pharmacological interest, labeled with luminescent complexes. The results obtained show that the luminescence response of the examined compound, even in dilute aqueous solutions is sufficient for fluorometric applications, such as luminescent labels in biomedical diagnostics and bioanalyses, as well as in laser scanning fluorescent microscopy.

In this report three types of optical polymer thin films deposited on glass substrates are investigated. Transmission spectra of the polymer samples are obtained in the range from 400 nm to 1500 nm. A laser microrefractometer has been used to measure the refractive indices of the examined materials at 406, 656, 910 and 1320 nm. Dispersion properties of the polymer films are analyzed on the base of the Cauchy-Schott's and Sellmeier`s approximations. Dispersion coefficients are calculated and dispersion charts in the visible and near infrared spectral regions are presented and compared. Abbe numbers of mean and partial dispersion of the polymer films are obtained. Calculation of refractive indices at many laser emission wavelengths in the considered spectral range is accomplished.

A strong laser-induced thermo-lens (LITL) effect is found in optically-transparent ion-implanted polymer upon irradiation by a cw laser with a power up to 100 mW (λ = 532 nm). The effect is observed in bulk polymethylmethacrylate (PMMA) implanted with silicon ions (Si⁺). A series of PMMA specimens is examined, subjected to low-energy (50 keV) Si⁺ implantation at various dosages in the range from 10¹⁴ to 10¹⁷ ions/cm². The thermo-lensing is unambiguously attributed to the modification of the subsurface region of the polymer upon the ion implantation. Having a gradient refractive-index in-depth profile, the subsurface organic-carbonaceous layer produced in the polymer by ion implantation, is responsible for the LITL effect observed in reflection geometry. The LITL occurs due to optical absorption of the ion-implanted layer of a thickness of about 100 nm buried in a depth ~ 100 nm, and subsequent laser-induced change in the refractive index of the Si⁺-implanted PMMA. Being of importance as considering photonic applications of ion-implanted optically-transparent polymers, the LITL effect in Si⁺-implanted PMMA is studied as a function of the implant dose, the incident laser power and incidence angle, and is linked to the structure formed in this ion-implanted plastic.

We study two-degree of freedom continuous nonlinear dynamical system introduced earlier³ for description of dual-core nonlinear directional coupler. The stationary points of this dynamical system are calculated as a function of dimensionless parameter that describes the material and pulse properties. The stability of obtained stationary points is studied. Stochastic motion has been observed

Description of optical system with saturable amplification, saturable losses and filtering^9-10 (see also^11-12) is studied. The aim of this work is to derive the system of ode's, that extends the earlier one^1-2, including additionally the temporal dependences of the saturated gain and losses. First, this extended system is applied to the case of fast changes in the amplification and losses^7-8. Regarding the dissipative solitons, earlier results^7-8 have been confirmed. In addition, nonlinear fixed points have been also studied and as result fronts were identified. Next, the extended system is applied to the case of temporal dependent amplification and losses^9-10. Linear and nonlinear fixed points of this system have been calculated and their stability identified. Typical solutions discussed earlier⁹ have been found. Fronts have been also revealed. Phase space interpretation of obtained coherent structures is given.

The influence of disordering of one-dimensional quasi-phase matched structures in nonlinear optical crystals on spectral characteristics of parametric processes is studied. Two different correlation functions are introduced for describing the random change of the sign of the second-order nonlinear susceptibility. The optimal conditions for quasi-phase matching of the interacting waves are found. The second aim of the paper is the investigation of important features of spontaneous parametric down-conversion (SPDC) in a homogeneous nonlinear slabs with emphasis of spatial inhomogeneity of interacting waves (in paraxial approximation). We estimate the absolute values of the emission rate of photon pairs produced by SPDC when all fields are in single Gaussian modes.

Using the spectral approach for description of the processes in nonlinear optics, by numerical calculations we obtain the dependences of the third harmonic generation (THG) on the output spectra and spectral energy, spectral width and spectral barycenter in the presence of quadratic and cubic spectral phase terms in the input. The main equation that describes the evolution of the spectral properties for the process of THG is derived. The suggested model could be applied also for analysis of a few order high harmonics.

Linear and nonlinear optical methods play an important role in surface science. On a microscopic level the theoretical understanding of the surface-specific second-order response is still incomplete. We discuss the calculation of linear and nonlinear optical response functions for Si(111) surfaces within a real-space and time-dependent approach. The electronic structure is modeled by cyclic-cluster configurations in real space using a tight-binding parametrization of the system hamiltonian. We apply a time-dependent formalism based on an equation of motion for the one-electron density matrix to obtain the linear S(τ) and nonlinear S(τ₁, τ₂) response functions for the optical polarization which also describe the corresponding response to ultrashort pulses. The Fourier transforms of S(τ) and S(τ₁, τ₂) are the frequency-dependent optical susceptibilities for the linear X⁽¹⁾(-ω;ω), and second-order response, X⁽²⁾(-ω₁-ω₂; ω₁,ω₂), for sum-frequency and second-harmonic (ω₁=ω₂) generation from surfaces. By a time-dependent Hartree-Fock method we treat the excitonic effects due to the Coulomb interaction between the photoexcited electrons and holes. Results for the spectral dependencies of the linear and nonlinear optical susceptibilities are presented.

We analyze a class of multicomponent nonlinear Schrödinger equations (MNLS) related to the symmetric BD.I-type symmetric spaces and their reductions. We briefly outline the direct and the inverse scattering method for the relevant Lax operators and the soliton solutions. We use the Zakharov-Shabat dressing method to obtain the two-soliton solution and analyze the soliton interactions of the MNLS equations and some of their reductions.

The new nonlinear crystal of magnesium sulfite hexahydrate (MgSO₃.6H₂O) belongs to the rare crystallographic class C₃ (without a symmetry centre), the other known only representative being sodium periodate (NaIO₄). There are some scarce data in the scientific papers about magnesium sulfite hexahydrate's physical properties. Single crystals of significant sizes (up to 40-50 mm) of MgSO₃.6H₂O as well as such, doped with Ni, Co, and Zn, for the time being are grown only by our own method developed in the Laboratory for Crystal growth at the Faculty of Physics of Sofia University. Recently we have observed the supposed presence of optical activity. Circular dichroism is not observed in pure MgSO₃.6H₂O. The results of the first ever investigations are presented, which demonstrate the presence of circular dichroism in MgSO₃.6H₂O doped with Co. The circular dichroism appears in the spectral range from 420 nm to 580 nm. The spectrum of circular dichroism demonstrates a well expressed structure - an isolated maximum at 470 nm and a combination of two overlapped maxima at ~ 495 nm and 520 nm respectively. The spectrum of the circular dichroism is compared with the absorption spectrum of MgSO₃.6H₂O:Co, along the direction (0001) and with the linear dichroism spectrum measured in direction(1210). It is shown in this way that the circular dichroism appears only in the spectral range of the optical absorption structure due to Co dopant. In the same spectral range can be observed also the linear dichroism determined by Co presence in MgSO₃.6H₂O.