Valentin I. Vlad

Stimulated brillouin scattering : fundamentals and applications

Spontaneous and stimulated scattering of the optical field. The one-dimensional SBS model-analytical solutions. Optical phase conjugation in SBS. Analytical solutions of the three-dimensional SBS model. SBS in optical fibers. Laser resonators with SBS mirrors. Optical solitons in SBS. Appendix 1: Selected materials for SBS. Appendix 2: Averaging of a Gaussian process in SBS. Appendix 3: Calculation of the SBS reflectivity with transverse effect corrections. Appendix 4: Calculation of SBS topological soliton solution in terms of the inverse problem in scattering theory

Saturated near-resonant refractive optical nonlinearity in CdTe quantum dots

In this work, we report on an experimental investigation of the nonlinear optical properties near the first electronic resonance of thiol-capped CdTe quantum dots (QDs) being in the strong confinement regime. Using a cw laser excitation in a Z-scan experimental setup, we show the presence of saturated Kerr-type nonlinear optical properties of the QDs, at low intensity levels. The large optical nonlinearity and the control of the linear and nonlinear optical properties by the size of the QDs are of special interest for applications in integrated nanophotonic devices.

Stationary self-confined beams at 633 nm in Bi12SiO20 crystals

Self-confinement of light beams at the wavelength of 633 nm is presented for the first time in Bi12SiO20 (BSO) photorefractive crystals. The 633 nm wavelength is slightly absorbed in BSO, not enough for efficient photorefractive nonlinearity. However, superposing on the 633 nm beam a second beam at 514.5 nm, two-step absorption processes can generate a hot free-carrier population in the conduction band. If a bias voltage is applied, the conducting electric field, responsible for the current flowing inside the crystal, is spatially modulated according to the red-beam profile and a spatial modulation of the material dielectric constant occurs. Efficient self-trapping is demonstrated both from the theoretical and experimental points of view.

3D-soliton waveguides in lithium niobate for femtosecond light pulses

We show that efficient waveguides can be written by bright spatial solitons in the volume of lithium niobate photorefractive crystals by cw and pulsed laser beams. Using high-repetition-rate femtosecond laser pulses, an efficient formation of soliton waveguides (SWGs) is possible, after accumulating a large number of pulses, because the characteristic photorefractive build-up time is much longer than the pulse period and the efficient two-photon absorption may contribute to the solitonic confinement. These results open the possibility of writing reconfigurable single SWGs and SWG arrays (with any spatial orientation and large range of periods) and optimally guiding the femtosecond pulsed laser beams through them, creating a graded refractive-index profile matched to the spatial beam profile. Our experiments also show a small increase in pulse duration (small dispersion) in these waveguides.

Experimental demonstration of (1+1)D self-confinement and breathing soliton-like propagation in photorefractive crystals with strong optical activity

We report the first experimental observation of (1+1)D self-confinement of laser beams in BSO crystals, which are photorefractive materials with strong optical activity (45??mm-1). The BSO crystal was biased along the [001] crystallographic direction while the light beam propagated along . Breathing of such beams has been monitored.

Complete characterization of (2 + 1)D soliton formation in photorefractive crystals with strong optical activity

We analyse the (2 + 1)D soliton formation in photorefractive crystals with strong optical activity. The complete characterization of a soliton in such a crystal requires a careful determination of its polarization state. In fact, even if the beam experiences an homogeneous propagation, the polarization dynamics require it to reach a well defined state which has been identified on the Poincare sphere. Experimental results for the soliton intensity profile and for its polarization dynamics are in good agreement with numerical simulations.

New treatment of the focusing method and tomography of the refractive index distribution of inhomogeneous optical components

A new mathematical formulation of the focusing method connecting the refractive index distribution of inhomogeneous optical components to the transversally transmitted light distribution is presented. This formulation has the following advantages: the characteristic singularity occurring in the focusing method is avoided; a single numerical integration is used instead of a double one, ensuring a faster data processing; the refractionless approximation is introduced only in the final step of calculations, leading to smaller errors; and the avoidance of a residual logarithmic singularity. An inverse functional transform connecting the intensity distribution in the near field to the refractive index distribution inside the object is deduced and used to obtain analytical test functions. The theoretical results are supported by computer simulations and experiments with gradient-index rods and optical preforms, which have shown improvements in the accuracy, stability, and speed of the method. Moreover, this mathematical formulation for the axisymmetric objects is generalized to a tomographic formulation for nonsymmetric objects using the inverse Radon transform. An efficient algorithm and suitable computer simulations are presented for this case.

Use of quasi-local photorefractive response to generated superficial self-written waveguides in lithium niobate

We report the formation of surface self-written waveguides by means of surface pyrolitons in lithium niobate. By a specific orientation of the crystal axis the quasi-local slow photorefractive response of lithium niobate was used to induce a self-confined beam exactly at the crystal-air interface. The mode profile of the photo-induced waveguide is strongly asymmetric due to the interface presence.

Optical limiting in CdTe nanocrystals embedded in polystyrene

In this work, we demonstrate optical functionalities obtained with CdTe nanocrystals embedded in polystyrene. These functionalities are based on our experimentally observed large, saturable, and controlled nonlinear optical properties of CdTe nanocrystals, in the case of strong quantum confinement and near resonant interaction with the excitation light. Our investigation considers the optical limiting functionality, presenting experimental proof of concepts. These types of functionalities are of special interest for integrated optical quantum dots devices with applications in imaging and telecommunications.

Z-Scan measurement of thermal optical nonlinearities

The thermal third order nonlinearity of a neutral density glass is measured using the Z-Scan method. The measurements are performed using two different laser configurations: a continuous wave laser at 532 nm and a femtosecond laser at 1060 nm. The measurements are used to determine the nonlinear refractive index, n2 and the thermo-optical coefficien dn/dt of the samples. The measurements in the two different laser configurations are in good agreement with the existing theory models.