Fundamental Light Beam Research Articles

Optical characterization of single nanostructured gradient index vortex phase masks fabricated by the modified stack-and-draw technique

The optical performance of nanostructured gradient index vortex phase masks (VPMs) in various external media was investigated. We utilized a modified stack-and-draw technique to fabricate the microscopic-size masks. Its structure contains spatially distributed sub-wavelength rods made of two thermally matched glasses with high and low refractive indices. Measured optical characteristics of the generated vortices with azimuthal phase distribution are presented. The numerical and experimental study shows that such VPMs with the same designed structure but varied thickness can convert fundamental light beam into optical vortices with different charges. Furthermore, we found that the mask performance is unaffected by the refractive index of the surrounding media making it an excellent candidate for microfluidic applications.

Generation and Transformation of Light Beams and Pulses, Containing Polarization Singularities, in Media with Nonlocality of Nonlinear Optical Response (Scientific Summary)

We discuss analytically found expressions, which relate the values of two parameters characterizing the topological type of linear and circular polarization singularities in nonparaxial light fields to the values of the complex amplitude components of the electric field and their first spatial derivatives. The necessary conditions are described for the formation of light pulse at the doubled frequency in the bulk of an isotropic gyrotropic medium with a frequency dispersion of quadratic nonlinearity and they impose constraints on the spatial transverse structure of the non-uniformly polarized incident radiation pulse and its time structure. The electric field of a light beam reflected from the surface of an isotropic gyrotropic medium with spatial dispersion of quadratic nonlinearity is investigated in the case of an arbitrary structure and geometry of incidence of non-uniformly polarized fundamental light beams. The ranges of the parameters of an elliptically polarized Gaussian beam and a medium with local and nonlocal cubic nonlinearity are determined, at which the lines of circular polarization singularity appear in cross sections of self-focused beam.

Interconversion between the orbital and spin angular momentum of light beams in three-wave mixing processes in the bulk of an isotropic chiral medium.

A phenomenological description of nondegenerate and degenerate three-wave mixing in the bulk of an isotropic chiral medium is given for the wide class of fundamental light beams with nonuniform polarization distribution and multimode transverse structure. Special attention is paid to the classical and quantum interpretation of interconversion between spin and orbital parts of the angular momentum of the input and output beams.

Theoretical investigation of impedance matching in the process of sum-frequency generation in an external resonator

The sum-frequency conversion efficiency is directly proportional to the product of two fundamental laser powers. Therefore, sum-frequency conversion efficiency is rather low when the fundamental beams pass through a nonlinear crystal only once. External resonant technique as an effective means of improving the powers of the fundamental light has been widely applied to the field of nonlinear frequency conversion. This technique can greatly improve the sum-frequency conversion efficiency and is particularly suitable for the situation in which the input power of the fundamental frequency lasers bas been limited. The implementation of high efficient sum-frequency generation in an external resonator requires that the fundamental frequency laser should be efficiently coupled to the external cavity. Therefore, the system needs to achieve impedance matching. In the part of theoretical analysis, first, we derive the enhancement factor when travelingwave cavity is resonant, and then, establish the theoretical models of doubly resonant and singly resonant sum-frequency generation in an external resonator respectively. The variation of enhancement factors as functions of reflectivity of the input couplers and power of the input fundamental light for doubly resonant and singly resonant sum-frequency systems is derived from Boyd-Kleinman theory in detail based on the theoretical models described in the text. The expressions of enhancement factors reflect the nonlinear correlation characteristics of two fundamental light beams in the process of sum-frequency generation. In the part of numerical simulation, firstly, we draw contour plots of output power as functions of reflectivity of the input couplers at two input frequencies in the doubly resonant sum-frequency system by theoretical simulation, and achieve an optimum reflectivity of the input couplers under the condition of different powers of input fundamental light. Secondly, we draw the contour plots of output power as functions of the reflectivity of the input coupler at the resonant frequency, and the input power of non-resonant frequency light in the singly resonant sum-frequency system by theoretical simulation, and achieve an optimum reflectivity of the input coupler at the resonant frequency. These optimum values enable the system to achieve impedance matching; consequently, the sum-frequency conversion efficiency is improved. Finally, this paper analyzes the influence of input power on the impedance matching, and shows that the optimal coupling mirror reflectivity of the resonant fundamental frequency will decrease with the increase of incident power of the other resonant or non-resonant fundamental frequency laser, otherwise, the resonant incident power of its own has less influence on the optimal coupling mirror reflectivity, whether the system undergoes doubly resonant or singly resonant sum-frequency. In addition, if the coupling mirror reflectivity exceeds the optimum value, the power of sum-frequency light will decrease rapidly, while if it is less than the optimum value, the power of sum-frequency light decreases relatively slowly. Therefore an input coupler that may yield over-coupling should be avoided. These results will have a certain guiding significance to related experiments.

Broadband cascading of second-order nonlinearity in randomized nonlinear photonic crystal

We study both experimentally and theoretically optical parametric process in one-dimensional nonlinear photonic crystal with random variation to the sign of the second-order nonlinear coefficient. We demonstrate that the structure randomness enables broadband third-harmonic generation via cascading of two second-order nonlinear parametric wave interactions with the overall conversion efficiency higher than previously recorded in fully disordered media such as the as-grown strontium barium niobate crystal. Furthermore, thanks to the one-dimensional modulation of the second-order nonlinearity, the emitted broadband harmonics preserve well the spatial intensity profile of the incident fundamental light beam, a feature critical for applications of optical frequency conversion but unavailable in two-dimensional fully random media due to the beam divergence caused by the simultaneous emission of noncollinear harmonic.

Interaction of two-dimensional orthogonally polarised super-Gaussian light beams in a photorefractive crystal

Propagation and interaction of orthogonally polarised two-dimensional super-Gaussian light beams is studied theoretically in a 4mm-symmetry photorefractive crystal in the drift regime when the external electric field is applied to the crystal in the direction of the optical axis. The output beams displaced with respect to each other in directions parallel and perpendicular to the direction of the external electric field strength vector are considered. It is shown that an auxiliary light beam polarised orthogonally to the fundamental light beam makes it possible to carry out efficiently address localisation of the fundamental beam propagating in a quasi-soliton regime. The crystal thicknesses are found, which are optimal from the viewpoint of maximisation of the fundamental light beam deviation. It is shown that 'square' super-Gaussian beams in the near-field diffraction region are focused at smaller values of the external electric field than those of the Gaussian beams.

SHG from bulk and surface of nanoparticle composites

Three wave mixing processes such as second harmonic generation (SHG) have proven to be a sensitive probe of buried interfaces. Recently SHG has been employed to monitor the surface of Si nanocrystals within a glass matrix. Due to the macroscopic homogeneity and centrosymmetry of the composite, its bulk SHG signal is of multipolar character, produced by the non-uniformity of the fundamental light beam, and can be enhanced several orders of magnitude by employing two interfering crossed beams. On the other hand, there are surface contributions to the SHG of the composite due to the relatively large inhomogeneities of the surface local field which act on the nanoparticles and to anisotropic particle shape fluctuations. These yield dipolar contributions which are therefore insensitive to the beam profile and are not enhanced in a two-beam geometry. We calculate the SHG from a thin nanocomposite material and compare the relative strength of its surface and bulk contributions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Second-harmonic generation device with integrated periodically domain-inverted regions and distributed Bragg reflector in a LiNbO3 channel waveguide

We propose a new structure of LiNbO3 guided wave second-harmonic generation device, in which the fundamental light beam is sufficiently confined by two distributed Bragg reflectors (DBRs) formed at input and output ports. The 40% increase in the conversion compared with a conventional device without DBR was achieved under a quasi-phase matching condition.