Shape and polarization distribution of non-circular conical diffraction beams from conjugate cascades.

Peculiar non-circularly shaped vector type beams can be obtained naturally by the conical diffraction phenomenon if specific manipulations in wavevector space are performed between optically biaxial crystals arranged in a cascade. We analyze in detail this situation by focusing on the general shapes and the polarization distribution. Both are shown to be correlated to the values of structure parameters introduced in this work. These control parameters depend on the conical diffraction cone aperture angle, on the crystal lengths, and on the magnification values due to x- and y-oriented cylindrical lenses placed between the crystals and coupling common conjugate planes. The local polarization is found to be always linear with the exception of regions where structures composing the pattern intersect or overlap, where elliptical or circular polarization can occur. The way in which the obtained patterns depend on the orientation of individual crystal samples around the common optical axis and on an eventual polarization filtering at various stages of the cascade is discussed as well. Theoretical and experimental findings agree well, as verified for the case of a cascade of two crystals.

Non-circularly shaped conical diffraction.

Waves with tailored shape and vectorial non-homogeneous polarization are of much interest due to the many prospects for relevant applications in the classical and quantum domains. Such vector beams can be generated naturally via conical diffraction in optically biaxial crystals. The recent strongly revived attention to this phenomenon is motivated by modern applications such as optical trapping, polarimetry or super-resolution imaging, partly enabled by new configurations increasing the beam complexity, like those with several crystals in cascade. However, up to now all beams generated by conical diffraction conserve at their sharpest plane the underlying circular shape connected with the planar section of light cones. Here we show that a proper manipulation in wave-vector space within a conical diffraction cascade produces vector beams with highly peculiar non-circular forms, leading to an interesting and reconfigurable platform for easily shaping all structured wave properties, increasing complexity and information content. The experimental observations are confirmed by numerical integration of a paraxial model incorporating the effects of the wave-vector space manipulation.

Polarization-independent optical isolator in a Sagnac-type configuration.

We propose a polarization-independent optical isolator that does not use walk-off between the two orthogonal polarizations. The design is based on two Faraday rotators in combination with two half-wave plates in a closed, Sagnac-interferometer-like configuration. An experimental prototype is tested successfully under variation of the input light polarization state with isolation level between 43 dB and 50 dB for all input polarizations (linear, circular, or elliptical).

Robust, efficient, and broadband SHG of ultrashort pulses in composite crystals.

We experimentally demonstrate efficient second-harmonic generation (SHG) of tunable ultrashort pulses of 100 femtoseconds, using a novel method based on composite segmented periodically poled (CSPP) design. The scheme was borrowed from the nuclear magnetic resonance (NMR) composite pulses (CP) of Shaka and Pines. Using CSPP, a broadband and efficient conversion over a bandwidth of 35 nm in very short interaction length was achieved. In addition, CSPP showed robustness to temperature changes up to 90°C. Two CSPP schemes with 15 and 31 segments and periodically poled design were implemented on Mg-doped LiNbO3 crystal. The CSPP performance was shown to be superior in all aspects. The experimental results are compared to numerical simulation with excellent agreement.

Complex beam shaping by cascaded conical diffraction with intercalated polarization transforming elements.

Cascaded conical diffraction where optical elements modifying the local polarization state are intercalated between the aligned biaxial crystals is analyzed theoretically in the framework of paraxial diffraction theory. The obtained expressions are verified and confirmed experimentally for the case of a two-crystal cascade intercalated by a polarizer or a wave plate. The present approach can be used to realize a variety of vector beams with complex beam shapes composed of concentric rings with strongly modulated azimuthal intensity distribution. A potentially very fast switching of the overall beam shape is possible if the intercalated elements are electro-optically tunable retarders.

Planar achromatic multiple beam splitter by adiabatic light transfer.

We introduce a novel achromatic and robust scheme for n-fold multiple beam splitting based on adiabatic light transfer in a planar geometry of coupled waveguides (WGs). The concept is experimentally verified for a one-to-three beam splitter by using a reconfigurable light-induced WG structure at two operating wavelengths. The demonstrated planar-type achromatic beam splitter opens new opportunities for the realization of ultra-high bandwidth on-chip photonic devices.

Lattice-controlled modulation instability in photorefractive feedback systems.

We study the modulation instability in a two-dimensional nonlinear single feedback system with a photonic lattice and reveal a sharp transition in the instability regimes as the lattice strength is increased. For a shallow lattice, the instability modes are enhanced parallel to the lattice wave vector, while in stronger lattices, these modes are suppressed.

Photorefractive acousto-optic imaging in thick scattering media at 790 nm with a Sn(2)P(2)S(6):Te crystal.

Acousto-optic imaging is based on ultrasound modulation of multiply scattered light in thick media. We experimentally demonstrate the possibility to perform a self-adaptive wavefront holographic detection at 790nm, within the optical therapeutic window where absorption of biological tissues is minimized. A high-gain Te-doped Sn(2)P(2)S(6) crystal is used for this purpose. Optical absorbing objects embedded within a thick scattering phantom are imaged by use of pulsed ultrasound to get a dynamic millimetric axial resolution. Our technique represents an interesting approach for breast cancer detection.

Light deflection and modulation through dynamic evolution of photoinduced waveguides.

Light induced waveguides produced by lateral illumination of a hotorefractive crystal show a complex dynamic evolution upon removal of the sustaining applied electric field. Using this effect, deflection and modulation of the guided light is realized by taking advantage of the screening and counter-screening of the space charge distribution. The spot separation upon deflection can exceed 10 times the original waveguide width. Numerical simulations of the refractive index evolution and beam propagation show a good agreement with the observations.

Near infrared photorefractive self focusing in Sn(2)P(2)S(6):Te crystals.

The experimental observation of photorefractive self focusing in Sn(2)P(2)S(6) : Te bulk crystals at 1.06 mum wavelength is presented. Steady state self focusing is reached as fast as 15 ms for an input peak intensity equal to 160 W/cm(2). Self focusing is maximum for input peak intensities around 15 W/cm(2) and is decreasing for intensities below and above this value.

Deep UV light induced, fast reconfigurable and fixed waveguides in Mg doped LiTaO(3).

Dynamic waveguides are induced beneath the surface of magnesium doped near-stoichiometric lithium tantalate by deep UV light at lambda = 257 nm using the interband photorefractive effect. The waveguides can be reconfigured in 10 ms at UV intensities of 100 mW/cm(2). We show the importance of the background illumination for the build-up of dynamic optical waveguides. We also present a new fixing process of the light-induced waveguide structures when the background light is absent. These quasi-fixed structures with dark decay times of several days are due to charges trapped in deep traps.