Diffraction-optimized aperiodic surface structures for enhanced current density in organic solar cells.

Novel diffractive surface structures are designed, evaluated, and optimized to achieve a significant increase in current density in organic solar cells. The surface relief phase gratings are patterned based on concentric rings, a Fermat's spiral, and an Archimedean spiral, respectively, and thus diffract incident light independent of its azimuthal angle, as they all exhibit circularly symmetric diffraction patterns. The grating pillars are arranged on the rings or spirals according to periodic or deterministic aperiodic Thue-Morse and Rudin-Shapiro sequences, tailoring the desired diffraction patterns. After evaluating the surface patterns against each other and further parameter optimization, a final current density enhancement of 5 % is achieved for one of the most promising patterns, the Thue-Morse sequence on an Archimedean spiral.

Electro-optical tunable waveguide Bragg gratings in lithium niobate induced by femtosecond laser writing.

We report the fabrication of femtosecond laser-induced, first-order waveguide Bragg gratings in lithium niobate in the low repetition rate regime. Type-II waveguides are written into an x-cut lithium niobate wafer and structured periodically to achieve narrowband reflections at wavelengths around 1550 nm. Additionally, electrodes are employed to allow for electro-optic tuning of the spectral response. We demonstrate wavelength control of the central reflection peak by applying a static external electric field. A maximum shift of the reflection peak of Δλ = 625 pm is observed.

Enhanced Cerenkov second-harmonic emission in nonlinear photonic structures.

We study experimentally and theoretically the Cerenkov-type second-harmonic generation in a one-dimensional nonlinear photonic crystal. We demonstrate that the power of emitted second-harmonic can be enhanced 270 times by varying the angle of incidence of the fundamental beam such that the reciprocal lattice vector of the crystal can be used to compensate for the phase mismatch in the transverse direction enabling interaction in the nonlinear Bragg diffraction regime.

Nonvolatile holographic storage in iron-doped lithium tantalate with continuous-wave laser light.

Holograms have been recorded in congruent LiTaO(3):Fe with continuous-wave laser light by use of a two-step process. Blue gating light (lambda=488 nm) sensitizes the crystals for holographic recording with red light (lambda=660 nm) of a diode laser. Refractive-index changes of as much as 1.0x10(-5) are achieved for intensities of the red light of 1 W/cm(2) . The saturation values are proportional to the intensity of the writing light. Nondestructive readout with red light is possible, and the holograms remain erasable for blue light.