1,721,218 research outputs found
Disorder-induced losses in photonic crystal waveguides with line defects
No full textA numerical analysis of extrinsic diffraction losses in two-dimensional photonic crystal slabs with line defects is reported. To model disorder, a Gaussian distribution of hole radii in the triangular lattice of airholes is assumed.. The extrinsic losses below the light line increase quadratically with the disorder parameter, decrease slightly-with increasing core thickness, and depend weakly on the hole radius. For typical values of the disorder parameter the calculated loss values of guided modes below the light line compare favorably with available experimental results
Second-harmonic generation in doubly-resonant microcavities with periodic dielectric mirrors
No full textStrong enhancement of second-harmonic generation (SHG) is expected in one-dimensional microcavities when double resonance for the pump and the harmonic fields, as well as phase matching, are achieved. The realization of a doubly resonant microcavity with dielectric mirrors made of nonbirefringent materials is difficult because of the refractive index dispersion of the constituent media. Here we present a powerful method, based on photonic crystal concepts like gap maps and their generalization to defect modes, for the
design of doubly resonant microcavities with periodic dielectric mirrors. The material dispersion is compensated
by using the angle of incidence as a tuning parameter, thanks to the polarization splitting of cavity modes.
The cavity enhancement of SHG increases exponentially with the number of periods in the dielectric mirrors
and can be much larger than in single-resonant microcavities with comparable -or even larger- quality factors. The roles of phase delay and of thin versus thick configurations in the dielectric mirrors, of the growth orientation, and of the polarization degrees of freedom in achieving double resonance with phase matching are discussed. Significant examples of doubly resonant SHG with high conversion efficiency are given for Al0.25Ga0.75As cavities with Al0.4Ga0.6As/Alox (oxidized AlAs) mirrors
Photonic crystal slabs with a triangular lattice of triangular holes investigated using a guided-mode expansion method
No full textL'articolo presenta una formulazione teorica di un metodo originale per calcolare i modi fotonici e le perdite nelle guide d'onda a cristallo fotonico.
Abstract: According to a recent proposal [S. Takayama et al., Appl. Phys. Lett. 87, 061107 (2005)], the triangular lattice of triangular air holes may allow us to achieve a complete photonic band gap in two-dimensional photonic crystal slabs. In this work we present a systematic theoretical study of this photonic lattice in a high-index membrane, and a comparison with the conventional triangular lattice of circular holes, by means of the guided-mode expansion method whose detailed formulation is described here. Photonic mode dispersion below and above the light line, gap maps, and intrinsic diffraction losses of quasiguided modes are calculated for the periodic lattice as well as for line and point defects defined therein. The main results are summarized as follows: (i) The triangular lattice of triangular holes does indeed have a complete photonic band gap for the fundamental guided mode, but the useful region is generally limited by the presence of second-order waveguide modes; (ii) the lattice may support the usual photonic band gap for even modes (quasi-TE polarization) and several band gaps for odd modes (quasi-TM polarization), which could be tuned in order to achieve doubly resonant frequency conversion between an even mode at the fundamental frequency and an odd mode at the second-harmonic frequency; (iii) diffraction losses of quasiguided modes in the triangular lattices with circular and triangular holes, and in line-defect waveguides or point-defect cavities based on these geometries, are comparable. The results point to the interest of the triangular lattice of triangular holes for nonlinear optics, and show the usefulness of the guided-mode expansion method for calculating photonic band dispersion and diffraction losses, especially for higher-lying photonic modes
Low-loss guided modes in photonic crystal waveguides
No full textWe study disorder-induced propagation losses of guided modes in photonic crystal slabs with line-defects. These losses are treated within a theoretical model of size disorder for the etched holes in the otherwise periodic photonic lattice. Comparisons are provided with state-of-the-art experimental data, both in membrane and Silicon-on-Insulator ( SOI) structures, in which propagation losses are mainly attributed to fabrication imperfections. The dependence of the losses on the photon group velocity and the useful bandwidth for low-loss propagation are analyzed and discussed for membrane and asymmetric as well as symmetric SOI systems. New designs for further improving device performances are proposed, which employ waveguides with varying channel widths. It is shown that losses in photonic crystal waveguides could be reduced by almost an order of magnitude with respect to latest experimental results. Propagation losses lower than 0.1 dB/mm are predicted for suitably designed structures, by assuming state-of-the-art fabrication accuracy
Gap maps and intrinsic diffraction losses in one-dimensional photonic crystal slabs
No full textA theoretical study of photonic bands for one-dimensional (1D) lattices embedded in planar waveguides with strong refractive index contrast is presented. The approach relies on expanding the electromagnetic field onthe basis of guided modes of an effective waveguide, and on treating the coupling to radiative modes by perturbation theory. Photonic mode dispersion, gap maps, and intrinsic diffraction losses of quasi guided modes are calculated for the case of self-standing membranes as well as for silicon-on-insulator structures. Photonic band gaps in a waveguide are found to depend strongly on the core thickness and on polarization, so that the gaps for transverse electric and transverse magnetic modes most often do not overlap. Radiative losses of quasiguided modes above the light line depend in a nontrivial way on structure parameters, mode index, and wave vector. The results of this study may be useful for the design of integrated 1D photonic structures with low radiative losses
- …
