Allan R. Cowan

Enhanced second-harmonic generation from planar photonic crystals

Strongly enhanced second-harmonic generation is observed from a two-dimensional square lattice GaAs/AlGaAs photonic crystal waveguide when the fundamental beam, the second-harmonic beam, or both beams resonantly couple to a leaky eigenmode. P-polarized second-harmonic spectra are obtained for s-polarized, 150-fs pump pulses that are tuned from 5000 to 5600 cm(-1) and directed along the gamma-chi direction of the crystal for various angles of incidence. Compared with off-resonant conditions, enhancements of >1200x in the second-harmonic conversion are observed for resonant coupling of both the fundamental and the second-harmonic fields to leaky eigenmnodes. The angular and spectral positions of the peaks are in good agreement with simulations.

Nonlinear transmission of 1.5 µm pulses through single-mode silicon-on-insulator waveguide structures

An 80 MHz pulse train of ~ 100 fs optical pulses centred at ~ 1.5 microm is propagated through a variety of high-index-contrast silicon-on-insulator waveguide structures less than 1 mm long. All-optical power limiting and negative differential transmission, based only on the intrinsic nonlinear response of the untextured waveguides near 1.5 microm, are demonstrated for average in-guide power levels of ~ 1 mW. Superlinear transmission is observed in a textured silicon waveguide for power levels less than 20 microW.

Resonant scattering and mode coupling in two-dimensional textured planar waveguides

A heuristic formalism is developed for efficiently determining the specular reflectivity spectrum of two-dimensionally textured planar waveguides. The formalism is based on a Greens function approach wherein the electric fields are assumed to vary little over the thickness of the textured part of the waveguide. Its accuracy, when the thickness of the textured region is much smaller than the wavelength of relevant radiation, is verified by comparison with a much less efficient, exact finite difference solution of Maxwells equations. In addition to its numerical efficiency, the formalism provides an intuitive explanation of Fano-like features evident in the specular reflectivity spectrum when the incident radiation is phase matched to excite leaky electromagnetic modes attached to the waveguide. By associating various Fourier components of the scattered field with bare slab modes, the dispersion, unique polarization properties, and lifetimes of these Fano-like features are explained in terms of photonic eigenmodes that reveal the renormalization of the slab modes due to interaction with the two-dimensional grating. An application of the formalism, in the analysis of polarization-insensitive notch filters, is also discussed.

Nonlinear optics in high refractive index contrast periodic structures

This paper reviews recent work regarding the nonlinear optical properties of high refractive index contrast (HRIC) periodic structures; photonic crystals formed in semiconductor hosts. After summarizing the generic impact of HRIC periodic texture on nonlinear optical responses, examples are provided from recent theoretical and experimental works on second harmonic generation and optical bistability in semiconductor-based planar photonic crystals and photonic crystal microcavities coupled to single-mode waveguides.

Near-infrared refractive index of thick, laterally oxidized AlGaAs cladding layers

The optical transmission in the range 900-1600 nm was measured through thick (/spl sim/1 /spl mu/m) layers of Al/sub 0.98/Ga/sub 0.02/As wet-oxidized at 375/spl deg/C on GaAs. The spectra are fit well by neglecting absorption, and using 1.61 for the refractive index.

Enhanced second harmonic generation in photonic crystal waveguides

A more than 103 enhancement of 2nd harmonic conversion has been observed by using both in-coming and out-going resonances with photonic eigenstates in a GaAs/Al-oxide, 2-D photonic crystal waveguide.

Nonlinear responses of 2D photonic crystal waveguides

This paper focuses on the third order nonlinear response of 1D waveguide modes, and how they nonlinearly couple to 0D localized photon states. A complete analytic treatment of this problem, that allows for a complex third order susceptibility and downstream, nonresonant scattering, suggests that optical bistability should be achievable at average power levels of only a few milliwatts. Furthermore, appropriately designed waveguides can exhibit strong optical limiting behaviour as shown by experimental results. This saturated transmission of pulses from an optical parametric oscillator is due to two-photon absorption in the silicon slab used to fabricate the waveguide.

Linear and nonlinear optical properties of planar photonic crystals

We have subsequently developed planar photonic crystals (PPCs) specifically for nonlinear optical conversion studies. In particular we designed and realized a square-lattice PPC that has bands that exhibit almost no dispersion along the entire /spl Gamma/-X direction of the square Brillouin zone. These flat bands offer tuning flexibility and conversion efficiency advantages when considering PPCs as hosts for 2 nd harmonic generation. We have recently extended our self-consistent scattering model to include 2 nd order polarization terms. This allows us to estimate the absolute external second harmonic generation efficiency in specular reflection when exciting the PPC from the top half space.