N. Ponnampalam

High index contrast waveguides in chalcogenide glass and polymer

We review various properties of chalcogenide glasses that make them promising materials for passive and active microphotonics. We then describe two processes for channel waveguide fabrication, using the chalcogenide glass As/sub 2/Se/sub 3/ as a core material and compatible polymers as claddings. In the first approach, waveguides are patterned directly in the chalcogenide film by photoexposure through a mask followed by selective wet etching. This technique has produced shallow rib waveguides with losses as low as 0.26 dB/cm and small modal area photonic wire waveguides with losses on the order of 10 dB/cm. In the second approach, waveguide patterning is achieved by using an organic photoresist as a mask for selective photodoping of silver into the chalcogenide glass. Selective wet etching produced strip waveguides with smooth and highly vertical sidewalls. We report preliminary light guiding results for these latter structures.

Chip-scale spectrometry based on tapered hollow Bragg waveguides

We describe a micro-spectrometer that exploits out-of-plane radiation at mode cutoff in a tapered leaky waveguide clad by omnidirectional Bragg reflectors. The device can be viewed as a side-coupled, tapered Fabry-Perot cavity. An effective-index transfer-matrix model reveals that optimal resolution is dependent on the reduction or mitigation of back-reflection and standing waves leading up to the cutoff point. We address this by insertion of low numerical aperture optics between the taper and the detector, and demonstrate an experimental resolution as small as approximately 1 nm and operating bandwidth >100 nm in the 1550 nm range, from a tapered waveguide with footprint approximately 50 microm x 500 microm. The device combines the small size of a Fabry-Perot instrument with the detector array compatibility and fixed optics of a grating-based instrument.

Small core rib waveguides with embedded gratings in As2Se3 glass.

Low-loss shallow-rib waveguides were fabricated using As2Se3 chalcogenide glass and polyamide-imide polymer. Waveguides were patterned directly in the As2Se3 layer by photodarkening followed by selective wet etching. Theory predicted a modal effective area of 3.5-4 microm2, and this was supported by near-field modal measurements. The Fabry-Perot technique was used to estimate propagation losses as low as ~0.25 dB/cm. First-order Bragg gratings near 1550 nm were holographically patterned in some waveguides. The Bragg gratings exhibited an index modulation on the order of 0.004. They were used as a means to assess the modal effective indices of the waveguides. Small core As2Se3 waveguides with embedded Bragg gratings have potential for realization of all-optical Kerr effect devices.

Improved omnidirectional reflectors in chalcogenide glass and polymer by using the silver doping technique.

We describe the fabrication and characterization of omnidirectional reflectors based on silver-doped chalcogenide glass and polymer. We deposited periodically alternating layers of thermally evaporated Ge33As12Se55 chalcogenide glass, sputtered silver, and spun-cast polyamide-imide polymer. The silver was subsequently dissolved into each adjacent chalcogenide glass layer, either by exposing the multilayer to visible light (photodoping) or by heating the sample. The resultant silver concentration within the chalcogenide glass layers is estimated to be ~20 at. %. Silver doping red-shifts the band edge of the glass, and produces an increase of ~0.3-0.4 in the refractive index. The glass retains good transparency in the near infrared after doping, and the technique enables the omnidirectional bandwidth to be increased from ~100 nm to ~200 nm in the 1550 nm wavelength region.

Hollow Bragg waveguides fabricated by controlled buckling of Si/SiO 2 multilayers

We describe integrated air-core waveguides with Bragg reflector claddings, fabricated by controlled delamination and buckling of sputtered Si/SiO2 multilayers. Thin film deposition parameters were tailored to produce a desired amount of compressive stress, and a patterned, embedded fluorocarbon layer was used to define regions of reduced adhesion. Self-assembled air channels formed either spontaneously or upon heating-induced decomposition of the patterned film. Preliminary optical experiments confirmed that light is confined to the air channels by a photonic band-gap guidance mechanism, with loss ~5 dB/cm in the 1550 nm wavelength region. The waveguides employ standard silicon processes and have potential applications in MEMS and lab-on-chip systems.

Guided self-assembly of integrated hollow Bragg waveguides

We describe the fabrication of integrated hollow waveguides through guided self-assembly of straight-sided, thin film delamination buckles within a multilayer system of chalcogenide glass and polymer. The process is based on silver photodoping, which was used to control both the stress and adhesion of the chalcogenide glass films. Straight, curved, crossing, and tapered microchannels were realized in parallel. The channels are cladded by omnidirectional dielectric reflectors designed for low-loss, air-core guiding of light in the 1550-1700 nm wavelength range. Loss as low as ~15 dB/cm was measured for channels of height ~2.5 mum, in good agreement with both an analytical ray optics model and finite difference numerical simulations. The loss is determined mainly by the reflectivity of the cladding mirrors, which is ~0.995 for the as-fabricated devices.

Self-assembled hollow waveguides with hybrid metal-dielectric Bragg claddings

We report on the fabrication and characterization of integrated hollow waveguides cladded by gold-terminated, omnidirectional Bragg reflectors. The hollow waveguide channels were realized by the controlled formation of straight-sided delamination buckles within a multilayer thin film stack. An optimized process produced low-defect, straight-sided buckles with base widths from 10 to 80 mum, and corresponding peak core heights from ~0.7 to ~4 mum, on a single sample. The waveguides described have upper and lower cladding mirrors of 4 and 5.5 periods, respectively. Gold termination of the cladding reflectors significantly reduces the propagation loss of air-guided modes. The minimum propagation loss is less than 4 dB/cm in the near infrared, corresponding to upper and lower cladding reflectance of ~ 0.999. The main details of the guiding mechanism are well approximated by a simple ray-optics model.

Out-of-plane coupling at mode cutoff in tapered hollow waveguides with omnidirectional reflector claddings

We describe the theoretical and experimental analysis of light propagation in tapered, air-core waveguides with omnidirectional reflector claddings. For light within the omnidirectional band, nearly vertical out-ofplane radiation at wavelength-dependent positions along the length of the taper was observed. The coupling positions correspond to the core sizes at which individual modes approach cutoff. The leaky nature and low scattering loss of the waveguides enabled the direct imaging of modal interference and standing waves. The out-coupling experiments were corroborated by in-coupling experiments and by a theoretical analysis. The mechanism described might find application to three-dimensional optical integration, on-chip spectroscopy, and wavelength division multiplexing.

High-finesse cavities fabricated by buckling self-assembly of a-Si/SiO 2 multilayers

Arrays of half-symmetric Fabry-Perot micro-cavities were fabricated by controlled formation of circular delamination buckles within a-Si/SiO(2) multilayers. Cavity height scales approximately linearly with diameter, in reasonable agreement with predictions based on elastic buckling theory. The measured finesse (F > 10(3)) and quality factors (Q > 10(4) in the 1550 nm range) are close to reflectance limited predictions, indicating that the cavities have low roughness and few defects. Degenerate Hermite-Gaussian and Laguerre-Gaussian modes were observed, suggesting a high degree of cylindrical symmetry. Given their silicon-based fabrication, these cavities hold promise as building blocks for integrated optical sensing systems.

Analysis and fabrication of hybrid metal-dielectric omnidirectional Bragg reflectors

We describe chalcogenide glass and polymer based Bragg reflectors with a metallic underlayer and use a transfer matrix model to analyze their performance. The angle-averaged reflectance of a hybrid mirror approaches unity for only a few periods and is much higher than that for a nonmetallized Bragg reflector or for the metallic layer alone. For an angle-averaged reflectance greater than 0.99, the addition of a metallic underlayer enables nearly a tripling of the omnidirectional bandwidth (from approximately 110 to approximately 305 nm) concurrent with a significant reduction in the number of required periods (from 10.5 to 4.5). Hybrid mirrors of 4.5 periods, with a 50 nm Au underlayer and overall thickness of approximately 2 microm, were fabricated atop silicon substrates and characterized. They exhibit an omnidirectional stop band in the 1450-1750 nm wavelength range, in good agreement with theoretical predictions.