T. W. Allen

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.

Photoluminescence and Thermal Properties of Er-DOPED As-Se-Ga-Ge Based Glasses

We studied the photoluminescence characteristics and thermal properties of Er-doped As-Se-Ga-Ge based chalcogenide glasses alloyed with different amounts of Ge (1 to 15 at. %) for use as optical amplifiers at 1550 v nm operation. We used a 980 v nm laser diode to optically excite the Er-doped glass samples. The 980 v nm pumping induced a 1550 v nm photoluminescence output signal. We measured the lifetime of the exited Er state in these chalcogenide glasses as a function of Er doping up to 3 at. %, and Ge composition from 1 to 15 at. %. We also carried out Temperature-Modulated Differential Scanning Calorimetry measurements to study the thermal stability of these glasses.

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.

Photoluminescence measurements of Er-doped chalcogenide glasses

We have investigated the photoluminescence (PL) emission spectra, photoluminescence lifetimes, and relative photoluminescence intensities of various Er-doped chalcogenide glass alloys, designed for possible use in optical waveguide amplifier applications at the telecommunications wavelength of 1550 nm. Bulk samples were prepared using melt-quenching techniques, and the samples were doped with varying amounts of Er2S3 or ErCl3, in the range of 0.3−3 at. %. Using a 980 nm pump beam, we measured the lifetime of the 1550 nm (4I13/2 to 4I15/2) luminescence and PL spectra. For two of the alloys, the luminescence efficiency as a function of erbium concentration was observed by scaling the luminescence intensity by the weight of each sample. For GaGeS, the luminescence lifetime exhibited quenching at concentrations of erbium greater than 1 at. %, but for GaGeSe, this behavior was not observed for concentrations as high as 3 at. %. However, the photoluminescence efficiency measurements for both glasses showed quen...

Nanocluster sensitized erbium-doped silicon monoxide waveguides

We describe the fabrication and characterization of micron-scale buried strip waveguides with erbium-doped (~ 1 at. %) silicon monoxide (SiO) cores and SiO(2) and polymer claddings. In spite of large core-cladding index offset (Deltan~0.4), propagation loss is as low as ~ 1 dB/cm. The cross-section for the (4)I(13/2) to (4)I(15/2) erbium transition was estimated as ~10(-20) cm(2), a factor of 2 to 3 higher than in silica glass. The annealed core material contains a high density of amorphous silicon nanoclusters, which act as efficient broadband sensitizers for erbium. Both a traditional co-propagating pump (980 nm wavelength) configuration and a transverse pump (532 nm wavelength, < 10 W/cm(2)) configuration were tested. In either case, free carrier absorption loss is the dominant pump-induced mechanism and approximately 15-20 % of the erbium population is invertible.

Wavelength interrogation of fiber Bragg grating sensors using tapered hollow Bragg waveguides.

We describe an integrated system for wavelength interrogation, which uses tapered hollow Bragg waveguides coupled to an image sensor. Spectral shifts are extracted from the wavelength dependence of the light radiated at mode cutoff. Wavelength shifts as small as ~10  pm were resolved by employing a simple peak detection algorithm. Si/SiO₂-based cladding mirrors enable a potential operational range of several hundred nanometers in the 1550 nm wavelength region for a taper length of ~1  mm. Interrogation of a strain-tuned grating was accomplished using a broadband amplified spontaneous emission (ASE) source, and potential for single-chip interrogation of multiplexed sensor arrays is demonstrated.

Conditions for admittance-matched tunneling through symmetric metal-dielectric stacks

We used the theory of potential transmittance to derive a general expression for reflection-less tunneling through a periodic stack with a dielectric-metal-dielectric unit cell. For normal-incidence from air, the theory shows that only a specific (and typically impractically large) dielectric index can enable a perfect admittance match. For off-normal incidence of TE-polarized light, an admittance match is possible at a specific angle that depends on the index of the ambient and dielectric media and the thickness and index of the metal. For TM-polarized light, admittance matching is possible within the evanescent-wave range (i.e. for tunneling mediated by surface plasmons). The results provide insight for research on transparent metals and superlenses.

Visible-band dispersion by a tapered air-core Bragg waveguide

We describe out-coupling of visible band light from a tapered hollow waveguide with TiO(2)/SiO(2) Bragg cladding mirrors. The mirrors exhibit an omnidirectional band for TE-polarized modes in the ~490 to 570 nm wavelength range, resulting in near-vertical radiation at mode cutoff positions. Since cutoff is wavelength-dependent, white light is spatially dispersed by the taper. Resolution on the order of 2 nm is predicted and corroborated by experimental results. These tapers can potentially form the basis for compact micro-spectrometers in lab-on-a-chip and optofluidic micro-systems.

Thermomechanical characterization of on-chip buckled dome Fabry–Perot microcavities

We report on the thermomechanical and thermal tuning properties of curved-mirror Fabry–Perot resonators, fabricated by the guided assembly of circular delamination buckles within a multilayer a-Si/SiO2 stack. Analytical models for temperature dependence, effective spring constants, and mechanical mode frequencies are described and shown to be in good agreement with experimental results. The cavities exhibit mode volumes as small as ∼10λ3, reflectance-limited finesse ∼3×103, and mechanical resonance frequencies in the MHz range. Monolithic cavity arrays of this type might be of interest for applications in sensing, cavity quantum electrodynamics, and optomechanics.

Assessing the maximum transmittance of periodic metal-dielectric multilayers

We show that the theory of potential transmittance (PT) is useful for problems involving photon tunneling through metal-dielectric stacks, regardless of whether the tunneling is mediated by Fabry–Perot or surface-plasmon resonances. A unifying principle is that, given a total thickness of metal, subdividing the metal into a larger number of thin films increases the maximum PT. For Fabry–Perot-based tunneling, we apply the concept of equivalent layers to stacks comprising dielectric-metal-dielectric unit cells and explore the conditions for impedance matching to an external air medium. This approach demonstrates that, to optimize transmittance, thicker metal films require higher-index dielectric spacers. For surface-plasmon-mediated tunneling, we confirm that the maximum transmittance also lies within the limits predicted by PT theory.