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.
The potential for a dielectric thin-film stack to provide omnidirectional reflection (near unity reflectance for all polarization states and incidence angles, over a finite range of wavelengths) was recognized only recently. [1][2][3] Nevertheless, such one-dimensional omnidirectional dielectric reflectors (ODRs) can provide some of the functionality promised by three-dimensional photonic crystals, while being comparatively simple to fabricate. They have been applied to the construction of low-loss hollow-core fibers [4] and integrated waveguides, [5,6] and to increase the emissivity of light-emitting diodes (LEDs).[7]An ODR is predicated on a pair of compatible dielectric materials that have transparency in the wavelength range of interest and sufficiently different refractive indices. For example, if the lower index material has a refractive index of n ∼ 1.5, an omnidirectional stop band can arise when the higher index material has n ∼ 2.3 or greater. It is an experimental challenge to identify material combinations that satisfy the refractive-index requirement, while also enabling practical implementation of a thermomechanically robust, stable, and uniform multilayer stack. Typical ODRs reported to date [1,2,[8][9][10] have employed a glass or semiconductor substrate as a base. However, ODR-based fibers have also been developed [11,12] and the fabrication of substrateless planar ODR mirrors (for the mid-IR) was reported anecdotally. [13] Of related interest, freestanding polymer-based optical filters were conceived several decades ago.[14] Moreover, efficient wide-angle mirrors based on highly birefringent polymers [15] are commercially available [16] in sheets ca. 50 lm thick. Freestanding optical filters have numerous potential applications, including reflectors for flexible organic LEDs, [17] laminates or wraps for light recycling in displays, [16] membranes in optomechanical light modulators, [18] pellicle-type beam splitters and filters, and sails on spacecraft propelled by radiation pressure. [19] Key to these applications is the flexibility, toughness, and fracture resistance of polymers, even in micrometer-scale thin sheets. These same properties motivate research and development of flexible electronic [20] and photonic [21,22] circuitry, using organic and sufficiently thin inorganic films.In this Communication, we describe a straightforward process for fabricating compliant but mechanically tough freestanding mirrors using commercially available chalcogenide glass and polymer materials. High-refractive-index contrast (Dn ∼ 1) between the materials enables near-unity, omnidirectional reflection from membranes less than 3 lm thick. The reflectors were successfully laminated (using surface-tension forces) on various flat and curved surfaces. Even after handling, bending, and re-lamination, the optical response of the reflectors was in excellent agreement with theoretical predictions. Optical filter response can be customized by manually stacking pieces of the mirror, and optomechanical tuning by stretchi...
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.
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.
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