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  • July 2012

Optics InfoBase > Spotlight on Optics > Chalcogenide glass layers in silica photonic crystal fibers

Chalcogenide glass layers in silica photonic crystal fibers

Published in Optics Express, Vol. 20 Issue 14, pp.14814-14824 (2012)
by Christos Markos, Spyros N. Yannopoulos, and Kyriakos Vlachos

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Spotlight summary: Silica photonic crystal fibers (PCF) have been of great interest to the optics community and have been used for a wide range of applications over the past decade. Their optical properties can be strongly modified by infiltrating their hollow channels with materials other than air, for example highly nonlinear glasses such as chalcogenides.

Chalcogenide glasses are non-oxide glasses that exhibit unique optical characteristics. They combine a large refractive index and high nonlinear coefficients (two to three orders of magnitude larger than silica) and offer a window of transparency extending from the visible up to the mid IR, making them promising candidates for nonlinear optics and IR applications.

So why not just omit silica fibers and draw a PCF directly from chalcogenide glass instead?

Drawing high-quality microstructured fibers from chalcogenide glasses is actually quite difficult, since they exhibit a rather steep viscosity-temperature profile and low mechanical stability compared to silica. In addition, many chalcogenide compositions are toxic and suffer from environmental degradation. A potential route of overcoming these drawbacks is to integrate chalcogenide glasses into silica PCF, the silica acting as a matrix that defines the fiber geometry and protects the chalcogenide glass, while light interacting with the chalcogenide allows making use of its optical properties.

Vlachos et al. present a simple and rather inexpensive way of integrating chalcogenide glasses into silica PCF. They dissolve bulk As2S3 chalcogenide glass in amine solvents, leading to the formation of As-S clusters, and fill the solution into the hollow channels of two different commercial silica PCF using capillary effect. Annealing at low temperature allows partially removing the solvent and gives rise to the formation of nanometer-thick amorphous As2S3 glass layers deposited inside the hollow channels. Off-resonance Raman spectra reveal the formation of the As2S3 glass network, overlaid with contributions from residual solvent. The filling process is rather easy to realize experimentally, and allows the fabrication of cm-long pieces of silica solid-core PCF with a thin film of chalcogenide glass deposited inside each cladding channel.

Launching white light from a broadband supercontinuum source into the silica core of the hybrid PCF and recording the transmitted spectrum, a series of transmission peaks and dips in the visible and near IR is observed. The transmission dips are a result of photonic passbands occurring when the light in the silica core can couple to Mie-like resonances in the high-index cladding strands, allowing the light from the core to escape. Guided modes only appear if the cladding provides a photonic bandgap, which is the case at the wavelength windows where recorded transmission spectra of the core exhibit transmission peaks. The position of the bandgaps and passbands can in principle be described by the anti-resonant reflecting optical waveguide (ARROW) model. However, due to the varying thickness of the chalcogenide layers and the influence of the annealing process and the residual solvent on the refractive index of the chalcogenide glass, this was not possible for the structures reported in this work.

The main advantage of this novel fabrication approach is really its simplicity, and the possibility of multi-layer deposition of different glass compositions, which is presently not possible with any other fabrication technique.

--Nicolai Granzow

Technical Division: Optoelectronics
ToC Category: Fiber Optics and Optical Communications
OCIS Codes: (060.2290) Fiber optics and optical communications : Fiber materials
(160.4330) Materials : Nonlinear optical materials
(060.4005) Fiber optics and optical communications : Microstructured fibers
(060.5295) Fiber optics and optical communications : Photonic crystal fibers

Posted on July 24, 2012

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