Jiri Orava

Embossing of chalcogenide glasses: monomode rib optical waveguides in evaporated thin films

Single-mode optical rib waveguides operating at telecommunication wavelengths are successfully patterned via a hot embossing technique in a thermally evaporated chalcogenide glass thin film on a chalcogenide glass substrate. Ellipsometry is used to measure the refractive index dispersion of the pressed film (As(40)Se(60)) and substrate (Ge(17)As(18)Se(65)).

Soft imprint lithography of a bulk chalcogenide glass

We report on large-surface-area micro-patterning of a bulk chalcogenide glass by a PDMS soft mould. Micrometre-scale (width ~4μm and depth ~0.8 μm) test patterns such as ribs, channels and a lens array are successfully imprinted into the surface of high refractive index As3S7 bulk glass at 225°C without any applied external pressure. The mean-square roughness of the patterned glass surface is in the range 3 – 10 nm. Soft imprinting of bulk chalcogenide glass is an efficient method for reliable fabrication of optical and photonic devices.

In-situ measurement of reversible photodarkening in ion-conducting chalcohalide glass

We report the kinetics of below band-gap light induced photodarkening in (80-x)GeS(2)-20Ga(2)S(3)-xAgI (x = 0 and 20 mol %) bulk chalcogenide glasses by measuring the time evolution of transmission spectra at every 10 milliseconds. The results prove clearly the enhancement of photosensivity upon doping of AgI compound in glasses. It is interesting to find that PD observed in AgI-doped glass totally disappears two hours later after the laser exposing even at room temperature. In significant contrast to 80GeS(2)-20Ga(2)S(3) glass that the metastable part of PD remains for a long time. We expect such a fast auto-recovery property in AgI-doped glass can be utilized for optical signal processing.

Viscosity of liquid Ag-In-Sb-Te: Evidence of a fragile-to-strong crossover

The temperature-dependent viscosity η(T) is measured for the equilibrium liquid of the chalcogenide Ag-In-Sb-Te (AIST), the first time this has been reported for a material of actual interest for phase-change memory. The measurements, in the range 829-1254 K, are made using an oscillating-crucible viscometer, and show a liquid with high fragility and low viscosity, similar to liquid pure metals. Combining the high-temperature viscosity measurements with values inferred from crystal growth rates in the supercooled liquid allows the form of η(T) to be estimated over the entire temperature range from above the melting point down to the glass transition. It is then clear that η(T) for liquid AIST cannot be described with a single fragility value, unlike other phase-change chalcogenides such as liquid Ge-Sb-Te. There is clear evidence for a fragile-to-strong crossover on cooling liquid AIST, similar to that analyzed in Te85Ge15. The change in fragility associated with the crossover in both these cases is rather weak, giving a broad temperature range over which η(T) is near-Arrhenius. We discuss how such behavior may be beneficial for the performance of phase-change memory. Consideration of the fragile-to-strong crossover in liquid chalcogenides may be important in tuning compositions to optimize the device performance.

Reversible photoinduced change of refractive index in ion-conducting chalcohalide glass

When irradiated by the focused Ar+ ion laser of below band-gap photon energy (2.41 and 2.54eV), a large increase of refractive index (e.g., Δn=6% at 1550nm) was observed in 60GeS2–20Ga2S3–20AgI bulk glass (with bandgap of 2.71eV). We found it surprisingly that such a large Δn quickly smeared out after the illumination even at room temperature, i.e., the refractive index will return to almost initial value within only 2h. It is fairly rare among chalcogenides ever reported. The plausible correlation between the photorefraction effect and the ion-conducting property of glasses was also suggested.

Sub-micrometer soft lithography of a bulk chalcogenide glass

We demonstrate, for the first time, time- and cost-effective replication of sub-micrometer features from a soft PDMS mold onto a bulk chalcogenide glass over a large surface area. A periodic array of sub-micrometer lines (diffraction grating) with period 625 nm, amplitude 45 nm and surface roughness 3 nm was imprinted onto the surface of the chalcogenide AsSe(2) bulk glass at temperature 225°C, i.e. 5°C below the softening point of the glass. Sub-micrometer soft lithography into chalcogenide bulk glasses shows good reliability, reproducibility and promise for feasible fabrication of various dispersive optical elements, anti-reflection surfaces, 2D photonic structures and nano-structured surfaces for enhanced photonic properties and chemical sensing.

Mid-infrared integrated optics: versatile hot embossing of mid-infrared glasses for on-chip planar waveguides for molecular sensing

Abstract. The versatility of hot embossing for shaping photonic components on-chip for mid-infrared (IR) integrated optics, using a hard mold, is demonstrated. Hot embossing via fiber-on-glass (FOG), thermally evaporated films, and radio frequency (RF)-sputtered films on glass are described. Mixed approaches of combined plasma etching and hot embossing increase the versatility still further for engineering optical circuits on a single platform. Application of these methodologies for fabricating molecular-sensing devices on-chip is discussed with a view to biomedical sensing. Future prospects for using photonic integration for the new field of mid-IR molecular sensing are appraised. Also, common methods of measuring waveguide optical loss are critically compared, regarding their susceptibility to artifacts which tend artificially to depress, or enhance, the waveguide optical loss.

Deposition techniques for chalcogenide thin films

Abstract: The chapter describes details of deposition techniques, namely thermal evaporation, sputtering, pulsed-laser deposition, chemical vapour deposition and spin-coating for chalcogenide thin films. We discuss key issues, advantages and influence of different deposition techniques and experimental conditions on properties of as-prepared amorphous chalcogenide thin films. The final chemical composition, structure and physico-chemical properties of the prepared films predetermine their application and vice versa. Their applications are in many fields such as (nano)electronics, (nano)optics, information storage, security, health protection and alternative energy sources.

Reversible migration of silver on memorized pathways in Ag-Ge40S60 films

Reversible and reproducible formation and dissolution of silver conductive filaments are studied in Ag-photodoped thin-film Ge40S60 subjected to electric fields. A tip-planar geometry is employed, where a conductive-atomic-force microscopy tip is the tip electrode and a silver patch is the planar electrode. We highlight an inherent “memory” effect in the amorphous chalcogenide solid-state electrolyte, in which particular silver-ion migration pathways are preserved “memorized” during writing and erasing cycles. The “memorized” pathways reflect structural changes in the photodoped chalcogenide film. Structural changes due to silver photodoping, and electrically-induced structural changes arising from silver migration, are elucidated using Raman spectroscopy. Conductive filament formation, dissolution, and electron (reduction) efficiency in a lateral device geometry are related to operation of the nano-ionic Programmable Metallization Cell memory and to newly emerging chalcogenide-based lateral geometry MEMS...

In-situ study of athermal reversible photocrystallization in a chalcogenide glass

The time-resolved Raman measurements reveal a three-stage mechanism of the photostructural changes in Ge25.0Ga9.5Sb0.5S65.0 (containing 0.5 at. % of Er3+) glass under continuous-above-bandgap illumination. These changes are reversible and effectively athermal, in that the local temperature rises to about 60% of the glass-transition temperature and the phase transitions take place in the glass/crystal and not in an equilibrium liquid. In the early stages of illumination, the glassy-network dimensionality changes from a predominantly 3-D to a mixture of 2-D/1-D represented by an increase in the fraction of edge-sharing tetrahedra and the emergence of homonuclear (semi)metallic bonds. This incubation period of the structural rearrangements, weakly thermally activated with an energy of ∼0.16 eV, facilitates a reversible photocrystallization. The photocrystallization rate in the glass is comparable to that achieved by thermal crystallization from supercooled liquid at large supercooling. Almost complete re-amo...