Plastic-clad chalcogenide glass fibers

Easy handling flexible mid-IR light conductors have been made of GeAsSe chalcogenide glass fibers coated with polyolefin. Optical transmission level is about 10 dB/m in the 4 - 11 micron band. Few watts of CO₂ laser power may be transmitted.     

Infrared transmitting chalcogenide glass ceramics

Infrared glass ceramics transmitting light from 2 to 15 μm have been produced from a Ga-Ge-Sb-Se glass. The glass composition is selected in the middle of the glass forming region and the annealing temperature must be close to T_g to avoid excessive crystal growth. A heating time of the order of two weeks is necessary to see evolution of the thermal expansion coefficient of the composite while keeping the optical transmission properties of the glass.     

Planar chalcogenide glass waveguides for IR evanescent wave sensors

Multi-layered chalcogenide glass waveguide structures have been fabricated for evanescent wave sensing of bio-toxins and other sensor applications. Thin films of Ge containing chalcogenides have been deposited onto Si substrates, with a-GeSe₂ as the lower cladding layer and a-GeSbSe as the core layer, to form the slab waveguide. The absence of a defined upper cladding layer enhances the leakage necessary to sense the target molecules. Modal refractive index is estimated from the m-lines. It is shown that photo-induced structural changes by 808 nm laser light in the core layer selectively enhance refractive index in the exposed regions, and thus provide a convenient method to form channel waveguides. A thin layer of Au has been deposited on top of the core layer for the attachment of linker molecules for biosensor application; ATR confirms this.     

Contactless molding of arrayed chalcogenide glass lenses

The moldable chalcogenide glass material has been developed recently. This study developed a new process to produce an arrayed chalcogenide glass lenses by a contactless molding process, which is considered to have a great potential for the mass production of arrayed glass lenses with low cost, ease of manufacture and free of surface defects. The stainless steel plate with arrayed through holes was employed as the molds. The selenium based chalcogenide glass plate was put on the mold and nitrogen gas was introduced into the closed chamber to allow gas pressure up to 4 kg/cm². IR heating allows the chalcogenide glass plate to approach the soft point, and forces glass flows into the arrayed through holes to form the arrayed glass lenses by viscoelastic deformation. A higher forming temperature, pressure and longer time duration tend to produce the arrayed lenses with a higher peak height and a smaller radius of curvature. The arrayed chalcogenide glass lenses with a peak height of 430 μm can be obtained at a forming temperature of 305 °C, a gas pressure of 1 kg/cm² and a time duration of 110 s. The contactless gas assisted molding system can avoid contact induced glass sticking and gas bubble problems. The surface qualities of molded lenses are much better than that of lenses molded by the traditional contact molding process.     

Vibrational entropy near glass transition in a chalcogenide glass and supercooled liquid

Raman spectroscopic measurements have been performed on Ge₂₀Se₈₀ glass and supercooled liquid at temperatures ranging between 298 and 500 K. Temperature dependent softening of vibrational mode frequencies has been used in conjunction with the available vibrational density of states data at ambient temperature to estimate the relative contributions of vibrational and configurational entropies across glass transition. Nearly 20% of the additional entropy above glass transition is estimated to be vibrational. Thermal expansion effect on vibrational mode softening is found to be insufficient to account for the anharmonic component of vibrational entropy implying possible coupling between the vibrational and configurational entropies at temperatures above T_g. These results may have important consequences in shaping our understanding of various aspects of glass transition.     

Design of Er3+-doped chalcogenide glass laser for MID-IR application

The feasibility of a photonic crystal fiber laser (PCF laser), made of a novel Er³⁺-doped chalcogenide glass and operating at the wavelength λ_s = 4.5 μm is investigated. The design is performed on the basis of spectroscopic and optical parameters measured on a fabricated Er³⁺-doped Ga₅Ge₂₀Sb₁₀S₆₅ chalcogenide bulk sample. The simulations have been performed by employing a home made numerical code that solves the multilevel rate equations and the power propagation equations via a Runge-Kutta iterative method. The numerical results indicate that a laser exhibiting slope efficiency close to the maximum theoretical one and a wide tunability in the wavelengths range where the atmosphere is transparent can be obtained.     

Surface biofunctionalization of nanostructured GeSbSe chalcogenide glass thin films

Thin nanostructured chalcogenide films were grown using the oblique angle deposition (OAD) technique and subsequently polymerized with thin poly(amino-p-xylylene) (PPX) films. Our objective was twofold, i.e., to use deposited polymeric thin films to allow the attachment of biomolecules to chalcogenide glass thin films, and at the same time, to increase surface area by OAD to enhance surface functionality. The effectiveness of this approach was evaluated by Fourier transform infrared spectroscopy (FTIR), together with a combination of fluorescent protein immobilization and confocal microscopy characterization. It is shown that the presence of amine groups on the surface of the polymer coated chalcogenide thin films yield a notable increment of surface coverage with proteins at large evaporation oblique angles which is expected to enhance detection performance of the film in biosensor applications.     

Multiphonon energy gap law in rare-earth doped chalcogenide glass

The parameters for multiphonon relaxation of rare earth (RE) ions in the sulfide glass Ge₂₅As_8.33Ga_1.67S₆₅ have been re-evaluated using the temperature dependence of the fluorescence lifetime to separate out true multiphonon decay from other non-radiative processes. It is found that for energy gaps to the next lowest level greater than 2500 cm⁻¹, other non-radiative processes become dominant over multiphonon decay. The most likely process for this additional non-radiative decay is energy transfer to vibrational impurities such as OH and SH. The newly derived parameters lead to an electron–phonon coupling parameter ε=0.058, which is more in line with other glass types than the previously accepted value of ε=0.36. These new multiphonon relaxation parameters and the existence of additional non-radiative decay mechanisms has implications for the modeling of chalcogenide-based active devices.     

Photo-induced phenomena in chalcogenide glass: Comparison with those in oxide glass and polymer

Two topics, the both being related with localized states in non-crystalline solids, are studied. One is a comparison of photo-induced phenomena in oxide, chalcogenide, and organic materials. Despite of different inorganic and organic structures, there exist many similarities in photo-induced phenomena, which can be ascribed to excitation of localized electronic states. The other topic concerns photo-induced phenomena induced by linear and non-linear excitation. A result on As₂S₃ demonstrates that band-gap excitation by one- and two-photon processes provides different changes. It is suggested that the two-photon process occurs resonantly at around localized states in amorphous materials, and the process plays important roles in the structural change.     

Structural heterogeneity in chalcogenide glass films prepared by thermal evaporation

GeSe₂ and Ge₂₈Sb₁₂Se₆₀ chalcogenide glass thin films have been deposited on single crystal silicon substrates by vacuum thermal evaporation. The surface morphology of these films has been investigated by field emission-scanning electron microscopy and atomic force microscopy, revealing heterogeneities in their microstructure consisting of granular regions ∼15–50 nm in size, which were coarser in the case of the GeSe₂ films. Typical RMS film surface roughness values were ∼0.9–1.3 nm.     

Extrusion of chalcogenide glass preforms and drawing to multimode optical fibers

We report on viscosity of a Ge₁₇As₁₈Se₆₅ glass over the temperature range of 280-420 °C and the successful co-extrusion and fiber-drawing of two chalcogenide glass boules to form a core/clad. pair. The co-extrusion produces a preform with optimum diameter stability and core/clad. glass ratio, and minimum defects at the core/clad. interface in the middle 120-200 mm region of a 270 mm long preform. Core/clad. fiber is drawn successfully from the extruded preform. An optical loss of 1.7 dB m⁻¹ at 1666 cm⁻¹ (6.0 μm) and 6.7 dB m⁻¹ at 6649 cm⁻¹ (1.55 μm) is reported.     

Evolution of chemical structure during silver photodiffusion into chalcogenide glass thin films

The change of chemical structure resulting after X-ray and photo-induced silver diffusion into chalcogenide glass (ChG) thin films is monitored by high resolution X-ray photoelectron spectroscopy (XPS). As₄₀S₆₀ and Ge₃₀Se₇₀ thin films, which are based on pyramids and tetrahedral structural units, are investigated as model materials. Survey, core level (As 3d, S 2p, Ge 3d, Ge 2p, Se 3d, Ag 3d_5/2, O 1s, C 1s) and valence band spectra have been recorded and analyzed. Reference point for the binding energy is established by the subsequent deposition of thin gold film on top of the measured samples. The chemical structure gradually changes during diffusion of silver in all the samples. The mechanism of change depends on the chemical composition, thickness of the diffused silver layer and conditions of irradiation. It is revealed that surface oxygen can play important role in the Ag photodiffusion process, leading to phase separation on the surface of the films. Photodiffusion of Ag into As₄₀S₆₀ film leads to the formation of a uniform ternary phase and arsenic oxides on the surface. The formation of ethane-like Ge₂(S_1/2)₆ units together with germanium oxidation are the main outcomes of X-ray induced Ag diffusion into Ge₃₀Se₇₀ film.     

Spin-coating of Ge23Sb7S70 chalcogenide glass thin films

Thin film Ge₂₃Sb₇S₇₀ chalcogenide glass has emerged as an important material system for photonic applications due to its high non-linear refractive index. However, one of the challenges is developing low-cost methods to deposit films of glassy material while retaining glass stoichiometry and high film quality. In this paper, we demonstrate a spin-coating technique for the deposition of such films. The dissolution mechanisms of Ge₂₃Sb₇S₇₀ in different solvents are studied in order to select the optimal solvent for film deposition. We show that the use of amine-based solvents allow the deposition of stoichiometric films in contrast to alkaline solutions. Films with low surface roughness (RMS roughness <5 nm) and controlled thickness (100–600 nm) can be deposited from solutions. We also show that annealing the films in vacuum decreases the amount of residual solvent, the presence of which is expected to lead to variation in optical properties of the thin films.     

Structural characterization of light-induced crystal growth in Se98Sb2 chalcogenide glass

The present paper reports the detailed study of crystallization morphology of light-induced crystal growth in Se₉₈Sb₂ chalcogenide glass using DSC, XRD and SEM techniques. Thermally-activated crystallization of the samples in powder form is analyzed by Differential Scanning Calorimetry (DSC) at different heating rates under non-isothermal conditions. The activation energy of crystallization has been calculated by analyzing the data using the classical Johnson–Mehl–Avrami (JMA) model.Amorphous thin films of Se₉₈Sb₂ are used for light-induced crystal growth. The d.c. conductivity of the films is taken as a characteristic quantity to measure the extent of light-induced crystal growth. X-ray diffraction (XRD) analysis has been carried out on Se₉₈Sb₂ samples for different illumination time and their diffractograms are analyzed to obtain information about various crystallographic aspects. Scanning electron microscopy has been used to confirm light-induced crystal growth.     

Critical analysis of endo-thermal effect in the glass transition process in chalcogenide glasses

Free volume model is widely used as a tool for understanding the nature of glass transition phenomenon and the related consequences. Recently, an analytical derivation of Kissinger's equation has been proposed in literature using free volume model. In this derivation, the glass transition activation energy has been assumed constant throughout the whole glass transition temperature range. The present paper investigates the applicability of free volume model for derivation of Kissinger's relation by checking the constancy of the glass transition activation energy (E_g) throughout the glass transition temperature range. Performing series of experiments, we have legalized the possibility for usage of the Kissinger relation for determination of the glass transition activation energy if the peak value of the endo-thermal effect is taken as temperature of glass transition.     

Glasses for lithography

The highest resolution in a lithographic process is often determined by the properties of the resist material. With the currently used polymeric resists, a resolution of better than 100 nm has been achieved under manufacturing conditions, but the future nanoscale devices will require a 10 times superior resolution. In this paper we present an overview of the resist materials, especially with regard to limiting resolution. In principle, inorganic resists should have higher limiting resolution than polymer resists due to smaller fundamental structural units and stronger bonds in the former. However, compositional and/or structural inhomogeneities may limit their ultimate resolution. New results are presented that indicate chalcogenide glasses as promising photo and electron beam resists, which also have the advantages of greater hardness, resistance to acids, easy fabrication in thin film form, and the unique phenomena like radiation enhanced diffusion.     

Kinetics of crystal growth of germanium disulfide in Ge0.38S0.62 chalcogenide glass

The crystal growth kinetics of GeS₂ in Ge_0.38S_0.62 glass has been studied by Differential Scanning Calorimetry (DSC) and microsopy. The linear crystal growth kinetics of both high temperature α-GeS₂ and low temperature β-GeS₂ polymorphs has been observed over a relatively broad range of temperatures, i.e. 420 < T < 494 °C that correspond to viscosity of supercooled melt: 3 × 10⁹ > η > 8 × 10⁵ Pa s. It seems that 2D nucleated growth is the most probable mechanism of crystallization for high temperature α-GeS₂ under these conditions. However, there are significant deviations for this model for the crystallization of low-temperature β-GeS₂. This might indicate some changes in crystal-melt interfacial energy or break down of Stokes–Einstein relation in that particular case. At temperatures below 500 °C the temperature range of directly observed crystal growth overlaps with isothermal DSC measurements. In this case overall crystallization kinetics can be described by the Johnson–Mehl–Avrami (JMA) nucleation-growth model for kinetic exponent n ≅ 4. The value of activation energy of nucleation estimated from these experiments E_N = 434 kJ mol⁻¹ is comparable with the activation energy of viscous flow in supercooled Ge_0.38S_0.62 melt (E_η = 478 kJ mol⁻¹). A more complex eutectic crystallization involving both GeS₂ and GeS phases has been observed at higher temperatures. This process is probably associated with secondary nucleation and cannot be described by a simple JMA model.     

Appearance of fast ionic conduction in AgI-doped chalcogenide glass powders prepared by mechanical milling

(AgI)_x(As₂Se₃)_1−x glass powders were successfully obtained up to 70 mol% AgI content by the mechanical milling method. Electrical conductivities, which show the exponential increase with addition of AgI, are comparable to the values of the corresponding melt-quenched glasses. The conductivity of 60 mol% AgI-doped glass shows great increase at the early period of the milling. A DSC signal for the β–α phases transition of crystalline AgI becomes weak at the medium stage of the milling, whereas glass transition appears clearly. EXAFS measurements at Ag, I, As, and Se K-edges have been carried out for (AgI)_0.6(As₂Se₃)_0.4 glasses with different period of the milling. The amorphization process of (AgI)_0.6(As₂Se₃)_0.4 system was accompanied by a decrease of the nearest neighboring coordination number of Ag at the medium stage of the milling. These results suggest that the structural disordering of AgI units in the glass matrix would be strongly related to the appearance of fast ionic conduction.     

Structural transformation on Se0.8Te0.2 chalcogenide glass

Using X-ray diffraction and differential scanning calorimetry (DSC), the structure and the crystallization mechanism of Se_0.8Te_0.2 chalcogenide glass has been studied. The structure of the crystalline phase has been refined using the Rietveld technique. The crystal structure is hexagonal with lattice parameter a = 0.443 nm and c = 0.511 nm. The average crystallite size obtained using Scherrer equation is equal 16.2 nm, so it lies in the nano-range. From the radial distribution function, the short range order (SRO) of the amorphous phase has been discussed. The structure unit of the SRO is regular tetrahedron with (r₂/r₁) = 1.61. The Se_0.8Te_0.2 glassy sample obeys the chemical order network model, CONM. Some amorphous structural parameters have been deduced. The crystallization mechanism of the amorphous phase is one-dimensional growth. The calculated value of the glass transition activation energy (E_g) and the crystallization activation energy (E_c) are 159.8 ± 0.3 and 104.3 ± 0.51 kJ/mol, respectively.     

Diffraction studies of glass structure: (V). The structure of some arsenic chalcogenide glasses

X-ray and neutron diffraction experiments have been performed on vitreous arsenic sulphide close to the composition As₂S₃ and on a series of vitreous arsenic selenides including compositions both arsenic- and selenium-rich with respect to As₂Se₃. The data have been analysed by direct examination of the diffraction patterns, by Fourier analysis and by comparison both with quasi-crystalline calculations based on the orpiment structure and with models of the first neighbour correlations using the experimental peak functions. Sheet structures occur in all the glasses, the sheet separations being larger than in the corresponding orpiment structures, and the intersheet correlation perpendicular to the sheets extends for 15 Å in arsenic sulphide and 20 Å in arsenic selenide. For the selenide glasses the sheet structure becomes more pronounced with increasing arsenic concentration. For all the glasses the data are consistent with full satisfaction of the respective two- and threefold covalency requirements of chalcogenide and arsenic atoms, excess atoms being incorporated into the structure by the formation of covalent bonds between like atoms. The distribution of arsenic-chalcogen bond lengths is shown to be symmetrical about the average values of 2.24 Å and 2.44 Å for sulphide and selenide respectively. In the selenide glasses close to the As₂Se₃ composition, the average first SeSe distance is 3.65 Å. The advantages and limitations of the experimental and analytical techniques employed are discussed and a simple model based on steric considerations is advanced to account for the influence of arsenic concentration on the inter-sheet correlation.