Jonathan D. Musgraves

Incorporation of luminescent CdSe/ZnS core-shell quantum dots and PbS quantum dots into solution-derived chalcogenide glass films

CdSe/ZnS core-shell quantum dots (CSQDs) and PbS quantum dots (QDs) were synthesized using a colloidal method and incorporated into Ge23Sb7S70 glass films via a solution-derived approach to film formation. Photoluminescence (PL) from the QDs inside the glass matrix was observed in the visible (CdSe/ZnS) and near-IR (PbS) regions. Properties of the QDs were found to be environment dependent, with the amine solvent partially quenching the luminescence. The PL lifetime of the CdSe/ZnS CSQDs and PbS QDs in the glass film was decreased to varying degrees from that of the QDs in pure chloroform. Monitoring the steady-state PL intensity and luminescence lifetime of PbS doped films showed that appropriate heat treatment of the deposited film increases the luminescence efficiency by removing residual solvent from the glass matrix.

Measurement of the refractive index dispersion of As2Se3 bulk glass and thin films prior to and after laser irradiation and annealing using prism coupling in the near- and mid-infrared spectral range

The prism coupling technique has been utilized to measure the refractive index in the near- and mid-IR spectral region of chalcogenide glasses in bulk and thin film form. A commercial system (Metricon model 2010) has been modified with additional laser sources, detectors, and a new GaP prism to allow the measurement of refractive index dispersion over the 1.5-10.6 μm range. The instrumental error was found to be ±0.001 refractive index units across the entire wavelength region examined. Measurements on thermally evaporated AMTIR2 thin films confirmed that (i) the film deposition process provides thin films with reduced index compared to that of the bulk glass used as a target, (ii) annealing of the films increases the refractive index of the film to the level of the bulk glass used as a target to create it, and (iii) it is possible to locally increase the refractive index of the chalcogenide glass using laser exposure at 632.8 nm.

Prony series spectra of structural relaxation in N-BK7 for finite element modeling.

Structural relaxation behavior of N-BK7 glass was characterized at temperatures 20 °C above and below T(12) for this glass, using a thermo mechanical analyzer (TMA). T(12) is a characteristic temperature corresponding to a viscosity of 10(12) Pa·s. The glass was subject to quick temperature down-jumps preceded and followed by long isothermal holds. The exponential-like decay of the sample height was recorded and fitted using a unique Prony series method. The result of his method was a plot of the fit parameters revealing the presence of four distinct peaks or distributions of relaxation times. The number of relaxation times decreased as final test temperature was increased. The relaxation times did not shift significantly with changing temperature; however, the Prony weight terms varied essentially linearly with temperature. It was also found that the structural relaxation behavior of the glass trended toward single exponential behavior at temperatures above the testing range. The result of the analysis was a temperature-dependent Prony series model that can be used in finite element modeling of glass behavior in processes such as precision glass molding (PGM).

Nanoscale optical features via hot-stamping of As2Se3 glass

Here we show our ability to fabricate two-dimensional (2D) gratings on chalcogenide glasses with peak-to-valley amplitude of ~200 nm. The fabrication method relies on the thermal nano-imprinting of the glass substrate or film in direct contact with a patterned stamp. Stamping experiments are carried out using a bench-top precision glass-molding machine, both on As2Se3 optically-polished bulk samples and thermally-evaporated thin films. The stamps consist of silicon wafers patterned with sub-micron lithographically defined features. We demonstrate that the fabrication method described here enables precise control of the glass’ viscosity, mitigates risks associated with internal structural damages such as dewetting, or parasitic crystallization. The stamping fidelity as a function of the Time-Force-Temperature regime is discussed, and further developments and potential applications are presented.

Chalcogenide glass thin-film optics for infrared applications

Chalcogenide glasses are increasingly used in infrared-transparent optical systems for space applications due to their relatively low density (compared to Ge or ZnSe), tunable spectral and thermo-mechanical properties, and molding capability. Remaining challenges include their application to thin-film optics and coatings. The high refractive indices of chalcogenide glasses (n ˃ 2.7) suggest the possibility for high reflecting coatings based on few periods of alternating layers with high index contrast. As2Se3 thin film deposited by thermal evaporation is investigated using ellipsometry which show optical properties consistent with bulk material. Also we demonstrate a novel method for fabrication of antireflection coating using porous chalcogenide. Possibility of negligible extinction coefficient and low refractive index of this porous coating promises broadband suppression of undesired Fresnel reflections at the interface from infrared optics.

Thermal properties of chalcogenide glasses

Abstract: Thermal analysis techniques provide powerful tools for both the fundamental understanding and practical applications of chalcogenide glasses. Differential scanning calorimetry, thermogravimetric, and thermomechanical analysis all provide insight not only into those properties of chalcogenide glasses which are critical for hot-forming applications, such as precision molding or fiber drawing, but also into the atomic-scale structural units that give rise to this material behavior. Similarly, a clear understanding of the viscosity–temperature relationship in chalcogenides is necessary for glass processing, from melting to fabrication, but also gives deep insight into fundamental structural differences between sulfide, selenide, and telluride glasses.

Compositional-tailoring of optical properties in IR transparent chalcogenide glasses for precision glass molding

The structural and optical properties of AsSe chalcogenide glass, starting with As40Se60, were studied as a function of Ge or Se additions. These elements provide broad glass forming options when combined with the host matrix to allow for compositional tuning of properties. Optimization of glass composition has been shown to produce bulk glasses with a thermoptic coefficient (dn/dT) equal to zero, as well as a composition which could demonstrate a net zero change in index after precision glass molding (PGM). The bulk glass density, coefficient of thermal expansion (CTE), refractive index, and dn/dT were measured for all bulk compositions, as was the refractive index after PGM. For the bulk glasses examined, both the refractive index (measured at discrete laser wavelengths from 3.4 to10.6 μm) and dn/dT were observed to decrease as the molecular percentage of either Ge or Se is increased. Compared to the starting glass’ network, additions of either Ge or Se lead to a deviation from the “optimally constrained” binary glass’ average coordination number = 2.4. Additions of Se or Ge serve to decrease or increase the average coordination number (CN) of the glass, respectively, while also changing the network’s polarizability. After a representative PGM process, glasses exhibited an “index drop” consistent with that seen for oxide glasses.1 Based on our evaluation, both the Gecontaining and Ge-free tielines show potential for developing unique compositions with either a zero dn/dT for the unmolded, bulk glass, as well as the potential for a glass that demonstrates a net zero “index drop” after molding. Such correlation of glass chemistry, network, physical and optical properties will enable the tailoring of novel compositions suitable for prototyping towards targeted molding behavior and final properties.

Chalcogenide-mold interactions during precision glass molding (PGM) of GeAsSe glasses

Five chalcogenide glasses in the GeAsSe ternary glass system were melted, fabricated into flats, and molded between planar, uncoated, binderless WC molds using a laboratory-scale precision glass molding machine. The five glasses originate at the binary arsenic triselenide (As40Se60) and are modified by replacing As with Se in 5 mol% increments, or by locking the As:Se ratio and adding Ge, also in 5 mol% increments. The glasses are separated into two groups, one for the Ge-free compositions and the other for the Ge-containing compositions. This effort analyzes the differences between the Ge-containing and the Ge-free glasses on the post-molded glass and mold surface behavior, as well as the mold lifetime. Fabrication features, such as scratch and/or dig marks were present on the glass and mold surfaces prior to the PGM process. White light interferometry analysis of the surfaces shows an overall reduction in the RMS roughness of the glass after molding, and an increase of the roughness of the molds, after 15 molding cycles. After molding, the quantity of observable defects, primarily deposits and dig marks are increased for both the glass and mold surfaces. Deposits found on the WC molds and glasses were analyzed using Electron Dispersive X-ray Spectroscopy (EDS) and showed no evidence of being due to material transfer between the WC molds and the glass constituents. In general the main observable difference in the analysis of the two post molded sets, despite the changes in chemistry, is the quantity of molding induced defects near the edge of the GeAsSe samples.