Peter Wachtel

Mid-infrared materials and devices on a Si platform for optical sensing

Abstract In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiNx waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.

Composition dependence of the viscosity and other physical properties in the arsenic selenide glass system

The viscosity of the AsxSe100−x family of glasses has been measured for 10 ≤ x ≤ 40 using beam bending and parallel plate viscometry, and fit with the Vogel-Fulcher-Tamann (VFT) viscosity model. Measurement of other physical properties of the glasses, including the density, glass transition temperature, and coefficient of thermal expansion has been conducted in order to accurately calculate the viscosity as a function of temperature and glass composition. The variation in fragility of the glasses is explained in the context of frozen-in configurational entropy in the glasses. This configurational entropy has minima at the endpoints of the one-dimensional network of amorphous selenium and the fully three-dimensional network of As40Se60, and an apparent maximum at the composition As30Se70. The frozen-in configurational entropy can be well described by a modified entropy of mixing of two solid solutions model, implying that the topological contribution to configurational entropy is nearly constant across the...

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).

Evidence of spatially selective refractive index modification in 15GeSe 2 -45As 2 Se 3 -40PbSe glass ceramic through correlation of structure and optical property measurements for GRIN applications

Thermally-induced nucleation and growth of secondary crystalline phases in a parent glass matrix results in the formation of a glass ceramic. Localized, spatial control of the number density and size of the crystal phases formed can yield ‘effective’ properties defined approximately by the local volume fraction of each phase present. With spatial control of crystal phase formation, the resulting optical nanocomposite exhibits gradients in physical properties including gradient refractive index (GRIN) profiles. Micro-structural changes quantified via Raman spectroscopy and X-ray diffraction have been correlated to calculated and measured refractive index modification verifying formation of an effective refractive index, neff, with the formation of nanocrystal phases created through thermal heat treatment in a multi-component chalcogenide glass. These findings have been used to define experimental laser irradiation conditions required to induce the conversion from glass to glass ceramic, verified using simulations to model the thermal profiles needed to substantiate the gradient in nanocrystal formation. Pre-nucleated glass underwent spatially varying nanocrystal growth using bandgap laser heating, where the laser beam’s thermal profile yielded a gradient in both resulting crystal phase formation and refractive index. The changes in the nanocomposite’s micro-Raman signature have been quantified and correlated to crystal phases formed, the material’s index change and the resulting GRIN profile. A flat, three-dimensional (3D) GRIN nanocomposite focusing element created through use of this approach, is demonstrated.

Performance Evaluation of a Bench-Top Precision Glass Molding Machine

A Dyna Technologies Inc. GP-5000HT precision glass molding machine has been found to be a capable tool for bridging the gap between research-level instruments and the higher volume production machines typically used in industry, providing a means to apply the results of rigorous instrumentation analysis performed in the lab to industrial PGM applications. The GP-5000HTs thermal and mechanical functionality is explained and characterized through the measurement baseline functionality and the associated error. These baseline measurements were used to determine the center thickness repeatability of pressed glass parts, which is the main metric used in industrial pressing settings. The baselines and the repeatability tests both confirmed the need for three warm-up pressing cycles before the press reaches a thermal steady state. The baselines used for pressing a 2 mm glass piece to a 1 mm target center thickness yielded an average center thickness of 1.001 mm and a standard deviation of thickness of 0.0055 mm for glass samples pressed over 3 consecutive days. The baseline tests were then used to deconvolve the sources of error of final pressed piece center thickness.

Engineering novel infrared glass ceramics for advanced optical solutions

Advanced photonic devices require novel optical materials that serve specified optical function but also possess attributes which can be tailored to accommodate specific optical design, manufacturing or component/device integration constraints. Multi-component chalcogenide glass (ChG) materials have been developed which exhibit broad spectral transparency with a range of physical properties that can be tuned to vary with composition, material microstructure and form. Specific tradeoffs that highlight the impact of material morphology and optical properties including transmission, loss and refractive index, are presented. This paper reports property evolution in a representative 20 GeSe2-60 As2Se3-20 PbSe glass material including a demonstration of a 1D GRIN profile through the use of controlled crystallization.

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.

Characterization of structural relaxation in As2Se3 for analysis of lens shape change in glass press mold cooling and post-process annealing

This study explores the structural relaxation behavior of As2Se3 by thermo mechanical analysis in order to characterize and eventually predict volume change in As2Se3 upon relaxation during cooling after precision glass molding (PGM) and annealing. A vertical beam of As2Se3 was placed in a thermo mechanical analyzer (TMA) and fully relaxed at a given temperature. The temperature was then quickly changed a given amount and the 1-D relaxation of the beam was measured until it reached equilibrium at the new temperature. The resultant curve was then fit with a Prony series which captured the relaxation data. The mathematical representation of the relaxation is then analyzed as a function of time, temperature, and quench rate and can be used to predict one dimensional (1-D) length change upon relaxation. A maximum of three terms is needed to describe the relaxation behavior and that number declines with an increase in temperature. This decay of the number of Prony terms needed to describe relaxation points to a structure that relaxes with less complexity as it approaches Tg. These trends can be converted to 3-D due to the amorphous and therefore typically isotropic nature of As2Se3 glass. This volume change information as a function of vital processing parameters can then be used to predict the change in shape of a work piece during cooling or post process annealing within a precision molding cycle. The mathematical representation of volume relaxation can then be applied to finite element models (FEM) of As2Se3 lenses or other optical elements.

Using design of experiments to improve precision glass moulding

A design of experiments (DOE) approach was used to characterise the effect of process parameters, including heating and cooling rates, soaking times, moulding viscosities, and forces applied during the moulding and cooling stages on the repeatability of the final thickness of moulded N-BK7 and LBAL35 glasses using a precision glass moulding (PGM) machine. Analysis of the DOE showed that process parameters that lengthen the overall time of the moulding process tend toward more repeatable final thicknesses. Using the ideal parameters found by the DOE, the error in the final thickness was held within ±0.05 mm.

IRradiance Glass: Technology Transfer from University to Industry

An overview of IRradiance Glass, Inc. and our development process for advanced chalcogenide glasses for infrared optics are presented. The physical and optical properties of chalcogenide glasses are reviewed, and some topics in infrared sensing are introduced to the reader. The impact of gradient refractive index (GRIN) materials on the functionality of optical elements is discussed, and the IRradiance Glass GRIN materials are described, explaining the technical challenges faced in moving this technology from the laboratory to the manufacturing floor. The chapter ends with a discussion of the technology transfer process, whereby intellectual property developed in a university setting is licensed to commercial entities for development. Future directions in both GRIN optics and technology transfer processes are discussed.