Seventeen infrared-transmitting GeAsSe chalcogenide glasses were fabricated to determine the role of chemistry and
structure on mid-wave infrared (MWIR) optical properties. The refractive index and thermoptic coefficients of
samples were measured at λ = 4.515 μm using an IR-modified Metricon prism coupler, located at University of
Central Florida. Thermo-optic coefficient (dn/dT) values were shown to range from approximately -40 ppm/°C to
+65 ppm/°C, and refractive index was shown to vary between approximately 2.5000 and 2.8000. Trends in
refractive index and dn/dT were found to be related to the atomic structures present within the glassy network, as
opposed to the atomic percentage of any individual constituent. A linear correlation was found between the quantity
(n-3•dn/dT) and the coefficient of thermal expansion (CTE) of the glass, suggesting the ability to compositionally
design chalcogenide glass compositions with zero dn/dT, regardless of refractive index or dispersion performance.
The tunability of these novel glasses offer increased thermal and mechanical stability as compared to the current
commercial zero dn/dT options such as AMTIR-5 from Amorphous Materials Inc. For IR imaging systems designed
to achieve passive athermalization, utilizing chalcogenide glasses with their tunable ranges of dn/dT (including zero)
can be key to addressing system size, weight, and power (SWaP) limitations.
This work reports a detailed study of the processing and photo-patterning of two chalcogenide glasses (ChGs) − arsenic trisulfide (As2S3) and a new composition of germanium-doped arsenic triselenide Ge5(As2Se3)95 − as well as their use for creating functional optical structures. ChGs are materials with excellent infrared (IR) transparency, large index of refraction, low coefficient of thermal expansion, and low change in refractive index with temperature. These features make them well suited for a wide range of commercial and industrial applications including detectors, sensors, photonics, and acousto-optics. Photo-patternable films of As2S3 and Ge5(As2Se3)95 were prepared by thermally depositing the ChGs onto silicon substrates. For some As2S3 samples, an anti-reflection layer of arsenic triselenide (As2Se3) was first added to mitigate the effects of standing-wave interference during laser patterning. The ChG films were photo-patterned by multi-photon lithography (MPL) and then chemically etched to remove the unexposed material, leaving free-standing structures that were negative-tone replicas of the photo-pattern in networked-solid ChG. The chemical composition and refractive index of the unexposed and photo-exposed materials were examined using Raman spectroscopy and near-IR ellipsometry. Nano-structured arrays were photo-patterned and the resulting nano-structure morphology and chemical composition were characterized and correlated with the film compositions, conditions of thermal deposition, patterned irradiation, and etch processing. Photo-patterned Ge5(As2Se3)95 was found to be more resistant than As2S3 toward degradation by formation of surface oxides.
Arsenic trisulfide (As2S3) is a chalcogenide (ChG) material with excellent infrared (IR) transparency (620 nm to 11 μm),
low phonon energies, and large nonlinear refractive indices. These properties directly relate to commercial and industrial
applications including sensors, photonic waveguides, and acousto-optics. Multi-photon exposure can be used to photopattern
thermally deposited As2S3 ChG glassy films of molecular clusters. Immersing the photo-patterned cross-linked
material into a polar-solvent removes the unexposed material leaving behind a structure that is a negative-tone replica of
the photo-pattern. Nano-structure arrays that were photo-patterned in single-layered As2S3 films through multi-photon
direct laser writing (DLW) resulted in the production of nano-beads as a consequence of a standing wave effect. To
overcome this effect, an anti-reflective (AR) layer of arsenic triselenide (As2Se3) was thermally deposited between the
silicon substrate and the As2S3 layer, creating a multi-layered film. The chemical composition of the unexposed and
photo-exposed multi-layered film was examined through Raman spectroscopy. Nano-structure arrays were photopatterned
in the multi-layered film and the resulting structure, morphology, and chemical composition were
characterized, compared to results from the single-layered film, and correlated with the conditions of the thermal
deposition, patterned irradiation, and etch processing.
Arsenic trisulfide (As2S3) is a transparent material from ~620 nm to 11 μm with direct applications in sensors, photonic
waveguides, and acousto-optics. As2S3 may be thermally deposited to form glassy films of molecular chalcogenide
(ChG) clusters. It has been shown that linear and multi-photon exposure can be used to photo-pattern thermally deposited
As2S3. Photo-exposure cross-links the film into a network solid. Treating the photo-patterned material with a polarsolvent
removes the unexposed material leaving behind a structure that is a negative-tone replica of the photo-pattern. In
this work, nano-structure arrays were photo-patterned in As2S3 films by multi-photon direct laser writing (DLW) and the
resulting structure, morphology, and chemical composition were characterized and correlated with the conditions of the
thermal deposition, patterned irradiation, and etch processing. Raman spectroscopy was used to characterize the
chemical structure of the unexposed and photo-exposed material, and near infrared ellipsometry was used to measure the
refractive index. Physical characterization including structure size and surface adhesion of nano-scale features is related
to the processing conditions.
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.
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.
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 <r> = 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.
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