Seeing glass in a new light

When we first learn about optics and manipulating light, we encounter glass—a hearty, low-cost material comprised mostly of silicon and oxygen, with a smattering of other elements. Silicate glasses work exceptionally well for lasers, imaging systems, sensors, and optical fibers that operate at visible wavelengths. Silicates absorb infrared (IR) light, so for IR applications, more exotic materials are used—like silicon, germanium, and zinc selenide. But these are crystalline, brittle, fragile, costly, and difficult to process.

Researchers have long sought alternative materials and processes for IR optics. Now, chalcogenide glasses (ChG) are coming into sharp focus as go-to materials for IR applications. Chalcogenides are elements found on the periodic table in the column starting with oxygen, and including below it sulfur, selenium, and tellurium. Chalcogenides are fascinatingly varied. Under ambient conditions, oxygen is a diatomic gas, yet sulfur is fluffy yellow molecular solid, and selenium and tellurium are long chains of atoms that form a semiconducting solid. Combining the heavier chalcogenides with silicon, germanium, and other elements yields new glasses with intriguing properties. Our understanding of ChG is becoming sufficiently mature that glass scientists and engineers can cook up novel compositions with optical, thermal, and mechanical properties that make them well suited for IR applications. Additionally, ChG compositions and processing can be tuned so that lasers and other direct-write methods can be used to 3D print useful optical forms—like lenses, gratings, and integrated photonics—opening entirely new capabilities in IR optics.

In a recently published article in the Journal of Optical Microsystems, a team of scientists and engineers from the United States highlights advances in patterning ChG using multiphoton lithography (MPL) and thermal scanning probe lithography (TSPL). These techniques offer spatial resolution that beats the classical diffraction limit and provides a route to nanophotonic devices. ChG-devices created by MPL and TSPL have applications in opto-electronics, detectors, sensors, photonics waveguides, acousto-optics, and optical data storage.

MPL can be used to create sub-micron-scale structures in thermally deposited ChG by exposing it with tightly focused femtosecond laser pulses. Unexposed material is then chemically etched away, revealing the photopatterned structure. Previously, structures created in single-layer As₂S₃ films were subject to delamination and interference effects. The authors optimized MPL and used multilayer ChG films to improve adhesion, suppress interference, and produce self-supporting cylindrical As₂S₃ structures with lateral widths down to 120 nm.

They then used the approach to fabricate the first IR lens created with MPL that focuses through gradient refractive index (GRIN). With conventional lenses, a large chunk of material is spatially shaped to bend light rays. GRIN lenses focus by spatially varying the refractive index across a thin surface. As a result, ChG GRIN lenses can be compact, lightweight, transmissive in the IR, low-aberration, and even optimized for various thermal and mechanical conditions.

The team also used TSPL to write structures in GSST—a ChG containing germanium, antimony, selenium, and tellurium. GSST is a phase-change material that can be thermally patterned without chemical etching. TSLP was used to create 100-nm-wide serpentine lines in GSST for the first time. Producing subwavelength features in GSST is of significant interest for photonic switches and active metasurfaces.

The team includes researchers from Ursinus College, the University of Central Florida, Lockheed Martin, and the Massachusetts Institute of Technology.

"We are a highly interdisciplinary team connected both in our interest in unique IR materials and in our motivation to create innovative routes to exciting new optical and photonic systems," says lead author Casey Schwarz, assistant professor of physics.

"The key to this technology is the young scientists we train," says author Stephen Kuebler, professor of chemistry and optics. "They have the interdisciplinary know-how needed to apply chalcogenide glasses and make stunning breakthroughs in optics."

Read the open access article by C. M. Schwarz et al., "Structurally and morphologically engineered chalcogenide materials for optical and photonic devices," J. Opt. Microsyst. 1(1), 013502 (2021), doi 10.1117/1.JOM.1.1.013502