Abstract
The development of photonic integrated circuits would benefit from a wider selection of materials that can strongly control near-infrared (NIR) light. Transition metal dichalcogenides (TMDs) have been explored extensively for visible spectrum optoelectronics; the NIR properties of these layered materials have been less-studied. The measurement of optical constants is the foremost step to qualify TMDs for use in NIR photonics. Here, we measure the complex optical constants for select sulfide TMDs (bulk crystals of MoS2, TiS2, and ZrS2) via spectroscopic ellipsometry in the visible-to-NIR range. We find that the presence of native oxide layers (measured by transmission electron microscopy) significantly modifies the observed optical constants and need to be modeled to extract actual optical constants. We support our measurements with density functional theory calculations and further predict large refractive index contrast between different phases. We further propose that TMDs could find use as photonic phase-change materials, by designing alloys that are thermodynamically adjacent to phase boundaries between competing crystal structures, to realize martensitic (i.e., displacive, order–order) switching.
Highlights
Polymorphism suggests that transitions between structural phases, such as the trigonal prismatic 2H and octahedral 1T, may be useful for optical switching.20,21 Transitions between 2H and 1T can be described by a simple translation of a plane of chalcogen atoms.22 The layered, van der Waals crystal structure suggests that martensitic transformation strain may be low, which is beneficial for switching energy and fatigue
We find that the presence of native oxide layers significantly modifies the observed optical constants and need to be modeled to extract actual optical constants
We support our measurements with density functional theory calculations and further predict large refractive index contrast between different phases
Summary
Polymorphism suggests that transitions between structural phases, such as the trigonal prismatic 2H and octahedral 1T (or distorted 1T’ and 1Td), may be useful for optical switching.20,21 Transitions between 2H and 1T can be described by a simple translation of a plane of chalcogen atoms (a martensitic transformation).22 The layered, van der Waals crystal structure suggests that martensitic transformation strain may be low, which is beneficial for switching energy and fatigue. We measure the complex optical constants for select sulfide TMDs (bulk crystals of MoS2, TiS2, and ZrS2) via spectroscopic ellipsometry in the visible-to-NIR range.
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