Background
Antireflection coatings (“ARC’s”) minimize undesired lens flare and glare and increase transmission at the interface between an optical material and its medium. While the inorganic fluorides, oxides, and sulfides that presently comprise conventional broadband ARC’s fulfill many if not all of their required functions, their brittleness is a significant limitation when applied to elastomeric substrates, which are being explored for advanced optical systems given the increasingly compact, flexible, and adaptive nature of high-performance optical and optoelectronic devices in recent years. On the other hand, polymer thin film coatings, possessing elastic properties and thermal expansivities comparable to the underlying substrate, are limited by their narrow range of refractive indexes. Increasing layer count can compensate for this, but solution processing and coextrusion techniques lack the key ability to monitor in situ nascent film thickness during fabrication, which complicates reproducible thickness control and tuning. Molecular layer deposition, chemical vacuum deposition (CVD), plasma-enhanced CVD, and substrate surface patterning are alternative approaches whose promise for implementation is hindered by high temperature requirements, production of highly reactive intermediates from energetic plasma, and complex multi-step fabrication processes/limited mechanical durability, respectively.
Technology Overview
Researchers prepared a fully polymeric two-layer ARC on a flexible substrate at room temperature utilizing initiated-chemical vapor deposition (iCVD), an established solvent-free vacuum deposition technique that can prepare highly uniform, multilayered thin film coatings on temperature-sensitive substrates with precise thickness control. iCVD enables additional layer deposition atop previous layers, without concerns over swelling or dissolution of the coatings, and precise thickness of each layer can be realized using in situ ellipsometry, reflectometry, and/or quartz crystal microbalance (QCM). Moreover, the polymer ARC design does not require complex surface texturing or patterning. Through precise control of the process chemistry, polymer film compositions can be carefully engineered to tune optical properties, and crosslinking can be incorporated as needed to modulate mechanical properties.
Benefits
Mechanical tests following deposition onto a membrane of thermoplastic polyurethane (TPU) elastomeric substrate revealed no sign of tensile fracture in this ARC at an aggressive strain value of 1.64% (<<1% is typical for flexible optics), while all inorganic coatings (namely MgF2) began to crack and eventually fractured into several fragments at 0.3%. Simulations with established iCVD polymer chemistries having larger index contrast revealed that reflectance can be further reduced to <1% or better. After 200 strain cycles applied, increased transmittance of the ARC-coated TPU substrate was still found, and any new small cracks did not span the entire coating, leaving most of the film intact; many cracks also exhibited crack closure and return to a smooth surface upon strain removal, a “self-healing” process foreign to the inorganic films.
Applications
- Wearable displays
- Adaptive AR/VR hardware
- Flexible photovoltaics