Physics / mechanism
Thin-film interference layers deposited on optical or semiconductor surfaces to suppress Fresnel reflection losses. A single-layer quarter-wave coating (optical thickness = λ/4, refractive index n = √(n_substrate × n_incident)) yields destructive interference of reflected waves. Multi-layer stacks (V-coats, broadband BBAR, rugate) extend bandwidth and hit <0.1% reflectance across 400–1600 nm. Key parameters: refractive index contrast, layer thickness uniformity (±0.5 nm tolerance matters), adhesion, laser damage threshold (LIDT), and environmental durability (MIL-C-48497). Deposition methods: ion-beam sputtering (IBS) for highest precision, PECVD for wafer-scale integration, ALD for conformal coverage on 3D features. SiN, SiO₂, TiO₂, MgF₂, and Al₂O₃ are the dominant materials.
Competitive landscape
Competing approaches include surface texturing (moth-eye nanostructures, black silicon) which achieves broadband low reflection without thin-film deposition but adds process complexity and scatters light at feature-scale defects. Gradient-index (GRIN) layers offer smooth impedance matching. Subwavelength grating (SWG) structures are CMOS-compatible but lithography-limited.
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Frontier (open questions)
- To be added.