Anomalous effective medium approximation breakdown in deeply subwavelength all-dielectric photonic multilayers

Area of Science:

• Optics and Photonics
• Materials Science

Background:

• Effective medium approximation (EMA) is widely used for modeling composite materials.
• All-dielectric multilayer structures with deep subwavelength features (period ≤λ/50) present unique challenges for EMA.

Purpose of the Study:

• To comprehensively analyze the applicability of EMA to deeply subwavelength all-dielectric multilayer structures.
• To identify limitations of EMA and conditions where it fails for these structures.

Main Methods:

• Theoretical analysis of dispersion relations for actual multilayer stacks.
• Comparison of optical properties (reflectance) between actual multilayers and their EMA models.
• Investigation of the influence of structural parameters like period variations and termination.

Main Results:

• Dispersion relations show only slight differences between actual multilayers and EMA predictions.
• Significant discrepancies in optical properties, particularly reflectance near the critical angle, are observed.
• Reflectance difference can reach up to 0.98, strongly dependent on period variations and termination.

Conclusions:

• EMA can be inaccurate for deeply subwavelength all-dielectric multilayers, despite similar dispersion relations.
• Subwavelength features significantly influence optical properties, especially near critical angles.
• Findings are crucial for accurate multilayer ellipsometry and advanced sensing applications.