Colossal optical anisotropy in wide-bandgap semiconductor CuAlO2

TL;DR

This paper reports that CuAlO2 exhibits unprecedented optical anisotropy and transparency across the entire visible spectrum, driven by its unique structure and excitonic properties, making it promising for advanced photonic applications.

Contribution

The study demonstrates for the first time that CuAlO2 has colossal optical anisotropy and transparency across the visible spectrum, supported by experimental measurements and theoretical analysis.

Findings

Maximum birefringence of 3.67 reported

Linear dichroism of 5.21 observed

Birefringence remains over 0.5 throughout visible range

Abstract

Colossal optical anisotropy in the entire visible spectrum is crucial for advanced photonic applications, enabling precise light manipulation without optical loss across a broad spectral range. Here, we demonstrate that CuAlO2 exhibits colossal optical anisotropy and transparency across the visible spectrum, enabled by its unique three-dimensional O-Cu-O dumbbell structure and two-dimensionally confined excitons. Using mm-sized single crystals, we independently measured ab-plane and c-axis optical properties, revealing maximum birefringence (= 3.67) and linear dichroism (= 5.21), the highest reported to date. CuAlO2 retains birefringence over 0.5 throughout the entire visible range and possesses a wide direct bandgap of 3.71 eV, surpassing the birefringence of commercial anisotropic crystals transparent in the visible spectrum. From the two-dimensional screened hydrogen model and first-principles calculations, we demonstrate that the colossal anisotropy arises from a unique excitonic Cu d-p transition confined to the atomic-thick layer. This colossal optical anisotropy and transparency across the entire visible spectrum makes CuAlO2 a promising candidate for future photonic technologies.