Bragg mirrors with modified period for sub-10-nm wavelengths

Abstract

Bragg mirrors with superlattices and depth grading of their period are studied numerically at sub-10-nm wavelengths, using La/B4C and Cr/Sc as bilayer materials at 6.64 and 3.12 nm, respectively. Superlattice-related reflection peaks (between normal incidence and total reflection) are very narrow, and sharpen with decreasing wavelength. Depth grading reduces and broadens the peaks. Together with weighting by source spectra, all-angle reflection is demonstrated. For the normal-incidence peaks, grading and weighting increases their overlap with source spectra and allows enhancement of the total collected power by up to 120% (at 6.64 nm) and 90% (3.12 nm) compared to ungraded mirrors. Interface roughness, determined from experimental data, approximately halves these enhancements.