Laterally Graded Rugate Filters in Porous Silicon

Abstract

Rugate optical reflectance filters with position dependent reflectance peaks in the visible to near infrared spectrum were realized in porous silicon (PS). Filters with strong reflection peaks, near 100%, no detectable higher order harmonics and suppressed sidebands compared to discrete layer filters were obtained by varying the current density continuously and periodically during etching. An in-plane voltage up to 1.5 V was used to obtain refractive index and periodicity change along the filter surface resulting in reflectance peak shifts of up to 100 nm/mm in the direction of the voltage drop. The effect of the lateral change in optical parameters on the filter characteristics is studied by varying the gradient and comparing measurements at different positions with measurements on a non-graded filter. We have observed extra features in the reflectance spectrum of these graded filters compared with reflectance from a non-graded filter which is likely caused by the gradient.

Porous silicon 2D photonic crystals

“…Recently, a few research groups have shown their results devoted to porous silicon structures with gradually changing optical parameters across the field of view [8][9][10]. Also, it was demonstrated that varying the applied potential across the backside of the wafer allows precise control of the local current density through wafers and consequently, the spectral position of the resonance peak.…”