Broadband enhancement of devices and microsystems for light harvesting and photocatalysis

Abstract

Plasmonic materials, stochastically roughened, can provide localized field enhancement under optical irradiation (light trapping). The use of plasmonic materials in semiconductor photocatalists also leads to drastic enhancement of their photoreactivity. Plasmonic enhancement can be seen in an extension of the useful range of solar spectrum, and in a high increase of the amount of collected light energy. The structures we propose here not only can be utilized for a vast majority of photocatalytic systems (including those microreactor-based,) but also for the enhancement of light harvesting devices of any kind, including all types of solar cells, as well as for ultrasensitive chemical sensing. Here we consider a structure with a roughened metallic film on the bottom and a roughened transparent conductive oxide (TCO) or silica glass layer on the top, separated by a thin dielectric layer. We fabricated experimental structures and simulated their electromagnetic properties by the finite element method. Thus we verified that the combined influence of two rough films significantly improves light trapping within the dielectric layer between them, while plasmonic effects lead to concentration of electromagnetic field into subwavelength volumes. Consequently, the radiation intensities are vastly increased within the dielectric layer, even for several orders of magnitude, compared to the incident beam. Another interesting property observed is that while both TCO and silica glass structures act as diffractive light traps well into the UV range, the TCO structure simultaneously acts as a plasmonic superabsorber for IR light, turning the increase of TCO opaqueness with wavelength into a benefit since it significantly expands the useful spectral range. Tuning of dopant levels in TCO or using alternative plasmonic materials, together with varying coarseness of the films can further be used to tailor the electromagnetic response.