Thin layer broadband porous chromium black absorber fabricated through wet-etching process

A thin layer porous chromium (Cr) black absorber was fabricated on a polyimide (PI) substrate with 2 inch diameter and 500 nm thickness. The chromium black was prepared by electron beam evaporation and wet-etching process. To optimize the parameters of the absorber, the Cr black was firstly fabricated on silicon and quartz wafers. A high average absorption of 93% over the whole visible spectrum (320 nm to 800 nm) was obtained by 3 min wet-etching of a 400 nm thick metal Cr film. The absorption was higher than 65% when the spectrum extended to near infrared from 800 to 1800 nm. The mechanism of the ultra-broadband absorption can be explained by the light trapping by numerous nanovoids formed inside the Cr film. The nanovoid acts as a blackbody cavity, where the incident light experienced multiple reflections. Using the optimized parameters obtained with silicon and quartz wafers, the Cr black absorber was fabricated on a PI film. Due to its porous structure (low density) and thin thickness, the Cr black/PI composite film showed a strong light absorption and a high optical thermal response. Compared to a PI film without Cr black layer, the average absorption of the composite film was increased from 5.0% to 93.4%, the optical thermal response was improved by 43.5 times. This property highlights its potential applications in various fields such as photo detection and thermal imaging.