Light-Harvesting Microconical Arrays For Enhancing Infrared Imaging Devices: Proposal And Demonstration

Light-harvesting low-index (n=1.6) microconical arrays are proposed for increasing the sensitivity and the signal-to-noise ratio (SNR) of mid-wave infrared (MWIR) focal plane arrays used in thermal cameras. The light is collected by the microcone's top base with diameter (D-t) and delivered to its bottom base with the wavelength-scale diameter (D-b), which is coupled to the photodetector mesa. The power enhancement factor (PEF) is defined as a ratio of the powers delivered to the photodetector with and without the microcone. By using numerical modeling, it is shown that in the 3D case the geometrical limit of PEF3-D=(D-t/D-b)(2) cannot be reached due to optical losses, but the values of PEF3D similar to 100 can be achieved in the MWIR range for slightly tapered (<= 10 degrees) microcones with narrow bottom bases (D-b <= 4 mu m) and significant height (h >= 120 mu m). To demonstrate the light concentrating capability, the microconical arrays with D-t/D-b=60 mu m/8 mu m and h=150 mu m were directly fabricated in photoresist by using a nanoscribe tool on top of the front-illuminated Ni/Si Schottky-barrier short-wave infrared photodetectors with 22 mu m mesas, and threefold enhancement in the photocurrent response was observed. Due to expected reduction of the thermal noise for compact photodetector mesas, the proposed approach permits an increase in the SNR and the operation temperature of the MWIR imaging devices.