Surface current confinement in circular ring optical antennas and its enhancement effect to the photoresponse of longwave infrared photodetectors

Abstract

In this paper, we report the analysis of surface current confinement in circular ring optical antennas and its enhancement effect to the photoresponse of quantum dot (QD) longwave infrared (LWIR) photodetectors. Circular ring optical antennas with various inner and outer diameters were simulated to determine the electric near-field (E-field) and surface current distribution. Over 80 times E-field enhancement was obtained when the width of the ring is as narrow as 2 nanometers (nm) where the surface current is strongly confined. In addition, the E-field (|E|) shows a direct linear relationship with the surface current density. The linear relationship is analyzed using an RLC circuit model. The E-field enhancement of the circular ring optical antenna is compared with that of the two-dimensional metallic subwavelength hole arrays structure (2DSHA) and is found to have higher enhancement with a broader spectral band coverage. The circular ring optical antenna enhanced LWIR QD infrared photodetectors were fabricated and measured. The ring optical antenna with stronger surface current confinement shows a higher photocurrent enhancement. The experimental results agree well with the simulation. Broader band photocurrent enhancement than the 2DSHA structure was also verified.