© (2006) AIP

Mid-infrared wavelengths in the vicinity of 10 μm are important for applications in night vision and astronomy research. At present, the common problem faced by the mid-infrared photodetectors is the high noise due to the thermal fluctuation in uncooled systems. Cooling down the detection systems not only induces additional cost, but also reduces the portability of the systems in real applications. One way to improve the signal-to-noise ratio without affecting the absorbed light power is to reduce the active detector volume.

With this aim in mind, a research team in Stanford University has exploited the use of surface plasmons1. The photodetector proposed by Z. Yu et al. consists of a 50-nm-wide and 1-μm-deep metal slit filled with an active detector material. This slit is surrounded by a series of periodic grooves and built on top of an insulating low-index oxide substrate. Two main mechanisms that enhance the light absorption are: (1) Fabry–Pérot resonances generated by the strong impedance mismatch between the modes in the slit and free-space propagating waves and (2) conversion of the incident electromagnetic waves into surface plasmons by the periodic grooves on the metal surface that can be led into the slit. By carefully choosing the geometric parameters, these two mechanisms can occur at the same wavelength. In this way, light can be collected from a much larger area and hence helps increase the absorbed energy. By adding 20 grooves with widths of 0.5 μm and depths of 0.6 μm on both sides of the slit, the team has numerically shown absorption enhancement of 250 times over the conventional photodetectors operating at 9.8 μm. This achievement suggests a promising way to reduce the active detector volume and hence increase the signal-to-noise ratio of mid-infrared photodetectors.