Design of midinfrared photodetectors enhanced by surface plasmons on grating structures

The authors propose to exploit the unique properties of surface plasmons to enhance the signal-to-noise ratio of midinfrared photodetectors. The proposed photodetector consists of a slit in a metallic slab filled with absorptive semiconductor material. Light absorption in the slit is enhanced due to Fabry-Perot resonances. Further absorption enhancement is achieved by surrounding the slit with a series of periodic grooves that enable the excitation of surface plasmons that carry electromagnetic energy towards the slit. Using this scheme, they design and optimize a photodetector operating at 0=9.8 m with a roughly 250 times enhancement in the absorption per unit of volume of semiconductor material compared to conventional photodetectors operating at the same wavelength. © 2006 American Institute of Physics. DOI: 10.1063/1.2360896 Midinfrared photodetectors and imaging systems operating in the vicinity of 10 m wavelength are important in applications ranging from night vision to astronomy research. 1 To suppress noise caused by thermal fluctuation, usually these detection systems have to be cooled, 1‐3 which greatly increases the cost and limits the portability. To improve signal/noise ratio, it is desirable to reduce the active detector volume from which the noise arises without affecting the absorbed light power. In this letter, we computationally explore the use of surface plasmons in these systems to improve the signal/noise ratio. The proposed structure, shown in Fig. 1a, consists of a metallic slit, surrounded by a linear grating structure. The entire structure is placed on top of an insulating oxide. The slit is filled with absorptive semiconductor material, and has a width that is far smaller than the operating wavelength. The metal regions serve both as electrical contacts and as a concentrator that funnels light into the deep-subwavelength slit. 4‐6 To maximize the concentration effects, we combine two distinct mechanisms that relate to the presence of the slit and the grating structure. 7 A metal slit supports a propagating TE mode, even when the width of the slit is at deepsubwavelength scales. Moreover, due to the strong impedance mismatch between the modes in the slit and free-space propagating waves, strong reflection can occur at the entrances to the slit. Consequently, with an appropriate choice of the length, the slit forms a Fabry-Perot resonator, and light absorption in the slit is resonantly enhanced. The grating, on the other hand, enhances the light absorption in the slit by converting incident electromagnetic EM waves into surface plasmons on the metal surface that can be funneled into the slit. 8 The area from which EM energy is collected can be substantially larger than the slit width and gives rise to significant enhancements in the absorbed energy. With a careful choice of the geometric parameters of the structure, these two mechanisms can be made to occur at the same wavelength, resulting in an optimized efficiency of the devices. To simulate the response of this structure for a normally incident EM plane wave, we use a two-dimensional finitedifference frequency-domain method, 9,10 which directly uses experimental data for the frequency-dependent dielectric constant of metals such as gold, 11 including both the real and imaginary parts, with no further approximation. For the detective materials in the slit, we have chosen to use HgCdTe MCT with a frequency independent dielectric permittivity of =12.5+i1.75. 2,11 The substrate is a low-index oxide