Impact of Duty Cycle and Nano-Grating Height on the Light Absorption of Plasmonics-Based MSM Photodetectors Farzaneh Fadakar Masouleh 1 , Narottam Das 2 , Senior Member, IEEE, and Hamid Reza Mashayekhi 1 1 Physics Group, University of Guilan, Rasht, Iran, 2 Department of Electrical and Computer Engineering, Curtin University, Perth, WA 6845, Australia e-mails: fafadakar@msc.guilan.ac.ir, narottam.das@curtin.edu.au, mashhr@guilan.ac.ir Abstract— We use finite difference time-domain (FDTD) method to calculate the light absorption enhancement of nano-grating assisted metal-semiconductor-metal photo-detectors (MSM-PDs). The simulated results show that the light absorption enhancement of nano-grating assisted MSM-PD is~9-times better than conventional MSM-PD. Index terms— Subwavelength aperture, duty cycle, nano-grating, surface plasmon polariton, FDTD simulation, MSM-PDs. I. INTRODUCTION The application of periodic structures on the metal- semiconductor-metal photodetectors (MSM-PDs) leads to effective light absorption and transmission through the subwavelength apertures. They have significant appeal in optical fiber communication, high-speed sampling, and chip to chip interconnects. The MSM-PD is a symmetrical semiconductor device which is equivalent to two back-to-back connected Schottky diodes [1]. There are two distinct mechanisms to produce transmission in one dimensional metal grating with narrow slits, which are the excitation of horizontal and vertical surface resonances. The horizontal surface resonances are excited by the periodic structure of the nano- gratings. The vertical surface resonances correspond to Fabry– Perot-like resonances of the fundamental TM guided wave in the slits [2]. The metallic gratings can exhibit absorption anomalies. One of these particularly remarkable anomalies is observed for p-polarized light only, and is due to surface plasmon polaritons (SPPs) excitations [3]. The light incident on the metal nano-grating is converted into propagating SPPs that can absorb the light efficiently in extremely thin (10’s~100’s of nm’s thick) layers. The extremely thin absorbing layers can act as a light concentrator which is essential for triggering the extra ordinary absorption (EOA) of light [4]. Subwavelength apertures have also been used to efficiently concentrate light into the deep subwavelength regions [5]. Finite-difference time-domain (FDTD) simulation results have demonstrated significant enhancement of light absorption for the design of ultrafast MSM-PDs [5-6]. II. DESIGNOF MSM-PD STRUCTURE Figure 1 shows a simple plasmonics-based MSM structure with gold (Au) nano-gratings etched on top of a layer of the same metal. The structure design is shown with three separate parts, namely, the metal nano-gratings (top part), the subwavelength apertures (middle part) and the substrate (bottom part). The momentum of surface plasmons can be easily changed by adding thin layers of material on the metal surface or by changing the dielectric constant of the material deposited on it. Here, the gold (Au) metal nano-gratings were deposited on top of the layer containing subwavelength apertures and the layer is only on the semiconductor (GaAs) substrates. Fig. 1. Schematic diagram of the MSM-PD structure with rectangular shaped nano-gratings on top of the subwavelength apertures. The subwavelength apertures are just on top of the semiconductor (GaAs) substrates. For a metal nano-grating period of Λ, the conservation of momentum in the direction parallel to the nano-gratings lead to the following relationship.  ௫ሺ௢௨௧ሻ . sin ߠൌ ௫ሺ௜௡ሻ . sin ߠേ  ੗ ൌ ఠ ௖ ට ఌ ೘ ′ ఌ ೏ ఌ ೘ ′ ାఌ ೏ (1) When the plasmonic excitations occur then the left side of equation (1) matches the wave vector of the excited SPP ( ܭ ௌ௉௉ ሻ. Here, m is an integer corresponding to the order of the outgoing diffracted beam,  ੗ ൌ 2п/Λ is the grating wave vector,  is the angular frequency of the incident light wave with θ as the angle of incidence and c is the speed of light. III. RESULTS AND DISCUSSION A. Impact of nano-grating height on LAEF In this sub-section, we will discuss the influence of nano- grating height on the light absorption enhancement. Fig. 2 shows the light absorption enhancement factor (LAEF) spectra for different nano-grating heights with 60% duty cycle and the NUSOD 2012 13 978-1-4673-1604-0/12/$31.00 ©2012 IEEE