Enhancement of Light Absorption in Plasmonic Based Photodetector with Double Nanograting Structure


 This era of high speed photonic system demand photodetectors to have large bandwidth, gain and improved light enhancement competence. Amongst the different light absorption enhancement methods being researched by the investigators, plasmonic has acquired increased attention in the last decades. Although single layer plasmonic supported metal semiconductor metal photodetector (MSM-PD) has been explored for higher light absorption efficiency, but exploration for double-layer structure is lacking in the literature. This paper presents the performance of plasmonic based photodetectors (PDs) with double layer of nanogratings optimized at wavelength of 1.4 µm for night vision applications. Proposed design of plasmonic supported photodetector with double nanograting reports Quenching factor (QF) of 92.14% and provides enhancement in light with the optimized height of subwavelength aperture (SWA) at 60 nm. This can be credited to fact that both top and bottom layer of grating contributes to light trapping.


Introduction
The modern era has observed an extensive concern of researchers for the expansion of nano-scale, high speed photonic devices [1]. Various models of photodetectors (PDs) have been explored by researchers for high bandwidth, gain and improved light enhancement capability. Plasmonic based metal semiconductor metal photodetectors (MSM-PDs) have been accepted as a promising contender for high speed photonic devices [2][3][4] and it has signi cant role for future-generation optoelectronic devices for high speed. In recent years, role of Surface Plasmons (SPs) in the plasmonic eld has become a major research topic that contributes the maximum output depending on the incident light. These are the oscillations of electron cloud present on the metal surface results in light con nement and light enhancement [5][6][7]. Various Plasmonic based PDs has been analyzed with a single layer of diverse groove with shapes viz. trapezoidal, rectangular, elliptical and triangular at wavelengths 986 nm and 827 nm [8][9][10]. Researchers have been proposed different nano structured MSM-PD models and compute the light absorption enhancement factor (LAEF) by changing the geometric properties of the device [11,12]. Most of the existing designs focus on the use of noble metal like gold (Au) with single layer of nanogratings on the metal surface [13]. These structures suffer with drawback of large amount back re ections of light. From literature, Tan et al. [1] have reported 1.5 LAEF with single nanogratings based plasmonic PD using gold material. Tan et al. [14] have presented 1.2 LAEF with double gratings based photodetector using gold material at 900 nm top rst grating width and also proposed the high absorption device by embedding metal nanoparticles with nanogratings [15]. Masouleh et al. [16] have obtained 1.56 LAEF at 1.4 µm using gold material for nanogratings with 450 nm SWA width and also compared the different plasmonic nanogratings pro le to enhance the light absorption in nanostructured device [17]. Daneshmandi et al. [18] have proposed extra ordinary transmission (EOT) structure based photodetector to improve the light transmission within the device whereas, Das et al. [19] have analyzed the effect of nanogratings phase-shift on device performance and examined that maximum device performance has been achieved at 0 o but it start decreases by increasing the phase-shift beyond 0 o . Fan et al. [20] have reported the plasmonic based MSM-PD with aluminum nanogratings and also obtained the light re ection factor (LRF). [21] Though the research community has provided numerous solutions for improving the device performance yet, MSM-PD devices suffer from re ection losses. Therefore, literature demands investigation of plasmonic supported MSM-PD with additional noble metals viz. silver. Improvisation of plasmonic supported MSM-PD structure with double layer of nanogratings for superior light absorption lacks in literature. This paper presents the plasmonic based PD with double nanogratings structure to enhance the light absorption. Proposed structure adds a bottom layer of metal gratings along with top metal gratings and SWA using silver as a cost effective alternative to gold. With the help of SWA light can not be dispersive in all angles [16] and it helps to re-emit the light from a very small area occupied by the SWA. The order of the paper as follows: Section I, i.e., "Introduction" is followed by Section II that describes the "structural design of the proposed double nanogratings layer based PD". Results obtained from the research work with all justi cations are presented in Section III. Finally, the "Conclusions" derived from research work are described in Section IV.

Photodetector Structure Design
This section describes the double nanograting structure of the proposed plasmonic based PD as shown in Fig uses silver (Ag) material for nanogratings, SWA and gallium arsenide (GaAs) as substrate whereas plasmonic supported PD is simulated using OptiFDTD software based on the Drude model [22].
For the better absorption, GaAs is mainly preferred over other semiconductor materials. Because of its better electrical properties and wide direct band gap (1.42 eV) for more light absorption and it also attained high responsivity due to high electron mobility. Noble material i.e., silver is used because it has less ohmic losses, stable in the environment, best conductor of heat and it is suitable for plasmons as compared to other materials. This work uses ( nite difference time domain) FDTD based algorithm for the study of optical properties of nanostructures [23] and simulations of the electromagnetic eld component. For the FDTD simulation, mesh size used is 5 nm and permittivity for GaAs is assumed to be real and taken from [24]. A periodic boundary condition along x direction, anisotropic perfectly matched layer (APML) in z direction is supposed for the simulations and structure consists of parallel grooves in xdirection made from perfect conductors.

Results And Discussion
In this section, the performance of plasmonic based PD has been analyzed, whereas the impact of SWA height at 40 nm, 60 nm and 80 nm has been studied and analyzed accordingly. Simulated refractive index plot, distribution of magnetic eld in x (H x ) and z direction (H x ) have been attained and depicted in Figs. 2-3, respectively. Vertical plane kept at position 0.125 is used for the input light and incoming light used which lies in near-infrared region from range 1.1 µm-1.55 µm with TM mode and θ = 0 0 . In this research work, we have obtained the LAEF by varying SWA height of values 40 nm, 60 nm and 80 nm and keeping SWA width constant at 50 nm. The term LAEF demonstrates the effect of nanogratings for light transmitted within the substrate through a coupling process based on resonance phenomenon. Table. 1 presents all the parameters for proposed double nanograting supported PD having rectangular grooves in the NIR region.
When the light is traversing with surface plasmon resonance the bottom layer prevents the loss by trapping the light using nano-gratings and thus increases absorption and bottom gratings further distribute the light in wider area of substrate. However, resonance occurs only when the frequencies of incoming light and electron cloud oscillations that are present on the surface matches with each other.
With the coupling effect, light is automatically absorbed when the surface plasmon resonance happens. Top gratings and SWA material Silver Refractive index layout with double nanograting structure design is shown in Fig. 2. However, top and bottom gratings enhance the light with SPPs due to excitation of SPPs with the interaction of light with the metal in nanostructures. Due to change in the refractive index, nanostructures help in increase the light transmission which passes through the SWA [25] whereas, Fig. 3 (a) and 3 (b) presents magnetic eld (H) along x and z axis. It is clearly noticed from Fig. 3 (a-b) that bottom grating distributes the maximum light within the substrate because of the light coupling process. If light transmission with double nanogratings through SWA increases then light absorption within the GaAs substrate also increases due to resonance phenomenon. So, the nanogratings are acting as plasmonic lenses or light concentrators [26] which is essential for triggering extraordinary optical absorption (EOA) of light into the substrate. For analyzing the design two performance metrics used includes QF and LAEF. LAEF, "the ratio of normalized power transmittance with double nanogratings to normalized power transmittance without top nanogratings" is calculated using Eq. (1) [27] whereas, QF describes the enhancement in light absorption from least amount to upper limit is calculated using Eq. (2) [28].
From the simulated results, Fig. 4 represents the LAEF spectra with variation in SWA height at a constant width of 50 nm. From results, we have obtained that optimized height of top and bottom nanogratings are 60 nm and 40 nm at which we achieved the maximum light absorption with highest QF by doing variations in the SWA height.

Declarations
Funding Authors declare that they did not receive any grant from funding sources for this research work.

Con ict of Interest/ competing interest
The authors declare that they have no con ict of interest.

Availability of data and material
The authors declare that data supporting the ndings of this study are available within the article.   Layout of the refractive index using OptiFDTD Figure 3 (a) Distribution of magnetic eld in x direction (Hx) (b) in z direction (Hz) with rectangular groove shape of silver material