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.
Research Article
Enhancement of Light Absorption in Plasmonic Based Photodetector with Double Nanograting Structure
https://doi.org/10.21203/rs.3.rs-505093/v1
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Nanogratings
plasmonics
surface plasmon polaritons (SPPs)
nanomaterials.
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 [2–4] and it has significant role for future-generation optoelectronic devices for high speed. In recent years, role of Surface Plasmons (SPs) in the plasmonic field 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 confinement and light enhancement [5–7]. 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 [8–10]. 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 reflections 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 first 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 profile 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 0o but it start decreases by increasing the phase-shift beyond 0o. Fan et al. [20] have reported the plasmonic based MSM-PD with aluminum nanogratings and also obtained the light reflection factor (LRF). [21] Though the research community has provided numerous solutions for improving the device performance yet, MSM-PD devices suffer from reflection 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 justifications are presented in Section III. Finally, the “Conclusions” derived from research work are described in Section IV.
This section describes the double nanograting structure of the proposed plasmonic based PD as shown in Fig. 1 which consists four parts i.e. top grating, SWA, bottom grating and substrate. This novel structure design is the advancement of the single grating structure to maximize the light absorption and reduce the light reflection. Initially, light is incident on the top metal gratings and coupled to top layer where it transmitted with surface plasmon by SWA. As incoming light hit the metal top surface, it induces electrons oscillations in the figure of surface plasmon with the help of metal-free electrons. With the help of these oscillations light is coupled with the bottom nanogratings and absorb in the substrate and generate electron-hole pairs within the substrate leads to light absorption enhancement. Proposed design 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 (finite difference time domain) FDTD based algorithm for the study of optical properties of nanostructures [23] and simulations of the electromagnetic field 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 x-direction made from perfect conductors.
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 field in x (Hx) and z direction (Hx) 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 = 00. 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.
Table 1
Parameters for double grating layer structure
|
Parameters |
Values |
|---|---|
|
Groove shape |
Rectangular |
|
Input wavelength |
1.1 µm – 1.55 µm |
|
Top Gratings height |
60 nm |
|
SWA width |
50 nm |
|
Bottom gratings height |
40 nm |
|
SWA height |
(40, 60, 80) nm |
|
Bottom gratings and Substrate material |
GaAs |
|
Number of gratings |
4 |
|
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 field (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.
Table 2 summarizes the performance parameters with maximum QF and LAEF over different heights of SWA at 1.4 µm. Hence from the simulated results, it is depicted that maximum LAEF of 2.2435 is achieved at optimized SWA height is 60 nm as compared to other heights in the presence of SPPs. It is concluded from 92.14 % QF that highest light enhancement is observed with SWA height of 60 nm for the proposed design as mentioned in Table 2. This can be credited from the fact that enhancement of light absorption depends on surface plasmon resonance and in double nanograting based photodetector both top and bottom layer contributes to enhanced light trapping. It is observed that light absorption highly depends upon nanogratings height and material. Proposed design because better results have vital role in night vision applications and can be utilized for future generation opto-electronic system. In the future this structure can be analyzed with other groove shapes for both top and bottom gratings to enhance light trapping further.
Table 2
Maximum LAEF with quenching factor (QF) at 1.4 µm input wavelength
|
Parameters |
Values |
|
|---|---|---|
|
Input wavelength for maximum LAEF |
1.4 µm |
|
|
SWA height (nm) |
Maximum LAEF |
QF (%) |
|
80 |
0.8 |
25 |
|
60 |
2.2435 |
92.14 |
|
40 |
0.99 |
89.89 |
Single layer plasmonic based photodetector needs more exploration for light absorption enhancement. Paper presents plasmonic based photodetector by adding one more grating of rectangular grooves and analyses the LAEF for a structure and design has been analyzed over varying SWA heights i.e., 40 nm, 60 nm and 80 nm keeping other parameters constant. Maximum LAEF is 2.2435 leading to QF of 92.14% has been achieved with SWA optimized height of 60 nm at 1.4 µm making it suitable for applications in night vision. It is interesting to note that the parameters of both top and bottom gratings are different from each other. But bottom metal gratings enhance the maximum light as compared to single grating structure. This can be credited to the fact that the bottom grating is minimizing light reflection and enhancing light trapping due to the nanogratings.
Funding
Authors declare that they did not receive any grant from funding sources for this research work.
Conflict of Interest/ competing interest
The authors declare that they have no conflict of interest.
Availability of data and material
The authors declare that data supporting the findings of this study are available within the article.
Code availability
Not applicable
Authors' contributions
Designing, Methodology, Data and Result Analysis, Writing-original draft preparation: Savita Kashyap; Manuscript review, Editing and Superivision: Harsimranjit Kaur
Ethics approval
Not Applicable
Consent to participate
Informed consent was obtained from all individual participants included in the study.
Consent for publication
Not Applicable
Acknowledgment
Authors gratefully acknowledge the members of Chitkara University, Punjab, India for their continuous support during the research work.
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