Plasmonic enhancement of the infrared radiation absorption in an ultrathin InSb layer

TL;DR

The paper addresses infrared detection in the MWIR window ($3-5\,\mu m$) using InSb, noting its advantages but also cooling-related noise for conventional detectors. It proposes a plasmonic structure comprising an InSb substrate with a gold grating to excite grating plasmon resonances that dramatically boost absorption in an ultrathin InSb film, analyzed with an improved RCWA approach and an integral-equation method. The results demonstrate a plasmonic resonance that increases total absorption by over a factor of ~10, with useful InSb absorption around $0.26$ and parasitic grating loss around $0.05$, and reveal strong near-field redistribution and tunability via grating geometry, including two-mode resonances and an analytic cue from Mikhailov theory for resonance frequency $\omega_p$. This work points to potential high-sensitivity, multi-color InSb detectors with reduced cooling requirements and outlines pathways for optimization through material choice, 2D gratings, and device integration.

Abstract

Indium antimonide (InSb) is a fundamental material for infrared radiation detectors based on interband transitions. Its narrow bandgap enables detection of infrared radiation within the $3-5 μm$ atmospheric window, while its high quantum efficiency ensures excellent sensitivity in InSb-based detectors. We propose a plasmonic structure that significantly enhances infrared absorption in an ultrathin InSb film. The resonant characteristics of this plasmonic enhancement effect could serve as a foundation for developing highly sensitive multi-color detectors.

Figures (5)

• Figure 1: Panel a: the schematic view of the proposed structure, Panel b: the dependence of the real and imaginary parts of InSb dielectric permittivity near the fundamental edge of absorption.
• Figure 2: The spectra of transmission, reflection (a), and absorption (b) of the plasmonic structure in comparison to the spectra of a bare substrate. Panel a: 1,2 are the reflection and absorption spectra of the plasmonic structure 1', 2' are the corresponding ones for the bare substrate, and panel b: 1, 2 are the absorption spectra of the grating structure and bare substrate, respectively. The thickness of the substrate $D = 300$ nm. The parameters of InSb are: $\alpha = 0.19\ eV^{3/2}$, the bandgap $E_g = 0.23\ eV$, the static dielectric permitivitty $\varepsilon_0 = 15.7$. The grating parameters are: the grating period $a = 1.5\ \mu m$, the grating depth $d = 100\ nm$, the metal conductivity $\sigma_0 = 3.7\times10^{17}\ s^{-1}$, the momentum relaxation time $\tau = 25\ fs$ the strip width $w_1 = 0.36\ \mu m$ and the opening width $w_2 = 1.14\ \mu m$.
• Figure 3: The spectra of absorption of the structure and its components in comparison to the spectra of a bare substrate: 1 - the bare substrate, 2 - the full structure absorption, 3 - the partial absorption in the InSb layer, 4 - the partial absorption in the metal grating. The parameters are the same as for Fig. \ref{['Fig2']}.
• Figure 4: Panel a: the spatial distribution of the time-averaged absorption density for the non-resonant ($\hbar\omega = 0.25$ eV) and Panel b: the resonant case ($\hbar\omega = 0.368$, which corresponds to the maximum of the absorption spectra).
• Figure 5: Panel a: The dependence of the spectra of absorption for the structures with different grating strip widths: 1, 2, 3, which correspond to 0.4, 0.36, and 0.3 $\mu m$, respectively. The other parameters are the same as for Fig. \ref{['Fig3']}. Panel b: the contour mapping of the partial absorption spectrum depending on the strip width.