Experimental Demonstration of Plasmon-Enabled Monolithic Bragg Reflectors for Infrared Light via Inverse Design

TL;DR

The paper tackles the challenge of achieving high-reflectivity mirrors in the mid-infrared (MIR) where conventional DBRs face strain, thermal, and growth limitations. It introduces plasmon-enabled DBRs (PE-DBRs) based on modulation-doped monolithic InP and leverages inverse-design optimization to maximize reflectivity while minimizing free-carrier absorption, achieving near-100% reflectivity in simulations. Experimentally, NU-PE DBRs designed for 5, 7, and 9 μm with a total thickness of ~14 μm exhibit peak reflectances up to 99% and bandwidths up to 18% of the design wavelength, with good agreement between FTIR measurements and simulations. The work demonstrates a scalable, junction-free, monolithic platform with enhanced thermal and electrical properties, and suggests paths to further reduce resistivity via targeted doping, broadening potential MIR applications in lasers, LEDs, and detectors; the approach is also extendable to other semiconductor materials.

Abstract

High-reflectivity mirrors in the mid-infrared (MIR) range are essential for next-generation optoelectronic devices but are still constrained by strain accumulation, poor thermal conductivity, and growth instability of thick multi-alloy stacks in conventional distributed Bragg reflectors (DBRs). We introduce plasmon-enabled DBRs (PE DBRs) based on modulation-doped monolithic InP, where plasmonic dispersion in highly doped layers provides a strong refractive-index contrast. Using inverse-design optimization targeting reduced free-carrier absorption and maximized reflectivity, we demonstrate that PE DBRs can achieve reflectivities approaching 100%. Experimentally grown 14 μm thick InP PE DBRs exhibit up to 99% reflectance with bandwidths reaching 18% of the design wavelength. The monolithic, junction-free configuration ensures low resistivity and enhanced thermal performance, offering a scalable platform for efficient plasmonic mirrors in MIR photonics, with potential applications in photodetectors, light-emitting diodes and lasers.

Figures (10)

• Figure 1: Cross-sectional schematics of uniform (U-) and nonuniform (NU-) plasmon-enabled (PE) DBR composed of pairs of doped (dark grey) and undoped (light grey) layers of InP, implemented on an undoped InP substrate. The red curve represents the amplitude of light incident from air. Two configurations are considered in this work: U-PE DBR with equal thicknesses of the layers in each pair, and NU-PE DBR with the thickness of the layers modified in each pair as shown in the figure.
• Figure 2: a) Real part and b) imaginary part of refractive index of InP. Values from panah2017 are marked by dots, lines represent dependency obtained by Drude model.
• Figure 3: a) Real part and b) imaginary part of the refractive index of InP based on experimental characteristics from panah2017, and c) reflectivity of a uniform InP layer when illuminated with infrared light incident from air side, all as a function of wavelength for different levels of carrier concentration indicated by colours. Vertical dashed lines indicate plasma frequency.
• Figure 4: a) Optimisation trajectory illustrating the evolution of reflectivity at the wavelength of 7µm for U-PE DBR and NU-PE DBR as a function of the number of iterations in the optimisation process with variable layer thicknesses. The initial PE DBR configuration assumes quarter-wavelength layer thicknesses. In the first stage (light-blue background), all DBR sections are identical, and the algorithm searches for the optimal thicknesses of the undoped and highly doped layers, resulting in the U-PE DBR design. In the second stage (light-pink background), the optimal U-PE DBR is used as the starting point, and the thickness of each individual layer is varied independently, leading to the optimised NU-PE DBR structure. Panels b), c) and d) show light intensity (red) in logarithmic scale and real refractive index (blue); b) depicts the initial PE DBR configuration, based on the quarter-wavelength layer thickness assumption, serving as the starting point for the optimization process; c) presents the U-PE DBR configuration obtained within first step of optimization procedure, d) presents the NU-PE DBR configuration obtained as the result of the optimization procedure.
• Figure 5: Calculated reflection spectra of optimised PE DBRs at the central wavelengths of 5, 7 and 9µm indicated by different colours in the case of a) U-PE DBR and b) NU-PE DBR; c) maximal reflectance of U-PE and NU-PE DBRs designed for the various wavelengths and d) difference in relative absorption between optimised NU-PE DBR and U-PE DBR versus number of DBR sections.