Can a vector beam be critically coupled leading to perfect absorption?

TL;DR

Problem: scalar critical coupling theories fail to capture polarization and orbital-angular-momentum effects in CC for structured light. Approach: develop a full-vector angular-spectrum framework with nonparaxial spectra and exact Fresnel coefficients via transfer-matrix formalism, applied to a four-layer absorber (silver-dielectric composite + spacer + DBR) with ε1 from Maxwell-Garnett theory, for Gaussian, LG (l=±1), and LCP-LG beams at normal and oblique incidences. Findings: vector-beam CC cannot be achieved in this planar stack due to off-axis components and spin–orbit interactions; oblique incidence yields complex spectra, polarization mixing, and significant beam shifts (Goos-Hänchen and Imbert-Fedorov) with orbital Hall effects; super-scattering can occur at certain angles; beam waist and OAM affect CC efficiency. Significance: provides a realistic, vectorial framework for designing absorbers with structured light, guiding experiments and enabling extensions to metasurfaces, photonic crystals, nonlinear media, and 2D materials.

Abstract

Critical coupling has emerged as a prominent area of research in recent years. However, most theoretical models are based on scalar theories (and occasionally coupled mode theories), which inadequately account for the polarization states of the incident light. To bridge this gap, we revisit the concept of critical coupling in planar multilayer structures using a full vectorial theory, where conventional plane wave illumination is replaced by well-defined vector beams with and without orbital angular momentum (OAM). Our investigation explores the possibility of complete absorption of monochromatic beams without and with intrinsic OAM (such as Gaussian and Laguerre-Gaussian (LG)), incident on the multilayer structure at normal or oblique incidence. A two-component metal-dielectric composite film is chosen as the absorbing layer in the system. Our results demonstrate a significant reduction in the intensities of the reflected and transmitted beams at normal incidence, with reduced efficiency for oblique incidence due to the lack of spatial overlap of multiply reflected components. Interestingly, we also observe super-scattering from the same structures when conditions for constructive interference of the various reflected components are satisfied. This work highlights the need to incorporate the vector nature of beams by retaining the complete polarization information of off-axis spatial harmonics in future studies.

Figures (9)

• Figure 1: (a) Schematic of the multilayered medium illuminated by a vector beam. (b) Real and (c) imaginary parts of the permittivity ($\epsilon_1$) of the composite material for various volume fractions ($\epsilon_m$) of silver. Total scattering R + T (red solid lines) as a function of wavelength for (d) normally and (e) obliquely incident ($\theta=45^{\circ}$) $s$-polarized plane waves.
• Figure 2: (a) Incident, (b) reflected, and (c) transmitted spectra of the $s-$polarized Gaussian beam incident normally on the structure. The line plots in (d)-(f) along the horizontal axis further illustrate the characteristics of these spectra presented in (a)-(c). Real-space (g) incident, (h) reflected, and (i) transmitted beams. Line plots (j)-(l) along the $x-$axis depict the nature of the beams corresponding to (g)-(i). Parameters for (a)-(l) are: $\epsilon_i=1.0$, $\epsilon_2=2.6244$, $\epsilon_h=2.25$, $\epsilon_a=5.7121$, $\epsilon_b=2.6244$, $\epsilon_f=2.25$, $d_1=0.01\mu m$, $d_2=0.677\mu m$, $f_m=0.05$, N=10. (m) Reflected beam spectra and (n) the corresponding reflected beams for different spacer thicknesses: $d_2 = 0.038\mu m, 0.422\mu m, 0.677\mu m$. Other parameters are the same as mentioned earlier. (o) Maximum value of the transmitted beam with increasing period of the DBR sublayers N.
• Figure 3: (a) Incident, (b) reflected, and (c) transmitted spectra of the $s-$polarized LG beam incident normally on the structure. The line plots in (d)-(f) along the horizontal axis further illustrate the characteristics of these spectra presented in (a)-(c). Real-space (g) incident, (h) reflected, and (i) transmitted beams. Line plots (j) and (l) along the $x-$axis depict the nature of the beams corresponding to (g) and (i). (k) Line plots along the principal (PD) and counter diagonal (CD) of the intensity profile in (h). Parameters mentioned in Fig. \ref{['fig:gaussiannormal']} are used for these simulations.
• Figure 4: (a) Incident, (b) reflected, and (c) transmitted spectra of the left circularly polarized LG beam incident normally on the structure. The line plots in (d)-(f) along the horizontal axis illustrate the characteristics of these spectra presented in (a)-(c). Real-space (g) incident, (h) reflected, and (i) transmitted beams. Line plots (j)-(l) along the $x-$axis depict the nature of the beams corresponding to (g)-(i). Parameters mentioned in Fig. \ref{['fig:gaussiannormal']} are used for these simulations.
• Figure 5: (a) Incident, (b) reflected, and (c) transmitted spectra of the $s-$polarized Gaussian beam incident obliquely at an angle of $45^{\circ}$. The line plots in (d)-(f) along the horizontal axis further illustrate the characteristics of these spectra presented in (a)-(c). Real-space (g) incident, (h) reflected, and (i) transmitted beams. Line plots (j)-(l) along the $x-$axis depict the nature of the beams corresponding to (g)-(i). White solid lines in (h) and (i) denote the GH (along $x$) shifts of the field profiles. Parameters are: $\epsilon_i=1.0$, $\epsilon_2=2.6244$, $\epsilon_h=2.25$, $\epsilon_a=5.7121$, $\epsilon_b=2.6244$, $\epsilon_f=2.25$, $d_1=0.01\mu m$, $d_2=0.754\mu m$, $f_m=0.05$, N=10.