Abstract
We present a computer code for calculating near- and far-field electromagnetic properties of multilayered spheres. STRATIFY is a one-of-a-kind open-source package that allows for efficient calculation of electromagnetic near-field, energy density, total electromagnetic energy, and radiative and non-radiative decay rates of a dipole emitter located in any (non-absorbing) shell (including a host medium), and fundamental cross-sections of a multilayered sphere, all within a single program. Because of its speed and broad applicability, our package is a valuable tool for analysis of numerous light scattering problems, including but not limited to fluorescence enhancement, upconversion, downconversion, second harmonic generation, and surface enhanced Raman spectroscopy. The software is available for download from GitLab as Code 1.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Multilayered spherical nanoparticles are fundamental building blocks for many vital applications in physics and chemistry: tailored scattering [1–7] and nonlinear optics [1,2,8,9], photonic crystals [10–14], sensing [15–17], photothermal cancer treatment [18–21], solar energy harvesting [22,23], photovoltaics [24], fluorescence [25–32] and upconversion [33–40] enhancement, surface-enhanced Raman scattering (SERS) [25,41,42], surface plasmon amplification by stimulated emission of radiation (spaser) [43–48], and cloaking [49–53]. To date, such matryoshkas with various combinations of dielectric and metal shells have well-established protocols for the efficient and controllable synthesis [6,54–59], greatly improved on initial synthesis attempts [3–5]. Theoretical and numerical studies in these fields are inevitable for a thoughtful design of experiment and reliable interpretation of measured data. The theory of electromagnetic light scattering from multilayered spheres has a long history since the pioneering work of Aden and Kerker for two concentric spheres [60]. An impressive number of closed-form solutions and discussions on coated [61,62] and general multilayered [63] spheres have been reported. There is a thorough understanding of scattering [64] and absorption [65] of light from multilayered spheres, including aspects for various types of illuminations [66–69], spontaneous decay rates of a dipole emitter in a presence of a multilayered sphere [26,27], electromagnetic energy [70] and near-field [71] distribution within and near the multilayered spheres, and the respective strategies for numerical implementation of the developed theories [72–75].
For a single shell, a number of features can be qualitatively understood from the quasi-static analysis [2], a full, quantitative understanding of the mechanisms underlying the applications of multilayered spheres is usually quite involved, requiring sophisticated and complex theoretical and numerical studies. The latter are often handled with commercial brute-force finite-difference time-domain (FDTD), finite-element-method (FEM) or boundary-element-method (BEM) solvers, which provide accurate results at relatively high computational price. However, spherical multilayered particles are computationally feasible per se due to the existence of the closed-form analytical solutions inherently convenient for computational implementation. Nonetheless, there are few freely available, user-friendly and comprehensive codes for light scattering from a general multilayered spheres based on these solutions [76–79]. In most of the cases, a very limited number of properties are calculated within a single freely available package. Fundamental cross-sections (i.e. scattering, absorption and extinction) and/or near fields are the usual choice implemented in freely available codes [77,80,81], and in a number of so-called “online Mie calculators” [82,83], which commonly handle only homogeneous spheres with a rare exceptions for core-shells and even more rarely for general multilayered spheres [84]. Computation of the orientation-averaged electric or magnetic field intensities (i.e. averaged over a spherical surface of a given fixed radius $r$), spontaneous decay rates [78] and other quantities are almost unavailable, yet generally very useful. Here we present free MATLAB code based on compact and easy-to-implement transfer-matrix formalism, which can be conveniently used to handle most of the problems of light scattering from multilayered spheres by using appropriate combinations and manipulations of transfer-matrices. The use of closed-form solutions significantly enhances the performance of the code compared with brute-force solvers without sacrificing the accuracy.
Our recursive transfer-matrix method (RTMM) has been inspired by the success of such a RTMM for planar stratified media developed by Abelès [85–87], summarized in Born & Wolf classical textbook [88, Sec.1.6], and can be seen as its analogue for spherical interfaces. From a historical perspective, the RTMM presented here had been developed as early as in 1998 and implemented in the F77 sphere.f code [76]. It was first applied to a number of different theoretical settings with external plane wave source [10,12,14], and successfully tested against experiment [6,13]. Later it was extended to incorporate a dipole source [26,27], energy calculations [70], and has enabled exhaustive optimization of plasmon-enhanced fluorescence [32].
The paper is organized as follows. In Sec. 2, we provide an overview of the RTMM for multilayered spheres and formulate the fundamental properties of spheres. In Sec. 3, we describe the developed code and benchmark it with robust BEM [89] implemented in freely available “MNPBEM” MATLAB package [90–93]. In Sec. 4, we discuss possible application of the code in SERS, plasmon-enhanced fluorescence, upconversion, and end with conclusive remarks and propose further developments of the package. Gaussian units are used throughout the paper.
2. Theory
2.1 Recursive transfer-matrix method
Consider a multilayered sphere with $N$ concentric shells as shown in Fig. 1. The sphere core counts as a shell with number $n=1$ and the host medium is the $n=N+1$ shell. Occasionally, the host medium will be denoted as $n=h$. Each shell has the outer radius $r_n$ and is assumed to be homogeneous and isotropic with scalar permittivity $\varepsilon _n$ and permeability $\mu _n$. Respective refractive indices are $\eta _n = \sqrt {\varepsilon _n \mu _n}$. We assume that the multilayered sphere is illuminated with a harmonic electromagnetic wave (either a plane wave or a dipole source) having vacuum wavelength $\lambda$. Corresponding wave vector in the $n$-th shell is $k_n = \eta _n\omega /c = 2\pi \eta _n/\lambda$, where $c$ is the speed of light in vacuum, and $\omega$ is frequency. Electromagnetic fields in any shell are described by the stationary macroscopic Maxwell’s equations (with time dependence $e^{-i\omega t}$ assumed and suppressed throughout the paper):
General solution for the electric field in the $n$-th shell, $1\leq n\leq N+1$, is [26, Eq. (8)]:
Given the general solution (6), the corresponding expansion of magnetic field ${\textbf {H}}$ follows from that of the electric field ${\textbf {E}}$ by the stationary macroscopic Maxwell’s Eqs. (1) on using relations (4) [26, Eqs. (9)–(10)]:
Very much as in the case of planar stratified media [85–88], our RTMM for spherical interfaces exploits to the maximum the property that once the coefficients $A_{pL}(n+1)$ and $B_{pL}(n+1)$ on one side of a $n$-th shell interface are known, the coefficients $A_{pL}(n)$ and $B_{pL}(n)$ on the other side of the shell interface can be unambiguously determined, and vice versa. Schematically,
The formalism becomes compact upon the use of the composite transfer matrices ${\cal T}_{p\ell }(n)$ and ${\cal M}_{p\ell }(n)$ defined as ordered (from the left to the right) products of the constituent raising and lowering $2\times 2$ matrices from Eqs. (10)–(13):
In order to unambiguously determine the expansion coefficients $A_{pL}(n)$ and $B_{pL}(n)$ in each shell, one has to impose two boundary conditions, which is the subject of the following section.
2.2 Boundary conditions
The total number of different sets of expansion coefficients comprising all $j$’s from the interval $1\leq j\leq N+1$ is larger by two than the number of corresponding equations. Therefore, boundary conditions have to be imposed to unambiguously determine the expansion coefficients at any shell. They are
- 1. The regularity condition of the solution at the sphere origin, which eliminates $h_{\ell }^{(1)}(0)\to \infty$ for $f_{p\ell }$ in Eq. (3) in the core region:
- 2. For a source located outside a sphere, the $A_{pL}(N+1)$ coefficients at any given frequency $\omega$ are equal to the expansion coefficients of an incident electromagnetic field in spherical coordinates.
Corresponding closed-form analytic expressions for applying the second boundary condition are presented in (i) Refs. [70, Eq. (10)], [96] for the plane electromagnetic wave, and in (ii) Ref. [26, Eqs. (44),(50)] for the electric dipole source. In the case of an elementary dipole radiating inside a sphere, there is no source outside a sphere, and the second boundary condition reduces to $A_{pL}(N+1)=0$ [26].
2.3 Far-field properties
Fundamental cross sections $\sigma$ (scattering, absorption and extinction) are determined as an infinite sum over polarizations ($p=E,M$) and all partial $\ell$-waves:
On using polarized scattering waves parallel and perpendicular to the scattering plane,
2.4 Near-field properties
For a given illumination, and, thus, known expansion coefficients $A_{pL}(n)$ and $B_{pL}(n)$, the electromagnetic field can be unambiguously defined by Eqs. (6) and (7). Below we consider more sophisticated near-field properties.
2.4.1 Electromagnetic energy
Total electromagnetic energy $W$ stored within a multilayered sphere is a sum of energies, $W_n$, stored within any given $n$-th shell, which in turn is an integral of an electromagnetic energy density $w_n(r)$ over the $n$-th shell:
Electric and magnetic components from Eq. (21) are nothing but the orientation-averaged electric and magnetic field intensities:
The radial integrations in (21) are performed by using Lommel’s integration formulas [70]:
2.4.2 Spontaneous decay rates
Radiative and nonradiative decay rates (normalized with respect to $\Gamma _\textrm {rad;0}$, the intrinsic radiative decay rate in the absence of a multilayered sphere) for a dipole emitter located in $n_d$-th shell at $r_d$ distance from a center of a sphere (see Fig. 1) are given by [26]:
For the detailed discussion of the spontaneous decay rates of a dipole emitter in a presence of a general multilayered sphere, we refer the reader to Ref. [26].
2.5 Convergence criteria
Numerical implementation of equations above, which involve infinite summation over $\ell$, requires truncation at some finite number $\ell _\textrm {max}$, which, as in any Mie theory based code, determines the total number of the vector spherical wave functions involved in calculations as $2[(\ell _\textrm {max}+1)^{2}-1]$. For far-field properties, Eqs. (19) and (20), the classic choice is the Wiscombe criterion [104]:
For the near-field, Eqs. (6), (7), and for the electromagnetic energy, Eqs. (21), (24), (27), slightly larger cut-off value is recommended [105]:
For the dipole source implied in Eq. (28), especially when considering non-radiative decay rates [26,106], the accuracy has to be set instead of $\ell _\textrm {max}$, since there are no general guidelines for choosing the latter in this case.
2.6 Electron free path correction
For thin metallic shell with thickness $(r_n - r_{n-1})$ less than a free electron path, the bulk permittivity $\varepsilon _{n,\textrm {bulk}}$ has to be corrected to take into account electron scattering from the shell surface [107]:
3. Computer code
3.1 Overview
Fundamental properties are calculated with the following functions:
- • t_mat.m calculates transfer matrices with Eqs. (10)–(13) and returns their ordered products (14). Any other function which returns electromagnetic properties (decay rates, electromagnetic energy and fields, far-field properties and etc) makes use of pre-calculated ordered products of transfer matrices for a better performance;
- • decay.m returns decay rates calculated with Eqs. (28). Decay rates are normalized (see Eqs. (29)) with respect to radiative decay rates of a dipole in a homogeneous medium (without a multilayered sphere) with the refractive index of a shell where the dipole is embedded, $\eta _d$, or with the refractive index of a host, $\eta _h$;
- • near_fld.m returns electric, ${\textbf {E}}({\textbf {r}}) = {\textbf {E}}_E({\textbf {r}}) + {\textbf {E}}_M({\textbf {r}})$, and magnetic, ${\textbf {H}}({\textbf {r}}) = {\textbf {H}}_E({\textbf {r}}) + {\textbf {H}}_M({\textbf {r}})$ near-field distributions given by Eqs. (6) and (7). We take the advantage of the spherical symmetry of the problem and pre-calculate computationally expensive ${\textbf {r}}$-dependent Bessel functions and $\cos \theta$-dependent associated Legendre polynomials only for unique values of ${\textbf {r}}$ and $\cos \theta$. Usually, the amount of these unique ${\textbf {r}}$ and $\cos \theta$ is significantly smaller than the respective total number of points in the rectangular mesh, which results in faster execution;
- • far_fld.m returns polarized scattering waves from Eq. (20);
- • crs_sec.m returns fundamental cross sections from Eq. (19);
- • el_fr_pth.m returns corrected permittivity according to Eq. (37).
3.2 Verification and performance
We have compared STRATIFY with freely available exact BEM-based [89] solver MNPBEM [90–93]. The latter package has been chosen for a benchmark since it is a freely available comprehensive MATLAB code capable of calculating most of the quantities considered in our code. For testing purposes, we have considered fundamental cross-sections, electric near-field distribution, and spontaneous decay rates of dipole emitter in the presence of matryoshkas composed of different combinations of Au and SiO${}_2$ layers. It can be easily seen from Fig. 2 that our code produces for spherical particles the same results as the general-purpose BEM method, but for significantly reduced computational time. This is not surprising in view of a number of exact analytic results underlying the spherical multilayered particles which are not available for general particle shapes. Any additional layer requires generation of an extra set of Bessel functions and introduces an additional multiplication by a $2\times 2$ matrix, which causes a linear scaling of computational time with the number of layers, $N$. For very large $x_N$ an increased cut-off $\ell _\textrm {max}$, which increases with $x_N$, may slow down the performance due to the necessity of calculating large sets of the Bessel functions and their derivatives.
4. Discussion and conclusions
The developed package is ready to be applied in a number of well-established applications of multilayered spheres in optics and photonics. Below we discuss the most common examples. Due to enormous electric field enhancement in multilayered metal-dielectric nanospheres, they are considered as good candidates for a number of applications. Squared electric field intensities from Eq. (24) or Eq. (27) are measure for performance of nanostructures in SERS [25,41,42,112–114] and second harmonic generation [8,9]. For fluorescence or upconversion enhancement, where the spontaneous decay rates of the dipole emitters are modified, the generic enhancement factor is nothing but a product of excitation rate enhancement (at excitation wavelength, $\lambda _\textrm {exc}$) and quantum yield (at emission wavelength, $\lambda _\textrm {ems}$):
Cloaking, Kerker effect [115], super- or optimally tuned scattering [52,116–118] and absorption [20,52,118,119], embedded photonic eigenvalues [120], spasing [44,47,121] and other intriguing phenomena [122] are easily understood from fundamental cross sections (19) and scattering patterns (20).
We have summarized a self-consistent and comprehensive RTMM theory reported earlier in our [26,70] for electromagnetic light scattering from general multilayered spheres composed of isotropic shells. Within the framework of RTMM, we have developed an efficient multi-purpose MATLAB package for calculating fundamental properties of multilayered spheres. Our package is one-of-a-kind freely available software which allows for a simultaneous calculation of a wide range of electromagnetic properties and is ready to be used for a broad number of applications in chemistry, optics and photonics, including optimization problems and machine-learning studies. We hope that the generalization presented here and corresponding MATLAB code will serve as a useful tool for photonics, physics, chemistry and other scientific communities and will boost the researches involving various kinds of multilayered spheres.
Extensions of our code for ultra-thin metallic shell characterized by nonlocal dielectric functions [123–127], an optically active shells [128–130], or including perfectly conducting boundary conditions at the sphere core are, following the theory developed in Ref. [26], rather straightforward. Further generalization of the package may include illumination with focused, Gaussian, or other beams [66–69]. An incorporation of magnetic dipole emitters [131–133] or modeling the effect of a multilayered sphere on the far-field radiation directivity of a dipole antenna should follow soon.
Disclosures
The authors declare no conflicts of interest.
The data for this work is available as Code 1 [134].
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