Abstract
The ability to control the laser modes within a subwavelength resonator is of key relevance in modern optoelectronics. This work deals with the theoretical research on optical properties of a PT-symmetric nano-scaled dimer formed by two dielectric wires, one is with loss and the other with gain, wrapped with graphene sheets. We show the existence of two non-radiating trapped modes which transform into radiating modes by increasing the gain–loss parameter. Moreover, these modes reach the lasing condition for suitable values of this parameter, a fact that makes these modes achieve an ultra high quality factor that is manifested on the response of the structure when it is excited by a plane wave. Unlike other mechanisms that transform trapped modes into radiating modes, we show that the variation of gain–loss parameter in the balanced loss–gain structure here studied leads to a variation in the phase difference between induced dipole moments on each wires, without appreciable variation in the modulus of these dipole moments. We provide an approximated method that reproduces the main results provided by the rigorous calculation. Our theoretical findings reveal the possibility to develop unconventional optical devices and structures with enhanced functionality.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
It is known that, under certain conditions, a plasmonic system can support trapped electromagnetics modes, which are electromagnetic non-radiating oscillations that stay localized inside the structure [1]. These plasmon modes are characterized by an ultra-high quality factor provided that the structural symmetry remains unbroken. In fact, the narrow resonances characterizing these mode excitations rely on the breaking degree of the symmetry which allows the trapped mode to couples with free photons [2]. In this sense, plasmon trapped modes can be considered as a kind of symmetry protected states. Due to Ohmic loss in the plasmonic material, the eigenfrequencies of the trapped states are complex valued. However, by introducing gain material elements into the structure it is possible to compensate this material loss and, as a consequence, to reach the lasing condition for which the eigenfrequency associated to an eigenmode is real valued [3].
Because of their fundamental properties as well as their potential applications, the study of hybrid systems composed of gain media and plasmonic materials, which are lossy, is a topic of continuous increasing interest [4–8]. In areas such as condensed matter and surface optics, the amplification of eigenmodes by stimulated emission of radiation has played a key role in the interpretation of a wide variety of experiments, the understanding of various fundamental properties of solids and the engineering of nanolaser devices [9–11]. In particular, new phenomena associated with parity-time (PT) symmetry have been observed in optical systems with balanced loss and gain [12,13]. These optical systems belong to a large family of non-Hermitian systems, which can have a real spectrum provided that the system be invariant under combined operations of parity (P) and time-reversal (T) symmetry [14,15].
Possibilities have been widened towards PT symmetric structures that incorporate graphene as plasmonic material. Doped graphene allows the propagation of surface plasmons with low Ohmic losses, i.e., with high quality factor, from terahertz to near-infrared range [16]. Moreover, the plasmon resonance spectrum, and consequently the optical responses of the system, can be tuned by varying the doped level on graphene. In this context, several works have focused on the influence that long-living and tunable surface plasmons have on graphene based strucures [17,18] and, in particular, on those that present PT-symmetry [19–22].
The present work contains all the ingredients above entered and focusing on the study of a sub-wavelength graphene plasmonic system with balanced volume losses. In particular, our system is composed of two parallel dielectric cylinders, one is with loss and the other with the same level of gain, both wrapped with graphene sheets. Here, the sole effect of graphene is to provide LSP resonances in sub-wavelength wires. Although other more complex structures such as an array of many or infinite cylinders can be designed to present PT-symmetry, our motivation is based in the simplicity of the dimer structure that we have chosen, since it allows us to understand the underlying physics behind the PT-symmetry.
Other dielectric and plasmonic structures (without graphene) with balanced gain and loss have been proposed and studied in the literature. For instance, exceptional points of resonant eigenmodes (whispering gallery eigenmodes) were examined in a system consisting of a finite number of parallel dielectric cylinders in [23]. By exploiting the dynamic characteristics of whispering gallery eigenmodes near the exceptional point, a route to single particle sensing was analyzed for two coupled resonators system [24]. Furthermore, full loss compensation with relative low gain level can also be reached by using the eigenmodes supported by two parallel wires made of plasmonic materials in the visible spectrum [25]. Recently, optical forces have been explored for two interacting parallel plane waveguides, which results from the coupling between the PT-eigenmodes propagating along the symmetry axis of the system [26]. Finally, an overview of recent advancements in the field of non-Hermitian as well as PT-symmetry optical systems can be found in [27], together with a systematic discussion of the key results that are useful to understand exceptional point dynamics.
We use a rigorous formalism based on Mie theory to calculate the eigenmodes dependence with the gain–loss parameter for both trapped and radiating eigenmodes. To do this, we solve the boundary value problem with the corresponding boundary conditions without an incident wave (homogeneous problem). The procedure requires the analytic continuation of the eigenvalue, the frequency in our case, in the complex plane. Analytic continuation is inevitable, even in the case of media with no intrinsic losses, since the open nature of our resonator generates non-null imaginary parts due to radiation losses.
In addition, as it is well established nowadays, the modal lasing analysis in an open plasmonic resonator, as our PT-symmetric structure, can be carried out by applying the lasing eigenvalue problem (see [28] and references therein), in which the modal eigenfrequencies are assumed to be real valued at the lasing condition (or lasing threshold). Following this concept, we solve the homogeneous problem to find the real valued eigenfrequencies, gain–loss parameters and chemical potentials for each modal lasing condition. Moreover, we study the eigenmode influence on the optical response of the system when it is excited by a plane wave near the lasing condition.
By using the quasistatic approximation valid in the long wavelength limit, we show that despite the graphene ohmic losses, a fact that gives rise to a complex spectrum, the structure exhibits a set of properties in common with a PT-symmetric system. For instance, two eigenmode branches coalesce at an exceptional point and, for the gain–loss parameter above certain threshold, these both branches are repelled in the direction of the imaginary part of frequency, making that one of these branches achieves the lasing condition. In addition, we demonstrate that branches eigenmode, which are trapped modes in case of null gain–loss parameter, transform into radiating modes that can be excited by plane wave incidence when the gain–loss parameter is increased. In previous works [29,30] it was reported mechanisms for controlling transitions from trapped to radiating eigenmodes based on producing a difference between the modulus of individual dipole moments on each particle forming the dimer, i.e., by producing a weakly asymmetry with respect to the center of the dimer modifying the dipole moment amplitudes. In addition, in [31] it was experimentally demonstrated that the modulus and phase of the coupling coefficient between the magnetic mode and the electric dipolar mode on a composite metamaterial can be efficiently manipulated for practical applications on radiation control. In particular, we show that the increment of the gain–loss parameter leads to a change in the phase difference between these individual dipole moments, maintaining their modulus values constant.
This paper is organized as follows. In section 2 we present a brief description of the rigorous method used in this work to calculate the scattering of a dimmer composed of two graphene wires. From this method, using the quasistatic approximation, we deduce analytical expressions for eigenfrequencies and eigenvectors as a function of geometrical and constitutive parameters that explain the main features calculated with the rigorous method. In section 3 we present results of two parallel dielectric cylinders tightly coated with a graphene layer, one of them with small inner losses and the other one with the same level of gain. Concluding remarks are provided in section 4. The Gaussian system of units is used and an $\mbox {exp}(-i\, \omega \, t)$ time-dependence is implicit throughout the paper, where $\omega$ is the angular frequency, $t$ is the time coordinate, and $i=\sqrt {-1}$. The symbols Re and Im are used for denoting the real and imaginary parts of a complex quantity, respectively.
2. Theory
2.1 Rigorous description of the fields and scattering efficiencies
We consider a cluster consisting of two parallel and non-overlapping cylindrical dielectric wires, one with gain, $\varepsilon _a=\varepsilon _1-i\varepsilon _i$ ($\varepsilon _i>0$), and the other with equal loss, $\varepsilon _b=\varepsilon _1+i\varepsilon _i$, as shown in Fig. 1. Both wires have the same radius $R_a=R_b=R$ and are wrapped with a graphene sheet. The system is embedded in a lossless and non-magnetic dielectric denoted as medium $v$ with permittivity $\varepsilon _v$. In this case, a PT symmetry around the central axis, denoted by $O$, is fulfilled. We assume that the radius $R$ is sufficiently large to describe the optical properties of the wires as characterized by the same local surface conductivity as planar graphene (see appendix A). Even though nonlocal effects can be observed in our optical system for frequencies lower than characteristic resonance frequencies, this is not interesting for our purposes (see appendix B). We denote by $r_j(\mathbf {r}),\,\phi _j(\mathbf {r})$ ($j=a,\,b$) the polar coordinates of a point at position $\mathbf {r}$ with respect to the local origin $O_j$. A plane wave radiation impinges on the wires with an angle of incidence $\phi _{inc}$ with respect to the $y$ axis. Although some enhanced optical effects related to invisibility modes are observed for $s$ polarization (electric field along the $z$ axis) [32], this work focus on $p$ polarization (magnetic fields along the $z$ axis) for which the electric field in the graphene coating induces electric currents directed along the azimuthal direction $\phi _j(\mathbf {r})$ and LSPs exist in the graphene circular cylinder. In this way, the incident magnetic field (along the $z$ axis) can be written in a system linked to the $j$-cylinder as [30],
Knowing the total electromagnetic field allows us to calculate the scattering cross sections. The time-averaged scattered power is calculated from the integral of the radial component of the complex Poynting vector flux through an imaginary cylinder of length $L$ of radius $\rho _0$ which envelops the graphene wire system (see Fig. 1),
It is known that the scattering cross section and the near to the cylinders field are strongly affected by complex singularities in the field amplitudes $b_{j m}$. Singularities occur at complex locations and they represent the frequency of the eigenmodes supported by the cylinders system. Complex frequencies of these modes are obtained by solving the homogeneous problem, i.e., by imposing the vectors $\overline {\mathbf {Q}}_a$ and $\overline {\mathbf {Q}}_b$ in Eq. (4) be zero. Then, a condition to determine eigenfrequencies is to require the determinant of the matrix in Eq. (4) to be zero,
2.2 Quasistatic approximation: a simple model based on two coupled electric dipoles
Although the rigorous treatment represented by Eq. (13) gives us all the kinematics and dynamics characteristics of the PT-symmetric eigenmodes, the method lacks of analytical expressions that explain the main dependencies with both geometrical and constitutive parameters. For the purpose of showing this dependencies, by applying the quasistatic method, here, we reduce the full treatment provided by the homogeneous part of Eq. (4) to a simple $2\times 2$ matrix description as follows.
Assuming that the radius $R$ of cylinders is much smaller than the wavelength $\lambda =2\pi c/\omega$, the problem can be treated using the dipole approximation where the dimer eigenfunctions calculated with (4) are, as a good approximation, a superposition of single plasmons with angular momentum $m=\pm 1$ linked to each cylinder. In this way, only four coordinates, $b_{j\pm 1}$ ($j=a,\,b$), define the amplitudes vector.
We first consider the case in which the induced dipole moments are along the $\pm x$ directions, i.e., the local magnetic field associated to each of the cylinders has a dependence $\approx \sin \phi _j$ ($j=a,\,b$). As a consequence, the amplitudes $b_{j 1}=b_{j -1}$ ($j=a,\,b$). Here, the subscript $j -1$ stand for cylinder $j$ and angular momentum $m=-1$. In this way, the matrix equation for the modal amplitudes reduces to a 2$\times$2 matrix system for amplitudes $b_{a 1}$ and $b_{b 1}$,
By taking into account the small argument $z=k_v R_{ab}$ in the Hankel functions, the off diagonal elements in the matrix in Eq. (14) are written as
We now consider the case of polarization along the $y$ axis, in which each of the induced dipole moments are along $\pm y$ direction, i.e., the local magnetic field associated to each of the cylinders has a dependence $\approx \cos \phi _j$ ($j=a,\,b$). As a consequence, the amplitudes $b_{j 1}=-b_{j -1}$ ($j=a,\,b$). In this case, the matrix equation for these amplitudes is
3. Results
We consider a system of two dielectric cylinders with permittivities $\varepsilon _a=2.13+i \varepsilon _i$ ($\varepsilon _i>0$), $\varepsilon _b=2.13-i\varepsilon _i$ for lossy and gain cylinders, respectively. The radii $R_a=R_b=R=0.03\mu$m and both cylinders are coated with a graphene monolayer and immersed in vacuum ($\varepsilon _v=1$). The graphene parameters are: temperature $T=300$K, chemical potentials $\mu _1=\mu _2=0.5$eV and the carriers scattering rates $\gamma _1=\gamma _2=0.1$meV. The positions of the cylinders are $\mathbf {r}^a=-0.05\mu$m$\hat {x}$, $\mathbf {r}^b=0.05\mu$m$\hat {x}$ (center to center distance $R_{ab}=0.1\mu$m) and the gap between them is $\Delta =0.04\mu$m. Fig. 3(a) shows the trajectory of the eigenfrequencies in the complex plane as a parametric function of the gain–loss parameter $\varepsilon _i$ calculated by solving the full retarded dispersion equation (FR). To solve this equation, we use a Newton–Raphson method adapted to treat complex variable. Four branches are observed, two of them ($+x+x$ and $+x-x$) corresponds to both cylinders polarized along the $x$ axis (dipole moments along the $x$ axis) while the other two branches ($+y-y$ and $+y+y$) corresponds to the case in which both cylinders are polarized in the $y$ axis (dipole moments along $y$ axis). We are using the notation of the asymptotic case when $\varepsilon _i=0$ for naming the dimer surface plasmons branches, so that $+y+y$ ($+x+x$) branch corresponds to the curve starting at the point for which both dipole moments move in phase on the $y$ axis ($x$ axis), point O” (point O”’) in Fig. 3(a), and the $+y-y$ ($+x-x$) branch corresponds to the curve starting at the point for which the dipole moments oscillating in opposite phase on the $y$ axis ($x$ axis), point O’ (point O) in Fig. 3(a). For instance, the branch $+x+x$ starts at frequency $\omega /c=0.8589226-i0.5789274\,10^{-3}\mu$m$^{-1}$ for $\varepsilon _i=0$, where both dipole moments are in phase, and it moves to the right side leaving away from the real axis. Moreover, the real part of this trajectory approaches asymptotically to the value $\omega /c=0.8868\mu$m$^{-1}$ corresponding to the real part of the eigenfrequency of a single graphene cylinder [36]. On the contrary, the branch $+x-x$ starts at $\omega /c=0.9107941-i0.253260710\,10^{-3}\mu$m$^{-1}$ for $\varepsilon _i=0$, with both dipole moments in opposite phase, and it approaches to the real axis where the lasing threshold at $\omega _{crit}/c \approx 0.8954615$m$^{-1}$ is reached for $\varepsilon _i=\varepsilon _{crit}=0.166$ (point A in Fig. 3(a)).
On the other hand, Fig. 3(a) also shows the trajectory of the branch corresponding to the polarization along the $y$ axis. The branch $+y-y$ starts at frequency $\omega /c=0.8594583-i0.25349871\,10^{-3}\mu$m$^{-1}$ for $\varepsilon _i=0$, where both dipole moments are in opposed phase, and it moves toward the right side, reaching the real axis at point C where the critical eigenfrequency $\omega _{crit}/c=0.8755407\mu$m$^{-1}$ for a critical value of the gain–loss parameter $\varepsilon _{crit}=0.1715275$. On the other hand, the $+y+y$ branch starts at frequency $\omega /c=0.9102954-i0.5560595\,10^{-3}\mu$m$^{-1}$ for $\varepsilon _i=0$ and it moves to the left side leaving away from the real axis. As in the $x$ polarized case, both branches are repelled in the direction of the imaginary axis allowing the $+y-y$ branch to achieve the lasing threshold at point C.
In order to understand the gain–loss compensation near the critical points at which the eigenfrequencies are almost real, in Fig. 3(b) we plot the scattering cross sections for a plane wave impinging at an angle $\phi =0$ (electric field along the $x$ axis) and $\phi =90^\circ$ (electric field along the $y$ axis). The corresponding gain–loss parameter is $\varepsilon _i=0.166$ for $\phi =0$, and $\varepsilon _i=0.1715$ for $\phi =90^\circ$, i.e., it values are near to the critical values at which lasing conditions are achieved. In the $\phi =0$ case, we observe that the scattering cross section is enhanced at frequency near $\omega /c \approx 0.895$m$^{-1}$ that agree well with the lasing frequency for the $+x-x$ branch calculated by solving the eigenmode problem (point A in Fig. 3(a)). Moreover, we observe another peak (less intense) near $\omega /c=0.873\mu$m$^{-1}$, a value that falls near the real part of the eigenfrequency of a state with $\varepsilon _i=0.166$ but corresponding to the $+x+x$ branch (point B in Fig. 3(a)). A similar behavior presents the scattering curve for $\phi =90^\circ$ and $\varepsilon _i=0.1715$. From Fig. 3(b) we observe a very sharp peak at frequency that coincides with the lasing frequency $\omega _{crit}/c \approx 0.8755407\mu$m$^{-1}$ calculated by solving the eigenmode problem (point C in Fig. 3(a)). Moreover, a second peak is observed at a frequency $\approx 0.894\mu$m$^{-1}$ associated to the excitation of the state on the $+y+y$ branch for $\varepsilon _i=0.1715$ (point D in Fig. 3(a)).
The question that arises from the above results is how eigenmodes on the $+x-x$ and $+y-y$ branches, which correspond to trapped modes for $\varepsilon _i=0$, can be excited with a plane wave by varying the gain–loss parameter $\varepsilon _i$. Furthermore, these branches reach their lasing threshold for a critical value of the gain–loss parameter. To find a response, we have calculated the eigenvectors, containing all the field amplitudes of the eigenmodes. In particular, we have verified that coefficients with $|m|\not =1$ are orders of magnitude less than those corresponding to $m=\pm 1$, suggesting that the dimer plasmons can be considered, as a good approximation, as a superposition of single plasmon with $m=\pm 1$ linked to each cylinder. As a consequence, to gain further insight into the underlying physics of these branches excitations, we applied the QA as follows. Without loss of generality, we consider the case for which the induced dipole moments on cylinders are in $\pm x$ direction. By replacing Eq. (26) into Eq. (24) and using Eq. (19), we find the following relation between the dimer amplitudes
In order to visualize the above behavior, in Figs. 5(a) and 5(b) we have plotted the spatial distribution of the magnetic field $H_s^{v}(\mathbf {r})$ (calculated by using the FR method) corresponding to the $+x-x$ mode for $\varepsilon _i=0$ (point O in Fig. 3(a)) and $\varepsilon _i=0.166$ (point A in Fig. 3(a)), respectively. The eigenfrequencies are $\omega /c=(0.9107941-i 0.2532607\,10^{-3})\mu$m$^{-1}$ (Fig. 5(a)) and $\omega /c=(0.8954615-i 0.2850863\,10^{-5})\mu$m$^{-1}$ (Fig. 5(b)). The map of Fig. 5(b) is characterized by four zones, two of them are in red (blue) pointing out that the magnetic field is in $-z$ ($+z$) direction. This field distribution corresponds to that of two identical dipole moments placed at the center of each cylinders and oscillating with opposite phase ($\Phi =\pi$). On the other hand, from Fig. 5(b) we observe two regions, one of them is red ($y>0$) and the other is blue ($y<0$). Moreover, the field intensity near the first cylinder is notably less than that near the second cylinder, indicating that, at this time, the dipole moment on the first cylinder is less than that corresponding to the second cylinder, as shown in Fig. 5(b). This behaviour is in accordance with the fact that the phase difference between both dipole moments $p_a$ and $p_b$ is near $\Phi =\pi /2$ for the lasing mode A and, as a consequence, this mode can be excited by plane wave incidence. Similarly, in Figs. 5(c) and Fig. 5(d), we plotted the magnetic field map for the $+y-y$ mode for $\varepsilon _i=0$ (point O’ in Fig. 3(a)) and $\varepsilon _i=0.1715$ (point C in Fig. 3(a)), respectively. The magnetic field map shown in Fig. 5(c) is similar to that of two dipole moments of equal amplitude oscillating with opposite phase ($\Phi =\pi$). On the contrary, as the gain–loss parameter reach the critical value $\varepsilon _i=0.1715$, the magnetic field distribution looks like that of two dipole moments oscillating out of phase (but not in opposed phase) and, as a consequence, the lasing mode B can be excited by plane wave incidence.
It is worth noting some similarities and differences between the eigenmode branches calculation by using the FR dispersion equation and those calculated by using the QA. On the one hand, the $+x+x$ and $+x-x$ branches in Fig. 3(a) are repelled in the direction of the imaginary axis, as predicted by the QA for a PT-symmetric system (Fig. 2), a feature that allows the $+x-x$ branch to achieve the lasing threshold at point A. Furthermore, the critical value (27) calculated by using the QA results $\varepsilon _{ep}=0.18$, a value that agree well with the gain–loss parameter for which the lasing threshold is achieved in Fig. 3(a). Moreover, the phase difference between the induced dipole moments on each cylinders calculated by FR and QA methods matches quite well. On the other hand, $+x+x$ and $+x-x$ branches in Fig. 3(a) do not start at points with the same value of their imaginary parts as occur for the branches in Fig. 2 and, as a consequence, these trajectories does not coalesce at an exceptional point as in Fig. 2. This is true because the lack of a radiation losses term in the QA, i.e., the radiation losses would prevent the system from having all the properties of a full loss compensated PT-symmetric structure, shown in Fig. 2, such as the existence of an exceptional point.
In order to study the behavior with the chemical potential on graphene, we set $\mu _a=\mu _b=\mu$ and vary the values of $\mu$. In Fig. 6 we have plotted the lasing frequency $\omega _{crit}$ and the corresponding gain–loss parameter $\varepsilon _{crit}$ as functions of $\mu$, calculated with the rigorous FR method. From Fig. 6(a) we can see that the lasing frequency for both $+x-x$ and $+y-y$ branches are increasing functions of $\mu$. This fact can be understood by taking into account that the real part of the eigenfrequency corresponding to a single graphene cylinder, which falls between the lasing frequencies for $+y-y$ and $+x-x$ branches, is proportional to $\sqrt {\mu }$ (see Eq. (20)). On the other hand, from Fig. 6(b) we see that the critical value of the gain–loss parameter for which the lasing condition is achieved is a decreasing function of the chemical potential $\mu$. This behaviour has a similarity with that presented by a single graphene cylinder for which has been demonstrated that the gain level to achieve the lasing condition decreases with the chemical potential value [37].
4. Conclusions
In conclusion, we have analytically studied the scattering and the eigenmode problems for a dimer composed of two graphene coated dielectric cylinders, one of them with loss and the other with the same level of gain. We have demonstrated the existence of two branches, corresponding to trapped modes when $\varepsilon _i=0$, that reach the lasing conditions for suitable values of the gain–loss parameter. While the phase difference between the induced dipole moments on individual cylinders changes from $\pi$ to a value near $\pi /2$ when the gain–loss parameter is incremented, a fact that provides the mode transformation from trapped to radiating modes, the modulus of each individual dipole moments maintains equals in between.
Other mechanisms to transform a trapped mode into a resonant observable which can be excited by a plane wave have been reported in other works. All these methods are based on the introduction of a small asymmetry with respect to the center of the dimer by producing dissimilar dipole moment modulus on each of the cylinders. Interestingly, here, we found that in the transformation from trapped to radiating eigenmodes on both $+x-x$ and $+y-y$ branches, the modulus of the individual dipole moments does not change, while it is changed the phase between them. A distinction between these two kind of mechanism, the phase variation and modulus variation mechanisms, to transform a trapped mode into a resonant observable has not been reported before to our knowledge. We believe that our results will be usefull for a deeper understanding of the PT-symmetric LSP characteristics and that the LSP effects we have demonstrated opens up possibilities for practical applications involving sub-wavelength laser structures.
Appendix A. Graphene conductivity
We consider the graphene layer as an infinitesimally thin, local and isotropic two-sided layer with frequency-dependent surface conductivity $\sigma (\omega )$ given by the Kubo formula [38], which can be read as $\sigma _{loc}= \sigma ^{intra}+\sigma ^{inter}$, with the intraband and interband contributions being
Appendix B. Nonlocal response in graphene conductivity
Nonlocal, or spatial dispersion effects, in graphene conductivity can present deviations from the results predicted by the local conductivity (A1) and (A2). In general, these effects appear when the graphene-based structure size is of a few nanometers. At this size scale, the surface plasmon propagates with low phase velocity $v_{SP}$ approaching to the electron Fermi velocity $v_{F} \approx 10^6\,m/s$. We can estimate the value of $v_F/v_{SP}$ using the quasistatic approximation as follows. Since the surface plasmon effective momentum $k_{SP}$, which is along the azimutal angle ($\phi$ axis), can be written as [36],
if we consider $m=1$ (dipolar order), the factorAppendix C. Developing of the right hand side in Eq. (25)
The right side of Eq. (25) can be calculated by using Eq. (18) as follows.
Disclosures
The authors declare no conflicts of interest.
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