Abstract
Spectral singularities appearing in parity-time (PT)-symmetric non-Hermitian optical systems have aroused a growing interest due to their new exhilarating applications, such as bifurcation effects at exceptional points and the coexistence of coherent perfect absorber and laser (so-called CPAL point). We introduce here how the concept of CPAL action provoked in PT-symmetric metasurfaces can be translated into practical implementation of a low-loss zero/low-index open channel supporting a nearly undamped fast-wave propagation. Such a PT-synthetic metachannel shows the capability to produce a high-directivity leaky radiation, with a tunable beam angle that depends on the gain-loss parameter. The proposed structure may enable new kinds of super-directivity antennas, as well as many applications that demand extreme dielectric properties, such as epsilon-near-zero (ENZ).
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Unusual points in the spectra of non-Hermitian physical systems, such as the exceptional point [1–8] and the merging point of coherent perfect absorber (CPA) and laser (i.e., CPAL point) [9–12], have attracted substantial attention in the past few years. At these points, Taylor series expansion become singular and fail to converge. These isolated singular points are generally missed in a generic situation and may require special engineering of a quantum [13], optical [1–12], acoustic [14] or electronic systems [15,16]. The CPAL systems with parity-time (PT) symmetry is of particular interest because they allow realization of laser and CPA within a single component [9–12]. Traditionally, a laser oscillator emits coherent outgoing radiations, whereas a CPA is its time-reversed counterpart that acts as a dark medium absorbing all incoming radiation. At a given CPAL wavelength, the two eigenvalues of the scattering matrix become infinity and zero, which corresponds to laser and CPA modes, respectively. These two modes with dramatically different scattering properties can be switched by altering the initial amplitude/phase offset of two counter-propagating incoming beams. The CPAL action has been theoretically proposed [9,10] and later experimentally demonstrated [11,12] using waveguides and coupled resonators with PT symmetry. In the context, PT-symmetric metasurfaces have been recently proposed to make a simplified, low-profile alternative to CPAL devices [17], negative-index media [18], optical sensing and imaging devices [19,20], as well as the unidirectional reflectionless channel biased at the exceptional point [21].
In this paper, we will propose new types of electromagnetic medium formed by PT-symmetric metasurfaces operating at the CPAL point. This system comprises a pair of active and passive metasurfaces with the spatial dependency of surface impedance given by ${{Z}_{s}}({y}) ={-} {R}\,\delta ({y} - {d}/2) + {R}\,\delta ({y} + {d}/2)$ (see Fig. 1(a)), where ± R are the surface resistances, d is the spacing between two metasurfaces, and $\delta$ is the Kronecker delta function. The spatially-distributed balanced gain ($- {R}$) and loss ($+ {R}$) form the basis of a PT-symmetric optical system [1]. While scattering from PT-symmetric metasurfaces has been studied for the above-mentioned applications [17–21], the guided-wave and leakage characteristics of a parallel-plate waveguide formed by PT-symmetric metasurfaces (Fig. 1(a)) is yet to be explored. Understanding basic characteristics and effective medium properties of such a PT-synthetic metachannel may lead to new physical phenomena and applications underlying them. In the following, we will show that this low-profile and unsophisticated metachannel can exhibit an extreme (effective) dielectric properties, such as epsilon-near-zero (ENZ) phenomena found in a dispersive lossy medium [6,7] or in a waveguide operating at its cutoff frequency [22,23]. Interestingly, the propagation constant of the guided transverse electric (TE) mode can be continuously varied from nearly zero to that of the background wavenumber (${k} = \omega \sqrt {\mu \varepsilon }$) by changing the dimensionless gain-loss parameter $\gamma = {R}/\eta$ ($\eta = \sqrt {\mu /\varepsilon }$ is the impedance of background medium). From the effective-medium perspective [22,23], the effective permittivity of the PT-synthetic metachannel can vary from ENZ to that of the background medium, i.e., $0 \le {\textrm{Re}} [{\varepsilon _{{eff}}}] \le \varepsilon$. However, unlike other ENZ and low-index media [6,7,22,23], the calculated ${\mathop{\rm Im}\nolimits} [{\varepsilon _{{eff}}}]$ related to the power attenuation rate or propagation loss can be vanishingly small. Additionally, the almost “undamped” fast wave propagating in the PT-synthetic metachannel can produce the coherent radiation leakage and form a highly directional beam, thanks to the large radiating aperture size of the metachannel. More interestingly, the beam angle can be reconfigured to any direction between broadside and end-fire by altering the gain-loss parameter (namely, the surface impedance profile of metasurfaces).
2. Results and discussion
In order to understand singularities in PT-symmetric metasurfaces, we first consider scattering of the TE-polarized plane wave from this composite structure [Fig. 1(b)], which can be described by the two-port transmission-line network (TLN) shown in the inset of Fig. 1(b) [17–20]. In the TLN model, the background medium has a tangential wavenumber and a characteristic impedance given by${{k}_{y}} = {k}\cos \alpha$ and ${Z} = \eta {k}/{{k}_{y}},$ where $\alpha$ is angle of incidence. The outgoing scattered waves and the incoming waves can be related by the scattering matrix (see Appendix A). Figure 1(c) presents the evolution of the two eigenvalues of scattering matrix as a function of $\gamma$ and $\alpha ,$ with the electrical length between the two metasurfaces $\Phi { = }{{k}_{y}}{d} = \pi /2.$ From Fig. 1(c), we find that CPAL points can exist when$\gamma = 1/(\sqrt 2 \cos \alpha )$, which makes eigenvalues become zero and infinity. Exceptional points are also observed in Fig. 1(c). Such branch point singularities divide the system into the exact symmetry phase with unimodular eigenvalues and the broken symmetry phases with non-unimodular ones. Moreover, eigenvalues coalesce at exceptional points [10].
Next, we will discuss the use of PT-symmetric metasurfaces as a waveguiding channel and will show that the CPAL point found in scattering events can shed light on tailoring effective medium properties of a PT-synthetic metachannel. According to the Lorentz reciprocity theorem [24–26], if a current density J1 placed at point r1 produces an electric field E1 at point r2, then by switching the position of source and observation, their product remains constant. In our case, we assume that the tangential electric field Es on PT-symmetric metasurfaces is induced by an incident plane wave sustained by the current density JFF placed in the volume VFF. Under the lasing condition, a source (Js) of arbitrary input amplitude can produce a huge Es. We can now apply reciprocity considerations to evaluate the radiated field EFF(x,y) produced by the equivalent surface current densities ${{\mathbf J}_{s}}({x})\delta ({y} \pm {d}/2)$ on metasurfaces (which is defined the ratio of the tangential electric field to the surface impedance) enclosed by the volume VMTS. The reciprocity formula yields
The eigenmodal solutions of this PT metachannel can be derived using the transverse-resonance relation that considers the TLN model of the transverse cross section of the waveguide, analogous to the one used for studying scattering of plane waves by PT-symmetric metasurfaces [the inset of Fig. 1(b)] [27]. In this case, the line has a transverse propagation constant ${{k}_{y}} = \sqrt {{{k}^2} - {\beta ^2}}$ and a characteristic impedance for the TE mode given by ${Z} = \eta {k}/{{k}_{y}},$ where $\beta$ is the longitudinal propagation constant. If the transverse resonance condition is satisfied, at any point along the y-axis, the sum of the input impedances seen looking to either side must be zero, i.e., ${Z}_{{in}}^{( + )} + {Z}_{{in}}^{( - )} = 0,$ where ${Z}_{{in}}^{( + )}$ and ${Z}_{{in}}^{( - )}$ are respectively input impedances seen looking to $+ \hat{{\boldsymbol {y}}}$ and $- \hat{{\boldsymbol {y}}}$ at any point on the resonant line, $- {d}/2 \le {y} \le {d}/2$ (see Appendix B). This yields the following dispersion equation:
We note further that a PT-synthetic metachannel locked at the CPAL point exhibits a fast-wave propagation behavior, i.e., $\beta < {k}$, and, thus, has a low effective permittivity given by ${\varepsilon _{{eff}}}/\varepsilon { } = { }si{n^2}\alpha .$ Fast waves propagating in the unbounded channel formed by metasurfaces will result in the radiation leakage and the beam angle measured from broadside $\alpha = {\sin ^{ - 1}}(\beta /{k}).$ The gain-loss parameter that controls the CPAL at a certain angle of incidence (scattering events) also governs the beam angle in the leaky-wave mode. We first consider a metachannel composed of PT-symmetric metasurfaces with $\gamma = 1/\sqrt 2$ and a height of one-quarter wavelength, which forms a CPAL for normally-incident waves at frequency f0. In accordance with the discussion above, when a PT-synthetic metachannel is excited by a waveguide port at f0, one can expect that $\beta \approx 0,$ and, thus, an ENZ medium with infinite phase velocity is achieved. Figures 3(a) and 3(b) show the calculated radiation pattern (see Appendix C) and electrical field distributions [28] for this unbounded metachannel at ${f} = {{f}_0} - \delta {f}$ (here, $\delta {f} = { }{10^{ - 4}}{{f}_0}\;$ which leads to $\beta /{k} = 0.005 - {j}0.010$). It is seen from Fig. 3(b) that inside the channel, a nearly constant phase distribution can be obtained due to the ENZ-like characteristics. Moreover, the nearly undamped fast-wave property with $\beta \sim 0$ results in a highly directive broadside radiation, as can be seen in Fig. 3(a). In the far (Fraunhofer) zone, the directivity of 2-D radiative apertures can be defined quantitively as the ratio of the maximum radiation intensity of the main lobe (${{U}_{\max }}$) to the average radiation intensity over all space [29]:
Moreover, changing the gain-loss parameter will alter the beam angle, as can be seen in the far-field radiation patterns in Fig. 3(c) and contour plots of electric field distributions in Fig. 3(b). For different targeted beam angles $\alpha =$ 0°, 30°, 45°, and 60°, surface resistances of two metasurfaces ($\gamma = 1/\sqrt 2 \cos \alpha$) and the spacing between them ($\Phi { = kd}\cos \alpha = \pi /2$) must be changed accordingly, in order to lock the system at the CPAL point. The radiation pattern is somehow bidirectional, due to the nature of unidirectional scattering in PT systems [21]. Compared with other ENZ medium made of metamaterials or Drude-dispersion materials, the proposed low-index metachannel may not only ease manufacturing complexity, but also greatly reduce the attenuation rate. The proposed structure may therefore facilitate the practice of ENZ-allowed applications (e.g., supercoupling and superluminal effect, energy squeezing, and enhanced nonlinear wave mixing [6,7,22,23], as well as leaky-wave emitters [7,27]). Leaky-wave antennas based on guided-wave devices with periodic grids/slots have been enormously studied in different spectral ranges. However, their effective aperture size is generally limited by the non-negligible attenuation rate, especially for optical applications. Besides, the occurrence of higher-order (Floquet) spatial harmonics could produce unintended grating lobes. These long-standing challenges may be addressed by the PT-synthetic leaky-wave structures, with homogeneous non-graded surfaces and contactless gain-loss interactions.
We also analyze radiation from an electric line source ($\overline {\textrm J} = {\boldsymbol {\hat{z}} }{{I}_0}\delta ({x})\delta ({y})\;\;[A/{m^2}]$) placed at the center of a PT-synthetic metachannel, as schematically shown in Fig. 4(a). Figures 4(b) and 4(c) show the far-field radiation pattern and contours of electric field distributions for the metachannel in Fig. 4(c), under excitation of a line source; here $\delta {f} = { }{10^{ - 1}}{{f}_0}.$ The electric field in the far zone can be obtained as an inverse Fourier transform [29]:
3. Conclusion
In conclusion, we have proposed the concept of a PT-synthetic metachannel exhibiting zero or low effective permittivity, for which the CPAL point offers a comprehensive guidance on tailoring the extreme effective permittivity. When this metachannel working at the CPAL point is fed by a waveguide port or line source, the leaky-wave mode can couple the (nearly undamped) guided fast wave into the background medium, resulting in an ultrahigh-directivity radiative leakage. In addition, the beam can be steered from broadside towards end-fire direction by controlling the gain-loss parameter. We envision that the proposed active component may be applied to many applications of interest in different spectra, including ultrahigh-directivity antennas or emitters with tunable radiating angles, as well as low-attenuation ENZ/low-index media with leaky-wave properties.
Appendix A. Coherent perfect absorber-laser using PT-symmetric metasurface
Consider scattering of a plane wave (wave vector ${\mathbf k}{ = }\hat{{\boldsymbol {x}}}{{k}_{x}} + \hat{{\boldsymbol {y}}}{{k}_{y}}$) from PT-symmetric metasurfaces, the elements of the scattering matrix can be calculated using the two-port transmission-line network (TLN) model in Fig. 1(b). The background medium has a characteristic impedance Z, and the two shunt surface resistances are separated by a portion of transmission line with a characteristic impedance Z and an electrical length $\Phi { = }{{k}_{y}}{d}{.}$ The surface resistances have opposite values, ±R. In this system, the dimensionless gain-loss parameter (or non-Hermiticity parameter) can be defined as $\gamma = {R}/{Z}{.}$ For the transverse electric (TE) plane wave incident at an arbitrary angle $\alpha$, the wave impedance is given by ${Z = }\eta /\cos \alpha$ and the propagation constant is ${{k}_{y}}{ = k}\cos \alpha ,$ where η is the characteristic impedance of background medium. Similar considerations apply to the transverse magnetic (TM) incidence, but with ${Z = }\eta \cos \alpha .$ Using the transfer matrix formalism, and assuming time-harmonic fields ejωt, the scattering parameters, involving transmission (t) and reflection (r) coefficients for bottom ($-$) and top (+) incidences are obtained as:
Appendix B. Eigenmodes in a PT-synthetic channel
Consider first the eigenmodes of the PT-synthetic channel in Fig. 1(a), a guided wave propagates along the x-axis with a factor${{e}^{ - {j}\mathrm{\beta }{x}}}.$ Electromagnetic fields can be separated into transverse electric (TE) and transverse magnetic (TM) fields with respect to a lateral coordinate. The TE mode has the following electric field distributions:
Appendix C. Radiation from a PT-synthetic channel under excitation of a waveguide port
The electric surface current density on a metasurface is induced by discontinuity of magnetic fields. For the PT-symmetric metasurface channel sketched in Fig. 5, surface current densities are given by:
${\mathbf A}(\bar{\rho }) = \int\!\!\!\int\limits_{s^{\prime}} {{{\mathbf J}_s}\frac{\mu }{{4{j}}}} {H}_0^{(2)}({{k|}\bar{\rho } - \bar{\rho }^{\prime}{|}} ){ds}^{\prime},$ where ${g}({\bar{\rho },\bar{\rho }^{\prime}} )= \frac{1}{{4{j}}}{H}_0^{(2)}({{k|}\bar{\rho } - \bar{\rho }^{\prime}{|}} )$ is the two-dimensional Green’s function, $\bar{\rho }^{\prime} = {x}^{\prime}\hat{\textrm x} + {y}^{\prime}\hat{\textrm y}$ and $\bar{\rho }{ = }\rho \hat{\rho } = {x}\hat{\textrm x} + {y}\hat{\textrm y}$ (ρ is the radial distance and $\hat{\rho } = \cos \phi {\hat{x}} + \sin \phi {\hat{y}}$) are the position vectors of the source and the observer, respectively, and${H}_0^{(2)}({\cdot} )$ is the Hankel function of the second kind. In the far zone, the electric and magnetic fields produced by sheet currents induced on the metasurfaces only have${\hat{z}}$ and $\hat{\phi }$ components in the cylindrical coordinates. Those constitute a transverse electromagnetic (TEM) wave propagating in the $\hat{\rho } -$ direction, given by:
Appendix D. Radiation from a PT-synthetic channels under excitation of an electric line source
The structure considered here is PT-symmetric metasurfaces excited by an electric line source along z with a time-harmonic dependence, embedded in the middle of the PT-symmetric metasurfaces, as sketched in Fig. 5. The transverse-equivalent network as in Fig. 6 can be used to model such an antenna [27,34–36]. The electric field in the background produced by a unit amplitude electric line source can be represented as an inverse Fourier transform [27,36]:
The characteristic impedance, Z, for the TE and TM polarizations have the following expressions:
Similarly, the far-zone electric field in the lower half-plane is given by:
whereAppendix E. Practical realization of PT-symmetric metasurfaces in the optical regime
According to impedance boundary conditions, a discontinuity on the tangential magnetic field on the metasurface is related to the induced averaged surface current by the surface impedance. For the normally-incident wave on a 2-D array of perfectly-conducting patches, the equivalent surface impedance can be approximately expressed as [37–39]:
Funding
National Science Foundation (1917678).
Disclosures
The authors declare no conflicts of interest.
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