Active polarization control with a parity-time-symmetric plasmonic resonator

Abstract

Control of the polarization state of light is essential for many technologies, but is often limited by weak light-matter interactions that necessitate long device path lengths or significantly reduce the signal intensity. Here, we investigate a nanoscale plasmonic aperture capable of modifying the polarization state of far-field transmitted light without loss in the probe signal. The aperture is a coaxial resonator consisting of a dielectric ring embedded within a metallic film; parity-time (PT) symmetric inclusions of loss and gain within the dielectric ring enable polarization control. Since the coaxial aperture enables near-thresholdless PT symmetry breaking, polarization control is achieved with realistic levels of loss and gain. Exploiting this sensitivity, we show that the aperture can function as a tunable waveplate, with the transmitted ellipticity of circularly polarized incident light changing continuously with the dissipation coefficient from pi/2 to 0 (i.e. linear polarization). Rotation of linearly polarized light with unity efficiency is also possible, with a continuously-tunable degree of rotation. This compact, low-threshold, and reconfigurable polarizer may enable nextgeneration, high-efficiency displays, routers, modulators, and metasurfaces.Recently, the concept of PT symmetry has been extended to the polarization degree of freedom. Employing a periodic array of specially designed subwavelength resonators, or meta-atoms, distinct amplification or dissipation rates can be achieved for spatially co-located but orthogonally polarized light. 37,42-44 These highly anisotropic systems have been shown to exhibit polarization phase transitions and polarization exceptional arXiv:1712.05383v2 [physics.optics]

Non-Hermitian Physics

“…Physically, the coalescence of eigenvectors near the spectral singularity thus leads to the chiral propagation in the positive direction [79]. When the two-dimensional vector space can be regarded as the polarization space in the basis of linear polarizations [189,190], the different chirality can represent the circular polarization in the opposite direction. We remark that if the gain-loss structure is reversed, i.e., if we set h = −2γ, the two right eigenvectors can be shown to coalesce into one having a negative group velocity given in Eq.…”

“…The chirality manifests itself as the breaking of mirror symmetry between clock-wise and counterclock-wise propagating modes in cavities [167,211,212]. The chiral property was also experimentally demonstrated in micro resonators [196], a coupled ferromagnetic waveguide [213], optical waveguides with integrated structures [214,215], and circularly polarized light [189,190]. It has been proposed that the concept of the chirality of EPs can be generalized or transferred to different types of physical phenomena such as the anomalous edge state emerging from the encircling of the EP in the momentum space [216], transverse zero spin-angular momentum of light [217], surface modes in Maxwell's equations [218], and the chiral polarization in the relativistic drift effect [219,220].…”

“…We here consider the case of two-spatial dimension r = (x, y) T , which is appropriate for many experimental systems. The first equation ( 189) is the continuity equation while the second one (190) corresponds to the equation of motion, in which the first term on the right-hand side suggests a preference for a nonzero constant speed |v| = α/β if α is positive while negative α results in the nonordered state |v| = 0. The terms including λ, λ 2,3 indicate the violation of the Galilean invariance due to the self-propulsions and are absent in the (nonactive) conventional fluid.…”

Single-mode lasers using parity-time-symmetric polarization eigenstates

“…Hence, Pancharatnam's discovery really appears to be a precursor to the recently proposed omnipolarizer [15]. The idea of PT-symmetry breaking in the polarization space is more recent and it generally aims at achieving active control of polarization [15][16][17][18][19][20][21], like compact polarization converters. It is generally realized with engineered metasurfaces or waveguides, wherein gain and loss are carefully balanced.…”

Bound states in the continuum and exceptional points in dielectric waveguide equipped with a metal grating

“…In optics and photonics, EPs in active or passive parity-time (PT) symmetric structures as well as those in non-PT symmetric structures have attracted a significant amount of interest [23,24] as such systems exhibit various exotic as well as practically important phenomena at or around the EP. For example, asymmetric mode switching [25], directional omni-polarizer [26], laser mode selection [27,28,29], unidirectional invisibility or reflectionlessness [30], directional total absorption [31], loss-induced transparency [32], polarization control [33], and enhancement of Sagnac sensitivity [34,35] have been proposed and/or demonstrated. They are expected to lead to a new paradigm of optical systems.…”