Chiral and degenerate perfect absorption on exceptional surfaces

Abstract

Engineering light-matter interactions using non-Hermiticity, particularly through spectral degeneracies known as exceptional points (EPs), is an emerging field with potential applications in areas such as cavity quantum electrodynamics, spectral filtering, sensing, and thermal imaging. However, tuning and stabilizing a system to a discrete EP in parameter space is a challenging task. Here, we circumvent this challenge by operating a waveguide-coupled resonator on a surface of EPs, known as an exceptional surface (ES). We achieve this by terminating only one end of the waveguide with a tuneable symmetric reflector to induce a nonreciprocal coupling between the frequency-degenerate clockwise and counterclockwise resonator modes. By operating the system at critical coupling on the ES, we demonstrate chiral and degenerate perfect absorption with squared-Lorentzian lineshape. We expect our approach to be useful for studying quantum processes at EPs and to serve as a bridge between non-Hermitian physics and other fields that rely on radiation engineering.

Linear response theory of open systems with exceptional points

“… 23 and shown to exhibit an exceptional surface with potential applications for sensing and controlling spontaneous emission 31 . In addition, it has been recently implemented using an on-chip microsphere resonator with the feedback realized using a fiber loop mirror 104 . It is described by the following set of equations: where, ω o is the resonant frequency, γ is the decay rate of the resonant mode into each waveguide.…”

“…The first step can be straightforward in simple systems. b In systems that exhibit exceptional hypersurfaces in the parameter space 23 , 104 , 125 – 127 , finding such a perturbation can be a very complex task since any perturbation that shifts the system along the surface will fail. In addition, taking the limit when ϵ → 0 involves the cancellation of several singular terms with opposite signs.…”

Response strengths of open systems at exceptional points

“…Note that the resulting spectral lineshape is not Lorentzian, see, e.g., Refs. [64][65][66]. The third factor is the spectral response strength associated to the EP, ξ.…”

“…A CW traveling wave that couples to the waveguide with the coupling rate γ ≥ 0 is reflected at the mirror with the field reflection coefficient r ≥ 0 and can therefore couple back into the microring where it propagates in the CCW direction. This model has been experimentally realized using a microsphere [66] and it has been proposed to use this model for EP-based optical amplifiers [67]. In the latter work the backscattering coefficient in the Hamiltonian (1) has been calculated to be…”

Topological engineering of terahertz light using electrically tunable exceptional point singularities

“…By contrast, at an EP both the eigenvalues and the associated eigenvectors coalesce, considerably modifying the energy landscape of the system and thus resulting in reduced dimensionality and skewed topology. This, in turn, enhances the system's response to perturbations (6)(7)(8)(9), modifies the local density of states leading to the enhancement of spontaneous emission rates (10,11), and leads to a plethora of counterintuitive phenomena such as loss-induced lasing (12), topological energy transfer (13), enhanced chiral absorption (14), linewidth enhancement in lasers (15), unidirectional emission in ring lasers (16), and asymmetric mode switching (17).…”