Towards a Broad-Band Coherent Perfect Absorption in systems without Scale-Invariance

Abstract

We experimentally and theoretically challenge the concept of coherent perfect absorption (CPA) as a narrow frequency resonant mechanism associated with scattering processes that respect scaleinvariance. Using a microwave platform, consisting of a lossy nonlinear resonator coupled to two interrogating antennas, we show that a coherent incident excitation can trigger a self-induced perfect absorption once its intensity exceeds a critical value. Importantly, a (near) perfect absorption persists for a broad band frequency range around the nonlinear CPA condition. Its origin is traced to a quartic behavior that the absorbance spectrum acquires in the proximity of a CPA associated with a new kind of exceptional point degeneracy related to the zeros of the nonlinear scattering operator.

Eigenfunction non-orthogonality factors and the shape of CPA-like dips in a single-channel reflection from lossy chaotic cavities

“…a cavity) with a small amount of loss which completely absorbs a monochromatic wave incident at a particular frequency [2]. Nonlinear effects may further help to make CPA's a broad-band phenomenon [3] increasing prospects of its practical implementation, including optical filters and switches or logic gates for use in optical computers. Recently, a CPA in a rectangular waveguide with randomly positioned scatterers and absorption due to a single antenna has been realised experimentally [4], paving the way for the construction of CPAs based on disordered cavities.…”

“…In another recent experiment, a CPA has been realised with a two-port microwave graph system, both with and without time-reversal symmetry [5], and yet in another way with chaotic cavity with programmable meta-surface inclusions [6]. In the latter setting the relevant framework is that of chaotic wave scattering, and the CPA state corresponds to an eigenstate of the scattering matrix S(ω) with zero eigenvalue at a real frequency 3 . This fact naturally motivates rising interest in a more general question of characterizing S-matrix complex zeros, as well as their manifestation in physical observables, which has not been systematically studied for wave chaotic systems with losses until very recently [7][8][9][10][11][12].…”

Exceptional points of PT-symmetric reflectionless states in complex scattering systems

“…A special case of RL-EPs found for purely incoming boundary conditions is the coalescence on the real axis of two zeros of the complete scattering matrix S(ω) known as a perfectly absorbing EP [19][20][21]. The phenomenon of coherent perfect absorption (CPA) occurs when absorption within a scattering medium balances the excitation rate of incoming channels [22][23][24][25].…”

“…The latter has been achieved in particular through purposeful perturbations of the scattering system, which can include additional constraints, such as on frequency or CPA wavefront [30,[32][33][34]. When two CPA-states coalesce, the broadened lineshape of the absorption spectrum indicates the existence of perfectly absorbing EPs [19][20][21].…”

“…As the penetration depth z increases, the RL-frequencies move closer until they coalesce for a critical perturbation (z = z EP ) to form RL-EPs. The two peaks on the spectrum of R(ν) found in the underperturbed regime (z < z EP ) transform into a single broadband one at z EP with a flattened quartic line shape which is characteristic of EPs [19][20][21]41]. Once two RL-eigenvalues have collapsed, they leave the real axis as complex-conjugate pairs.…”