[Paper Review] CPA-lasing associated with the quasibound states in the continuum in asymmetric non-Hermitian structures

Denis V. Novitsky, Adrià Canós Valero+4 more|arXiv (Cornell University)|May 23, 2022

Quantum Mechanics and Non-Hermitian PhysicsPhysics and Astronomy70 references53 citations

TL;DR

This paper proposes a mechanism for CPA-lasing—simultaneous coherent perfect absorption and lasing—induced by quasibound states in the continuum (quasi-BIC) in asymmetric non-Hermitian photonic structures. By introducing geometric or non-Hermitian asymmetry in a trilayer system with epsilon-near-zero (ENZ) media, the authors demonstrate a linear dependence of the quality factor on the inverse asymmetry parameter and observe a counter-intuitive loss-induced lasing-like response at the CPA-lasing point.

ABSTRACT

Non-Hermitian photonic systems with loss and gain attract much attention due to their exceptional abilities in molding the flow of light. Introducing asymmetry to the $\mathcal{PT}$-symmetric system with perfectly balanced loss and gain, we reveal the mechanism of transition from the quasibound state in the continuum (quasi-BIC) to the simultaneous coherent perfect absorption (CPA) and lasing in a layered structure comprising epsilon-near-zero (ENZ) media. Two types of asymmetry (geometric and non-Hermitian) are analyzed with the scattering matrix technique. The effect of the CPA-lasing associated with the quasi-BIC is characterized with the unusual linear dependence of the quality factor on the inverse of asymmetry parameter. Moreover, the counter-intuitive loss-induced-lasing-like behavior is found at the CPA-lasing point under the non-Hermitian asymmetry. The reported features of non-Hermitian structures are perspective for sensing and lasing applications.

Motivation & Objective

• To investigate the transition from quasi-BIC to CPA-lasing in non-Hermitian photonic systems with loss and gain.
• To explore the role of geometric and non-Hermitian asymmetry in enabling CPA-lasing in epsilon-near-zero (ENZ) multilayer structures.
• To characterize the unusual linear dependence of the quality factor on the inverse asymmetry parameter in such systems.
• To identify and analyze the counter-intuitive loss-induced lasing-like behavior at the CPA-lasing point under non-Hermitian asymmetry.
• To demonstrate the potential of these structures for applications in sensing and tunable lasing.

Proposed method

• Employing the scattering matrix technique to analyze the poles and zeros of the scattering matrix in non-Hermitian systems.
• Modeling a trilayer structure with an ENZ dielectric spacer and outer layers of loss and gain media with permittivity ε±(ω) = 1 ± iγ − ω²p/ω² using the Drude-Lorentz model.
• Introducing geometric asymmetry via varying the thickness of the gain layer (d− = αd+) relative to the loss layer (d+), with α as the asymmetry parameter.
• Introducing non-Hermitian asymmetry by varying the non-Hermiticity parameter γ in the gain layer (γ− = βγ+), while keeping the loss layer fixed.
• Using the transfer-matrix method to compute reflection spectra and identify resonance features such as Fano profiles and CPA-lasing points.
• Analyzing the transition from a quasi-BIC dip to a CPA-lasing peak via changes in the asymmetry parameter α or β, with fixed incidence angle θ = 23.881° and γ = 0.001.

Experimental results

Research questions

• RQ1 How does geometric asymmetry (differing thicknesses of loss and gain layers) affect the transition from quasi-BIC to CPA-lasing in non-Hermitian ENZ structures?
• RQ2 What is the role of non-Hermitian asymmetry (varying γ in gain layer) in enabling CPA-lasing and inducing loss-induced lasing-like behavior?
• RQ3 Why does the quality factor exhibit a linear dependence on the inverse of the asymmetry parameter in this system?
• RQ4 At what critical asymmetry parameter does the transition from a reflection dip (quasi-BIC) to a peak (CPA-lasing) occur?
• RQ5 Can the CPA-lasing point be directly associated with the quasi-BIC resonance, and what are the implications for sensing and lasing applications?

Key findings

• The CPA-lasing point emerges at a critical geometric asymmetry parameter αt ≈ 1.0056, where the system transitions from a reflection dip to a peak due to slightly increased gain over loss.
• The quality factor of the resonance shows a linear dependence on the inverse of the asymmetry parameter, a behavior not observed in symmetric systems.
• A Fano profile is observed at the CPA-lasing point (αt ≈ 1.0056), indicating interference between a wide Fabry-Perot resonance and a narrow plasmonic resonance.
• Under non-Hermitian asymmetry, a counter-intuitive loss-induced lasing-like response is observed at the CPA-lasing point, where lasing-like amplification occurs despite net loss.
• The CPA-lasing is directly linked to the quasi-BIC, with strong light amplification and high Q-factor, making the system promising for sensing and tunable lasing applications.
• The transition from quasi-BIC to CPA-lasing is driven by the competition between loss and gain, with the system maintaining a stable, high-Q resonance near the critical asymmetry point.