Electromagnetically induced transparency at a chiral exceptional point (1911.03552v1)

Abstract: Electromagnetically induced transparency, as a quantum interference effect to eliminate optical absorption in an opaque medium, has found extensive applications in slow light generation, optical storage, frequency conversion, optical quantum memory as well as enhanced nonlinear interactions at the few-photon level in all kinds of systems. Recently, there have been great interests in exceptional points, a spectral singularity that could be reached by tuning various parameters in open systems, to render unusual features to the physical systems, such as optical states with chirality. Here we theoretically and experimentally study transparency and absorption modulated by chiral optical states at exceptional points in an indirectly-coupled resonator system. By tuning one resonator to an exceptional point, transparency or absorption occurs depending on the chirality of the eigenstate. Our results demonstrate a new strategy to manipulate the light flow and the spectra of a photonic resonator system by exploiting a discrete optical state associated with specific chirality at an exceptional point as a unique control bit, which opens up a new horizon of controlling slow light using optical states. Compatible with the idea of state control in quantum gate operation, this strategy hence bridges optical computing and storage.

• The paper presents a novel method of using chiral exceptional points to control optical interference via electromagnetically induced transparency.
• It combines coupled mode theory, numerical simulations, and experiments with microtoroid and microdisk resonators to validate the EP tuning strategy.
• The findings offer advances in integrated photonics, paving the way for robust optical storage and slow light generation with improved noise resilience.

Electromagnetically Induced Transparency at a Chiral Exceptional Point

The paper presents a detailed exploration of electromagnetically induced transparency (EIT) phenomena at chiral exceptional points (EPs) in photonic systems, specifically utilizing a configuration of indirectly coupled whispering-gallery-mode (WGM) microresonators. The work combines both theoretical and experimental approaches to demonstrate control over optical states using exceptional points, aiming to bridge the gap between optical computing and storage through the manipulation of light flow and resonance spectra.

EIT is a well-documented quantum interference effect utilized extensively in applications such as slow light generation, optical storage, and enhanced nonlinear interactions. Traditional EIT configurations involve complex control of external parameters, introducing noise and instability. The current research circumvents these issues by harnessing the unique properties of EPs, which provide a level of control over optical chirality previously unattainable in integrated photonic systems.

In their experiments, the authors employ two microresonators, a microtoroid and a microdisk, coupled to a fiber taper waveguide. By tuning one resonator to an EP, they facilitate chiral-induced transparency (CIT) or absorption depending on the eigenstate's chirality. The paper specifies two types of EPs based on chirality: EP_ with chirality -1, where the backscattering from counterclockwise (CCW) to clockwise (CW) is zero, and EP+ with chirality +1, where backscattering from CW to CCW is zero. These conditions are realized through the precise application of a nanotip, which perturbs the resonant modes of the microtoroid resonator, leading to asymmetric coupling.

The authors' methodology includes deriving coupled mode equations to describe the system's dynamics and utilizing a numerical model for simulation purposes. The experimental setup provides evidence of the predicted behaviors, showing how tuning to specified EPs can effectively enable or suppress optical interference. In EP+, a narrow transparency window is achieved through destructive interference within the optical loop, while EP_ results in a unique absorption profile, showcasing the elimination of interference due to the breakdown of the loop.

Key numerical results include the tuning of transmission spectra through the manipulation of phase angles and coupling strengths between the resonators and the fiber taper. Notable outcomes include the ability to achieve both exceptional-point-assisted transparency (EPAT) and exceptional-point-assisted absorption (EPAA), highlighting a new form of state control through discrete chiral optical modes.

The theoretical implications of this research extend to the design of robust and efficient photonic systems capable of high-fidelity optical storage and information processing. The practical implications suggest advancements in integrated photonics, such as slow light generation and optical logic gates, becoming feasible through electrically controlled refractive index modulation. This proposes a CMOS-compatible approach for future devices, significantly enhancing the operability of photonic systems in a noise-resilient manner.

As the field of non-Hermitian photonics continues to evolve, this work underscores the potential of EPs as a fundamental tool in developing next-generation photonic technologies. Future research directions could involve exploring different material systems and configurations to expand the range of achievable optical phenomena, as well as integrating these findings into more complex photonic circuits for advanced functionalities.