Chirality-enabled unidirectional light emission and nanoparticle detection in parity-time-symmetric microcavity

Abstract

Achieving unidirectional emission and manipulating waves in a microcavity are crucial for information processing and data transmission in next-generation photonic circuits (PCs). Here we show how to impose twin microcavities with opposite chirality by incorporating parity-time (PT) symmetry to realize unidirectional emission. Our numerical calculation results show that the opposite chirality in microcavities stems from the asymmetric coupling of the clockwise (CW) and counterclockwise (CCW) components carried by the attached waveguide to the left-or right-sided microcavities, respectively. Notably, by engineering PT symmetry in the coupled system via the gain-loss control, the clockwise component of the lossy cavity could be selectively suppressed, which leads to the unidirectional emission with an extinction ratio of up to −52 dB. Furthermore, the chirality and PT-symmetry breaking enabled unidirectional emission is extremely sensitive to external scatters, allowing the detection of nanoparticles with an ultrasmall radius of 5-50 nm by recording the extinction ratio change. The proposed system provides a simple yet general way to manipulate the standing waves in a microcavity and will be essential for advancing the potentials of the microcavity in PCs.

Variable optical chirality in atomic assisted microcavity*

“…[1] Because of the mirror reflection geometry symmetry, the microcavity with a pair of standing wave modes possesses chiral symmetry, exhibiting that the transmission or reflection spectra are identical at the two ports of the input-output taper fiber nearby the WGM microcavity. The manipulating of optical waves to break the chiral symmetry in a microcavity is vital to both the research of microcavity photonics and the development of integrated optical devices such as unidirectional lasers, [2][3][4] highly sensitive sensors, [5,6] and all-optical memories. [7,8] Chiral symmetry breaking is derived from breaking either parity [1,3,9,10] or time-reversal symmetry.…”

“…[3] An open system described by Non-Hermitian Hamiltonian exhibits many intriguing physical phenomena at a singularity, called EP, at which the eigenvalues and the corresponding eigenvectors coalesce. [4][5][6][7][8][9][10][11][12][13][14][15][16] Chirality is a fascinating asymmetric phenomenon in the vicinity of the EP, where the cavity retains only one mode (CW or CCW) and exhibits the maximum chirality value 1 or −1. [3,17] On the other hand, chiral symmetry breaking in an optical microcavity will also appear when the time-reversal symmetry is broken such as rotation.…”