However, their unidirectional nature in nonreciprocal devices is still elusive because such trapping states are easily destroyed when the symmetry of an optical system is broken. They have found applications in numerous areas, including optical nonlinearity, light emitters, and nano-sensors. Recently studied bound states in the continuum (BICs) enable perfect localization of light and enhance light–matter interactions although systems are optically open. The proposed theories and novel chiral-coupling interface will be beneficial to the design of more compact and flexible chiral-coupling systems for diverse applications.

Two first-principles-based SPP models for the reciprocal and original problems are built up to verify the decisive role of the FP resonance in achieving the chiral coupling. A unidirectivity up to 99.9% along with a high coupling efficiency and enhancement of spontaneous emission rate is achieved. The FP resonance provides flexible design freedoms which can modulate the chirality of the T-spin (and the resultant chiral-coupling directionality) to flip or disappear. Based on the theories, we propose an indirect chiral-coupling system on the platform of surface plasmon polariton (SPP), with a nanocavity supporting Fabry–Perot (FP) resonance of dual SPP modes serving as a novel coupling interface. Here, we present an intuitive and rigorous approach for analyzing the direct/indirect chiral coupling, and thereout, derive some general relations between the chiral-coupling directionality and the T-spin of the field or emitter. However, this indirect chiral coupling still undergoes limitations in flexibility and miniaturization, and the underlying physical mechanisms are to be clarified. This chiral coupling can be largely enhanced in terms of unidirectivity, efficiency, and spontaneous emission rate by introducing resonant modes as coupling interfaces. The chiral coupling of an emitter to waveguide mode, i.e., the propagation direction of the excited waveguide mode is locked to the transverse spin (T-spin) of a circularly polarized emitter, has exhibited unprecedented applications in nanophotonics and quantum information processing. In the limit of large cooperativity, the input noise is instead transmitted in an unaltered way in the allowed direction in particular one has only vacuum noise in the output rf port in the case of optical-to-rf conversion. We also study the transducer noise and show that mechanical thermal noise is always reflected back onto the isolated port. We show that perfect nonreciprocal transduction can be obtained in the limit of large cooperativity in both directions, from optical to rf and vice versa. In our protocol, we apply a bichromatic drive to the cavity mode and a single-tone drive to the rf resonator, and use the relative phase between the drive tones to obtain nonreciprocity. The nonreciprocal transmission is obtained by interference of two dissipative pathways of transmission between the two electromagnetic modes established through two distinct intermediate mechanical modes.

Here we propose a scheme enabling nonreciprocal conversion between optical and radio-frequency (rf) photons using exclusively optomechanical and electromechanical interactions.

Nonreciprocal systems breaking time-reversal symmetry are essential tools in modern quantum technologies enabling the suppression of unwanted reflected signals or extraneous noise entering through detection ports.

Uses for an optical isolator