Abstract We propose a scheme for realizing broadband and tunable transmission non-reciprocity by utilizing two-photon near-resonant transitions in thermal atoms as single-photon far-detuned transitions can be eliminated. Our basic idea is to largely reduce the Doppler broadenings on a pair of two-photon, probe and coupling, transitions and meanwhile make the only four-photon transition Doppler-free (velocity-dependent) for a forward (backward) probe field. One main advantage of this scheme lies in that the transmission non-reciprocity can be realized and manipulated in a frequency range typically exceeding 200 MHz with isolation ratio above 20 dB and insertion loss below 1.0 dB by modulating an assistant field in frequency and amplitude. The intersecting angle between four applied fields also serves as an effective control knob to optimize the nonreciprocal transmission of a forward or backward probe field, e.g. in a much wider frequency range approaching 1.4 GHz.
Abstract We propose a scheme for realizing broadband and tunable transmission non-reciprocity by utilizing two-photon near-resonant transitions in thermal atoms as single-photon far-detuned transitions can be eliminated. Our basic idea is to largely reduce the Doppler broadenings on a pair of two-photon, probe and coupling, transitions and meanwhile make the only four-photon transition Doppler-free (velocity-dependent) for a forward (backward) probe field. One main advantage of this scheme lies in that the transmission non-reciprocity can be realized and manipulated in a frequency range typically exceeding 200 MHz with isolation ratio above 20 dB and insertion loss below 1.0 dB by modulating an assistant field in frequency and amplitude. The intersecting angle between four applied fields also serves as an effective control knob to optimize the nonreciprocal transmission of a forward or backward probe field, e.g. in a much wider frequency range approaching 1.4 GHz.
We present a further investigation on the optical nonreciprocity based on four-wave mixing (FWM) by leveraging coherently driven cold atoms, as a beneficial supplement to the usual cavity-waveguide solution in virtue of the on-demand tunability and broadband advantage. We consider two parameter regimes unexplored yet, where the probe and coupling fields are set with vanishing or large detunings while the dressing field is kept on resonance. Numerical results reveal that the forward (backward) probe exhibits a high transmittance (is strongly absorbed) in the resonant regime, but may be largely converted into an FWM field (propagate almost freely) in the far-detuned regime. The optimization of such a reversible unidirectional transmission featured by large isolation ratios and low insertion losses can be attained only when the coupling and dressing fields fulfill different scaling relations in amplitude, which has been well explained via analytical results. It is also viable to achieve reversible unidirectional transmission for a probe field of fixed large detunings by simply tuning the coupling field in a wide spectral range. Our findings provide additional insights into the FWM-based optical nonreciprocity and may enable more powerful unidirectional devices in quantum networks.
Optical nonreciprocity is an essential property for a wide range of applications, such as building nonreciprocal optical devices that include isolators and circulators. The realization of optical nonreciprocity relies on breaking the symmetry associated with Lorentz reciprocity, which typically requires stringent conditions such as introducing a strong magnetic field or a high-finesse cavity with nonreciprocal coupling geometry. Here we discover that the collision effect of thermal atoms, which is undesirable for most studies, can induce broadband optical nonreciprocity. By exploiting the thermal atomic collision, we experimentally observe magnet-free and cavity-free optical nonreciprocity, which possesses a high isolation ratio, ultrabroad bandwidth, and low insertion loss simultaneously. The maximum isolation ratio is close to 40 dB, while the insertion loss is less than 1 dB. The bandwidth for an isolation ratio exceeding 20 dB is over 1.2 GHz, which is 2 orders of magnitude broader than typical resonance-enhanced optical isolators. Our work paves the way for the realization of high-performance optical nonreciprocal devices and provides opportunities for applications in integrated optics and quantum networks.
Abstract Optical nonreciprocity provides a firm basis for direction-dependent control of light transport. We propose a scheme to manipulate two counter-propagating light pulses based on dynamically controlled dual-channel optical nonreciprocity originating from the Doppler effect. This scheme is implemented in a four-level tripod-type thermal atomic medium driven by two counter-propagating control fields. Symmetric or asymmetric propagation and storage scenarios for the two probe pulses can be established by adjusting the detunings and intensities of the control fields. In particular, complete separation of the forward and backward transmission windows enables the realization of a frequency-tunable bidirectional optical isolator. Doppler shifts make the tripod system behave as two weakly coupled three-level Λ-type channels associated mainly with the forward and backward probe fields. Our proposal may have potential applications in designing direction-dependent all-optical devices for manipulating multiple optical pulses.
We theoretically investigate narrowband nonreciprocal transmission in a Doppler-broadened five-level M-type atomic medium. After Doppler averaging, the M-type system retains a narrow near-resonant electromagnetically induced transparency window that is narrower than that of a standard three-level Λ system. This spectral narrowing is associated with the decreasing separation between the two inner dressed states as atomic velocity increases. The transparency window exhibits pronounced direction-dependent transmission, with high forward transmission and strong suppression in the reverse direction. The transparency linewidth and forward-backward probe transmission can be controlled via two independent driving fields, while the central transparency frequency is kept essentially unchanged. The resulting narrowband nonreciprocity supports high forward transmission together with strong suppression in the reverse direction, which is suitable for frequency-selective optical filtering and nonreciprocal optical transmission in thermal atomic media. Our scheme with these specific features is geared towards enabling compact vapor-cell-based nonreciprocal devices for quantum network applications.
Atomic absorption-induced insertion loss poses a fundamental limitation on signal transmission in optical nonreciprocity (ONR) systems. We propose a lossless and magnetic-field-free ONR scheme based on dual-Raman interference in two far-detuned Λ-type three-level systems. In the forward direction, destructive interference cancels the imaginary part of the susceptibility while retaining a large phase shift, enabling absorption-free transmission. In contrast, Doppler shifts arising from atomic thermal motion disrupt the Raman resonances in the backward direction, suppressing both absorption and phase modulation. This direction-dependent optical response yields nonreciprocal phase shifts which, when implemented in a Mach-Zehnder interferometer, enable an optical isolator with 100% forward transmittance and an isolation of up to 37 dB.
Owing to the potential applications in all-optical quantum information processing and quantum optical networks, magnet-free optical non-reciprocity transmission has attracted great interest and has been studied in many fields, such as parity-time-symmetry enhanced nonlinearity, optomechanical systems, photonic crystal, cold atomic Bragg lattices, chiral quantum optics, and hot atoms. In particular, the random thermal motion of hot atoms can be a useful resource to realize optical non-reciprocity. Here in this work, based on the susceptibility-momentum-locking of atomic thermal motion and the strong coupling characteristics of cavities, a magnetic-free optical reciprocity-nonreciprocity transmission conversion scheme is designed and realized through the atom-cavity compound system. Theoretical and experimental analysis show that the coupling field conditions determine the nonreciprocity of the system. Under the action of single traveling-wave field, the nonreciprocity in hot atoms depends on the propagation direction of the coupling field due to the Doppler effect. Therefore, by changing the opening and closing of the opposite coupling field, the two-way single channel optical nonreciprocal transmission based on intracavity electromagnetically induced transparency can be controlled. When the two coupling fields propagate simultaneously in the opposite directions, however, the cavity transmission changes from single-dark-state to double-dark-state peaks, in which the reciprocity outputs depend on the frequency difference between the two coupling fields. By tuning the frequency difference, the two-way multi-channel reciprocal-nonreciprocal transmission regulation based on double dark polar peaks can be realized. The study can be applied to all-optical quantum devices and quantum information processing, such as optical transistors, optical switching and routing, and quantum gate manipulation.