Mediated by the interactions with quantum vacuum fields, a probe laser field propagating in a nonlinear optical medium can generate new pair of light fields over a broad spectral range via spontaneous parametric process. Such process is inherently independent of the incident direction of light and reciprocal thus far, due to the direction-independent field-vacuum interactions. In this work, we experimentally demonstrate within sodium atomic vapors that such spontaneous parametric process can be nonreciprocal by unidirectionally coupling it to another pumped four-wave mixing process. Thanks to the broad bandwidth of the spontaneous parametric process, in combination with the Doppler and power-induced broadening of atomic energy levels, we achieve optical isolation with isolation ratio >25 dB over a bandwidth larger than 100 GHz. Considering that both spontaneous parametric processes and the pumped four-wave mixing have been realized in diverse solid photonic platforms, the demonstrated concept can motivate further explorations in the design of integrated magnetic-free broadband optical nonreciprocity via the interactions between nonlinear optical processes.
Mediated by the interactions with quantum vacuum fields, a probe laser field propagating in a nonlinear optical medium can generate new pair of light fields over a broad spectral range via spontaneous parametric process. Such process is inherently independent of the incident direction of light and reciprocal thus far, due to the direction-independent field-vacuum interactions. In this work, we experimentally demonstrate within sodium atomic vapors that such spontaneous parametric process can be nonreciprocal by unidirectionally coupling it to another pumped four-wave mixing process. Thanks to the broad bandwidth of the spontaneous parametric process, in combination with the Doppler and power-induced broadening of atomic energy levels, we achieve optical isolation with isolation ratio >25 dB over a bandwidth larger than 100 GHz. Considering that both spontaneous parametric processes and the pumped four-wave mixing have been realized in diverse solid photonic platforms, the demonstrated concept can motivate further explorations in the design of integrated magnetic-free broadband optical nonreciprocity via the interactions between nonlinear optical processes.
Nonreciprocal optical devices are important in wide areas, ranging from optical telecommunications to signal processing. Here, we experimentally realize the magnet-free and cavity-free optical nonreciprocity based on phase-matched four-wave mixing (FWM) in hot atoms. Specifically, the isolation of more than 20 dB and insertion loss less than 1 dB without using any optical cavity can be achieved simultaneously. Our results provide general insights into a broad class of parametrically modulated nonreciprocal behavior, paving the way toward optimal nonreciprocal devices based on the nonlinear FWM process.
A scheme for magnetic-free optical nonreciprocity in an ensemble of four-level cold atoms is proposed by exploiting the directional four-wave mixing effect. Using experimentally achievable parameters, the nonreciprocal optical responses of the system can be observed and the conversion on nonreciprocal transmission and nonreciprocal phase shift can be implemented. These nonreciprocal phenomena originate from the directional phase matching, which breaks the time-reversal symmetry and dynamic reciprocity of the cold atomic system. Moreover, by embedding the cold atoms into a Mach-Zehnder interferometer and choosing proper parameters, a two-port optical isolator with an isolation ratio of 79.70 dB and an insertion loss of 0.35 dB and a four-port optical circulator with a fidelity of 0.9985 and a photon survival probability of 0.9278 can be realized, which shows the high performance of isolation and circulation. The proposal may enable a new class of optically controllable cavity-free nonreciprocal devices in optical signal processing at the low light level.
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.
Optical nonreciprocity and nonreciprocal devices such as optical diodes have broad and promising applications in various fields, ranging from optical communication to signal process. Here, we propose a magnet-free nonreciprocal scheme based on the four-wave mixing (FWM) effect in semiconductor quantum dots (SQDs). Via controlling the directions of the coupling fields, the probe field can achieve high transmission in the forward direction within a certain frequency range due to the FWM effect. And the transmission of the probe field in the backward direction undergoes significant reduction, as the FWM effect is absent. The calculation results show a wide nonreciprocal transmission window with isolation greater than 12 dB and insertion loss lower than 0.08 dB. The influences of the Rabi frequencies of the coupling fields, the medium length, and the decay rates on the nonreciprocal propagation of the probe field are also studied, showing the requirements of these parameters for good nonreciprocal performances. Our work may offer an insight for developing optical nonreciprocal devices based on the FWM process and the SQD system.
Temporal dissipative solitons are central to nonlinear photonics, yet conventional platforms such as optical fibers and microresonators provide limited dispersion tunability and weak nonlinearities. Here, we demonstrate a highly tunable soliton platform based on atom–light interactions in a rubidium vapor optical parametric oscillator. A continuous-wave pump drives four-wave mixing in the vapor, generating probe and conjugate fields, one of which is fed back through a fiber loop to form the oscillator. By tuning the pump intensity and detuning, we control the dispersive properties and third-order nonlinearity of the medium, enabling the formation of diverse nonlinear states including bright and dark solitons, soliton crystals, dark–bright soliton pairs, platicons, and breather solitons. Unlike conventional systems, solitons emerge spontaneously without cavity scanning or pulsed excitation. The system isolates gain, dispersion, and nonlinearity within the atomic medium, allowing flexible cavity-length tuning without modifying other operating parameters. We introduce the technique of characterizing the solitonic states through the linewidth of the output intensity power spectrum, which remains independent of cavity length and is limited only by the experimental apparatus, indicating a high degree of phase coherence. This contrasts with conventional mode-locked states, for which the linewidth scales inversely with the square of the cavity length. With pulse durations of ∼100 ns determined by the gain bandwidth of the four-wave mixing, the solitons can be directly detected for rapid and complete characterization. This platform offers a versatile route for exploring nonlinear optical dynamics and developing applications in quantum photonics and communication.
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.