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Physical Review Applied

Multiwavelength Magnetic-Free Optical Isolator by Optical Pumping in Warm Atoms

Yiqi Hu, Shicheng Zhang, Yihong Qi, Gongwei Lin, Yueping Niu, Shangqing Gong

Published Nov 4, 2019• 41 citations
Publisher

Abstract

Achieving multiwavelength nonreciprocal optical devices, which can realize nonreciprocal (perfect one-way) transmission for two or more wavelengths at the same time, without magnetic materials is challenging, but is highly desirable for densely integrated multiwavelength photonic components. The authors experimentally realize multiwavelength magnetic-free optical nonreciprocity using optical pumping of warm ${}^{85}$Rb atoms, with the assistance of Doppler effect. This scheme may have advantages in decreasing the number of optical isolators, and thus can reduce the complexity of an integrated multiwavelength system.

DOI
10.1103/physrevapplied.12.054004

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Nonreciprocal light propagation is essential to control the direction of the light flow. Here, we report the realization of magnetic-free optical nonreciprocity using a simple two-level system driven by a pump field in warm atoms. In our experiment, we not only demonstrate less than 0.5 dB of insertion loss and up to 20 dB of isolation but also provide flexible and reconfigurable operations of the isolation bandwidth, frequency, and direction. Nonreciprocal scheme with these characteristics may find important applications in photonic devices.
Nonreciprocal optical devices, such as isolators and circulators, provide highly desirable functionalities in routing signals. This motivates the active research on high-efficiency nonreciprocal devices, especially for miniaturized and integrable magnetic-free schemes. All the present experimental demonstrations of circulators in optical domains are based on the high-QQQ factor cavity. In this work, we demonstrate a cavity-free optical circulator with low insertion loss by making use of a far-detuned nonlinear Raman process with the assistance of the atomic Doppler effect. Due to the large detuning, the excited state can be adiabatically eliminated, resulting in nearly vanishing absorption. An optical circulator with average transmission of more than $85\mathrm{%}$ and fidelity above 0.980.980.98 is well achieved in an experiment with hot atoms. Our scheme shows the possibility of cavity-free nonreciprocal optical devices with low insertion loss.
Nonreciprocal transmission of structured light has possible applications in high-capacity optical communication and high-dimensional signal processing. Here, we demonstrate the magnet-free and cavity-free optical nonreciprocity of an orbital-angular-momentum (OAM) beam using optical-pumping technology with the assistance of the Doppler effect in hot atoms. By using optical pumping to induce a direction-dependent population transfer of the ground-state atoms, we realize optical nonreciprocity for the signal laser that is carrying OAM. Interestingly, an isolation ratio close to 30 dB and maximum transmission over 0.86 can be achieved simultaneously without the use of an optical cavity. Due to the unique amplitude and helical phase profile of the structured light, our proposal may open an avenue for exploring the singularity characteristics of optical nonreciprocity using structured light.
Optical nonreciprocity is an essential concept in optical signal processing and communication. The traditional way to achieve nonreciprocity is using magneto-optical nonreciprocal polarization rotation, which is incompatible with existing on-chip integration due to the applied magnetic field. Here, we theoretically and experimentally realize unidirectional polarization rotation with a magnetic-free mechanism using atomic ensembles. Free-space isolators and circulators are further formed based on the nonreciprocity of polarization rotation. As only one direction of the signal changes its polarization, the forward transmission and backward isolation can be adjusted separately. Compared with the existing magnetic-free circulators that rely on high-Q factor cavities or Mach–Zehnder interferometers, we experimentally realized circulators in free space. This dramatically simplifies the magnetic-free circulator structure and provides better stability.
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.