Passive Nonlinear Optical Isolators Bypassing Dynamic Reciprocity

Yiqi Hu, Yihong Qi, Yu You, Shicheng Zhang, Gongwei Lin, Xiaolin Li, Jiangbin Gong, Shangqing Gong, and Yueping Niu

Phys. Rev. Applied 16, 014046 – Published 19 July 2021

Abstract

Magnetic-free optical isolators are critical components for the realization of integrated optical systems. The underlying physics of passive nonlinear optical isolators is not solely about breaking the Lorentz reciprocity without requiring any external bias. Indeed, one major obstacle to the operation of self-Kerr-type nonlinear optical isolators is the so-called dynamic reciprocity, of which a serious outcome is that a backward signal cannot be isolated in the presence of a forward strong signal. In this work, we advocate the concept of velocity-selective nonlinearity to bypass such dynamic reciprocity. Using a proof-of-principle platform with warm rubidium atoms in an asymmetric cavity, we experimentally achieve the isolation of a backward signal in the presence of a strong forward signal, with experimental observations in agreement with our theoretical simulations. This work has thus made one essential step towards functioning Kerr-type passive optical isolators.

Received 17 November 2020
Revised 10 June 2021
Accepted 24 June 2021

DOI:https://doi.org/10.1103/PhysRevApplied.16.014046

Physics Subject Headings (PhySH)

Light propagation, transmission & absorption Nonlinear optics Photonics Thermal properties

Alkali metals Atomic ensemble Devices

PT-symmetry

Nonlinear DynamicsAtomic, Molecular & Optical

Authors & Affiliations

Yiqi Hu ^1,2,†, Yihong Qi^1,†, Yu You ^1,2, Shicheng Zhang¹, Gongwei Lin¹, Xiaolin Li¹, Jiangbin Gong ³, Shangqing Gong^1,4, and Yueping Niu ^1,4,*

¹School of Physics, East China University of Science and Technology, Shanghai 200237, China
²School of materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
³Department of Physics, National University of Singapore, 117542, Singapore
⁴Shanghai Engineering Research Center of Hierarchical Nanomaterials, Shanghai 200237, China

^*niuyp@ecust.edu.cn
^†Y. Hu and Y. Qi contributed equally to this work.

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Vol. 16, Iss. 1 — July 2021

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Images

Figure 1
Operating principle for bypassing dynamic reciprocity. (a) Two-port nonlinear isolator with spatially asymmetric structure. Without “velocity-selective nonlinearity,” forward and backward signals will interact with the same group of atoms at near-zero velocity (labeled NL). Large intensity signal from the forward direction (left to right) will “open a channel” for transmission of the weak signal from the backward direction (right to left). Then, the backward signal cannot be isolated if the forward signal is present at the same time. This so-called “dynamic reciprocity,” as discovered in Ref. [17], causes serious limitations for passive nonlinear isolators. (b) With “velocity-selective nonlinearity,” forward and backward signals selectively interact with two subensembles of atoms with different peak velocities, $v$ and $- v$ (labeled NL1 and NL2, respectively). Forward large-intensity signal can only open the transmission channel in NL1 and will not affect the weak backward signal propagating in NL2. Dynamic reciprocity is therefore bypassed. (c) Theoretical simulated transmission of forward (red line) and backward (blue line) signals when they are injected simultaneously.
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Figure 2
Experimental setup and results. (a) Schematic of experimental setup. $M 1$ , $M 2$ , and $M 3$ are cavity mirrors and form an asymmetric cavity. $Rb$ atoms are used as the nonlinear medium. Forward and backward signals are detected by photodetectors 2 (PD2) and 1 (PD1), respectively. (b) Normalized transmission of the forward (red circles) and backward (blue squares) signals. Forward and backward signals are incident separately (panel 1) or simultaneously (panel 2) without the mechanism of velocity-selective nonlinearity. Forward and backward signals are incident separately (panel 3) or simultaneously (panel 4) with our proposed velocity-selective nonlinearity in action. Forward and backward signal intensities are fixed at 0.6 mW. (c) Normalized transmission of forward and backward signals as a function of $Δ$ when they are injected simultaneously. Results from theoretical simulations are presented here as solid lines.
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Figure 3
Experimental observation of the dependence of signal transmission on light intensity, corresponding to the case of panel 4 in Fig. 2. (a) Normalized transmission of forward (red dot) and backward (blue square) signals. (b) Normalized transmission of the backward signal with its intensity fixed at 0.6 mW. Error bars show standard deviation. Results from theoretical simulations are presented as solid lines.
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