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Results in Physics

Efficient optical isolator via dual-Raman process with chiral nonlinearity

Mingzhi Han, Yuan He, Qianzhu Li, Xiaoyun Song, Yudou Wang, Aihong Yang, Qingtian Zeng, Yandong Peng

Published Feb 14, 2023• 2 citations
Publisher

Abstract

A scheme for optical nonreciprocity is proposed via dual-mode Raman process in thermal atoms. We investigate a far-detuned nonlinear Raman process, with the assistance of the Doppler effect to achieve chiral optical nonlinearity. The resulting Kerr nonlinear absorption is subject to a direction-dependent Doppler shifts seen by the thermal atoms. Moreover, the inter-channel Raman interference could significantly enhance Kerr nonlinear absorption for probe field, leading to the enhancement of the optical nonreciprocity. Simulation results show that the isolation ratio could reach 41 dB and the insertion loss is about 0.16 dB, with bandwidth up to 2π ×48 MHz. Furthermore, the sensitivity of the system is investigated under different strengths of the control and Raman fields.

DOI
10.1016/j.rinp.2023.106288

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A scheme for optical nonreciprocity is proposed via dual-mode Raman process in thermal atoms. We investigate a far-detuned nonlinear Raman process, with the assistance of the Doppler effect to achieve chiral optical nonlinearity. The resulting Kerr nonlinear absorption is subject to a direction-dependent Doppler shifts seen by the thermal atoms. Moreover, the inter-channel Raman interference could significantly enhance Kerr nonlinear absorption for probe field, leading to the enhancement of the optical nonreciprocity. Simulation results show that the isolation ratio could reach 41 dB and the insertion loss is about 0.16 dB, with bandwidth up to 2π ×48 MHz. Furthermore, the sensitivity of the system is investigated under different strengths of the control and Raman fields.
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.
Optical nonlinearity has been widely used to try to produce optical isolators. However, this is very difficult to achieve due to dynamical reciprocity. Here, we show the use of the chiral cross-Kerr nonlinearity of atoms at room temperature to realize optical isolation, circumventing dynamical reciprocity. In our approach, the chiral cross-Kerr nonlinearity is induced by the thermal motion of N-type atoms. The resulting cross phase shift and absorption of a weak probe field are dependent on its propagation direction. This proposed optical isolator can achieve more than 30 dB of isolation ratio, with a low loss of less than 1 dB. By inserting this atomic medium in a Mach-Zehnder interferometer, we further propose a four-port optical circulator with a fidelity larger than 0.9 and an average insertion loss less than 1.6 dB. Using atomic vapor embedded in an on-chip waveguide, our method may provide chip-compatible optical isolation at the single-photon level of a probe field.
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.
Hot atoms have been demonstrated to be a promising platform for the manipulation of optical fields. However, the thermal motion of atoms results in the broadening of spectral lines, which is accompanied by a reduction in the light-atom interaction. In this study we propose a nonreciprocal optical system that significantly enhances the strength of the light-atom interaction by unidirectionally compensating for the broadening associated with two-photon processes, employing inhomogeneous light shifts in a $\mathrm{\ensuremath{\Lambda}}$ system. Utilizing the density matrix formalism, we provide a detailed description of the dynamics and examine the nonreciprocity in two distinct detuned regions. In the far-detuned region, a room-temperature nonreciprocity with an isolation of 37.3dB37.3\phantom{\rule{0.28em}{0ex}}\mathrm{dB}37.3dB and a transmission of 0.99 can be achieved. Furthermore, in the near-resonance region, the ultrabroad bandwidth of the nonreciprocal window can reach gigahertz. Our findings pave the way for the realization of high-performance optical nonreciprocity and present opportunities for applications in integrated optics and quantum networks.
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.
Multi-band signal propagation and processing play an important role in quantum communications and quantum computing.In recent years, optical nonreciprocal devices such as an optical isolator and circulator are proposed via various configurations of atoms, metamaterials, nonlinear waveguides, etc.In this work, we investigate all-optical controlled nonreciprocity of multi-band optical signals in thermal atomic systems.Via introducing multiple strong coupling fields, nonreciprocal propagation of the probe field can happen at some separated frequency bands, which results from combination of the electromagnetically induced transparency (EIT) effect and atomic thermal motion.In the proposed configuration, the frequency shift resulting from atomic thermal motion takes converse effect on the probe field in the two opposite directions.In this way, the probe field can propagate almost transparently within some frequency bands of EIT windows in the opposite direction of the coupling fields.However, it is well blocked within the considered frequency region in the same direction of the coupling fields because of destruction of the EIT.Such selectable optical nonreciprocity and isolation for discrete signals may be greatly useful in controlling signal transmission and realizing selective optical isolation functions.