image: a, schematic of the improved experimental apparatus with an extra Rb vapor cell. The backward probe purified through Cell2 was used to characterize the isolation ratio in Cell1. b, transmittance of the backward probe against its polarization, which is controlled by the angle of the QWP (near port 2) with a forward signal power of 150 mW and a backward probe power of 1 mW. The zero angle corresponds to a linear polarization. The four lines show the corresponding theoretical predictions under different conditions, while the dots are the experimental results. Shaded areas denote noise floors from different causes. c, the improved measurement of the isolation ratio (red circles) by using the NLNR effect for circular-polarization purification and an etalon to eliminate laser background noise, compared to the results without purification (blue diamonds). These two results correspond to 45° in b . The highest isolation ratio reaches 63.4 dB with a 2.1 GHz bandwidth for 60 dB isolation. The black line is the theoretical prediction of the ideal isolation ratio, while the red and blue lines are the results considering two different noise floors to fit the experimental data.
Credit: Zhu-Bo Wang et al.
In the rapidly advancing field of photonic technology, recent discoveries in light-matter interactions with broken time-reversal symmetry promise to transform our approach to non-reciprocal optical systems. A study published in Light: Science & Applications introduces a novel concept harnessing nonlinear non-reciprocal susceptibility (NLNR) to achieve high-performance optical isolation, setting new records in the process. The work was completed by a team of scientists, led by Professor Chang-ling Zou from University of Science and Technology of China, Hefei, China.
Conventional methods of achieving optical non-reciprocity typically rely on magneto-optical media or nonlinear optics effects, often requiring conditions such as external magnetic fields and precise phase matching. The latest research takes a bold step forward by leveraging intrinsic NLNR responses, achieving ideal optical isolation without such requirements.
Setting a new benchmark, the study demonstrates a record-breaking isolation ratio of 63.4 dB—currently the highest reported for magnetic-free optical isolation. This remarkable achievement highlights the potential of the NLNR mechanism in overcoming limitations associated with traditional approaches. Additionally, this device has an isolation bandwidth exceeding 20 dB of 12.5 GHz, which is more than an order of magnitude greater than the bandwidth of previous isolators that also used atomic ensembles as the medium.
Central to these advancements is the concept of self-induced isolation, where the inherent properties of the medium facilitate non-reciprocity through the input signal itself. The researchers demonstrate that combining the signal's Kerr-type optical nonlinearity with spatial asymmetry effectively blocks counter-propagating light while allowing forward light transmission.
It is important to note that although the self-induced non-reciprocal isolator has achieved breakthrough performance in magnetic-free isolation, it still requires the presence of forward light to isolate the backward light. To address this, the research team further placed the self-induced non-reciprocal medium within an asymmetric cavity. This improvement allows for the blockage of backward light, provided that its intensity remains below a specific threshold, even in the absence of forward light. Since this threshold is significantly higher than the typical reflective light intensity encountered in practical applications, this design effectively realizes a magnetic-free and passive ideal isolator.
The physical mechanism of self-induced non-reciprocity is not only applicable to rubidium atomic ensembles but can also be extended to other atomic and molecular systems. This opens the potential for realizing non-reciprocal devices in the ultraviolet, mid-infrared, or terahertz frequency ranges. In the field of integrated optics, coupling between evanescent waves from waveguides and gas atoms in free space holds promise for the development of on-chip magnetic-free non-reciprocal devices, making these application prospects highly anticipated.
Article Title
Self-induced optical non-reciprocity