Self-induced optical non-reciprocity

Abstract

Non-reciprocal optical components are indispensable in optical applications, and their realization without any magnetic field has attracted increasing research interest in photonics. Exciting experimental progress has been achieved by either introducing spatial-temporal modulation of the optical medium or combining Kerr-type optical nonlinearity with spatial asymmetry in photonic structures. However, extra driving fields are required for the first approach, while the isolation of noise and the transmission of the signal cannot be simultaneously achieved for the other approach. Here, we propose the mechanism of nonlinear non-reciprocal susceptibility for optical media and experimentally realize the self-induced isolation of optical signals without any external bias field. The self-induced isolation by the input signal is demonstrated with an extremely high isolation ratio of 63.4 dB, a bandwidth of 2.1 GHz for 60 dB isolation, and a low insertion loss of ~1 dB. Furthermore, the new mechanism allows novel functional optical devices, including polarization purification and non-reciprocal leverage. A complete passive isolator is realized by introducing an asymmetry cavity. It is demonstrated that the 70 μW signal could lever the non-reciprocity and realize a 30 dB isolation of the backward laser with a power 100 times higher. The demonstrated nonlinear non-reciprocal medium provides a versatile tool to control light and deepen our understanding of light-matter interactions and enables applications ranging from topological photonics to unidirectional quantum information transfer in a network.

Fig. 1. Schematic diagram of reciprocal and non-reciprocal optical media.

a The regular medium that is transparent for both forward (blue arrow) and backward (red arrow) propagating light. b The medium under spatial-temporal modulation due to an external drive (ω_p). The non-reciprocity is induced by the directional coherent conversion (ω₀ → ω₀ + ω_p) for the forward signal. c, d Nonlinear non-reciprocal (NLNR) medium. The input signal induces non-reciprocal responses of the medium, so the direction of the isolation could be switched when changing the direction of the input signal

Fig. 2. Experimental setup and characterization of the isolation capability.

a Schematic of the experimental apparatus. LP linear polarizer, QWP quarter wave plate, BS beam spillter, PD photo detector. The kernel device of self-induced non-reciprocity is composed of a 10 mm Rb vapor cell filled with buffer gas, two LPs and two QWPs. The inset on the vapor cell denotes the energy structure of ⁸⁷Rb, with the energy levels $∣g⟩$ and $∣e⟩$ denoted 5²S_1/2 F = 2 and 5²P_1/2 F = 2, respectively. Blue and red arrows represent the regulated σ⁺ and σ⁻ polarization of the forward and backward light, respectively. b Forward σ⁺ and backward σ⁻ transmission at 81 °C under two circumstances: applying a 5 Gauss bias magnetic field or using a magnetic shield. c Isolation spectra under different temperatures. The highest isolation 39 dB is reached when the temperature is >93 °C, and a 12.5 GHz bandwidth for 20 dB isolation is realized at 103 °C. For the results in both (b, c) the forward power is 100 mW, and the backward power is 10 μW. d Maximum isolation ratio under different forward and backward powers at 84 °C. Colored numbers beside the lines represent the forward power: 0.01, 0.1, 1, 10, 100 mW

Fig. 3. Ultrahigh isolation via optical circular-polarization purification.

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 θ = 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

Fig. 4. Cavity-induced non-reciprocal leverage.

a The experimental apparatus of an asymmetric traveling wave cavity, comprising four mirrors and an Rb vapor cell inside. b Backward transmission spectra for the σ⁺ and σ⁻ probes when the cavity is resonantly driven by a forward signal laser. The powers of the forward laser and backward laser are both 7 mW. Here the effective cavity length is about l = 40 cm and the corresponding free spectral range (FSR) of the cavity is about 0.372 GHz. c Forward and backward transmission when only one laser is on. Solid lines are the theoretical prediction while dots correspond to the measured results. The blue shadow area indicates the power range where the cavity-induced isolation of the backward probe is effective even if there is no forward signal. d A contrast of the dependence of isolation on the forward signal power for cavity and free space configurations. The backward laser power is 7 mW for both lines and the data of free space configurations is measured separately, which has a certain deviation in temperature compared with Fig. 2d