Self-induced optical non-reciprocity

Abstract

Non-reciprocal optical components are indispensable in optical applications, and their realization without any magnetic field arose increasing research interests 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 experimentally demonstrate a new concept of nonlinear non-reciprocal susceptibility for optical media and realize the completely passive 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 around 1 dB. Furthermore, novel functional optical devices are realized, including polarization purification and non-reciprocal leverage. The demonstrated nonlinear non-reciprocity 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.

Figures (6)

• Figure 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 ($\omega_{\mathrm{p}}$). The non-reciprocity is induced by the directional coherent conversion ($\omega_{0}\rightarrow \omega_0+\omega_{\mathrm{p}}$) 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.
• Figure 1: Schematic of the experimental setup for the isolation measurement without a cavity. The red beams denote the free-space optical paths of both the forward signal and backward probe, the gray lines represent the optical fibers, and the yellow lines are electric cables. SAS: saturated absorption spectrum. TA: tapered amplifier. PBS: polarization beam splitter. BS: beam splitter. QWP: quarter wave plate.
• Figure 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 $10\,\mathrm{mm}$ Rb vapor cell filled with buffer gas, two LPs and two QWPs. The inset on the vapor cell denotes the energy structure of $^{87}$Rb, with the energy levels $\left|g\right\rangle$ and $\left|e\right\rangle$ denoted 5$^{2}S_{1/2}$$F=2$ and 5$^{2}P_{1/2}$$F=2$, respectively. Blue and red arrows represent the regulated $\sigma^{+}$ and $\sigma^{-}$ polarization of the froward and backward light, respectively. b, Forward $\sigma^{+}$ and backward $\sigma^{-}$ transmission at $81\,^{\circ}\mathrm{C}$ under two circumstances: applying a $5\,\mathrm{Gauss}$ bias magnetic field or using a magnetic shield. c, Isolation spectra under different temperatures. The highest isolation $39\,\mathrm{dB}$ is reached when the temperature is higher than $93\,^{\circ}\mathrm{C}$, and a $12.5\,\mathrm{GHz}$ bandwidth for $20\,\mathrm{dB}$ isolation is realized at $103\,^{\circ}\mathrm{C}$. For the results in both b and c, the forward power is $100\,\mathrm{mW}$, and the backward power is $10\,\mu\mathrm{W}$. d, Maximum isolation ratio under different forward and backward powers at $84\,^{\circ}\mathrm{C}$. Colored numbers beside the lines represent the forward power: 0.01, 0.1, 1, 10, $100\,\mathrm{mW}$.
• Figure 2: Definition of the isolation ratio.a, The system isolation ratio is defined as the ratio between the forward transmittance and the backward transmittance. $T_{\mathrm{f(b)},\pm}$ is the transmission of the forward (backward) laser with an H-polarization from the input and output ports. The polarization of the light when coupled with the atoms depends on the incident direction due to the quarter wave plates. b,c, The experimentally measured isolation ratio is defined as the transmittance ratio between the backward and forward probes that are $\sigma^{+}$-polarized and $\sigma^{-}$-polarized when coupled with the atoms.
• Figure 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 $150\,\mathrm{mW}$ and a backward probe power of $1\,\mathrm{mW}$. 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 $\theta=45^{\circ}$ in b. The highest isolation ratio reaches $63.4\,\mathrm{dB}$ with a $2.1\,\mathrm{GHz}$ bandwidth for $60\,\mathrm{dB}$ 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.