Fig. 4: Experimental observations.
a Measured spectra of spontaneous parametric signals and b transmitted probe as a function of the probe detuning for only a unidirectional probe beam applied (red), the co-propagation case (blue) and counter-propagation case (green). The inset in a shows the observed pumped FWM spectrum. c Nonreciprocal transmission contrast calculated from b. d Bandwidth (BW) for isolation ratio >25 dB versus the probe power when two probe beams (E1 and E1′ in Fig. 1) propagate simultaneously in opposite directions. The square are experimental data, while the solid line represents a fitting and provides a guide to the eye. The probe power is 150 μW in (a–c). The power of both pump beams are 3.75 mW
