1.State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi University, Taiyuan 030006, China 2.Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Fund Project:Project supported by the National Basic Research Program of China (Grant Nos. 2017YFA0304203, 2016YFF0200104) and the National Natural Science Foundation of China (Grant No. 61975104)
Received Date:15 January 2021
Accepted Date:04 March 2021
Available Online:06 May 2021
Published Online:20 May 2021
Abstract:Based on the Electromagnetically-Induced-Transparency (EIT) effect of cesium Rydberg atoms, the dispersion of the probe light will experience a drastically change while the absorption is diminished, as the frequency of it is resonated with that of the corresponding atomic transition. In this case, as the light pulse propagates in the atomic medium, the group velocity of the pulse will be slowed. In the cesium atoms 3-ladder-level system ($ 6{\rm S}_{1/2}\rightarrow6{\rm P}_{3/2}\rightarrow49{\rm D}_{5/2} $),the frequency of the probe light is locked at the resonance transition of $ 6{\rm S}_{1/2}\rightarrow6{\rm P}_{3/2} $, while the transmission signal of 852 nm probe light is measured by scanning the coupling light frequency near the transition of $ 6{\rm P}_{3/2}\rightarrow49{\rm D}_{5/2} $, We observed the EIT phenomenon and explored the relationship between the power of coupling laser and linewidth of the EIT signal. The experimental results show that the linewidth of the EIT signal is proportional to the power of the coupling laser. Then under the two-photon resonance condition, the deceleration of the probe light pulse caused by the steep change of the dispersion curve is observed. We also systematically investigate the influences of coupling optical power and temperature of vapor cell on the slowing down of light pulse. The experimental results show that the weaker the coupled light was, the longer the delay time; and the higher the temperature of the atomic gas chamber was, the more obvious the deceleration effect would be observed, those of which are consistent with the theoretical calculations. The investigation of the deceleration of optical pulses based on the Rydberg Electromagnetically-Induced-Transparency is important for understanding the coherence mechanism of 3-ladder-level system and some potential applications, such as in Rydberg-atom-based electric field metrology. This research provides a new tool for the measurement of pulsed microwave electric field through the optical pulse deceleration effect. Keywords:Rydberg atoms/ electromagnetically-induced-transparency/ 3-ladder-level-system/ deceleration of optical pulses
$ V_{\rm g} = \frac{c}{1+{g}^{2} N /\left|\varOmega_{\rm {c}}\right|^{2}}, $
其中, c为光在真空中的传播速度, g为探测光场与原子的耦合常数, N为原子密度, $ \varOmega_{\rm {c}} $为耦合光的拉比频率. 可以看出, 光脉冲的减速效应强烈依赖于耦合光的拉比频率$ \varOmega_{\rm {c}} $和原子密度N, 在耦合光的拉比频率较小或原子密度较大的情况下, 减速效应越明显. 如前所述, 输入光脉冲的半高宽为15 μs, 且脉冲光的载波频率处在EIT介质的透明窗口内. 同时保持原子气室温度为25 ℃, 探测光的功率为160 μW不变, 研究改变耦合光功率时, 探测光相对于参考光的减速效应, 结果如图5所示. 图5(a)是耦合光功率为5 mW时, 输出光脉冲与参考光脉冲的对比. 图5(a)中橙色线是输出光脉冲的实验结果, 蓝色线为输出光脉冲的理论结果. 同时添加了辅助线便于分辨输出光脉冲与参考光脉冲之间的延迟. 图5(a) (1)中, 竖点虚线为输入脉冲的峰值中心位置, 将其设置为时间零点并贯穿到图5(a) (2)中, 方便与减速后的光脉冲中心位置作比较. 两条虚线所夹阴影范围为参考脉冲的半高全宽(FWHM), 通过上升沿和下降沿的变化也可以明显地看到光速减慢. 在图5(a) (2)中用竖短实线标注输出光脉冲的峰值中心位置, 它相对参考脉冲峰值中心位置的偏移就是延迟时间$ \tau $. 根据(18)式以及拉比频率的定义, 延迟时间$ \tau $会随着耦合光功率线性变化, 如图5(c)所示. 可以看出, 在耦合光功率为5 mW的情况下, 探测光脉冲相对参考光脉冲的延迟时间达到最大值, 为522 ns. 之后在一定范围内随着耦合光功率的增加, 延迟时间在慢慢减小, 两者呈现良好的线性关系. 所以在耦合光功率越低时, EIT窗口越窄, 导致了EIT介质色散曲线的变化更为陡峭, 使得延迟时间更大, EIT效应也逐渐变弱. 图 5 耦合光功率和温度对输出光脉冲的影响 (a) 耦合光功率为5mW时输出光脉冲与参考光脉冲的对比(强度上归一化); (b)温度为40℃时输出光脉冲与参考光脉冲的对比(强度上归一化); (c) 延迟时间随耦合光功率的变化; (d)延迟时间随原子密度的变化 Figure5. Change of output pulse with coupling power and tempreature: (a) Comparison of the output optical pulse and the reference optical pulse when the coupling optical power is 5mW (Normalization of intensity); (b) Comparison of the output optical pulse and the reference optical pulse when the temperature is 40℃(Normalization of intensity); (c) delay time vary with coupling power; (d) delay time vary with atom density.