摘要
光与原子相互作用中量子相干导致的电磁感应透明(EIT)效应是量子光学中的一个重要研究内容。由于EIT介质具有透射高、色散强、非线性效应大等特性,在光减速、光量子存储、共振非线性增强、非经典光的产生、光与原子纠缠等方面具有重要的应用,所以EIT效应受到了人们的关注并且被广泛深入地研究。
本文介绍了EIT效应的基本概念,回顾了EIT研究的发展历史。主要介绍了我们基于EIT进行的研究工作,包括光减速、光存储、光致旋光效应以及多能级原子中的非线性效应。
1)研究了EIT介质中的光减速和光存储。在充缓冲气体的铷泡中,使用线宽压窄的激光器,将光脉冲群速度减慢到8000m/s。在光减速的基础上,通过EIT动力学过程,进行了光脉冲的存储与释放,存储时间达到了100μs。在双人型四能级系统中,使用780nm或795nm“读”控制光读出存储在原子中的光脉冲,并用光栅将读出的795nm和780nm信号在空间上分离,实现了存储信息在空间两个不同通道上的可控释放。
2)在多能级EIT系统中观察到了线偏振探针光的偏振面旋转。采用右旋圆偏振耦合光与原子作用,产生了对左旋和右旋探针光分量非对称的EIT系统,这种非对称的EIT系统对左旋与右旋探针光分量的折射率不相同,由此导致了光致旋光效应。在实验上观察到了15mW的耦合光就可导致探针光偏振面旋转达45°。提出一种测量偏振面旋转的新方法,消除了吸收对转角测量的影响,大大提高了测量精度。
3)采用左旋圆偏振探针光、左旋圆偏振耦合光和右旋圆偏振触发光与~(87)Rb原子的D1线作用,构建了四能级Tripod结构。在该系统中观察到了分别由耦合光和触发光在探针光吸收谱上导致的两个EIT窗口(Two EIT),研究了由这两个EIT窗口相互作用引起的EIT信号增强现象。实验结果表明,当触发光与耦合光的失谐不同时,在探针光的吸收光谱上观察到了两个EIT窗口,一个由强的耦合光形成,一个由弱的触发光形成。当触发光的失谐与耦合光的失谐接近或相等时,耦合光形成的EIT信号会明显增大。使用半经典理论对EIT信号增强现象进行了数值计算,结果与实验很好的相符。
4)在四能级Tripod系统中,研究了基于双EIT(double EIT)窗口的弱光交叉Kerr非线性效应。我们在实验上观察到了耦合光在探针光和触发光的吸收谱上同时导致的EIT窗口(Double EIT),并利用Mach-Zehnder干涉仪对探针光与触发光之间相互作用导致的交叉Kerr非线性相移进行了测量。实验结果显示,当探针光和触发光的透射都超过60%时,触发光的交叉Kerr非线性系数可达到2×10~(-5) cm~2/W。
其中创新性的工作包括:
Ⅰ.在双人型四能级系统中,采用795nm或780nm的读控制光,使存储在EIT介质中的信号以两个不同波长的光脉冲释放出来。用光栅将这两个不同波长的释放光脉冲在空间上分离,实现了存储信息在空间两通道上的可控释放。
Ⅱ.在非对称EIT系统中观察到了光致旋光效应。该系统中,探针光的两个圆偏振分量都与耦合光形成EIT,所以探针光的圆二向色性较小。在实验中,15mW的耦合光就可使探针光偏振面旋转角达到45°。我们进一步对探针光偏振旋转做了理论分析和数值计算,结果表明主要是探针光左旋和右旋分量EIT数目的不对称导致了旋光效应。
Ⅲ.通过选择合适圆偏振的探针光、耦合光和触发光与原子的Zeeman子能级作用,在~(87)Rb的D1线构建了四能级Tripod系统。在该系统中观察到了分别由耦合光和触发光在探针光的吸收谱上导致的两个EIT窗口以及两个EIT窗口相互作用产生的EIT信号增强现象。
Ⅳ.在Tripod系统中,观察到了耦合光在探针光和触发光的吸收谱上同时导致的EIT窗口。利用Mach-Zehnder干涉仪对探针光和触发光之间的交叉Kerr非线性相移进行了测量。在弱光情况下观察到了显著的非线性相移,交叉Kerr非线性系数可达到2×10~(-5) cm~2/W。所采用的Mach-Zehnder干涉仪由两个偏振位移器构成,信号光和参考光之间的光程差对镜子的振动不敏感,使交叉Kerr非线性相移能够被精确地测量。
The electromagnetically induced transparency (EIT) effect induced by quantum interference in the interaction of atoms with light is one of important fields in quantum optics. Under the condition of EIT, the atomic medium exhibits unique optical properties: high transmission, steep dispersion and enhanced nonlinearity. Due to these unique properties, the EIT effect has been found applications in light speed reduction, optical quantum storage, generation of nonclassical light, realization of the entanglement between light and atoms. The EIT effect has attracted a great deal of attention and been studied extensively.
The dissertation introduces the basic concepts on EIT effect and briefly reviews the history of research on EIT. Then it presents our main works which include light speed reduction, light storage, polarization rotation of a linearly polarized light and nonlinear effects in multi-level atomic medium.
1) The light speed reduction and light storage in EIT medium are studied. Using the linewidth-narrowed laser, the group velocity of optical pulse is slowed down to 8000m/s in Rb cell with buffer gas. On the basis of light speed reduction, we experimentally realized the light storage and release by using dynamics process of EIT. The storage time of light pulse can be up to 100μs. By controllably turning on the retrieve control pulses at either 795 or 780nm to read the stored optical pulses in a four lever double A-type system, and further separating spatially these readout pulses through a grating, we realized a controllable releasing for stored light information into two separate photonic channels.
2) The phenomenon of polarization rotation of a weak, linearly-polarized optical field is observed in a multi-level EIT system in rubidium atoms. By choosing circularly-polarized coupling beam to interact with atoms, the symmetry in number of EIT subsystems seen by the left- and right-circularly-polarized components of the weak probe beam can be broken, which makes the refrective indices of left- and right-circularly-polarized components of the probe beam different and leads to the polarization rotation of the probe beam. In the experiment, a large polarization rotation angle (up to 45 degrees) has been achieved with a coupling beam power of only 15mW. In addition, a new method to measure the polarization rotation angle is proposed, which can eliminate the influence of absorption on the measurement of polarization rotation angle and improve the measurement precision.
3) By choosing left circularly-polarized probe, left circularly-polarized coupling beam and right circularly-polarized trigger beam to interact with the energy levels of D1 line in ~(87)Rb atoms, a tripod system is formed. We observed the two EIT windows induced by coupling and trigger beams respectively, and studied the enhanced EIT signal due to the interaction between the two EIT windows in the four-level tripod system. The two EIT dips produced by a strong coupling beam and a weak trigger beam are observed in the absorption spectrum of the probe field when the frequency detuning of trigger beam is different from that of the coupling beam. When the frequency detuning of trigger beam is near or equal to that of the coupling beam, the total depth of the EIT dip created by coupling beam clearly becomes larger. We have made numerical calculation for the enhanced EIT signal through semiclassical theory, these results are agreement with experimental results.
4) The cross-Kerr nonlinear effect between two weak beams based on double EIT is investigated in a tripod system. We have observed the simultaneous EIT windows for probe and trigger fields (double EIT), and measured the cross-phase modulation (XPM) between the two fields using Mach-Zehnder interferometer. The experimental results show that the XPM coefficient of trigger beam is larger than 2×10~(-5)cm~2/W when the accompanying transmissions of probe and trigger beams are higher than 60%.
The characterized works among the above are as follows:
Ⅰ. We realized a controllable releasing of stored optical pulses into two spatial optical channels. In a four level double A-type system, the light information stored in the EIT medium is released into optical pulse at two different wavelength by turning on the retrieve control pulses at either 795 or 780nm. These readout pulses are further separated spatially through a grating.
Ⅱ. The phenomenon of optically induced polarization rotation is observed in an asymmetry EIT system. In the system, both of the two orthogonal polarized components of the probe field form EIT with coupling field, so the circular dichroism of the probe field is small. In the experiment, a large polarization rotation angle (up to 45 degrees) has been achieved with a coupling beam power of only 15mW. We have made detailed theoretical analyses and numerical calculation for the polarization rotation, which show that the asymmetry in the number of EIT subsystems for the two circularly probe components is the dominant mechanism to cause the polarization rotation of the linearly polarized probe beam.
Ⅲ. A four-level tripod system is formed in the D1 line in ~(87)Rb atoms by choosing proper circularly-polarized probe, coupling and trigger beams. We observed the two EIT windows induced by coupling and trigger beams respectively, and studied the enhanced EIT signal due to the interaction between the two EIT windows in the four-level tripod system.
Ⅳ. In a four-level tripod system, the simultaneous EIT windows for probe and trigger fields (double EIT) are observed. The cross-phase modulation (XPM) between the probe and trigger fields are measured by using a Mach-Zehnder interferometer. The obvious XPMs are observed at low intensities, the XPM coefficient is larger than 2×10~(-5)cm~2/W. The Mach-Zehnder interferometer is formed by using two beam displacing polarizers, and the optical path differences between the signal and reference beams are insensitive to the vibration of mirrors. So the measurements for cross-Kerr phase shifts are precision.
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