Extension of the concept of lasing-without-inversion to nonlinear optical processes

本项目旨在通过扩展“无种群反转的激光”思想，探索一种非线性光学混频过程的新方法。关键思想是在三能级系统中强耦合上两个状态，导致两个被修饰的状态衰变成一个相同的连续体。使用这种修饰态系统，原子介质的光学响应可以获得比裸原子系统更大的自由度。我们以氢原子为测试介质，研究了共频产生过程，产生了103 nm的相干辐射。在这个项目中，我们已经证明了3p-2s跃迁的强耦合产生了破坏性干涉，从而降低了共振3p*Is跃迁的线性磁化率。同时，2s和1s态的双光子耦合产生建设性干涉，共振增强了3p*1s （Lyman-beta）跃迁和频产生的非线性磁化率。实验结果清楚地证明了理论预测。当3p-2s耦合的拉比频率超过介质的多普勒尾时，由于诱导透明，共振上的光子信号被显著抑制。当原子密度为10^<15> cm^<-3>，相互作用长度为3.5 cm时，产生的莱曼- β辐射迅速增长250倍，导致转换效率为2 × 10^<-4>。当耦合频率与共振失谐时，可以控制介质的折射率以满足和频产生的相位匹配条件。

This project aims to exploit a new method in nonlinear optical frequecy mixing processes, by extending an idea of "lasing without population inversion". The key idea is to strongly couple the upper two states in a three level system, leading to two dressed states which are decaying to an identical continuum. Using such dressed state system, optical responses of the atomic medium can obtain much more freedom than that for bare atomic system. We use the atomic hydrogen as a test medium, and have investigated the sumfrequency generation process, generating coherent 103 nm radiation.In this project, we have shown that strong coupling at the 3p-2s transition creates destructive interference to reduce the linear susceptibility at the resonant3p*Is transition. Simultaneously, two-photon coupling of the 2s and 1s states produces constructive interference to resonantly enhance the nonlinear susceptibility for sum-frequency generation at the 3p*1s (Lyman-beta) transition.Experimental results clearly demonstrate the theoretical predictions.When the Rabi frequency for the 3p-2s coupling exceeds the Doppler tail of the medium, the photoion signals on resonance are suppressed significantly because of induced transparency.The generated Lyman-beta radiation grows rapidly by a factor of 250 at an atom density of 10^<15> cm^<-3> and interaction length of 3.5 cm, resulting in a conversion efficiency of 2x10^<-4>.It has also shown that, when the coupling frequency is detuned from the resonance, the refractive index of the medium can be controlled to satisfy the phase-matching condition for sum-frequency generation.