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

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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.