ELECTROMAGNETICALLY INDUCED TRANSPARENCY IN FOUR-WAVE MIXING SCHEMES


I. Küçükkara, M. Anscombe and J. P. Marangos

Laser Optics and Specroscopy Group
The Blackett Laboratory
Imperial College Of Science Technology and Medicene, London,SW7 2BW


ABSTRACT



      Electromagnetically Induced Transparency (EIT) atomic coherence phenomenon that occurs as a result of laser induced quantum interference within atomic transition levels. In an EIT process probability amplitudes may cancel each other via destructive interference and this eliminates the absorption of an atomic media.

Four-wave mixing (FWM) is a non-linear optical parametric process that arises from the third order term in the dipole susceptibility. We used EIT in two different fully resonant four-wave mixing schemes. These schemes were conducted in krypton gas media exploiting the Electromagnetically Induced Transparency (EIT) to enhance the efficiency of non-linear optical processes. The generated radiations were in the vacuum ultra-violet (VUV) and extreme ultra-violet (XUV) regions.

In order to see effect of EIT, the Rabi splitting due to the coupling laser has to exceed the Doppler width of the gas. This requires the use of pulsed lasers. In sum-difference frequency mixing scheme (for VUV generation), single mode (<500MHz) coupling field at 759nm was singly resonant with the 4p55p [1,1/2]0 to 4p5s 0[1,1/2]1 and was provided by a Nd:YAG pumped optical parametric oscillator (OPO). Pump field at 212nm was in two-photon resonance with 4p6 1S0 ground state to 4p55p [0,1/2]0 excited state and it was supplied by an excimer pumped dye laser. The generated radiation in the region of VUV was at 123.5nm.

Extreme ultra-violet generation was based on a sum frequency scheme. The pump field the same as VUV experiment but the coupling field was at 423nm and was in singly resonant with 4p55p [0,1/2]0 to the autoionising state 17s 1[1/2]0. The field was supplied by a second harmonic crystal, which was pumped by the OPO system at 846nm. The generated XUV field was the radiation at 85nm transition from the autoionising state to the ground state.