| Electromagnetically induced transparency (EIT) is a technique for improving the transmission of laser beams through otherwise absorbing or refractive media. A major potential application of EIT is gas-phase nonlinear frequency conversion with maximal atomic coherence: using EIT, one is able to adiabatically drive atoms into a maximal coherent state and these coherently-driven and phased atoms act as a frequency converter, enabling efficient sum or difference frequency conversion when beating against another applied frequency.; However, previous practices of EIT has been limited to ideal three-state Raman atomic systems with lasers on Raman resonance. Most often, atoms have additional angular momenta that enlarge the manifold of coupled states (for example, hyperfine splittings), and achieving EIT in such systems is important for practical applications of EIT in nonlinear frequency conversions. We have theoretically found and experimentally demonstrated methods for obtaining EIT in such systems: by tuning the frequencies of the laser beams to coincide with the centers of gravity of the split transitions, we are able to obtain transparency and high-quality beam transmission in atoms with hyperfine structure, and this idea will apply to other types of splittings. This progress allows us to perform efficient frequency conversion experiments with maximal coherence in a lead heatpipe. Efficient generation of VUV radiation with near unity conversion efficiency has been demonstrated. In a recent experiment, we have efficiently generated coherent infra-red radiation using this method and the same scheme can be used to generate widely-tunable, coherent, mid-to-far radiation with a tabletop setup. |
