1. 1 Coherent processes in metastable helium at room temperature Thomas Lauprêtre Fabienne Goldfarb Fabien Bretenaker Laboratoire Aimé Cotton, Orsay, France School of Physical Sciences, Jawaharlal Nehru University, Delhi, India Rupamanjari Ghosh Santosh Kumar

2. 2 Summary Electromagnetically Induced Transparency Experimental set-up Dispersion in a cavity Tripod system

3. 3 Electromagnetically Induced Transparency ? Fact: Optical transition is made transparent for a resonant field (otherwise opaque medium) How it happens: A quantum interference effect, induced by a control field on a second transition

4. 4 One optical transition Λ system  Electromagnetically Induced Transparency (EIT) Width of transparency window

5. 5 EIT and Slow Light Kramers-Kronig Strong positive dispersion Slow Light ! Kash & al, PRL, 1999: 90 m.s -1 in Rb Hau & al, Nautre, 1999: 17 m.s -1 in cold Na

6. 6 Summary EIT Experimental set-up Dispersion in a cavity Tripod system

7. 7 Metastable 4 He 1S01S0 3P13P1 3S13S1 m =10 RF discharge   pp pp cc  cc Lifetime ~8000s  polarization selected Λ system

8. 8 Experimental results Width at half maximum (kHz) Coupling intensity (W.m -2 ) Group delay (µs) Coupling intensity (W.m -2 )  Group velocity around 8 km.s -1 ! Goldfarb, F. & al., EPL (Europhysics Letters), 2008, 82, 54002 Ghosh, J. & al., Phys.Rev.A, 2009

9. 9 Summary EIT Experimental set-up Dispersion in a cavity Tripod system

10. 10 Dispersion in cavity Why? Enhance sensitivity of cavity based sensors (ring laser gyroscope…)  Positive dispersion reduces the linewidth of a resonator Main parameter: lifetime of photons in the cavity

11. 11 Lifetime of photons 2 different points of view 1) Phase velocity Resonant cavity: monochromatic field 2) Group velocity  Gaussian pulse  Δt ∞ ? ΔtΔt

12. 12 Experiment

13. 13 Global results Measured decay time ~ a few µs ~150 ns with phase velocity Group velocity ! τ cav

14. 14 Summary EIT Experimental set-up Dispersion in a cavity Tripod system

15. 15 Tripod system 1S01S0 3P03P0 3S13S1 RF discharge m =10 Santosh Kumar, School of Physical Sciences, Jawaharlal Nehru University, Delhi, India

16. 16 Tripod system ΩPΩP ΩPΩP ΩCΩC m y =10 0 (kHz) ΩCΩC ΩCΩC ΩPΩP m y =10 0 Laser π σ

17. 17 Outlook Negative dispersion inside our cavity  Fast light  Negative group velocity Further investigations on Tripod system

18. 18

19. 19 0<v g <cc<v g v g <-c-c<v g <0