Papers Published: 
September 2013 

Nonequilibrium Phase Transition in a Dilute Rydberg Ensemble: Phys. Rev. Lett. 111 113901 
Abstract: 

We demonstrate a nonequilibrium phase transition in a dilute thermal atomic gas. The phase transition, between states of low and high Rydberg occupancy, is induced by resonant dipoledipole interactions between Rydberg atoms. The gas can be considered as dilute as the atoms are separated by distances much greater than the wavelength of the optical transitions used to excite them. In the frequency domain, we observe a meanfield shift of the Rydberg state which results in intrinsic optical bistability above a critical Rydberg number density. In the time domain, we observe critical slowing down where the recovery time to system perturbations diverges with critical exponent α=0.53±0.10. The atomic emission spectrum of the phase with high Rydberg occupancy provides evidence for a superradiant cascade. 
Key topics: 

Dipoledipole interactions, nonequilibrium dynamics, phase transition, superradiance 
Links: 

You can find the preprint here. 



December 2012 

Maximal refraction and superluminal propagation in a gaseous nanolayer: Phys. Rev. Lett. 109 233001 
Abstract: 

We present an experimental measurement of the refractive index of high density Rb vapor in a gaseous atomic nanolayer. We use heterodyne interferometry to measure the relative phase shift between two copropagating laser beams as a function of the laser detuning and infer a peak index n = 1.26 ± 0.02, close to the theoretical maximum of 1.31. The large index has a concomitant large index gradient creating a region with steep anomalous dispersion where a subnanosecond optical pulse is advanced by >100 ps over a propagation distance of 390 nm, corresponding to a group index of (1.0 ± 0.1)x10^{5}, the largest negative group index measured to date. 
Key topics: 

Fast light, group velocity, refractive index, group index, dipoledipole interactions, quantitative spectroscopy, pulse propagation 
Links: 

You can find the preprint here. 



April 2012 

Cooperative Lamb Shift in an Atomic Vapor Layer of Nanometer Thickness: Phys. Rev. Lett. 108, 173601 (2012) 
Abstract: 

We present an experimental measurement of the cooperative Lamb shift and the Lorentz shift using a nanothickness atomic vapor layer with tunable thickness and atomic density. The cooperative Lamb shift arises due to the exchange of virtual photons between identical atoms. The interference between the forward and backward propagating virtual fields is confirmed by the thickness dependence of the shift, which has a spatial frequency equal to twice that of the optical field. The demonstration of cooperative interactions in an easily scalable system opens the door to a new domain for nonlinear optics. 
Key topics: 

Cooperative effects, cooperative Lamb shift, superradiance, dipoledipole interactions, spectroscopy 
Links: 

You can find the preprint here. 



March 2012 
Subnatural linewidths in twophoton excitedstate spectroscopy: Phys. Rev. A 85, 033830 (2012). 
Abstract: 

We investigate, theoretically and experimentally, absorption on an excitedstate atomic transition in a thermal vapor where the lower state is coherently pumped. We find that the transition linewidth can be subnatural, that is, less than the combined linewidth of the lower and upper state. For the specific case of the 6P3/2→7S1/2 transition in room temperature cesium vapor, we measure a minimum linewidth of 6.6 MHz compared with the natural width of 8.5 MHz. Using perturbation techniques, an expression for the complex susceptibility is obtained which provides excellent agreement with the measured spectra. 
Key topics: 

Excited state spectroscopy, Doppler averaging, laser locking, narrow resonances 
Links: 

You can find the preprint here. 



January 2012 
Polarization spectroscopy of an excited state transition: Opt. Lett. 37, 1, 118 (2012). 
Abstract: 

We demonstrate polarization spectroscopy of an excited state transition in room temperature cesium vapor. An anisotropy induced by a circularly polarized pump beam on the D2 transition is observed using a weak probe on the 6P3/2 to 7S1/2 transition. When the D2 transition is saturated, a subnatural linewidth feature is observed which theoretical modeling shows is enhanced by Doppler averaging. Polarization spectroscopy provides a simple modulationfree signal suitable for laser frequency stabilization to excited state transitions. 
Key topics: 

Excited state spectroscopy, Doppler averaging, laser locking, narrow resonances 
Links: 

You can find the preprint here. 



October 2010 

Effect of buffer gas on electromagnetically induced transparency in a ladder system using thermal rubidium vapor: PRA 82, 045806 
Abstract: 

We report on the first observation of electromagnetically induced transparency (EIT) in a ladder system in the presence of a buffer gas. In particular we study the 5S1/2  5P3/2  5D5/2 transition in thermal rubidium vapor with a neon buffer gas at a pressure of 6 Torr. In contrast to the line narrowing effect of buffer gas on {\Lambda}systems we show that the presence of the buffer gas leads to an additional broadening of (32 +/ 5) MHz, which suggests a cross section for Rb(5D5/2)Ne of {\sigma} = (7 +/ 1) x 1019 m2. However, in the limit where the coupling Rabi frequency is larger than k the collisional dephasing a strong transparency feature can still be observed. 
Key topics: 

Electromagnetically induced transparency, ladder system, collisional dephasing 




Papers Just Submitted: 
May 2012 

Threephoton electromagnetically induced transparency using Rydberg states: arXiv:1205.2499 
Abstract: 

We demonstrate electromagnetically induced transparency (EIT) in a fourlevel cascade system where the upper level is a Rydberg state. The observed spectral features are subDoppler and can be enhanced due to the compensation of Doppler shifts with AC Stark shifts. A theoretical description of the system is developed which agrees well with the experimental results and an expression for the optimum parameters is derived. 
Key topics: 

Rydberg states, EIT 
Links: 

You can find the preprint here. 

September 2011 

Optical transmission through a dipolar layer: arXiv:1109.3669 
Abstract: 

The interaction between light and matter is fundamental to developments in quantum optics and information. Over recent years enormous progress has been made in controlling the interface between light and single emitters including ions, atoms, molecules, quantum dots and ensembles. For many systems, interparticle interactions are typically negligible. However, if the emitters are separated by less than the emission wavelength, resonant dipoledipole interactions modify the radiative decay rate and induce a splitting or shift of the resonance. Here we map out the transition between individual dipoles and a strongly interacting ensemble by increasing the density of atoms confined in a layer with thickness much less than the emission wavelength. We find two surprising results: whereas for a noninteracting ensemble the opacity increases linearly with atomic density, for an interacting ensemble the opacity saturates, i.e., a thin dipolar layer never becomes opaque regardless of how many scatterers are added. Secondly, the relative phase of the dipoles produces an abrupt change in the optical transmission around the thickness λ/4. 
Key topics: 

Saturation of opacity, cooperative effects, dipoledipole interactions, linebroadening, spectroscopy 
Links: 

You can find the preprint here, and related supplementary information here. 



Recent Posters: 
Cooperative interactions in nmthickness thermal Rb vapour 
Presented at ICAP 2012, Paris 
Click on the image for the full pdf version. 

Optical properties of a dipolar layer 
Presented at Quamp 2011, Oxford 
Click on the image for the full pdf version. 
