Strong Interactions and Rydberg Excitation in Thermal Vapours
Rubidium: Kate Whittaker, Robert Bettles, James Keaveney, Ifan Hughes & Charles Adams
Caesium: Christopher Wade, Nikola Sibalic, James Keaveney, Kevin Weatherill & Charles Adams
Thermal atomic vapours are a useful tool for probing many areas of fundamental physics, and have many applications beyond their increasingly common use as an atomic frequency reference for laser locking. We investigate thermal Rb and Cs vapours confined in very short (in the direction of light propagation) vapour cells, which offers many advantages. Firstly, the cell thickness is small compared to the Rayleigh range of a focussed beam, meaning that there is a quasi-uniform intensity profile across the entire length of the cell, so that all atoms see the same Rabi frequency. With tight focussing, very large Rabi frequencies are possible with only moderate amounts (typically tens of mW) of laser power. Secondly, the optical setup required is much simpler than for a cold-atom setup, since there is no need for vacuum systems or complex optical and magnetic trapping. Finally, the atomic number densities can be many orders of magnitude higher than for cold atoms, meaning that interactions between atoms become important, even for low-lying excited states. These properties make this project an exciting and dynamic area of research - a very 'hot topic' (ahem...) indeed!
The thermal vapours project is divided into two research areas - one focussing on high density rubidium (Rb) in nano-scale vapour cells, and the other on Rydberg excitation in caesium (Cs) using a three-photon ladder scheme. Further information on each of the two sub-projects can be found in the links on the left.
Recent figures
December 2012
Autler-Townes splitting in CW spectroscopy provides a direct measure of the Rabi frequency in the system. In a vee-type 3-level system in Rb, the coupling laser splits the ground state (in this case Rb-85, F=3). The plot is experimental data, where we observe a Rabi frequency of over 3 GHz for the highest coupling laser power. Click on the image for a high-res pdf.
Our first Rabi oscillation signals in hot Rb in a vee-type energy level configuration on the sub-nanosecond timescale. The Fourier transform on the inset shows a strong peak at 1 GHz, the bandwidth limit of our detection equipment. Time to go shopping? Click on the image for a high-res pdf.
Fast light data signals. If our optical pulse travels through vacuum, it arrives as the red trace. Sending it through the cell causes the peak to appear earlier, indicating superluminal propagation. Indeed, this data is consistent with a group index of (-1.0 ± 0.1) × 105, the largest negative group index ever measured (to our knowledge). Though this may seem a bit odd, if we look at the total integrated photon counts, we see that it's still more likely to detect a photon in the vacuum pulse, preserving causality! Click on the image for a high-res pdf.
Plot of the theoretical maximum refractive index of Rb vapour. Even though the medium is a gas, near resonance the index change can be very large, owing to the high atomic density. However, due to dipole-dipole interactions which are dominant at high density, the index (and susceptibility) saturates, with a maximum of 1.31 for the Rb D2 line, at around 350°C. Click on the image for a high-res pdf.
Phase shift due to a single atomic layer. We measure the relative phase shift of two copropagating beams to investigate the refractive index of the atomic medium. For these conditions, the medium is on average only one atom thick, yet there is still a measurable phase shift and thus a significant refractive index change (for an atomic vapour). Click on the image for a high-res pdf.
Three-photon Rydberg EIT theoretical and experimental data for Rabi frequency coupling ratios; 0.4 (left), 0.8 (middle, optimum) and 1.6 (right). It is clear that the optimum Rabi frequency coupling ratio leads to a velocity insensitive resonance between the ground and Rydberg state. Click on the image for a high-res pdf.
Plot of the theoretical group index as a function of detuning and temperature (density) showing regions of both slow and fast (anomalous dispersion) light. We hope to see evidence of this fast light in the Rb nano-cell soon. Click on the image for a high-res pdf.
Fluorescence decay from the 4 mm cell after initial excitation with a 1 ns pulse (large spike at t = 165 ns). The fluorescence then falls off with the expected 27 ns decay time of the 5p-state. Inset shows the same decay on a log scale. This was taken using almost the same method as for the CW data below (see December graph).
December 2011
Fluorescence from a 4mm thin cell, collected on an avalanche photodiode (APD) module as the laser is scanned across the D2 line. The plot shows the evolution of signal to noise as total counts increase. Red, 1k counts; Yellow, 10k counts; Green, 100k counts; Blue, 500k counts; Black, 25M counts. For clarity, the maximum counts have been normalised to 0.2 (R), 0.4 (Y), 0.6 (G), 0.8 (B) and 1 (K). This was a test of the APD and counting method (no Labview required!). Click on the image for a high-res pdf.
December 2012 Maximal refraction and superluminal propagation in a gaseous nanolayer now published in Physical Review Letters!
November 2012 We have observed Rabi oscillations for the first time on the sub-nanosecond timescale in thermal Rb.
July 2012 We have observed fast light in dense thermal Rb, with the largest negative group index ever measured.
May 2012 Doppler-compensated three photon electromagnetically induced transparency preprint now available on the arXiv!
March 2012 Cooperative Lamb shift in a atomic vapor layer of nanometer thickness has now been accepted for publication in Physical Review Letters!
January 2012 Polarization spectroscopy of an excited state transition has now been published in Optics Letters.
The cooperative Lamb shift in an atomic nanolayer paper has been submitted. You can find the preprint here.
December 2011 The hunt for superradiance takes a step forward with the setup of the APD modules (see graph of the week).
November 2011 Polarization spectroscopy of an excited state transition has been accepted for publication in Optics Letters!
We have observed the Cooperative Lamb Shift in our Rb thin cell, and it fits to theory!
October 2011 Chris, James and Kev took part in the Celebrate Science festival, demonstrating a real laser cooling setup with a pyramid MOT, some marbles and a golf ball! Find out more about AtMol outreach activities.
September 2011 Optical transmission through a dipolar layer paper submitted. You can find the preprint here, and related supplementary information here.
August 2011 Polarization spectroscopy of an excited state transition paper submitted. You can find the preprint here.
We welcome our IASTE summer student, Julia Gontcharov to our lab. She will be working on the Rb experiment, taking data and then setting up a Rydberg EIT experiment (780nm/480nm) in another of our thin cells.
Content © James Keaveney and Christopher Carr, Durham University 2011/2012