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
We investigate high density vapours of rubidium (Rb) confined in layers which are typically much smaller than the excitation wavelength (780 or 795 nm). The short length of these cells means we see many interesting effects which are normally not present in standard (cm-length) vapour cells. We are interested in exploring fundamental physics in these extreme conditions.
Our investigation covers:
  • Atom-atom interactions. This is the main focus of our work. We focus our efforts in a regime where interactions between the atoms play a dominant role in the spectral properties, leading to cooperative effects such as the Cooperative Lamb shift and superradiance, which we can observe either in the steady state through spectroscopic investigation, or in the time domain via time resolved fluorescence measurements or by looking at coherent dynamics (Rabi oscillations).
  • Atom-surface interactions. Due to the tight confinement in extremely thin vapour cells, surface interactions (van der Waals) become important. This leads to broadening and shifts of resonances, which become significant when the cell is shorter than λ/2π.
  • Non-linear optics. Effects such as electromagnetically induced transparency (EIT) scale with atomic density, so using thin cells again provides an appealing solution. In addition to atomic density, the high Rabi frequencies achievable through tight focussing combined with a uniform intensity across the cell (since the cell is much smaller than the Rayleigh length of the beam focus) are two very attractive properties.
  • Although the aim of this project is not explicitly application-focussed, there are some interesting applications using thin cells. These include:
  • A switchable, tunable narrowband optical filter using a near-100% transparency window with a ladder EIT system.
  • A single photon amplifier, again exploiting the properties of EIT to rotate a probe pulse with a single atomic excitation from a single photon.
  • A Faraday rotator/isolator based on a high density atomic ensemble in a high magnetic field.
  • Click on any of the images below for higher resolution versions.
    Experimental Details
    The experimental setup for this experiment is extremely simple. We use pump-probe Doppler-free spectroscopy through a standard Rb vapour cell as a frequency reference, with a Fabry Perot Interferometer (FPI) to correct for any non-linearity in the laser scan. The remaining beam is then focussed into our extremely thin cell and the transmitted light collected on a photodiode. Not shown are waveplates and additional density filters, resulting in a linearly polarised, weak probe beam passing through the experimental cell.
    We have a long-running collaboration with the group of David Sarkisyan at the Armenian academy of sciences, who construct our thin cells. The cell shown on the left has sapphire windows, and is filled with Rb vapour.
    A photograph of the cell windows reveals a Newton's Rings interference pattern, resulting from the slight curvature of the windows. This leads to a wedge-shaped profile over the cell, with a thickness that is variable between 30 nm and 2 μm. Even with this variable thickness, the angle between the two surfaces is less than 0.1 mrad, meaning we still have two near-parallel surfaces.
    We use a heater with two heating elements to pseudo-independently heat the windows and Rb resevoir. Control over both is required to prevent Rb vapour condensing on the cell windows. With this, we can heat to a resevoir temperature of around 350°C.
    From the top, you can see the cell inside the heater.
    At this temperature, we have more than 6 orders of magnitude higher atomic density than a room temperature vapour cell! The plot on the left shows the dependence of atomic number density and interatomic spacing on temperature, calculated from formulae in Steck's datasheets.
    Atom-Atom Interactions
    Dipole-dipole interactions are the key to many of the interesting effects we study. These interactions cause the atomic ensemble to change from acting collectively as N individual atoms (low density, or large dipolar separation) to acting as a cooperatively coupled system.
    Using a two-atom basis Hamiltonian, we calculate the splitting (on the D1 line) of energy levels due to dipole-dipole interactions as a function of atomic spacing.
    The plot on the left is experimental data showing the evolution from sharp Dicke-narrowed peaks to a dipole-dipole interaction dominated regime. The experimental data is for a thickness L = λ/2 = 390 nm, from 140°C (top) to 330°C (bottom).
    A surprising result of dipole-dipole interactions is the saturation of the susceptibility, implying the counter-intuitive result that adding more scatterers to a medium does not necessarily make it more opaque. Insted, interactions broaden the lineshape so much that eventually a uniform opacity will be reached. For more detailed information, click here.
    The interactions lead to cooperative effects, essentially where the atoms can no longer be thought of as individuals. In this case the interaction leads to a modification of the radiative lifetime and a shift of the resonance lines called the Cooperative Lamb shift, which for our geometry has the characteristic decaying oscillatory behaviour seen in this figure. This is the first direct observation of this shift.
    Atom-Surface Interactions
    Atom surface interactions in our cells follow a van der Waals (1/R^3) potential. In our cell, we have a potential from each of the two cell windows, leading to a combined potential as shown on the right.
    Analysis of transmission spectra taken at constant atomic density (temperature) for various cell lengths leads to this plot, where we have fitted a 1/r3 fit line. This allows us to calculate a value of the C3 coefficient betweeen the Rb 5P3/2 and sapphire.
    Electromagnetically Induced Transparency
    Electromagnetically induced transparency in a 3-level atomic system is a result of quantum interference between excitation pathways. In an atomic system like the one shown here, a weak probe beam (780nm) couples the ground and intermediate states (5S and 5P), while a strong coupling beam couples intermediate to excited states (5P to 5D). The interference gives rise to a window around the two-photon resonance where the medium is more transparent than would be the case in the absence of the strong coupling field. For a comprehensive review of EIT, see the review by Fleischhauer, Imamoglu and Marangos.
    The plot on the right shows theory curves for a three level system like the one above. The red curve shows a probe-only spectrum, where we see the familiar Doppler-broadenend absorption profile. The solid blue line is for zero detuning of the coupling beam, at moderately low power, while the green solid line shows the spectrum for a 200 MHz detuned coupling beam for the same power. The dashed blue line is for zero coupling detuning, with considerably more power. Here the transparency window is power broadened, but has near 100% transmission on resonance.
    We use the 5S,5P,5D system in Rb which gives a strong coupling on the excited state transition, and in counter-propagating beam geometry gives a near Doppler-free configuration. This allows us to see very narrow transparency windows (for a ladder system) with very high contrast. With strong coupling power, we see near-100% transmission in an otherwise optically thick area, using a 4mm long vapour cell. This has potential as a tunable ultra-narrowband optical filter.
    Content © James Keaveney, Durham University 2011/2012. Last updated 12/03/2012.