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Cooperative interactions in dense thermal Rb vapour confined in nm-scale cells

J. Keaveney

Published Jan 1, 2013• 8 citations
OpenAlex

Abstract

This thesis presents an investigation of the fundamental interaction between light and matter, realised with a rubidium vapour confined in a cell whose thickness (in the propagation direction) is less than the optical wavelength. This confinement allows observation of spectroscopic features not found in longer cells, such as Dicke narrowing. These effects are measured experimentally and a theoretical model is developed, which allows the characterisation of the medium in terms of the atomic electric susceptibility. Interactions between the atoms and their surroundings, whether this be the walls of the cell or other nearby atoms, are explored. In the frequency domain we ob- serve broadening and shifts of the spectral features due to these interactions. The atom-surface interaction shifts the spectral lines, following the expected 1/r^3 van-der-Waals behaviour. The interatomic dipole-dipole interactions are more complex, and we find cooperative effects play an important role. We present an experimental verification of the full spatial dependence of the cooperative Lamb shift, which follows the theoretical prediction made 40 years ago, an important demonstration of coherent interactions in a thermal ensemble. The interactions also play a role in determining the refractive index of the medium, limiting the maximum near-resonant index to n = 1.31. Using heterodyne interferometry, we experimentally measure an index of n = 1.26± 0.02. This index enhancement leads to large bandwidth regions where a significant slow- or fast-light effect is present. We verify the fast-light effect in the time domain by observing the superluminal propagation of a sub- nanosecond optical pulse, and measure the group index of the medium to be ng = −1.0 ± 0.1 × 10^5, the largest negative group index measured to date. We investigate the radiative decay rate using time-domain fluorescence, and we observe radiation trapping effects in a millimetre-thickness vapour. Fi- nally, we present results on sub-nanosecond coherent dynamics in the system which are achieved by pumping the medium with a strong optical pulse.

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High optical nonlinearities can be achieved at the single photon level by coupling the photon states to strongly interacting Rydberg excitations under the conditions of electromagnetically induced transparency. The nonlinear response in Rydberg quantum optics comes as a direct result of the long range dipole-dipole interactions via Rydberg blockade and interaction induced dephasing. In this thesis, Rydberg quantum optics experiments are performed with two spatially separated mediums with a nonlinear response at the single photon level. Two ultracold rubidium atomic clouds are tightly confined by a pair of in-vacuo aspheric lenses such that only a few Rydberg excitations can exist in each one of the clouds simultaneously. The long range character of the dipole-dipole interactions leads to the generation of quantum states of light. The effective photon-photon interactions are directly observed as an anti-correlation in the simultaneous photon retrieval as a result of the non resonant van der Waals interactions. The collective Rydberg excitations, stored in the non-overlapping mediums, experience an additional spatial non-uniform phase. In addition, we experimentally characterize the cooperative optical response of a cold atomic medium at a single photon level. This aims to exploit the enhancement of the spontaneous emissions decay rate (superradiance) as a dependence of the number of atoms involved in the ensemble. The observed atomic response in two different regimes gives a further understanding of the system dynamics and unlocks a promising potential route towards the implementation in scalable, multichannel quantum optical devices.

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T. Peyrot, James Keaveney, Yvan R. P. Sortais +5 more

When many light emitters are subjected to an electromagnetic field with a wavelength λ, they may react collectively to the field. The resonant resulting, light-induced dipole-dipole interactions between atoms should lead to a collective frequency shift of the atomic lines. This shift, unfortunately named the cooperative or collective Lamb shift (CLS) despite its classical nature, depends on the shape of the sample. I will present a new investigation of the origin and validity of the CLS. To do so, we have measured the transmission resonance line shape of a dense hot vapor of potassium atoms confined in a slab with nanometer thickness. Using a complete model, we extract a density-dependent shift and width of the bulk atomic medium resonance (See Fig. 1(a)), deconvolved from the cavity effect [1].

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 1, 2019•0 citations
This thesis presents an investigation of the fundamental interaction between light and matter, realised with a rubidium vapour confined in a cell whose thickness (in the propagation direction) is less than the optical wavelength. This confinement allows observation of spectroscopic features not found in longer cells, such as Dicke narrowing. These effects are measured experimentally and a theoretical model is developed, which allows the characterisation of the medium in terms of the atomic electric susceptibility. Interactions between the atoms and their surroundings, whether this be the walls of the cell or other nearby atoms, are explored. In the frequency domain we ob- serve broadening and shifts of the spectral features due to these interactions. The atom-surface interaction shifts the spectral lines, following the expected 1/r^3 van-der-Waals behaviour. The interatomic dipole-dipole interactions are more complex, and we find cooperative effects play an important role. We present an experimental verification of the full spatial dependence of the cooperative Lamb shift, which follows the theoretical prediction made 40 years ago, an important demonstration of coherent interactions in a thermal ensemble. The interactions also play a role in determining the refractive index of the medium, limiting the maximum near-resonant index to n = 1.31. Using heterodyne interferometry, we experimentally measure an index of n = 1.26± 0.02. This index enhancement leads to large bandwidth regions where a significant slow- or fast-light effect is present. We verify the fast-light effect in the time domain by observing the superluminal propagation of a sub- nanosecond optical pulse, and measure the group index of the medium to be ng = −1.0 ± 0.1 × 10^5, the largest negative group index measured to date. We investigate the radiative decay rate using time-domain fluorescence, and we observe radiation trapping effects in a millimetre-thickness vapour. Fi- nally, we present results on sub-nanosecond coherent dynamics in the system which are achieved by pumping the medium with a strong optical pulse.
Alkali vapors confined in nano-scale cells are promising tools for future integrated atom-based sensor. In this thesis, we investigate the interaction between light and an ensemble of atoms confined in a nano-geometry. We focus on the different processes that can modify the optical response of the atomic ensemble and possibly affect the sensitivity of a sensor based on that technology. First, we study the non-local response of atoms to a light excitation due the atomic motion in thermal vapors. When the distance over which the atoms relaxes is larger than the size of the cell, the optical response depends on the size of the system. We have observed that for transmission spectroscopy, this leads to a periodic modification of the optical response with a period equal to the wavelength of the optical transition. Subsequently we showed that when the density of atom increases, the atomic response becomes local again. In this dense regime, the resonant dipole-dipole interaction in a sub-wavelength geometry leads to collective frequency shifts of the spectral lines. We demonstrate that these shifts were induced by the cavity formed by the cell walls, hence clarifying a long-standing issue. We developed a model to extract the density shifts deconvolved from the cavity effects. Close to a surface, the optical response is also affected by the van der Waals atom-surface interaction. We introduced a new method to extract precisely the strength of this interaction. We also developed a new generation of super-polished glass nano-cells and we presented promising spectroscopic signals. Finally, using these cells, we have compared transmission and off-axis spectroscopic techniques.
This thesis presents the study of thermal alkali-metal vapours confined in a layer with a sub-micron thickness. This confinement enables the study of high density media without the loss of signal present in usually thermal vapours, but also has additional effects on spectra acquired from the system. Such effects include the suppression of the Doppler broadening and the interaction of the atoms with nearby surfaces. Herein, we present a study of this atom-surface interaction in both Rubidium and Caesium atoms, demonstrating that the interaction follows a power law of 1/rα1/r^\alpha1/rα, where α=3.02±0.06\alpha = 3.02 \pm 0.06α=3.02±0.06. We also study Rabi oscillations at high densities, driving GHz Rabi oscillations in a Rb vapour at densities up to 101710^{17}1017~cm−3^{-3}−3. We find that the results do not have sufficient agreement with an optical Bloch simulation, but Maxwell Bloch simulations indicate the possible presence of simultons; simultaneously propagating solitons. Such phenomena have not yet been observed out of crystalline media. We also present a study of causality relations in atomic media. We first discuss the equivalency of the Hilbert transform to the Kramers-Kronig relations, well known in signal processing, but rarely applied in atomic physics. We then demonstrate that the Hilbert transform can be applied to atomic transmission spectra to quickly generate refractive index spectra. The final section of this thesis fully details the successful design and fabrication of vapour cells with a thickness of 500-1500~nm. The cells are assembled using a combination of traditional scientific glass blowing techniques and thermally annealed optically contacted plates. We fully outline the production process, and then present evidence of their successful functionality and longevity.
This thesis presents a comparative study of the measured and calculated absolute absorption and dispersion properties of the Rb D lines through a dense thermal vapour in the absence and presence of an applied magnetic field. A detailed theoretical model valid in the weak-probe regime is calculated. The model uses a matrix representation of the atomic Hamiltonian including the magnetic field interaction for Rb in the completely uncoupled basis. Numerical diagonalisation allows the frequency detunings and transition strengths to be calculated. The lineshape of each transition is modelled as a Voigt profile, a convolution of the inhomogeneous and homogeneous profiles. The medium’s susceptibility is found by summing over all the electric-dipole-allowed transitions. For dense thermal vapours a modification to the homogeneous linewidth of each transition, which grows linearly with the number density of atoms, arises due to resonant dipole-dipole interactions between identical atoms in superpositions of the ground and excited terms. In the presence of an applied magnetic field we investigate the Stokes parameters of light propagating through a dense thermal vapour. For fields larger than 0.33 T we enter the hyperfine Paschen-Back regime on the Rb D lines. We present a compact optical isolator based on an atomic vapour, exploiting the spectral region of high transmission and large dispersion where we would normally expect absorption on the Rb D lines. Frequency up-conversion is shown in the fluorescence measurements over the visible and near infra-red regions for strong excitation. Low density transfer arises due to the energy-pooling effect between two identical atoms in their first excited terms. At high densities resonant dipole-dipole interactions give rise to a threshold for the energy transfer. We characterise the threshold behaviour with increasing number density.
This thesis presents theoretical models and results of numerical simulations describing the propagation of optical pulses through dense, thermal atomic vapours. In particular we investigate the nonlinear effects of optical solitons due to self-induced transparency (SIT) in two-level systems, optical simultons in V-type three-level systems and electromagnetically induced transparency (EIT) in Λ-type systems, including the storage and retrieval of dark-state polaritons. An investigation is made into two-photon excitation of the 5D states of rubidium in a high-intensity beam including the hyperfine structure of the relevant atomic levels. Decay from these states to the 6P manifolds is ruled out as a cause of experimentally observed fluorescence due to the amount of power broadening associated with intensities necessary to provide any significant level of population in these highly excited states. We combine the nonlinear effects of optical solitons and EIT to explain experimentally observed steepened pulses in a V-type system in a micron-length cell. We explain the behaviour as the early formation of a simulton pulse drawn from a CW probe field by a strong coupling pulse, due to coherent population trapping. We predict that in a longer cell it may be possible to facilitate propagation of matched pulses, even when the transitions in the system have different propagation coefficients, as long as decoherence from collision broadening can be controlled. The fact that weak pulses can propagate with this scheme suggests an approach to achieving transparent propagation of single or few photon pulses distinct from, but related to, both SIT and EIT.
High optical nonlinearities can be achieved at the single photon level by coupling the photon states to strongly interacting Rydberg excitations under the conditions of electromagnetically induced transparency. The nonlinear response in Rydberg quantum optics comes as a direct result of the long range dipole-dipole interactions via Rydberg blockade and interaction induced dephasing. In this thesis, Rydberg quantum optics experiments are performed with two spatially separated mediums with a nonlinear response at the single photon level. Two ultracold rubidium atomic clouds are tightly confined by a pair of in-vacuo aspheric lenses such that only a few Rydberg excitations can exist in each one of the clouds simultaneously. The long range character of the dipole-dipole interactions leads to the generation of quantum states of light. The effective photon-photon interactions are directly observed as an anti-correlation in the simultaneous photon retrieval as a result of the non resonant van der Waals interactions. The collective Rydberg excitations, stored in the non-overlapping mediums, experience an additional spatial non-uniform phase. In addition, we experimentally characterize the cooperative optical response of a cold atomic medium at a single photon level. This aims to exploit the enhancement of the spontaneous emissions decay rate (superradiance) as a dependence of the number of atoms involved in the ensemble. The observed atomic response in two different regimes gives a further understanding of the system dynamics and unlocks a promising potential route towards the implementation in scalable, multichannel quantum optical devices.
When many light emitters are subjected to an electromagnetic field with a wavelength λ, they may react collectively to the field. The resonant resulting, light-induced dipole-dipole interactions between atoms should lead to a collective frequency shift of the atomic lines. This shift, unfortunately named the cooperative or collective Lamb shift (CLS) despite its classical nature, depends on the shape of the sample. I will present a new investigation of the origin and validity of the CLS. To do so, we have measured the transmission resonance line shape of a dense hot vapor of potassium atoms confined in a slab with nanometer thickness. Using a complete model, we extract a density-dependent shift and width of the bulk atomic medium resonance (See Fig. 1(a)), deconvolved from the cavity effect [1].