Selective reflection is a high-resolution spectroscopic method that allows the probing of atomic and molecular gases in the near field of dielectric cell windows. It is a sensitive technique for measuring interactions between excited atoms and surfaces, thus providing a unique tool to probe resonant effects in Casimir-Polder physics. The theoretical modelling of selective reflection spectra has thus far employed the infinite Doppler approximation which assumes that the Doppler shift dominates over the atom-surface interactions as well as the homogeneous transition linewidth. Here we extend the theoretical treatment of selective reflection spectra to include the effects of finite Doppler width. This allows us to analyse the regime of very large Casimir-Polder interactions, relevant for Rydberg atom spectroscopy, and revisit previous analyses of experiments with low-lying excited states. Further, this will aid in determining the signatures of quantum friction in vapor cell spectroscopy.
Selective reflection is a high-resolution spectroscopic method that allows the probing of atomic and molecular gases in the near field of dielectric cell windows. It is a sensitive technique for measuring interactions between excited atoms and surfaces, thus providing a unique tool to probe resonant effects in Casimir-Polder physics. The theoretical modelling of selective reflection spectra has thus far employed the infinite Doppler approximation which assumes that the Doppler shift dominates over the atom-surface interactions as well as the homogeneous transition linewidth. Here we extend the theoretical treatment of selective reflection spectra to include the effects of finite Doppler width. This allows us to analyse the regime of very large Casimir-Polder interactions, relevant for Rydberg atom spectroscopy, and revisit previous analyses of experiments with low-lying excited states. Further, this will aid in determining the signatures of quantum friction in vapor cell spectroscopy.
The Casimir-Polder interaction between atoms and macroscopic surfaces is a fundamental prediction of quantum electrodynamics of importance for precision measurements, quantum technologies and our understanding of the electromagnetic properties of materials. In the near field, Casimir-Polder interactions are described as the interaction of the fluctuating atomic dipole with its surface-induced image, which evolves as −C3/z3 where z is the atom-surface distance and C3 is the van derWaals coefficient. In this thesis, we study boththeoretically and experimentally the Casimir-Polder interaction between a highly excited caesium atom (Rydberg atom) and a dielectric surface. Our theoretical study extends previous studies to include higher-order terms in the atom-surface interaction, beyond the dipole-dipole interaction term investigated in the seminal work of Casimir and Polder. In particular, we include quadrupole-quadrupole and dipole-octupole interaction terms with a distance dependence of −C5/z5 giving explicit calculations of the C5 coefficients for most common alkali atoms. We also describe a new selective reflection experiment probing Cs Rydberg atoms (15 − 17D3/2 and 16 − 17S1/2) at nanometric distances away from a sapphire surface. The experiment is performed in an all-sapphire vapour cell using stepwise excitation spectroscopy that includes a pumping step at 6S1/2 → 6P1/2 (894nm) and a selective reflection probing step at 6P1/2 → nS1/2,nD3/2 with n = 15 - 17 (∼ 510nm). To analyze our experimental spectra, we extend selective reflection theory beyond the infinite Doppler approximation, including in our analysis the Maxwell-Boltzmann distribution of atomic velocities. Fitting our theoretical lineshape model to our experimental spectra allows us to extract the C3 van der Waals coefficient for the probed caesium excited states. This C3 measurement exceeds the theoretical predictions of Casimir-Polder theory by a factor of about 1.5 - 2, depending on the probed state. Finally, we describe a new thin-cell experiment that aims to illuminate the reported disagreement between theory and experiment for the C3 coefficient and provide a first experiment
Selective reflection spectroscopy at normal incidence provides signals with sub-Doppler resolution, linear with optical intensity. Frequency modulated (FM) selective reflection probes atomic vapours at distances comparable to the excitation wavelength (~λ/2π) and is used extensively to probe the Casimir-Polder interactions between an excited state atom and a macroscopic surface. Extending selective reflection spectroscopy to molecular gases allows probing a thin layer of molecular gas next to a surface. This is a very attractive prospect that allows envisaging high-resolution molecular spectroscopy and molecular frequency references in a compact and miniaturised apparatus, such as a thin cell [1] or a photonic crystal [2]. Additionally, it paves the way for spectroscopic probing of the Casimir-Polder interaction with molecules. The molecule-surface interaction has been the object of extensive theoretical investigations, focusing on the effects of molecular orientation and chirality. However, experimental tests are few and comparison with theoretical predictions has been challenging [3].
The Casimir-Polder interaction between atoms and macroscopic surfaces is a fundamental prediction of quantum electrodynamics of importance for precision measurements, quantum technologies and our understanding of the electromagnetic properties of materials. In the near field, Casimir-Polder interactions are described as the interaction of the fluctuating atomic dipole with its surface induced image, which evolves as -C3/z3 where z is the atom-surface distance and C3 is the van der Waals coefficient. In this thesis, we study both theoretically and experimentally the Casimir-Polder interaction between a highly excited cesium atom (Rydberg atom) and a dielectric surface. Our theory extends previous studies to[3]include higher-order terms in the atom-surface interaction, beyond the dipole-dipole interaction term investigated in the seminal work of Casimir and Polder. In particular, we include quadrupole-quadrupole and dipole-octupole interaction terms with a distance dependence of ?C5/z5 giving explicit calculations of the C5 coefficients for most common alkali atoms. We also describe a new selective reflection experiment probing Cs Rydberg atoms (15-17D3/2 and 16-17S1/2) at nanometric distances away from a sapphire surface. The experiment is performed in an all-sapphire vapour cell using stepwise excitation spectroscopy that includes a pumping step at 6S1/2?6P1/2 (894nm) and a selective reflection probing step at 6P1/2?nS1/2 , nD3/2 with n=15-17 (~510nm). In order to analyze our experimental spectra, we extend selective reflection theory beyond the infinite Doppler approximation, including in our analysis the Maxwell-Boltzmann distribution of atomic velocities. Fitting our experimental spectra with our theoretical models allows us to extract the C3 van der Waals coefficient for the probed cesium excited states. The experimental results exceed our theoretical predictions (by a factor of about 1.5-2, depending on the probed state). Finally, we describe a new thin-cell experiment that aims to illuminate the reported disagreement between theory and experiment for the C3 coefficient and provide a first experimental measurement of the C5 coefficient (experimental study of higher-order interactions).
Highly excited (Rydberg) atoms have exaggerated properties making them extremely sensitive to external electromagnetic fields and interacting strongly with their environment. Atomic vapor cells represent an attractive platform for studying Rydberg atoms and fabricating quantum devices. For example, Rydberg atoms in vapor cells have been used as sensitive detectors of electric fields of frequencies ranging from DC up to the THz range but also as single photon sources for quantum technology applications exploiting collective phenomena due to the Rydberg blockade effect [1]. Rydberg atoms also find applications in fundamental physics, in particular for the measurement of dispersive interactions of the Casimir-Polder type (atom-surface interactions) [2]. One major advantage of Rydberg atoms is that they expose limitations in the traditional perturbative approach of Casimir-Polder (CP) theory [3, 4]. Indeed, in the extreme near-field, the dipole approximation breaks down and higher-order terms need to be considered. We report on extensive experimental measurements of the Rydberg-surface interaction using spectroscopy in cesium vapor nanocells of a thickness ranging roughly from 200-700nm, as well as selective reflection spectroscopy on a macroscopic cesium all-sapphire cell. Atoms are first excited to the Cs(6P) level with a 894nm pump laser and subsequently a green laser ≈ 510nm probes Rydberg nD or nS states, where the principal quantum number n ranges between 15-17. Our experiments evidence the dipole-dipole term of the Casimir-Polder interaction providing a measurement of the C3 coefficient for cesium Rydberg states. Furthermore, our experiment clearly evidences an additional interaction that induces shifts and broadens the linewidth of the probed transitions in the vicinity of the dielectric windows of our cells. We believe that this interaction is due to electric fields that are either generated by patch charges (trapped on the surface or induced by the excitation lasers), or by cesium adsorbants. We show that the different polarizability (of opposing sign) between S and D Rydberg states can be exploited to extract quantitative measurements of the strength and distance scaling (z-dependence) of such parasitic electrostatic interactions.Our experiments suggest that the sensitivity of Rydberg atoms to external electric fields could provide a unique tool for probing electrostatic interactions in the vicinity of surfaces. This could allow systematic error corrections in Casimir-Polder experiments with excited or even ground state atoms that aim at putting bounds on the existence of non-Newtonian gravity [5]. We are currently exploring the possibility of coating the internal window interfaces with conducting 2D material such as graphene to reduce the influence of parasitic charges. This could allow us to probe atoms closer to the surface, where quadrupole interactions could be experimentally attainable for the first time [4].References[1] H. Kubler, J. P. Shaffer, T. Baluktsian, R. Loew, T. Pfau, Nat. Photon., 4, 112–116 (2013).[2] V. Sandoghdar et al., Phys. Rev. Lett 68, 3432–3435 (1993).[3] J. A. Crosse et al., Phys. Rev. A 82, 3010901 (2010).[4] B. Dutta et al., Phys. Rev. Res. 6, L022035 (2024).[5] A. Laliotis, B-S. Lu, M. Ducloy, D. Wilkowski, AVS Quantum Sci. 3, 043501 (2021).
Electric-dipole-forbidden transitions play an important role in quantum sensing, quantum information, and fundamental tests in physics. As such, the development of novel and sensitive spectroscopic methods is of major interest. Here, we present a Doppler-free selective reflection experiment on the 62S1/2–52D5/2 electric-quadrupole transition of cesium vapor at the vicinity of a sapphire window. The spectrum is obtained by precision experiments overcoming limitations due to the small-signal amplitude of forbidden transitions. The observed sub-Doppler lines allow us to measure the collisional broadening of the electric-quadrupole line. The interaction of cesium atoms with the sapphire surface of the cell is also evidenced, but, due to its weak contribution, a quantitative analysis remains challenging. Our experiment paves the way for further studies of the Casimir–Polder interaction between exotic excited-state atoms and dielectric surfaces.
Selective reflection spectroscopy at an interface with a low-density resonant vapor, especially when combined with a frequency modulation technique, is a high-resolution Doppler-free tool for probing atoms interacting with a surface. We analyze different types of relevant surface interaction, emphasizing the spectral consequences of a van der Waals surface attraction associated to a z−3 potential dependence (z: distance to the wall). We present detailed results of two series of experiments at a Cs vapor/dielectric window interface on the 6S1/2-6P3/2 (λ=852 nm) resonance line and on the 6S1/2-7P second resonance line (λ=455 nm and 459 nm). Lineshape analysis at various pressures consistently shows that a van der Waals-type surface attraction has to be considered to interpret strong lineshape distortions and resonance shift. The attractive strengths are found to be equal respectively to ≈2 kHz μm3 and ≈20 kHz μm3, independently of the considered hyperfine component, within the experimental accuracy. It yields also typical parameters of pressure broadening and shift, which are shown to originate in collisional processes, at densities where the medium is opaque. Theoretical expectations for the VW strength are discussed on the basis of the results of atomic theory. The predicted values are smaller, by a typical factor of 2, than those deduced from the experiments. The validity of the theory, when applied to a dielectric interface, is discussed and seems questionable when the frequency of virtual atomic transitions involved in the van der Waals attraction potential lies in the dielectric window absorption range.