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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Spectroscopic Probing of Retardation Effects in the Casimir-Polder Interaction: A Theoretical Study
摘要： Spectroscopic probing of retardation effects in the Casimir-Polder interaction: a theoretical study. Lennard-Jones theory describes the atom-surface interaction as the instantaneous interaction between a fluctuating dipole and its image predicting a surface induced shift of the atomic energy levels given by -C3/z3, where z is the atom-surface distance and C3 is the van der Waals (vdW) coefficient. In Casimir-Polder theory, demonstrated experimentally with ground state atoms in ~1μm thick metallic cavities [1], atom-surface interactions arise from the modification of vacuum fluctuations next to a dielectric boundary. More recently, Casimir-Polder theory has been tested with ground state cold atoms at intermediate distances from a dielectric surface [2]. Nevertheless, testing the limits of the van der Waals law in the extreme near field remains an important experimental challenge. Additionally, excited state atoms are also of fundamental importance, as their interaction with surfaces can also be of resonant nature. This is of particular interest when atomic dipole transitions couple resonantly to surface polaritons, allowing for exotic near field effects. Selective reflection spectroscopy is a major experimental method testing Casimir-Polder interaction of excited state atoms in the near field. Up to now, experimental results have been interpreted exclusively under the prism of the van der Waals approximation. Here, we show calculations of the fully retarded, spectroscopically relevant, Casimir-Polder potentials for the 6S1/2→6P1/2 and 6S1/2→5D5/2 (Fig.1a) Cs transitions (difference between the Casimir-Polder potentials between probed states) taking into account temperature corrections [3]. The dipole forbidden 6S1/2→6D5/2 transition was recently probed by selective reflection spectroscopy [4]. We demonstrate that accounting for a fully retarded potential leads to significantly different predictions of selective reflection spectra compared to a -C3/z3 vdW approximation. Surprisingly, a vdW model when allowing for an adjustable, ad hoc, van der Waals coefficient can accommodate these differences (Fig1b). However, careful analysis shows that this ad hoc coefficient is not a constant but strongly depends on transition linewidth (collisional broadening) as seen in Fig1c. This is because the contribution of atoms to the experimental spectra depends on their relative detuning, i.e. Casimir-Polder shift divided by the transition linewidth. As the linewidth increases, atoms closer to the surface become more ‘resonant’ with the excitation lasers increasing their contribution [5]. Our analysis shows that by increasing the collisional broadening in selective reflection spectroscopy one can tune the experimental probing depth thus probing atoms that are closer to the surface. This can provide an important experimental tool for measuring the effects of retardation in atom-surface potentials of low lying atomic transitions using selective reflection spectroscopy. Retardation effects can have an impact in atom-metamaterial interactions where plasmons can be tuned in resonance with D1 and D2 lines of alkali atoms at near infrared wavelengths [6].
关键词： van der Waals coefficient,selective reflection spectroscopy,Casimir-Polder interaction,atom-surface interaction,retardation effects
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Probing Molecules Next to Surfaces
摘要： 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]. Here, we present selective reflection measurements on polyatomic molecules in gaseous form. We probe rovibrational molecular transitions of NH3 and SF6 using a Quantum Cascade Laser (QCL) at ~10.6μm. Reflection spectroscopy is performed in a vacuum chamber with transparent ZnSe windows. In a separate chamber, we perform simultaneous saturated absorption measurements, to get molecular frequency references in the volume. We also use an auxiliary set-up to lock the QCL laser, either on the derivative (after FM demodulation) of a Doppler linear absorption, or on the wings of the NH3 linear absorption (direct signal). This allows us to eliminate a frequency drift of the QCL source due to temperature fluctuations. By tuning the molecular pressure and therefore the absorption profile, the latter method (lock on the direct signal) allows stable frequency scanning for hundreds of MHz. A system of electronic valves allows us to empty and refill the chamber with molecules within tens of seconds. Detecting the difference between signals, as well as using multiple vibrating mirrors in our set-up, eliminates to about 0.1ppm an interferometric parasitic background, typical in infrared spectroscopy. Fig.1 shows our experimental results obtained for the isolated saP(1) transition of NH3 (Fig. 1a) and a multitude of transitions of SF6, mostly unidentified in molecular databases (Fig. 1b). Linear selective reflection allows us to pinpoint these transitions and easily determine their relative amplitude. At sufficiently low molecular pressure, the frequency resolution of our measurements is limited to ~0.5MHz essentially by laser linewidth. This allows partially resolving the hyperfine structure of NH3. The dotted curves represent theoretical predictions of selective reflection spectra, with transition amplitude adjustments, accounting for FM and laser linewidth. We are working on the fabrication of thin cells using ZnSe windows for rovibrational transmission spectroscopy in the mid-infrared, as well as glass windows, for probing C2H2 at telecommunication wavelengths. Due to the Dicke narrowing effect, thin cells are a step towards compact high-resolution frequency references. Furthermore, achieving, nanometric molecular confinement, defined by cell thickness, instead of wavelength (λ/2π for selective reflection, here ~1.5μm) will allow us to measure the Casimir-Polder interaction with molecules and to study the thermal coupling and energy transfer between rovibrational molecular transitions and surface polaritons [4].
关键词： SF6,Casimir-Polder interactions,molecular gases,Quantum Cascade Laser,NH3,Selective reflection spectroscopy
更新于2025-09-16 10:30:52