Surface-plasmon-based dispersive detection and spectroscopy of ultracold atoms

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Surface-plasmon-based dispersive detection and spectroscopy of ultracold atoms

Matthias Mildner, Claus Zimmermann, and Sebastian Slama

Phys. Rev. Research 3, 013005 – Published 4 January 2021

Abstract

The paper reports on the optical detection and spectroscopy of ultracold atoms near a gold surface. A probe light field is used to excite surface plasmon polaritons. The refractive index of the atomic gas shifts the plasmon resonance and changes the reflected light power. Thus, the sensitivity of the detection is plasmonically enhanced. Absorption of photons from the evanescent wave is avoided by detuning the laser from atomic resonance which makes the detection scheme potentially nondestructive. The spectrum of the signal is determined by a Fano resonance. We show that atoms can be detected nondestructively with single atom resolution for typical parameters in cold atom experiments. Thus, the method is suitable for quantum nondemolition measurements of matter wave amplitudes. Experimentally, we measure a technically limited sensitivity of 30 atoms and extend the detection scheme to dispersively image the atom cloud near the surface.

Received 8 September 2020
Accepted 15 December 2020

DOI:https://doi.org/10.1103/PhysRevResearch.3.013005

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic spectra Cold atoms & matter waves Near-field optics Plasmonics Quantum optics

Atomic, Molecular & Optical

Authors & Affiliations

Matthias Mildner, Claus Zimmermann, and Sebastian Slama

Physikalisches Institut and Center for Quantum Science, Eberhard Karls Universität Tübingen, D-72076 Tübingen, Germany

Article Text

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Vol. 3, Iss. 1 — January - March 2021

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Images

Figure 1
(a) Experimental setup: A near resonant $p$ -polarized light field with intensity $I_{in}$ and wave number $k_{1}$ is reflected by total internal reflection at a glass-gold-vacuum interface. It excites a plasmonically enhanced evanescent wave. Atoms are placed in a layer at a distance $z_{B}$ from the surface with atomic density $ρ ろー (z)$ . The interaction of the atoms with the evanescent wave changes the reflection of the light field. (b) Intensity reflectivity $R$ as function of incidence angle ${θ しーた}_{in}$ without atoms, i.e., $ρ ろー = 0$ (black dashed curve), and for an atomic density of $ρ ろー = 1 \times 10^{13} {cm}^{- 3}$ (black solid curve) at a detuning of $δ でるた = - 30 Γ がんま$ , and distance $z_{B} = 100 nm$ . We further used the refractive index of glass $n_{1} = 1.51$ and the dielectric constant of gold $ɛ_{2} = n_{2}^{2} = - 22.9 + 1.42 i$ valid at 780-nm wavelength. The maximum number of atoms $N_{atom}^{\max}$ (red curve, right axis) that can be detected in a QND measurement is determined from Eq. (30).
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Figure 2
The maximum number of atoms $N_{atom}^{\max}$ that can be detected in a QND measurement increases with the atomic density $ρ ろー$ . For fixed density a maximum is reached for a certain detuning $δ でるた$ .
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Figure 3
(a) Measured change in reflectivity with detuning $δ でるた = 12.6 Γ がんま$ , incident intensity $I_{in} = 14.2 I_{sat}$ , and incidence angle of ${θ しーた}_{in} = 43 . 07^{\circ}$ . The signal is fitted by a Gaussian function representing the density distribution of the cloud during its reflection at the surface. (b) Measurements (asterisks) and simulated spectra (lines). The black spectrum (5 times magnified) is recorded with an incident intensity as used in (a) and atom density $ρ ろー = 6.3 \times 10^{11} {cm}^{- 3}$ . The spectrum is largely power-broadened and reduced in height due to light scattering which also leads to the plateau around $δ でるた = 0$ . This plateau has been included in the simulation as Lorentzian suppression of the signal height. For the red data points the intensity is reduced to $I_{in} = 0.012 \times I_{sat}$ far below saturation intensity, the atom density is $ρ ろー = 5 \times 10^{11} {cm}^{- 3}$ . Error bars indicate the statistical error of the fitted peak height of four measurements each. For large incident intensity the achieved resolution (blue squares, right axis, red line is a guide to the eye) has been determined from curves as shown in a). In the center where the signal is zero due to light scattering the resolution diverges.
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Figure 4
(a) The image of the atom cloud has been taken using the reflected probe beam (atom density $ρ ろー = 6.1 \times 10^{12} {cm}^{- 3}$ , probe laser intensity $I_{in} = 0.02 I_{sat}$ , detuning $δ でるた = - 8.5 MHz$ , incidence angle ${θ しーた}_{in} = 42 . 54^{\circ}$ , and exposure time $400 μ みゅー s$ ). The experimental resolution is $x_{\min} = 12 μ みゅー m$ . The inset shows a spectrum taken from pixels in the center of the atom cloud.
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