Coupling Thermal Atomic Vapor to Slot Waveguides

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Coupling Thermal Atomic Vapor to Slot Waveguides

Ralf Ritter, Nico Gruhler, Helge Dobbertin, Harald Kübler, Stefan Scheel, Wolfram Pernice, Tilman Pfau, and Robert Löw

Phys. Rev. X 8, 021032 – Published 4 May 2018

Abstract

We study the interaction of thermal rubidium atoms with the guided mode of slot waveguides integrated in a vapor cell. Slot waveguides provide strong confinement of the light field in an area that overlaps with the atomic vapor. We investigate the transmission of the atomic cladding waveguides depending on the slot width, which determines the fraction of transmitted light power interacting with the atomic vapor. An elaborate simulation method has been developed to understand the behavior of the measured spectra. This model is based on individual trajectories of the atoms and includes both line shifts and decay rates due to atom-surface interactions that we have calculated for our specific geometries using the discrete dipole approximation. Furthermore, we investigate density-dependent effects on the line widths and line shifts of the rubidium atoms in the subwavelength interaction region of a slot waveguide.

Received 9 November 2017
Revised 6 February 2018

DOI:https://doi.org/10.1103/PhysRevX.8.021032

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 Photonics

Atomic gases Atoms

Atomic, Molecular & Optical

Authors & Affiliations

Ralf Ritter¹, Nico Gruhler^2,3, Helge Dobbertin⁴, Harald Kübler¹, Stefan Scheel⁴, Wolfram Pernice^2,3, Tilman Pfau¹, and Robert Löw^1,*

¹5. Physikalisches Institut and Center for Integrated Quantum Science and Technology, Universität Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany
²Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
³Institute of Physics, University of Münster, 48149 Münster, Germany
⁴Institut für Physik, Universität Rostock, Albert-Einstein-Straße 23, 18059 Rostock, Germany

^*r.loew@physik.uni-stuttgart.de

Popular Summary

Atomic vapor cells—light sources coupled to clear containers filled with ultrapure atomic gas—find use in various applications ranging from microwave detection to gyroscopes, and they are employed commercially for wavelength referencing, magnetometry, and atomic clocks. Interfacing room-temperature gases with chip-scale photonic circuits enables the realization of micrometer-sized units with light sources, interconnections, active media, and detectors all on a single platform. Previous implementations relied on regular waveguides, which limit how efficiently the atoms can interact with light. Here, we explore the interplay between thermal atoms and so-called “slot waveguides,” which provide much higher optical intensity at the position of the atoms, thereby offering enhanced interaction strength.

A slot waveguide confines light in a very narrow, subwavelength channel. We developed an optical chip containing slot waveguides with different slot widths, ranging from 30 nm to 250 nm. The different widths determine the power density in the vapor region, which allows us to systematically investigate the interaction between rubidium atoms and the guided modes. We complement our studies with a detailed theory and simulation model, which is envisaged to serve as a toolbox for future investigations of hybrid systems combining atomic vapor and nanophotonic devices.

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Vol. 8, Iss. 2 — April - June 2018

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Images

Figure 1
Waveguide structure and mode profiles. (a) Schematic of the slot waveguides. The probe light $P_{in}$ is coupled into the waveguide via a grating coupler and guided to the slot section. The output $P_{out}$ is detected with a photo multiplier tube. (b) FIB/SEM image of the waveguide cross section. The white patches are sputtered gold to prevent charge buildup during imaging. (c)–(e) Mode profiles of the fundamental TE mode for a closed (c), partially open (d), and fully open (e) slot waveguide. (f)–(h) Corresponding atomic decay rates $κ$ into the fundamental mode normalized by the total decay rate $Γ$ .
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Figure 2
Transmission spectrum of the $w_{d} = 75 nm$ , $l = 200 μ m$ slot waveguide with an atom density of $N \approx 3.5 \times 10^{14} {cm}^{- 3}$ . The red curve is a fit of our theory model. The dotted lines indicate the transition frequencies from the ${Rb}^{85}$ $S_{1 / 2}$ $F = 2$ (i), $F = 3$ (ii) and ${Rb}^{87}$ $S_{1 / 2}$ $F = 1$ (iii), $F = 2$ (iv) to the $P_{3 / 2}$ states.
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Figure 3
Line shape parameters of the experimental spectra and Monte Carlo simulation results versus design slot width $w_{d}$ . Black squares show experimental data and the red areas correspond to simulation results, taking into account the uncertainty of the CP potential calculations. The gray shaded area indicates the interval $50 nm \leq w_{d} \leq 125 nm$ , where the slot is not yet completely developed. (a) Center frequency $ω_{0}$ of the $D_{2}$ spectrum relative to the center frequency obtained in a reference cell. (b) Voigt width $γ_{V}$ (FWHM). (c) Optical depth of the ${Rb}^{85}$ $5 S_{1 / 2}, F = 3 \to 5 P_{3 / 2}, F = 4$ transition. The simulation results for the OD are normalized to the maximum values of the measured OD and presented as relative OD (right ordinate) for a better comparison with the experimental data.
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Figure 4
Simulation result for the $w_{d} = 150 nm$ slot waveguide. The black dots show the scattered power $P_{sc}$ normalized by the input power $P_{in}$ as a function of detuning $Δ$ . Also shown is the corresponding Voigt fit (red line), from which we extract the relevant line shape parameters.
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Figure 5
Casimir-Polder line shift (a) and modified decay rate (b) plotted in the cross section of a waveguide with $w_{d} = 75 nm$ . Note that, in (a), the color scale corresponds to the logarithm of the absolute value of the shift. In the vicinity of the surface, the transition frequency is redshifted. However, at larger distances, a positive resonant potential contribution cancels (dark blue region) and overcomes the negative off-resonant potential such that the total shift becomes positive and the line is blueshifted. Further, the total decay rate $Γ$ of the $5^{2} P_{3 / 2}$ state (b) approaches its vacuum value of $Γ_{0}$ at large distances.
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Figure 6
Density-dependent measurements in the 50-nm slot waveguides. (a) Lorentzian linewidth for the $l = 30 μ m$ (blue circles) and $l = 200 μ m$ (green triangles) devices. (b) Corresponding line shift with linear fit.
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