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Optics Express

Monitoring Raman emission through state population in cold atoms confined inside a hollow-core fiber

Taehyun Yoon, Zhenghao Ding, Jeremy Flannery, Fereshteh Rajabi, Michal Bajcsy

Published Jun 11, 2019• 3 citations
Publisher

Abstract

We study the spontaneous Raman emission in an ensemble of laser-cooled three-level Λ-type atoms confined inside a hollow-core photonic-bandgap fiber using a novel approach to observe the process. Instead of detecting the emitted light, we measure the number of atoms in the ground state as a function of Raman pump time, which eliminates the need to suppress the pump photons with a high-resolution filter. We describe how this measurement can be used to detect superradiant emission from the atomic ensembles and estimate the number of atoms required to observe Raman superradiance in atomic clouds inside a hollow-core fiber.

DOI
10.1364/oe.27.017592

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•May 23, 2019•0 citations
We study the spontaneous Raman emission in an ensemble of laser-cooled three-level Λ-type atoms confined inside a hollow-core photonic-bandgap fiber using a novel approach to observe the process. Instead of detecting the emitted light, we measure the number of atoms in the ground state as a function of Raman pump time, which eliminates the need to suppress the pump photons with a high-resolution filter. We describe how this measurement can be used to detect superradiant emission from the atomic ensembles and estimate the number of atoms required to observe Raman superradiance in atomic clouds inside a hollow-core fiber.
Cold atoms confined inside a hollow core photonic crystal fiber with core diameter of a few photon wavelengths are a promising medium for studying nonlinear optical interactions at extremely low light levels. The high electric field intensity per photon, large optical depths, and interaction lengths not limited by diffraction are some of the unique features of this system. We describe recent progress in our experiment that uses a combination of magnetic trapping and a red-detuned optical dipole trap to transfer cold rubidium atoms into the hollow-core fiber. We present data on transfer efficiencies as well as preliminary experiments towards nonlinear optics with few photon pulses.
We present a thorough experimental investigation of the loading process of laser-cooled atoms from a magneto-optical trap into an optical dipole trap located inside a hollow-core photonic bandgap fiber, followed by propagation of the atoms therein. This, e.g., serves to identify limits to the loading efficiency and thus optical depth which is a key parameter for applications in quantum information technology. Although only limited access in 1D is available to probe atoms inside such a fiber, we demonstrate that a detailed spatially resolved characterization of the loading and trapping process along the fiber axis is possible by appropriate modification of probing techniques combined with theoretical analysis. Specifically, we demonstrate the loading of up to $2.1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms with a transfer efficiency of 2.12.1%2.1 during the course of 50 ms and a peak loading rate of $4.7\ifmmode\times\else\texttimes\fi{}{10}^{3}$ atoms ${\mathrm{ms}}^{\ensuremath{-}1}$ resulting in a peak atomic number density on the order of ${10}^{12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Furthermore, we determine the evolution of the spatial density (profile) and ensemble temperature as it approaches its steady-state value of $T=1400\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$, as well as loss rates, axial velocity and acceleration. The spatial resolution along the fiber axis reaches a few millimeters, which is much smaller than the typical fiber length in experiments. We compare our results to other fiber-based as well as free-space optical dipole traps and discuss the potential for further improvements.
We report loading of laser-cooled cesium atoms into a hollow-core photonic-band gap fiber and confining the atoms in the fiber's $7\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-diameter core with a magic-wavelength dipole trap at $\ensuremath{\sim}935$ nm. The use of the magic wavelength removes the ac Stark shift of the 852 nm optical transition in cesium caused by the dipole trap in the fiber core and suppresses the inhomogeneous broadening of the atomic ensemble that arises from the radial distribution of the atoms. This opens the possibility to continuously probe the atoms over timescales of a millisecond---approximately 1000 times longer than what was reported in previous works, because the dipole trap does not have to be modulated. We describe our atom-loading setup and its unique features and present spectroscopy measurements of the cesium's D2{D}_{2}D2​ line in the continuous-wave dipole trap with up to $1.7\ifmmode\times\else\texttimes\fi{}{10}^{4}$ loaded inside the hollow-core fiber.
In this thesis, I am intending to understand the cooperative effect of an ensemble of quantum emitters, which constitutes the preliminary elements of our current experimental investigations towards realization of an ultra-narrow linewidth superriant laser. In the first part of the thesis, I investigate the basics of the theory of superradiance (SR), which includes the full derivation of the Hamiltonian and the Lindblad equation for an ensemble of two-level atoms in both free-space and a single-mode waveguide. In addition, I construct the simulations for observing the transition from single-atom uncorrelated spontaneous emission to superradiance in various physical settings, as well as a simulation for the understanding of the cooperative effects of an ensemble of two-level atoms inside an optical cavity. Then, in the second part of the thesis, I introduce the experimental progress we have been making to observe SR with an ensemble of laser-cooled Cs atoms inside a hollow-core photonic crystal fiber (HCPCF). In our experiment, the Cs atoms, initially cooled using a magneto-optical trap (MOT), are guided and confined inside a short piece of HCPCF with a magic-wavelength dipole trap. Currently we have successfully implemented a novel detection methods for studying superradiance.