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Optics Express

Investigation of the transit-time broadening in optical nanofiber based spectroscopy

Yun Qi, Wei Jin, Hoi Lut Ho

Published Jan 22, 2020• 1 citation
Publisher

Abstract

Optical nanofiber is a widely adopted platform for highly efficient light-matter interaction by virtue of its exposed evanescent field with high light intensity. However, the strongly constrained mode field with the wavelength-scale size makes the light-matter interaction time limited in consideration of the random thermal motion of warm molecules, which results in considerable transit-time dephasing and thus line broadening. Here we report a systematic study of the transit-time effect associated with the optical nanofibers. Both simulation and experiment for nanofibers exposed in acetylene demonstrate the considerable transit-time broadened linewidth in the low-pressure range.

DOI
10.1364/oe.387343

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In this thesis, we demonstrate the measurement of laser inducedfluorescence (LIF) emission spectra of strongly driven atoms using optical nanofibers.For the fluorescence emission spectrum measurements, we develop and demonstrate anew sensitive and high resolution method consisting of optical heterodyne techniqueand photon correlation spectroscopy. The key point of the method is to extract theinformation of first-order field correlation function from the measured second-orderintensity correlation function. The Fourier transform of the field correlation functionthen gives the fluorescence emission spectrum.The atoms emitting fluorescence in the guided-modes of the nanofiber lie very closeto the nanofiber surface and are subjected to atom-surface interactions. It has beenreported in earlier experiments on nanofiber, that such surface interactions can bemodified by irradiating the MOT-atoms overlapped with the nanofiber with UV lasers.After such irradiations, MOT-atoms are kept from falling into the surface-potentialand behave almost like free-space atoms around the nanofiber. We first perform ourfluorescence spectra measurements under this free-atom conditions. The observedexcitation spectrum lineshape shows a free-atom like Lorentzian behavior, with avery small tail in the red-detuned side (red-tail). The red-tail has been attributedto photoassociation process, due to atoms making transition from free ground statesto bound excited states of the surface potential. Under this conditions, the emissionspectra measured for on-resonant and off-resonant excitations show the well-knownthree-peak Mollow-triplet spectrum of free-space two-level atoms. The observationconfirms that the near surface atoms behave like free-space atoms. 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The evolution of thespectrum lineshape has been attributed to the changing nanofiber surface condition.The spectrum shape is found to depend on the flux of background Cs-atoms. Wespeculate that this might be due to the adsorption of hot background atoms ontothe nanofiber surface, which changes the surface condition of the nanofiber locallyand helps in loading of cold MOT-atoms into the shallow surface potential. We havefound that the effect of UV laser irradiation is to photoionize the MOT-atoms, andthe generated photoelectrons might help in the desorption of adsorbed atoms on thesurface. We discuss the roles of adsorption and desorption of atoms on the surfaceconditions. We have shown that such changing surface condition can be controlledby adjusting the hot background atom flux and the UV laser intensity. Under suchcontrolled surface condition, we have measured the emission spectrum. The observedemission spectrum under the broad red-tail condition can be explained only by includingthe surface interactions. We also discuss qualitatively the phonon-mode densityfor nanofiber at room temperature and the phonon-mediated decay of atoms in thesurface potential.This work may open up the way for using optical nanofibers as a tool to investigatethe laser-induced fluorescence spectrum of strongly driven atoms, and to study atomsurfaceinteractions. In the future, we hope to create optical dipole traps around thenanofibers by introducing trap laser lights through the guided-modes and performfluorescence measurements of these trapped atoms.

•Jan 1, 2011•0 citations
Optical nanofiber is a widely adopted platform for highly efficient light-matter interaction by virtue of its exposed evanescent field with high light intensity. However, the strongly constrained mode field with the wavelength-scale size makes the light-matter interaction time limited in consideration of the random thermal motion of warm molecules, which results in considerable transit-time dephasing and thus line broadening. Here we report a systematic study of the transit-time effect associated with the optical nanofibers. Both simulation and experiment for nanofibers exposed in acetylene demonstrate the considerable transit-time broadened linewidth in the low-pressure range.
In a chip-based waveguide, the confined nature of the optical field and proximity of the waveguide's surface must not be ignored. The short transit time of warm atoms through highly confined light fields can strongly influence the observed atomic absorption spectra. Most theoretical studies have focused on fields with Gaussian intensity profiles, but is that realistic? Here the authors model the transit-time-limited interaction between atomic gas and an exponentially decaying evanescent field, and find spectral lineshapes unlike those for a Gaussian field. These simulations compare well to experiments on Rb vapor and the evanescent guided modes of an exposed-core optical fiber.
New effects can arise in quantum physics when there is strong coupling, either between atoms and light or between different quantum systems. This thesis examines an optical nanofiber atom trap as a mediator of atom-light interactions and a potential element of a hybrid quantum system. The evanescent field around the sub-wavelength waist of an optical nanofiber possesses a small mode area that increases the cooperativity between atoms and the mode, in a manner analogous to traditional cavity QED. We demonstrate trapping of 87^{87}87Rb atoms with an optical nanofiber, confining hundreds of atoms with typical trap lifetimes of tens of milliseconds. We then employ single photon counting techniques to study untrapped ensembles of cold atoms around the nanofiber. A first experiment uses intensity autocorrelations of resonance fluorescence emitted into the nanofiber mode to observe a transition from classical to nonclassical photon statistics. Measuring the correlations on longer timescales reveals the motion of atoms through the optical mode, and we develop a correspondence between the transit time and atomic cloud temperature. A second experiment measures Purcell enhancement of spontaneous emission of atoms near the nanofiber by correlating their fluorescence with a known trigger event. The spontaneous decay rate of an atom near a dielectric is modified by the induced dipole and by a change in the modes of the vacuum electromagnetic field. Our observed enhancement of 6.5±0.9%6.5\pm0.9\%6.5±0.9% over the free-space rate matches well with what one finds from mode simulations in our system.
In this thesis I present a developing project to elaborate a scientific platform towards the realization of light-matter interaction based on tapered optical nanofiber (TNF) that enables optical fields to interface with trapped neutral atoms in their vicinity. In such a configuration, the guided light exhibits a large portion of evanescent field into the vacuum thus laser cooled atoms can couple to the fiber mode. By trapping and cooling atoms around the nanofiber I investigated the interaction of the atoms with the field. My work presented in this thesis includes: Calculating the evanescent modes propagating through our fiber; Implementing fiber pulling setup using oxyhydrogen flame and analyzing nanofiber’s thermal dynamics and degradation; Constructing a Rubidium 87 MOT and characterizing its critical parameters; Realizing some atom-light interface through an experiment on coupling spontaneously emitted photons from MOT into TNF; Calculating two-color dipole trap parameters for our nanofiber.
Tapered optical fiber with subwavelength diameter waist, an optical nanofiber, provides a unique and versatile platform for manipulating atoms and photons. In the guided modes of the nanofibers, the optical field can be tightly confined in the transverse direction while enabling strong interaction with the surrounding medium in the evanescent region. Combining laser-cooled atoms with nanofibers has enabled surprising quantum optics experiments, e.g. the efficient channeling of emission from single atoms into the fiber-guided modes, spectroscopy of near-surface atoms, high optical depth with an array of atoms optically trapped around the nanofiber, atomic memories and Bragg reflectors for fiber-guided photons, chiral light-matter interaction etc. In addition, using moderate longitudinal confinement in nanofiber cavities has enabled strong coupling between a single atom and fiber-guided photons. In this article, I review some of the key experimental demonstrations on the “atom＋nanofiber” platform.
Fluorescence of atoms has been the subject of interest in quantum optical measurementsfor many years. The atomic fluorescence is routinely used as a tool to detectatoms, to generate photons and to generate non-classical states of light. One of thewidely used methods to investigate the atomic fluorescence is laser-induced fluorescencespectroscopy. This thesis presents the development of optical nanofibers as atool for the measurement of fluorescence spectrum of atoms.Optical nanofibers are ultrathin optical fibers, with diameters less than the propagationwavelength of the fiber, with a silica core and a vacuum clad. Such opticalnanofibers can support only the single propagating guided-mode. The guided-modesof the nanofiber is confined in the transverse direction of the nanofiber and a partof the propagating field lies outside the fiber, in the evanescent region of the fiber.The mode density of the electromagnetic field is modified by the nanofiber. Due tothe modification of mode density, the spontaneous emission of atoms is modified bythe optical nanofibers and the confinement of the guided-modes results in a strongchanneling of spontaneous emission of atoms into the guided-modes of the nanofiber.Such a property of nanofibers can be used to study fluorescence of atoms around thenanofiber.Optical nanofibers are prepared by the adiabatic tapering of single-mode opticalfibers. Our experiments are performed by overlapping cold Cesium atoms, from amagneto-optical trap (MOT), with the optical nanofiber and observing the fluorescencethrough the guided-modes of the nanofiber. The nanofiber suppresses with highprobability any coupling of scattering light from the excitation laser into the guidedmodesand high coupling efficiency of fluorescence into the guided-modes enables us tomeasure the fluorescence spectrum of atoms present around the nanofiber with a goodsignal-to-noise ratio. In this thesis, we demonstrate the measurement of laser inducedfluorescence (LIF) emission spectra of strongly driven atoms using optical nanofibers.For the fluorescence emission spectrum measurements, we develop and demonstrate anew sensitive and high resolution method consisting of optical heterodyne techniqueand photon correlation spectroscopy. The key point of the method is to extract theinformation of first-order field correlation function from the measured second-orderintensity correlation function. The Fourier transform of the field correlation functionthen gives the fluorescence emission spectrum.The atoms emitting fluorescence in the guided-modes of the nanofiber lie very closeto the nanofiber surface and are subjected to atom-surface interactions. It has beenreported in earlier experiments on nanofiber, that such surface interactions can bemodified by irradiating the MOT-atoms overlapped with the nanofiber with UV lasers.After such irradiations, MOT-atoms are kept from falling into the surface-potentialand behave almost like free-space atoms around the nanofiber. We first perform ourfluorescence spectra measurements under this free-atom conditions. The observedexcitation spectrum lineshape shows a free-atom like Lorentzian behavior, with avery small tail in the red-detuned side (red-tail). The red-tail has been attributedto photoassociation process, due to atoms making transition from free ground statesto bound excited states of the surface potential. Under this conditions, the emissionspectra measured for on-resonant and off-resonant excitations show the well-knownthree-peak Mollow-triplet spectrum of free-space two-level atoms. The observationconfirms that the near surface atoms behave like free-space atoms. Also, the quantitativeagreement between the observed spectra with the theoretical calculationsdemonstrates the validity of the optical nanofiber method for measuring fluorescenceemission spectrum of atoms.As mentioned above, since we are observing near-surface atoms, fluorescence measurementsbased on optical nanofibers may become a unique tool to probe atomsurfaceinteractions. In earlier experiments using nanofiber, signatures of atomsurfaceinteraction have been observed in the excitation spectrum of atoms, in theform of a broad red-tail. The lineshape of the excitation spectrum is understood as aresult of a process where atoms approaching the nanofiber surface fall into the shallowsurface potential, forming long-range atom-surface bound states. In this thesis,we have systematically investigated the evolution of the lineshape of the excitationspectrum from a Lorentzian shape to a broad red-tail condition. The evolution of thespectrum lineshape has been attributed to the changing nanofiber surface condition.The spectrum shape is found to depend on the flux of background Cs-atoms. Wespeculate that this might be due to the adsorption of hot background atoms ontothe nanofiber surface, which changes the surface condition of the nanofiber locallyand helps in loading of cold MOT-atoms into the shallow surface potential. We havefound that the effect of UV laser irradiation is to photoionize the MOT-atoms, andthe generated photoelectrons might help in the desorption of adsorbed atoms on thesurface. We discuss the roles of adsorption and desorption of atoms on the surfaceconditions. We have shown that such changing surface condition can be controlledby adjusting the hot background atom flux and the UV laser intensity. Under suchcontrolled surface condition, we have measured the emission spectrum. The observedemission spectrum under the broad red-tail condition can be explained only by includingthe surface interactions. We also discuss qualitatively the phonon-mode densityfor nanofiber at room temperature and the phonon-mediated decay of atoms in thesurface potential.This work may open up the way for using optical nanofibers as a tool to investigatethe laser-induced fluorescence spectrum of strongly driven atoms, and to study atomsurfaceinteractions. In the future, we hope to create optical dipole traps around thenanofibers by introducing trap laser lights through the guided-modes and performfluorescence measurements of these trapped atoms.