Nanofibers are a valuable tool for interfacing photons with atomic media through its guided mode which propagates evanescent to the fiber. However, the nanofiber modifies the natural decay rate of the atomic dipoles due to the Purcell effect. It is vital to study these atom-fiber interactions to characterize the system. This thesis delineates an experimental technique to obtain the spontaneous emission rate of a cloud of Rubidium-87 atoms in the presence of a nanofiber. The atoms are prepared in a magneto optical trap and the emissions are monitored by balanced homodyne detection. The temporal autocorrelation function of the homodyne’s photocurrent reproduces the exponential profile of spontaneous emission. The measured time constant of the profile is 26.3 ns which is almost twice that of free space emission. The modified decay rate depends on the atom’s orientation and distance from the nanofiber surface. We theoretically estimate the modified decay rate and model the expected decay profile for our ensemble of atoms which agrees well with our measurements. To our knowledge, this is the first experimental demonstration of spontaneous emission being measured by homodyne detection.
Nanofibers are a valuable tool for interfacing photons with atomic media through its guided mode which propagates evanescent to the fiber. However, the nanofiber modifies the natural decay rate of the atomic dipoles due to the Purcell effect. It is vital to study these atom-fiber interactions to characterize the system. This thesis delineates an experimental technique to obtain the spontaneous emission rate of a cloud of Rubidium-87 atoms in the presence of a nanofiber. The atoms are prepared in a magneto optical trap and the emissions are monitored by balanced homodyne detection. The temporal autocorrelation function of the homodyne’s photocurrent reproduces the exponential profile of spontaneous emission. The measured time constant of the profile is 26.3 ns which is almost twice that of free space emission. The modified decay rate depends on the atom’s orientation and distance from the nanofiber surface. We theoretically estimate the modified decay rate and model the expected decay profile for our ensemble of atoms which agrees well with our measurements. To our knowledge, this is the first experimental demonstration of spontaneous emission being measured by homodyne detection.
We perform balanced homodyne detection of the electromagnetic field in a single-mode tapered optical nanofiber surrounded by rubidium atoms in a magneto-optical trap. Resonant fluorescence of atoms into the nanofiber mode manifests itself as increased quantum noise of the field quadratures. The autocorrelation function of the homodyne detector's output photocurrent exhibits exponential fall-off with a decay time constant of 26.3±0.6 ns, which is consistent with the theoretical expectation under our experimental conditions. To the best of our knowledge, this is the first experiment in which fluorescence into a tapered optical nanofiber has been observed and measured by balanced optical homodyne detection.
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 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% over the free-space rate matches well with what one finds from mode simulations in our system.
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.
We perform balanced homodyne detection of the electromagnetic field in a single-mode tapered optical nanofiber surrounded by rubidium atoms in a magneto-optical trap. Resonant fluorescence of atoms into the nanofiber mode manifests itself as increased quantum noise of the field quadratures. The autocorrelation function of the homodyne detector's output photocurrent exhibits exponential fall-off with a decay time constant of 26.3±0.6 ns, which is consistent with the theoretical expectation under our experimental conditions. To our knowledge, this is the first experiment in which fluorescence has been observed and measured by balanced optical homodyne detection.
We experimentally and theoretically investigate collective radiative effects in an ensemble of cold atoms coupled to a single-mode optical nanofiber. Our analysis unveils the microscopic dynamics of the system, showing that collective interactions between the atoms and a single guided photon gradually build up along the atomic array in the direction of propagation of light. These results are supported by time-resolved measurements of the light transmitted and reflected by the ensemble after excitation via nanofiber-guided laser pulses, whose rise and fall times are shorter than the atomic lifetime. Superradiant decays more than 1 order of magnitude faster than the single-atom free-space decay rate are observed for emission in the forward-propagating guided mode, while at the same time, no speed-up of the decay rate is measured in the backward direction. In addition, position-resolved measurements of the light that is transmitted past the atoms are performed by inserting the nanofiber-coupled atomic array in a 45-m-long fiber ring resonator, which allow us to experimentally reveal the progressive growth of the collective response of the atomic ensemble. Our results highlight the unique opportunities offered by nanophotonic cold atom systems for the experimental investigation of collective light-matter interaction.
This thesis investigates the cascaded photon interactions and spectral properties of a waveguide quantum electrodynamics system, with a focus on the interaction between spontaneous emission and a distant ensemble of laser-cooled Cesium atoms coupled to an optical nanofiber. We fabricate optical nanofibers with a diameter of approximately 400 nm using a tapering machine and integrate the nanofibers into the system in which the atoms in a magneto-optical trap interact with the nanofiber mode. We explore how spontaneous emission from a first atomic ensemble, reflected by a distant fiber Bragg grating mirror, interacts with a second ensemble, forming a cascaded configuration. Spectral measurements reveal broadening effects consistent with reabsorption and dephasing mechanisms. These effects are incorporated into theoretical models, yielding agreement with experimental observations and providing insight into non-Markovian dynamics. Additionally, we implement a two-color dipole trap and evaluate the coupling efficiency of trapped atoms to the optical nanofibers.