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Hollow Bessel Tunnel Guide: Theory and Experiment

Narupon Chattrapiban

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

The main objectives of this thesis were to create and investigate properties and applications of all-optical atomic guides, which are potential building blocks for atom optics experiments. Specifically, we investigated experimentally and theoretically the evolution of the transverse and longitudinal atomic distributions in blue-detuned Bessel tunnels. These tunnels provide confining potentials that are comparable to or higher than the average kinetic energy of cold hundreds of microKelvin) ensembles. The new work highlighted in this thesis include investigations of the following: 1) the generation of Bessel beams using a 2D spatial light modulator (SLM) (both experimental measurement and numerical simulations were performed), 2) the efficiency of the SLM for converting light from a Gaussian beam to a Bessel beam, 3) the behavior of several propagating structures that might be useful for guiding atoms, 4) the atom-tunnel dynamics, 5) an atom switch and delay line (along with measurements of the switch efficiency), and 6) the single-mode propagation in an idealized tunnel. To the best of our knowledge, the work in this thesis constitutes the first time that (1) an atom switch and a delay line have been demonstrated using blue detuned unnels and (2) the atom-tunnel dynamics and thephase-space distribution of a non-interacting dilute gas in a non-equilibrium state has been tracked experimentally.

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The main objectives of this thesis were to create and investigate properties and applications of all-optical atomic guides, which are potential building blocks for atom optics experiments. Specifically, we investigated experimentally and theoretically the evolution of the transverse and longitudinal atomic distributions in blue-detuned Bessel tunnels. These tunnels provide confining potentials that are comparable to or higher than the average kinetic energy of cold hundreds of microKelvin) ensembles. The new work highlighted in this thesis include investigations of the following: 1) the generation of Bessel beams using a 2D spatial light modulator (SLM) (both experimental measurement and numerical simulations were performed), 2) the efficiency of the SLM for converting light from a Gaussian beam to a Bessel beam, 3) the behavior of several propagating structures that might be useful for guiding atoms, 4) the atom-tunnel dynamics, 5) an atom switch and delay line (along with measurements of the switch efficiency), and 6) the single-mode propagation in an idealized tunnel. To the best of our knowledge, the work in this thesis constitutes the first time that (1) an atom switch and a delay line have been demonstrated using blue detuned unnels and (2) the atom-tunnel dynamics and thephase-space distribution of a non-interacting dilute gas in a non-equilibrium state has been tracked experimentally.
This thesis is concerned with the guiding of cold atoms using optical forces, which is of great importance in the field of atom optics. Atomic beams can be used for precision sensor equipment, building nano-scale structures, construction of quantum computers and to further the understanding of the properties of atoms. Atoms are guided along light beams using the dipole force; there are two regimes under which this force works. Typically red-detuned guides are used (atoms are attracted towards the light) such guides, however, require large detuning and high powers. In this thesis we investigate the use of blue-detuned (atoms are repelled from the light) hollow light beams of moderate power (a few hundred mW) and confine atoms in the dark centre of the beams. Several magneto-optical traps (MOTs) have been constructed to exploit different guiding geometries. Hollow beams have been generated using a variety of methods; in particular the use of a computer controlled spatial light modulator (SLM) has provided great versatility and simplicity to the experimental arrangements. First, experiments were performed with a low-velocity intense source (LVIS) of atoms. A co-linear LG beam significantly enhances the observed flux, however, considerable difficulties are encountered loading atoms into oblique guides. Imaging a hole in the walls of the light tube was used to improve the loading efficiency. Second, guiding a free-falling atom cloud is performed using a non-diffracting Bessel beam. It is found that while the potential of the Bessel beam is steeper than equivalent LG beams the power distribution across the beam severely limits its usefulness. The next study investigated higher-order LG guide beams generated with an SLM. High order modes have a narrower profile so confine the atoms with less interaction with the guide beam, leading to a more natural guide (as opposed to a pushing force). Finally the SLM was used to create non-trivial beam shapes for beam splitters and interferometers.
In this dissertation we report the results of two experimental projects with laser-cooled rubidium atoms: I. Application of Bessel beams for atom optics, and II. Spectroscopic measurements of Rydberg blockade effect. The first part of the thesis is devoted to the development of new elements of atom optics based on blue-detuned high-order Bessel beams. Properties of a 4th order Bessel beam as an atomic guide were investigated for various parameters of the hollow beam, such as the detuning from an atomic resonance, size and the order of the Bessel beam. We extended its application to create more complicated interferometer-type structures by demonstrating a tunnel lock, a novel device that can split an atomic cloud, transport it, delay, and switch its propagation direction between two guides. We reported a first-time demonstration of an atomic beam switch based on the combination of two crossed Bessel beams. We achieved the 30% efficiency of the switch limited by the geometrical overlap between the cloud and the intersection volume of the two tunnels, and investigate the heating processes induced by the switch. We also showed other applications of crossed Bessel beams, such as a 3-D optical trap for atoms confined in the intersection volume of two hollow beams and a splitter of the atomic density. The second part of this dissertation is devoted to the spectroscopic measurements of the Rydberg blockade effect, a conditional suppression of Rydberg excitations depending on the state of a control atom. We assembled a narrow-linewidth, tunable, frequency stabilized laser system at 480 nm to excite laser-cooled rubidium atoms to Rydberg states with a high principal quantum number n ~ 50 through a two-photon transition. We applied the laser system to observe the Autler-Townes splitting of the intermediate 5p state and used the broadening of the resonance features to investigate the enhancement of Rydberg-Rydberg interactions in the presence of an external electric field.
The longitudinal and transverse evolution of thermal clouds have been studied experimentally and theoretically in blue-detuned hollow tunnels. Tunnels based on axicon generation and holographic phase-mask generation have been investigated. A simple model is presented that (1) accounts for longitudinal acceleration and (2) shows that a cloud confined in a tunnel with a potential having a Bessel mode distribution will absorb fewer photons than it would confined in a comparable tunnel with a Laguerre-Gaussian mode distribution. The longitudinal and transverse profiles are fit to analytical distributions functions from which we extract transverse and longitudinal temperatures of the cloud. We find the two temperatures to be very different, with the transverse temperature being as much as five times colder. Finally, we studied the energy level structure within a Bessel potential theoretically and found that single-mode propagation is possible.
Firstly the interaction of a three-level /spl Lambda/-type atom with a blue-detuning dark hollow beam is described in the dressed-atom picture. The dipole potential and radiation pressure forces are derived, and the spontaneous emission processes between both adjacent manifolds are analyzed in detail. Then through Monte Carlo simulation, the relations between the guiding efficiency and the detuning in varieties of conditions are acquired. We found that the guiding efficiency also depends on guiding direction relative to the propagation direction of the hollow beam except for the intensity and configuration of the hollow beam and pressure of the chamber, when both directions are opposite, the optimal detuning is in high detuning region, the value is similar to the experimental value. Furthermore, we calculated the final velocity distribution of atoms guided by the hollow beam, we found that in same direction case, even though in low detuning region the guiding efficiency is high, but the radial and longitudinal coherence are seriously destroyed, the most atoms are much heated. It is not useful in application to atom optics experiments. The main factors are the radiation pressure forces and photon scattering effects. Finally we found that the atoms can only be guided in the high detuning region, and their coherence will be held whether same or opposite direction.
We explore a scheme for guiding cold atoms through a hollow Bessel beam generated by a single axicon and a lens from a two-dimensional magneto-optical trap toward a science chamber. We compare the Bessel beam profiles measured along the optical axis to a numerical propagation of the beam’s wavefront, and we show how it is affected by diffraction during the passage through a long narrow funnel serving as a differential pumping tube between the chambers. We derive an approximate analytic expression for the intensity distribution of the Bessel beam and the dipolar optical force acting on the atoms. By a Monte-Carlo simulation based on a stochastic Runge–Kutta algorithm of the motion of atoms initially prepared at a given temperature, we show that a considerable enhancement of the transfer efficiency can be expected in the presence of a sufficiently intense Bessel beam.
We present a general theory of atom guiding in a blue-detuned hollow laser beam. Using the dressed-atom approach, we obtain the mean dipole gradient force, the radiation pressure force, and the momentum diffusion coefficients for three-level Λ-type cold atoms. Using Monte Carlo simulation, we calculate the guiding efficiencies and the final velocity distributions of atoms for various conditions. We find that the guiding efficiency depends not only on the intensity and detuning of the guiding hollow beam but also on the atom-guiding direction with respect to the propagation direction of the hollow laser beam. Comparing our analyses with recent experimental results, we find that they are mutually consistent. The results that we present can also be applied to atom guiding by hollow optical fibers.