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Numerical Study of Periodic Adiabatic Rapid Passage Sequences

Zhang, HE

Published Dec 1, 2015• 0 citations
OpenAlex

Abstract

Laser cooling refers to the techniques that cool atomic and molecular samples down to near absolute zero through the interaction with laser fields. Atoms absorb a photon and the transferred momentum generates force to lower the atom's velocity. The Doppler shift is associated with the motion of the atom, in which a moving atom always observes a slightly different frequency than the laser at rest. Based on the two-level system theory, this thesis studied the process of multiple sequences of adiabatic rapid passage by simulating the process of the interaction between optical forces and helium atoms. By studying the force map in parameter space, trying to understand the relationship between the average force and relevant laser parameters $\Omega_0$ and $\delta_0$ in the Rabi frequency. Furthermore, we studied the relationship between the average force and velocity by comparing the dragged atom model and moving atom model. We also studied the process of multiple sequences of adiabatic rapid passage by changing spontaneous emission, sweep range, pulse number for the interaction, exploring the distance between peaks that appear in the velocity distribution. The observational results will be presented in the thesis as well as the explanation. | 86 pages

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Optical Forces from Periodic Adiabatic Rapid Passage Sequences on Metastable Helium Atoms

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Over the past 30 years, optical manipulation of neutral atoms has been primarily performed with a monochromatic laser beam. The simplest tool for the control of atomic motion is the radiative force exerted by a monochromatic laser on a twolevel atom. The radiative force arises from absorption followed by spontaneous emission, and its magnitude is limited by the atom's excited state lifetime. The coherent momentum exchanges between light elds and atoms can be exploited to produce long-range optical forces much greater than the radiative force through the use of absorption-stimulated emission processes. Adiabatic Rapid Passage (ARP) is a long-existing method to invert the population of a two-level nuclear spin system. Its extension to the optical domain necessitates a frequency chirped light pulse to interact with a two-level atom via the dipole interaction. I will first present a numerical study of the properties of optical forces on moving atoms derived from purely stimulated processes produced by multiple ARP sequences. This will be followed by experimental observations of long-range ARP forces much larger than the radiative force in metastable helium. Sequences of properly timed laser pulses may be used for rapid deceleration of neutral atomic (or molecular) beams. | 171 pages

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We present a numerical study of the properties of optical forces on moving atoms derived from purely stimulated processes produced by multiple adiabatic rapid-passage sequences. The optical Bloch equations are solved for a carefully timed sequence of frequency-swept pulses that can produce a force much larger than the ordinary radiative force. We describe the effects of the sweep range, peak intensity, sweep direction, number of pulses, atomic velocity, and spontaneous emission. Since the momentum of thermal atoms is much larger than that transferred by a single absorption-stimulated emission cycle, multiple repetitions are needed to make a significant velocity change.

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The use of optical forces to precisely control the motion of atoms has played a major role in expanding the field of atomic physics. However, in a majority of experiments, these optical forces are limited in strength by the inherent properties of the chosen atoms, mainly the rates of spontaneous emission, required to return the atoms to their ground state. It has been shown that the process of absorption, followed by stimulated-emission, can be used to provide a more rapid return to the atomic ground state. Through careful design of the experiment, a coherent exchange of momentum can take place, exerting a large force on the atoms. Adiabatic Rapid Passage (ARP) is a technique commonly used in the magnetic resonance community for the population inversion of a two-level spin system. This technique has been expanded to the optical regime and used to invert the electronic states of an atomic system, allowing for control over both absorption and stimulated emission. It has been previously demonstrated that this scheme can be used to generate large forces on stationary atoms. An in-depth study of this ARP force will be presented with a strong emphasis on experiments designed to measure the dependence of the ARP force on atomic velocity. The experiments use the $2^3S\rightarrow2^3P$ transition in Helium at $\lambda=1.083\unit{\um}$, and results show, that by using a properly-tailored pulse sequence, the ARP force can exerts a large force on atoms over a large velocity range, making it a potential candidate to decelerate neutral atomic (or molecular) beams Numerical simulations of multiple ARP processes on two-level atoms were run with the hopes of gaining further insight into the subsequent force. These simulations have unveiled some interesting questions about the ARP process. These questions pertain to spontaneous emission's effect on the force and the influence of the phase of the optical pulses on the atomic state. While the simulations exhibit some overall similarities with the experimental results quantitative agreement is generally poor. Explanation for this disagreement is presented along with a roadmap for future work. | 91 pages

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The usual form of electromagnetic forces is given by but for neutral atoms q = 0. The next order of force is the dipole term, but this also vanishes because neutral atoms have no inherent dipole moment. However, a dipole moment can be induced by a field, and this is most efficient if the field is alternating near the atomic resonance frequency. Since these frequencies are typically in the optical range, dipole moments are efficiently induced by shining nearly resonant light on the atoms. Laser cooling and trapping rely on the interaction between laser light and atoms to exert a controllable force on the atoms, and many sophisticated schemes have been developed using the special properties of the interaction. This chapter begins with the simplest schemes for exerting optical forces on atoms, namely a single-frequency light field interacting with a two-level atom. Although this is the simplest possible scheme, it is pedagogically valuable because it shows many of the features that will be encountered further on. The ultimate temperature using such a scheme is limited, but lower temperatures can be achieved with multilevel atoms. This topic is addressed later in the chapter. Two kinds of optical forces When an atom absorbs nearly resonant light, it makes a transition to the excited state, and its subsequent return to the ground state can be either by spontaneous or by stimulated emission. The nature of the optical force that arises from these two different processes is quite different, and will be described separately [1]. The spontaneous emission case is different from the familiar quantum mechanical calculations using state vectors to describe the state of the system, because spontaneous emission causes the state of the system to evolve from a pure state into a mixed state (see Exercise 6.9). Spontaneous emission is an essential ingredient for the dissipative nature of the optical forces in monochromatic light, and the density matrix is needed to describe it (see Chap. 6). When an atom absorbs light, its energy excites the atom and its angular momentum changes the electron's orbit. Both of these are internal atomic properties. However, the linear momentum of the light, can change only the external, translational motion of the atom.

Cambridge University Press eBooks•Dec 31, 2015•0 citations
Laser cooling refers to the techniques that cool atomic and molecular samples down to near absolute zero through the interaction with laser fields. Atoms absorb a photon and the transferred momentum generates force to lower the atom's velocity. The Doppler shift is associated with the motion of the atom, in which a moving atom always observes a slightly different frequency than the laser at rest. Based on the two-level system theory, this thesis studied the process of multiple sequences of adiabatic rapid passage by simulating the process of the interaction between optical forces and helium atoms. By studying the force map in parameter space, trying to understand the relationship between the average force and relevant laser parameters Ω0\Omega_0Ω0​ and δ0\delta_0δ0​ in the Rabi frequency. Furthermore, we studied the relationship between the average force and velocity by comparing the dragged atom model and moving atom model. We also studied the process of multiple sequences of adiabatic rapid passage by changing spontaneous emission, sweep range, pulse number for the interaction, exploring the distance between peaks that appear in the velocity distribution. The observational results will be presented in the thesis as well as the explanation. | 86 pages
Over the past 30 years, optical manipulation of neutral atoms has been primarily performed with a monochromatic laser beam. The simplest tool for the control of atomic motion is the radiative force exerted by a monochromatic laser on a twolevel atom. The radiative force arises from absorption followed by spontaneous emission, and its magnitude is limited by the atom's excited state lifetime. The coherent momentum exchanges between light elds and atoms can be exploited to produce long-range optical forces much greater than the radiative force through the use of absorption-stimulated emission processes. Adiabatic Rapid Passage (ARP) is a long-existing method to invert the population of a two-level nuclear spin system. Its extension to the optical domain necessitates a frequency chirped light pulse to interact with a two-level atom via the dipole interaction. I will first present a numerical study of the properties of optical forces on moving atoms derived from purely stimulated processes produced by multiple ARP sequences. This will be followed by experimental observations of long-range ARP forces much larger than the radiative force in metastable helium. Sequences of properly timed laser pulses may be used for rapid deceleration of neutral atomic (or molecular) beams. | 171 pages
We present a numerical study of the properties of optical forces on moving atoms derived from purely stimulated processes produced by multiple adiabatic rapid-passage sequences. The optical Bloch equations are solved for a carefully timed sequence of frequency-swept pulses that can produce a force much larger than the ordinary radiative force. We describe the effects of the sweep range, peak intensity, sweep direction, number of pulses, atomic velocity, and spontaneous emission. Since the momentum of thermal atoms is much larger than that transferred by a single absorption-stimulated emission cycle, multiple repetitions are needed to make a significant velocity change.
The use of optical forces to precisely control the motion of atoms has played a major role in expanding the field of atomic physics. However, in a majority of experiments, these optical forces are limited in strength by the inherent properties of the chosen atoms, mainly the rates of spontaneous emission, required to return the atoms to their ground state. It has been shown that the process of absorption, followed by stimulated-emission, can be used to provide a more rapid return to the atomic ground state. Through careful design of the experiment, a coherent exchange of momentum can take place, exerting a large force on the atoms. Adiabatic Rapid Passage (ARP) is a technique commonly used in the magnetic resonance community for the population inversion of a two-level spin system. This technique has been expanded to the optical regime and used to invert the electronic states of an atomic system, allowing for control over both absorption and stimulated emission. It has been previously demonstrated that this scheme can be used to generate large forces on stationary atoms. An in-depth study of this ARP force will be presented with a strong emphasis on experiments designed to measure the dependence of the ARP force on atomic velocity. The experiments use the 23S→23P2^3S\rightarrow2^3P23S→23P transition in Helium at $\lambda=1.083\unit{\um}$, and results show, that by using a properly-tailored pulse sequence, the ARP force can exerts a large force on atoms over a large velocity range, making it a potential candidate to decelerate neutral atomic (or molecular) beams Numerical simulations of multiple ARP processes on two-level atoms were run with the hopes of gaining further insight into the subsequent force. These simulations have unveiled some interesting questions about the ARP process. These questions pertain to spontaneous emission's effect on the force and the influence of the phase of the optical pulses on the atomic state. While the simulations exhibit some overall similarities with the experimental results quantitative agreement is generally poor. Explanation for this disagreement is presented along with a roadmap for future work. | 91 pages
Periodic adiabatic rapid passage from counterpropagating light pulses is used in a parameter domain outside of the conventionally expected successful range to produce large optical forces on atoms by coherent control of the momentum exchange between the light and atoms. Both the magnitude and velocity range are much larger than those of the usual radiative force. We observed the force on metastable He atoms by the deflection of the atomic beam with periodic pulse sequences. We have also been able to map out the force magnitude in the two-dimensional parameter space of peak Rabi frequency and sweep range, and we find qualitative agreement with our model.
We study the force exerted on two-level atoms by short, counterpropagating laser pulses. When the counterpropagating pulses overlap each other partially, multiphoton adiabatic processes are possible in several configurations, which amplify the force exerted on the atoms. We investigate the practical usefulness of such multiphoton adiabatic transitions for the manipulation of the atoms' mechanical state. In particular, we compare the efficiency of a pair of constant frequency, oppositely detuned laser pulses and that of a pair of frequency-chirped pulses. We also consider the case of prolonged exposure to a sequence of laser pulses for a duration that is comparable to or much larger than the spontaneous lifetime of the atoms. We use numerical methods to calculate the reduction of the force and the heating of the atomic ensemble when spontaneous emission cannot be neglected during the interaction. In addition, we derive simple approximate formulas for the force and the heating, and compare them to the numerical results.
The usual form of electromagnetic forces is given by but for neutral atoms q = 0. The next order of force is the dipole term, but this also vanishes because neutral atoms have no inherent dipole moment. However, a dipole moment can be induced by a field, and this is most efficient if the field is alternating near the atomic resonance frequency. Since these frequencies are typically in the optical range, dipole moments are efficiently induced by shining nearly resonant light on the atoms. Laser cooling and trapping rely on the interaction between laser light and atoms to exert a controllable force on the atoms, and many sophisticated schemes have been developed using the special properties of the interaction. This chapter begins with the simplest schemes for exerting optical forces on atoms, namely a single-frequency light field interacting with a two-level atom. Although this is the simplest possible scheme, it is pedagogically valuable because it shows many of the features that will be encountered further on. The ultimate temperature using such a scheme is limited, but lower temperatures can be achieved with multilevel atoms. This topic is addressed later in the chapter. Two kinds of optical forces When an atom absorbs nearly resonant light, it makes a transition to the excited state, and its subsequent return to the ground state can be either by spontaneous or by stimulated emission. The nature of the optical force that arises from these two different processes is quite different, and will be described separately [1]. The spontaneous emission case is different from the familiar quantum mechanical calculations using state vectors to describe the state of the system, because spontaneous emission causes the state of the system to evolve from a pure state into a mixed state (see Exercise 6.9). Spontaneous emission is an essential ingredient for the dissipative nature of the optical forces in monochromatic light, and the density matrix is needed to describe it (see Chap. 6). When an atom absorbs light, its energy excites the atom and its angular momentum changes the electron's orbit. Both of these are internal atomic properties. However, the linear momentum of the light, can change only the external, translational motion of the atom.