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EXCITATION AND PHOTOIONIZATION OF LASER-COOLED ATOMS IN THE AC-DRIVEN MAGNETO-OPTICAL TRAP (AC-MOT), USING RESONANT AND NON-RESONANT RADIATION.

John Chidowerem Agomuo

Published Mar 27, 2015• 0 citations
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Abstract

ABSTRACTCandidate?s Names: John AgomuoDegree Title: PhDThesis Title: Excitation And Photoionization Of Laser-Cooled Atoms In The AC Driven Magneto-Optical Trap (AC-MOT), Using Resonant And Non-Resonant Radiation.Date: 19/12/2014Results of experiments involving the manipulation of atoms using laser radiation are presented. A high-density trap of potassium atoms was created using an AC-driven magneto-optical trap (AC-MOT), loaded from a Zeeman-slowed atomic beam. The AC-MOT was operated in a pulsed mode, with a trapping laser that was detuned by -4?, where ? is the natural line-width of the trapping transition. Under these conditions, the AC-MOT trapped an average of 3.63�0.02�?10?^8 atoms, which were cooled to a mean temperature of 275?K�33?K.Using the AC-MOT, the absolute photoionization cross-section of cold potassium atoms was determined using ionizing laser radiation at 450.4 nm obtained from a high-power diode laser. The measured photoionization cross-section from the 42P3/2 excited state was obtained by measuring the loss rate and the lifetime of the AC-MOT during the loading and decay processes. Using these methods, the cross section was determined to be 1.49�0.13 Mb, and 2.38�0.18 Mb respectively. Precise energy levels of high-n Rydberg states of potassium have also been measured using stepwise-excitation of the trapped atoms, with the intermediate state being the 42P1/2 state that was excited by resonant laser radiation at 389,286,368.12 MHz. Excitation from the 42P1/2 state to the n^2 S_(1/2) states was carried out using blue radiation from a dye laser, for transitions to Rydberg states with principal quantum numbers ranging from n = 18 to ~200. For excitation to the n^2 D_(3/2) states, transitions up to Rydberg states with n ranging from 18 to 167 were studied. Due to the strong interaction between Rydberg atoms, the measured energy levels deviated from their predicted theoretical values at high principal quantum numbers (n?130 for the S-states, and n?100 for the D-states). For principal quantum numbers greater than these values, the spectral lines were found to both broaden and shift in energy, and new features in the Rydberg spectra were observed. These new features have been attributed to the dipole-forbidden n2P1/2 and n2P3/2 states, which cannot be excited directly by the laser. These states must hence be excited due to interplay between the highly excited Rydberg atoms that is occurring due to their large effective size and close proximity in the AC-MOT.

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Spectroscopy of ultra-cold 39K Rydberg Atoms in an AC-Driven magneto-optical Trap (AC-MOT)

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Results from measurement of the energy levels of ultra-cold Rydberg states of 39 K in an AC-Magneto-Optical Trap are presented. Rydberg atoms are produced by stepwise excitation from the laser-excited 4 2 P 1 / 2 states using a tunable blue dye laser. Energy level data for principal quantum numbers n = 18 up to n = 199 are reported. For n = 18 to n = 46 , the measured levels are in good agreement with the available data from the National Institute of Standards and Technology (NIST) list of atomic spectral lines. New values for the parameters in the Rydberg-Ritz formula were then calculated from a fit to the measured energy level data. Atomic transition energies for n = 47 to n = 199 are also reported for the first time. Shifts in the levels for excitation to both the 2 S 1 / 2 and 2 D 3 / 2 states at principal quantum number n > 100 are found when compared to calculations derived from the revised Rydberg-Ritz formula. These deviations are due to the greater probability of overlap between the wavefunctions of the excited atoms in the ensemble, resulting in strong interaction between the Rydberg atoms which are in close proximity. For n > 167, the D-state energies were unresolved.

Journal of Quantitative Spectroscopy and Radiative Transfer•Feb 27, 2026•1 citation

Studying Cold Potassium Rydberg Atoms with an AC-MOT

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A new method is described for the detection and study of cold Rydberg atoms. Cold atoms held in an AC-driven magneto optical trap (AC-MOT) are selectively laser excited to a Rydberg state in a stepwise process. Ions or electrons from interactions between Rydberg atoms are then electrostatically extracted and detected via a channel electron multiplier. The apparatus is capable of studying Rydberg atoms from n = 15 to n ∼ 220.

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Long-lived states in cold Rydberg gases

D. Feldbaum, S.K. Dutta, Alisa I. Walz‐Flannigan +2 more

Summary form only given. Laser-cooled atoms can be excited into Rydberg states to study Rydberg gases at high densities and low atomic velocities. Dense clouds of such atoms undergo virtually complete ionization when their density exceeds a critical value dependent on the principal quantum number n. We report a new regime in which the excited Rydberg population spontaneously evolves into long-lived high-l states, the lifetimes of which exceed the natural lifetimes of the initially excited Rydberg levels by two orders of magnitude. /sup 87/Rb atoms are collected and cooled to 50 /spl mu/K in a magneto optical trap (MOT), which is periodically turned off for 50 ms. 1 ms after the shutdown of the MOT the Rydberg atoms are created via a two-step excitation process using a 5 /spl mu/s long diode laser pulse.

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This thesis discusses an approach to excite potassium atoms to very highly excited states (Rydberg states), and then tune their energy levels to induce resonant collisions between atoms. Potassium gas is super-cooled to 1 mK and confined to a small volume in a magneto-optical trap. A 405 nm laser diode, electronically locked to a potassium vapor cell via Doppler free spectroscopy, excites these atoms from the 4s1/2 state (ground state) to the 5p3/2 state. A 978 nm laser then excites the 5p3/2 to nd3/2 or nd5/2 transition, creating Rydberg atoms. Since there is no ground state reference for this transition, an alternative method must be used to lock the laser's frequency. A scanning transfer cavity lock transfers the stability of a 632 nm HeNe reference laser to the unstable 978 nm laser diode by passing both lasers through a Fabry-Perot interferometer. Once Rydberg atoms have been created, their energy levels are tuned with an external electric field so that collisions between atoms cause resonant atomic transitions to different Rydberg states. The Rydberg atoms are ionized with a second, stronger electric field, and detected by microchannel plate detectors. Rydberg atoms with different principal quantum numbers ionize at different electric field strengths, so a successful collision signal has multiple peaks from the microchannel plates.

•Jan 1, 2016•1 citation

Dynamics and applications of excited cold atoms

BJ Bert Claessens

In a Magneto-Optical Trap (MOT), realized for the first time in 1987, one can trap and cool neutral atoms to temperatures below a mK. The invention of this device caused a revolution in atomic physics. With an MOT collision and spectroscopy experiments could be performed with unprecedented accuracy. The main breakthrough came about 10 years later when the atom from an MOT were used to create a Bose Einstein Condensate (BEC), a source of coherent matter. All this work has been rewarded with no less than two Nobel prizes: in 1997, Chu, Cohen-Tannoudji and Phillips received the Nobel prize for the practical and theoretical development of laser cooling, and in 2001 Cornell, Ketterle and Wieman received the Nobel prize for the realization of a Bose Einstein Condensate in a dilute gas. In 1999 Killian et al. took trapped atoms in an entirely new direction when they succeeded in creating the very first Ultracold Plasma (UCP) from the atoms in an MOT. The temperature of such a plasma is more than 1000 times lower than the temperatures in conventional cold plasmas. As a result, this type of plasmas received a lot of attention, both theoretically and experimentally. So far studies of UCPs have mainly been of a fundamental kind. In this thesis we study both the fundamental aspects of atoms excited from a magneto-optical as well as the feasibility of using the electrons and ions from a UCP as a source for charged particle accelerators. The work in this thesis therefore pioneers the line between atom physics, plasma physics and accelerator physics. The first part of this thesis deals with an experimental study of the effect of an ionizing laser on cold neon atoms. Here we study the loss of atoms from a Magneto-Optical Trap caused by photionization. From these measurements we deduce a new value for the near threshold photoionization cross section 2:05±0:25x10-18 cm2 at ¿ = 351 nm and 2:05±0:25x10-18 cm2 at ¿ = 364 nm, which is a factor of four more accurate than previous measurements. The measured valuesagree with earlier measurements but differ signi¯cantly from theoretical values. The value of this cross section has a practical relevance for the realization of a continuous ion source. The second part of this thesis has been conducted at the University of Virginia. We study the role of dipole-dipole interactions in the formation of a UCP from a gas of cold Rydberg atoms, and the effect of dipole-dipole interactions on the broadening of microwave transitions. We find that the broadening scales linearly with the density en scales with the fourth power of n, the principal quantum number. These results show the great importance of dipole-dipole interactions in gas of cold Rydberg atoms and are also relevant for the possible realization of a quantum computer from Rydberg atoms. In a third part we studied the potential of using a UCP as a source for electron accelerators. This was done based upon elaborate simulations performed with the General Particle Tracer Code. The main result of these simulations is that, based upon realistic parameters, the brightness, i.e., the figure of merit for charged particle sources defined as the current divided by the emittance squared, of such a source can be up to two orders of magnitude larger than the brightness of conventional sources, which is a major advance. The emittance is a measure for the the focusability of the bunch, and scales as p T, with T the electron temperature. The increase in brightness is mainly attributed to the low electron temperatures of approximately 10 K in an UCP, resulting in an initial emittance of 0.01 mm mrad, compared to 1 mm mrad in a conventional source. With experimentally proven techniques it is possible to extract 10 nC/s from the source, which is a competitive repetition rate. In the next part we describe the design, construction and testing of the first setup with which we want to build an electron accelerator based upon cold atoms. For this we first built a vapor-cell magneto-optical trap, in which we can trap over 109 rubidium atoms with a temperature of 200 ¹K, straight from a background vapor, we also designed an atomic beam that will be implemented in the near future and can provide an increased repetition rate. In the last chapter we show how Rydberg atoms excited from an MOT, evolve into an UCP spontaneously. We extracted a minimum of 1 pC from the plasma. As a main result, we were one of the ¯rst to show how the ions and electrons from a UCP can be imaged on a phosphor screen. From these images we can deduct an upper value for the electron and the ion temperature, we extract for both the electrons and the ions an upper temperature of 40 K, when we translate this temperature to an initial emittance, we obtain a value of 0.01 mm mrad, proving the potential of the electrons extracted from atoms excited out of an MOT as a low emittance source. Our measured current of 1 ¹A is not yet competitive and results from the small electric field, but is expected to increase dramatically in the near future. We will implement a 30 kV 10 ns risetime accelerator with which it should be possible to get a current of about 50 mA. Furthermore we will implement the proper diagnostics to measure the proper charge, current and emittance. In the very last chapter we speculate charged particle sources from cold atoms might advance the state of the art in achievable brightness for both electron and ion beams.

•Jan 1, 2006•1 citation

Photoionization for Na and Li cold atoms in a MOT

Volker Wippel, C. Binder, W. Huber +4 more

Summary form only given. Photoionization of cold atoms in their first excited state has been applied to measure the photoionization cross section in rubidium magneto-optical trap (MOT), and cesium MOT. The modifications of the MOT loading rate and of the steady state number of trapped atoms allow one to measure the dependence of the cold atom photoionization cross section on the laser wavelength for absorption from the excited atomic state. We report here on measurements of photoionization cross-sections of both laser-cooled sodium and lithium atoms at various photoionization wavelengths.

Conference on Lasers and Electro-Optics•Aug 29, 2005•0 citations
ABSTRACTCandidate?s Names: John AgomuoDegree Title: PhDThesis Title: Excitation And Photoionization Of Laser-Cooled Atoms In The AC Driven Magneto-Optical Trap (AC-MOT), Using Resonant And Non-Resonant Radiation.Date: 19/12/2014Results of experiments involving the manipulation of atoms using laser radiation are presented. A high-density trap of potassium atoms was created using an AC-driven magneto-optical trap (AC-MOT), loaded from a Zeeman-slowed atomic beam. The AC-MOT was operated in a pulsed mode, with a trapping laser that was detuned by -4?, where ? is the natural line-width of the trapping transition. Under these conditions, the AC-MOT trapped an average of 3.63�0.02�?10?^8 atoms, which were cooled to a mean temperature of 275?K�33?K.Using the AC-MOT, the absolute photoionization cross-section of cold potassium atoms was determined using ionizing laser radiation at 450.4 nm obtained from a high-power diode laser. The measured photoionization cross-section from the 42P3/2 excited state was obtained by measuring the loss rate and the lifetime of the AC-MOT during the loading and decay processes. Using these methods, the cross section was determined to be 1.49�0.13 Mb, and 2.38�0.18 Mb respectively. Precise energy levels of high-n Rydberg states of potassium have also been measured using stepwise-excitation of the trapped atoms, with the intermediate state being the 42P1/2 state that was excited by resonant laser radiation at 389,286,368.12 MHz. Excitation from the 42P1/2 state to the n^2 S_(1/2) states was carried out using blue radiation from a dye laser, for transitions to Rydberg states with principal quantum numbers ranging from n = 18 to ~200. For excitation to the n^2 D_(3/2) states, transitions up to Rydberg states with n ranging from 18 to 167 were studied. Due to the strong interaction between Rydberg atoms, the measured energy levels deviated from their predicted theoretical values at high principal quantum numbers (n?130 for the S-states, and n?100 for the D-states). For principal quantum numbers greater than these values, the spectral lines were found to both broaden and shift in energy, and new features in the Rydberg spectra were observed. These new features have been attributed to the dipole-forbidden n2P1/2 and n2P3/2 states, which cannot be excited directly by the laser. These states must hence be excited due to interplay between the highly excited Rydberg atoms that is occurring due to their large effective size and close proximity in the AC-MOT.
Results from measurement of the energy levels of ultra-cold Rydberg states of 39 K in an AC-Magneto-Optical Trap are presented. Rydberg atoms are produced by stepwise excitation from the laser-excited 4 2 P 1 / 2 states using a tunable blue dye laser. Energy level data for principal quantum numbers n = 18 up to n = 199 are reported. For n = 18 to n = 46 , the measured levels are in good agreement with the available data from the National Institute of Standards and Technology (NIST) list of atomic spectral lines. New values for the parameters in the Rydberg-Ritz formula were then calculated from a fit to the measured energy level data. Atomic transition energies for n = 47 to n = 199 are also reported for the first time. Shifts in the levels for excitation to both the 2 S 1 / 2 and 2 D 3 / 2 states at principal quantum number n > 100 are found when compared to calculations derived from the revised Rydberg-Ritz formula. These deviations are due to the greater probability of overlap between the wavefunctions of the excited atoms in the ensemble, resulting in strong interaction between the Rydberg atoms which are in close proximity. For n > 167, the D-state energies were unresolved.
Summary form only given. Laser-cooled atoms can be excited into Rydberg states to study Rydberg gases at high densities and low atomic velocities. Dense clouds of such atoms undergo virtually complete ionization when their density exceeds a critical value dependent on the principal quantum number n. We report a new regime in which the excited Rydberg population spontaneously evolves into long-lived high-l states, the lifetimes of which exceed the natural lifetimes of the initially excited Rydberg levels by two orders of magnitude. /sup 87/Rb atoms are collected and cooled to 50 /spl mu/K in a magneto optical trap (MOT), which is periodically turned off for 50 ms. 1 ms after the shutdown of the MOT the Rydberg atoms are created via a two-step excitation process using a 5 /spl mu/s long diode laser pulse.
This thesis discusses an approach to excite potassium atoms to very highly excited states (Rydberg states), and then tune their energy levels to induce resonant collisions between atoms. Potassium gas is super-cooled to 1 mK and confined to a small volume in a magneto-optical trap. A 405 nm laser diode, electronically locked to a potassium vapor cell via Doppler free spectroscopy, excites these atoms from the 4s1/2 state (ground state) to the 5p3/2 state. A 978 nm laser then excites the 5p3/2 to nd3/2 or nd5/2 transition, creating Rydberg atoms. Since there is no ground state reference for this transition, an alternative method must be used to lock the laser's frequency. A scanning transfer cavity lock transfers the stability of a 632 nm HeNe reference laser to the unstable 978 nm laser diode by passing both lasers through a Fabry-Perot interferometer. Once Rydberg atoms have been created, their energy levels are tuned with an external electric field so that collisions between atoms cause resonant atomic transitions to different Rydberg states. The Rydberg atoms are ionized with a second, stronger electric field, and detected by microchannel plate detectors. Rydberg atoms with different principal quantum numbers ionize at different electric field strengths, so a successful collision signal has multiple peaks from the microchannel plates.
In a Magneto-Optical Trap (MOT), realized for the first time in 1987, one can trap and cool neutral atoms to temperatures below a mK. The invention of this device caused a revolution in atomic physics. With an MOT collision and spectroscopy experiments could be performed with unprecedented accuracy. The main breakthrough came about 10 years later when the atom from an MOT were used to create a Bose Einstein Condensate (BEC), a source of coherent matter. All this work has been rewarded with no less than two Nobel prizes: in 1997, Chu, Cohen-Tannoudji and Phillips received the Nobel prize for the practical and theoretical development of laser cooling, and in 2001 Cornell, Ketterle and Wieman received the Nobel prize for the realization of a Bose Einstein Condensate in a dilute gas. In 1999 Killian et al. took trapped atoms in an entirely new direction when they succeeded in creating the very first Ultracold Plasma (UCP) from the atoms in an MOT. The temperature of such a plasma is more than 1000 times lower than the temperatures in conventional cold plasmas. As a result, this type of plasmas received a lot of attention, both theoretically and experimentally. So far studies of UCPs have mainly been of a fundamental kind. In this thesis we study both the fundamental aspects of atoms excited from a magneto-optical as well as the feasibility of using the electrons and ions from a UCP as a source for charged particle accelerators. The work in this thesis therefore pioneers the line between atom physics, plasma physics and accelerator physics. The first part of this thesis deals with an experimental study of the effect of an ionizing laser on cold neon atoms. Here we study the loss of atoms from a Magneto-Optical Trap caused by photionization. From these measurements we deduce a new value for the near threshold photoionization cross section 2:05±0:25x10-18 cm2 at ¿ = 351 nm and 2:05±0:25x10-18 cm2 at ¿ = 364 nm, which is a factor of four more accurate than previous measurements. The measured valuesagree with earlier measurements but differ signi¯cantly from theoretical values. The value of this cross section has a practical relevance for the realization of a continuous ion source. The second part of this thesis has been conducted at the University of Virginia. We study the role of dipole-dipole interactions in the formation of a UCP from a gas of cold Rydberg atoms, and the effect of dipole-dipole interactions on the broadening of microwave transitions. We find that the broadening scales linearly with the density en scales with the fourth power of n, the principal quantum number. These results show the great importance of dipole-dipole interactions in gas of cold Rydberg atoms and are also relevant for the possible realization of a quantum computer from Rydberg atoms. In a third part we studied the potential of using a UCP as a source for electron accelerators. This was done based upon elaborate simulations performed with the General Particle Tracer Code. The main result of these simulations is that, based upon realistic parameters, the brightness, i.e., the figure of merit for charged particle sources defined as the current divided by the emittance squared, of such a source can be up to two orders of magnitude larger than the brightness of conventional sources, which is a major advance. The emittance is a measure for the the focusability of the bunch, and scales as p T, with T the electron temperature. The increase in brightness is mainly attributed to the low electron temperatures of approximately 10 K in an UCP, resulting in an initial emittance of 0.01 mm mrad, compared to 1 mm mrad in a conventional source. With experimentally proven techniques it is possible to extract 10 nC/s from the source, which is a competitive repetition rate. In the next part we describe the design, construction and testing of the first setup with which we want to build an electron accelerator based upon cold atoms. For this we first built a vapor-cell magneto-optical trap, in which we can trap over 109 rubidium atoms with a temperature of 200 ¹K, straight from a background vapor, we also designed an atomic beam that will be implemented in the near future and can provide an increased repetition rate. In the last chapter we show how Rydberg atoms excited from an MOT, evolve into an UCP spontaneously. We extracted a minimum of 1 pC from the plasma. As a main result, we were one of the ¯rst to show how the ions and electrons from a UCP can be imaged on a phosphor screen. From these images we can deduct an upper value for the electron and the ion temperature, we extract for both the electrons and the ions an upper temperature of 40 K, when we translate this temperature to an initial emittance, we obtain a value of 0.01 mm mrad, proving the potential of the electrons extracted from atoms excited out of an MOT as a low emittance source. Our measured current of 1 ¹A is not yet competitive and results from the small electric field, but is expected to increase dramatically in the near future. We will implement a 30 kV 10 ns risetime accelerator with which it should be possible to get a current of about 50 mA. Furthermore we will implement the proper diagnostics to measure the proper charge, current and emittance. In the very last chapter we speculate charged particle sources from cold atoms might advance the state of the art in achievable brightness for both electron and ion beams.
Summary form only given. Photoionization of cold atoms in their first excited state has been applied to measure the photoionization cross section in rubidium magneto-optical trap (MOT), and cesium MOT. The modifications of the MOT loading rate and of the steady state number of trapped atoms allow one to measure the dependence of the cold atom photoionization cross section on the laser wavelength for absorption from the excited atomic state. We report here on measurements of photoionization cross-sections of both laser-cooled sodium and lithium atoms at various photoionization wavelengths.