Curtis Bradley | The Petroleum Institute (original) (raw)
Papers by Curtis Bradley
Physical Review A, 2000
We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and... more We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and two repumping lasers tuned to intercombination transitions, over 10 6 atoms were trapped and average densities of over 10 16 m −3 were obtained. Non-exponential loss of atoms is observed at high densities indicating inelastic collisions between trapped atoms. Over a range of trapping conditions, the density-dependent trap loss rate constant β is extracted from fluorescence decay curves and found to have values in the range of 10 −15 m 3 s −1 , much larger than is observed in alkali traps.
Reading Psychology, 2021
Abstract There is a worldwide concern about science and engineering undergraduate students, parti... more Abstract There is a worldwide concern about science and engineering undergraduate students, particularly those who are English language learners, struggling with their entry-level content courses. From a teacher cognition perspective and drawing on semi-structured interviews with chemistry, mathematics and physics teachers at a university in the Middle East, we sought to gain insights into the extent to which language is seen as interfering with content learning and how this issue is being addressed. Focusing particularly on challenges students face with processing written text in English, since this emerged through thematic analysis as the investigated content teachers’ primary concern, we explore reported practices and highlight needs for in-service professional development. We suggest that science and mathematics instructors may need heightened syntactical and morphological awareness to prepare them to more effectively help their students to decode dense word problems. They may also need consciousness-raising, so that they can see themselves as language teachers too.
Laser focusing of atoms during deposition has been demonstrated as an effective method for creati... more Laser focusing of atoms during deposition has been demonstrated as an effective method for creating surface nanostructures. For example, the induced dipole interaction for atoms in a laser standing-wave can exert sufficient force to confine gas atoms into nanometer-sized regions. We are building an experiment to explore the fundamental limits of this type of nanoscale atom manipulation. Initially, we will
A new method for the rapid detection and tracking of individual microscopic and nanoscale particl... more A new method for the rapid detection and tracking of individual microscopic and nanoscale particles is demonstrated, based on inexpensive, low-power, diode-laser light sources and photodiode receivers. Using a swept standing-wave laser probe and a phase-sensitive detector, single micrometer-sized particles are simply and easily monitored via the scattered light in the far-field. Also, strong signals are obtained in the forward-scattering direction via the resulting modulation of the transmitted probe beam power. We will report on our verification and testing of the method via detailed measurements and theoretical modeling of signals from mounted and oriented microfibers. Also, we will give results of particle sizing measurements for distributions of metallic and nonmetallic micro-particles, with comparative sizing obtained from optical and electron microscopy. Further, we will discuss and demonstrate extensions of this technique to a novel and highly sensitive form of optical phase-...
Chromium atoms are focused onto a substrate via the dipole force exerted while traveling through ... more Chromium atoms are focused onto a substrate via the dipole force exerted while traveling through an optical standing wave. This produces a pattern of parallel chromium lines as narrow as 28 nm FWHM and spaced by 212.78 nm (half the vacuum wavelength of the light used to form the standing wave.) We present a study of the factors contributing to the feature size of these nanostructures. The factors can be separated into two categories: atom-optical parameters and chromium growth considerations. We present calculations and measurements representing an extensive exploration of the effects of laser power, atomic collimation, free-flight distance, substrate temperature and chromium thickness. Qualitative agreement is seen between trends in observed feature sizes and the theoretically modelled atomic flux distribution. The observed linewidths are larger than theoretical predictions and this difference is attributed to chromium growth behavior.
Atomic chromium vapor sublimated from a set of hot Cr-plated tungsten filaments is collected and ... more Atomic chromium vapor sublimated from a set of hot Cr-plated tungsten filaments is collected and cooled in a magneto-optic trap. The trap laser is tuned to excite the ^52Cr ^7S3 arrow ^7P^circ4 cooling transition at 425.55 nm (vacuum wavelength). The performance and dynamics of such a trap are strongly driven by the kHz rate for decay of the ^7P_4^circ excited state to the ^5D metastable states, the dominant trap loss mechanism. The number of trapped atoms results from a balance between this loss rate and the rate for loading into the trap from the filaments. Improved trap lifetime can be obtained by repumping atoms into the cooling transition via injection of additional laser beams tuned to the appropriate ^5D arrow ^7P^circ transitions. Laser-cooled and trapped chromium vapor is considered as a potential source of monochromatic and spatially coherent atoms for experiments in atom nanolithography.
Optical Tweezers are becoming an increasingly common tool for the manipulation of microscopic mat... more Optical Tweezers are becoming an increasingly common tool for the manipulation of microscopic materials. In these applications, subtle variations in either the laser field or the microscopic media can produce significant changes in the resulting interaction force. We intend to study these forces in a simple model system using microscopic gas bubbles suspended in a liquid. We are building a fluid jet apparatus to form an adjustable continuous stream of microscopic monomorphic bubbles. Focused laser beams will be used to deflect and control the bubble stream, allowing detailed studies of the interactions between the laser field, bubbles, and surrounding liquid. We will report on experimental progress with our jet source and initial imaging of our bubble stream.
Journal of Research of the National Institute of Standards and Technology, 2003
With the drive toward miniaturization of a wide range of technologies, in fields ranging from ele... more With the drive toward miniaturization of a wide range of technologies, in fields ranging from electronics, to magnetics, to chemistry and biotechnology, dimensional metrology on the nanometer scale has become an increasingly important area for research. One of the key elements of dimensional measurement on any scale is the development of length standards of appropriate dimensions. If well-characterized devices are to be manufactured reproducibly, it is important to be able to measure them with confidence, and this involves using well-understood length standards on a suitable scale. On the nanometer scale, length standards pose particular challenges because many effects such as thermal expansion, material graininess, and material
Physical Review B, 1999
We p r e s e n t calculations of surface growth in laser-focused nanostructure fabrication. We s ... more We p r e s e n t calculations of surface growth in laser-focused nanostructure fabrication. We s h o w that theoretical predictions of the structure pro le's shape depend sensitively on the model used to describe the growth, and also on the parameters chosen within the model. This sensitivity illustrates that growth e ects can play a major role in laser-focused atomic deposition, and also suggests that this process could be utilized for studies of surface growth mechanisms.
Technical Digest. Summaries of Papers Presented at the Quantum Electronics and Laser Science Conference
ABSTRACT We have constructed a magneto-optical trap for chromium atoms. The motivation for this w... more ABSTRACT We have constructed a magneto-optical trap for chromium atoms. The motivation for this work lies in the usefulness of chromium for atom-optically controlled deposition, and the fact that a magneto-optically trapped sample of atoms provides a high-brightness monochromatic source of atoms. Further motivation exists in the possible production of quantum-degenerate Bose and Fermi gases with different isotopes of Cr
ABSTRACT Time-Lapse Microgravity can reveal relatively small underground fluid displacements via ... more ABSTRACT Time-Lapse Microgravity can reveal relatively small underground fluid displacements via the redistribution of density-contrast at fluid boundaries. The method has, for example, been successfully demonstrated in underground natural gas storage and carbon sequestration studies, and has clear potential for hydrological research and applications such as mining and hydrocarbon reservoir management. For surveys based on modern spring-type gravimeters, the technique's basic measurement resolution is limited by instrument drift, offset errors, and viscoelastic strain hysteresis of the sensor. Despite the sophistication of modern instruments, actual field performance of gravimeters still depends on operator technique and survey conditions. To explore the ultimate resolution of field microgravity (and time-lapse measurements, in particular), we have made repeated gravity surveys of a shallow aquifer storage and recovery test site in the UAE, where injection produces a 'water mound' - localized vertical water-level shifts - monitored by a set of instrumented wells. Based on field measurements and additional laboratory testing of our Scintrex CG-5 gravimeter, we find that the main limit on measurement resolution is from orientation strain hysteresis - a variable amplitude error that decays in approximately 30 minutes, typically in response to orientation during transport between measurement stations. Still, carefully conducted surveys (during the summer, in the desert conditions of the UAE) demonstrate time-lapse microgravity resolution of about 3 microGals, corresponding to a water-level shift of about 0.3 m for this aquifer. In this paper, we will discuss what a 'careful survey' requires and present our detailed testing and survey results.
![Research paper thumbnail of Evidence of Bose-Einstein Condensation in an Atomic Gas with Attractive Interactions [Phys. Rev. Lett. 75, 1687 (1995)]](https://attachments.academia-assets.com/113164680/thumbnails/1.jpg)
](Physical Review Letters, 1997
Physical Review A, 1995
Accurate measurements are presented of the rate of trap-loss-producing collisions between ultraco... more Accurate measurements are presented of the rate of trap-loss-producing collisions between ultracold magneto-optically trapped Li atoms for a range of trap laser intensities and frequencies. Intensities from near the atomic saturation intensity to well above it are investigated. At low intensities, fine-structure-changing collisions cause trap loss with a rate constant of-10 ' cm /s. At sufficiently high intensity, the trap can be deep enough to effectively freeze out the dominant fine-structure-changing collisions as a loss mechanism, enabling an accurate comparison of the radiative escape loss rate with theory. At the lowest intensities of this radiative escape regime, the measured loss rates compare favorably with those calculated using an optical Bloch equation theory and a three-dimensional model of trap depth. However, the intensity dependence of the measured rates does not show the saturation predicted by the optical Bloch equation theory. It is shown that reliable knowledge of trap depth is necessary to accurately compare experiment with theory.
Physical Review A, 2000
We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and... more We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and two repumping lasers tuned to intercombination transitions, over 10 6 atoms were trapped and average densities of over 10 16 m −3 were obtained. Non-exponential loss of atoms is observed at high densities indicating inelastic collisions between trapped atoms. Over a range of trapping conditions, the density-dependent trap loss rate constant β is extracted from fluorescence decay curves and found to have values in the range of 10 −15 m 3 s −1 , much larger than is observed in alkali traps.
Physical Review A, 1995
We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1X10 ground-sta... more We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1X10 ground-state lithium atoms have been confined in a nonzero magnetic-field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of approximately 3 X 10 cm. The trapped-atom lifetime is 240 s, limited by collisions with background gas. This trap provides an environment in which quantum-statistical effects, atomic collisions, and other ultralow-temperature phenomena can be studied.
Physical Review A, 1997
Recent experiments have used forced evaporative cooling to produce Bose-Einstein condensation in ... more Recent experiments have used forced evaporative cooling to produce Bose-Einstein condensation in dilute gases. The evaporative cooling process can be optimized to provide the maximum phase-space density with a specified number of atoms remaining. We show that this global optimization is approximately achieved by locally optimizing the cooling efficiency at each instant. We discuss how this method can be implemented, and present the results for our 7 Li trap. The predicted behavior of the gas is found to agree well with experiment. ͓S1050-2947͑97͒07005-4͔
Optics Letters, 1993
We explored the suitability of visible III-V single-mode cw diodes for difference-frequency gener... more We explored the suitability of visible III-V single-mode cw diodes for difference-frequency generation of tunable infrared radiation by mixing a red single-mode cw III-V diode laser with a tunable single-frequency cw Ti:sapphire laser in AgGaS 2. More than 1 ILW of cw tunable, infrared (A 5 ,um), narrow-band coherent radiation was generated with type I noncritical phase matching. The wavelength and output-power characteristics of this novel tunable all-solid-state laser source are described, and we demonstrate the applicability of the source to high-resolution molecular spectroscopy by obtaining a test spectrum. The feasibility of a more compact solidstate cw laser spectrometer based on the mixing of two single-mode diode lasers (808 and 690 nm) as pump sources in AgGaS 2 is shown (infrared power generated-3 nW).
Optics Letters, 1992
We demonstrate a chirp-cooling technique that uses a laser with multiple FM sidebands to slow lit... more We demonstrate a chirp-cooling technique that uses a laser with multiple FM sidebands to slow lithium atoms from initial velocities of as much as 1880 m/s to final velocities near zero. Compared with the use of a single sideband, this multisideband technique significantly greater flux of slowed atoms.
Physical Review A, 2000
We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and... more We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and two repumping lasers tuned to intercombination transitions, over 10 6 atoms were trapped and average densities of over 10 16 m −3 were obtained. Non-exponential loss of atoms is observed at high densities indicating inelastic collisions between trapped atoms. Over a range of trapping conditions, the density-dependent trap loss rate constant β is extracted from fluorescence decay curves and found to have values in the range of 10 −15 m 3 s −1 , much larger than is observed in alkali traps.
Reading Psychology, 2021
Abstract There is a worldwide concern about science and engineering undergraduate students, parti... more Abstract There is a worldwide concern about science and engineering undergraduate students, particularly those who are English language learners, struggling with their entry-level content courses. From a teacher cognition perspective and drawing on semi-structured interviews with chemistry, mathematics and physics teachers at a university in the Middle East, we sought to gain insights into the extent to which language is seen as interfering with content learning and how this issue is being addressed. Focusing particularly on challenges students face with processing written text in English, since this emerged through thematic analysis as the investigated content teachers’ primary concern, we explore reported practices and highlight needs for in-service professional development. We suggest that science and mathematics instructors may need heightened syntactical and morphological awareness to prepare them to more effectively help their students to decode dense word problems. They may also need consciousness-raising, so that they can see themselves as language teachers too.
Laser focusing of atoms during deposition has been demonstrated as an effective method for creati... more Laser focusing of atoms during deposition has been demonstrated as an effective method for creating surface nanostructures. For example, the induced dipole interaction for atoms in a laser standing-wave can exert sufficient force to confine gas atoms into nanometer-sized regions. We are building an experiment to explore the fundamental limits of this type of nanoscale atom manipulation. Initially, we will
A new method for the rapid detection and tracking of individual microscopic and nanoscale particl... more A new method for the rapid detection and tracking of individual microscopic and nanoscale particles is demonstrated, based on inexpensive, low-power, diode-laser light sources and photodiode receivers. Using a swept standing-wave laser probe and a phase-sensitive detector, single micrometer-sized particles are simply and easily monitored via the scattered light in the far-field. Also, strong signals are obtained in the forward-scattering direction via the resulting modulation of the transmitted probe beam power. We will report on our verification and testing of the method via detailed measurements and theoretical modeling of signals from mounted and oriented microfibers. Also, we will give results of particle sizing measurements for distributions of metallic and nonmetallic micro-particles, with comparative sizing obtained from optical and electron microscopy. Further, we will discuss and demonstrate extensions of this technique to a novel and highly sensitive form of optical phase-...
Chromium atoms are focused onto a substrate via the dipole force exerted while traveling through ... more Chromium atoms are focused onto a substrate via the dipole force exerted while traveling through an optical standing wave. This produces a pattern of parallel chromium lines as narrow as 28 nm FWHM and spaced by 212.78 nm (half the vacuum wavelength of the light used to form the standing wave.) We present a study of the factors contributing to the feature size of these nanostructures. The factors can be separated into two categories: atom-optical parameters and chromium growth considerations. We present calculations and measurements representing an extensive exploration of the effects of laser power, atomic collimation, free-flight distance, substrate temperature and chromium thickness. Qualitative agreement is seen between trends in observed feature sizes and the theoretically modelled atomic flux distribution. The observed linewidths are larger than theoretical predictions and this difference is attributed to chromium growth behavior.
Atomic chromium vapor sublimated from a set of hot Cr-plated tungsten filaments is collected and ... more Atomic chromium vapor sublimated from a set of hot Cr-plated tungsten filaments is collected and cooled in a magneto-optic trap. The trap laser is tuned to excite the ^52Cr ^7S3 arrow ^7P^circ4 cooling transition at 425.55 nm (vacuum wavelength). The performance and dynamics of such a trap are strongly driven by the kHz rate for decay of the ^7P_4^circ excited state to the ^5D metastable states, the dominant trap loss mechanism. The number of trapped atoms results from a balance between this loss rate and the rate for loading into the trap from the filaments. Improved trap lifetime can be obtained by repumping atoms into the cooling transition via injection of additional laser beams tuned to the appropriate ^5D arrow ^7P^circ transitions. Laser-cooled and trapped chromium vapor is considered as a potential source of monochromatic and spatially coherent atoms for experiments in atom nanolithography.
Optical Tweezers are becoming an increasingly common tool for the manipulation of microscopic mat... more Optical Tweezers are becoming an increasingly common tool for the manipulation of microscopic materials. In these applications, subtle variations in either the laser field or the microscopic media can produce significant changes in the resulting interaction force. We intend to study these forces in a simple model system using microscopic gas bubbles suspended in a liquid. We are building a fluid jet apparatus to form an adjustable continuous stream of microscopic monomorphic bubbles. Focused laser beams will be used to deflect and control the bubble stream, allowing detailed studies of the interactions between the laser field, bubbles, and surrounding liquid. We will report on experimental progress with our jet source and initial imaging of our bubble stream.
Journal of Research of the National Institute of Standards and Technology, 2003
With the drive toward miniaturization of a wide range of technologies, in fields ranging from ele... more With the drive toward miniaturization of a wide range of technologies, in fields ranging from electronics, to magnetics, to chemistry and biotechnology, dimensional metrology on the nanometer scale has become an increasingly important area for research. One of the key elements of dimensional measurement on any scale is the development of length standards of appropriate dimensions. If well-characterized devices are to be manufactured reproducibly, it is important to be able to measure them with confidence, and this involves using well-understood length standards on a suitable scale. On the nanometer scale, length standards pose particular challenges because many effects such as thermal expansion, material graininess, and material
Physical Review B, 1999
We p r e s e n t calculations of surface growth in laser-focused nanostructure fabrication. We s ... more We p r e s e n t calculations of surface growth in laser-focused nanostructure fabrication. We s h o w that theoretical predictions of the structure pro le's shape depend sensitively on the model used to describe the growth, and also on the parameters chosen within the model. This sensitivity illustrates that growth e ects can play a major role in laser-focused atomic deposition, and also suggests that this process could be utilized for studies of surface growth mechanisms.
Technical Digest. Summaries of Papers Presented at the Quantum Electronics and Laser Science Conference
ABSTRACT We have constructed a magneto-optical trap for chromium atoms. The motivation for this w... more ABSTRACT We have constructed a magneto-optical trap for chromium atoms. The motivation for this work lies in the usefulness of chromium for atom-optically controlled deposition, and the fact that a magneto-optically trapped sample of atoms provides a high-brightness monochromatic source of atoms. Further motivation exists in the possible production of quantum-degenerate Bose and Fermi gases with different isotopes of Cr
ABSTRACT Time-Lapse Microgravity can reveal relatively small underground fluid displacements via ... more ABSTRACT Time-Lapse Microgravity can reveal relatively small underground fluid displacements via the redistribution of density-contrast at fluid boundaries. The method has, for example, been successfully demonstrated in underground natural gas storage and carbon sequestration studies, and has clear potential for hydrological research and applications such as mining and hydrocarbon reservoir management. For surveys based on modern spring-type gravimeters, the technique's basic measurement resolution is limited by instrument drift, offset errors, and viscoelastic strain hysteresis of the sensor. Despite the sophistication of modern instruments, actual field performance of gravimeters still depends on operator technique and survey conditions. To explore the ultimate resolution of field microgravity (and time-lapse measurements, in particular), we have made repeated gravity surveys of a shallow aquifer storage and recovery test site in the UAE, where injection produces a 'water mound' - localized vertical water-level shifts - monitored by a set of instrumented wells. Based on field measurements and additional laboratory testing of our Scintrex CG-5 gravimeter, we find that the main limit on measurement resolution is from orientation strain hysteresis - a variable amplitude error that decays in approximately 30 minutes, typically in response to orientation during transport between measurement stations. Still, carefully conducted surveys (during the summer, in the desert conditions of the UAE) demonstrate time-lapse microgravity resolution of about 3 microGals, corresponding to a water-level shift of about 0.3 m for this aquifer. In this paper, we will discuss what a 'careful survey' requires and present our detailed testing and survey results.
Physical Review Letters, 1997
Physical Review A, 1995
Accurate measurements are presented of the rate of trap-loss-producing collisions between ultraco... more Accurate measurements are presented of the rate of trap-loss-producing collisions between ultracold magneto-optically trapped Li atoms for a range of trap laser intensities and frequencies. Intensities from near the atomic saturation intensity to well above it are investigated. At low intensities, fine-structure-changing collisions cause trap loss with a rate constant of-10 ' cm /s. At sufficiently high intensity, the trap can be deep enough to effectively freeze out the dominant fine-structure-changing collisions as a loss mechanism, enabling an accurate comparison of the radiative escape loss rate with theory. At the lowest intensities of this radiative escape regime, the measured loss rates compare favorably with those calculated using an optical Bloch equation theory and a three-dimensional model of trap depth. However, the intensity dependence of the measured rates does not show the saturation predicted by the optical Bloch equation theory. It is shown that reliable knowledge of trap depth is necessary to accurately compare experiment with theory.
Physical Review A, 2000
We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and... more We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and two repumping lasers tuned to intercombination transitions, over 10 6 atoms were trapped and average densities of over 10 16 m −3 were obtained. Non-exponential loss of atoms is observed at high densities indicating inelastic collisions between trapped atoms. Over a range of trapping conditions, the density-dependent trap loss rate constant β is extracted from fluorescence decay curves and found to have values in the range of 10 −15 m 3 s −1 , much larger than is observed in alkali traps.
Physical Review A, 1995
We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1X10 ground-sta... more We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1X10 ground-state lithium atoms have been confined in a nonzero magnetic-field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of approximately 3 X 10 cm. The trapped-atom lifetime is 240 s, limited by collisions with background gas. This trap provides an environment in which quantum-statistical effects, atomic collisions, and other ultralow-temperature phenomena can be studied.
Physical Review A, 1997
Recent experiments have used forced evaporative cooling to produce Bose-Einstein condensation in ... more Recent experiments have used forced evaporative cooling to produce Bose-Einstein condensation in dilute gases. The evaporative cooling process can be optimized to provide the maximum phase-space density with a specified number of atoms remaining. We show that this global optimization is approximately achieved by locally optimizing the cooling efficiency at each instant. We discuss how this method can be implemented, and present the results for our 7 Li trap. The predicted behavior of the gas is found to agree well with experiment. ͓S1050-2947͑97͒07005-4͔
Optics Letters, 1993
We explored the suitability of visible III-V single-mode cw diodes for difference-frequency gener... more We explored the suitability of visible III-V single-mode cw diodes for difference-frequency generation of tunable infrared radiation by mixing a red single-mode cw III-V diode laser with a tunable single-frequency cw Ti:sapphire laser in AgGaS 2. More than 1 ILW of cw tunable, infrared (A 5 ,um), narrow-band coherent radiation was generated with type I noncritical phase matching. The wavelength and output-power characteristics of this novel tunable all-solid-state laser source are described, and we demonstrate the applicability of the source to high-resolution molecular spectroscopy by obtaining a test spectrum. The feasibility of a more compact solidstate cw laser spectrometer based on the mixing of two single-mode diode lasers (808 and 690 nm) as pump sources in AgGaS 2 is shown (infrared power generated-3 nW).
Optics Letters, 1992
We demonstrate a chirp-cooling technique that uses a laser with multiple FM sidebands to slow lit... more We demonstrate a chirp-cooling technique that uses a laser with multiple FM sidebands to slow lithium atoms from initial velocities of as much as 1880 m/s to final velocities near zero. Compared with the use of a single sideband, this multisideband technique significantly greater flux of slowed atoms.