1,721,010 research outputs found
Broadband boundary effects on Brownian motion
Get PDFBrownian motion of particles in confined fluids is important for many applications, yet the effects of the boundary over a wide range of time scales are still not well understood. We report high-bandwidth, comprehensive measurements of Brownian motion of an optically trapped micrometer-sized silica sphere in water near an approximately flat wall. At short distances we observe anisotropic Brownian motion with respect to the wall. We find that surface confinement not only occurs in the long time scale diffusive regime but also in the short time scale ballistic regime, and the velocity autocorrelation function of the Brownian particle decays faster than that of a particle in bulk fluid. Furthermore, at low frequencies the thermal force loses its color due to the reflected flow from the no-slip boundary. The power spectrum of the thermal force on the particle near a no-slip boundary becomes flat at low frequencies. This detailed understanding of boundary effects on Brownian motion opens a door to developing a 3D microscope using particles as remote sensors.Sid W. Richardson FoundationR. A. Welch Foundation F-1258Physic
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High-sensitivity tracking of optically trapped particles in gases and liquids : observation of Brownian motion in velocity space
Get PDFtextThe thermal velocity fluctuations of microscopic particles mediate the transition from microscopic statistical mechanics to macroscopic long-time diffusion. Prior to this work, detection methods lacked the sensitivity necessary to resolve motion at the length and time scales at which thermal velocity fluctuations occur. This dissertation details two experiments which resulted in velocity measurement of the thermal motion of dielectric microspheres suspended by an optical trap in gases and liquids. First, optical tweezers were used to trap glass microspheres in air over a wide range of pressures and a detection system was developed to track the trapped microspheres' trajectories with MHz bandwidth and <100 fm/rt(Hz) position sensitivity. Low-noise trajectory measurements allowed for observation of fluctuations in the instantaneous velocity of a trapped particle with a signal to noise ratio (SNR) of 26 dB, and provided direct verification of the equipartition theorem and of the Maxwell-Boltzmann velocity distribution for a single Brownian particle. Next, the detection technology was further optimized and used to track optically trapped silica and barium titanate glass microspheres in water and acetone with >50 MHz bandwidth and <3 fm/rt(Hz) sensitivity. Brownian motion in a liquid is influenced by hydrodynamic, time-retarded coupling between the particle and the fluid flow its motion generates. Our measurements allowed for instantaneous velocity measurement with an SNR of up to 16 dB and confirmed the Maxwell Boltzmann distribution for Brownian motion in a liquid. The measurements also revealed several unusual features predicted for Brownian motion in the regime of hydrodynamic coupling, including faster-than-exponential decay of the velocity autocorrelation function, correlation of the thermal force and non-zero cross-correlation between the particle's velocity and the thermal force preceding it.Physic
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Direct study of quantum statistics in a degenerate Bose gas
Get PDFtextThis dissertation describes the experimental setup for obtaining low dimensional
Bose-Einstein condensate in a single optical box trap. This novel optical trap
type can provide strong confinement in two directions comparable to that which
is possible in an optical lattice, but allows single-site control and addressibility.
The trap consists of a crossed pair of elongated Hermite-Gaussian TEM01 mode
beams supplemented by Gaussian beam end-caps producing a “particle-in-a-box”
type geometry. Single atom detection with nearly unit quantum efficiency has been
demonstrated and is fully integrated with the new trap.
Using this system, we have directly observed sub-Poissonian atom number
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statistics for a degenerate Bose gas with numbers as small as 20 atoms. The observed
reduction of number fluctuations is nearly a factor of two below the Poissonian (shot
noise) limit. Current limit to Fock state creation for even smaller atom number is
also discussed.Physic
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Investigations of laser-induced neuronal guidance
Get PDFOne of the long-standing goals of neuroscience has been to look for ways in
which to control neuronal growth. It has been discovered recently that placing a focused
near-infrared laser spot at the leading edge of a neuronal growth cone elicits a turn in the
direction of the laser spot [Ehrlicher et al., PNAS 99: 16024 (2002)]. Optical guidance
has the advantage over conventional neuronal guidance methods that neither steric strain
nor chemical gradients are necessary to its functionality. Although the success of optical
guidance suggests potential therapeutic applications, our first priority is to decipher the
mechanism underlying the phenomenon in order better to understand it. Several
hypotheses are presented as well as preliminary evidence supporting biased diffusion as
an appropriate model for the system.Physic
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Pulsed magnetic slowing of supersonic beams
No full textSupersonic beams provide a source of cold atoms where laser cooling is not applicable. Although the atoms' temperature in the co-moving frame is in the subkelvin range their velocity is on the order of several hundreds of meters per second. This thesis describes the experimental realization of a novel method to slow atoms and molecules with permanent magnetic moments using pulsed magnetic fields. The method is suitable for most elements since most atomic species are paramagnetic, and can also be applied to certain molecules, as well as electronically excited metastable states and most radicals. We show the slowing of metastable neon in a proof of principle experiment where the mean velocity is reduced from 461.0[plus or minus] 7.7 m/s to 403 [plus or minus]16 m/s in 18 stages. A second setup with 64 stages is now operating and allows us to stop metastable neon in principle. Preliminary results showing slowing from 447[plus or minus]3m/s to 136[plus or minus]5m/s with an efficiency of up to 3.9% are included here, and the slower has been shown to generate atoms as slow as 50 m/s. We find that the slowing efficiency depends strongly on the switching phase. In addition to the experimental results described above, we present simulations of a moving trap which allows trapping and decelerating at the same time.Physic
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Spatially resolved single atom detection of neutral atoms
Get PDFThis thesis describes several new ideas towards spatially resolved single atom detection, using a magic wavelength and spatially resolved stimulated Raman transitions. After an introduction to the theory of AC-Stark shifts and the magic wavelength, we will present the calculations on the magic wavelength for Sodium and Rubidium. We conclude the first chapters by summarizing the experimental efforts made in order to determine the magic wavelength in Sodium. The theory for stimulated Raman transitions will be described in chapter 5 and evaluated in chapter 6 for Sodium atoms, including a prediction for the ideal pulse area and the spatial resolution achievable using magnetic field gradients.Physic
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Experiments with a Bose-Einstein condensate in a quasi-1D magnetic waveguide
No full texttextThis thesis is primarily a comprehensive discussion of the development
of two experimental studies: the quantum transport and effects of heating
of ultracold atoms. It specifically provides details of the manipulation and
control of ultracold atoms in magnetic waveguides, optical lattices, and optical
billiards. The design, construction, and implementation of experimental
apparati are also outlined and additional experimental tests are summarized,
including the realization of a macroscopic transport (> 20 cm) system for ultracold
atoms and transmission of ultracold atoms through a random optical
potential.
The first experiment is a study of the quantum transport for atoms
confined in a periodic potential. These results include a comparison made of
thermal and BEC initial conditions. Here, observation of ballistic transport
is made for all values of well depth and initial conditions, and the expansion
rates for thermal atoms are shown to be in excellent agreement with a singleparticle
model. For weak wells (V0/ER ≤ 6), the expansion of the BEC is
also in excellent agreement with single-particle theory, using an effective temperature
model based on single (non-interacting) particle theory. For deep
wells (V0/ER ≥ 6), a crossover is observed to a new regime for the BEC case,
indicating the importance of interactions on quantum transport.
The second experiment is a study of the effect of different heating rates
on a dilute Bose gas confined in a quasi-1D finite, leaky box. An optical
kicked-rotor is used to transfer energy to the atoms while two repulsive optical
beams are used to confine the atoms. The average energy of the atoms is
localized after a large number of kicks and the system reaches a nonequilibrium
steady state. A numerical simulation of the experimental data suggests that
the localization is due to energetic atoms leaking over the barrier. Our data
also indicates a correlation between collisions and the destruction of the BoseEinstein
condensate fraction and an exponential decay in phase space density.Physic
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Experiments with Bose-Einstein condensation in an optical box
Get PDFtextThis dissertation details the experimental methods used to produce a
Bose-Einstein condensate (BEC) in an optical box trap. This novel optical
trap type has strong confinement in two directions comparable to that which
is possible in an optical lattice, yet produces individually addressable condensates.
The optical trap is based on an pair of elongated Hermite-Gaussian
TEM01 mode beams and tailored by designer box wall beams. Using this
method, we have succeeded in producing individual highly confined lower dimensional
condensates. The box trap is integrated with single atom detection
capability, paving the way for studies of quantum atom statistics.Physic
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Nanowire sharpening : application to field ionization
Get PDFNanowires show potential for a wide range of fields, from developing next generation solar cells, to detecting viruses, to field ionizing gasses. Their uses in such disparate fields are due to the extreme flexibility in which nanowires can be manufactured and customized to order. The heart of a nanowire, however, is it’s tip. Here surface charges accumulates in extreme densities when the wire is biased, consequently
producing large electric fields that are then used in creative ways for exciting applications.
My aim for this thesis is three-fold. First, I intend to establish a context for
nanowires. Why were these structures studied in the first place? What are some
exciting application areas? What makes nanowires unique? Etc... This will set the stage for subsequent sections, and provide the salt and pepper that will make the main course more flavorful. The second is to present an overview of relevant results from the literature that I will later build upon. There are analytical models of varying complexity examining the type of protrusions we are interested in here, as well as numerous numerical studies. We will look at them to get a deeper intuition for whats happening, and use them as a basis for comparison later on. Finally, I will present my results and discuss their consequences.
As we will see, sharpening a cylindrical-post nanowire of height H = 1um,
starting radius of curvature r₀ = 100nm, and base size b = 2 · r₀ = 200nm can
enhance the field further by an order of 100. In addition, the dielectrophoretic (DEP) force present due to strong electric field gradients can significantly alter cooled gas beam trajectories towards the nanowire tips. Non-cooled gas beams also display room temperature trajectory deviations for species with large polarizability to mass
ratios. This leads to the conclusion that at lower temperatures (and even at room
temperature) the field ionization cross section of a nanowire array is significantly increased under certain conditions when taking into account DEP coupling.Physic
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Elastic slowing of supersonic beams
No full textIn this thesis we describe a new method of producing an intense, cold beam of slow noble gas atoms. A beam produced by a pulsed cryogenic supersonic nozzle is reflected from a single crystal atom mirror mounted on the tip of a spinning rotor. Simulations show that fluxes of 10¹¹ He atoms per second at velocities of 50m/s and temperatures of less than 20¹K in the longitudinal direction are feasible. We describe in detail the apparatus built in our laboratory. We are able to tune the speed of a He-Ne mixture from 265m/s to 510m/s while maintaining a cold, monochromatic beam. We plan to reduce the speed of the reflected beam further to under 100m/s in the future by simple modifications of our system.Physic
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