Technical Highlights
- Four Vector Correlations in Collisions. The formalism and
experiments for a four vector correlation, collisional process have been
achieved for the first time. In an atomic beam of calcium atoms, two atoms are
excited with a laser, which aligns the directions of the atomic orbitals,
defining two vectors. A collision in a beam defines a third vector from the
relative velocity, and the fourth vector is determined by the final state
alignment after a collisional energy transfer process. Up until now, there have
been only brief hints of such four vector correlations in other studies, but
never actual measurements of cross section values. In this study, the complete
formalism to extract four-vector correlation cross sections has been developed
and experiments performed to obtain a large number of the cross section values
for the first time. The experiments are performed on the system of two excited
Ca atoms colliding to form one higher excited state and one ground state atom.
There are 27 formal cross sections for the four vector process, and relative
values for 18 of these have been obtained in the measurements here.
(S.R. Leone)
- Surfactant-Controlled Nanodot Growth. Nanoscale semiconductor
devices present numerous possibilities for future studies of quantum
confinement and single electron devices. The growth of germanium on silicon is
a classic system in which nanodot growth occurs spontaneously because of the
mismatch in lattice spacings. Typically a few layers of germanium will grow
first, layer by layer, and then islands of germanium form to produce nanodots
of varying sizes. In recent experiments, control of the size of the nanodots
has been demonstrated by use of an arsenic "surfactant" during
molecular beam epitaxy deposition of germanium on Si (100). Experiments
are performed with controlled molecular beam epitaxy of germanium on silicon,
with varying amounts of arsenic surfactant deposited first on the surface.
Atomic force microscopy is used to investigate the island size after growth.
Laser ionization mass spectrometry and reflection high-energy electron
diffraction are used to correlate the desorbing fluxes of arsenic dimers and
tetramers with the onset of island growth. The results show a dramatic
alteration of the island size and density with application of modest amounts
of arsenic surfactant in the range of 0.25 to 1.0 monolayers.
(S.R. Leone)
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Figure 1. Germanium islands grown on Si(100) using arsenic as a
surfactant. |
- Infrared Near Field Optical Microscopy. A new, infrared, near field,
optical microscope has been constructed and used successfully to interrogate
photoresist polymer films with wavelength tunability. The results show great
promise for probing with molecular group specificity in various materials. A
new method has been devised to pull small diameter fiber tips in
infrared-transparent fibers. This method uses a two-step laser heating and
pulling technique, so that the fiber is first pulled bluntly to an intermediate
diameter and then pulled to a very small diameter tip to form the probe. The
fiber is a zirconium-aluminum fluoride material, which can be successfully
coated with metal to form the transparent waveguide tip. A three micron,
tunable, color center laser is transmitted through the fiber with an efficiency
of one part in 10000, and the improvement in spatial resolution is over a
factor of six better than the diffraction limit of the light at these infrared
wavelengths. (S.R. Leone)
Figure 2. Fiber tips for infrared near-field microscopy. A new, two-step
method, for pulling fluoride fibers leads to 200 nm tips (left) with
enhanced infrared transmissivity (right).
- Optical Frequency Comb Generator -- Optical Frequency Shifter. There
have been considerable efforts toward converting the Optical Frequency Comb
Generator into a versatile and dependable laboratory tool. The first version
had several weaknesses, mainly optical inefficiency and the sensitivity of
cavity tuning and alignment to temperature. The PID temperature controllers on
both the crystal modulator case and on the optical mounting plate substantially
eliminated these thermal problems. The optical inefficiency arose because of
the high frequency-conversion efficiency of the 10.6 Ghz phase modulator:
for a coherent optical input frequency, something like 1/3 of the power is
"lost" by scattering to another sideband frequency. The cure is to
add another mirror in the input line, also of ~99 % reflectivity, and
form an auxiliary Fabry-Perot cavity with the nominal Input Mirror. With PZT
control, this auxiliary cavity can be locked on a transmission fringe for the
input laser frequency, leading to a resonant transmission above 50 %. The
resulting apparatus has the new and elegant property that an input cw laser
field is "transmitted" with an efficiency of ~10 %, but with a
controllable frequency shift of up to about 2 THz, in ~10 GHz steps
controllable with rf precision. The optical spectral purity of the output beam
is good, containing only about -20 dB of the strongest non-selected
(neighbor) component. (J.L. Hall).
Figure 3. Apparent measured frequency of the a10 component of
R(56) at 532 nm. The data of 4/18/98 are shown with their own baseline.
June measurements were made to see limits of offsets caused by pre-filter
mistuning, maladjustment of tracking filter gains, etc. Corrections for rf
standard frequency and 633 nm I2-stabilized laser offsets are
not yet applied.
- NICE OHMS Spectroscopy, and Lasers Stabilized to HCCD and Iodine. It
has been interesting to compare lasers stabilized onto iodine resonances using
modulation transfer method with lasers stabilized to HCCD resonances using the
Noise-Immune, Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy
(NICE-OHMS) method. (See Fig. 4) The HCCD-stabilized system shows only 2
to 4 times more short-term noise than the I2 system, a
remarkable success considering the green I2 transition strength is
half a million times stronger than the P(5) line of the
C2HD (υ2 + 3 υ3)
overtone band. The fact that amazing frequency stability is being achieved with
such an extremely weak reference transition is a direct result of the
spectrometer's ultra-high detection sensitivity. For concreteness we show the
frequency stability achieved with the two approaches.
Figure 4. (top) Stability of beat between I2 stabilized
and HCCD-stabilized lasers. The improving ultrasensitive detection of a weak
overtone resonance of molecular HCCD permits progressively better results on
the laser stabilization. (bottom) The heterodyne reference laser is
stabilized on an I2 transition at 532 nm using modulation
transfer spectroscopy. This reference laser has a stability
~5 × 10-14 at 1 s, from beating experiments with
two I2-stabilized systems.
The NICE-OHMS spectrometer naturally provides laser frequency discrimination
information from both the cavity resonance and the molecular transition. Thus,
it is an ideal system for simultaneously achieving good short- and long-term
frequency stabilization. The laser frequency basically tracks the cavity
resonance with a precision of a few mHz with a fast servo loop. The vibration
noise and the long-term drift of the cavity can be eliminated by stabilizing to
the intracavity molecular transition. (J.L. Hall)
- Apertureless Near Field Scanning Optical Microscopy. With the
ever-decreasing scale of electronic components and chip design, there is an
ever-increasing need to develop efficient optical methods to measure properties
of nanoscale objects with sizes well below the diffraction limit of light.
There have been considerable breakthroughs in this area based on near field
scanning optical microscopy (NSOM), which has conventionally been achieved by
using metal coated, optical fiber tips to confine the light in rapidly tapered,
optical fibers. However, this method is inherently limited by the skin depth of
light in the metal cladding (≈ 12 nm for Al), which, even for
optimum cases, yields only 20 nm to 30 nm resolution and, under
typical operating conditions, more on the order of 100 nm resolution. An
alternative, actively explored in the Nesbitt and Gallagher groups, is to
develop new methods in apertureless near field scanning optical
microscopy, which already have demonstrated resolution improvements down to the
2 nm to 3 nm length scale. This effort is based on a combination of:
i) evanescent wave excitation of molecules/nanostructures on a prism
surface; ii) sharp Si or Ag coated Si structures guided by atomic force
microscopy (AFM) to condense the evanescent electric fields in the vicinity of
the tip; and iii) resonant light scattering or fluorescence detection of
the molecules/nanostructures subsequent to the excitation event. This method
has been used to image Au nanospheres by resonant scattering of 543 nm
light near the Au plasmon resonance, and to determine that the combination of
AFM tip + particle leads to a scattering enhancement of over
4000-fold from that of the bare Au nanospheres. (D.J. Nesbitt and
A. Gallagher).
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Figure 5. Apertureless AFM/NSOM fluorescence image of a dye doped
polystyrene nanosphere (~80 nm) with the corresponding atomic force image
below. Full size image. |
- Fluorescence Based Near Field Imaging Method. We have been extending
near field imaging methods into the fluorescence domain, where the Si and Ag
coated AFM tips are used to influence the near field excitation of dye
molecules doped in latex nanospheres. The resulting fluorescence is imaged with
a high numerical aperture microscope and fiber coupled avalanche photodiode
combination. What is observed is considerable spatial structure in the
fluorescence NSOM images as a function of tip-molecule distance, indicating
both enhancement as well as quenching effects due to the presence of the tip.
Especially interesting in the NSOM images are the effects due to the AFM probe
tip blocking the molecular fluorescence, which therefore represents the casting
of a vastly sub-diffraction-limited "shadow" of a near field light source by a
nanoscale object. These measurements represent a world record for the highest
spatial resolution achieved to date with near field excitation and fluorescence
detection. The results provide unprecedented, experimental benchmarks for
comparison with near field theory, as well as a completely new method of near
field optical imaging by spatially shadowing specific fluorescent sites on the
nm length scale. (D.J. Nesbitt and A. Gallagher)
- State-to-State Reaction Dynamics in Crossed Molecular Beams.
Chemistry is a discipline of enormous technological and economic importance and
yet a detailed understanding of how the simplest of chemical reactions occur
still represents a state-of-the-art area of experimental and theoretical
chemical physics research. There has been major progress in this area by
exploiting slit discharges as an intense source of jet cooled F atoms for
state-to-state reactive scattering studies in crossed supersonic jets. As the
initial target, the focus has been on the classic
F + H2 → HF (v,J) + H
reaction, where the nascent rovibrational product states of HF are detected via
direct IR laser absorption methods. The powerful advantage of such a high
resolution, laser based approach is that it offers
10-4 cm-1 spectral resolution on the product states,
which is 5 to 6 orders of magnitude better than previous crossed beam time
of flight studies. These studies have yielded for the first time, collision
free, fully quantum state resolved, product state HF (v,J) distributions
from the F + H2 reaction. These can now be studied as a
function of center of mass collision energy. The experimentally observed
distributions are in good qualitative agreement with state-of-the art
ab initio/dynamics calculations. The data also reveal significant
discrepancies between theory and experiment, providing the first experimental
evidence that multiple electronic potential surfaces (i.e., with both ground
state, spin orbit excited F atoms) are involved in the reaction event. This
evidence for the importance of non-adiabatic processes in this simplest
of "benchmark" systems implies a much richer range of dynamics for chemical
reaction systems than heretofore suspected. (D.J. Nesbitt).
- Second Harmonic Generation from Si(100)/SiO2. The
Si(100)/SiO2 interface is critical to the semiconductor industry.
The channel region in a MOSFET is Si(100), while the gate dielectric consists
of SiO2. The thickness of the gate dielectric must scale in
proportion to the scaling of the channel length. The current generation of
integrated circuits has oxide thickness of approximately 9 nm, but within
2 to 3 generations the thickness will shrink to 4 nm. The necessity for
extremely thin oxides has made roughness at the Si/SiO2 interface a
topic of increasing importance in the industry.
Optical second harmonic generation has been shown to be sensitive to roughness
at this interface. In materials with bulk inversion symmetry, second harmonic
generation is only dipole allowed at an interface or surface, making it a
highly interface/surface selective technique. However, the underlying physics
that gives rise to the roughness sensitivity is not understood. We are
investigating why second harmonic generation is sensitive to roughness. Our
experiments are focused on measuring the spectral dependence of the signal.
There are resonant structures that can be attributed to various features in the
band structure of silicon.
The preliminary results indicate a previously unknown resonance in the second
harmonic spectrum. It is manifest as a strong increase in the signal at the
blue end of the spectral region that the incident laser can tune over. Most
strikingly, this resonance shows different symmetry properties than adjacent
spectral regimes. The symmetry properties suggest that the signal is arising
from step edges at the interface. If confirmed, this would be a very
interesting result because we will have identified interface features that
influence the second harmonic spectrum. Efforts are underway to extend the
tuning range of the incident pulses using an optical parametric oscillator.
This will necessitate the use of a reference signal to allow comparison of
results from the optical parametric oscillator and laser.
(S.T. Cundiff)
- Isolation System. New, more sensitive seismometers with
interferometric readouts have improved the performance of our low-frequency,
active isolation system. The low-frequency, active isolation system comprises
multiple, suspended stages with six seismometers on each to sense the
diminished disturbances that penetrate the passive suspensions. Control systems
cause actuators to counter the residual disturbances on each stage. The
"preliminary" stage, which operates outside of vacuum, has optical
imaging readouts of its seismometers. It is fully operational and meets its design
performance goals of a factor 100 isolation down to 1 Hz. The two
main stages are housed in a vacuum system that is supported by the
preliminary stage and has seismometers with both low sensitivity, imaging
readouts for quieting and high sensitivity, interferometric readouts for
maximum disturbance reduction. The first main stage now has operating
interferometers on all six seismometers. The second main stage has not been
instrumented with seismometers, but has a complete suspension and dummy
payload. We have demonstrated 70 dB of isolation in vertical and
horizontal degrees of freedom with both the preliminary and the first main
stage control loops active. This demonstrates that such stages can be stacked,
that they are stable in a stacked configuration and that high gains are
simultaneously achievable on both stages. The demands on seismometer
sensitivity increase at each inner stage because the remaining disturbances
are reduced by another outer stage. Research continues on seismometer
improvements and technical challenges stemming from the intricate control
systems. (J.E. Faller)
- New, Small, Absolute, and Highly Portable Gravity Instrument. Work
is progressing extremely well in the development of a new, mechanical,
cam-based, free-fall (i.e., dropping) system. This is to be integrated in a
small, fast, portable, and simple-to-use absolute g instrument. A
two-centimeter free-fall drop is created by using a rotating cam to drive the
dropping chamber. This approach results in a high measurement rate of
3 drops per second. The feasibility of this approach has been
demonstrated, and presently a refined and evacuable apparatus is under
construction. At the same time, and in the interest of both simplicity and
instrumental cost, an alternate, passive spring system is being worked on as a
possible replacement for the traditionally used "super spring."
During the last year a major conceptual breakthrough occurred in regard to this
development. By employing a double cam with the second half of the cam driving
an auxiliary mass, the center of mass of the instrument does not move. Thus its
motion can be used to cancel out ground-sensed weight changes of the apparatus
that could heretofore systematically bias the determination. This double cam
system has been implemented and the concept successfully demonstrated. This
development is being carried out as a part of an ongoing CRADA with Micro-g
Solutions, the commercial supplier of our previously developed FG-5 gravimeter.
(J.E. Faller).
- Gravity Measurement. The nearly 1% discrepancy between the recent
PTB results and the "accepted" value of "big" G,
the Newtonian constant of gravitation, is of considerable interest to the
standards community. The fact that the PTB measurement appears to have been
competently and thoughtfully carried out makes the discrepancy even more
intriguing. NIST
personnel, along with collaborators from the National Geodetic Survey,
Micro-g Solutions, and JILA, have used an FG-5 absolute gravimeter
together with a movable 500 kg tungsten mass which surrounds the dropping
chamber to measure G through the effect this large mass has on the
measured value of g in the free-fall region. The measurement has been
carried out and the data analysis is now completed. Though the idea of
measuring the small mass-induced Δg effect on top of g
itself might appear almost
impossibly difficult, the fact that the signal can be modulated by moving the
source mass every 10 or 20 min to either increase or decrease the measured
value of g makes it possible to measure well into the noise floor.
Though the "expected" accuracy (given the integration time and the expected
FG-5 precision) should be 5 or 6 parts in 104, instrumental drifts
of unknown origin have limited the accuracy obtained to about 0.14%. The final
result is in reasonable agreement with the presently accepted CODATA value and
does not support the PTB outlying value. However, the value is about 0.2%
higher than the CODATA value which, incidentally, puts us a bit closer to the
PTB result. (J.E. Faller)
- Non-linear Dynamics of Intense Femtosecond Pulse Propagation. A
clear picture of the propagation of femtosecond laser pulses is fundamentally
important to many scientific and technological applications. Although some
applications rely primarily on the delta-function qualities of a femtosecond
pulse (e.g., time-resolved gating, transmission of binary data), at a more
fundamental level pulse propagation encompasses much more than the simple
transport of energy. It is a basic fact of the Maxwell equations that the
manner in which a field propagates is fundamentally tied (via the polarization)
to the properties of the medium in which it travels. From the standpoint of
femtosecond diagnostics, this implies that if the electric field can be
accurately characterized after propagation through a medium of interest, then
valuable information about the medium and the propagation process may be
obtained. Work has focused on both measuring and modeling the non-linear
dynamics of the propagation of intense femtosecond pulses in bulk media.
Measurements of the fields of such pulses have been performed with
frequency-resolved optical gating (FROG), providing a means to observe the
evolution of both the temporal amplitude and phase of femtosecond pulses as
they undergo rapid broadening and splitting while propagating in fused silica.
These accurate measurements have initiated the development of a more complete,
modified, non-linear Schrödinger equation (NLSE), which includes
contributions of physical mechanisms such as Raman non-linearities, space-time
coupling, nonlinear shock effects, and non-paraxiality. The improved model
successfully predicts temporal asymmetries observed in the measurements. In
addition, the technique of spectral interferometry has been applied to full
beam measurements, permitting the measurement of the full (temporal plus
spatial) electromagnetic field on a femtosecond time scale for the first time.
(T.S. Clement).
- Bose-Einstein Condensation. When a gas is made sufficiently cold, it
undergoes a phase transition into a Bose-condensed state: the sample becomes
highly coherent, with most of the atoms participating in a macroscopic common
wavefunction. The atoms in a condensate are in close analogy with photons in a
laser beam. It was NIST/JILA scientists in 1995 who first observed the
phenomenon of Bose-Einstein condensation in a gas. Since then the field has
grown very rapidly, to the point where there are now hundreds of technical
papers published every year on the topic.
Work at JILA on BEC in the past year has concentrated in two main areas,
tunable interactions and mixed condensates.
Tunable interactions: Many of the properties of a Bose-Einstein condensate are
determined by the interactions between atoms. To the extent one can
deliberately modify the nature of the interactions, one has intimate control
over the behavior of the condensate. JILA theorists predicted that in the
presence of an ambient magnetic field of about 15.0 mT (150 gauss),
two-body interactions between Rb-85 atoms should go through a resonance.
Experiments at JILA recently confirmed that the rate of elastic collisions
between two ultracold Rb-85 atoms in a 15.5 mT (155 gauss) field is
at least a factor of 10,000 higher than it is in a 16.7 mT (167 gauss
field). The 16.7 mT magnetic fields at the heart of an atomic collision
are on the order of tens of Tesla (hundreds of thousands of gauss). That a mT
change in the ambient field could have such a profound effect on a collision
says a lot about the power of resonance effects in ultracold collisions.
Efforts are now underway to create a condensate in Rb-85 in order to exploit
this so-called "Feshbach resonance" in BEC studies.
Mixed Condensates: A series of studies on the behavior of condensate mixtures
has been performed, with the two components corresponding to two different
hyperfine states of Rb-87. Interestingly, the same atomic physics theory that
predicts the "pressure" in the condensate (the self-repulsion of the condensate
atoms) also yields a prediction for the pressure-shift of the Rubidium clock
transition. Thus, some of JILA's fluid-dynamical studies on mixed condensates
have provided a sensitive confirmation of the accuracy of the atomic theory
that predicts the ultimate limit to the accuracy of Rubidium-based atomic
clocks. (E.A. Cornell).
- Cooling a Dilute Gas of Fermionic Atoms to Quantum Degeneracy
Substantial progress has been made toward the goal of cooling a dilute gas of
fermionic atoms to quantum degeneracy. Building on techniques used to produce
BEC in a dilute atomic gas, we have implemented a double-MOT apparatus for
cooling and trapping potassium. It has a fermionic isotope in addition to two
that are bosons. Features of the apparatus include the use of diode lasers
exclusively and the successful implementation of a novel, enriched source for
the fermionic isotope 40K. In the first stage of cooling, we
have trapped 40K atoms in a magneto-optical trap (MOT) with four
orders of magnitude higher number of atoms than any previous work. In
preparation for the second stage of cooling, we have transferred 70% of these
atoms into a magnetic trap, and seen lifetimes that are sufficiently long for
successful evaporative cooling. Since evaporative cooling depends heavily on
elastic collision rates that are not well known for potassium, we are
performing measurements of cold collision rates and observing the suppression
of s-wave collisions in a spin-polarized sample due to Fermi-Dirac statistics.
Once quantum degeneracy is reached, additional effects of the quantum
statistics and the formation of a Fermi sea will be studied through
measurements of thermodynamics, light scattering, and collisional properties of
the dilute fermionic gas. (D.S. Jin).
- Strontium Atom Trapping. Strontium atom trapping offers some
outstanding opportunities for metrology and studying the physics of cold
collisions. Since Sr singlet states have no fine structure and the principle
isotope has no hyperfine structure, Sr provides exceptionally clean cut tests
of trap collision theories. The triplet states of Sr offer an opportunity for
analysis of trap behavior, very low temperature cooling, metrology and possibly
Bose condensation without evaporative cooling. We have developed a Sr trap,
based on resonance-line trapping from a thermal vapor in a sapphire-window
cell. Trap loss due to leakage of excited 51P atoms (Sr*) to the
triplet manifold is prevented with lasers that recycle metastable atoms back to
the ground state, resulting in high trapped-atom densities and storage times.
The trap velocity distribution is nondestructively measured using
intercombination line fluorescence, which allows an exacting study of the
dependence of trap temperature and cloud-cooling rates on detuning and power of
the trapping beams. From the transient response of resonance-line and
intercombination-line fluorescence, we deduce the various radiative rates
within the lowest nine states of Sr. We have also determined the Sr-Sr*
collisional loss rate coefficient from trap-loading time dependence and power
and density dependencies. This is dominated by excitation of the attractive,
non-radiating molecular state, which gains a dipole moment at large atomic
separation due to radiation retardation. This provides an exceptionally
sensitive test of retardation effects in molecular spectra, as well as of trap
loss theories due to the exactly known molecular potentials. Atoms trapped at
mK temperatures in the resonance-line trap are also transiently loaded into an
intercombination-line trap with ~50% efficiency using broad-band, red-detuned
cooling. This yields high Sr densities at ľK temperatures, as well as the
opportunity to measure cooling rates and unique forms of Ramsey fringes.
(A.C. Gallagher).
- Particle Growth in Thin-Film Deposition Discharges. Particles grow
in the discharges used to deposit thin semiconducting films. Some of these
particles escape to the films, causing a deterioration in their electronic
properties. To understand the causes and behavior of these particles, their
growth and transport to the substrate has been studied in the types of
discharges used to make amorphous silicon, thin-film transistors and
photovoltaics. Light scattering by the suspended particles, during and
immediately after extinguishing the discharge, has been analyzed to yield
particle growth rates and densities for the very small particles
(<30 nm diameter) that escape to the films. This has yielded clear
insights regarding the species trapped in the discharge and the reactions
causing them to grow. These results have also demonstrated that particle
trapping and growth is an inevitable consequence of the plasma chemistry used
to produce the films. However, this does not mean that particles will
inevitably escape to the growing films. The observations and related models
have identified important spatial variations in plasma potential, as well as
thermophoretic effects, that can be used to mitigate the escape of particles to
the films. (A.C. Gallagher).
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