Chairs
Gerhard Abstreiter
Walter-Schottky-Inst
Technical Univ of Munchen
Garching, D-85748 GERMANY
49-89-28912770
Yusihiko Arakawa
Univ of Tokyo
Tokyo, 106 JAPAN
81-3-34781139
David Awschalom
Physics Dept
Univ of California-Santa Barbara
Santa Barbara, CA 93106
805-893-2121
Daniel Gammon
Naval Research Laboratory
Code 6876
Washington, DC 20375
202-404-4533
Lukas Novotny
Pacific Northwest National Lab
MS K8-88 PO Box 999
Richland, WA 99352
509-376-4294
Symposium Support
- Army Research Office
- Office of Naval Research
* Invited paper
SESSION AA1: SEMICONDUCTOR HETEROSTRUCTURES
Chair: David D. Awschalom
Monday Morning, April 5, 1999
Salon 1 (M)
8:30 AM *AA1.1
NEAR-FIELD SPECTROSCOPY OF A GATED TWO-DIMEN- SIONAL
ELECTRON GAS. G. Eytan, Y. Yayon, M. Rappaport, H. Shtrikman and I. Bar-Joseph
, The Weizmann Institute of Science, Dept of Condensed Matter Physics,
Rehovot, ISRAEL.
We study the spatial distribution of the photoluminescence
(PL) of a gated two-dimensional electron gas with sub-wavelength resolution.
This is done using a low temperture near-field scanning optical microscope:
a tapered optical fiber tip is scanned in the near-field region of the
sample surface, and collects the PL emitted by the sample, through a semi-transparent
gate. The sample studied is a modulation doped single GaAs/AlGaAs quantum
well, and the experiment is conducted at 4.2K. The collected PL is analyzed
in a spectrometer. The spectral line of the negatively charged exciton,
X-, which is formed by binding a photo-excited electron-hole
pair to a native electron, serves as an indicator for the local presence
of charge. The local luminescence intensity of this line is directly proportional
to the number of electrons under the tip. We observe large spatial fluctuations
in the X- intensity in the gate voltage range, where the electron
conductivity exhibits a sharp drop. The amplitude of these fluctuations
increases and the Fourier spectrum extends to lower spatial frequencies
as the gate voltage becomes more negative. We show that the fluctuations
are due to the statistical distribution of localized electrons in the random
potential of the remote ionized donors. We use these fluctuations to image
the electron and donor distribution in the plane.
9:00 AM *AA1.2
NEAR FIELD SPECTROSCOPY OF INDIUM GALLIUM NITRIDE HETEROSTRUCTURES.
D.K. Young , P.A. Crowell, M.P. Mack, S. Keller, E.L. Hu and D.D. Awschalom,
University of California, Santa Barbara, CA.
The efficient optical properties of Indium Gallium Nitride
(InGaN) in the blue region of the spectrum despite its large dislocation
density (1010/cm2) suggests strong carrier localization
preventing non-radiative recombination. A lack of understanding of the
role of defects and their optical properties has limited the commercial
applicability of InGaN laser diodes, which suffer from short laser life
and mutimode emission. Near-field scanning optical microscopy is used to
image photo- and electro-luminescence from InGaN heterostructures and laser
diodes, respectively, with spatial resolution of 
100nm
for T = 20-300 K. Photoluminescence from InGaN/GaN single and multiple
quantum wells (QW) is seen to vary spatially on sub-micron length scales,
without any spectroscopic signature of quantum dots. Strong correlations
are found between emission sites and defects in single and multiple quantum
well samples on a sub-micron scale. Incorporation of multiple QWs into
InGaN based laser diodes and their waveguide properties are investigated
by imaging the electroluminescence below and above the lasing threshold
along their cross section at room temperature. Broad spectral emission
near the active region reveals inefficient lasing as well as absorption
and reemission of the lasing mode from a strain compensating layer. Near-field
measurements have also shown a relationship between modal emission and
waveguide structure.
Work supported by AFOSR, ONR, and the NSF Science and
Technology Center for Quantized Electronic Structures. We thank A.C. Abare,
M. Hansen, L.A. Coldren, and S.P. Denbaars for their collaboration in the
laser diode work.
9:30 AM AA1.3
RAMAN IMAGING OF SEMICONDUCTOR NANOSTRUCTURES USING SOLID
IMMERSION LENSES. C.D. Poweleit , L. Shi and J. Menendez, Arizona State
University, Department of Physics and Astronomy, Tempe, AZ.
Near field optical microscopy (NSOM) has been successfully
combined with photoluminescence spectroscopy to gain deeper insight into
the electronic structure of nanostructured systems. Unfortunately, second-order
optical techniques such as Raman scattering, with its very small scattering
cross sections, have shown the current limitations of a fiber based NSOM
technique with weak light scattering systems. To overcome this problem,
we have developed an alternative near-field technique which eliminates
some of the intrinsic limitations of NSOM-based spectroscopy. We combine
a conventional microscope with solid immersion lenses (SILs) to obtain
Raman images with an unprecedented combination of spatial and energy resolution.
SILs are particularly adapted to Raman spectroscopy because they lead to
an increase in the collected signal. Moreover, by using the spectrometer
as a tunable bandpass filter we can obtain Raman images of wide areas with
no need to use point-by-point scanning. We present Raman images of silicon
wires, silicon microstructures, and semiconductor quantum dots. We show
that Raman images of quantum dots can be successfully collected even when
the dot density is so low that no characteristic Raman signal is observed
in a conventional Raman experiment.
9:45 AM AA1.4
IMAGING OF TWO DIMENSIONAL AND ZERO DIMENSIONAL EIGENSTATES
IN A NARROW QUANTUM WELL USING A SOLID IMMERSION MICROSCOPE. Qiang Wu and
Robert D. Grober, Yale University, Dept of Applied Physics, New Haven,
CT; D. Gammon, Naval Research Laboratory, Washington, DC.
We report a low temperature, spectroscopic, imaging study
of a 2.8 nm GaAs/AlGaAs quantum well using a sapphire solid immersion lens.
This solid immersion technique achieves 250 nm spatial resolution with
unity transmission efficiency. Our photoluminescence studies yield images
of the well known sharp-line local emission spectrum indicative of these
narrow quantum wells. These images allow us to document the density and
distribution of dots. We observe that the dots can be sorted spectrally
into two distributions, referred to in the literature as ten monolayer
and eleven monolayer wide regions, and that these distributions spatially
anticorrelate. Photoluminescence excitation (PLE) spectroscopy shows evidence
for a two-dimensional free exciton state which persists throughout the
entire sample. We have studied this eigenstate using a novel PLE diffusion
technique where we generate images by raster scanning the pump while monitoring
the emission from a particular dot. This diffusion study confirms the non-local
nature of this eigenstate. Our study indicates that the excitonic nature
of this narrow quantum well is mostly two dimensional with occasional imperfections
that yield the zero dimensional dots. While these dots define the PL spectrum,
they constitute only a small fraction of the sample.
10:30 AM *AA1.5
NEAR-FIELD SCANNING OPTICAL MICROSCOPY INVESTIGATIONS
OF ORGANIC ELECTRONIC MATERIALS. David M. Adams, Donald B. O'Connor, Paul
F. Barbara , Univ of Texas, Dept of Chemistry and Biochemistry, Austin,
TX.
NSOM investigation of a variety of organic materials such
as organic semiconductor heterojunctions and other organic charge transfer
media, has lead to new information on the energy transfer and electron
transfer properties of these materials. Additionally, the coupling of topographic
and optical information in the NSOM experiment has allowed for a direct
correlation of local morphology and optical properties. A variety of different
NSOM fluorescence experiments including polarization microscopy, time-resolved
NSOM and other more complex NSOM experiments have been shown to be promising
approaches to understanding the spatially-resolved photophysical and electronics
properties of a broad range of organic materials.
11:00 AM AA1.6
NEAR-FIELD SCANNING OPTICAL MICROSCOPY STUDY OF CARRIER
RECOMBINATION AT SURFACES AND INTERFACES. M.K. Herndon and R.T. Collins
, Physics Department, Colorado School of Mines, Golden, CO; D.J. Friedman,
National Renewable Energy Laboratory, Golden, CO.
A near-field scanning optical microscope (NSOM) has been
used to record spatially resolved photocurrent images in pn junction structures
made from single crystal and polycrystalline semiconductors. The magnitude
of the photocurrent provides a sensitive measure of minority carrier recombination
effects. Single crystal GaAs junctions were cleaved and examined edge-on
allowing the depletion region to be directly probed. The magnitude of the
photocurrent varied systematically across the depletion region. This is
attributed to a competition between recombination at the free surface and
collection of carriers by the depletion region. Variations in the photocurrent
images after sulfur passivation of the surface and as excitation wavelength
was varied from 2.7 eV to near the GaAs bandgap were observed and interpreted
in terms of changes in surface recombination. Samples with embedded heterojunctions
exhibited features in the photocurrent image which could be attributed
to the heterolayer. Varying the excitation wavelength through the bandgap
of the heterolayer showed that the features arose primarily from changes
in carrier collection efficiency near the layer, rather than from absorption
in the layer. Photocurrent and topographic images of the front faces of
polycrystalline devices were also recorded and correlations between grain
structure and collection efficiency were observed. This work was supported
by the NSF.
11:15 AM AA1.7
ENHANCED NEAR-FIELD RAMAN SPECTROSCOPY. Claire E. Jordan
, Lee J. Richter, Richard R. Cavanagh and Stephan J. Stranick.
Near-field Raman spectroscopy can be used to obtain chemical
specificity with the subwavelength spatial resolution of near-field scanning
optical microscopy (NSOM). We report detailed measurements of near-field
Raman spectra from a single crystal diamond sample. These measurements
allow us to access the limits of using conventional aluminum coated apertured
probes for near-field Raman spectroscopy. In order to discriminate between
the near-field contributions to the Raman signal and bulk scattering, the
Raman intensity has been measured as a function of the sample-probe separation.
The Raman intensity shows about a factor of seven increase for sample-probe
separations of approximately 10 nm compared to signals measured at separations
greater than 100 nm, indicating that near-field contributions are present
in these Raman spectra. The functional form of the increase in the Raman
signal with decreasing sample-probe separation is expected to depend on
the aperture size of the near-field probe. This has the potential to provide
a simple in situ means of measuring aperture sizes of NSOM probes. Comparisons
between this predicted size and the aperture size measured by scanning
electron microscopy are made. Due to both the relatively low Raman cross
sections and the poor throughput of aluminum coated probes, relatively
long integration times (5 minutes)
are required to obtain high quality spectra. For this reason we are investigating
methods of modifying the probes to enhance the near-field Raman signal.
Preliminary experiments indicate that an aluminum coated apertured probe
modified by over-coating it with a rough layer of silver shows a greater
enhancement in the near-field Raman intensity than is observed for typical
NSOM probes coated only with aluminum.
SESSION AA2: THIN FILMS, POLYMERS, AND MOLECULES
Chair: Lukas Novotny
Monday Afternoon, April 5, 1999
Salon 1 (M)
1:30 PM *AA2.1
NANOMETER-SCALE POLARIZATION DYNAMICS IN FERROELECTRIC
THIN FILMS. Charles Hubert, Jeremy Levy , University of Pittsburgh, Dept.
of Physics and Astronomy, Pittsburgh, PA; Adrian C. Carter, Wontae Chang,
Steven W. Kierchoefer, James S. Horwitz, Douglas B. Chrisey, Naval Research
Laboratory, Washington, DC; Hua Jiang, NZ Applied Technologies.
Ferroelectric thin films are attractive materials for
a number of applications including frequency-agile microwave electronics
and non-volatile high-density storage. We have developed two high-resolution
optical techniques for studying ferroelectric polarization dynamics in
ferroelectric thin films. Confocal scanning optical microscopy (CSOM) is
used to image the ferroelectric polarization of BaxSr1-xTiO3
(BST) thin films at room temperature with sub-micrometer spatial resolution.
These films appear to show coexisting paraelectric and ferroelectric phases,
which may be related to local strain or compositional variations in the
film. These variations may be responsible for the inhomogeneous thermal
broadening of the ferroelectric phase transition; in particular, dielectric
loss in thin films may be dominated by a relatively small fraction of nanometer-sized
regions. The second technique, apertureless near-field scanning optical
microscopy (ANSOM), is used to probe polarization dynamics on scales as
small as 30 . ANSOM images of
BST films show rich detailed polarization structure which is not related
to topographic features. ANSOM has provided, we believe, the first real-space
images of polar nanodomains in these materials. Finally, time-resolved
images of domain motion on nanosecond and picosecond time scales will be
presented.
This work is supported by DARPA and NSF grant DMR9701725.
2:00 PM *AA2.2
IMAGING LOCAL MICROWAVE MATERIAL PROPERTIES USING SCANNING
NEAR-FIELD MICROWAVE MICROSCOPY. B.J. Feenstra , D.E. Steinhauer, C.P.
Vlahacos, John Lee, S. Aggarwal, R. Ramesh, M. Rajeswari, T. Venkatesan,
F.C. Wellstood and Steven M. Anlage, University of Maryland, Materials
Research Science and Engineering Center and Center for Superconductivity
Research, College Park, MD.
Many commercial applications require devices to operate
at frequencies within the GHz range. For optimum device performance, the
materials used need to be characterized at the operating frequencies. In
addition, as the size of devices shrinks, local information about material
properties becomes increasingly significant. We will present the use of
a scanning near-field microwave microscope for measuring microwave material
properties down to length scales of approximately 1 m.
The versatility of the microscope will be demonstrated using several examples.
First, we will show spatially resolved, quantitative images of the dielectric
constant, , and loss tangent, ,
of ferroelectric thin films. Results were obtained in the ferroelectric
as well as in the paraelectric state. Furthermore, the local tunability
of a 1
area can be measured by studying the hysteretic behavior under the influence
of an applied dc-bias. A second example will be the existence of a large
magnetoresistive (MR) effect at GHz frequencies and room temperature in
colossal magnetoresistance (CMR) thin films. At high frequencies (
10 GHz) and moderate applied magnetic fields (
T), we find an MR-effect which is considerably larger than the effect expected
on the basis of observations made for the dc-resistivity.
2:30 PM AA2.3
QUANTITATIVE ELECTRICAL IMPEDANCE MAPPING WITH 100 NM
RESOLUTION BY SCANNING EVANESCENT MICROWAVE MICROSCOPE. X.D. Xiang , C.
Gao and Fred Duewer, Lawrence Berkeley National Laboratory, Berkeley, CA.
We have developed a novel scanning evanescent microwave
microscope (SEMM) capable of mapping the complex electrical impedance (of
any materials) quantitatively with sub-micron resolution. The microwave
frequency range was chosen because this is the relevant frequency range
for most electronic applications. A 100 nm resolution on dielectric materials
(/106) has been demonstrated.
Since the scanned tip is a part of a high Q resonator, the SEMM has very
high sensitivity (f/f
10-7 and /
10-4). More importantly, we have performed a theoretical near-field
analysis that yielded analytic solutions for both insulating and
conducting materials. The theory enables quantitative local measurements
of complex dielectric constant of insulators or conductivity of conductors.
The theory also allows quantitative microscopy of scanning capacitance,
electrostatic force and electrical charge microscopes. Applications of
SEMM to study ferroelectrics, dielectrics, metals, semiconductors, magnetic
materials and biological samples will be discussed.
3:15 PM AA2.4
NANOSCALE OPTICAL PROPERTIES AND STRUCTURE OF MONOLAYER
FILMS. Steven R. Cordero , Kenneth D. Weston, Steven K. Buratto, Department
of Chemistry, University of California at Santa Barbara, CA.
Supported monolayer films, prepared by the Langmuir-Blodgett
(LB) technique, of the phospholipid dipalmitoylphosphatidylcholine (DPPC)
stained with various fluorescent probes have been examined with fluorescence
near-field scanning optical microscopy (NSOM). These NSOM images provide
high spatial resolution (10-100 nm) well beyond the diffraction-limited
resolution (400nm) of conventional
fluorescence microscopy. Our images show a variety of interesting features
such as grain boundaries, subdomains, sheared domains, and collapsed structures.
In addition, our topography images obtained via simultaneous shear force
microscopy provide important insight into the relationship between film
morphology and optical properties. We have used these insights to direct
new self-assembly methods for optical materials such as silicon nanoparticles
and semi-conductor quantum dots at the air-water interface based on LB
techniques.
3:30 PM AA2.5
SELF-ORGANIZED CHARGE TRAPPING MATERIALS: SPATIALLY RESOLVING
NANOSCOPIC STRUCTURE AND ELECTRO-OPTIC CHARGING/DISCHARGING IN ZINC PORPHYRIN
ASSEMBLIES WITH NSOM. David M. Adams , Josef Kerimo, Chong-yang Liu, Allen
J. Bard, Paul F. Barbara.
Sandwich cells consisting of the photoconductive material
zinc-octakis(-decoxyethyl) porphyrin
(ZnODEP) deposited between conducting indium tin oxide (ITO) coated glass
slides have been shown to behave as charge storage devices suitable for
electro-optic memory applications. The present study utilizes near-field
scanning optical microscopy (NSOM) to spatially resolve the complex morphologies
and photophysics of thin films of ZnODEP. The electro-optic charging/discharging
of distinct domains are investigated by spatially and temporally resolving
the charge induced fluorescence quenching and by monitoring the near-field
probe-sample distance. These studies show that the operation of a molecular
based charge trapping device can be simulated in the NSOM microscope when
a bias voltage is applied between the aluminum coated near-field probe
and the conducting substrate. The simulated device is [electrode (Al probe)/insulator
(impurity layer)/photoconductor (ZnODEP)/electrode (ITO glass)]. These
results demonstrate that NSOM is an effective analytical method for the
spatially resolved study of the rates and efficiencies of charging/discharging
in electro-optic materials.
3:45 PM AA2.6
LOCAL OPTICAL FIELDS AT FRACTAL SURFACES. Z. Charles
Ying, K. Banerjee, W.D. Bragg and Jane G. Zhu , New Mexico State University,
Department of Physics, Las Cruces, NM.
The optical field at a fractal surface, due to its unique
geometry, is highly non-uniform; there exist areas of nanometer dimensions
where the local field exceeds the incident field by several orders of magnitude.
Such local field variations can be observed using the near-field optical
technique. In our study, we have synthesized two classes of fractal materials,
nanoparticle aggregates and metal-insulator films near percolation threshold,
using the laser-ablation technique. The products are characterized using
transmission electron microscopy and optical absorption spectroscopy in
the ultraviolet and visible ranges. The optical and microstructural properties
of the material are affected by the growth conditions, such as buffer-gas
pressure and laser intensity. The local optical field and its correlation
with morphology are investigated using a near-field optical microscopy
with atomic force microscopy (AFM) capability. Local variations of optical
field are observed for both types of fractal materials. AFM images of metal-insulator
films are essentially featureless due to their fine geometry, while near-field
optical images recorded simultaneously show clearly the areas of high and
low intensity. These observations provide unambiguous experimental proof
for the existence of local-field variations of nanometer dimensions. We
have also demonstrated that the local-field variations at a fractal surface
can be photomodified by laser irradiation at moderate powers.
4:00 PM AA2.7
NOVEL TIP-SAMPLE DISTANCE FEEDBACK CONTROL METHODS IN
A SCANNING EVANESCENT MICROWAVE MICROSCOPE. Fred Duewer , C. Gao, I. Takeuchi
and X.D. Xiang, Lawrence Berkeley National Laboratory, CA.
The image response of all scanned probe based microscopes
depends on both tip-sample distance and physical properties. It is important
to be able to separate topography and physical properties of samples. This
requires measurements of multiple independent signals and detailed knowledge
about the functional dependence of signals with regard to the tip-sample
distance and physical properties of samples. We have demonstrated this
capability in our scanning evanescent microwave microscope (SEMM). Our
SEMM can access multiple independent signals, such as the change in resonant
frequency and quality factor, etc. simultaneously. Furthermore, we have
obtained analytic solutions of the near-field interaction between evanescent
electromagnetic waves between the tip and sample. Combining these capabilities,
we demonstrated that topography and physical properties could be separated
and quantitatively determined during real-time scanning. Case demonstrations
on conducting and ferroelectric materials will be discussed. Tip-sample
distance regulation can be achieved over distances ranging from microns
to nanometers. Local physical properties, such as conductivity, complex
dielectric constant and nonlinear dielectric constant, of different samples
can be quantitatively determined.
4:15 PM AA2.8
NANOSCALE INVESTIGATION OF THE OPTICAL PROPERTIES OF
TRIS-8-HYDROXYQUINOLINE ALUMINUM (ALQ3) FILMS. Grace M. Credo
, Steven K. Buratto, UC Santa Barbara, Dept. of Chemistry, Santa Barbara,
CA.
For the past decade, thin films of the luminescent organic
semiconductor tris-8-hydroxyquinoline aluminum (Alq3) have been
widely studied due to their tremendous potential as the active layer in
organic light-emitting devices. Despite the numerous spectroscopy techniques
applied to Alq3 films, the dependence of the optical properties
on film morphology, particularly on a sub-micron level, remain poorly understood.
The principal reason for this is that previous studies rely on far-field
spectroscopy techniques which average over many morphological domains.
In order to overcome this drawback, we use near-field scanning optical
microscopy (NSOM) to probe carrier transport and diffusion length in Alq3
vacuum-deposited films with 10-100 nm resolution, the length scale of many
interesting structural domains. We use concurrent shear force microscopy
(an analog to atomic force microscopy, AFM) to correlate morphology (crystalline
vs. amorphous regions) to intensity variations in our fluorescence images
as well as variations in the localized fluorescence spectra. Our results
lead to a better understanding of how the nanoscale structure in Alq3
affects its optical properties.
4:30 PM AA2.9
NEAR-FIELD SCANNING OPTICAL MICROSCOPY OF CONDUCTING
PHASE-SEPARATED POLYMER FILMS. Jeeseong Hwang 1, Alamgir Karim2,
Connie Gettinger3 and Lori S. Goldner1, 1Optical
Technology Division, Department of Physics, National Institute of Standards
and Technology (NIST), Gaithersburg, MD; 2Polymers Division,
NIST; 3Corporate Processing Technology Center, 3M Company, St.
Paul, MN.
Many aspects of phase separation in multi-component systems
have been revealed through recent investigations by microscopy techniques
of thin films of polymer blend. These studies have mostly focussed on the
kinetics of phase separation and the resulting morphological evolution.
However, there is little information available on the properties of ultrathin
phase separated films, such as electric, mechanical, or optical behavior
at a mesoscopic or molecular scale. This study aims at investigating these
aspects of phase separated polymer blend films in which one of the polymer
components is conducting. A home-built near-field scanning optical microscope
(NSOM) was used to investigate polymer blend films at different stages
of phase separation. Transmission and transmitted fluorescence images were
taken. Samples consisted of thin films of a poly(octyl-thiophene)/polystyrene
blend deposited either on bare borosilicate glass substrates or on indium-tin-oxide
(ITO)-coated glass substrates. The films were prepared by spin-casting
from a dilute mixture of the blend in toluene. The poly(octylthiophene)
has a measurable electrical conductivity that results in an excellent optical
contrast with polystyrene, which has a relatively high optical transmission
coefficient. The NSOM utilizes a straight, aluminum-coated tapered single
mode optical fiber tip, controlled by either an optical or a mechanical
feedback mechanism to monitor and regulate tip-sample distance. We report
on the poly(octylthiophene)/polystyrene phase separated domains structures
and their associated characteristics in these films. The influence of annealing
on the phase separated structures and kinetics, and changes in macroscopic
parameters such as film thickness and relative fraction of two polymers
on the characteristics of spinodal decomposition is considered. The effect
of the ITO substrate on the blend is also considered and is important in
the development of these materials for opto-electronic applications.
SESSION AA3/W3: JOINT SESSION:
NEAR-FIELD SPECTROSCOPY OF QUANTUM DOTS, WIRES AND METALS
Chairs: Daniel Gammon and J. P. LeBurton
Tuesday Morning, April 6, 1999
Golden Gate C2 (M)
8:30 AM *AA3.1/W3.1
TRANSFORMATION OF A QUANTUM WIRE INTO QUANTUM DOTS. Joel
Hasen , Loren N. Pfeiffer, Aron Pinczuk, Song He, Ken W. West and Brian
Dennis, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.
We report the first spatially resolved photoluminescence
(PL) images of an isolated GaAs single quantum wire. The wire is formed
at the T-intersection of two quantum wells and has an atomically smooth
nominal cross-section 70 x 66 .
The images reveal several new effects: (i) At 4K the PL is dominated by
sharp 80 to 150 eV wide peaks
spatially localized along the quantum wire. Such sharp peaks are a signature
of excitons localized in a series of shallow quantum dot states distributed
along the length of the wire. (ii) At the site of an isolated quantum dot,
we observe an unusual decrease in the relaxation rate of excitons, such
that they radiate from higher energy states before relaxing to their ground
state. We argue that this is a direct observation of an exciton relaxation
bottleneck. The limited spatial extent of the localized excitons prevents
the emission of high energy phonons. When the energy level separation between
states exceeds the maximum allowable phonon energy the exciton must relax
via higher ordered processes such as multiphonon emission. (iii) As the
temperature is raised beyond 20 K, the sharp peaks decrease in intensity
and are overtaken by a broad 5 meV peak. It appears that the excitons have
enough thermal energy to escape the quantum dot states and therefore are
free to move along the quantum wire.
9:00 AM *AA3.2/W3.2
LOCALIZED EXCITONS: PROBING ONE QUANTUM DOT AT A TIME.
Jeff Guest , Physics Dept, Univ. of Michigan, Ann Arbor, MI; D. Gammon,
E.S. Snow, D.S. Katzer, D. Park, Naval Research Laboratory, Washington,
DC; N.H. Bonadeo, J. Erland, D.G. Steel, Harrison M. Randall, Laboratory
of Physics and Center for Ultrafast Optical Science, Univ. of Michigan,
Ann Arbor, MI.
We have used near-field optical spectroscopy to probe
the spectra of individual quantum dots that are formed by the interface
fluctuations in narrow quantum wells. We have observed the discrete atomic-like
ground and excited-state spectra of the quantum dots with homogeneously-broadened
lines that are as narrow as a few tens of a microeV. Such high-resolution
spectroscopy has allowed us to observe fine structure splittings and hyperfine
effects resulting from the interaction of the exciton spin and the spin
of the lattice nuclei. The extraordinarily narrow spectral linewidths are
the result of long coherence times of the localized excitons. The excited
state linewidths and the measured temperature dependence has been understood
in terms of exciton phonon interactions. We have directly measured the
coherence time of the quantum dot excitons by using coherent transient
spectroscopy. We find excellent agreement between the homogeneous linewidth
measured in CW spectroscopy and the coherence time measured with transient
spectroscopy. We also observe quantum beating by exciting a coherent superposition
of two spin states separated by a small fine structure splitting. This
experiment demonstrates the coherent control of superpositions of exciton
states in single quantum dots. We have also measured the nonlinear spectra
of quantum dot excitons. This experiment opens up a new direction of research
for direct measurements of exciton dynamics and optical nonlinearities
in quantum dots. These examples of advanced spectroscopies on individual
excitons are the first steps toward what may eventually lead to in its
maturity coherent optical control of quantum dots comparable to what is
now possible in atoms. If this is to happen it will be necessary to further
develop not only the optical techniques, but also the quantum dot material
systems themselves.
9:30 AM AA3.3/W3.3
PAULI-BLOCKING IMAGING OF SINGLE STRAIN-INDUCED SEMICONDUCTOR
QUANTUM DOTS. C. Obermuller, A. Deisenrieder, G. Abstreiter, Walter Schottky
Institut, Technical University, Munich, GERMANY; S. Grosse, J. Feldmann,
K. Karrai , Center for Nano-Science (CeNS) at the Ludwig Maximilians University,
Munich, GERMANY; H. Lipsanen, M. Sopanen, and Ahopelto, Optoelectronic
Laboratory, Helsinki University of Technology, FINLAND.
The photoluminescence (PL) of InP strained single semiconductor
quantum dots in a GaInAs/GaAs quantum well is measured at low temperature
(4.2 K) using near-field scanning optical microscopy (NSOM). The mapping
of the PL originating from the first three confined level of 8 individual
dots is performed of an area of 1.4 x 1.4 micrometers. The spatial resolution
of the PL of the lowest energy level is found to be limited to about 500nm,
i.e. the diffusion length of the excitons. In contrast, the mapping of
the PL of higher excited state shows a much improved spatial resolution
of the order of 150 nm which is the instrumental resolution. This effect
is understood in terms of Pauli-blocking of the dot level filling. We also
report on in-situ (i.e. at 4.2 K) mechanical manipulation of single dot
stressor field by adjusting the tip-sample shear force interaction. The
dot potential can be fine tuned in a controlled but irreversible way to
be shallower. This way the color of the luminescence can be accurately
adjusted toward shorter wave lengths.
10:15 AM *AA3.4/W3.4
MAGNETOSPECTROSCOPY OF SINGLE SELF-ASSEMBLED InGaAs QUANTUM
DOTS IN GaAs. Artur Zrenner , Markus Markmann, Frank Findeis, Gerhard Bohm,
Gerhard Abstreiter, Walter Schottky Institut, Garching, GERMANY.
Self-assembled InGaAs quantum dots (QDs) have been investigated
by optical near-field spectroscopy through shadow masks. With this technique
we have analysed the optical properties of single QDs as a function of
excitation power and magnetic field. This allows us to identify unambiguously
the ground and excited states of a given QD. With increasing excitation
power we were able to populate a QD with up to 4 excitons. Besides the
single exciton ground state we have been able to observe also a discrete
biexciton line and emissions from higher exciton complexes. Due to the
narrow emission linewidth we fully resolve diamagnetic/orbital effects
and the Zeeman splitting in photoluminescence (PL) experiments at high
magnetic fields. This gives us precise information about the many body
eigenstates in the QD. Besides such PL investigations on a partially occupied
QD, we have determined for the same QD the eigenenergies also for the empty
configuration by magneto-PL excitation spectroscopy (PLE). In our PLE experiments
we find absorption from the first excited state of the QD. In addition
we observe equally strong, discrete phonon-assisted absorption under the
participation of InAs and GaAs LO-Phonons. From our PL experiment and by
comparison between the PL and PLE experiments we can further determine
the few-body correlation energies of the filled QD for occupancies with
2, 3, and 4 excitons.
10:45 AM AA3.5/W3.5
LATERAL COUPLING OF SELF-ASSEMBLED QUANTUM DOTS STUDIED
BY NEAR-FIELD SPECTROSCOPY. H.D. Robinson and B.B. Goldberg, Boston Univ.,
Boston, MA; J.L. Merz, Notre Dame Univ., South Bend, IN.
Lateral coupling between spatially separated zero-dimensional
states has been observed in a system of In0.55Al0.45As
self-assembled quantum dots. The experiment was performed by taking photoluminescence
excitation (PLE) spectra in the near-field at 4.2 K. The high spatial resolution
afforded by the near-field technique allows us to resolve individual dots
in a density regime where interactions between neighboring dots become
apparent. In the PLE spectra, narrow resonances are observed in the emission
lines of individual dots. A fraction of these resonances occur simultaneously
in several emission lines, originating from different quantum dots. This
is evidence of interdot scattering of carriers, which additional data show
to be mediated by localized states below the wetting layer band edge. Near-field
PLE data from several other III-V and II-VI self-assembled dot samples
will also be presented.
11:00 AM AA3.6/W3.6
OPTICAL NEAR-FIELD PROPERTIES OF LITHOGRAPHICALLY DESIGNED
METALLIC NANOPARTICLES. J.C. Weeber , J.R. Krenn, A. Dereux, J.P. Goudonnet,
Laboratoire de Physique, Universite de Bourgogne, Dijon, FRANCE; G. Schider,
F.R. Ausseneg, Institut für Experimental Physik, Karl-Franzen Universtat,
Graz, AUSTRIA; Ch. Girard, CEMES-CNRS, Toulouse, FRANCE.
Metallic particles can sustain electromagnetic modes known
as Localized Surface Plasmons (LSP) which account for most of their optical
properties. Over the last decade, the experimental study of LSP was restricted
to the analysis of far-field spectrum of large ensembles of particles.
Today, the improvement of near-field microscopy techniques allows the observation
of the LSP in the vicinity of particles arrays or isolated particles. In
this work, we use a Photon Scanning Tunneling Microscope (PSTM) to investigate
the near-field optical properties of metallic nanoparticles. The nanoparticles
are obtained by an electron beam lithography technique in order to control
precisely their shapes and dimensions. The experimental results are compared
with simulated images computed in the framework of the Green's dyadic formalism.
We investigate in detail the specific near-field optical properties of
one-dimension (1D) metallic nanostructures such as nanowires or chains
of particles. we show that 1D sub-wavelength resonant structures are convinient
to achieve the propagation of light over distances larger than the excitation
wavelength.
11:15 AM AA3.7/W3.7
NEAR-FIELD OPTICAL IMAGING OF ELECTROMIGRATION DAMAGES
IN PASSIVATED METAL STRIPS. E. Bonera , A. Borghesi, Laboratorio MDM -
INFM, Agrate Brianza (MI), ITALY; C. Caprile, STMicroelectronics, Agrate
Brianza (MI), ITALY.
Electromigration is one of the main failure mechanism
that limit the miniaturization of microelectronics devices. As consequence
of the high current densities in the interconnections, hillocks and voids
are formed and their evolution can modify the electrical performances of
device till failure. To characterize electromigration damages, today's
failure analisys techniques require to remove the protection passivation
to allow scanning electron microscope or focused ion beam microscope imaging,
but the removal process itself can damage the surface of the metal strips.
Due to the optical transparency of the passivation near-field scanning
optical microscopy can be used to overcome this problem. We succeeded in
obtaining the first near-field images in super-resolution (<150 nm)
of electromigration-damaged metal structures without complete removal of
the passivation. The latter was thinned by chemical etching to 100-200
nm to allow the evanescent waves to reach the metal structures and illuminate
a subwavelength zone of the sample. Near-field images show the presence
of hillocks and voids of dimensions down to 250 nm under the thinned passivation
which can be due only to electromigration, and in this sense are more reliable
than the usual scanning electron microscope images.
11:30 AM AA3.8/W3.8
NEAR-FIELD IMAGING OF FIBER BRAGG GRATINGS. J. Mills
, C.W.J. Hillman, L. Reekie, W.S. Brocklesby, Optoelectronics Research
Centre, University of Southampton, Southampton, UNITED KINGDOM; B.H. Blott,
Department of Physics & Astronomy, University of Southampton, Southampton,
UNITED KINGDOM.
Fiber Bragg gratings are an important component of modern
telecommunications systems. Characterisation of gratings is usually performed
by interrogating the whole grating in either reflection or transmission
along the fiber, and there is considerable interest in possible errors
and defects in the gratings. In these experiments we have used near-field
optical techniques to characterise Bragg gratings on a microscopic scale.
Using D-fibers in order to access the evanescent fields normally within
the cladding of the fiber, direct imaging of the standing wave patterns
formed when the propagating laser is on resonance with the grating has
been performed. Changes in patterns with laser wavelength can be observed,
and compared with theories of grating reflectivity which predict superstructure
on the standing wave patterns. The SNOM tip can also be used to study the
free-space patterns formed by the phase masks used to write the gratings
into the core of the fiber. Our images of these patterns agree well with
theoretical predictions developed from earlier work, and clearly show the
effect of errors in writing wavelength on the visibility of fringes.
11:45 AM AA3.9/W3.9
FIELD ENHANCED SCANNING OPTICAL MICROSCOPE WITH NANOMETRIC
RESOLUTION. Andrea V. Bragas , Oscar E. Martínez, Lab de Electrónica
Cuántica, Dept de Física, Facultad de Ciencias Exactas y
Naturales, Universidad de Buenos Aires, ARGENTINA.
In this work we present optical images with nanometric
resolution, obtained with an optical imaging technique that uses as a local
probe the laser field enhanced at the tunnel junction of an STM. The STM
tip is illuminated with a focused laser radiation and the light scattered
from the tip-sample region is detected by means of a PIN photodiode and
filtered from the background by dithering the tip-sample voltage and using
a lock-in amplifier tuned at the dithering frequency. Images with polycrystalline
gold show a resolution better than .
The optical field enhancement at the tip of an STM microscope has been
measured recently and the possible enhancement mechanisms are being analyzed.
Two major probable contributions to it could be the laser field enhancement
at the tip considered as a metallic nanostructure (similar to the enhancement
in surface enhanced Raman experiments) and a pure quantum origin mechanism
arising from the overlap of the wave functions of the tip and sample. The
net result is a dipolar radiation coming from the tip-sample region that
is highly sensitive to the tip sample distance providing a contrast mechanism
for high resolution images. In order to avoid optical spurious signal produced
by the light scattering in the tip or sample, tip-sample voltage modulation
was performed instead of tip-sample distance modulation. With this experimental
conditions 5nm resolution optical images are recorded, where the resolution
limitation is only due to the time constant integration of the lockin amplifier.
SESSION AA4: PROTEINS AND POLYMERS
Chair: James M. Kikkawa
Tuesday Afternoon, April 6, 1999
Salon 1 (M)
1:30 PM *AA4.1
A NOVEL SCHEME FOR HIGH RESOLUTION NEAR-FIELD MICROSCOPY:
TWO-PHOTON FLUORESCENCE IMAGING WITH AN ILLUMINATED METAL TIP. Erik Sanchez
, Lukas Novotny, X. Sunney Xie, Pacific NW National Laboratory, Richland,
WA.
We have demonstrated a new scheme for near-field fluorescence
imaging using a metal tip illuminated with femtosecond laser pulses using
proper polarization. The strongly enhanced electric field at the small
metal tip (30 nm end diameter) results
in a highly localized excitation source for molecular fluorescence. Excitation
of the sample is provided by two-photon absorption using a modelocked Ti-Sapphire
laser. Two-photon excitation provides better image contrast than one-photon
excitation due to the quadratic intensity dependence. The spatial resolution
for the fluorescence imaging is approximately on the order of the tip diameter,
which is better than the conventional fiber based technique. This scheme
is well suited for imaging biological samples, such as individual proteins
in lipid membranes. We have used this technique to image fragments of thylakoid
membranes as well as dye aggregates with spatial resolutions up to 30 nm.
2:00 PM *AA4.2
METALLIC PROBES FOR FIELD-ENHANCED NEAR-FIELD SCANNING
OPTICAL MICROSCOPY. Satoshi Kawata , Osaka Univ., Dept. of Applied Physics,
Osaka, JAPAN.
We developed NSOMs with metallic probes which enhance
local field near the sample structure. A metalized cantilever is used in
an atomic force microscope with the evanescent-field illumination through
an objective lens of the numerical aperture larger than unity. Another
NSOM with a metallic probe will be also described, which is coupled with
laser-trapping technology for near-field imaging and spectroscopy. Experimental
results of NSOM imaging for fluorescent molecules and labeled DNAs, obtained
with the developed NSOM will be shown. A golden bead is used as a probe
to interact efficiently with sample structure based on the local field
enhancement mechanism. The position control of the probe with a feedback
system, the application of NSOM to two-photon spectroscopy, and the numerical
analysis of field-enhancement mechanism will be also described.
2:30 PM AA4.3
NEAR-FIELD SCANNING OPTICAL MICROSCOPY STUDIES OF ALKYL-SUBSTITUTED
POLYFLUORENE THIN FILMS. Julie Teetsov , Eun-Soo Kwak, Laura Deschenes,
David A. Vanden Bout, Univ of Texas, Austin, TEXAS.
Polyfluorene is an excellent candidate for the luminescent
material in polarized light-emitting devices because of its rigid rod structure
and thermotropic liquid crystalline properties. The fluorescence behavior
of polyfluorene thin films is directly related to interpolymer interactions
which are influenced by liquid crystalline ordering and polymer chain aggregation
and which can be dramatically affected by annealing conditions and film
thickness. While the fluorescent properties of rigid rod polymers have
recently been investigated on a macroscopic scale, little is known about
how interpolymer interactions affect fluorescence behavior on a sub-micron
scale. We are using near-field scanning optical microscopy (NSOM) to study
the fluorescence properties of a series of alkyl-substituted polyfluorenes
as a function of film morphology in order to determine how the length of
the alkyl chain affects local ordering and aggregation. Polarized fluorescence
NSOM images of annealed and pristine films of various thickness show sub-micron
ordering that can be correlated with emission from aggregate versus non-aggregate
species The length of the alkyl chain directly affects the degree of sub-micron
liquid crystalline ordering and aggregation and polarized fluorescence
efficiency data show that the extent of polarization also changes with
the length of the alkyl chain.
2:45 PM AA4.4
NEAR-FIELD SCANNING OPTICAL MICROSCOPY OF CONJUGATED
POLYMER FILMS. Jessie DeAro , Paul Carson, Jonathan Sexton, Steve Buratto,
University of California at Santa Barbara, Dept. of Chemistry, Santa Barbara,
CA.
We will present the results of the application of Near-Field
Scanning Optical Microscopy (NSOM) and Near-Field Optical Spectroscopy
(NFOS) to the investigation of the mesoscale (10-100 nm) optical, transport
and photochemical properties of semiconducting polymers. Conjugated polymers,
such as poly(p-phenylene vinylene) (PPV) and its derivatives are quasi-one-dimensional
luminescent materials with optical and transport properties which are strongly
dependent on the polymer morphology. Results of photoluminescence, linear
dichroism, photo-oxidation and photoconductivity NSOM experiments of non-oriented
and stretch-oriented neat polymer films show that these properties depend
strongly on the local morphology of the polymer film on a 50nm scale. NSOM
experiments of polymer blends have investigated the role of phase separation
on the mesoscale optical properties of the film. Blends of stretch-oriented
MEH-PPV and ultra high-density polyethylene show phase separation on a
50nm scale directly related to the film morphology. Spatial hole burning
NSOM experiments have been done to measure carrier diffusion as well as
used as a tool for nanoscale photo-patterning of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene
vinylene) (MEH-PPV) thin films. The resulting photo-oxidation pattern is
sensitive not only to ambient conditions but also to heat transport, carrier
diffusion and local film morphology. Photoluminescence (PL) spectra can
be collected concurrently with the photo-oxidation patterning, showing
the changing profile of the PL emission versus time exposed to light. We
will also discuss single polymer molecule experiments of MEH-PPV on glass
as well as in stretch-oriented polyethylene.
3:00 PM AA4.5
DEVELOPMENT OF CHEMICALLY CONTROLLED ATOMIC FORCE MICROSCOPY
TIPS AND TEST SAMPLES. Ruth Ellen Thomson , Paul Rice, Shane Roark, John
Moreland and Todd Ruskell, National Institute of Standards and Technology,
Boulder, COLORADO.
Recent work on coating atomic force microscope (AFM) tips
with self-assembled monolayers (SAMs) of thiols has enabled the AFM to
probe the chemical nature of a samples by using functionalized tips with
specific chemical properties. [1] We have coated AFM tips with both hydrophobic
and hydrophilic SAMs. We have also developed test samples that are prepared
using the same process as is used for the tips, providing an easy alternative
to the complicated photo-patterned or contact-printed samples previously
reported. We have determined the relative pull-off forces between the functionalized
tips and SAM covered samples under ambient conditions. By measuring the
pull-off forces on the test samples we can reject tips that have defects
in the SAMs at the apex of the tip and thus are not acceptable for use
in AFM studies. We have found that this step of checking the pull-off force
between the tip and the test samples is an important step that must be
performed for each tip prepared. By using the criteria of rejecting tips
that do not exhibit the expected pull-off force on the test samples we
have found that the SAM coated tips must be used the same day as they are
coated. Other factors that affect the quality of the tips include the ambient
humidity, the age of the thiol solutions, and the length of exposure of
the gold surfaces to air prior to coating with SAMs. We have used these
functionalized tips in a variety of imaging modes including phase contrast
imaging, lateral force imaging and force-mode imaging. We have found that
knowing and controlling the chemical nature of the tip is a key element
in understanding the contrast produced in these imaging techniques. 1.
A. Noy, D. V. Vezenov and C. M. Lieber, Annual Reviews of Materials Science
27, 381-421 (1997).
SESSION AA5/J10/K6: JOINT SESSION:
SPIN DYNAMICS AND TRANSPORT
Chair: Hans-Christoph Siegmann
Wednesday Morning, April 7, 1999
Salon 1-3 (M)
8:30 AM *AA5.1/J10.1/K6.1
MACROSCOPIC SPIN TRANSPORT IN GALLIUM ARSENIDE. J.M.
Kikkawa and D.D. Awschalom, Department of Physics, University of California,
Santa Barbara, CA.
Spin precession measurements uncover extremely long transverse
electron spin lifetimes (>100 ns) in low-doped n-type GaAs, arising from
a weak entanglement of electron spin coherence and orbital motion at low
temperatures. The relative roles of spin decoherence and dephasing in such
systems may be sensitively probed using a new pump-probe technique known
as resonant spin amplification (RSA), wherein electron spin precession
is driven into resonance with the duty cycle of optical spin injection
[1]. In the resonant condition, spin from successive optical pump pulses
interferes constructively and yields an accumulation of spin at the injection
site. We complement this method by applying an in-plane electric field
that deconstructs spin resonances into individual spin packets. A technique
of non-local, time-resolved Faraday rotation images the displacement of
these spins over distances exceeding 100 m
in low-mobility GaAs, revealing that spins involved in RSA are non-localized
despite a close proximity to the metal-insulator transition [2]. We find
that dragging coherent spins by their charge minimally impacts their decoherence,
but generates an effective magnetic field that dephases spin precession
near zero magnetic field. Spatially-resolved studies of the spin profile
show that spin diffusion in these systems involves not only carrier diffusion
but also pure spin diffusion and that an asymmetrical broadening of the
spin packet occurs during transport.
[1] J.M. Kikkawa and D.D. Awschalom, ``Resonant Spin
Amplification in n-type GaAs'', Phys. Rev. Lett. 80, p. 4313-6 (1998).
[2] J.M. Kikkawa and D.D. Awschalom, ``Dragging Spin
Coherence in GaAs'', submitted for publication (1998).
We thank ONR N00014-97-1-0575, ARO DAAG55-98-1-0366 and
NSF STC DMR91-20007 for support.
9:00 AM *AA5.2/J10.2/K6.2
MAGNETIZATION REVERSAL IN MICRON-SIZED MAGNETIC THIN
FILMS. R.H. Koch , J.G. Deak, D.W. Abraham, P.L. Trouilloud, R.A. Altman,
Yu Lu, W. J. Gallagher, IBM Thomas J. Watson Research Center, Yorktown
Heights, NY; and R. E. Scheuerlein, K.P. Roche and S.S.P. Parkin, IBM Almaden
Research Center Almaden, CA.
We have measured and simulated the dynamics of magnetization
reversal in 5 nm by 0.8 by 1.6 m
Ni60Fe40 thin films. The films measured form the
upper electrode of a spin-polarized tunnel junction so that the magnetization
direction of the film can be probed by measuring the tunneling resistance
of the junction. When a magnetic field pulse is applied, the time to switch
the film magnetization changes from greater than 10 ns to less than 500
ps as the pulse amplitude is increased from the coercive field to 10 mT
and beyond. We have simulated these transitions using micromagnetic modeling
of the exact experimental conditions. The simulations agree well with the
experimental measurements and indicate complex dynamical behavior, that
is a mixture of domain wall motion, magnetization rotation, and ferromagnetic
resonance.
9:30 AM *AA5.3/J10.3/K6.3
MAGNETIZATION DYNAMICS STUDIES WITH SOLID IMMERSION LENS
MICROSCOPY. Mark Freeman , Greg Ballentine, Wayne Hiebert, Andrzej Stankiewicz,
University of Alberta, Dept of Physics, Edmonton, CANADA.
The dynamics of the magnetization in small ferromagnetic
structures is a topic of considerable current interest. Some of the interest
stems from rapid advances in magnetic recording technology, where dynamics
may dictate the ultimate limits in speed and storage density. High speed
magneto-optic imaging is being used in an effort to gain new insight into
micromagnetic dynamics. Ultrafast measurements are performed in pump-probe
experiments analogous to electro-optic sampling. Nonequilibrium magnetic
states are excited by the pump beam using a photoconductively switched
electromagnetic circuit, and the relaxation of these states detected optically
though the Faraday or Kerr effect of the magnetization on the polarization
of the probe beam. Spatial imaging is added because the processes are inherently
nonuniform.
Applications to magnetics include studies of magnetization
reversal[1], and spatio-temporal observations of modes of oscillation in
ferromagnetic resonance[2]. The difficulties of understanding nonlocal
and nonlinear processes such as these provides fascinating challenges.
The ultimate goal is a simultaneous combination of spatial and temporal
resolution sufficient to directly record all of the relevant dynamics.
For most materials of interest this requires the highest spatial resolution
that can reasonably be achieved. The solid immersion lens offers many favorable
characteristics for such experiments[3]. Enhanced resolution is obtained
without a big loss in the photon budget and while maintaining the ability
to resolve all vector components of the magnetization. It is also portable
to hostile environments and offers the potential for future improvements
through very high index lens materials.
1. A. Stankiewicz, W.K. Hiebert, G.E. Ballentine, K.W.
Marsh, and M.R. Freeman, IEEE Trans. Mag. 34, 1003 (1998)
2. W.K. Hiebert, A. Stankiewicz, and M.R. Freeman, Phys.
Rev. Lett 79, 1134 (1997).
3. J.A.H. Stotz and M.R. Freeman, Rev. Sci. Instr. 68,
4468 (1997).