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Spring 1999 logo1999 MRS Spring Meeting & Exhibit

April 5-9, 1999 | San Francisco
Meeting Chairs: Katayun Barmak, James S. Speck, Raymond T. Tung, Paul D. Calvert

Symposium AA—Near-Field Scanning Optical Microscopy and Spectroscopy


Gerhard Abstreiter
Technical Univ of Munchen
Garching, D-85748 GERMANY

Yusihiko Arakawa
Univ of Tokyo
Tokyo, 106 JAPAN

David Awschalom
Physics Dept
Univ of California-Santa Barbara
Santa Barbara, CA 93106

Daniel Gammon
Naval Research Laboratory
Code 6876
Washington, DC 20375

Lukas Novotny
Pacific Northwest National Lab
MS K8-88 PO Box 999
Richland, WA 99352

Symposium Support

  • Army Research Office
  • Office of Naval Research

* Invited paper

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 $\sim$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 ($\approx$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.

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 $\AA$. 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 $\mu$m. The versatility of the microscope will be demonstrated using several examples. First, we will show spatially resolved, quantitative images of the dielectric constant, $\epsilon_r$, and loss tangent, $\tan \delta$, of ferroelectric thin films. Results were obtained in the ferroelectric as well as in the paraelectric state. Furthermore, the local tunability of a $\sim$$\mu m^2$ 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 ($\sim$ 10 GHz) and moderate applied magnetic fields ($\sim 0.2$ 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

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 ($\lambda$/106) has been demonstrated. Since the scanned tip is a part of a high Q resonator, the SEMM has very high sensitivity ($\Delta$f/f   10-7 and $\Delta \epsilon$/$\epsilon$   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 ($\sim$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

Sandwich cells consisting of the photoconductive material zinc-octakis($\beta$-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

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

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.

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 $\AA$ x 66 $\AA$. The images reveal several new effects: (i) At 4K the PL is dominated by sharp 80 to 150 $\mu$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 $\lambda /100$. 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.

Chair: James M. Kikkawa
Tuesday Afternoon, April 6, 1999
Salon 1 (M)
1:30 PM *AA4.1

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 ($\sim$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

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

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).

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 $\mu$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 $\mu$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).