AVS1997 Session NS-WeM: Near-Field Optics and Nanoparticles

Wednesday, October 22, 1997 8:20 AM in Room K

Wednesday Morning

Start	Invited?	Item
8:20 AM		NS-WeM-1 Low-Temperature Near-Field Scanning Optical Microscopy Study of III-V Quantum Heterostructures. D. Kulik, H.B. Yu, H. Htoon, J. Keto, C.K. Shih, O. Baklenov, B.G. Streetman (University of Texas, Austin) We have built a Near-Field Scanning Optical Microscopy and Spectroscopy (NSOM/S) system which operates at high vacuum and liquid helium temperatures. Samples under investigation are MBE grown InGaAlAs quantum dots and AlGaAs/GaAs multiple quantum wells. Spectroscopy on both samples has been performed over a wide range of temperatures. The influence of the temperature on the luminescence quantum efficiency has been studied. Currently, we are performing the spatially resolved spectroscopy study on the InGaAlAs quantum dots. Topography versus Spectroscopy results will be presented.
8:40 AM		NS-WeM-2 Near-Field Optics at Optic Fiber End-Faces S.Z. Lo, D.P. Tsai (National Chung Cheng University, Taiwan) Near-field optics at fiber end-faces was studied using a tapping-mode scanning near-field optical microscope (SNOM). Topography and near-field intensity images at the end-faces were obtained simultaneously but independently. Both collection and emission modes of the near-field fiber probe were used. A brief and gentle sample process on fiber end-faces in a saturated solution of ammonium bifuoride was able to show the structures of fibers. Fibers of step-index, graded-index, single-mode, multi-mode, circular or non-circular geometry were imaged with a superior resolution. The topographic feature generated by different local chemistry is closely related to the local doping concentration and the distribution of refractive index. For a fiber with known refractive index distribution our results have demonstrated the topographic image of an etched fiber end-face corresponds to the refractive index distrubution. Using this novel method, the correlation between fiber structures and their near-field intensity profiles of propagating modes can be acquired directly. On the other hand, noncoherent light source was used for the study of multi-mode fibers as well. The interference of the coherence was avoided in this case. Furthermore, fiber endfaces and their near-field optical intensity profiles can served as a well known sample stage for samples smaller than the diameter of optical fiber core. Interesting changes of the near-field optical intensity profiles were observed because of the existence of the samples at fiber end-faces. The samples of human's red blood cells and latex particles have been successfully imaged and studied.
9:00 AM		NS-WeM-3 Optical Writing and Reading on Phase-Change Thin Film using an Internal Reflection Mode Scanning Near-Field Optical Microscope D.P. Tsai, W.R. Guo, Y.Y. Lu (National Chung Cheng University, Taiwan) An internal reflection mode scanning near-field optical microscope has been successfully developed to perform optical writing and reading on Ge₂₁Sb₅₃Te₂₆ phase-change thin film. A 2 X 2, 50-50 single-mode directional fiber coupler and a 20 mW, wavelength of 532 nm laser were used. Near-field optical fiber prboe at one end of the directional fiber coupler was used to deliver the writing optical energy, and to collect the reflected optical signals for reading as well. The structure of phase-change thin film is ZnS-SiO₂ (2nm)/Ge₂₁Sb₅₃Te₂₆ (20nm)/ZnS-SiO₂ (20nm)/Al(80nm)/ glass substrate, the ZnS-SiO₂ is the protective layer with a high refractive index, the optical recording layer is the as-deposited amorphous Ge₂₁Sb₅₃Te₂₆ thin film. The optical energy deliver from the near-field optical fiber probe can locally increase the temperature (more than 300°C)of the amorphous Ge₂₁Sb₅₃Te₂₆ thin film to form crystal phase writing bits. Our internal reflection-mode reading results showed the reflectivity in the writing region (crystal phase) is two times higher than the as-desposited background in amorphous phase. The diameter of the writing bits in the range of 400 nm to 100 nm can be easily written and resolved. No surface deformation happened on the surface of the phase-change thin film during the writing and reading, this was imaging simultaneously by atomic force microscope. The root-mean-square roughness of the surface of our phase-change thin film measured in an area of 10 X 10 microns² showed a value less than 5 nm. Results showed the smoothness of the surface topography and distinct reflectivity of two phases lead to very good contrast, and possibly higher phase-change recording density as well.
9:20 AM		NS-WeM-4 Near-Field Optical Microscopy Studies of Gold Nanoparticles S.J. Stranick, L.J. Richter, R.R. Cavanagh (National Institute of Standards & Technology) We have recently developed a near-field scanning optical microscope (NSOM) based on reflective optics for operation from the visible to the mid infrared. The scanner lies inside of a Au coated ellipsoidal cavity, with the Al-coated near-field-aperture-probe at one foci of the cavity and a microscope objective at the other foci, allowing for efficient collection of reflected light. The instrument can simultaneously acquire reflection NSOM, transmission NSOM, and non-optical shear force topographic images. We have used the microscope to study the optical scattering properties of dilute layers of size selected Au nanoparticles with average particle sizes ranging from 15 to 100 nm.¹ Pronounced 'wave-like' patterns are observed in both transmission and reflection images of large (100 nm) particles (or of large clusters of smaller particles) recorded with 488 nm light. The intensity oscillations have a nominal period of half the excitation wavelength and extend for up to a micrometer from isolated scatters. The wave-like patterns are not observed in images of small (15 nm), isolated particles. A model for the wave-patterns, in terms of multiple scattering between the particles and the tip, will be discussed. ¹The samples were provided by M. Natan and coworkers, Chemistry Dept., Pennsylvania State University
9:40 AM	Invited	NS-WeM-5 Semiconductor Nanocrystallites: From Artificial Atoms to Quantum Dot Heterostructures M.G. Bawendi (Massachusetts Institute of Technology) Small crystallites of semiconductors which are in the nanometer size range have the crystalline structure of the bulk but an electronic structure which is quantized and molecular in nature. They are often coated with a variety of functional groups and they can be manipulated as if they were large molecules with a crystalline inorganic core. The synthesis of these particles represents a chemical route to quantum dots or artificial atoms. In this talk we focus on nanocrystallites of the II-VI semiconductor CdSe. Cadmium selenide quantum dots are a particularly appealing model system. The bonding in II-VI semiconductors is ionic enough that it is relatively straightforward to synthesize high quality samples. Size quantization of the electronic levels in the quantum dots results in a lowest transition and fluorescence which can be tuned across the visible, from a red bulk band gap at 1.8 eV for particles ~10 nm in diameter to a clean blue at ~2.6 eV for particles ~1.5 nm in diameter. The electronic structure of these quantum dots has been explored using a variety of size selective optical techniques, including fluorescence line narrowing, photoluminescence excitation, and single dot spectroscopy. The picture that emerges is of quantum confined, atomic like states which are delocalized throughout the particle. Single dot spectroscopy at 10 K confirms that the fluorescence linewidth is atomic like (< 0.120 meV, resolution limited). Cadmium selenide quantum dots can be manipulated to form a number of structures. They can be combined with polymers or simply spin coated to form thin films. These thin films can be combined with charge transporting semiconducting polymers or semiconductor thin films to make light emitting or photovoltaic devices. Finally the dots can be close packed into both glassy and crystalline structures with long range translational symmetry. Energy transfer from dot to dot as well as charge transport are observed in these solids.
10:20 AM		NS-WeM-7 Zinc Nanocrystals Embedded in Fused Silica: Radiation Effects of 266 nm Picosecond Light Pulses A. Ueda, J. Chen, Z. Gu, R. Mu, Y.S. Tung, W.E. Collins, D.O. Henderson (Fisk University); C.W. White, J.G. Zhu, R.A. Zuhr (Oak Ridge National Laboratory) Zinc ions were implanted into fused silica hosts at doses from 1 x 10¹⁶ to 3 x 10¹⁷ ions/cm² and at energy of 160 keV. The substrates were annealed in 5% H₂ + 95% Ar and 5% O₂ + 95% Ar at temperatures from 100 to 500°C. The electronic and vibrational spectra were measured before and after the annealing treatments. A peak near 230 nm is observed for the as-implanted substrates and increases in intensity with annealing temperature. This peak is assigned to the surface plasmon absorption of zinc nanocrystals. Annealing in an oxygen atmosphere leads to the appearance of new peaks at 4.3 and 4.7 eV for the 3 x 10¹⁶ ions/cm² and 1 x 10¹⁷ ions/cm² samples, respectively. These peaks are tentatively assigned to quantum confined zinc oxide nanocrystals corresponding to different particle sizes. The infrared spectra show the appearance of peak near 1000 cm^-1 that increases with ion dose and is attributed to the stretching vibration of an Si-O--- dangling produced from ion beam damage. Annealing the samples causes this peak decrease indicating the recovery of the glass network structure. Irradiation of the samples with 5 mJ ps pulses of 266 nm light causes a photo-darkening of the samples in the irradiated region. Optical microscopy reveals that the irradiated samples have domains where material has accumulated and appear to represent complex interference patterns resulting from constructive and destructive interference of the light interacting with the near surface region.
10:40 AM		NS-WeM-8 The Origin of Photon Absorption above Surface Plasmon Resonance of Gold Colloids Formed in Silica via Ion Implantation Z. Gu, R. Mu, A. Ueda, Y.S. Tung, D.O. Henderson (Fisk University); C.W. White, R.A. Zuhr (Oak Ridge National Laboratory) Gold nanocrystals or clusters embedded in dielectric hosts have illustrated many intriguing and unique optical properties dissimilar to those of the bulk. These properties have been attributed to i) a three dimensionally confining potential; ii) dielectric confinement from the host matrix and iii) interactions at host-guest interfaces. Numerous studies have shown that the classical and semi-classical theories can successfully predict the position of the surface plasmon absorption (SPA) of gold nanoparticles in various hosts. However, the theoretically predicted absorption cross section at high photon energy, i.e., hv > 2.5 eV shows a considerable and consistent deviation with the experimental results in the literature. In fact, the theoretically predicted values are always lower than the experimental results. In order to understand the nature of the absorption at hv > 2.5 eV in Au ion implanted fused silica, both theoretical modeling and controlled experiments have been conducted. Our recent results presented in this paper, seem to suggest that the broad region of strong absorption at hv > 2.5 eV is mainly due to the modified electronic interband transitions in Au nanocrystals rather than the defects in silica resulting from ion implantation. For an annealed sample, the silica defects do have a relatively well-defined peak around 5.4 eV. The absorption due to the ultrasmall gold clusters possibly formed in silica may not be the main source responsible for the high energy photon absorption observed in the spectra. The study of the interband transition of gold nanocrystals may also help us to understand why metal colloids can fluoresce.
11:00 AM		NS-WeM-9 Multilayered Films of Nanoparticles in a Single Step through Amphiphilic Templating J.S. Devlin, B.G. Higgins (University of California, Davis) Amphiphilic templating is used to organize cadmium sulfide nanoparticles between bilayers of alkyltrimethylammonium chloride. X-ray diffraction shows the composite layers to be approximately 4 nm thick. The structure is analogous to the highly regular layered structure of films formed from pure surfactant solutions, but with an expanded interlayer spacing resulting from the presence of the inorganic component. Films are produced in a single solution coating step under ambient conditions. Film thickness can be varied from tens to hundreds of layers. Almost monodisperse CdS nanoparticles of adjustible size are synthesized by a simple solution-based procedure using capping agents. UV-vis absorption and fluorescensce spectroscopy show evidence of size quantization effects. Ordered films of nanoparticles are of interest for novel electroluminescent devices.
11:20 AM		NS-WeM-10 Rigorous Nanometric Dipole-Surface Formalism and its Application to Molecule Fluorescence M. Xiao (Institute of Physics-UNAM, Mexico) We have recently established the full retarded and full spectrum reflection field propagator in dipole-surface system [1]. Using the new formalism, we demonstrate that the field distribution can be exactly computed everywhere in the half space in nanometric dipole-surface systems for arbitrary dipole-surface distances, and the propagating and evanescent components can be separately and exactly calculated. We have found theoretically and numerically significant differences between the rigorous formalism and various approaches in the literature including our earlier work. We have applied our theory to study fluorescence of single molecule, which appears of current interest in the near field optics. Using our rigorous formalism, we demonstrate theoretically that radiation of a near-surface molecule (treated as a point dipole) may be enhanced during its fluorescence decay. Firstly, we point out that the Chance-Prock-Silbey theory [2] is not accurate when the dipole-surface coupling is strong. The field has to be calculated self-consistently. Secondly, we criticize the use of lifetime to describe the molecular fluorescence for strong couplings. We have found that in case of weak coupling the decay is exponential, thus the decay can be described with the lifetime. In this case our theory gives the accurate lifetime dependency on the dipole-surface distance. We have also found that in case of strong coupling the decay is not exponential, thus the decay cannot be described with the lifetime, no matter if the field is calculated self-consistently or not. In this case the decay curve may be resonantly enhanced, which is due to the configurational resonances in the dipole-surface system. This finding provides a theoretical explanation to the experimentally observed phenomenon in [3], that the fluorescence of single molecule may be enhanced during the decay. [1] M. Xiao, Chem. Phys. Lett. 270 (1997) 169. [2] R.R. Chance, A. Prock, and R. Silbey, J. Chem. Phys. 60 (1974) 2744. [3] W.P. Ambrose, P. M. Goodwin, J.C. Martin, and R.A. Keller, Phys. Rev. Lett. 72 (1994) 160.
11:40 AM		NS-WeM-11 Experimental Study of Two Surface Plasmon Interference: Some Spectroscopic Aspects M. Xiao, R. Machorro, J.M. Siqueiros (Institute of Physics-UNAM, Mexico) Work is reported on experimental studies of two-plasmon interference. Experiments are carried out with two different methods to launch the surface plasmon. In the first experiment, the technique of attenuated total reflection in Kretschmann configuration is used to launch the plasmons on the surface of a Silver thin film, which is covered on the flat surface of a semicylindrical prism. The incident light beam impinges on the film from the solid side. Near the critic incident angle, surface plasmon occurs on the air side of the film. The reflected light is measured in far field. In the spectrum of the reflected light, there appears an absorption peak, which is the indication that the plasmon is launched. A cylindrical mirror is then used to send back the reflected light beam to the same area of the film, which launches another surface plasmon in the opposite direction. The thickness of the film is optimized to ensure a near 50% energy partition by both plasmons, which improves the contrast in the interference fringes. The twice-reflected beam is measured in the incident direction with help of a beam splitter. The wavelength of the incident light is scanned. The measured spectroscopic curves are used to discuss some important aspects of the device, such as, the interference of the two plasmons, the influence of the angles of the incident beam as well as the mirrored beam, the geometric accuracy of the meeting area of the plasmons. Our findings are helpful for further studies of the interference in the near field region. These studies are of current interest because of the possibility to use the scanning near field optical microscopes to closely study the plasmons and the plasmon polaritons on a rough surface. Finally, similar experiments are carried out with another technique that uses a well fabricated metallic grating to produce the plasmon. Some results are presented in comparison with the first technique.