AVS1996 Session NS-MoA: Photonics in Scanned Probe Microscopy

Monday, October 14, 1996 1:30 PM in Room 202 A/B

Monday Afternoon

Start	Invited?	Item
1:30 PM	Invited	NS-MoA-1 Near- and Far-Field Imaging Spectroscopies J. Trautman, J. Macklin (SEQ) Recent progress in near-field scanning optical microscopy/spectroscopy will be reviewed. In particular, our efforts to push the technique into the ultraviolet and to develop new imaging modes will be covered. Direct comparison to classical optical methods will be made using examples from time-resolved spectroscopy of single molecules and quantum dots.
2:10 PM		NS-MoA-3 Scanning Near-field Optical Microscope for Single Molecule Imaging at Cryogenic Temperatures G. Tarrach, M. Bopp, M. Lieb, A. Meixner (University of Basel, Switzerland) The construction of a scanning near-field optical microscope (SNOM) for fluorescence imaging of single molecules at low temperature imposes strong engineering problems due to the difficult environment and the limited access. The most important requirements are: i) Efficient optical detection to achieve the best possible photon statistics of the low fluorescence signal, ii) strong background rejection for maximum contrast, iii) extremely high stability of the SNOM itself and with respect to the detection optics outside the cryostat, iv) compact design for minimum heat load and size, v) versatile and reliable adjustment of tip, sample, and optics even at low temperature to avoid needless warming up.We report on the construction of such a microscope with confocal light detection. Mirror optics inside the cryostat makes the instrument independent of the medium in which it is immersed. A parabolic mirror covers 83 % of the upper half of the solid angle, while 21 % of the photons emitted into the lower half are collected by an additional plane mirror. Versatile xy coarse motion of the sample and z approach are realized using piezoelectric motors. These provide excellent stability while the overall size is kept small.Results with partially opaque samples demonstrate simultaneous reflection and transmission microscopy in the near field. Further, fluorescence images of samples with rhodamine 6G will be shown.
2:30 PM		NS-MoA-4 Super-Resolution Fluorescence Imaging of Single Dye Molecules in Thin Polymer Films and on Glass Surfaces M. Bopp, G. Tarrach, A. Lieb, A. Meixner (University of Basel, Switzerland) We report on the imaging of single dye molecules in thin polymer films and on glass surfaces under ambient conditions by means of scanning near-field optical microscopy (SNOM or NSOM). The long-term mechanical stability and high detection efficiency of our instrument allow the imaging of single dye molecules over several hours with a high signal-to-noise ratio. The local excitation by the near-field tip, the low excitation power and the fact that the dye molecules are embedded in the three dimensional polymer structure drastically reduce the photodestruction process compared to conventional microscopy. The observation of many molecules in the same film allows to probe them under identical experimental conditions and to statistically analyze their global behavior. We find that the molecules do move and rotate within the polymer film. The single dye molecules act as nanometer-sized probes to investigate the local structure of the polymer. These results are compared to those when the molecules are adsorbed on quartz glass surfaces in a two dimensional fashion.
2:50 PM		NS-MoA-5 Mie Scattering in Nanoscale Particles as Observed by Near-field Scanning Optical Microscopy (NSOM) S. Tang (DuPont Central Research and Development, U. S. A.); S. Stranick, A. McGhie (DuPont Central Research and Development); K. Grabar, M. Natan (Pennsylvania State University) NSOM has been used to observe the physical and optical properties of individual and aggregated nanometer-scale gold colloidal particles held on silanized (aminopropylmethyldiethoxysilane) glass slides for the first time. These samples are not stable for the contact or tapping mode imaging of the conventional atomic force microscope. With NSOM, however, simultaneous topographical shear force and optical absorption images readily reveal the true dimensions (obtained from TEM) of the 30-nm diameter gold particles. In 488 nm wavelength light, the aggregates appear to be "less dark", i.e., absorbing less light than the individual particles. Simulations based on Mie scattering theory for individual nanoscale gold particles predict that the absorption of 488 nm light by these particles peaks around 30 nm, whereas scattering (in the forward direction) is more favored as the particle increases in diameter. These simulations are in excellent agreement with our NSOM observations. Results on these well-defined gold spheres are useful for understanding more complex systems such as TiO2 particles. *Permanent address: NIST, Gaithersburg, MD 20889
3:10 PM		NS-MoA-6 Optical Fiber Structures Studied by a Tapping-mode Scanning Near-field Optical Microscope D. Tsai (National Chung Cheng University, Taiwan) A tapping-mode scanning near-field optical microscope (SNOM) using a bent optical fiber probe has been successfully developed to study the structures of a variety of optical fibers. Fibers of step index,graded index, single-mode, multi-mode, circular or noncircular geometry were imaged with a superior resolution. Both topography and near-field intensity images of fiber structures at the endfaces were obtained simultaneously but independently by the same tapered and bent fiber probe. A brief and gentle sample process on fiber endfaces in a saturated solution of ammonium bifluoride was able to show the structures of fibers in terms of the topographic features due to different local chemistry that is closely related to the local doping concentrations. Since the refractive index distribution corresponds to the doping concentration inside the fiber, the topographic image represents the refractive index distribution, and the optical image shows the near-field propagating intensity profile of it. Experimental results of normal fiber and polarization maintaining fibers showed the correlation between fiber structures and their near-field intensity of propagating modes can be acquired directly.
3:30 PM		NS-MoA-7 Polarization of Light Emitted from the STM Tunnel Junction D. Pierce, A. Davies, J. Stroscio, R. Celotta (National Institute of Standards & Technology) We present a complete characterization of the polarization of light emitted from the STM tunnel junction by measuring the Stokes parameters and thereby determining the degree of linearly polarized, circularly polarized and unpolarized light. Both Fe(001) and Au(111) surfaces were investigated with W(111) tips. Of order 100 photons/sec, depending on the tip, are collected from the point source into 0.2 sr at 1 nA owing to inelastic tunneling events. A Fourier analysis was made of the photon intensity generated by rotating a quarter wave retarder in front of a fixed linear analyzer. Great care was taken to minimize spurious polarization signals in these sensitive measurements. For both Fe(001) and Au(111), the light is predominantly linearly polarized in the plane formed by the tip axis and light emission direction. There is a significant unpolarized component of the emitted light which depends on the tip-sample distance and appears to be tip dependent. We will discuss the degree of circular polarization from magnetic surfaces. This work was supported in part by the Office of Naval Research.
3:50 PM		NS-MoA-8 Luminescence from GaAs(100) Excited by a Scanning Tunneling Microscope I. Chizhov (Pennsylvania State University); G. Lee, R. Willis (University of Pennsylvania); D. Lubyshev, D. Miller (Pennsylvania State University) We have studied luminescence induced by scanning tunneling microscope (STM) of atomically clean GaAs(100) samples under ultra high vacuum (UHV) conditions. A 1.5 micron thick layer of heavily doped (n-type, N\sub D\=8x10\super -18\ cm\super -3\) GaAs was grown by molecular beam epitaxy (MBE) on a semiinsulating GaAs(100) substrate. The doped layer was subsequently capped with a thick As overlayer to protect the surface during its transfer through ambient atmosphere to the UHV STM chamber equipped with a light collection system. After decapping the surface exhibited a number of previously observed reconstructions: 2x4/c(2x8), 4x6 and 4x2 as confirmed by LEED and STM. The luminescence arises from the excitation of carriers across the bandgap induced by the tunelling current followed by their radiative recombination. Maps of luminescence intensity as a function of lateral position of the STM tip (photon maps) were recorded. Arsenic related features of the size of ~20 nm appear as dark protrusions in the photon maps, indicating increased local non-radiative recombination. Photon maps acquired simultaneously with the STM images taken at positive and negative bias show significant difference in contrast. Mechanisms responsible for the STM induced luminescence and contrast observed in photon maps as well as the prospects of applications of this technique to the characterization of local optical properties of the surfaces will be discussed.
4:10 PM		NS-MoA-9 Selective Scanning Tunneling Microscope-Induced Photon Emission from Self-assembled Monolayer and Dye-coated Surfaces S. Evoy, F. Pardo, P. St. John, H. Craighead (Cornell University) We have studied the scanning tunneling microscope-induced photon-emission from metallic and semiconductor surfaces in air. With a tip bias of -1.8 V and an average tunnel current of 2 nA, photon emission from bare gold surfaces averages several thousand counts per second, corresponding to an estimated quantum efficiency of \eta\ = 10\super -5\. Photon images show good correlation with surface topography. The emission process was found to be sensitive to surface adsorbates [1], opening new possibilities for nanoscale luminescence studies. Gold surfaces were selectively covered with an octadecanethiol self-assembled monolayer using microcontact printing, and slightly etched in a cyanide salt solution in order to enhance the topographic visibility of the pattern. Photon emission is selectively detected from the unprotected areas. We have also studied the scanning tunneling luminescence from dye molecules on surfaces. Dilute solutions of rhodamine 6G were spin-coated on p-GaAs. The stained surfaces show an increase of photon emission of nearly an order of magnitude compared to the unstained ones. This work was supported by ARPA, the Tri-Services Center on Atomic Scale Spectroscopy, and the Cornell Materials Science Center. [1] R. Berndt et al, Surf. Sci. 307-309, 1033 (1994)
4:30 PM		NS-MoA-10 Near-field Optical Recording on Cyanine Dye Layer of a Commercial Compact Disk-recordable (CD-R) D. Tsai, C. Huang (National Chung Cheng University, Taiwan) A tapping mode or a shear force mode AFM combined with a homemade SNOM were used to image and deliver the optical energy to the recording surface of an uncovered commercial compact disk-recordable (CD-R). The recording surface is the cyanine dye layer which was spun and coated on polycarbonate substrate of a commercial compact disk. Two different wavelengths, 543 nm or 650 nm were separately used to couple into our homemade bent or straight fiber probes to do the near-field optical recording. Results showed the size of writing bits can be less than 40 nm in diameter. Ultra high density optical writing were able to be achieved in a relatively slow speed (less than 100 Hz). However, the effective aperture of near-field probe and the dithering motion produced by the bimorph can be readily disclosed. We have found the heating of the metal coating which forms the near-field optical aperture at the end of fiber probe plays an important role in the upraised edge of writing bits as well.