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Bert Hecht

Nano-Optics and Bio-Photonics Group

Physics Institute
Experimental Physics 5
University of Würzburg
Am Hubland
97074 Würzburg
Germany
hecht@physik.uni-wuerzburg.de
Bert Hecht studied Physics at the University of Konstanz where he graduated in 1993 with the diploma thesis: "Mikroskopie und Spektroskopie im Optischen Nahfeld" supervised by Prof. Dr. O. Marti / Prof. Dr. J. Mlynek. He then joined the IBM Zurich Research Laboratory in Rüschlikon and worked with D.W. Pohl in the area of near-field optics. In 1996 he got his Ph.D. from the University of Basel (Prof. Dr. H.J. Güntherodt) for the thesis "Forbidden Light Scanning Near-Field Optical Microscopy". Between 1996 and 2001 he was with the Physical Chemistry Laboratory (Prof. U.P. Wild) at the Swiss Federal Institute of Technology (ETH) working on optical single-molecule spectroscopy in combination with scanning probe techniques. In 2002 he received the venia legendi in Physical Chemistry from the Swiss Federal Institute of Technology. In 2001, Bert Hecht was awarded a Swiss National Science Foundation professorship at the Institute of Physics at the University of Basel where he was a member of the National Competence Center for Research in Nanoscale Science (NCCR Nano).

Since October 2006 Bert Hecht is associate professor at the Physics Institute, Experimental Physics 5, of the University of Würzburg. He is applying Nano-Optics and Bio-Photonics to investigate complex matter at the nanoscale.

Books

2006

Journal articles

2010
Jer-Shing Huang, Johannes Kern, Peter Geisler, Pia Weinmann, Martin Kamp, Alfred Forchel, Paolo Biagioni, Bert Hecht (2010)  Mode Imaging and Selection in Strongly Coupled Nanoantennas.   Nano Lett Apr  
Abstract: The number of eigenmodes in plasmonic nanostructures increases with complexity due to mode hybridization, raising the need for efficient mode characterization and selection. Here we experimentally demonstrate direct imaging and selective excitation of the "bonding" and "antibonding" plasmon mode in symmetric dipole nanoantennas using confocal two-photon photoluminescence mapping. Excitation of a high-quality-factor antibonding resonance manifests itself as a two-lobed pattern instead of the single spot observed for the broad "bonding" resonance in accordance with numerical simulations. The two-lobed pattern is observed due to the fact that excitation of the antibonding mode is forbidden for symmetric excitation at the feedgap, while concomitantly the mode energy splitting is large enough to suppress excitation of the "bonding" mode. The controlled excitation of modes in strongly coupled plasmonic nanostructures is mandatory for efficient sensors, in coherent control as well as for implementing well-defined functionalities in complex plasmonic devices.
Notes:
2009
Jer-Shing Huang, Thorsten Feichtner, Paolo Biagioni, Bert Hecht (2009)  Impedance matching and emission properties of nanoantennas in an optical nanocircuit.   Nano Lett 9: 5. 1897-1902 May  
Abstract: An experimentally realizable prototype optical nanocircuit consisting of a receiving and an emitting nanoantenna connected by a two-wire optical transmission line is studied using finite-difference time- and frequency-domain simulations. To optimize the coupling between optical nanocircuit elements we apply impedance matching concepts in analogy to radio frequency technology. We show that the degree of impedance matching, and in particular the impedance of the emitting nanoantenna, can be inferred from the experimentally accessible standing wave pattern on the transmission line. We demonstrate the possibility of matching the nanoantenna impedance to the transmission line by variations of the antenna length and width realizable by modern microfabrication techniques. The radiation efficiency of the emitting antenna also depends on its geometry but is independent of the degree of impedance matching. The case study presented here provides the basis for experimental realizations of general optical nanocircuits based on readily available gold nanostructures and a large variety of derived novel devices.
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P Biagioni, J S Huang, L Duò, M Finazzi, B Hecht (2009)  Cross resonant optical antenna.   Phys Rev Lett 102: 25. Jun  
Abstract: We propose a novel cross resonant optical antenna consisting of two perpendicular nanosized gold dipole antennas with a common feed gap. We demonstrate that the cross antenna is able to convert propagating fields of any polarization state into correspondingly polarized, localized, and enhanced fields and vice versa. The cross antenna structure therefore opens the road towards the control of light-matter interactions based on polarized light as well as the analysis of polarized fields on the nanometer scale.
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2008
Wilfried Grange, Philippe Haas, Andreas Wild, Michael Andreas Lieb, Michel Calame, Martin Hegner, Bert Hecht (2008)  Detection of transient events in the presence of background noise.   J Phys Chem B 112: 23. 7140-7144 Jun  
Abstract: We describe a method to detect and count transient burstlike signals in the presence of a significant stationary noise. To discriminate a transient signal from the background noise, an optimum threshold is determined using an iterative algorithm that yields the probability distribution of the background noise. Knowledge of the probability distribution of the noise then allows the determination of the number of transient events with a quantifiable error (wrong-positives). We apply the method, which does not rely on the choice of free parameters, to the detection and counting of transient single-molecule fluorescence events in the presence of a strong background noise. The method will be of importance in various ultra sensing applications.
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P Biagioni, J N Farahani, P Mühlschlegel, H - J Eisler, D W Pohl, B Hecht (2008)  A simple method for producing flattened atomic force microscopy tips.   Rev Sci Instrum 79: 1. 016103 Jan  
Abstract: We describe a simple and reliable procedure for obtaining a flat plateau on top of standard silicon nitride atomic force microscopy tips by scanning them over the focus of a high-numerical-aperture objective illuminated by near-infrared ultrashort laser pulses. Flattened tips produced this way exhibit a plateau that is parallel to the substrate when the cantilever is mounted. They represent a valid and cost-effective alternative to commercially available plateau tips.
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2007
J N Farahani, H J Eisler, D W Pohl, M Pavius, P Fluckiger, P Gasser, B Hecht (2007)  Bow-tie optical antenna probes for single-emitter scanning near-field optical microscopy   NANOTECHNOLOGY 18: 19. - MAR 28  
Abstract: A method for the fabrication of bow-tie optical antennas at the apex of pyramidal Si3N4 atomic force microscopy tips is described. We demonstrate that these novel optical probes are capable of sub-wavelength imaging of single quantum dots at room temperature. The enhanced and confined optical near-field at the antenna feed gap leads to locally enhanced photoluminescence (PL) of single quantum dots. Photoluminescence quenching due to the proximity of metal is found to be insignificant. The method holds promise for single quantum emitter imaging and spectroscopy at spatial resolution limited by the engineered antenna gap width exclusively.
Notes: Times Cited: 2
2006
Y Lill, M A Lill, B Fahrenkrog, K Schwarz-Herion, S Paulillo, U Aebi, B Hecht (2006)  Single hepatitis-B virus core capsid binding to individual nuclear pore complexes in HeLa cells   BIOPHYSICAL JOURNAL 91: 29. 3123-3130 OCT  
Abstract: We investigate the interaction of hepatitis B virus capsids lacking a nuclear localization signal with nuclear pore complexes (NPCs) in permeabilized HeLa cells. Confocal and wide-field optical images of the nuclear envelope show well-spaced individual NPCs. Specific interactions of capsids with single NPCs are characterized by extended residence times of capsids in the focal volume which are characterized by fluorescence correlation spectroscopy. In addition, single-capsid-tracking experiments using fast wide-field fluorescence microscopy at 50 frames/s allow us to directly observe specific binding via a dual-color colocalization of capsids and NPCs. We find that binding occurs with high probability on the nuclear-pore ring moiety, at 44 +/- 9 nm radial distance from the central axis.
Notes: Times Cited: 1
P Mühlschlegel, J Toquant, D W Pohl, B Hecht (2006)  Glue-free tuning fork shear-force microscope   REVIEW OF SCIENTIFIC INSTRUMENTS 77: 12. - JAN  
Abstract: A scanning near-field optical microscope without any glued parts is described. Key elements are the optical fiber probe/tuning fork junction and the piezotube scanner assembly. In both cases, fixation is achieved by means of controlled pressure and elastic deformation. The avoidance of glued connections was found to improve the Q factor of the shear-force sensor as well as to facilitate the replacement of the fiber probe and other parts of the scanner head. We present approach curves and shear-force images that demonstrate the performance and stability of the system. (c) 2006 American Institute of Physics.
Notes: Times Cited: 0
V A Azov, A Beeby, M Cacciarini, A G Cheetham, F Diederich, M Frei, J K Gimzewski, V Gramlich, B Hecht, B Jaun, T Latychevskaia, A Lieb, Y Lill, F Marotti, A Schlegel, R R Schlittler, P J Skinner, P Seiler, Y Yamakoshi (2006)  Resorcin[4]arene cavitand-based molecular switches   ADVANCED FUNCTIONAL MATERIALS 16: 84. 147-156 JAN 19  
Abstract: Resorcin[4]arene cavitands with four quinoxaline bridges are a family of macrocycles that adopt, at elevated temperature, a contracted, vase-type conformation, capable of guest inclusion, whereas at low temperature they switch to all expanded, kite-type conformation with a large flat surface. The present investigations lay the foundation for the use of such dynamic cavitands as miniaturized mechanical grippers for supramolecular construction at the single-molecule level. New vase-kite switching modes, stimulated by pH changes or stoichiometric metal-ion complexation, have been discovered and monitored by H-1 NMR and optical absorption spectroscopy. The solid-state geometries of the two states have been revealed by X-ray crystallography, and the kinetics and thermodynamics of the switching processes in solution as well as their solvent dependency has been investigated in. great detail. Monolayers of the cavitand in the vase form have been studied by scanning tunneling microscopy at molecular resolution; conformational switching is also observed in Langmuir monolayers at the air/water interface. Synthetic protocols have been developed for preparation of partially and asymmetrically bridged resorcin[4]arene cavitands, which are also shown to undergo conformational switching. These synthetic advances pave the way to new, dynamic molecular receptors for steroids, tetrathiofulvalene-bridged grippers with the potential to undergo electrochemically induced conformational switching, and systems with greatly extended, rigid cavity walls functionalized at the termini by dipyrrometheneboron difluoride dyes. The latter cavitands are shown by fluorescence resonance energy transfer to undergo geometrically precisely defined motions between a contracted (approximate to 7 angstrom linear extension) and a strongly expanded (approximate to 7 nm linear extension) state.
Notes: Times Cited: 5
J Y P Butter, B R Crenshaw, C Weder, B Hecht (2006)  Single-molecule spectroscopy of uniaxially oriented terrylene in polyethylene   CHEMPHYSCHEM 7: 25. 261-265 JAN 16  
Abstract: Single terrylene molecules doped into linear low-density polyethylene can be oriented by tensile deformation of the matrix. In measurements on ensembles at ambient and on single terrylene molecules at cryogenic temperature, strong orientation along the stretching direction was observed by polarization-resolved confocal microscopy. At cryogenic temperatures narrow and spectrally stable zero-phonon lines were found. The low saturation intensity of 0.07 Wcm(-2) is consistent with an uniaxial orientation of terrylene in the sample plane.
Notes: Times Cited: 1
J Y P Butter, B Hecht (2006)  Fast determination of saturation intensity and maximum emission rate by single-emitter imaging   OPTICS EXPRESS 14: 18. 9350-9357 OCT 2  
Abstract: We investigate the dependence of the spot size in single-emitter confocal imaging on the degree of saturation. We show that single-emitter spots are broadened and flattened significantly already at excitation intensities well below saturation. The resulting single-emitter spot shapes thus deviate significantly from the excitation point spread function. We show and support by Monte Carlo simulations that fitting of a single spot is sufficient to extract the saturation intensity and the maximum emission rate of a single emitter with high accuracy. Our results will be of interest in all areas of single-emitter studies. (c) 2006 Optical Society of America.
Notes: Times Cited: 1
S Karotke, A Lieb, B Hecht (2006)  Stark-shift microscopy of single emitters   APPLIED PHYSICS LETTERS 89: 18. - JUL 10  
Abstract: We study the interaction of a biased, metallized tip in close proximity to single fluorescent molecules at cryogenic temperature. By scanning over the sample, the tip's inhomogeneous electric field induces Stark shifts of the zero-phonon lines of nearby molecules. When illuminated with an off-resonant laser, molecules are tuned into resonance for specific tip positions located on circular patterns around the molecules' spatial positions. The origins of circles belonging to different molecules can be determined with high precision. We demonstrate a spatial resolution of neighboring individual molecules of similar to 50 nm using a tip scanned in a distance of similar to 3 mu m above the sample. (c) 2006 American Institute of Physics.
Notes: Times Cited: 0
J Y P Butter, B Hecht (2006)  Aperture scanning near-field optical microscopy and spectroscopy of single terrylene molecules at 1.8 K   NANOTECHNOLOGY 17: 23. 1547-1550 MAR 28  
Abstract: Single-molecule imaging and spectroscopy using an aperture scanning near-field optical microscope operating at 1.8 K in a helium bath cryostat is demonstrated. From near-field images at constant excitation frequency, the orientation of single molecules can be deduced. Spectral information is obtained using both near-field and confocal excitation schemes by scanning the excitation frequency at a fixed sample position. Differences between near-field and confocal spectra are discussed in terms of the position with respect to the aperture and the molecular orientation.
Notes: Times Cited: 2
B Hecht, P Mühlschlegel, J N Farahani, H J Eisler, D W Pohl, O J F Martin, P Biagioni (2006)  Prospects of resonant optical antennas for nano-analysis   CHIMIA 60: 21. A765-A769  
Abstract: Suitably shaped metal nanostructures act as resonant optical antennas that efficiently collect light and confine it to a subwavelength volume. Vice versa, light emission from nano volumes can be enhanced by coupling to antenna structures. We give a short introduction to antenna theory and discuss recent experiments that show the feasibility of achieving strong field enhancement using resonant dipole antennas for near infrared wavelengths. By scanning an optical antenna fabricated at the apex of an AFM tip over individual quantum dots, we observe enhanced emission of the latter while it is in close proximity of the antenna feed gap. Resonant optical antennas hold promise to be applied for spectroscopic characterization of nano structures with high spatial resolutions and single-molecule sensitivity.
Notes: Times Cited: 0
Jacqueline Y P Butter, Brent R Crenshaw, Christoph Weder, Bert Hecht (2006)  Single-molecule spectroscopy of uniaxially oriented terrylene in polyethylene.   Chemphyschem 7: 1. 261-265 Jan  
Abstract: Single terrylene molecules doped into linear low-density polyethylene can be oriented by tensile deformation of the matrix. In measurements on ensembles at ambient and on single terrylene molecules at cryogenic temperature, strong orientation along the stretching direction was observed by polarization-resolved confocal microscopy. At cryogenic temperatures narrow and spectrally stable zero-phonon lines were found. The low saturation intensity of 0.07 W cm(-2) is consistent with an uniaxial orientation of terrylene in the sample plane.
Notes:
Yoriko Lill, Markus A Lill, Birthe Fahrenkrog, Kyrill Schwarz-Herion, Sara Paulillo, Ueli Aebi, Bert Hecht (2006)  Single hepatitis-B virus core capsid binding to individual nuclear pore complexes in Hela cells.   Biophys J 91: 8. 3123-3130 Oct  
Abstract: We investigate the interaction of hepatitis B virus capsids lacking a nuclear localization signal with nuclear pore complexes (NPCs) in permeabilized HeLa cells. Confocal and wide-field optical images of the nuclear envelope show well-spaced individual NPCs. Specific interactions of capsids with single NPCs are characterized by extended residence times of capsids in the focal volume which are characterized by fluorescence correlation spectroscopy. In addition, single-capsid-tracking experiments using fast wide-field fluorescence microscopy at 50 frames/s allow us to directly observe specific binding via a dual-color colocalization of capsids and NPCs. We find that binding occurs with high probability on the nuclear-pore ring moiety, at 44 +/- 9 nm radial distance from the central axis.
Notes:
J Y P Butter, B Hecht, B R Crenshaw, C Weder (2006)  Absorption and fluorescence of single molecules.   J Chem Phys 125: 15. Oct  
Abstract: Simultaneous detection of single molecules by absorption and fluorescence is demonstrated using confocal microscopy at cryogenic temperature. Dynamical processes such as blinking and spectral jumping of single emitters are observed in both detection channels. The relative magnitude of fluorescence and absorption varies between molecules. In particular, we observe molecules that do not emit detectable Stokes-shifted fluorescence but show a strong absorption signal. The fact that coherent resonant scattering underlies the absorption process is demonstrated by a correlation between small linewidth and large absorption amplitude.
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2005
N A Janunts, K S Baghdasaryan, K V Nerkararyan, B Hecht (2005)  Excitation and superfocusing of surface plasmon polaritons on a silver-coated optical fiber tip   OPTICS COMMUNICATIONS 253: 24. 118-124 SEP 1  
Abstract: We theoretically study the transfer of energy from a fiber mode to surface plasmon polaritons at the silver/air interface in the taper region of a silver-coated fiber tip. We describe conditions, under which a transfer efficiency from the fiber mode to the surface plasmon polariton of up to 46% can be achieved. Propagation of the surface plasmon polaritons towards the tip of the tapered structure leads to a strong subwavelength localization of the excitation enabled by a substantial increase of the surface plasmon polariton wave vector approaching the tip. The resulting strong field enhancement and confinement has a tremendous potential for applications in ultrahigh resolution tip-enhanced near-field optical microscopy. (c) 2005 Elsevier B.V. All rights reserved.
Notes: Times Cited: 15
J N Farahani, D W Pohl, H J Eisler, B Hecht (2005)  Single quantum dot coupled to a scanning optical antenna : A tunable superemitter   PHYSICAL REVIEW LETTERS 95: 32. - JUL 1  
Abstract: The interaction of a single quantum dot with a bowtie antenna is demonstrated for visible light. The antenna is generated at the apex of a Si3N4 atomic force microscopy tip by focused ion beam milling. When scanned over the quantum dot, its photoluminescence is enhanced while its excited-state lifetime is decreased. Our observations demonstrate that the relaxation channels of a single quantum emitter can be controlled by coupling to an efficiently radiating metallic nanoantenna.
Notes: Times Cited: 50
P Mühlschlegel, H - J Eisler, O J F Martin, B Hecht, D W Pohl (2005)  Resonant optical antennas.   Science 308: 5728. 1607-1609 Jun  
Abstract: We have fabricated nanometer-scale gold dipole antennas designed to be resonant at optical frequencies. On resonance, strong field enhancement in the antenna feed gap leads to white-light supercontinuum generation. The antenna length at resonance is considerably shorter than one-half the wavelength of the incident light. This is in contradiction to classical antenna theory but in qualitative accordance with computer simulations that take into account the finite metallic conductivity at optical frequencies. Because optical antennas link propagating radiation and confined/enhanced optical fields, they should find applications in optical characterization, manipulation of nanostructures, and optical information processing.
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Yoriko Lill, Karen L Martinez, Markus A Lill, Bruno H Meyer, Horst Vogel, Bert Hecht (2005)  Kinetics of the initial steps of G protein-coupled receptor-mediated cellular signaling revealed by single-molecule imaging.   Chemphyschem 6: 8. 1633-1640 Aug  
Abstract: We report on an in vivo single-molecule study of the signaling kinetics of G protein-coupled receptors (GPCR) performed using the neurokinin 1 receptor (NK1R) as a representative member. The NK1R signaling cascade is triggered by the specific binding of a fluorescently labeled agonist, substance P (SP). The diffusion of single receptor-ligand complexes in plasma membrane of living HEK 293 cells is imaged using fast single-molecule wide-field fluorescence microscopy at 100 ms time resolution. Diffusion trajectories are obtained which show intra- and intertrace heterogeneity in the diffusion mode. To investigate universal patterns in the diffusion trajectories we take the ligand-binding event as the common starting point. This synchronization allows us to observe changes in the character of the ligand-receptor-complex diffusion. Specifically, we find that the diffusion of ligand-receptor complexes is slowed down significantly and becomes more constrained as a function of time during the first 1000 ms. The decelerated and more constrained diffusion is attributed to an increasing interaction of the GPCR with cellular structures after the ligand-receptor complex is formed.
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2004
Y Lill, B Hecht (2004)  Single dye molecules in an oxygen-depleted environment as photostable organic triggered single-photon sources   APPLIED PHYSICS LETTERS 84: 17. 1665-1667 MAR 8  
Abstract: We demonstrate, that under nitrogen atmosphere, 20% of single DiIC(18)(3) (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) molecules in poly(methylmethacrylate) show an extremely low photobleaching quantum yield of (4.66+/-0.07)x10(-8) together with a reasonably short triplet lifetime. We exploit these properties to demonstrate that the system can be used to produce a triggered single-photon source based exclusively on organic materials. (C) 2004 American Institute of Physics.
Notes: Times Cited: 9
Bert Hecht (2004)  Nano-optics with single quantum systems.   Philos Transact A Math Phys Eng Sci 362: 1817. 881-899 Apr  
Abstract: This paper reviews the recent progress in using single quantum systems, here mainly single fluorescent molecules, as local probes for nano-optical field distributions. We start by discussing the role of the absorption cross-section for the spatial resolution attainable in such experiments and its behaviour for different environmental conditions. It is shown that the spatial distribution of field components in a high-numerical aperture laser focus can be mapped with high precision using single fluorescent molecules embedded in a thin polymer film on glass. With this proof-of-principle experiment as a starting point, the possibility of mapping strongly confined and enhanced nano-optical fields close to material structures, e.g. sharp metal tips, is discussed. The mapping of the spatial distribution of the enhanced field at an etched gold tip using a single molecule is presented as an example. Energy transfer effects and quenching are identified as possible artefacts in this context. Finally, it is demonstrated that the local quenching at a sharp metal structure nevertheless can be exploited as a novel contrast mechanism for ultrahigh-resolution optical microscopy with single-molecule sensitivity.
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2003
W Trabesinger, A Kramer, M Kreiter, B Hecht, U P Wild (2003)  Single-molecule near-field optical energy transfer microscopy with dielectric tips.   J Microsc 209: Pt 3. 249-253 Mar  
Abstract: The fluorescence lifetime and the fluorescence rate of single molecules are recorded as a function of the position of a Si3N4 atomic force microscopy tip with respect to the molecule. We observe a decrease of the excited state lifetime and the fluorescence rate when the tip apex is in close proximity to the molecule. These effects are attributed to the fact that the dielectric tip converts non-propagating near-fields to propagating fields within the dielectric tip effectively quenching the fluorescence. The spatial extension of the quenching area is of subwavelength dimensions. The results are discussed in terms of molecular fluorescence in a system of stratified media. The experiment provides surprising new insights into the interactions between a fluorescent molecule and a dielectric tip. The methodology holds promise for applications in ultra high-resolution near-field optical imaging at the level of single fluorophores.
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J P Renault, A Bernard, A Bietsch, B Michel, H R Bosshard, E Delamarche, M Kreiter, B Hecht, U P Wild (2003)  Fabricating arrays of single protein molecules on glass using microcontact printing   JOURNAL OF PHYSICAL CHEMISTRY B 107: 61. 703-711 JAN 23  
Abstract: Microcontact printing biomolecules from elastomeric micropatterned stamps onto surfaces is a versatile method to prepare surfaces for diagnostic applications. We show how to create patterns of proteins having a lengthscale lower than 100 nm using high-resolution microcontact printing. The elastomeric stamps used have meshes composed of 100- and 40-nm-wide lines, arrays of 100 x 400 nm(2) features, and arrays of 100-nm-wide,posts. The spherical geometry of the posts on the stamps contributes to reduce the printed areas below the effective size of the molded features. Proteins adsorb onto the hydrophobic surface of the stamp during the inking step, and by varying the concentration of the protein solutions, it is possible to adsorb a single or a few protein molecules, such as antibodies (fluorescently labeled) or green fluorescence proteins, on each of the elements forming the high-resolution pattern of the stamp. The transfer of the proteins from the stamp to a hydrophilic glass surface occurs during the printing step. Characterization of the printed patterns using atomic force microscopy and fluorescence confocal microscopy reveals sites unoccupied or occupied by one or more protein molecules that are located within 50 nm of the expected printed locations. The placement of a small number of protein molecules on a surface at precise locations is the key to localizing and identifying single proteins and might constitute a method of choice to study single protein molecules on surfaces.
Notes: Times Cited: 59
V A Azov, F Diederich, Y Lill, B Hecht (2003)  Synthesis and conformational switching of partially and differentially bridged resorcin[4]arenes bearing fluorescent dye labels - Preliminary communication   HELVETICA CHIMICA ACTA 86: 22. 2149-2155  
Abstract: We report the synthesis of modified Cram-type cavitands bearing one or two fluorescent labels for single-molecule spectroscopic studies of vase-kite conformational switching (Scheme 3). Syntheses were performed by stepwise bridging of the four couples of neighboring H-bonded OH groups of resorcin[4]arene bowls (Schemes 2 and 3). The new substitution patterns enable the construction of a large variety of future functional architectures. H-1-NMR Investigations showed that the new partially and differentially bridged cavitands feature temperature- and pH-triggered vase-kite conformational isomerism similar to symmetrical cavitands with four identical quinoxaline bridges (Table). It was discovered that vase-kite switching of cavitands is strongly solvent-dependent.
Notes: Times Cited: 20
M Prummer, B Sick, B Hecht, U P Wild (2003)  Three-dimensional optical polarization tomography of single molecules   JOURNAL OF CHEMICAL PHYSICS 118: 24. 9824-9829 JUN 1  
Abstract: We apply the concept of tomography to polarization-sensitive optical microscopy of single fluorophores to determine the three-dimensional orientation of molecular absorption dipoles with isotropic sensitivity. Wide-field microscopy provides the opportunity to monitor simultaneously three-dimensional rotation and two-dimensional translation of many molecules in parallel. For orientation determination the molecules are illuminated from different directions of incidence with linearly polarized light. In each exposure the excitation along a particular projection of the absorption dipole on the electric field leads to a distinct fluorescence intensity. Five exposures are sufficient to determine the full orientation of the fluorophores. To demonstrate the potential of the method we determine the orientation and position of individual immobilized lipid membrane markers. The shot-noise-limited isotropic angular resolution is 2degrees. For time-resolved studies the bandwidth can be expanded up to 200 Hz. (C) 2003 American Institute of Physics.
Notes: Times Cited: 14
2002
A Kramer, W Trabesinger, B Hecht, U P Wild (2002)  Optical near-field enhancement at a metal tip probed by a single fluorophore   APPLIED PHYSICS LETTERS 80: 25. 1652-1654 MAR 4  
Abstract: The optical near-field in the vicinity of a metal tip is mapped using a single-molecule optical probe. We observe an enhancement of the fluorescence signal by a factor of 5.7+/-0.3, clearly larger than the fourfold enhancement that can arise from constructive interference if the tip acts as a simple mirror. Considering the tip apex as a nanoparticle of nonregular shape, we suggest that, in the case of gold tips, the enhancement is due to resonant plasmon excitation. Consistently, no enhancement has been observed using Pt/Ir tips. (C) 2002 American Institute of Physics.
Notes: Times Cited: 44
W Trabesinger, C G Hubner, B Hecht, U P Wild (2002)  Continuous real-time measurement of fluorescence lifetimes   REVIEW OF SCIENTIFIC INSTRUMENTS 73: 7. 3122-3124 AUG  
Abstract: We report on the continuous measurement of fluorescence lifetimes at low light levels. Fluorescence photons following pulsed excitation generate a pulse sequence with exponentially distributed amplitudes and interphoton times at the output of a time-to-amplitude converter. This sequence is turned into a continuous step function and is time averaged with an adjustable bandwidth. For a single-exponential decay, our approach yields identical results as would be obtained from fitting fluorescence decays, while being a real-time technique. The proposed technique performs especially well at low count rates. We demonstrate the applicability of the method at the example of confocal fluorescence lifetime imaging of single molecules. (C) 2002 American Institute of Physics.
Notes: Times Cited: 6
W Trabesinger, A Kramer, M Kreiter, B Hecht, U P Wild (2002)  Single-molecule near-field optical energy transfer microscopy   APPLIED PHYSICS LETTERS 81: 28. 2118-2120 SEP 9  
Abstract: The nano-optical interaction between a sharp tip and a single dipolar emitter is investigated. Changes of the excited state lifetime and the fluorescence rate of single molecules are recorded simultaneously as a function of the tip position relative to the molecule. A subdiffraction-limited area of decreased fluorescence and shortened lifetime is observed for gold-coated Si3N4 tips. The results are discussed in terms of molecular fluorescence in a system of stratified media. The outlined methodology holds promise for applications in ultrahigh-resolution near-field optical imaging at the level of single fluorophores. (C) 2002 American Institute of Physics.
Notes: Times Cited: 23
A Kramer, J M Segura, A Hunkeler, A Renn, B Hecht (2002)  A cryogenic scanning near-field optical microscope with shear-force gapwidth control   REVIEW OF SCIENTIFIC INSTRUMENTS 73: 24. 2937-2941 AUG  
Abstract: We present a scanning near-field optical microscope designed for nanoscale optical imaging and spectroscopy as well as simultaneous tuning fork shear-force topographic imaging at cryogenic temperatures. The whole setup is immersed in superfluid helium (T=1.8 K). In this medium we observe resonance frequency fluctuations of the tuning fork sensor with an amplitude of Deltanuapproximate to5%-10% of the full width at half-maximum of the resonance. Possible reasons for the occurrence of the frequency fluctuations are discussed. A stable gapwidth feedback can still be achieved if the set value of the frequency shift is chosen slightly larger than the fluctuation amplitude. As an example we demonstrate shear-force topographic imaging of a silicon grating in superfluid helium. (C) 2002 American Institute of Physics.
Notes: Times Cited: 6
M Kreiter, M Prummer, B Hecht, U P Wild (2002)  Orientation dependence of fluorescence lifetimes near an interface   JOURNAL OF CHEMICAL PHYSICS 117: 20. 9430-9433 NOV 22  
Abstract: The fluorescence lifetime of single DiI-dye molecules in a 20 nm polymer film on glass is measured as a function of the orientation of the absorption dipole moment. A strong dependence of the lifetime on the orientation of the dye molecules relative to the polymer/air interface is found. Molecules with a dipole moment perpendicular to the interface exhibit a lifetime which is by a factor of 2.1+/-0.1 longer than the lifetime of molecules with parallel dipole moments. The general trend of the results is in good agreement with theoretical predictions. However there are significant deviations which are attributed to varying molecular environments. (C) 2002 American Institute of Physics.
Notes: Times Cited: 19
2001
J M Segura, G Zumofen, A Renn, B Hecht, U P Wild (2001)  Tip-induced spectral dynamics of single molecules   CHEMICAL PHYSICS LETTERS 340: 16. 77-82 MAY 25  
Abstract: Manipulation of spectral dynamics of single molecules (SM) by a metallized scanning probe tip is demonstrated. The Stark effect of the zero-phonon lines of single pentacene molecules in a p-terphenyl host at 1.8 K is investigated by applying a voltage to the tip in contact with the sample. The measured Stark shifts exhibit a plateau and the line widths depend on the electric field. These anomalies are explained by a model based on two-level systems with field-dependent double-well potentials. The experimental data show that the two-level systems are induced by the tip. (C) 2001 Elsevier Science B.V. All rights reserved.
Notes: Times Cited: 5
W Trabesinger, A Renn, B Hecht, U P Wild, A Montali, P Smith, C Weder (2001)  Molecular rearrangements observed by single-molecule microscopy   SYNTHETIC METALS 124: 5. 113-115 OCT 3  
Abstract: We have applied quantitative single-molecule photoluminescence microscopy for directly observing molecular rearrangements in a polymeric material subjected to tensile deformation. The system examined is a blend of fluorescent conjugated polymer molecules embedded in polyethylene, known to exhibit strong phase separation. Statistical analysis of the sizes of phase-separated domains reveals that during tensile deformation large domains of fluorescent molecules are transformed into smaller clusters and ultimately isolated single molecules. In addition to elucidating the nature of the stretch-induced morphology change, our work underscores the fundamental value of single-molecule techniques for material analysis [1]. (C) 2001 Elsevier Science B.V. All rights reserved.
Notes: Times Cited: 0
B Sick, B Hecht, U P Wild, L Novotny (2001)  Probing confined fields with single molecules and vice versa.   J Microsc 202: Pt 2. 365-373 May  
Abstract: Single dye molecules are used as local probes to map the spatial distribution of the squared electric field components in the focus of a high numerical aperture lens. Simulated field distributions are quantitatively verified by experimentally obtained fluorescence excitation maps. We show that annular illumination can be used to engineer the field distribution in the focus at a dielectric/air interface such that electric field components in all directions acquire comparable magnitudes. The 3D orientation of molecular absorption dipoles can be determined by comparing measured to simulated image patterns. The presence of longitudinal electric field components in a focus is of particular interest in tip-enhanced scanning near-field optical microscopy.
Notes:
W Trabesinger, B Hecht, U P Wild, G J Schütz, H Schindler, T Schmidt (2001)  Statistical analysis of single-molecule colocalization assays.   Anal Chem 73: 6. 1100-1105 Mar  
Abstract: In chemical assays, specific molecular recognition events result in close physical proximity of two molecular species, e.g., ligands and receptors. Microscopy techniques that are able to image individual molecules allow for achieving a positional accuracy far beyond the resolution limit Therefore, independent position determination, e.g., by dual-color microscopy, becomes possible, permitting determination of intermolecular distances beyond the resolution limit. Nonzero measured distances occur due to experimental inaccuracies in case of a recognition event or due to accidental close proximity between ligand-receptor pairs. Using general statistical considerations, finite measured distances between single ligand-receptor pairs are directly translated into probabilities for true molecular recognition or mere accidental proximity. This enables a quantitative statistical analysis of single recognition events. It is demonstrated that in a general assay, even in the presence of strong unspecific background, the probability for a certain diagnosis and a measure for its reliability can be extracted from the observation of a few binding events. The power of the method is demonstrated at the example of a single-molecule DNA hybridization assay. Our findings are of major importance for future assay miniaturization and assaying with minute amounts of analyte.
Notes:
L Fleury, J M Segura, G Zumofen, B Hecht, U P Wild (2001)  Photon statistics in single-molecule fluorescence at room temperature   JOURNAL OF LUMINESCENCE 94: 25. 805-809 DEC  
Abstract: The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of inter-photon time distribution, second-order correlation function, and Mandel's Q-function. Nonclassical photon statistics is observed and a reverse intersystem crossing is detected that accelerates linearly with the applied laser power. Rate equations for the time evolution of the molecular level populations are shown to be appropriate for the analysis of the observations. (C) 2001 Elsevier Science B.V. All rights reserved.
Notes: Times Cited: 0
2000
M Prummer, C G Hubner, B Sick, B Hecht, A Renn, U P Wild (2000)  Single-molecule identification by spectrally and time resolved fluorescence detection   ANALYTICAL CHEMISTRY 72: 12. 443-447 FEB 1  
Abstract: A method to identify single molecules rapidly and with high efficiency based on simple probability considerations is proposed. In principle, any property of a detected photon in a single-molecule fluorescence experiment, e.g., emission wavelength, arrival time after pulsed excitation, and polarization, can be analyzed within the framework of the outlined methodology. Monte Carlo simulations show that less than 500 photons are needed to assign an observed single molecule to one out of four species with a confidence level higher than 99.9%. We show that single dye molecules of four different dyes embedded in a polymer film can be identified with time-correlated single-photon counting spectrally resolved in two channels.
Notes: Times Cited: 35
B Sick, B Hecht, L Novotny (2000)  Orientational imaging of single molecules by annular illumination   PHYSICAL REVIEW LETTERS 85: 24. 4482-4485 NOV 20  
Abstract: The absorption dipole orientation of single fluorescent molecules is determined by mapping the spatial distribution of the squared electric field components in a high-numerical-aperture laser focus. Annular illumination geometry and the vicinity of a plane dielectric/air interface strongly enhance the longitudinal field component and the transverse fields perpendicular to the polarization direction. As a result, all three excitation field components in the focus are of comparable magnitude. The scheme holds promise to monitor rotational diffusion of single molecules in complex environments.
Notes: Times Cited: 104
L Fleury, J M Segura, G Zumofen, B Hecht, U P Wild (2000)  Nonclassical photon statistics in single-molecule fluorescence at room temperature   PHYSICAL REVIEW LETTERS 84: 24. 1148-1151 FEB 7  
Abstract: The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy Photon arrival times are analyzed in terms of interphoton time distributions, second order correlation functions, and the variance of the photon number probability distribution. Antibunching at short times and bunching behavior for longer time is observed, associated with sub- and super-Poissonian statistics, respectively. A rate-equation analysis of the molecular level populations indicates an accelerated reverse intersystem crossing.
Notes: Times Cited: 110
J M Segura, A Renn, B Hecht (2000)  A sample-scanning confocal optical microscope for cryogenic operation   REVIEW OF SCIENTIFIC INSTRUMENTS 71: 25. 1706-1711 APR  
Abstract: A sample-scanning confocal optical microscope for single-molecule imaging and spectroscopy working at superfluid helium temperature, liquid nitrogen, and room temperature is described. An optical resolution of 800 nm full width at half maximum as well as a detection efficiency of approximate to 3.5% are achieved. The sample scanner features an exceptionally large scan range of 23 mu m at 1.8 K. A position sensor allows for continuous observation of the scanner motion and for a correction of piezoelectric hysteresis and creep at 77 K and at room temperature. Coarse positioning of the sample in x-y is achieved by an inertia drive with high reproducibility and nanometer precision. We demonstrate combined high-resolution confocal imaging and spectroscopy of single molecules at 1.8 K. (C) 2000 American Institute of Physics. [S0034-6748(00)00704-8].
Notes: Times Cited: 9
R Rodrigues-Herzog, F Trotta, H Bill, J M Segura, B Hecht, H J Güntherodt (2000)  Optical microscopy of single ions and morphological inhomogeneities in Sm-doped CaF2 thin films   PHYSICAL REVIEW B 62: 25. 11163-11169 OCT 15  
Abstract: We have investigated the luminescence of CaF2, thin films doped with very low concentrations of Sm2+ ions using scanning confocal optical microscopy at low temperatures. The film morphology was studied independently by atomic force microscopy. The Sm2+ ions are homogeneously distributed in the films and show photobleaching. Unexpectedly, on the film surface strongly luminescent small topographic features are observed that are found to contain Sm3+ by spectral analysis. The formation of Sm3+ is probably due to the presence of oxygen during film growth. In the lowest doped films on-off blinking behavior of isolated luminescent spots provides strong evidence for the first observation of single ions in a crystal.
Notes: Times Cited: 5
W Trabesinger, A Renn, B Hecht, U P Wild, A Montali, P Smith, C Weder (2000)  Single-molecule imaging revealing the deformation-induced formation of a molecular polymer blend   JOURNAL OF PHYSICAL CHEMISTRY B 104: 46. 5221-5224 JUN 8  
Abstract: Photoluminescence microscopy of conjugated polymer molecules in a polyethylene host has been applied to monitor the transition of a phase-separated polymer blend into a molecular dispersion induced by solid-state tensile deformation. Statistical analysis of conjugated polymer cluster sizes as a function of the degree of matrix deformation shows that phase-separated domains of conjugated polymer transform into smaller clusters and single molecules as the degree of matrix deformation increases. Concomitantly, the conjugated guest molecules tend to adopt the preferential orientation of the surrounding matrix. We demonstrate that single-molecule detection can be readily extended and applied to probing molecular dispersion, orientation: and morphology in polymer-polymer blends.
Notes: Times Cited: 14
A Montali, A R A Palmans, M Eglin, C Weder, P Smith, W Trabesinger, A Renn, B Hecht, U P Wild (2000)  Phase behavior and anisotropic optical properties of photoluminescent polarizers   MACROMOLECULAR SYMPOSIA 154: 15. 105-116 APR  
Abstract: The phase behavior and anisotropic optical properties of tensile deformed blends of a photoluminescent polymer guest in an ultra-high molecular weight polyethylene matrix were studied on the level of single molecules by means of scanning confocal optical microscopy. It is shown that upon tensile deformation of the blends, the system transforms from a phase-separated system into a quasimolecular solid solution. The influence of this phase transition on the anisotropic optical properties of oriented blend films was also investigated with polarized steady-state photoluminescence spectroscopy. We show that well-dissolved guest molecules tend to reach higher degrees of orientation at lower draw ratios of the blend films compared to guests that phase-separate from the matrix polymer. Dichroic ratios in emission in the range of 50 were observed in optimized blend films based on photoluminescent oligomers and linear low density polyethylene.
Notes: Times Cited: 6
B Hecht, B Sick, U P Wild, V Deckert, R Zenobi, O J F Martin, D W Pohl (2000)  Scanning near-field optical microscopy with aperture probes : Fundamentals and applications   JOURNAL OF CHEMICAL PHYSICS 112: 130. 7761-7774 MAY 8  
Abstract: In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool. (C) 2000 American Institute of Physics. [S0021-9606(00)70316-3].
Notes: Times Cited: 142
L Fleury, J M Segura, G Zumofen, B Hecht, U P Wild (2000)  Nonclassical photon statistics in single-molecule fluorescence at room temperature   Phys Rev Lett 84: 6. 1148-1151 Feb  
Abstract: The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of interphoton time distributions, second order correlation functions, and the variance of the photon number probability distribution. Antibunching at short times and bunching behavior for longer times is observed, associated with sub- and super-Poissonian statistics, respectively. A rate-equation analysis of the molecular level populations indicates an accelerated reverse intersystem crossing.
Notes:
1999
R Stockle, C Fokas, V Deckert, R Zenobi, B Sick, B Hecht, U P Wild (1999)  High-quality near-field optical probes by tube etching   APPLIED PHYSICS LETTERS 75: 17. 160-162 JUL 12  
Abstract: A method called tube etching for the fabrication of near-field optical probes is presented. Tip formation occurs inside a cylindrical cavity formed by the polymer coating of an optical fiber which is not stripped away prior to etching in hydrofluoric acid. The influence of temperature, etchant concentration, and fiber type on the tip quality is studied. A tip formation mechanism for the given geometry is proposed. The procedure overcomes drawbacks of the conventional etching techniques while still producing large cone angles: (i) tips with reproducible shapes are formed in a high yield, (ii) the surface roughness on the taper is drastically reduced, and (iii) the tip quality is insensitive to vibrations and temperature fluctuations during the etching process. After aluminum coating, optical probes with well-defined apertures are obtained. Due to the smooth glass surface the aluminum coating is virtually free of pinholes. (C) 1999 American Institute of Physics. [S0003-6951(99)03228-3].
Notes: Times Cited: 112
1998
L Fleury, B Sick, G Zumofen, B Hecht, U P Wild (1998)  High photo-stability of single molecules in an organic crystal at room temperature observed by scanning confocal optical microscopy   MOLECULAR PHYSICS 95: 25. 1333-1338 DEC 20  
Abstract: Single molecules have been observed in a molecular crystal at room temperature using scanning confocal fluorescence microscopy. Individual terrylene molecules in a p-terphenyl crystal show high photo-stability with a corresponding ensemble-averaged photo-destruction quantum efficiency lower than 1.2 x 10(-8). On average, with a detection efficiency of 6.5%, a stable fluorescence signal of 10(5) photons s(-1) could be detected from single molecules during 1 min of continuous photo-excitation. Both irreversible and reversible abrupt fluorescence intensity jumps to the background have been observed. The experimental data indicate that diffusing quenchers at low concentration in the crystal contribute to fluorescence bleaching of single molecules. All molecular fluorescence signals follow a typical power saturation law with a mean saturation count rate of 4.3 x 10(5) photons s(-1). The large photo-stability allows for long illumination times and high emission rates of single molecules trapped in a crystal at room temperature. A potential application as single organic quantum light sources under ambient conditions is conceivable.
Notes: Times Cited: 27
B Hecht, H Bielefeldt, D W Pohl, L Novotny, H Heinzelmann (1998)  Influence of detection conditions on near-field optical imaging   JOURNAL OF APPLIED PHYSICS 84: 33. 5873-5882 DEC 1  
Abstract: The process of image formation in transmission mode scanning near-field optical microscopy is analyzed both theoretically and experimentally. Changes in the dielectric and topographic properties of the sample influence not only the total transmitted intensity, but also its angular distribution in the far field. This opens up an additional source of optical information about the sample. Some of this additional information is retrieved by separate but simultaneous detection of the radiation emitted at angles smaller (allowed light) and larger (forbidden light) than the critical angle of total internal reflection, respectively. Different experimental setups and their respective advantages are discussed. High resolution, constant height mode optical images of test structures are compared with theoretical predictions. Forbidden-light optical images frequently provide enhanced resolution and/or contrast as compared to allowed light images. For small phase objects, in contrast to amplitude objects, a contrast reversal between forbidden and allowed light images is observed. (C) 1998 American Institute of Physics. [S0021-8979(98)02923-5]
Notes: Times Cited: 24
L Novotny, B Hecht, D W Pohl (1998)  Implications of high resolution to near-field optical microscopy   ULTRAMICROSCOPY 71: 6. 341-344 MAR  
Abstract: This paper presents problems inherent to high-resolution near-field optical microscopy. It is shown on an easily understandable level, that high lateral confinement of optical fields (a prerequisite for high-resolution microscopy) leads to a fast decay of the fields. Consequently, the optical probe has to be brought very close to the sample surface, increasing the sensitivity to artifacts. Highly confined optical fields are strongly sensitive to variations in the probe-sample separation. The resulting optical images are, therefore, dominated by topographical variations and do not represent the optical properties of the sample surface. (C) 1998 Published by Elsevier Science B.V. All rights reserved.
Notes: Times Cited: 19
1997
L Novotny, B Hecht, D W Pohl (1997)  Interference of locally excited surface plasmons   JOURNAL OF APPLIED PHYSICS 81: 50. 1798-1806 FEB 15  
Abstract: Surface plasmon interactions on a finite silver layer are theoretically investigated using a coupled dipole formalism. The studied system consists of several protruding particles located on the surface of the layer that are scanned with an optical probe. An optical scan-image of the silver surface is obtained by assigning the recorded far-field radiation to the momentary position of the optical probe. Both, probe and protrusions are considered as single dipolar panicles. Interferences of the locally excited surface plasmons can be recorded by detecting the radiation emitted into the lower half-space at angles beyond the critical angle of total internal reflection (forbidden light). The resulting scan images show excellent agreement with recent experimental measurements. The theory of the coupled dipole formalism using Green's functions of a layered reference system is outlined and electromagnetic properties of surface plasmons are discussed. (C) 1997 American Institute of Physics.
Notes: Times Cited: 45
B Hecht, H Bielefeldt, Y Inouye, D W Pohl, L Novotny (1997)  Facts and artifacts in near-field optical microscopy   JOURNAL OF APPLIED PHYSICS 81: 42. 2492-2498 MAR 15  
Abstract: Near-field optical (NFO) microscopes with an auxiliary gap width regulation (shear force, tunneling) may produce images that represent the path of the probe rather than optical properties of the sample. Experimental and theoretical evidence leads us to the conclusion that many NFO results reported in the past might have been affected or even dominated by the resulting artifact. The specifications derived from such results for the different types of NFO microscopes used therefore warrant reexamination. We show that the resolving power of aperture NFO microscopes, 30-50 nm, is of genuine NFO origin but can be heavily obscured by the artifact. (C) 1997 American Institute of Physics.
Notes: Times Cited: 261
D W Pohl, H Bach, M A Bopp, V Deckert, P Descouts, R Eckert, H J Güntherodt, C Hafner, B Hecht, H Heinzelmann, T Huser, M Jobin, U Keller, T Lacoste, P Lambelet, F MarquisWeible, O J F Martin, A J Meixner, B Nechay, L Novotny, M Pfeiffer, C Philipona, T Plakhotnik, A Renn, A Sayah, J M Segura, B Sick, U Siegner, G Tarrach, R Vahldieck, U P Wild, D Zeisel, R Zenobi (1997)  Optical microscopy in the nano-world   CHIMIA 51: 38. 760-767 OCT  
Abstract: Scanning near-field optical microscopy (SNOM) is an optical microscopy whose resolution is not bound to the diffraction limit. It provides chemical information based upon spectral, polarization and/or fluorescence contrast images. Details as small as 20 nm can be recognized. Photophysical and photochemical effects can be studied with SNOM on a similar scale. This article reviews a good deal of the experimental and theoretical work on SNOM in Switzerland.
Notes: Times Cited: 4
1996
B Hecht, H Bielefeldt, L Novotny, Y Inouye, D W Pohl (1996)  Local excitation, scattering, and interference of surface plasmons   PHYSICAL REVIEW LETTERS 77: 19. 1889-1892 AUG 26  
Abstract: The optical probe of a scanning near-held optical microscope is shown to act as a point source of surface plasmon (SP) polaritons on gold and silver films. Plasmon excitation manifests itself by emission of light in the direction of the SP resonance angle, originating from an area with the shape of a dipole radiation pattern whose extension is given by the SP decay length. Interaction with selected, individual surface inhomogeneities gives rise to characteristic modifications of the emitted radiation, which provide detailed information about SP scattering, reflection, and interference phenomena.
Notes: Times Cited: 140
D W Pohl, L Novotny, B Hecht, H Heinzelmann (1996)  Radiation coupling and image formation in scanning near-field optical microscopy   THIN SOLID FILMS 273: 32. 161-167 FEB  
Abstract: The propagation of light through an aperture scanning near-field optical microscope is studied. Rules for optimizing the confinement and throughput of aperture near field probes are derived. The radiation from the aperture through simple sample structures, a flat substrate and a substrate with stripe-like elevations and depression, into classically allowed and forbidden directions is determined quantitatively. It is shown that small amplitude and phase objects can be distinguished by comparing ''allowed'' and ''forbidden'' light images.
Notes: Times Cited: 21
H Heinzelmann, T Lacoste, T Huser, H J Güntherodt, B Hecht, D W Pohl (1996)  Instrumental developments and recent experiments in near-field optical microscopy   THIN SOLID FILMS 273: 20. 149-153 FEB  
Abstract: Recent advances in the understanding of light propagation in small dimensions as well as in instrumentation make scanning near-field optical microscopy (SNOM) a very promising tool for studying optical phenomena on a nanometer scale. In this talk, we will demonstrate experiments carried out with the recently developed tunneling near-field optical microscope. We found superior image contrast, as compared with images taken with conventional aperture SNOM, along with the high resolution commonly achieved with fiber probes. This work was motivated by the theoretical investigations presented in Dr. Pohl's talk. We will further describe two recently built instruments. The first is a scanning tunneling optical microscope combined with a scanning force microscope. The second instrument is an aperture-type SNOM mounted on the sample stage of a conventional inverted optical microscope. Of particular interest to us is imaging with polarization contrast. One of the goals is to study liquid-crystal films which have been micropatterned with the help of a force microscope tip. These samples are promising as waveguides and potential electro-optical devices. Additionally, they represent very convenient test samples for polarization SNOM.
Notes: Times Cited: 8
1995
L Novotny, D W Pohl, B Hecht (1995)  Light confinement in scanning near-field optical microscopy   ULTRAMICROSCOPY 61: 22. 1-9 DEC  
Abstract: Light propagation and light confinement in scanning near-field optical microscopy (SNOM) are studied by means of the multiple multipole method (MMP). Helmholtz' vector wave equation is solved in three dimensions for four different near-field optical probes: a bare glass tip, a metal-clad aperture probe, an ''entirely coated'' tip and a novel plasmon probe. The latter two are capable of producing a near-field light spot of less than 20 nm (FWHM) extension. The spots have a single intensity peak at the centre, in contrast to the field distribution behind an aperture, which is doubly peaked. The effect of various parameters on light throughput and confinement is investigated for these probes.
Notes: Times Cited: 59
H Heinzelmann, B Hecht, L Novotny, D W Pohl (1995)  FORBIDDEN LIGHT SCANNING NEAR-FIELD OPTICAL MICROSCOPY   JOURNAL OF MICROSCOPY-OXFORD 177: 14. 115-118 FEB  
Abstract: Near-field optics (NFO) opens the door to light microscopy techniques with resolutions well beyond the diffraction limit, The richness of optical investigations is now applicable on a near-molecular level, Among the novel scanning near-field optical microscopy (SNOM) schemes, the most prominent representatives are aperture SNOM and scanning tunnelling optical microscopy (STOM or PSTM). New experimental and theoretical work has to be performed to study the phenomena specific to NFO. One such example is the angular dependence of light emission in aperture SNOM. The detection of radiation at angles greater than the critical angle of total internal reflection alpha(c)=arcsin(n(-1)), where n is the sample refractive index, can represent a microscopy scheme that combines the respective advantages of both aperture SNOM and STOM, Recent experiments have demonstrated the expected exponential dependence of light intensity on gap width (for fixed emission angle alpha > alpha(c)). The decay length as a function of a is in agreement with the Fresnel description of the evanescent field when total reflection occurs at an interface, These investigations were additionally motivated by calculations based on the multiple multipole method.
Notes: Times Cited: 7
B Hecht, H Heinzelmann, D W Pohl (1995)  COMBINED APERTURE SNOM/PSTM - BEST OF BOTH WORLDS   ULTRAMICROSCOPY 57: 13. 228-234 FEB  
Abstract: Aperture scanning near-field optical microscopy (SNOM/NSOM) and scanning tunneling optical microscopy (STOM, also known as PSTM) are integrated into an instrument which combines the advantages of both schemes. As a result, more near-field optical information can be obtained and new modes of operation become possible. Scan images of a glass surface with a fine grating relief demonstrate some of the capabilities of the new 'TNOM' technique.
Notes: Times Cited: 38
H Heinzelmann, T Huser, T Lacoste, H J Güntherodt, D W Pohl, B Hecht, L Novotny, O J F Martin, C V Hafner, H Baggenstos, U P Wild, A Renn (1995)  SCANNING NEAR-FIELD OPTICAL MICROSCOPY IN BASEL, RUSCHLIKON, AND ZURICH   OPTICAL ENGINEERING 34: 38. 2441-2454 AUG  
Abstract: The concepts of near-field optical microscopy and experimental and theoretical work carried out in Switzerland over the last 10 years are reviewed. After a description of the pioneering experiments of the mid-1980s, we focus on the recent efforts of the three Swiss laboratories currently working in the field in close collaboration. This newly refreshed initiative in near-field optics is supported by the Swiss Priority Program Optique.
Notes: Times Cited: 19
P Bauer, B Hecht, C Rossel (1995)  Piezoresistive cantilevers as optical sensors for scanning near-field microscopy   ULTRAMICROSCOPY 61: 8. 127-130 DEC  
Abstract: To combine the high lateral resolution of AFM with near-field optical measurements, we propose the use of microfabricated piezoresistive cantilevers as miniaturized photosensitive elements. The silicon-based sensors consist of a p-doped piezoresistive path, which also includes the tip. The resistance of this path can be changed either by pressure on the lever or by light. For combined optical and topographical measurements, an evanescent field is created. Because the AFM tip is the only part of the cantilever that is exposed to the evanescent field, the tip can be used as a near-field optical probe. It is possible to extract the exponential decay of the evanescent field from combined force/optical measurements. To decouple optical and topographical information, the intensity of the evanescent field is modulated and the optical signal is measured with the lock-in technique.
Notes: Times Cited: 10
L Novotny, D W Pohl, B Hecht (1995)  SCANNING NEAR-FIELD OPTICAL PROBE WITH ULTRASMALL SPOT SIZE   OPTICS LETTERS 20: 10. 970-972 MAY 1  
Abstract: A novel Light-emitting probe for scanning near-field optical microscopy is investigated theoretically. The three-dimensional vectorial Helmholtz equation is solved for the new probe geometry by using the multiple multipole method. The novel probe consists of a dielectric tip that is entirely metal coated. It provides a single near-field spot that can be smaller than 20 nm (FWHM). The dependence on tip radius, taper angle, and metal thickness in front of the tip is investigated for the power transmission through the probe as well as for the spot size.
Notes: Times Cited: 137
B Hecht, D W Pohl, H Heinzelmann, L Novotny (1995)  ''Tunnel'' near-field optical microscopy : TNOM-2   ULTRAMICROSCOPY 61: 20. 99-104 DEC  
Abstract: Light emitted from the aperture of a near-field optical probe in the close vicinity of a dielectric object propagates in classically ''forbidden'' as well as ''allowed'' directions; the two zones are separated by the critical angle for total internal reflection. The new ''tunnel'' near-field optical microscopy (TNOM) technique makes use of forbidden and allowed radiation, in contrast to standard scanning near-field optical microscopy (SNOM or NSOM), which records only the allowed light. Scan images obtained with allowed and forbidden light are complementary to some extent; the latter, however, provide high contrast and resolution even in situations in which standard SNOM/NSOM shows little or no contrast. The influence of topography on image formation is analyzed and discussed.
Notes: Times Cited: 13
1994
1993
O Marti, H Bielefeldt, B Hecht, S Herminghaus, P Leiderer, J Mlynek (1993)  NEAR-FIELD OPTICAL MEASUREMENT OF THE SURFACE-PLASMON FIELD   OPTICS COMMUNICATIONS 96: 14. 225-228 FEB 15  
Abstract: The intensity of the evanescent electromagnetic wave of optically excited surface plasmons was measured directly using a scanning tunneling optical microscope (STOM) setup. When resonant coupling of the driving field to the surface plasmons was achieved, the measured intensity was increased by a factor of 30 larger than the corresponding evanescent wave intensity on a bare glass surface, in agreement with the theoretical prediction. Experimental results are presented for three laser wavelengths (514 nm, 633 nm, 670 nm). Possible applications of the technique to study surface plasmon fields are discussed.
Notes: Times Cited: 42
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