Papers by Norbert Scherer
arXiv (Cornell University), Feb 14, 2018
Rapid and accurate volumetric imaging remains a challenge, yet has the potential to enhance under... more Rapid and accurate volumetric imaging remains a challenge, yet has the potential to enhance understanding of cell function. We developed and used a multifocal microscope (MFM) for 3D snapshot imaging to allow 3D tracking of insulin granules labeled with mCherry in MIN6 cells. MFM employs a special diffractive optical element (DOE) to simultaneously image multiple focal planes. This simultaneous acquisition of information determines the 3D location of single objects at a speed only limited by the array detector's frame rate. We validated the accuracy of MFM imaging/tracking with fluorescence beads; the 3D positions and trajectories of single fluorescence beads can be determined accurately over a wide range of spatial and temporal scales. The 3D positions and trajectories of single insulin granules in a 3.2um deep volume were determined with imaging processing that combines 3D decovolution, shift correction, and finally tracking using the Imaris software package. We find that the motion of the granules is superdiffusive, but less so in 3D than 2D for cells grown on coverslip surfaces, suggesting an anisotropy in the cytoskeleton (e.g. microtubules and action).
Proceedings of the National Academy of Sciences of the United States of America, Feb 14, 2019
Three-Dimensional and Multidimensional Microscopy: Image Acquisition and Processing XXV, 2018
An interferometric fluorescent microscope and a novel theoretic image reconstruction approach wer... more An interferometric fluorescent microscope and a novel theoretic image reconstruction approach were developed and used to obtain super-resolution images of live biological samples and to enable dynamic real time tracking. The tracking utilizes the information stored in the interference pattern of both the illuminating incoherent light and the emitted light. By periodically shifting the interferometer phase and a phase retrieval algorithm we obtain information that allow localization with sub-2 nm axial resolution at 5 Hz.
Optics Letters, 2018
Accurate and rapid particle tracking is essential for addressing many research problems in single... more Accurate and rapid particle tracking is essential for addressing many research problems in single molecule and cellular biophysics and colloidal soft condensed matter physics. We developed a novel three-dimensional interferometric fluorescent particle tracking approach that does not require any sample scanning. By periodically shifting the interferometer phase, the information stored in the interference pattern of the emitted light allows localizing particles positions with nanometer resolution. This tracking protocol was demonstrated by measuring a known trajectory of a fluorescent bead with sub-5 nm axial localization error at 5 Hz. The interferometric microscopy was used to track the RecA protein in Bacillus subtilis bacteria to demonstrate its compatibility with biological systems.
Three-Dimensional and Multidimensional Microscopy: Image Acquisition and Processing XXV, Feb 23, 2018
Rapid and accurate volumetric imaging remains a challenge, yet has the potential to enhance under... more Rapid and accurate volumetric imaging remains a challenge, yet has the potential to enhance understanding of cell function. We developed and used a multifocal microscope (MFM) for 3D snapshot imaging to allow 3D tracking of insulin granules labeled with mCherry in MIN6 cells. MFM employs a special diffractive optical element (DOE) to simultaneously image multiple focal planes. This simultaneous acquisition of information determines the 3D location of single objects at a speed only limited by the array detector's frame rate. We validated the accuracy of MFM imaging/tracking with fluorescence beads; the 3D positions and trajectories of single fluorescence beads can be determined accurately over a wide range of spatial and temporal scales. The 3D positions and trajectories of single insulin granules in a 3.2um deep volume were determined with imaging processing that combines 3D decovolution, shift correction, and finally tracking using the Imaris software package. We find that the motion of the granules is superdiffusive, but less so in 3D than 2D for cells grown on coverslip surfaces, suggesting an anisotropy in the cytoskeleton (e.g. microtubules and action).
Optics Express, 2018
Realizing both high temporal and spatial resolution across a large volume is a key challenge for ... more Realizing both high temporal and spatial resolution across a large volume is a key challenge for 3D fluorescent imaging. Towards achieving this objective, we introduce an interferometric multifocus microscopy (iMFM) system, a combination of multifocus microscopy (MFM) with two opposing objective lenses. We show that the proposed iMFM is capable of simultaneously producing multiple focal plane interferometry that provides axial superresolution and hence isotropic 3D resolution with a single exposure. We design and simulate the iMFM microscope by employing two special diffractive optical elements. The point spread function of this new iMFM microscope is simulated and the image formation model is given. For reconstruction, we use the Richardson-Lucy deconvolution algorithm with total variation regularization for 3D extended object recovery, and a maximum likelihood estimator (MLE) for single molecule tracking. A method for determining an initial axial position of the molecule is also proposed to improve the convergence of the MLE. We demonstrate both theoretically and numerically that isotropic 3D nanoscopic localization accuracy is achievable with an axial imaging range of 2um when tracking a fluorescent molecule in three dimensions and that the diffraction limited axial resolution can be improved by 3-4 times in the single shot wide-field 3D extended object recovery. We believe that iMFM will be a useful tool in 3D dynamic event imaging that requires both high temporal and spatial resolution.
Nature Communications, 2018
The photons in circularly polarized light can transfer their quantized spin angular momentum to m... more The photons in circularly polarized light can transfer their quantized spin angular momentum to micro- and nanostructures via absorption and scattering. This normally exerts positive torque on the objects wher the sign (i.e., handedness or angular direction) follows that of the spin angular momentum. Here we show that the sign of the optical torque can be negative in mesoscopic optical matter arrays of metal nanoparticles (NPs) assembled in circularly polarized optical traps. Crossover from positive to negative optical torque, which occurs for arrays with different number, separation and configuration of the constituent particles, is shown to result from many-body interactions as clarified by electrodynamics simulations. Our results establish that both positive and negative optical torque can be readily realized and controlled in optical matter arrays. This property and reconfigurability of the arrays makes possible programmable materials for optomechanical, microrheological and bio...
Nano letters, Jan 2, 2018
The tremendous progress in nanoscience now allows creating static nanostructured materials for a ... more The tremendous progress in nanoscience now allows creating static nanostructured materials for a broad range of applications. A further goal is to achieve dynamic and reconfigurable nanostructures. One approach involves nanoparticle-based optical matter, but so far studies have only considered spherical constituents. A non-trivial issue is that nanoparticles with other shapes are expected to have different local electromagnetic field distributions and interactions with neighbors in optical matter arrays. Therefore, one would expect their dynamics to be different as well. This paper reports directed-assembly of ordered arrays of gold nanoplatelets in optical line traps demonstrating reconfigurability of the array by altering the phase gradient via holographic beam shaping. The weaker gradient forces and resultant slower motion of the nanoplatelets as compared with plasmonic (Ag and Au) nanospheres allows precise study of their assembly and disassembly dynamics. Both temporal and spat...
Nano letters, Nov 8, 2017
We examine the formation and concomitant rotation of electrodynamically bound dimers (EBD) of 150... more We examine the formation and concomitant rotation of electrodynamically bound dimers (EBD) of 150 nm diameter Ag nanoparticles trapped in circularly polarized focused Gaussian beams. The rotation frequency of an EBD increases linearly with the incident beam power, reaching mean values of ∼4 kHz for relatively low incident powers of 14 mW. Using a coupled-dipole/effective polarizability model, we reveal that retardation of the scattered fields and electrodynamic interactions can lead to a "negative torque" causing rotation of the EBD in the direction opposite to that of the circular polarization. This intriguing opposite-handed rotation due to negative torque is clearly demonstrated using electrodynamics-Langevin dynamics simulations by changing particle separations and thus varying the retardation effects. Finally, negative torque is also demonstrated in experiments from statistical analysis of the EBD trajectories. These results demonstrate novel rotational dynamics of na...
We describe an apparatus using dynamic holographic optical tweezers which is capable of trapping ... more We describe an apparatus using dynamic holographic optical tweezers which is capable of trapping and aligning a single micron scale anisotropic ZnO particle for x-ray Bragg diffraction experiments. The optical tweezers demonstrate enough stability to perform coherent x-ray diffraction imaging.
The Journal of Physical Chemistry C, 2014
Optical forces acting on metallic nanoparticles can be used to organize mesoscale arrays for vari... more Optical forces acting on metallic nanoparticles can be used to organize mesoscale arrays for various applications. Here, we show that silver nanoparticles can be deposited as ordered arrays and chains on chemically modified substrates using a simple and facile optical trapping approach that we term "optical printing". The deposited patterns show preferred separations between nanoparticles resulting from their electrodynamic coupling (i.e., optical binding) in the electromagnetic field of the optical trapping beam. Centrosymmetric optical traps readily allow simultaneous deposition of nanoparticle pairs and triples maintaining the interparticle geometries present in solution. Repositioning an optical line trap with small intercolumn separations allows selectively sampling low and high energy parts of the interparticle potentials. We find that the preferred particle arrangements controllably change from rectangular and triangular to near-field aggregates as one forces the separation to be small. The separation affects the interactions. Interpretation of the results is facilitated by electrodynamic simulations of optical forces. This optical printing approach, which enables efficient fabrication of dense nanoparticle arrays with nanoscale positional precision, is being employed for quantum optics and enhanced sensing measurements.
Physical Review Letters, 2015
Optical matter can be created using the intensity gradient and electrodynamic (e.g., optical bind... more Optical matter can be created using the intensity gradient and electrodynamic (e.g., optical binding) forces that nano-and micro-particles experience in focused optical beams. Here we show that the force associated with phase gradient is also important. In fact, in optical line traps the phase gradient force is crucial in determining the structure and stability of optical matter arrays consisting of Ag nanoparticles (NPs). NP lattices can be repeatedly assembled and disassembled simply by changing the sign of the phase gradient. The phase gradient force induces strain in optically bound Ag NP lattices, causing structural transitions from 1-D "chains" to 2-D lattices, and even to amorphous structures. The structural transitions and dynamics of driven transport are well described by electrodynamics simulations and a drift-diffusion Langevin equation. Optical matter, ordered assemblies of small particles drawn together and stabilized by electrodynamic inter-particle forces in an optical field, represents a unique type of material [1]. Since the first observation of optical binding between two dielectric microparticles by Burns et al. [2], theoretical and experimental investigations of optical binding have significantly enriched the examples of optical matter [3-12]. Notably, Grzegorczyk et al. [13] recently made a major step toward laser-trapped mirrors [14] by assembling a monolayer of ~150 optically bound polystyrene microspheres at a dielectric surface in water. Assembling such an extended optical matter system is challenging. One needs a spatially extended optical beam to illuminate the particles with sufficient intensity to overcome Brownian random forces. Both the gradient forces that confine the (individual) particles, and the optical binding forces that can unite the particles into arrays, are linearly proportional to the intensity. Apart from increasing the laser power to enhance optical binding, metal NPs provide an alternative solution for creating large scale optical matter assemblies. Metal NPs exhibit much stronger light scattering, per unit volume, compared to dielectric particles, leading to "ultra-strong" optical binding forces [15]. The structure of optically bound metal NPs can be controlled by adjusting the intensity gradient of the trapping beam [16]. If other optical
The Journal of Chemical Physics, 2014
Quantifying the interactions in dense colloidal fluids requires a properly designed order paramet... more Quantifying the interactions in dense colloidal fluids requires a properly designed order parameter. We present a modified bond-orientational order parameter, \documentclass[12pt]{minimal}\begin{document}$\bar{\psi }_{6}$\end{document}ψ¯6, to avoid problems of the original definition of bond-orientational order parameter. The original bond-orientational order parameter can change discontinuously in time but our modified order parameter is free from the discontinuity and, thus, it is a suitable measure to quantify the dynamics of the bond-orientational ordering of the local surroundings. Here we analyze \documentclass[12pt]{minimal}\begin{document}$\bar{\psi }_{6}$\end{document}ψ¯6 in a dense driven monodisperse quasi-two-dimensional colloidal fluids where a single particle is optically trapped at the center. The perturbation by the trapped and driven particle alters the structure and dynamics of the neighboring particles. This perturbation disturbs the flow and causes spatial and te...
Optics Express, 2006
Multiparticle tracking with scanning confocal and multiphoton fluorescence imaging is increasingl... more Multiparticle tracking with scanning confocal and multiphoton fluorescence imaging is increasingly important for elucidating biological function, as in the transport of intracellular cargo-carrying vesicles. We demonstrate a simple rapid-sampling stochastic scanning multiphoton multifocal microscopy (SS-MMM) fluorescence imaging technique that enables multiparticle tracking without specialized hardware at rates 1,000 times greater than conventional single point raster scanning. Stochastic scanning of a diffractive optic generated 10x10 hexagonal array of foci with a white noise driven galvanometer yields a scan pattern that is random yet space-filling. SS-MMM creates a more uniformly sampled image with fewer spatio-temporal artifacts than obtained by conventional or multibeam raster scanning. SS-MMM is verified by simulation and experimentally demonstrated by tracking microsphere diffusion in solution.
The Journal of Physical Chemistry C, 2007
We observe spontaneous formation of hexagonal and square arrays of sub-10-nm Au nanoparticles by ... more We observe spontaneous formation of hexagonal and square arrays of sub-10-nm Au nanoparticles by asymmetric heating of a novel precursor material, alkanethiol-passivated atomic Au cluster solutions, on amorphous carbon films. Nanoparticle shape, array symmetry, and lattice constant are controlled by substrate temperature and solvent composition. It is shown that definition of the array lattice points precedes complete particle formation, ruling out an entropy-driven "hard sphere"-type self-assembly process. Mechanisms for monolayer array formation are discussed.
The Journal of Physical Chemistry B, 2001
Two-pulse second-order interferometric autocorrelation responses of single Ag nanoparticles are r... more Two-pulse second-order interferometric autocorrelation responses of single Ag nanoparticles are reported. The surface plasmon-enhanced second harmonic generation interferogram for single Ag colloids shows significant broadening with respect to the laser pulse second-order autocorrelation. The interferometric autocorrelation response is described and analyzed with a density matrix formalism that incorporates (phenomenologically) the population and the polarization relaxation of the surface plasmon. The total dephasing time (T 2 ) of the surface plasmon in single Ag colloids is determined to be 10 fs. The present results are compared to previously reported values obtained from ensemble studies of Ag particles and from the line width of the absorption spectrum. Extension of the present ultrafast measurements, the first performed on single nanoparticles, to single molecules studies is discussed.
The Journal of Physical Chemistry A, 2009
Interparticle forces that can be driven by applied (optical) fields could lead to the formation o... more Interparticle forces that can be driven by applied (optical) fields could lead to the formation of new particle arrangements when assembled in arrays. Furthermore, the potentially large interactions and large local fields associated with plasmon excitations in anisotropic nanoparticles can lead to enhanced nonlinear responses and applications for sensing. These and other applications would benefit from simulations of spectra and forces arising from plasmonic interactions. We present the results of rigorous three-dimensional, finitedifference, time-domain calculations of near-and far-field properties of pairs of Au bipyramidal nanoparticles in three different configurations: side-by-side, head-to-tail, and face-on. The absorption and scattering spectra depend strongly on the geometry as well as on the interparticle separation, as intuitively expected from a dipole coupling picture. Bipyramidal dimers in head-to-tail and face-on geometries exhibit an increasingly red-shifted (longitudinal) plasmon resonance with decreasing separation, whereas side-by-side dimers exhibit a blue shift. Large resonant field enhancements at the gap between particles in a head-to-tail configuration indicate the strong coupling of plasmonic modes. The Maxwell stress tensor formalism is employed to calculate the optical force one particle exerts on the other. Both significant attraction and weak repulsion can be obtained, depending on the relative arrangement of the particles. The force between bipyramids in the head-to-tail configuration can be greater than 10 times the force between pairs of Au nanospheres with the same volume. Experimental linear scattering spectra of particles trapped using the plasmon-resonance-based optical trapping method are found to be consistent with two particles trapped in the side-by-side configuration. † Part of the "George C. Schatz Festschrift".
Proceedings of the National Academy of Sciences, 2004
The equilibrium folding of the catalytic domain of Bacillus subtilis RNase P RNA is investigated ... more The equilibrium folding of the catalytic domain of Bacillus subtilis RNase P RNA is investigated by single-molecule fluorescence resonance energy transfer (FRET). Previous ensemble studies of this 255-nucleotide ribozyme described the equilibrium folding with two transitions, U-to-I eq -to-N, and focused on the I eq -to-N transition. The present study focuses on the U-to-I eq transition. Comparative ensemble measurements of the ribozyme construct labeled with fluorescein at the 5′ end and Cy3 at the 3′ end show that modifications required for labeling do not interfere with folding and help to define the Mg 2+ concentration range for the U-to-I eq transition. Histogram analysis of the Mg 2+ -dependent single-molecule FRET efficiency reveals two previously undetermined folding intermediates. The single-molecule FRET trajectories exhibit non-two-state and nonergodic behaviors at intermediate Mg 2+ concentrations on the time scale of seconds. The trajectories at intermediate Mg 2+ conce...
Optics Letters, 2006
We demonstrate three-dimensional trapping and orientation of individual Au nanorods by using lase... more We demonstrate three-dimensional trapping and orientation of individual Au nanorods by using laser light slightly detuned from their longitudinal plasmon mode. Detuning to the long-wavelength side of the resonance allows stable trapping for several minutes, with an exponential dependence of trapping time on laser power (consistent with a Kramer's escape process). Detuning to the short-wavelength side causes repulsion of the rods from the laser focus. Alignment of the long axis of the rods with the trapping laser polarization is observed as a suppression of rotational diffusion about the short axis.
Nature Communications, 2014
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Papers by Norbert Scherer