We achieve controllable noninterferometric rotation of a bored helical beam by introducing a phase shift exclusively to the annular helical region of the phase. We present a derivation based on the decomposition of the beams, which shows that a constant phase shift of ΔΦ between the bore and the surrounding helical phase with topological charge ℓ will rotate the intensity profile by −ΔΦ/ℓ about its center. The effect of the phase shifting is verified with experiments. This technique is simple, while it preserves the transverse intensity profiles of the beams. Our report may find applications in optical manipulation and trapping.
Friday, February 26, 2010
Controllable rotation of optical beams with bored helical phases
Stein Alec Baluyot and Nathaniel Hermosa, II
We achieve controllable noninterferometric rotation of a bored helical beam by introducing a phase shift exclusively to the annular helical region of the phase. We present a derivation based on the decomposition of the beams, which shows that a constant phase shift of ΔΦ between the bore and the surrounding helical phase with topological charge ℓ will rotate the intensity profile by −ΔΦ/ℓ about its center. The effect of the phase shifting is verified with experiments. This technique is simple, while it preserves the transverse intensity profiles of the beams. Our report may find applications in optical manipulation and trapping.
We achieve controllable noninterferometric rotation of a bored helical beam by introducing a phase shift exclusively to the annular helical region of the phase. We present a derivation based on the decomposition of the beams, which shows that a constant phase shift of ΔΦ between the bore and the surrounding helical phase with topological charge ℓ will rotate the intensity profile by −ΔΦ/ℓ about its center. The effect of the phase shifting is verified with experiments. This technique is simple, while it preserves the transverse intensity profiles of the beams. Our report may find applications in optical manipulation and trapping.
Observation of bistability of trapping position in aerosol optical tweezers
Kerry J. Knox, Daniel R. Burnham, Lowell I. McCann, Shawntel L. Murphy, David McGloin, and Jonathan P. Reid
Bistability in the axial trapping position of aqueous aerosol droplets has been observed for the first time, to our knowledge, in optical tweezers. The behavior has been observed for two distinct trapping configurations, with the trapping beam oriented either along the vertical or horizontal axis. This represents the first report of the optical tweezing of aerosol droplets with a horizontally propagating laser beam. Side imaging was used in conjunction with imaging in the plane of the optical trap. Droplet sizing was performed using cavity-enhanced Raman spectroscopy or by applying a circular regression routine to the acquired images. Predictions from a theoretical model of optical forces are shown to be in good agreement with the experimental observations of bistability in the trapping position. These studies have significance both for the rigorous interpretation of data obtained using aerosol optical tweezers and for the modeling of aerosol optical traps.
Bistability in the axial trapping position of aqueous aerosol droplets has been observed for the first time, to our knowledge, in optical tweezers. The behavior has been observed for two distinct trapping configurations, with the trapping beam oriented either along the vertical or horizontal axis. This represents the first report of the optical tweezing of aerosol droplets with a horizontally propagating laser beam. Side imaging was used in conjunction with imaging in the plane of the optical trap. Droplet sizing was performed using cavity-enhanced Raman spectroscopy or by applying a circular regression routine to the acquired images. Predictions from a theoretical model of optical forces are shown to be in good agreement with the experimental observations of bistability in the trapping position. These studies have significance both for the rigorous interpretation of data obtained using aerosol optical tweezers and for the modeling of aerosol optical traps.
Optical manipulation of plasmonic nanoparticles, bubble formation and patterning of SERS aggregates
Zuwei Liu, Wei Hsuan Hung, Mehmet Aykol, David Valley and Stephen B Cronin
We present an optical method for patterning SERS (surface-enhanced Raman spectroscopy)—enhancing aggregates of gold nanoparticles, using a focused laser beam to optically trap the nanoparticles in suspension. At high laser powers, heat generated from the plasmonic excitation causes boiling of the aqueous suspension and the formation of gaseous bubbles of water vapor. By measuring the Raman peak of the hydroxyl bond of water, the temperature in the laser spot during the aggregation can be determined in situ. The hydrophilic nanoparticles are found to aggregate at the liquid–vapor interface. By allowing the suspension to dry, a ring of gold nanoparticles is deposited on the substrate, producing a highly SERS-active region. These aggregates are studied using optical microscopy, scanning electron microscopy and micro-Raman spectroscopy.
Thursday, February 25, 2010
Human actin mutations associated with hypertrophic and dilated cardiomyopathies demonstrate distinct thin filament regulatory properties in vitro
Edward P. Debold, Walid Saber, Yaser Cheema, Carol S. Bookwalter, Kathleen M. Trybus, David M. Warshaw and Peter VanBuren
Two cardiomyopathic Mutations were expressed in human cardiac actin, using a Baculovirus/insect cell system: E99K is associated with hypertrophic cardiomyopathy whereas R312H is associated with dilated cardiomyopathy. The hypothesis that the divergent phenotypes of these two cardiomyopathies are associated with fundamental differences in the molecular mechanics and thin filament regulation of the underlying actin mutation was tested using the in vitro motility and laser trap assays. In the presence of troponin (Tn) and tropomyosin (Tm), beta-cardiac myosin moved both E99K and R312H thin filaments at significantly slower velocities than wild type (WT) at maximal Ca++. At submaximal Ca++, R312H thin filaments demonstrated significantly increased Ca++ sensitivity (pCa(50)) when compared to WT. Velocity as a function of ATP concentration revealed similar ATP binding rates but slowed ADP release rates for the two actin mutants compared to WT. Single molecule laser trap experiments performed using both unregulated (i.e. actin) and regulated thin filaments in the absence of Ca++ revealed that neither actin mutation significantly affected the myosin's unitary step size (d) OF duration Of Strong actin binding (t(on)) at 20 mu M ATP. However, the frequency of individual strong-binding events in the presence of Tn and Tm, was significantly lower for E99K than WT at comparable myosin surface concentrations. The cooperativity of a second myosin head binding to the thin filament was also impaired by E99K. In conclusion, E99K inhibits the activation of the thin filament by myosin strong-binding whereas R312H demonstrates enhanced calcium activation.
Two cardiomyopathic Mutations were expressed in human cardiac actin, using a Baculovirus/insect cell system: E99K is associated with hypertrophic cardiomyopathy whereas R312H is associated with dilated cardiomyopathy. The hypothesis that the divergent phenotypes of these two cardiomyopathies are associated with fundamental differences in the molecular mechanics and thin filament regulation of the underlying actin mutation was tested using the in vitro motility and laser trap assays. In the presence of troponin (Tn) and tropomyosin (Tm), beta-cardiac myosin moved both E99K and R312H thin filaments at significantly slower velocities than wild type (WT) at maximal Ca++. At submaximal Ca++, R312H thin filaments demonstrated significantly increased Ca++ sensitivity (pCa(50)) when compared to WT. Velocity as a function of ATP concentration revealed similar ATP binding rates but slowed ADP release rates for the two actin mutants compared to WT. Single molecule laser trap experiments performed using both unregulated (i.e. actin) and regulated thin filaments in the absence of Ca++ revealed that neither actin mutation significantly affected the myosin's unitary step size (d) OF duration Of Strong actin binding (t(on)) at 20 mu M ATP. However, the frequency of individual strong-binding events in the presence of Tn and Tm, was significantly lower for E99K than WT at comparable myosin surface concentrations. The cooperativity of a second myosin head binding to the thin filament was also impaired by E99K. In conclusion, E99K inhibits the activation of the thin filament by myosin strong-binding whereas R312H demonstrates enhanced calcium activation.
Trapping two types of particles using a double-ring-shaped radially polarized beam
Yaoju Zhang, Biaofeng Ding, and Taikei Suyama
An optical-trap method based on the illumination of a double-ring-shaped radially polarized beam (R-TEM11*) is proposed. The numerical results based on the vector diffraction theory show that a highly focused R-TEM11* beam not only can produce a bright spot but also can form an optical cage in the focal region by changing the truncation parameter β, defined as the ratio of the radius of the aperture to the waist of the beam. The radiation forces acting on Rayleigh particles are calculated by using the Rayleigh scattering theory. The bright spot generated by the R-TEM11* beam with a β value close to 2 can three-dimensionally trap a particle with a refractive index larger than that of the ambient. An optical cage or three-dimensional dark spot generated by the R-TEM11* beam with a β value close to 1.3 can three-dimensionally trap a particle with refractive index smaller than that of the ambient. Because the adjustment of the truncation parameter can be actualized by simply changing the radius of a circular aperture inserted in the front of the lens, only one optical-trap system in the present method can be used to three-dimensionally trap two types of particles with different refractive indices.
An optical-trap method based on the illumination of a double-ring-shaped radially polarized beam (R-TEM11*) is proposed. The numerical results based on the vector diffraction theory show that a highly focused R-TEM11* beam not only can produce a bright spot but also can form an optical cage in the focal region by changing the truncation parameter β, defined as the ratio of the radius of the aperture to the waist of the beam. The radiation forces acting on Rayleigh particles are calculated by using the Rayleigh scattering theory. The bright spot generated by the R-TEM11* beam with a β value close to 2 can three-dimensionally trap a particle with a refractive index larger than that of the ambient. An optical cage or three-dimensional dark spot generated by the R-TEM11* beam with a β value close to 1.3 can three-dimensionally trap a particle with refractive index smaller than that of the ambient. Because the adjustment of the truncation parameter can be actualized by simply changing the radius of a circular aperture inserted in the front of the lens, only one optical-trap system in the present method can be used to three-dimensionally trap two types of particles with different refractive indices.
Thursday, February 18, 2010
Scheme for Information Erasure in a Double-Well Potential
Wang Xin-Xin and Bao Jing-Dong
We design an experimental scheme to realize one-bit information erasure and restoring processes by considering an overdamped colloidal particle in a double-well optical trap, which is added by a controllable laser tweezer. Using the Monte Carlo method, we simulate numerically the Langevin equation to calculate the mean work spent during the entire process and validate the entropy production fluctuation theory. Our result shows that the distribution of entropy production becomes narrow with increasing temperature and becomes stationary, represents the diminishing extent of irreversibility.
We design an experimental scheme to realize one-bit information erasure and restoring processes by considering an overdamped colloidal particle in a double-well optical trap, which is added by a controllable laser tweezer. Using the Monte Carlo method, we simulate numerically the Langevin equation to calculate the mean work spent during the entire process and validate the entropy production fluctuation theory. Our result shows that the distribution of entropy production becomes narrow with increasing temperature and becomes stationary, represents the diminishing extent of irreversibility.
Direct physical study of kinetochore–microtubule interactions by reconstitution and interrogation with an optical force clamp
Andrew D. Franck, Andrew F. Powers, Daniel R. Gestaut, Trisha N. Davis and Charles L. Asbury
We detail our use of computer-controlled optical traps to study interactions between kinetochore components and dynamic microtubules. Over the last two decades optical traps have helped uncover the working principles of conventional molecular motors, such as kinesin and dynein, but only recently have they been applied to study kinetochore function. The most useful traps combine sensitive position detectors and servo-control, allowing them to be operated as force clamps that maintain constant loads on objects as they move. Our instrument, which is among the simplest designs that permits force clamping, relies on a computer-controlled piezoelectric stage and a single laser for trapping and position detection. We apply it in motility assays where beads coated with pure microtubule-binding kinetochore components are attached to the tips of individual dynamic microtubules. Like kinetochores in vivo, the beads remain tip-attached, undergoing movements coupled to filament assembly and disassembly. The force clamp provides many benefits over instruments that lack feedback control. It allows tension to be applied continuously during both assembly- and disassembly-driven movement, providing a close match to the physiological situation. It also enables tracking with high resolution, and simplifies data interpretation by eliminating artifacts due to molecular compliance. The formation of persistent, load-bearing attachments to dynamic microtubule tips is fundamental to all kinetochore activities. Our direct, physical study of kinetochore–microtubule coupling may therefore furnish insights into many vital kinetochore functions, including correction of aberrant attachments and generation of the ‘wait-anaphase’ signals that delay mitosis until all kinetochores are properly attached.
Highly selective separation of DNA fragments using optically directed transport
Avital Braiman, Fedor Rudakov, and Thomas Thundat
We present a design that allows selective separation of biomolecules of a particular size without performing complete separation of the sample by size. By focusing a laser beam onto a photoelectrode in contact with an electrolyte medium, a highly localized andoptically controlled photoelectrophoretic trap is created. Moving the light beam along the photoelectrode consequently moves the trap. We demonstrate that by manipulating the speed of the photoelectrophoretic trap biomolecules of a particular size can be selectively separated from the mixture. We achieve a qualitative agreement between our experimental results and numerical simulations.
We present a design that allows selective separation of biomolecules of a particular size without performing complete separation of the sample by size. By focusing a laser beam onto a photoelectrode in contact with an electrolyte medium, a highly localized andoptically controlled photoelectrophoretic trap is created. Moving the light beam along the photoelectrode consequently moves the trap. We demonstrate that by manipulating the speed of the photoelectrophoretic trap biomolecules of a particular size can be selectively separated from the mixture. We achieve a qualitative agreement between our experimental results and numerical simulations.
Resolution of the Abraham-Minkowski Dilemma
Stephen M. Barnett
The dilemma of identifying the correct form for the momentum of light in a medium has run for a century and has been informed by many distinguished contributions, both theoretical and experimental. We show that both the Abraham and Minkowski forms of the momentum density are correct, with the former being the kinetic momentum and the latter the canonical momentum. This identification allows us to explain why the experiments supporting each of the rival momenta gave the results that they did. The inclusion of dispersion and absorption provides an interesting subtlety, but does not change our conclusion.
The dilemma of identifying the correct form for the momentum of light in a medium has run for a century and has been informed by many distinguished contributions, both theoretical and experimental. We show that both the Abraham and Minkowski forms of the momentum density are correct, with the former being the kinetic momentum and the latter the canonical momentum. This identification allows us to explain why the experiments supporting each of the rival momenta gave the results that they did. The inclusion of dispersion and absorption provides an interesting subtlety, but does not change our conclusion.
Wednesday, February 17, 2010
Direct Force Measurements on Double-Stranded RNA in Solid-State Nanopores
Michiel van den Hout, Igor D. Vilfan, Susanne Hage and Nynke H. Dekker
Solid-state nanopores can be employed to detect and study local structure along single molecules by voltage driven translocation through the nanopore. Their sensitivity and versatility can be augmented by combining them with a direct force probe, for example, optical tweezers. Such a tool could potentially be used to directly probe RNA secondary structure through the sequential unfolding of duplex regions. Here, we demonstrate the first application of such a system to the study of RNA by directly measuring the net force on individual double-stranded RNA (dsRNA) molecules. We have probed the force on dsRNA over a large range of nanopore sizes from 35 nm down to 3.5 nm and find that it decreases as the pore size is increased, in accordance with numerical calculations. Furthermore, we find that the force is independent of the distance between the optical trap and the nanopore surface, permitting force measurement on quite short molecules. By comparison with dsDNA molecules trapped in the same nanopores, we find that the force on dsRNA is on the order of, but slightly lower than, that on dsDNA. With these measurements, we expand the possibilities of the nanopore-optical tweezers to the study of RNA molecules with potential applications to the detection of RNA-bound proteins, the determination of RNA secondary structure, and the processing of RNA by molecular motors.
Solid-state nanopores can be employed to detect and study local structure along single molecules by voltage driven translocation through the nanopore. Their sensitivity and versatility can be augmented by combining them with a direct force probe, for example, optical tweezers. Such a tool could potentially be used to directly probe RNA secondary structure through the sequential unfolding of duplex regions. Here, we demonstrate the first application of such a system to the study of RNA by directly measuring the net force on individual double-stranded RNA (dsRNA) molecules. We have probed the force on dsRNA over a large range of nanopore sizes from 35 nm down to 3.5 nm and find that it decreases as the pore size is increased, in accordance with numerical calculations. Furthermore, we find that the force is independent of the distance between the optical trap and the nanopore surface, permitting force measurement on quite short molecules. By comparison with dsDNA molecules trapped in the same nanopores, we find that the force on dsRNA is on the order of, but slightly lower than, that on dsDNA. With these measurements, we expand the possibilities of the nanopore-optical tweezers to the study of RNA molecules with potential applications to the detection of RNA-bound proteins, the determination of RNA secondary structure, and the processing of RNA by molecular motors.
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