Zhe Zhang, Haifeng Li, Peng Li, Kebin Shi, Perry Edwards,Fiorenzo Omenetto, Mark Cronin-Golomb, Guizhong Zhang, and Zhiwen Liu
We report a novel method for characterizing the stiffness of white light supercontinuum tweezers, in which the nonlinear photonic crystal fiber used for supercontinuum generation is also utilized as an effective confocal pinhole to track the motion of a trapped bead and as a scan head to realize rapid scanning of the optical trap. By measuring the phase of the bead’s motion in following the trap, a lateral stiffness value of about 7.9 μN/m was obtained with supercontinumm power of about 75mW. Our technique can potentially allow for trap stiffness calibration along an arbitrary direction in three dimensions.
DOI
Friday, December 3, 2010
Simultaneous calibration of optical tweezers spring constant and position detector response
Nathalie Westbrook, Antoine Le Gall, David Dulin, André Villing,Philippe Bouyer, Karen Perronet, and Koen Visscher
We demonstrate a fast and direct calibration method for systems using a single laser for optical tweezers and particle position detection. The method takes direct advantage of back-focal-plane interferometry measuring not an absolute but a differential position, i.e. the position of the trapped particle relative to the center of the optical tweezers. Therefore, a fast step-wise motion of the optical tweezers yields the impulse response of the trapped particle. Calibration parameters such as the detector’s spatial and temporal response and the spring constant of the optical tweezers then follow readily from fitting the measured impulse response.
DOI
We demonstrate a fast and direct calibration method for systems using a single laser for optical tweezers and particle position detection. The method takes direct advantage of back-focal-plane interferometry measuring not an absolute but a differential position, i.e. the position of the trapped particle relative to the center of the optical tweezers. Therefore, a fast step-wise motion of the optical tweezers yields the impulse response of the trapped particle. Calibration parameters such as the detector’s spatial and temporal response and the spring constant of the optical tweezers then follow readily from fitting the measured impulse response.
DOI
Thursday, December 2, 2010
All-Optical Optomechanics: An Optical Spring Mirror
S. Singh, G. A. Phelps, D. S. Goldbaum, E. M. Wright, and P. Meystre
The dominant hurdle to the operation of optomechanical systems in the quantum regime is the coupling of the vibrating element to a thermal reservoir via mechanical supports. Here we propose a scheme that uses an optical spring to replace the mechanical support. We show that the resolved-sideband regime of cooling can be reached in a configuration using a high-reflectivity disk mirror held by an optical tweezer as one of the end mirrors of a Fabry-Perot cavity. We find a final phonon occupation number of the trapped mirror n̅ =0.56 for reasonable parameters, the limit being set by our approximations, and not any fundamental physics. This demonstrates the promise of dielectric disks attached to optical springs for the observation of quantum effects in macroscopic objects.
DOI
The dominant hurdle to the operation of optomechanical systems in the quantum regime is the coupling of the vibrating element to a thermal reservoir via mechanical supports. Here we propose a scheme that uses an optical spring to replace the mechanical support. We show that the resolved-sideband regime of cooling can be reached in a configuration using a high-reflectivity disk mirror held by an optical tweezer as one of the end mirrors of a Fabry-Perot cavity. We find a final phonon occupation number of the trapped mirror n̅ =0.56 for reasonable parameters, the limit being set by our approximations, and not any fundamental physics. This demonstrates the promise of dielectric disks attached to optical springs for the observation of quantum effects in macroscopic objects.
DOI
Robust Mechanosensing and Tension Generation by Myosin VI
Peiying Chuan, James A. Spudich and Alexander R. Dunn
Myosin VI is a molecular motor that is thought to function both as a transporter and as a cytoskeletal anchor in vivo. Here we use optical tweezers to examine force generation by single molecules of myosin VI under physiological nucleotide concentrations. We find that myosin VI is an efficient transporter at loads of up to 2 pN but acts as a cytoskeletal anchor at higher loads. Our data and the resulting model are consistent with an indirect coupling of global structural motions to nucleotide binding and release. The model provides a mechanism by which load may regulate the dual functions of myosin VIin vivo. Our results suggest that myosin VI kinetics are tuned such that the motor maintains a consistent level of mechanical tension within the cell, a property potentially shared by other mechanosensitive proteins.
DOI
Myosin VI is a molecular motor that is thought to function both as a transporter and as a cytoskeletal anchor in vivo. Here we use optical tweezers to examine force generation by single molecules of myosin VI under physiological nucleotide concentrations. We find that myosin VI is an efficient transporter at loads of up to 2 pN but acts as a cytoskeletal anchor at higher loads. Our data and the resulting model are consistent with an indirect coupling of global structural motions to nucleotide binding and release. The model provides a mechanism by which load may regulate the dual functions of myosin VIin vivo. Our results suggest that myosin VI kinetics are tuned such that the motor maintains a consistent level of mechanical tension within the cell, a property potentially shared by other mechanosensitive proteins.
DOI
Optical manipulation of light-absorbing particles takes to the air
Ashley G. Smart
The idea that light can move matter is not new—Johannes Keplersuspected as much some 400 years ago when he noticed that the tails of comets always point away from the Sun. That suspicion was formalized in 1871 with James Clerk Maxwell's prediction of radiation pressure—the force imparted on a body by refracted, reflected, or absorbed light—and confirmed in 1900, when Pyotr Lebedev observed the effect in experiments.
DOI
The idea that light can move matter is not new—Johannes Keplersuspected as much some 400 years ago when he noticed that the tails of comets always point away from the Sun. That suspicion was formalized in 1871 with James Clerk Maxwell's prediction of radiation pressure—the force imparted on a body by refracted, reflected, or absorbed light—and confirmed in 1900, when Pyotr Lebedev observed the effect in experiments.
DOI
Towards detection and identification of circulating tumour cells using Raman spectroscopy
U. Neugebauer, T. Bocklitz, J. H. Clement, C. Krafft and J. Popp
Body fluids are easily accessible and contain valuable indices for medical diagnosis. Fascinating tools are tumour cells circulating in the peripheral blood of cancer patients. As these cells are extremely rare, they constitute a challenge for clinical diagnostics. In this contribution we present the Raman spectroscopic-based identification of different single cells in suspension that are found in peripheral blood of cancer patients including healthy cells like leukocytes and erythrocytes, and tumour cells like leukaemic cells and cells originating from solid tumours. Leukocytes and erythrocytes were isolated from the peripheral blood of healthy donors while myeloid leukaemia cells (OCI-AML3) and breast carcinoma derived cells (MCF-7, BT-20) were obtained from cell cultures. A laser emitting 785 nm light was used for optical trapping the single cells in the laser focus and to excite the Raman spectrum. Support vector machines were applied to develop a supervised classification model with spectra of 1210 cells originating from three different donors and three independent cultivation batches. Distinguishing tumour cells from healthy cells was achieved with a sensitivity of >99.7% and a specificity of >99.5%. In addition, the correct cell types were predicted with an accuracy of approximately 92%.
DOI
Body fluids are easily accessible and contain valuable indices for medical diagnosis. Fascinating tools are tumour cells circulating in the peripheral blood of cancer patients. As these cells are extremely rare, they constitute a challenge for clinical diagnostics. In this contribution we present the Raman spectroscopic-based identification of different single cells in suspension that are found in peripheral blood of cancer patients including healthy cells like leukocytes and erythrocytes, and tumour cells like leukaemic cells and cells originating from solid tumours. Leukocytes and erythrocytes were isolated from the peripheral blood of healthy donors while myeloid leukaemia cells (OCI-AML3) and breast carcinoma derived cells (MCF-7, BT-20) were obtained from cell cultures. A laser emitting 785 nm light was used for optical trapping the single cells in the laser focus and to excite the Raman spectrum. Support vector machines were applied to develop a supervised classification model with spectra of 1210 cells originating from three different donors and three independent cultivation batches. Distinguishing tumour cells from healthy cells was achieved with a sensitivity of >99.7% and a specificity of >99.5%. In addition, the correct cell types were predicted with an accuracy of approximately 92%.
DOI
Angular and position stability of a nanorod trapped in an optical tweezers
Paul B. Bareil and Yunlong Sheng
We analyze the trap stiffness and trapping force potential for a nano-cylinder trapped in the optical tweezers against its axial and lateral shift and tilt associated to the natural Brownian motion. We explain the physical properties of the optical trapping by computing and integrating the radiation stress distribution on the nano-cylinder surfaces using the T-matrix approach. Our computation shows that the force stiffness to the lateral shift is several times higher than that to the axial shift of the nano-cylinder, and lateral torque due to the stress on the side-face is 1-2 orders of magnitude higher than that on the end-faces of a nano-cylinder with the aspect ratio of 2 – 20. The torque due to the stress on the nano-cylinder surface is 2-3 orders of magnitude higher than the spin torque. We explain why a nano-cylinder of low aspect ratio is trapped and aligned normal to the trapping beam axis.
DOI
We analyze the trap stiffness and trapping force potential for a nano-cylinder trapped in the optical tweezers against its axial and lateral shift and tilt associated to the natural Brownian motion. We explain the physical properties of the optical trapping by computing and integrating the radiation stress distribution on the nano-cylinder surfaces using the T-matrix approach. Our computation shows that the force stiffness to the lateral shift is several times higher than that to the axial shift of the nano-cylinder, and lateral torque due to the stress on the side-face is 1-2 orders of magnitude higher than that on the end-faces of a nano-cylinder with the aspect ratio of 2 – 20. The torque due to the stress on the nano-cylinder surface is 2-3 orders of magnitude higher than the spin torque. We explain why a nano-cylinder of low aspect ratio is trapped and aligned normal to the trapping beam axis.
DOI
Wednesday, December 1, 2010
Optical forces from an evanescent wave on a magnetodielectric small particle
M. Nieto-Vesperinas and J. J. Saenz
We report the first study on the optical force exerted by an evanescent wave on a small sphere with both electric and magnetic responses to the incident field, immersed in an arbitrary nondissipative medium. New expressions and effects from their gradient, radiation pressure, and curl components are obtained owing to the particle induced electric and magnetic dipoles, as well as to their mutual interaction. We predict possible dramatic changes in the force depending on the host medium, the polarization, and the nature of the surface wave.
DOI
We report the first study on the optical force exerted by an evanescent wave on a small sphere with both electric and magnetic responses to the incident field, immersed in an arbitrary nondissipative medium. New expressions and effects from their gradient, radiation pressure, and curl components are obtained owing to the particle induced electric and magnetic dipoles, as well as to their mutual interaction. We predict possible dramatic changes in the force depending on the host medium, the polarization, and the nature of the surface wave.
DOI
Mathieu beams as versatile light moulds for 3D micro particle assemblies
C. Alpmann, R. Bowman, M. Woerdemann, M. Padgett, and C. Denz
We present tailoring of three dimensional light fields which act as light moulds for elaborate particle micro structures of variable shapes. Stereo microscopy is used for visualization of the 3D particle assemblies. The powerful method is demonstrated for the class of propagation invariant beams, where we introduce the use of Mathieu beams as light moulds with non-rotationally-symmetric structure. They offer multifarious field distributions and facilitate the creation of versatile particle structures. This eneral technique may find its application in micro fluidics, chemistry, biology, and medicine, to create highly efficient mixing tools, for hierarchical supramolecular organization or in 3D tissue engineering.
DOI
We present tailoring of three dimensional light fields which act as light moulds for elaborate particle micro structures of variable shapes. Stereo microscopy is used for visualization of the 3D particle assemblies. The powerful method is demonstrated for the class of propagation invariant beams, where we introduce the use of Mathieu beams as light moulds with non-rotationally-symmetric structure. They offer multifarious field distributions and facilitate the creation of versatile particle structures. This eneral technique may find its application in micro fluidics, chemistry, biology, and medicine, to create highly efficient mixing tools, for hierarchical supramolecular organization or in 3D tissue engineering.
DOI
Sorting colloidal particles into multiple channels with optical forces: Prismatic optical fractionation
Ke Xiao and David G. Grier
Brownian particles drifting through a periodically structured force landscape can become entrained by the landscape’s symmetries. What direction a particular particle takes can depend strongly on subtle variations in its physical properties. Consequently, a homogeneously structured force field can sort a mixture of particles into spatially separated fractions, much as an optical prism refracts light into its component wavelengths. When the force landscape is implemented with structured light fields, such continuous multichannel sorting may be termed prismatic optical fractionation. We describe experimental and numerical studies of colloidal spheres’ transport through periodic arrays of optical tweezers, which reveal an important role for three-dimensional motion in determining a drifting particle’s fate. These studies also demonstrate sorting on the basis of statistically locked-in transport, in which Brownian fluctuations contribute to direction selection.
DOI
Brownian particles drifting through a periodically structured force landscape can become entrained by the landscape’s symmetries. What direction a particular particle takes can depend strongly on subtle variations in its physical properties. Consequently, a homogeneously structured force field can sort a mixture of particles into spatially separated fractions, much as an optical prism refracts light into its component wavelengths. When the force landscape is implemented with structured light fields, such continuous multichannel sorting may be termed prismatic optical fractionation. We describe experimental and numerical studies of colloidal spheres’ transport through periodic arrays of optical tweezers, which reveal an important role for three-dimensional motion in determining a drifting particle’s fate. These studies also demonstrate sorting on the basis of statistically locked-in transport, in which Brownian fluctuations contribute to direction selection.
DOI
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