O Otto, J L Gornall, G Stober, F Czerwinski, R Seidel and U F Keyser
We have developed an optical tweezer setup, with high-speed and real-time position tracking, based on a CMOS camera technology. Our software encoded algorithm is cross-correlation based and implemented on a standard computer. By measuring the fluctuations of a confined colloid at 6000 frames s − 1, continuously for an hour, we show our technique is a viable alternative to quadrant photodiodes. The optical trap is calibrated by using power spectrum analysis and the Stokes method. The trap stiffness is independent of the camera frame rate and scales linearly with the applied laser power. The analysis of our data by Allan variance demonstrates single nanometer accuracy in position detection.
DOI
Sunday, March 13, 2011
Saturday, March 12, 2011
Understanding Optical Trapping Phenomena: A Simulation for Undergraduates
Mas, J.; Farré, A.; Cuadros, J.; Juvells, I.; Carnicer, A.
Optical trapping is an attractive and multidisciplinary topic that has become the center of attention to a large number of researchers. Moreover, it is a suitable subject for advanced students that requires a knowledge of a wide range of topics. As a result, it has been incorporated into some syllabuses of both undergraduate and graduate programs. In this paper, basic concepts in laser trapping theory are reviewed. To provide a better understanding of the underlying concepts for students, a Java application for simulating the behavior of a dielectric particle trapped in a highly focused beam has been developed. The program illustrates a wide range of theoretical results and features, such as the calculation of the force exerted by a beam in the Mie and Rayleigh regimes or the calibration of the trap stiffness. Some examples that are ready to be used in the classroom or in the computer lab are also supplied.
DOI
Optical trapping is an attractive and multidisciplinary topic that has become the center of attention to a large number of researchers. Moreover, it is a suitable subject for advanced students that requires a knowledge of a wide range of topics. As a result, it has been incorporated into some syllabuses of both undergraduate and graduate programs. In this paper, basic concepts in laser trapping theory are reviewed. To provide a better understanding of the underlying concepts for students, a Java application for simulating the behavior of a dielectric particle trapped in a highly focused beam has been developed. The program illustrates a wide range of theoretical results and features, such as the calculation of the force exerted by a beam in the Mie and Rayleigh regimes or the calibration of the trap stiffness. Some examples that are ready to be used in the classroom or in the computer lab are also supplied.
DOI
Optical tweezers: wideband microrheology
Daryl Preece, Rebecca Warren, R M L Evans, Graham M Gibson, Miles J Padgett, Jonathan M Cooper and Manlio Tassieri
Microrheology is a branch of rheology having the same principles as conventional bulk rheology, but working on micron length scales and microlitre volumes.
Optical tweezers have been successfully used with Newtonian fluids for rheological purposes such as determining fluid viscosity. Conversely, when optical tweezers are used to measure the viscoelastic properties of complex fluids the results are either limited to the material's high-frequency response, discarding important information related to the low-frequency behaviour, or they are supplemented by low-frequency measurements performed with different techniques, often without presenting an overlapping region of clear agreement between the sets of results. We present a simple experimental procedure to perform microrheological measurements over the widest frequency range possible with optical tweezers. A generalized Langevin equation is used to relate the frequency-dependent moduli of the complex fluid to the time-dependent trajectory of a probe particle as it flips between two optical traps that alternately switch on and off.
DOI
Microrheology is a branch of rheology having the same principles as conventional bulk rheology, but working on micron length scales and microlitre volumes.
Optical tweezers have been successfully used with Newtonian fluids for rheological purposes such as determining fluid viscosity. Conversely, when optical tweezers are used to measure the viscoelastic properties of complex fluids the results are either limited to the material's high-frequency response, discarding important information related to the low-frequency behaviour, or they are supplemented by low-frequency measurements performed with different techniques, often without presenting an overlapping region of clear agreement between the sets of results. We present a simple experimental procedure to perform microrheological measurements over the widest frequency range possible with optical tweezers. A generalized Langevin equation is used to relate the frequency-dependent moduli of the complex fluid to the time-dependent trajectory of a probe particle as it flips between two optical traps that alternately switch on and off.
DOI
Simultaneous transfer of linear and orbital angular momentum to multiple low-index particles
Vincent Ricardo Daria, Mary Ann Go and Hans-A Bachor
We demonstrate simultaneous transfer of linear and orbital angular momentum (OAM) to hollow glass microbeads using a dynamic array of optical vortices. Previous reports have shown that the transfer of OAM is due to light scattering which creates a tangential force on a particle and causes it to move on a circular orbit around a vortex. In this paper we describe a case with reduced frictional force, as the low-index particle is pinned to the wall of the sample cell. This results in a more efficient transfer of OAM, which sets a hollow microbead into orbital motion around the optical vortex. We show that the localized OAM carried by each vortex in the array can be independently transferred to one microbead trapped per vortex. Finally, we present novel demonstrations showing simultaneous transfer of both orbital angular and linear momentum to multiple microbeads.
DOI
We demonstrate simultaneous transfer of linear and orbital angular momentum (OAM) to hollow glass microbeads using a dynamic array of optical vortices. Previous reports have shown that the transfer of OAM is due to light scattering which creates a tangential force on a particle and causes it to move on a circular orbit around a vortex. In this paper we describe a case with reduced frictional force, as the low-index particle is pinned to the wall of the sample cell. This results in a more efficient transfer of OAM, which sets a hollow microbead into orbital motion around the optical vortex. We show that the localized OAM carried by each vortex in the array can be independently transferred to one microbead trapped per vortex. Finally, we present novel demonstrations showing simultaneous transfer of both orbital angular and linear momentum to multiple microbeads.
DOI
Moving into the cell: single-molecule studies of molecular motors in complex environments
Claudia Veigel and Christoph F. Schmidt
Much has been learned in the past decades about molecular force generation. Single-molecule techniques, such as atomic force microscopy, single-molecule fluorescence microscopy and optical tweezers, have been key in resolving the mechanisms behind the power strokes, 'processive' steps and forces of cytoskeletal motors. However, it remains unclear how single force generators are integrated into composite mechanical machines in cells to generate complex functions such as mitosis, locomotion, intracellular transport or mechanical sensory transduction. Using dynamic single-molecule techniques to track, manipulate and probe cytoskeletal motor proteins will be crucial in providing new insights.
Thursday, March 10, 2011
Optimizing the optical trapping stiffness of holographically trapped miJourcrorods using high-speed video tracking
D B Phillips, D M Carberry, S H Simpson, H Schäfer, M Steinhart, R Bowman, G M Gibson, M J Padgett, S Hanna and M J Miles
Dielectric microrods can be trapped horizontally in pairs of holographically controlled optical traps. External forces acting on these microrods are registered via the rotational and translational displacement of the microrod relative to the traps. In the following paper we demonstrate accurate, real-time tracking of this displacement in two dimensions. The precision of the method is estimated and the translational and rotational stiffness coefficients of the trapped microrod are evaluated by analysing the thermal motion and the Stokes drag. The variation of these stiffness coefficients relative to the displacement of the traps from the ends of the microrods is measured, and optimal trapping conditions are located.
DOI
Dielectric microrods can be trapped horizontally in pairs of holographically controlled optical traps. External forces acting on these microrods are registered via the rotational and translational displacement of the microrod relative to the traps. In the following paper we demonstrate accurate, real-time tracking of this displacement in two dimensions. The precision of the method is estimated and the translational and rotational stiffness coefficients of the trapped microrod are evaluated by analysing the thermal motion and the Stokes drag. The variation of these stiffness coefficients relative to the displacement of the traps from the ends of the microrods is measured, and optimal trapping conditions are located.
DOI
Tailored leaky plasmon waves from a subwavelength aperture for optical particle trapping on a chip
M. S. Muradoglu, Tuck Wah Ng, Adrian Neild, and Ian Gralinski
Optical forces available on a chip that possess features of strong trapping at the subwavelength scale, in a coplanar geometry, and at specific and selective locations portend many useful applications. We demonstrate here a two-pronged approach to accomplish this. First, the plasmon fields emanating from a subwavelength aperture are manipulated so that they leak maximally to the sides on a surface through the use of tailored corrugations. Second, the location of secondary corrugation at some distance permits reflection of these leaky waves, which, with the coherence property of light used, generate optical standing wave fields capable of strong optical trapping. The linear optical forces generated with this scheme are presented here.
DOI
Optical forces available on a chip that possess features of strong trapping at the subwavelength scale, in a coplanar geometry, and at specific and selective locations portend many useful applications. We demonstrate here a two-pronged approach to accomplish this. First, the plasmon fields emanating from a subwavelength aperture are manipulated so that they leak maximally to the sides on a surface through the use of tailored corrugations. Second, the location of secondary corrugation at some distance permits reflection of these leaky waves, which, with the coherence property of light used, generate optical standing wave fields capable of strong optical trapping. The linear optical forces generated with this scheme are presented here.
DOI
Optical aberration compensation in a multiplexed optical trapping system
T Čižmár, H I C Dalgarno, P C Ashok, F J Gunn-Moore and K Dholakia
In this paper we discuss optical aberrations within a multiplexed optical trapping system. We analyze two of the most powerful methods for optical trap multiplexing: time-shared beam steering and holographic beam shaping in a tandem system with an acousto-optic deflector and spatial light modulator. We show how to isolate and correct for the aberrations introduced by these individual optical components using the spatial light modulator and demonstrate the enhancement this provides to optical trapping.
DOI
In this paper we discuss optical aberrations within a multiplexed optical trapping system. We analyze two of the most powerful methods for optical trap multiplexing: time-shared beam steering and holographic beam shaping in a tandem system with an acousto-optic deflector and spatial light modulator. We show how to isolate and correct for the aberrations introduced by these individual optical components using the spatial light modulator and demonstrate the enhancement this provides to optical trapping.
DOI
Alternative modes for optical trapping and manipulation using counter-propagating shaped beams
D Palima, T B Lindballe, M V Kristensen, S Tauro, H Stapelfeldt, S R Keiding and J Glückstad
Counter-propagating beams have enabled the first stable three-dimensional optical trapping of microparticles and this procedure has been enhanced and developed over the years to achieve independent and interactive manipulation of multiple particles. In this work, we analyse counter-propagating shaped-beam traps that depart from the conventional geometry based on symmetric, coaxial counter-propagating beams. We show that projecting shaped beams with separation distances previously considered axially unstable can, in fact, enhance the axial and transverse trapping stiffnesses. We also show that deviating from using perfectly counter-propagating beams to use oblique beams can improve the axial stability of the traps and improve the axial trapping stiffness. These alternative geometries can be particularly useful for handling larger particles. These results hint at a rich potential for light shaping for optical trapping and manipulation using patterned counter-propagating beams, which still remains to be fully tapped.
DOI
Counter-propagating beams have enabled the first stable three-dimensional optical trapping of microparticles and this procedure has been enhanced and developed over the years to achieve independent and interactive manipulation of multiple particles. In this work, we analyse counter-propagating shaped-beam traps that depart from the conventional geometry based on symmetric, coaxial counter-propagating beams. We show that projecting shaped beams with separation distances previously considered axially unstable can, in fact, enhance the axial and transverse trapping stiffnesses. We also show that deviating from using perfectly counter-propagating beams to use oblique beams can improve the axial stability of the traps and improve the axial trapping stiffness. These alternative geometries can be particularly useful for handling larger particles. These results hint at a rich potential for light shaping for optical trapping and manipulation using patterned counter-propagating beams, which still remains to be fully tapped.
DOI
Near-field optical trapping with an actively locked cavity
N J van Leeuwen, L J Moore, W D Partridge, R Peverall, G A D Ritchie and M D Summers
This paper details the construction and performance of a resonant cavity evanescent wave trap for the trapping and assembling of microparticles. The technique employs a low finesse resonator which incorporates total internal reflection (TIR) at the silica/water interface to generate an evanescent field which is coherently scattered by silica microparticles present in suspension to form optically bound structures. Circulating powers of 14 W over an area of 150 µm × 75 µm were generated with a 400 mW Nd:YAG source. This approach allows some degree of control over the shape of the evanescent field by locking to higher-order cavity modes.
DOI
This paper details the construction and performance of a resonant cavity evanescent wave trap for the trapping and assembling of microparticles. The technique employs a low finesse resonator which incorporates total internal reflection (TIR) at the silica/water interface to generate an evanescent field which is coherently scattered by silica microparticles present in suspension to form optically bound structures. Circulating powers of 14 W over an area of 150 µm × 75 µm were generated with a 400 mW Nd:YAG source. This approach allows some degree of control over the shape of the evanescent field by locking to higher-order cavity modes.
DOI
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