15,338 research outputs found
A Step-by-step Guide to the Realisation of Advanced Optical Tweezers
Since the pioneering work of Arthur Ashkin, optical tweezers have become an
indispensable tool for contactless manipulation of micro- and nanoparticles.
Nowadays optical tweezers are employed in a myriad of applications
demonstrating the importance of these tools. While the basic principle of
optical tweezers is the use of a strongly focused laser beam to trap and
manipulate particles, ever more complex experimental set-ups are required in
order to perform novel and challenging experiments. With this article, we
provide a detailed step- by-step guide for the construction of advanced optical
manipulation systems. First, we explain how to build a single-beam optical
tweezers on a home-made microscope and how to calibrate it. Improving on this
design, we realize a holographic optical tweezers, which can manipulate
independently multiple particles and generate more sophisticated wavefronts
such as Laguerre-Gaussian beams. Finally, we explain how to implement a speckle
optical tweezers, which permit one to employ random speckle light fields for
deterministic optical manipulation.Comment: 29 pages, 7 figure
Quantum limited particle sensing in optical tweezers
Particle sensing in optical tweezers systems provides information on the
position, velocity and force of the specimen particles. The conventional
quadrant detection scheme is applied ubiquitously in optical tweezers
experiments to quantify these parameters. In this paper we show that quadrant
detection is non-optimal for particle sensing in optical tweezers and propose
an alternative optimal particle sensing scheme based on spatial homodyne
detection. A formalism for particle sensing in terms of transverse spatial
modes is developed and numerical simulations of the efficacy of both quadrant
and spatial homodyne detection are shown. We demonstrate that an order of
magnitude improvement in particle sensing sensitivity can be achieved using
spatial homodyne over quadrant detection.Comment: Submitted to Biophys
Linear microrheology with optical tweezers of living cells 'is not an option'!
Optical tweezers have been successfully adopted as exceptionally sensitive transducers for microrheology studies of complex fluids. Despite the general trend, in this article I explain why a similar approach should not be adopted for microrheology studies of living cells. This conclusion is reached on the basis of statistical mechanics principles that indicate the unsuitability of optical tweezers for such purpose
Simulation of superresolution holography for optical tweezers
Optical tweezers manipulate microscopic particles using foci of light beams. Their performance is therefore limited by diffraction. Using computer simulations of a model system, we investigate the application of superresolution holography for two-dimensional (2D) light shaping in optical tweezers, which can beat the diffraction limit. We use the direct-search and Gerchberg algorithms to shape the center of a light beam into one or two bright spots; we do not constrain the remainder of the beam. We demonstrate that superresolution algorithms can significantly improve the normalized stiffness of an optical trap and the minimum separation at which neighboring traps can be resolved. We also test if such algorithms can be used interactively, as is desirable in optical tweezers
Theory of Optical Tweezers
We derive a partial-wave (Mie) expansion of the axial force exerted on a
transparent sphere by a laser beam focused through a high numerical aperture
objective. The results hold throughout the range of interest for practical
applications. The ray optics limit is shown to follow from the Mie expansion by
size averaging. Numerical plots show large deviations from ray optics near the
focal region and oscillatory behavior (explained in terms of a simple
interferometer picture) of the force as a function of the size parameter.
Available experimental data favor the present model over previous ones.Comment: 4 pages, 3 figure
Dynamic stereo microscopy for studying particle sedimentation
We demonstrate a new method for measuring the sedimentation
of a single colloidal bead by using a combination of optical tweezers and a stereo microscope based on a spatial light modulator. We use optical tweezers to raise a micron-sized silica bead to a ïŹxed height and then release it to observe its 3D motion while it sediments under gravity. This experimental procedure provides two independent measurements of bead diameter and a measure of FaxĂ©nâs correction, where the motion changes due to presence of the boundary
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