Analysis of the Transverse Force in the Rayleigh and Mie Approximations for a Capture Beam TEM00 and TEM*01 (original) (raw)
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Optical tweezers 3D photonic force spectroscopy.pdf
Since optical tweezers trapped microspheres can be used as an ultrasensitive force measurements technique, the knowledge of its theoretical description is of utmost importance. However, even the description of the incident electromagnetic fields under very tight focusing, typical of the optical trap, is not yet a closed problem. Therefore it is important to experimentally obtain whole accurate curves of the force as a function of wavelength, polarization and incident beam 3D position with respect to the center of the microsphere. Theoretical models for optical forces such as the Generalized Lorenz-Mie theory, can then be applied to the precisely evaluated experimental results.
Calculation and optical measurement of laser trapping forces on non-spherical particles
Journal of Quantitative Spectroscopy and Radiative Transfer, 2001
Optical trapping, where microscopic particles are trapped and manipulated by light is a powerful and widespread technique, with the single-beam gradient trap (also known as optical tweezers) in use for a large number of biological and other applications. The forces and torques acting on a trapped particle result from the transfer of momentum and angular momentum from the trapping beam to the particle. Despite the apparent simplicity of a laser trap, with a single particle in a single beam, exact calculation of the optical forces and torques acting on particles is difficult. Calculations can be performed using approximate methods, but are only applicable within their ranges of validity, such as for particles much larger than, or much smaller than, the trapping wavelength, and for spherical isotropic particles. This leaves unfortunate gaps, since wavelength-scale particles are of great practical interest because they are readily and strongly trapped and are used to probe interesting microscopic and macroscopic phenomena, and non-spherical or anisotropic particles, biological, crystalline, or other, due to their frequent occurance in nature, and the possibility of rotating such objects or controlling or sensing their orientation. The systematic application of electromagnetic scattering theory can provide a general theory of laser trapping, and render results missing from existing theory. We present here calculations of force and torque on a trapped particle obtained from this theory and discuss the possible applications, including the optical measurement of the force and torque.
Force measurements of optical tweezers in electro-optical cages
Applied Physics A: Materials Science & Processing, 1998
We introduce a new device for measuring optically induced forces on microparticles and cells. It uses highfrequency electric field cages (octopoles); dielectrophoretic force calculations allow calibration of optical tweezers. A microsystem in the form of a microscope slide was fabricated by photolithography and electrically connected to a 4-phase high-frequency generator. The possibilities and advantages of a combined electro-optical trap are exemplified by the manipulation of latex beads and human erythrocytes. The system clearly detects misalignments of optical tweezers. In spite of instabilities it is possible to obtain a force-versus-power calibration. Further possibilities of the technique are discussed.
Absolute calibration of forces in optical tweezers
Physical Review A, 2014
Optical tweezers are highly versatile laser traps for neutral microparticles, with fundamental applications in physics and in single molecule cell biology. Force measurements are performed by converting the stiffness response to displacement of trapped transparent microspheres, employed as force transducers. Usually, calibration is indirect, by comparison with fluid drag forces. This can lead to discrepancies by sizable factors. Progress achieved in a program aiming at absolute calibration, conducted over the past 15 years, is briefly reviewed. Here we overcome its last major obstacle, a theoretical overestimation of the peak stiffness, within the most employed range for applications, and we perform experimental validation. The discrepancy is traced to the effect of primary aberrations of the optical system, which are now included in the theory. All required experimental parameters are readily accessible. Astigmatism, the dominant effect, is measured by analyzing reflected images of the focused laser spot, adapting frequently employed video microscopy techniques. Combined with interface spherical aberration, it reveals a previously unknown window of instability for trapping. Comparison with experimental data leads to an overall agreement within error bars, with no fitting, for a broad range of microsphere radii, from the Rayleigh regime to the ray optics one, for different polarizations and trapping heights, including all commonly employed parameter domains. Besides signaling full first-principles theoretical understanding of optical tweezers operation, the results may lead to improved instrument design and control over experiments, as well as to an extended domain of applicability, allowing reliable force measurements, in principle, from femtonewtons to nanonewtons.
Optical tweezers (OT) rely on the radiation pressure to trap and manipulate microscopic particles and living microorganisms. Because the optical forces vary from hundreds of femto to tens of pico Newtons, OT can be used as an ultra sensitive force measurement tool to study interactions involving very small forces. We use a double tweezers to perform ultra sensitive measurement of the force due to the scattering of light as a function of its wavelength, in other words, to perform a Force Spectroscopy. Our results show not only the Mie resonances but also a selective coupling to either the TE, TM or both microsphere modes using the light polarization and the beam positioning. Mie resonances have usually been observed by scattering measurements. Very few reports of levitation experiments observed these resonances directly through the force. The double tweezers system has the advantage and flexibility of a stable restorative force measurement system. The experimental results show excellent agreement with Gaussian shaped beam partial wave decomposition theory. The understanding of the optical scattering forces in dielectric microspheres under different incident beam conditions is important as they have been used as the natural force transducer for mechanical measurements. Our results show how careful one has to be when using optical force models for this purpose. The Mie resonances can change the force values by 30-50 %. Also the results clearly show how the usually assumed azimuthal symmetry in the horizontal plane no longer holds because the beam polarization breaks this symmetry.
Optical tweezers (OT) rely on the radiation pressure to trap and manipulate microscopic particles and living microorganisms. Because the optical forces vary from hundreds of femto to tens of pico Newtons, OT can be used as an ultra sensitive force measurement tool to study interactions involving very small forces. We use a double tweezers to perform ultra sensitive measurement of the force due to the scattering of light as a function of its wavelength, in other words, to perform a Force Spectroscopy. Our results show not only the Mie resonances but also a selective coupling to either the TE, TM or both microsphere modes using the light polarization and the beam positioning. Mie resonances have usually been observed by scattering measurements. Very few reports of levitation experiments observed these resonances directly through the force. The double tweezers system has the advantage and flexibility of a stable restorative force measurement system. The experimental results show excellent agreement with Gaussian shaped beam partial wave decomposition theory. The understanding of the optical scattering forces in dielectric microspheres under different incident beam conditions is important as they have been used as the natural force transducer for mechanical measurements. Our results show how careful one has to be when using optical force models for this purpose. The Mie resonances can change the force values by 30-50 %. Also the results clearly show how the usually assumed azimuthal symmetry in the horizontal plane no longer holds because the beam polarization breaks this symmetry.
Force spectroscopy and two photon excited luminescence in an optical tweezers system
Optical Trapping and Optical Micromanipulation II, 2005
Up to now optical spectroscopies have analyzed the scattered light or the heat generated by absorption as a function of the wavelength to get information about the samples. Among the light matter interaction phenomena one that has almost never been used for spectroscopy is the direct photon momenta transfer. Probably because the forces involved are very small, varying from hundreds of femto to tens of pico Newtons. However, the nowadays very popular Optical Tweezers can easily accomplish the task to measure the photon momenta transfer and may be the basis for the Optical Force Spectroscopy. We demonstrate its potential as such a tool by observing more than eight Mie resonance peaks of a single polystyrene microsphere, and showed the capability to selective couple the light to either the TE, TM or both microsphere modes depending of the beam size, the light polarization and the beam positioning. The Mie resonances can change the optical force values by 30-50 %. Our results also clearly show how the beam polarization breaks the usually assumed azimuthal symmetry by Optical Tweezers theories. We also obtained the spectrum from the two photon excited luminescence using the Optical Tweezers to hold a single bead suspended and a femtosecond Ti:sapphire laser for the non-linear excitation. This spectrum shows the pair of peaks due to both TE and TM spherical cavity modes. We have been able to observe more than 14 Mie resonance peaks in the TPE luminescence. Our results are in good agreement with optical force calculations using Maxwell stress tensor and partial wave decomposition of the incident beam approximated to a 3 th order gaussian beam.
Force Measurements with Optical Tweezers
Springer Handbook of Nanotechnology, 2010
An optical tweezer is a scientific instrument that uses a focused laser beam to provide an attractive or repulsive force, depending on the index mismatch, to physically hold and move microscopic dielectric objects.
Optical force calculations in arbitrary beams by use of the vector addition theorem
Journal of the Optical Society of America B, 2005
We derive and apply formulas that employ the vector-wave addition theorem and rotation matrices for quantitative calculations of both radial and axial optical forces exerted on particles trapped in arbitrarily shaped tweezer beams. For the tightly focused beams encountered in optical tweezers, we shall highlight the importance of formulating the optical forces and beam symmetries in terms of the irradiance and total beam power. A major interest of the addition theorem treatment of optical forces is that it opens up the possibility of modeling a wide variety of beam shapes while automatically ensuring that the beams satisfy the Maxwell equations. In some of the first numerical applications of our method, we shall illustrate that resonance effects play an important role in the axial trapping position of particles comparable in size with the wavelength of the trapping beam.