Optics: measuring and testing – Shape or surface configuration
Reexamination Certificate
2001-11-15
2004-12-21
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Shape or surface configuration
C356S625000
Reexamination Certificate
active
06833923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a method for detecting an object and/or the area immediately surrounding an object and/or the interior of an object with regard to physical and/or chemical and/or biological problems. More particularly, the invention relates to a method that uses a scanning particle within a potential trap to scan the object or the area immediately surrounding or its interior wherein at least one scanning-dependent position of the scanning particle is detected.
2. Description of Related Art
As is well known in the art, photons carry momentum equal to the product of photon wavelength and Planck's action quantum. The momentum carried by the photons can be transferred to matter. A. Ashkin (“Acceleration and trapping particles by radiation pressure”, Physical Review Letters 24(4), pp 156-159 [1970]) was able to show that small particles within a substantially collimated laser beam do move towards the zone of the strongest field (highest intensity). By using a slightly divergent laser beam pointing away from the center of the earth, Ashkin compensated for gravity and trapped glass particles having diameters of approximately 20 &mgr;m in a position wherein the force of radiation pressure balances gravity. See A. Ashkin, J M Dziedzik, “Optical levitation by radiation pressure”, Applied Physics Letters, 19(8), pp 283-285 [1971]. A further experiment showed that two laser beams pointing at one another create a 3-D potential that traps the particle in all three spatial coordinates. See A. Ashkin, J M Dziedzik, “Observation of radiation pressure trapping of particles by alternating light beams”, Physical Review Letters 54(2), pp 145-147, [1985]. Furthermore, a highly focused laser beam using lenses of high NA (small f
umbers) was shown to be able to retain particles in aqueous media in all three spatial coordinates. See A. Ashkin, J M Dziedzik, J E Bjorkholm, S Chu “Observation of a single-beam gradient force optical trap for dielectric particles”, Optics Letters 11(5), pp 288-290 [1986]. When the laser beam is moved, the particles follow the focal zone of the trapping beam.
Based on such work, measuring devices commonly referred to as optical tweezers were built into microscopes to measure small forces that occur during the interaction between proteins. See K. Visscher, G J Brakenhoff, “Single beam optical trapping integrated in a confocal microscope for biological applications”, Cytometry 12, pp 486-491 [1991]. In such measurements, it is extremely critical to use sensors that detect the particle position relative to the geometric focal point of the laser trapping beam. Cameras may be used in the simplest cases. See L. Malmqvist, H M Hertz, “Trapped particle microscopy”, Optics Communication 94, pp 19-24 [1992]. Malmqvist and Hertz built a near field microscope having optical tweezers wherein a trapped particle scatters light and is considered a light source passing through an object to be observed. The light source approaches objects to be observed within a few nanometers. In this method, the object is irradiated by the near field induced by the particle rather than the far field of the microscope lens. The image is recorded as a shadow in the far field.
Another method uses a quasi-heterodyne interferometer to measure particles at the geometrical focus of the trapping beam. See W Denk, W W Webb, “Optical measurement of picometer displacements of transparent microscopic objects” Applied Optics, 29(16), pp 2382-2391 [1990]. The light scattered forward and the unscattered light interfere in a diode so that a one-dimensional position of the particle is measured relative to the geometric focus of the trapping beam. In principle, the accuracy of determining the position of the particle depends only on the signal-to-noise ratio of the detection method. It should be noted that noise is meant in the terms of the stochastic phenomenon rather than in the acoustic sense.
Some conventional methods use cameras as well as quadrant photodiodes, cathode beam cameras, CCD or CID cameras, and spatially resolving secondary electron multipliers, for example, to determine the positions of small particles to sub-pixel accuracy. See M J Saxton and K Jacobson, “Single-particle tracking: applications to membrane dynamics”, Annual Review of Biophysics and Biomolecular Structures 26, pp 373-399 [1997].
Ghislain and Webb describe an instrument wherein a glass bit is first caught using optical tweezers and is then moved over a surface. The above mentioned interferometer detects the object excursion. See L P Ghislain, WW Webb, “Scanning force microscope based on an optical trap”, Optics Letters, 18(19), pp 1678-1680 [1993]. The topological change is inferred from the change in scattering intensity.
A force microscope based on the principle of the optical tweezers is disclosed by U.S. Pat. No. 5,445,011 wherein a probe is deflected microscopically and the excursion is determined. The instrument disclosed by U.S. Pat. No. 5,445,011 may be less effective than a conventional force microscope and may not offer the desired advantages since instrument's resolution is limited by thermal vibrations of the probe. See A L Stout, W W Webb, “Optical force microscopy”, Methods in Cell Biology 55, pp 99-116 [1998].
In a scientific paper presented by the Applicants it was shown that the position of a trapped fluorophore-marked latex particle can be determined using the intensity of the fluorescence. See E L Florin, J K H Hörber, E H K Stelzer, “High-resolution axial and lateral position sensing using two-photon excitation of fluorophores by a continuous-wave Nd/YAG laser”, Applied Physics Letters 69(4), pp 446-448 [1996]. The field intensity applied to a particle in optical tweezers changes with its position in the point spread function. Also, the fluorescence intensity varies as a function of particle position along the optical axis. The axial position can be determined at an accuracy better than 8 nm.
Regarding fluorescence detection of the axial position of a particle trapped in optical tweezers, the particle should be trapped, not at the geometric focus but, and depending on particle size, behind the geometric focus as seen in the direction of the beam. See T Wohland, A Rosin, E H K Steizer, “Theoretical determination of the influence of the polarization on forces exerted by optical tweezers”, Optik, 102(4), pp 180-190 [1996]. As a rule, this will be the case with known optical tweezers because of the radiation pressure on the particle. Because of the different interaction intensity, even in axially symmetric potentials, it is possible to determine the particle position along the optical axis. See A Pralle, M Prummer, E-L Florin, E H K Steizer, J K H Hörber “Three-dimensional high-resolution position tracking for optical tweezers by forward scattered light:” Microscopy Research and Techniques 44(5), pp 378-386 [1999].
Another conventional method used to determine a particle position within an optical potential is based on the analysis of an interference pattern derived, for example, from a plane conjugate to an image plane or Fourier plane. See A Pralle, M Prummer, E-L Florin, E H K Stelzer, J H K Hörber, “Three-dimensional high-resolution particle tracking for optical tweezers by forward scattered light”, Microscopy Research Techniques 44(5), pp 378-386, [1999]. An interference pattern between unscattered laser light and laser light scattered at the particle is analyzed by a quadrant diode and the position of the particle constituting the scattering center is determined relative to a geometric focus of the optical tweezers.
The forces of the photons, i.e., optical force, acting on a particle trapped by optical tweezers may be calibrated by analyzing the thermal noise of the particle positions within the optical tweezers' trapping potential. See E-L Florin, A Pralle, E H K Stelzer, J K H Hörber, “Photonic fo
Florin Ernst-Ludwig
Hörber Heinrich J. K.
Stelzer Ernst H. K.
Europaisches Laboratorium fur Molekularbiologie (EMBL)
Font Frank G.
Merlino Amanda
Rothwell Figg Ernst & Manbeck
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