Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
  • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
  • G21K1/006—Manipulation of neutral particles by using radiation pressure, e.g. optical levitation

Definitions

• This invention relates to a method and laser tweezers for orienting a body immobilized by a laser beam and further relates to a complemental apparatus for the laser tweezers.
• the manipulating tool based on this phenomenon are the laser tweezers (A. Ashkin, Science, vol.210, pages 1081- 1088, 1980).
• laser tweezers are obtained by means of a microscope, wherein the focusing element is the microscope objective.
• the tool is extremely well adapted for grasping and manipulating microscopic bodies having dimensions comparable to the wavelength of light.
• the tool has been used particularly in the field of biology, since the size of cells and cell organisms falls into such a micron range.
• the order of magnitude of forces appearing when using lasers of easily attainable power (a few tens of mW) is sufficient to overcome the Brownian motion of cells floating in water.
• the effect of the molecules exerting a biological force also falls within the order of magnitude
• the laser tweezers do not orient the body they grasp; yet, in practice, a suitable control of the spatial orientation of the immobilized specimen is also of significance.
• a spatial orientation an asymmetry of the trapping is necessary.
• an orientation of a spherical body consisting of an isotropic material is not possible which is typical in case of synthetic spheres.
• a further degree of freedom is the control of the direction of light propagation by a rotation about an axis.
• the grasped body is of an optically birefringent material, and the light of the laser tweezers is polarized. If the light is linearly polarized, the specimen orients itself, and the optical axis of the birefringent material will lie in the plane of polarization.
• a restoring torque is exerted on the grasped body in case it deviates from its position of equilibrium. In case the light is circularly polarized, the body is exposed to a continuous torque which causes the body to rotate.
• the directio of rotation depends on the direction of the circular polarization, that is, the direction of rotation may be reversed at will (M.E.J.Friese, T.A.Nieminen, N.R.Heckenberg and H.Rubinsztein-Dunlop, Nature, vol.394, pages 348-350, 1998).
• the laser tweezers In case the light of the laser tweezers is not circularly polarized, but has phase singularity, it is capable of generating a torque and thus can rotate the body which the laser tweezers grasped (H.He, M.E.J.Friese, N.R.Heckenberg and H.Rubinsztein-Dunlop, Phys. Rev. Lett, vol.75, pages 826-829, 1995).
• a further mechanism can cause rotation in case the light itself does not carry a pulse torque, but the grasped body is helical, and the light, as it scatters on the body, acquires a pulse torque.
• This effect causes the body to rotate.
• This phenomenon dynamically corresponds to a wind-driven propeller (R.C.Gauthier, Appl. Phys. Lett., vol.67, pages 2269-2271 , 1995; E.Higurashi, R.Sawada and T.lto, Appl. Phys. Lett, vol.72, pages 2951-2953, 1995; and P.Galajda and P.Ormos, Appl. Phys. Lett, vol.78, pages 249-251, 2001).
• An asymmetric intensity distribution of the trapping light bundle may also effect an orientation of the trapped body (A.T.O'Neil and M.J.Padgett, Opt. Lett., vol.27, pages 743-745, 2002).
• Such an asymmetry may be achieved in a simplest manner by using an aperture having the shape of an oblong rectangle.
• the force of all orienting mechanisms have the same order of magnitude: in case of typical parameters (a laser intensity of 10 mW, the birefringence of calcite, a size in the micron range, etc.) the order of magnitude of the torque is 10 "19 - 10 "17 Nm, dependent upon the various parameters.
• the rotation may be expediently effected by a ⁇ /2 plate positioned in the light path.
• the body to be oriented is converted into a flat body before immobilizing it in the laser tweezers.
• the conversion into a flat body is achieved by affixing the body, which by itself has no asymmetry or is significantly smaller than the wavelength of the linearly polarized light, to an asymmetric carrier whose size is comparable to the wavelength of light.
• the carrier is obtained by polymerizing a resin placed into the sample liquid.
• the essential aspect of the laser tweezers according to the invention resides in that they are provided with means for rotating the polarization plane of the light beam.
• a means is a rotatable ⁇ /2 plate placed in the light path.
• a rotatable ⁇ /4 plate is positioned in the light path.
• the units which rotate the ⁇ /2 and ⁇ /4 plates are computer-controlled.
• the apparatus serving for complementing the laser tweezers includes a rotatable ⁇ /2 plate, and if required, a rotatable ⁇ /4 plate, both positioned into the light path of the light source.
• the invention is based on the observation that if the laser tweezers are formed by a linearly polarized light, flat bodies orient themselves therein.
• the orientation is such that the large body side is aligned with the polarization direction of the light.
• the force of the orientation has a magnitude with which, in case of an intensity of 10 mW, it can easily overcome the Brownian motion of a body having the size of a few microns.
• the orienting effect resulting from the solution according to the invention extends the possibilities of utilization of laser tweezers, namely, a simple orientation and spatial manipulation of a body grasped by the laser tweezers.
• the orienting effect can be directly utilized for orienting and manipulating elongate, non-spherical bodies, such as most living cells.
• the body to be oriented does not have the shape necessary to be oriented by the above- outlined mechanism (either because it is spherically symmetrical, or because its size is much smaller than the wavelength of light), the body to be oriented has to be affixed to a suitable auxiliary body permitting an orientation.
• the direction of orientation may be altered by changing the polarization plane of the light constituting the laser tweezers, and the force of the orienting polarization effect may be altered without changing the trapping force of the laser tweezers.
• a ⁇ /4 plate is placed in the path of the polarized light, the plate, dependent upon its orientation with respect to the orientation of the polarization plane of the light, renders the light linearly polarized or circularly polarized.
• the flat body is not oriented, but the force of the trapping does not significantly change.
• the force of the trapping may be controlled between limit values, that is, between a maximum and a zero value measured in the linearly polarized light.
• both plates may be placed in the light path of the laser tweezers regardless of the tweezers' design. Both plates may be rotated and placed into a desired position independently from one another by computer-controlled motors.
• the apparatus thus offers the possibility to orient, manipulate and rotate bodies which meet the above- described criteria, independently from the trapping force of the laser tweezers.
• Figure 1 is an image of a flat body oriented in laser tweezers.
• Figure 2 is a diagram illustrating the difference (shift) between the polarization plane and the direction of orientation as a function of the rotational velocity as the body grasped by the laser tweezers is rotated by the polarization plane of the light.
• Figure 3 is a diagram explaining the Fresnel equations characterizing the refraction and reflection of polarized light.
• Figure 4 is a diagram showing the computation results concerning the magnitude of the torque exerted on the flat body by the polarized light as a function of the relative orientation between the polarization plane and the body.
• Figure 5 is a perspective view of an apparatus according to the invention.
• Figure 6 is a schematic side elevational view illustrating the adaptation of the apparatus of Figure 5 to a microscope.
• Figure 7 is a perspective view illustrating the adaptation of the apparatus of Figure 5 to a microscope.
• Example 1 Flat bodies were obtained, having defined geometrical parameters and oriented by linearly polarized light.
• the arrow N in Figure 1 indicates the direction of polarization.
• the orienting torques were determined for different, relative positions between the body and the polarization plane of the light. Measuring was based on the following principle: As a starting point, the orienting torque is zero in the position of equilibrium, and, deviating from such a position, a restoring torque is generated which increases as the deviation increases. Thus, the restoring torque increases, as the angle formed between the direction of polarization and the longitudinal direction of the body increases.
• the polarization plane of the laser light may be arbitrarily set by a ⁇ /2 plate 1 (shown in Figures 5 and 6) placed in the light path.
• a rotation of the ⁇ /2 plate 1 evidently entrains the body into rotation too, since its orientation follows the direction of polarization.
• a perfect, ideal following by the body is impeded by the friction of the liquid: the direction of polarization set by the ⁇ /2 plate 1 does not agree entirely with the orientation of the body.
• the friction generated during rotation may be overcome by the body by the difference which develops between the orientation of the body and the direction of polarization.
• the body lags behind the polarization, that is, it has a phase lag.
• the phase lag may be determined for any revolution by determining the rotational phase of the ⁇ /2 plate 1 and the body.
• the rotational resistance affecting the body may be computed based on the known laws of hydrodynamics.
• the phenomenon is based on that the refraction and reflection depend on the polarization of the light. An exact, detailed description thereof would be very complex, because in the instant case the dimension of the grasped body falls into the wavelength range of the laser tweezers 2. Under these circumstances an exact computation of the light scattering is not solved for bodies of complex shape. As an approximation it is assumed that the wavelength of the light is smaller than the characteristic dimensions of the body.
• the laser tweezers 2 are produced by a great number (100,000) of light rays.
• the distribution and direction of the light rays correspond to the real intensity and direction distribution of the laser tweezers.
• For each individual light ray it is examined how the light ray enters into a mutual effect with the grasped body, that is, whether the ray impinges on the surface of the body. If such an impingement does take place, then one part of the light is reflected from the body surface, while the other part continues to propagate in the medium after refraction.
• the equations yield the vector-related reflection coefficients and transmission coefficients of the electric field intensity in case of a polarization direction which is perpendicular or parallel to the plane of incidence. Based on the above equations the course of the post-refraction ray may be computed for each polarized ray, that is, the momentum change may be determined. The momentum changes to which the individual rays undergo exert a force on the body, and by adding up the momentum changes, the entire force (torque) acting on the body may be determined.
• auxiliary bodies ensure that any object may be oriented.
• Carriers may be made in numerous ways; the Example which follows describes the making of such bodies from a material that polymerizes when exposed to light.
• the bodies were made by introducing the light
• the resin was a Norland NOA 63 substance which polymerizes when excited by a light having a wavelength of less than 340 nm.
• the light of 514 nm wavelength is, in focus, of sufficient intensity to cause a two-photon excitation; thus the resin is polymerized in a very small volume.
• a three- dimensional body of arbitrary shape in this instance the shape of a flat cross) may be drawn by moving the focus.
• the specimen to be oriented may be affixed to the body and then may be manipulated at will by the laser tweezers 2 (which may be a conventional, home-made arrangement or may be mounted on the microscope 3) complemented by a simple apparatus 4 according to the invention. It is the essence of the complemental apparatus 4 that it holds a unit, such as a ⁇ /2 plate 1 , in the light path of the laser tweezers 2 and rotates the polarization plane of the light beam.
• the complemental apparatus 4 may be adapted practically to any laser tweezers 2. It is a condition for its use that the light of the laser tweezers 2 be polarized.
• the light bundle producing the laser tweezers 2 is typically a single-mode laser light, that is, it is linearly polarized. Should this not be the case, the light should be polarized by suitable methods.
• the essential components of the apparatus 4 are two optical elements which are placed in the light path of the laser tweezers 2 and whose orientation may be varied by electric motors 5 and 6, controlled by a computer 7.
• the first element as viewed in the direction of light propagation, is a ⁇ /4 plate 8 which, dependent upon the angle it forms with the polarization direction of the linearly polarized light, either transforms the linearly polarized light into a circularly polarized light, or leaves it linearly polarized.
• the role of the element 8 is to switch the orienting effect on or off.
• the linearly polarized light orients the flat body.
• a circularly polarized light does not have such an effect; in case of a circularly polarized light the plane of polarization rotates at the frequency of light.
• a body of micron size cannot follow such a rotation.
• the orienting effect may be switched without appreciably affecting the trapping force of the laser tweezers 2. This represents a very significant and advantageous property.
• the second element is a ⁇ /2 plate 1 which rotates the direction of polarization of the linearly polarized light passing therethrough.
• the orientation of the grasped body may be set.
• the ⁇ /4 plate 8 and the ⁇ /2 plate 1 are mounted on respective, bearing- supported holders 9 and 10.
• the holders 9 and 10 are rotated by respective motors 5 and 6 via respective toothed belts 11 and 12.
• the motors 5 and 6 are, with the intermediary of a control 13, connected to a computer 7 for controlling the rotation of both optical elements 8 and 1.
• the jig in which the optical elements and their drive are mounted may be installed in the light path of the laser tweezers 2. In case of home-made laser tweezers 2 such a solution is evident. If the laser tweezers 2 are mounted on the microscope 3, the apparatus 4 is installed in the microscope 3, whereby the ⁇ /4 plate 8 and the ⁇ /2 plate 1 take the place of the fluorescence filters, as illustrated in Figures 6 and 7.
• Figure 6 clearly shows the path of the light emanating from a laser light source 14. After being deflected by mirrors 15, the light first passes through the ⁇ /4 plate 8 and then through the ⁇ /2 plate 1.
• the apparatus 4 significantly widens the possibilities of application of the laser tweezers 2.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Engineering & Computer Science (AREA)
• General Engineering & Computer Science (AREA)
• High Energy & Nuclear Physics (AREA)
• Microscoopes, Condenser (AREA)
• Lasers (AREA)

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Citations (1)

• 2002
  • 2002-09-20 HU HU0203115A patent/HU224857B1/hu not_active IP Right Cessation
• 2003
  • 2003-09-22 WO PCT/HU2003/000073 patent/WO2004027784A1/en not_active Application Discontinuation
  • 2003-09-22 AU AU2003267677A patent/AU2003267677A1/en not_active Abandoned

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