Abstract:
A combination for use in optical trapping is provided, comprising, in series: an adaptable reflective optical element for sculpting a laser beam to produce a sculpted beam; a beam splitter for splitting the sculpted beam into a first and a second sculpted beam; a micro lens array for dividing the first sculpted beam into an array of beamlets to produce a plurality of focal points; relay optics; and a focusing lens; and, in parallel: a wavefront curvature sensing device for accepting and analyzing the second sculpted beam, and reporting to a computer.

Description:
FIELD 
   The present technology is directed to optical tweezers and use thereof. More specifically, the technology is directed to optical tweezers that permit real-time control. 
   BACKGROUND 
   An optical trap is an optical tool that utilizes the gradient forces of a focused beam of light to manipulate particles with dielectric constants higher than a surrounding media. To minimize energy, the particles move to where an electric field is the strongest. 
   Optical trapping is used to manipulate particles, such as cells and nucleic acids. Trapped particles are typically suspended in a fluid medium. The fluid can create unwanted optical aberrations in the optical path and degrade performance. 
   There are a number of different approaches used for creating optical traps. In one approach, a single laser beam is focused to create one optical trap. The beam remains stationary and a device used to hold the particles, such as a microscope slide, is mechanically moved. In this technique, the optical system is static and any movement is made through mechanical means such as a precise motorized stage. Only a single trap can be created with this approach. 
   In another approach, a single laser beam is focused to create one optical trap and the trap is moved in two dimensions by means of a scanning mirror. For example, the mirror can be moved at a high speed by two actuators under computer control. A single particle can be trapped and then moved by the scanning system. Furthermore, multiple particles (typically up to 8 effectively) can be trapped by rapidly scanning the laser beam between the trapped particles. Essentially, rapidly moving the beam using the scanning mirror results in “time sharing” of one laser beam over multiple particles. Only a single trap can be created with this approach. 
   In yet another approach, multiple laser beams are created using a spatial light modulator (SLM). The multiple beams can then be controlled by the SLM. The SLM is composed of cells, each of which can be addressed electronically from the computer. The computer can then control the refractive index of each cell which in turn controls properties of each beam. Using this approach, multiple particles can be trapped and individually controlled. However, this method is indirect and typically requires a complex and relatively slow algorithm to compute trap positions. 
   Grier et al (U.S. Publication No. 2005/0001063) disclose a method and apparatus for laterally deflecting and separating a flow of particles using a static array of optical tweezers. While the beam can be controlled, control is not done in real-time. 
   It is an object of the present technology to address deficiencies in the prior art and to provide other advantages. 
   SUMMARY 
   The present technology is a multiple beam system for optical trapping. Multiple optical traps are created that are individually controllable by a computer. The disclosed method provides direct real time control of each trap through combination of deformable mirror motion and Shack Hartmann Wave-Front Sensor (SHWFS) data. Therefore, particles can be moved at a high speed, thereby permitting analyses where real-time control is advantageous or required such as cell sorting and manipulation of micro-devices (pumps and valves). Further, the technology provides for a greater degree of beam control thereby allowing more sophisticated control of each individual trap. 
   In one embodiment, a combination for use in optical trapping is provided, the combination comprising, in series: an adaptable reflective optical element for sculpting a laser beam to produce a sculpted beam; a beam splitter for splitting the sculpted beam into a first and a second sculpted beam; a micro lens array for dividing the first sculpted beam into an array of beamlets to produce a plurality of focal points; relay optics; and a focusing lens; and, in parallel: a wavefront curvature sensing device for accepting and analyzing the second sculpted beam and reporting to a computer. 
   In one aspect of the embodiment, the wavefront curvature sensing device is a Shack Hartmann Wave-front Sensor. 
   In another aspect, the adaptable reflective optical element comprises a reflective, dynamic surface and actuators. 
   In another aspect, the adaptable reflective optical element is a deformable mirror. 
   In another aspect, the focusing lens is an objective lens. 
   In another aspect, the objective lens has a numerical aperture greater than unity. 
   In another aspect, the focusing lens forms an array of high gradient optical regions. 
   In another aspect, there are at least four actuators per optical trap. 
   In another aspect, there are five actuators per optical trap. 
   In another aspect, at least four actuators are configured to manipulate positions of each region of the array of high gradient optical regions. 
   In another aspect, five actuators are configured to manipulate the positions of each region of the array of high gradient optical regions. 
   In another aspect, the combination further comprises a second beam splitter and a real-time visual monitor. 
   In another aspect, the real-time visual monitor is a video monitor. 
   In another aspect, the wavefront curvature sensing device is an interferometer. 
   In another aspect, the adaptable reflective optical element is a spatial light modulator. 
   In a second embodiment, a method for real time optical trapping is provided. The method comprising: 
   sculpting an incident laser beam; 
   splitting a resulting sculpted laser beam into a first sculpted beam and a second sculpted beam; 
   subdividing the first sculpted beam into a plurality of beamlets; 
   transferring and focusing said beamlets to produce a plurality of focal points; 
   concomitantly accepting and analyzing the second sculpted beam; 
   reporting information to a controller; and 
   moving the focal points by resculpting said incident laser beam on the basis of instructions from said controller, thereby optically trapping at least one particle in real time. 
   In one aspect, the sculpting is effected by at least four actuators per optical trap. 
   In another aspect, the sculpting is effected by five actuators per optical trap. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a schematic of an embodiment of a system for generating multiple moveable optical traps for manipulating small particles in accordance with the disclosed technology. 
       FIG. 2  is an exemplary deformable mirror of the technology of  FIG. 1 . 
       FIG. 3  is an exemplary micro lens array (MLA) of the technology of  FIG. 1 . 
       FIG. 4  is a Shack Hartmann Wave-Front Sensor, which is an exemplary wavefront curvature sensing device of the technology of  FIG. 1 . 
       FIG. 5  is an objective lens, which is exemplary focusing optics of the technology of  FIG. 1 . 
       FIG. 6  is a schematic illustrating creation of optical traps by a micro lens array and movement of each trap to a new position, the new position based on a sculpted wavefront of the laser beam formed by a deformable mirror and received by the micro lens array. 
       FIG. 7  is a schematic illustrating registration between a micro lens array used to create optical traps, a Shack Hartmann Wave-Front Sensor micro lens array, and actuators of a deformable mirror of the disclosed technology. 
       FIG. 8  is a flowchart showing how computer software can control the system. 
       FIG. 9  is a schematic of another embodiment of a system for generating multiple moveable optical traps for manipulating small particles in accordance with the disclosed technology. 
   

   DETAILED DESCRIPTION 
   A system for creating an optical trap, generally referred to as  10 , is shown in  FIG. 1 . The system  10  includes an adaptable reflective optical element  12  such as a deformable mirror situated upstream from a beam splitter  14 , a micro lens array (MLA)  16 , relay optics  18 , and focusing optics  20 . Focusing optics  20  is typically an objective lens with a very high numerical aperture (NA), typically greater than unity. A transparent vessel  22  for containing a sample is located downstream from the focusing optics  20  in the system  10 . On a second path, a wavefront curvature sensing device  24  such as a Shack Hartmann Wave-Front Sensor (SHWFS) is located downstream from the beam splitter  14  and upstream from a computer  26 . The light source for the system  10  is a laser  28 . Any suitable laser can be used. Useful lasers include, but are not limited to, solid state lasers, diode pumped lasers, gas lasers, dye lasers, alexanderite lasers, free electron lasers, VCSEL lasers, diode lasers, Ti-Sapphire lasers, doped YAG lasers, doped YLF lasers, diode pumped YAG lasers, and flash lamp-pumped YAG lasers. Diode-pumped Nd:YAG lasers operating between 10 mW and 10 W are preferred. 
     FIG. 2  shows a cross-sectional view of a deformable mirror  12 . The deformable mirror  12  is an exemplary adaptable reflective optical element that can be used in system  10 . The deformable mirror  12  has an array of actuators  30  positioned beneath a reflective, dynamic surface  32  and the actuators  30  are under control of a computer  26 . The computer  26  can address each actuator  30  and precisely control its length. Therefore, as the computer  26  addresses each actuator  30 , their length changes. In this manner, motions of the actuators  30  alter the shape of the reflective, dynamic surface  32 . Therefore, the entire shape of the reflective, dynamic surface  32  can be changed. An exemplary deformable mirror device is a phase only deformable mirror such as the “multi-DM” manufactured by Boston Micromachines of the USA or the “mirao” manufactured by Imagine Eyes of France. Another exemplary adaptable reflective optical element is Texas Instruments digital light processor (DLP) employing micromirrors. These dynamic optical devices have an encodable reflective surface in which a computer sculpts the wave-front surface formed therein. 
     FIG. 3  shows an exemplary MLA  16  that can be used in system  10 . MLA  16  includes an array of micro-lenses  34 . The micro-lenses  34  of the MLA (called lenslets  34 ) typically all have the same focal length. 
     FIG. 4  is a side perspective and a front perspective view of a SHWFS  24 . The SHWFS  24  is an exemplary wavefront curvature sensing device that can be used in system  10 . The SHWFS  24  has an array of micro-lenses  36  positioned in front of a digital imaging camera  38 . The SHWFS  24  can be used for measuring a phase profile of a wavefront such as a wavefront generated by an adaptable reflective optical element (e.g., deformable mirror  12 ). 
     FIG. 5  is a side-view of an objective lens  20 . The objective lens  20  is an example of focusing optics that can be used in system  10 . The objective lens  20  has a back aperture  40 . 
   With regard to  FIG. 1 , operation of the system  10  using the exemplary devices shown in  FIGS. 2-5  can be described as follows. A laser beam  42  is directed off a deformable mirror  12 , which is positioned in a plane conjugate to the planar surface at the back aperture  40  of the objective lens  20  (see  FIG. 5 ). The laser beam  42  can be described as having a wavefront with a phase profile. The wavefront of the laser beam  42  is sculpted by the deformable mirror  12  by changing the phase profile, thereby forming a sculpted laser beam  44  that is then directed to the beam splitter  14 . 
   The beam splitter  14  transmits a portion of the sculpted beam  44 , thereby forming a first sculpted beam, to the MLA  16 . The MLA  16  can be a static optical element that takes the incoming first sculpted laser beam and subdivides it into an array of smaller optical beams termed beamlets.  FIG. 6  is an illustration of micro-lenses  34  of an MLA  16  forming beamlets  46 . The number and spacing of the beamlets  46  is dependent on the number of MLA micro-lenses  34  in the MLA  16 . Each of the beamlets  46  is brought to a focus at a focal point  48  behind or downstream of the MLA  16 . Positions of focal points  48  are determined by focal lengths of the MLA micro-lenses  34 . An adaptable reflective optical element upstream from the MLA  16  can control the positions of the focal points  48  of each beamlet  46  by adjusting the wavefront of the sculpted laser beam that is incident on the MLA  16 . For example, in system  10 , a deformable mirror  12  can be used to adjust or sculpt the wavefront of the laser beam  42  to form a sculpted wavefront  44  that is received by the MLA  16 . In this manner, the deformable mirror  12  can directly control both the motion and the positions of the focal points  48  of the MLA  16  by adapting the reflective dynamic surface  32  of the deformable mirror  12 . 
   The sculpting by the deformable mirror  12  individually or collectively can move the focal points  48  to a desired set of new spatial positions. As shown in  FIG. 6 , if a plane wave  50  with an unsculpted wavefront phase  52  is received by the MLA  16  then each beamlet  46  is brought to a focus on the axis  54  of each MLA micro lens  34 . If the wavefront phase  56  has been adjusted by a deformable mirror, (i.e., a sculpted wavefront phase  56 ) then the positions of the focal points  48  can be offset from the MLA micro lens axis  54 . The degree to which focal points  48  can be offset from the axis  54  is based on the curvature of the sculpted wavefront  56 . For example, in system  10 , the shape of a deformable mirror reflective dynamic surface  32  of deformable mirror  12  imparts a sculpted wavefront  56  onto the laser beam  42  thereby forming sculpted beam  44 . The sculpted wavefront  56  in turn dictates the positions of the beamlet focal points  48  in a plane behind the MLA, the motion of optical traps in the direction of the beam, and clockwise or counter-clockwise rotation of the traps in the plane. 
   In system  10 , an array of focal points  48  of the MLA  16  ( FIG. 6 ) is transferred by relay optics  18  to the back aperture  40  of the objective lens  20  ( FIG. 5 ). The focal points  48  are relayed and focused into a vessel  22  by the objective lens  20 . The vessel  22  includes particles to be captured. The objective lens  20  focuses beamlets formed by the MLA  16 , such as beamlets  46 , in a high gradient optical region that is capable of trapping particles. The vessel  22  is constructed of transparent material, to allow the beamlets  46  to pass through and to reduce interference with the formation of the optical traps. 
   In  FIG. 1 , a portion of the sculpted laser beam  44  is also directed by the beam splitter  14  into a wavefront curvature sensing device  24  such as an exemplary Shack Hartmann Wave-Front Sensor (SHWFS)  24 , thereby forming a second sculpted beam. The SHWFS  24  is configured to measure the phase of the wavefront generated by the deformable mirror  12 . Wavefront phase information is then transmitted to the computer  26  where the information is utilized by control software. 
   For fine control and manipulation of trapped particles, the SHWFS  24  can have more wavefront sample points than the number of optical traps created by the MLA  16 . As shown in  FIG. 7 , the SHWFS micro-lens array  36  can be registered to the actuators  30  of the deformable mirror  12  at the corners of the SHWFS micro lens array  36 . This registration allows precise motion of the optical traps  37  and also the ability to apply specific modes to each of the beamlets  46  in the traps, such as defocus or helical modes. For clarity, only four optical traps  37  are shown in  FIG. 7 , but it should be understood that a larger array of such optical traps can be created by the MLA  16 . 
   The number of actuators  30  per optical trap  37  is related to the number of degrees of freedom of an optical trap. For example, four actuators  30  per optical trap provide control of the optical trap motion in a plane; five actuators  30  per optical trap are needed to change the optical trap depth. The number of lenses  36  in the SHWFS micro lens array  24  is directly related to the number of actuators  30 , because the registration of the actuators  30  and the SHWFS lenses  36  is preferably done such that the actuators  30  are at the corners  39  of the SHWFS lenses  36 . 
   The number of optical traps  37  is given by the number of lenses  34  in the MLA  16 , the number of actuators  30  is given by the number of degrees of freedom of the optical traps, and the number of lenses  36  of the SHWFS  24  is given by the registration between the SHWFS  24  and the deformable mirror  12 . 
   The number of actuators  30  is dependent on the deformable mirror  12  design and on the mirror manufacturer. For example, deformable mirrors made by Boston Micro Machines can have 140 actuators. Other deformable mirrors include 1024 actuators. The number of lenses  36  in the SHWFS micro-lens array  24  is also a function of design. If the SHWFS micro-lens array  24  has more lenses than the MLA  16 , oversampling of the wavefront shape after it leaves the deformable mirror  12  is possible. 
     FIG. 8  is a flowchart showing how the system  10  can be controlled. To trap small particles, an operator  102  and/or the computer  26  adjust the deformable mirror  12  to direct the movement of each optical trap to acquire a selected small particle and trap it. A video camera system  66  can observe the position of the traps. The plurality of optical traps containing particles can then be configured and reconfigured. Using the video camera data, the position and identity of one or more of the trapped particles can be monitored. Control software can be employed to accept SHWFS data and to determine the shape of the reflective, dynamic surface  32  of deformable mirror  12 . The operator  102  can determine desired positions for the optical traps either manually or through a computer-based algorithm. The control software then utilizes the determined information, the SHWFS data, and current positions of the optical traps to compute shapes of the reflective, dynamic surface  32  for moving the traps to the desired positions. 
   The foregoing are embodiments of the disclosed technology. As would be known to one skilled in the art, variations that do not alter the scope of the technology are contemplated. For example, suitable adaptable reflective optical elements having a time dependent aspect to their function include, but are not limited to, variable computer generated diffractive patterns, variable phase shifting materials, variable liquid crystal phase shifting arrays, micro-mirror arrays, piston mode micro-mirror arrays, spatial light modulators, electro-optic deflectors, acousto-optic modulators, deformable minors, reflective MEMS arrays. Through use of a dynamic phase patterning optical element, the features of a surface can be encoded to form a hologram and altered such as by a computer to effect a change in the hologram which can in turn affect the number of beamlets, the phase profile of at least one of the beamlets, and the location of at least one of the beamlets. The video camera system can be replaced with any suitable real time visual monitor. The SHWFS could be replaced with, for example, but not limited to, an interferometer or any suitable wavefront curvature sensing device. The deformable minor could be replaced with, for example, but not limited to a spatial light modulator (SLM). The particles can be contained in, for example, but not limited to, a fluid medium that is held in a small cavity created between two microscope slides. The system can further comprise a second beam splitter.  FIG. 9 , for example, illustrates a system  10 ′ with a second beam splitter  92  and a real-time visual monitor  94 . 
   Aspects of the disclosed technology, such as selective generation and control of an optical tweezer system, may be useful in a variety of commercial applications, such as, optical circuit design and manufacturing, nanocomposite material construction, fabrication of electronic components, opto-electronics, chemical and biological sensor arrays, assembly of holographic data storage matrices, rotational motor, mesoscale or nanoscale pumping, energy source or optical motor to drive a micro electrical mechanical system (MEMS), facilitation of combinatorial chemistry, promotion of colloidal self-assembly, manipulation of biological materials, interrogating biological material, concentrating selected biological material, investigating the nature of biological material, and examining biological material. 
   In view of the many possible embodiments to which the disclosed principles may be applied, it should be recognized that the illustrated embodiments are only examples and should not be taken as limiting in scope. Rather, the scope is defined by the following claims. We therefore claim all that comes within the scope and spirit of these claims.