46892-AC6
Direct Measurement of Casimir Forces in Near-critical Binary Fluids
An optical tweezers corresponds to a tightly focused laser beam, which can trap a micron-sized dielectric object, such as a colloidal bead. A dual-trap tweezers permits the trapping and micromanipulation of two such beads, and the determination of forces acting between them, including the critical Casimir forces that are the subject of this program. Over the last year, we have designed and constructed a precision dual-trap optical tweezers apparatus: The beam from a highly-stable, 3 Watt, 1064 nm-wavelength laser is used for trapping. This beam is split into two by means of a polarizing beam splitter, so that there are two independently-steerable beams with orthogonal polarizations. One of the beams may be steered manually by means of a mirror located in a plane conjugate to the laser focus. The other is steered by means of an acousto-optic deflector (AOD). Both beams are expanded and parallelized, and then used to overfill the back aperture of a high-numerical aperture microscope objective. The result is two diffraction-limited laser foci a few microns below the microscope coverslip, each of which is capable of trapping a one-micron-diameter dielectric bead with a maximum trapping force on the order of 100 pN. The force on the bead is measured by means of quadrant-photodiode-based back-focal-plane interferometry (BFPI), which monitors in real-time the displacement of each bead from the center of its trap by imaging the back focal plane of the microscope condenser onto a quadrant photodiode. In addition, to the trapping laser beams, orthogonally-polarized beams from a much-weaker 30 mW 785-nm laser are brought to a focus in the same plane, permitting us to accurately monitor the beads' absolute positions, also via BFPI. For the planned studies, it will be necessary for us to control and monitor the beads' positions with high precision. This may be accomplished via piezoelectric or galvanometric mirrors, electro-optic deflectors (EODs), or AODs. Each of these devices changes the beam's propagation direction in a plane conjugate to the focus, thus controlling the trap position. Each has its own advantages and disadvantages. In particular, AODs are fast (they have a low rise-time of 1.5 microsecond per mm of beam-width), they are capable of displacing a trap over distances greater than 10 microns, and they are relatively inexpensive. However, AODs show a non-linear diffraction efficiency with deflection angle and, for nanometer resolution, require radio-frequency (RF) power with a precise frequency. To precisely control and monitor our beads' positions, we have devote considerable effort to developing and constructing a programmable direct-digital-synthesis-based (DDS-based) controller for acousto-optic deflectors (AODs). Our controller corrects for an AOD's inevitably non-linear diffraction efficiency versus diffraction angle, and provides superior stability, functionality and configurability for optical trapping applications. To demonstrate the positional resolution possible with this controller within our current setup, we have carried a standard resolution test in which the trapping beam causes a bead to step through the focus of the 785 nm-wavelength detection laser. The bead's position relative to the fixed center of the detection laser's focus is then determined by BFPI. Using computer control to step the bead in steps of 8 nanometers, 2 nanometers, and 1 nanometers reveals that in each case, steps of the correct size are readily resolvable. A manuscript describing our DDS-based controller for AOD's is about to be submitted to Review of Scientific Instruments.
