Feedback enhanced optical tweezers, based on Proportional and Integral (PI) control, are routinely used for increasing the stiffness of optical traps. Digital implementation of PI controller, using DSP or FPGA, enables easy maneuverability of feedback gains. In this paper, we report occurrence of a peak in the thermal noise power spectrum of the trapped bead as the proportional gain is cranked up, which imposes a limit on how stiff a trap can be made using position feedback. We explain the reasons for the deviant behavior in the power spectrum and present a mathematical formula to account for the anomaly, which is in very good agreement with the experimental observations. Further, we present a new method to do the closed loop system identification of feedback enhanced optical tweezers by applying a frequency chirp. The system model thus obtained greatly predicts the closed loop behavior of our feedback based optical tweezers system.
In this paper, we report a novel approach to functionalize the tip of a micro-cantilever by selectively positioning
a functionalized polystyrene bead using optical tweezers as a lever arm. We present a design that consists
of an Atomic Force Microscope (AFM) combined with an optical tweezers setup to study specific interactions
between complementary protein molecules. A BSA protein coated polystyrene bead, held stationary in an optical
trap, is chemically grafted to AFM cantliever tip functionalized by the complementary protein- anti BSA. This
arrangement also provides a flexible means of reversibly and irreversibly fusing polystyrene beads thermally at
desired specified locations on the micro-cantilever, by heating the silicon tip with the focused laser beam of the
optical tweezers. We use optical tweezers as a micro-manipulation tool for grafting pre-specified number of beads
to cantilever in a controlled fashion as against the other widely used methods where an aggregate of molecules
are chemically attached. Further, we study the changes in cantilever's resonant frequency and find it in good
agreement with the expected change due to the additional bead mass. This study opens up opportunities in the
area of biosensors by providing a method to standardize the calibration of chemically modified cantilevers.