We present the results and progress of research to create a multiplex chemical sensor based on Au catalyzed vapor-liquid-solid (VLS) silicon nanowires deployed as resonant mass sensors. Each element of this sensor has a single VLS wire grown in close proximity to a Si photodiode. Together they create a Fabry-Pérot interferometer that allows for the sensitive detection of the beam’s resonant motion. Small changes in mass on the cantilever that occur as a result of chemical absorption on the functionalized Au surface shift the resonant frequency. Our integrated approach will allow large reductions in system complexity for this sensor class.
Techniques to measure the trapping force in an optical tweezers without any prior assumptions about the trap
shape have been developed. The response of a trapped micro or nanoparticle to a step input is measured and
then used to calculate the trapping force experienced by the particle as a function of it's position in the trap. This
method will provide new insight into the trapping behavior of nanoparticles, which are more weakly bound than
microparticles and thereby explore larger regions of the trapping potential due to Brownian motion. Langevin
dynamics simulations are presented to model the system and are used to demonstrate this technique. Preliminary
experimental results are then presented to validate the simulations. Finally, the measured trapping forces, from
simulations and laboratory experiments, are integrated to recover the trapping potential.
System identification methods are presented for the estimation of the characteristic frequency of an optically trapped particle. These methods are more amenable to automated on-line measurements and are believed to be less prone to erroneous results compared to techniques based on thermal noise analysis. Optical tweezers have been shown to be an effective tool in measuring the complex interactions of micro-scale particles with piconewton resolution. However, the accuracy of the measurements depends heavily on knowledge of the trap stiffness and the viscous drag coefficient for the trapped particle. The most commonly referenced approach to measuring the trap stiffness is the power spectrum method, which provides the characteristic frequency for the trap based on the roll-off of the frequency response of a trapped particle excited by thermal fluctuations. However, the reliance on thermal fluctuations to excite the trapping dynamics results in a large degree of uncertainty in the estimated characteristic frequency. These issues are addressed by two parameter estimation methods which can be implemented on-line for fast trap characterization. The first is a frequency domain system identification approach which combines swept-sine frequency testing with a least-squares transfer function fitting algorithm. The second is a recursive least-squares parameter estimation scheme. The algorithms and results from simulation studies are discussed in detail.
The National Aeronautics and Space Administration (NASA) plans to develop optical communication terminals for future spacecraft, especially in support of high data rate science missions and manned exploration of Mars. Future, very long-range missions, such as the Realistic Interstellar Explorer (RISE)1, will need optical downlink communications to enable even very low data rates. For all of these applications, very fine pointing and tracking is also required, with accuracies on the order of ± 1 μrad or less and peak-to-peak ranges of ± 10 mrad or more. For these applications, it will also be necessary to implement very compact, lightweight and low-power precision beam-steering technologies. Although current commercial-off-the-shelf devices, such as macro-scale piezo-driven tip/tilt actuators exist, which approach mission requirements, they are too large, heavy, and power consuming for projected spacecraft mass and power budgets. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has adopted a different approach to beam-steering in collaboration with the National Institute of Standards and Technology (NIST). We are testing and planning to eventually package a highly accurate large dynamic range meso-scale position transducer under development at NIST. In this paper we will describe a generic package design of an optical communications terminal incorporating the NIST prototype beam-steerer. We will also show test results comparing the performance of the NIST prototype meso-scale position beam-steerer to a commercial macro-tip/tilt actuator using a quad-cell tracking sensor.
Beam steering accuracy is critical to the successful operation of optical communications systems, especially those which take place over extreme length scales, such as for an interstellar spacecraft. In this paper, a novel beam steering mechanism and several control system approaches for ultra-precision beam steering are discussed. The beam steering mechanism is a nanopositioning device which utilizes a parallel cantilever configuration and a piezoelectric actuator to obtain extremely high positioning accuracy with minimal parasitic errors. A robust motion controller is presented for this mechanism which is designed to compensate for modeling uncertainty. This controller is intended for use with feedback from the nanopositioner’s built-in capacitance probe. Due to the need to track the trajectory of the steered beam, two additional control approaches are presented which combine the robust motion controller with additional feedback for the actual beam displacement. These multi-loop control approaches provide a level of robustness to thermal effects and vibrations which could not be obtained from a single sensor and feedback loop. Simulation results are provided for each of the control designs.
Conference Committee Involvement (1)
Optomechatronic Sensors and Instrumentation IV
18 November 2008 | San Diego, California, United States