We have integrated a microfluidic magnetic trap platform with an external magnetic force microscope (MFM) cantilever. The MFM cantilever tip serves as a magnetorobotic arm that provides a translatable local magnetic field gradient to capture and move magnetic particles with nanometer precision. The MFM electronics have been programmed to sort an initially random distribution of particles by moving them within an array of magnetic trapping elements. We measured the maximum velocity at which the particles can be translated to be 2.2mm∕s±0.1mm∕s, which can potentially permit a sorting rate of approximately 5500particles∕min. We determined a magnetic force of 35.3±2.0pN acting on a 1μm diameter particle by measuring the hydrodynamic drag force necessary to free the particle. Release of the particles from the MFM tip is made possible by a nitride membrane that separates the arm and magnetic trap elements from the particle solution. This platform has potential applications for magnetic-based sorting, manipulation, and probing of biological molecules in a constant-displacement or a constant-force mode.
We have developed a micromachined fluid-cell platform that consists of patterned magnetic thin-film elements supported on a thin silicon–nitride membrane. In the presence of an external magnetic field, the field gradients near the magnetic elements are sufficiently large to trap magnetic particles that are separated from the patterned films by a 200 nm thick nitride membrane. The two main applications of this fluid-cell platform are to provide a means to control and position magnetic microparticles, which can be tethered to biological molecules, and also to sort superparamagnetic microparticles based on their size and magnetic susceptibility. We determine the characteristic trapping forces of each trap in the array by measuring the Brownian motion of the microparticle as a function of applied external field. Typical force constants and forces on the superparamagnetic particles are 4.8×10−4±0.7×10−4 N/m and 97±15 pN, respectively.
Ultrafast pulse shapes are used to control simultaneously the optimal population transfer coefficients and rotational wave-packet quantum interferences in the E 1 ⌺ g ϩ state of Li 2 ( E ϭ9, J E ϭ27 and 29͒. By dividing the spectral bandwidth of the ultrafast pulses into multiple ''control domains'' centered on each resonant wavelength, the population transfer coefficients can be manipulated independently of the wave-packet interferences to maximize the Li 2 photoionization yield at arbitrary short pump-probe time delays. To investigate the population transfer coefficients with and without wave-packet interferences, respectively, the pump polarization is set to be either parallel to or at the magic angle ͑ϳ55°͒ relative to the probe polarization. A comparison is made between phases that are symmetric and antisymmetric about the resonances. The effects of resonant and nonresonant frequencies are separately established and quantified. It is estimated that up to 90% of the possible nonresonant Rabi oscillations can be brought into phase simultaneously for each rovibrational state in the wave packet, while at the same time a constant phase offset added to one of the control domains establishes the phase of the wave-packet interference.
Optical phase manipulation of nonresonant frequencies is investigated as a method of achieving optimal population transfer during resonant impulsive stimulated Raman scattering. Wave packets containing quantum beats between an initially prepared rovibrational level in the A(1Σu+) electronic state of Li2 and states populated via a resonance-enhanced rotational Raman process are created using a shaped ultrafast pulse centered near 800 nm. Study of these wave packets allows a quantitative comparison of population transfer as a function of applied phases in the ultrafast pulse. Two cases are explored to determine the ability to enhance population transfer: one with a wide state spacing [A(νA=11, JA=28)-A(11,30) at 50.1 cm−1] and one with a narrow spacing [A(11,8)-A(11,10) at 16.6 cm−1]. In both cases, several different phase masks are applied to the wave packet preparation pulse to enhance the population transferred to the newly formed state of interest. One phase mask involves the application of a −90° phase shift to the nonresonant optical frequencies that lie between the resonant transition frequencies, resulting in an optimal phase relationship between pairs of nonresonant frequencies contributing to the Stokes–Raman excitation. Another extends the phase modification to the nonresonant frequencies lying outside the two resonant transitions to allow constructive enhancement from a larger range of frequencies. Significant population enhancements, up to a factor of ∼12, of the newly formed A(11,30) and A(11,10) states are demonstrated. In addition, the dependence on the state spacing and therefore the extent to which nonresonant frequencies affect the population transferred in the stimulated Raman process are demonstrated.
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