Direct Measurement of Anharmonic Decay Channels of a Coherent Phonon.

We report channel-resolved measurements of the anharmonic coupling of the coherent A_{1g} phonon in photoexcited bismuth to pairs of high wave vector acoustic phonons. The decay of a coherent phonon can be understood as a parametric resonance process whereby the atomic displacement periodically modulates the frequency of a broad continuum of modes. This coupling drives temporal oscillations in the phonon mean-square displacements at the A_{1g} frequency that are observed across the Brillouin zone by femtosecond x-ray diffuse scattering. We extract anharmonic coupling constants between the A_{1g} and several representative decay channels that are within an order of magnitude of density functional perturbation theory calculations.

Highly parallel magnetic tweezers by targeted DNA tethering.

Single-molecule force-spectroscopy methods such as magnetic and optical tweezers have emerged as powerful tools for the detailed study of biomechanical aspects of DNA-enzyme interactions. As typically only a single molecule of DNA is addressed in an individual experiment, these methods suffer from a low data throughput. Here, we report a novel method for targeted, nonrandom immobilization of DNA-tethered magnetic beads in regular arrays through microcontact printing of DNA end-binding labels. We show that the increase in density due to the arrangement of DNA-bead tethers in regular arrays can give rise to a one-order-of-magnitude improvement in data-throughput in magnetic tweezers experiments. We demonstrate the applicability of this technique in tweezers experiments where up to 450 beads are simultaneously tracked in parallel, yielding statistical data on the mechanics of DNA for 357 molecules from a single experimental run. Our technique paves the way for kilo-molecule force spectroscopy experiments, enabling the study of rare events in DNA-protein interactions and the acquisition of large statistical data sets from individual experimental runs.

Simultaneous magnetic manipulation and fluorescent tracking of multiple individual hybrid nanostructures.

Controlled transport of multiple individual nanostructures is crucial for nanoassembly and nanodelivery but is challenging because of small particle size. Although atomic force microscopy and optical and magnetic tweezers can control single particles, it is extremely difficult to scale these technologies for multiple structures. Here, we demonstrate a "nano-conveyer-belt" technology that permits simultaneous transport and tracking of multiple individual nanospecies in a selected direction. The technology consists of two components: nanocontainers, which encapsulate the nanomaterials transported, and nanoconveyer arrays, which use magnetic force to manipulate individual or aggregate nanocontainers. This technology is extremely versatile. For example, nanocontainers encapsulate quantum dots or rods and superparamagnetic iron oxide nanoparticles in <100 nm nanocontainers, the smallest magnetic composites to have been simultaneously moved and optically tracked. Similarly, the nanoconveyers consist of patterned microdisks or zigzag nanowires, whose dimensions can be controlled through micropatterning. The nanoconveyer belt technology could impact multiple fields, including nanoassembly, biomechanics, nanomedicine, and nanofluidics.

Ultrafast disordering of vanadium dimers in photoexcited VO2.

Many ultrafast solid phase transitions are treated as chemical reactions that transform the structures between two different unit cells along a reaction coordinate, but this neglects the role of disorder. Although ultrafast diffraction provides insights into atomic dynamics during such transformations, diffraction alone probes an averaged unit cell and is less sensitive to randomness in the transition pathway. Using total scattering of femtosecond x-ray pulses, we show that atomic disordering in photoexcited vanadium dioxide (VO2) is central to the transition mechanism and that, after photoexcitation, the system explores a large volume of phase space on a time scale comparable to that of a single phonon oscillation. These results overturn the current understanding of an archetypal ultrafast phase transition and provide new microscopic insights into rapid evolution toward equilibrium in photoexcited matter.

Skewed brownian fluctuations in single-molecule magnetic tweezers.

Measurements in magnetic tweezers rely upon precise determination of the position of a magnetic microsphere. Fluctuations in the position due to Brownian motion allows calculation of the applied force, enabling deduction of the force-extension response function for a single DNA molecule that is attached to the microsphere. The standard approach relies upon using the mean of position fluctuations, which is valid when the microsphere axial position fluctuations obey a normal distribution. However, here we demonstrate that nearby surfaces and the non-linear elasticity of DNA can skew the distribution. Through experiment and simulations, we show that such a skewing leads to inaccurate position measurements which significantly affect the extracted DNA extension and mechanical properties, leading to up to two-fold errors in measured DNA persistence length. We develop a simple, robust and easily implemented method to correct for such mismeasurements.

Magnetic forces and DNA mechanics in multiplexed magnetic tweezers.

Magnetic tweezers (MT) are a powerful tool for the study of DNA-enzyme interactions. Both the magnet-based manipulation and the camera-based detection used in MT are well suited for multiplexed measurements. Here, we systematically address challenges related to scaling of multiplexed magnetic tweezers (MMT) towards high levels of parallelization where large numbers of molecules (say 10(3)) are addressed in the same amount of time required by a single-molecule measurement. We apply offline analysis of recorded images and show that this approach provides a scalable solution for parallel tracking of the xyz-positions of many beads simultaneously. We employ a large field-of-view imaging system to address many DNA-bead tethers in parallel. We model the 3D magnetic field generated by the magnets and derive the magnetic force experienced by DNA-bead tethers across the large field of view from first principles. We furthermore experimentally demonstrate that a DNA-bead tether subject to a rotating magnetic field describes a bicircular, Limaçon rotation pattern and that an analysis of this pattern simultaneously yields information about the force angle and the position of attachment of the DNA on the bead. Finally, we apply MMT in the high-throughput investigation of the distribution of the induced magnetic moment, the position of attachment of DNA on the beads, and DNA flexibility. The methods described herein pave the way to kilo-molecule level magnetic tweezers experiments.