9 research outputs found
Sub-harmonic resonant excitation of confined acoustic modes at GHz frequencies with a high-repetition-rate femtosecond laser
We propose sub-harmonic resonant optical excitation with femtosecond lasers
as a new method for the characterization of phononic and nanomechanical systems
in the gigahertz to terahertz frequency range. This method is applied for the
investigation of confined acoustic modes in a free-standing semiconductor
membrane. By tuning the repetition rate of a femtosecond laser through a
sub-harmonic of a mechanical resonance we amplify the mechanical amplitude,
directly measure the linewidth with megahertz resolution, infer the lifetime of
the coherently excited vibrational states, accurately determine the system's
quality factor, and determine the amplitude of the mechanical motion with
femtometer resolution
Laser microfluidics: fluid actuation by light
The development of microfluidic devices is still hindered by the lack of
robust fundamental building blocks that constitute any fluidic system. An
attractive approach is optical actuation because light field interaction is
contactless and dynamically reconfigurable, and solutions have been anticipated
through the use of optical forces to manipulate microparticles in flows.
Following the concept of an 'optical chip' advanced from the optical actuation
of suspensions, we propose in this survey new routes to extend this concept to
microfluidic two-phase flows. First, we investigate the destabilization of
fluid interfaces by the optical radiation pressure and the formation of liquid
jets. We analyze the droplet shedding from the jet tip and the continuous
transport in laser-sustained liquid channels. In the second part, we
investigate a dissipative light-flow interaction mechanism consisting in
heating locally two immiscible fluids to produce thermocapillary stresses along
their interface. This opto-capillary coupling is implemented in adequate
microchannel geometries to manipulate two-phase flows and propose a contactless
optical toolbox including valves, droplet sorters and switches, droplet
dividers or droplet mergers. Finally, we discuss radiation pressure and
opto-capillary effects in the context of the 'optical chip' where flows,
channels and operating functions would all be performed optically on the same
device
Lattice Dynamics of Laser Excited Ferroelectric
We investigated the lattice dynamics of the prototypic ferroelectric barium titanate close to its ferroelectric-paraelectric phase transition aiming at a better understanding of the atomistic nature of the transition. The usage of time-resolved X-ray techniques allows to disentangle lattice motion and unit cell changes, which, in part, relate to the ferroelectric polarization. In the quasi-static case both the electrical and the laser excitation show a mean-field, simple thermal behaviour, while for time scales shorter than nanoseconds the impulsive nature of the excitation becomes visible
Laser microfluidics: fluid actuation by light
The development of microfluidic devices is still hindered by the lack of robust fundamental building blocks that constitute any fluidic system. An attractive approach is optical actuation because light field interaction is contactless and dynamically reconfigurable, and solutions have been anticipated through the use of optical forces to manipulate microparticles in flows. Following the concept of an 'optical chip' advanced from the optical actuation of suspensions, we propose in this survey new routes to extend this concept to microfluidic two-phase flows. First, we investigate the destabilization of fluid interfaces by the optical radiation pressure and the formation of liquid jets. We analyze the droplet shedding from the jet tip and the continuous transport in laser-sustained liquid channels. In the second part, we investigate a dissipative light-flow interaction mechanism consisting in heating locally two immiscible fluids to produce thermocapillary stresses along their interface. This opto-capillary coupling is implemented in adequate microchannel geometries to manipulate two-phase flows and propose a contactless optical toolbox including valves, droplet sorters and switches, droplet dividers or droplet mergers. Finally, we discuss radiation pressure and opto-capillary effects in the context of the 'optical chip' where flows, channels and operating functions would all be performed optically on the same device