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Multimode nanomechanics in conservative and non-conservative force fields

Event Details

Event Dates: 

Monday, May 15, 2017 - 11:00am

Seminar Location: 

  • JILA X317

Speaker Name(s): 

Laure De Lepinay

Speaker Affiliation(s): 

Institut Neel, Centre national de la recherche scientifique, Grenoble FR
Seminar Type/Subject

Scientific Seminar Type: 

  • Other

Seminar Type Other: 

JILA Internal Meeting

Event Details & Abstract: 

Force sensors based on nanomechanical oscillators do not only exhibit exceptional sensitivities, they are also very readily affected by the vectorial character of the investigated force fields. This particularly holds for nearly degenerate multidimensional oscillators such as nanotubes or nanowires, optically trapped particles, or nanobeams under stress. We study the phenomenology of the mechanical dressing of a 2D oscillator (a singly clamped SiC nanowire oscillating along two transverse directions with quasi-degenerated eigenfrequencies) by a 2D force field through an analytical model. This model reveals a new physical richness specific of dimensions greater than one. To investigate these predictions, we developed a method to measure all the components of the nanowire's motion in 2D, based on the optical detection of two non-collinear projections of the motion on a quadrant photodiode. We then first tested our understanding of the mechanical dressing by measuring all vectorial components of an electrostatic force field. The automatization of the measurement procedure allowed to map out the force and observe all predicted signatures in such a conservative force field. Second, we  characterized the dressing phenomenology in a static, non-conservative, rotational force field established by immersing the nanowire in a tightly focused optical field. Dramatic modifications of both the probe thermal noise and driven dynamics were observed, as the eigenmodes lose their initial orthogonality in regions of strong vorticity. As the system is driven out of equilibrium, anomalous thermal noise spectra strongly deviating from the normal mode expansion were measured as well as an excess of noise which violates the equality of the fluctuation-dissipation theorem. This violation was characterized by measuring both the susceptibility and the thermal noise of the system using a protocol adapted to its multidimensionality. While the Harada-Sasa equality very generally relates the fluctuation-dissipation relation violation to the entropy production rate, we developed another, specific approach that suggests a geometrical explanation to the violation in rotationally coupled systems based on the emergence of transverse susceptibilities. Finally, we theoretically derived the existence of a noise compression phenomenon in non-conservative force fields. The system can therefore be used both as an ultrasensitive measurement technique for imaging bidimensional force fields and as a simple platform to test stochastic thermodynamic equalities out of equilibrium. 

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