Stability of optically bound cluster, in the Lyapunov sense, is governed by the force constant matrix (or stiffness matrix), which is the first order Taylor Series Expansion about equilibrium, and it determined the stability and vibration frequencies.
The openness of light matter interaction, i.e. the incident wave propagates, the scattered wave propagates outward, and absorption, caused non-conservative forces, making the force constant matrix non-symmetric (non-Hermitian). This opens up the possibility of having unstable vibrational modes with complex frequencies when the system is driven beyond its exceptional point, i.e., the non-conservative force is strong compare to the “gap” between adjacent vibrational frequencies.
Noting that in large scale optical binding, the “gap” goes to zero. The Lorentz force alone will not be sufficient in general to ensure the stability of optically bound structure. Other forces, such as hydrodynamic damping in water, is essential.
Fabrication of 3D chiral nanoplasmonic structures is always challenging, while the principles for their chiroptical properties are still ambiguous.
We will present a combined experimental and theoretical study on 3D chiroplasmonic activity of silver nanospirals (AgNSs), fabricated on sapphire by low temperature glancing angle deposition. AgNSs exhibit bisignated CD spectra in the UV-visible range, in the form of not only individual AgNSs or an array. Compared to individual AgNSs, the array of AgNSs show CD with an intensity 3 order of magnitude higher. It is demonstrated the engineering of chiroplasmonic CD via adjusting spiral parameters, including spiral pitches, number of turns and handedness.
Finite element simulations were performed and are in good agreement with the experiments. A LC theory is also employed to explain the difference of chiroplasmonic CD in the UV and visible region.
The transverse force profile of a particle in an optical trap is important for the designs of optical trapping-based force
transducers. We mapped these force profiles for micron-size polystyrene beads using a pair of overlapping optical traps
produced by two highly focused Gaussian beams with unequal intensity; the stronger trap serves as a force transducer to
measure the force of the weaker trap in both linear and nonlinear regimes. For particles with size smaller or comparable
to the laser wavelength, the force profiles follow closely the gradient of the Gaussian profile, but as the particle size
increases, the force profiles deviate from the shape of the gradient of Gaussian for the distance beyond the position of the
maximum force. The distance from the center of the trap to the position of the maximum trapping force was found to
increase linearly with the particle size. The experimental results are in good agreements with our theoretical model,
based on a combination of the Mie theory, vector Debye integral, and Maxwell stress tensor; except that the
experimental particle-size dependence of the maximum trapping forces was found to be weaker than that predicted by
Optical binding has been proposed to be responsible for the cluster formation of micron size dielectric
spheres in coherent light fields. However, a direct measurement of the forces involved in binding is
missing. We report an experimental study of optical binding forces between two optically trapped
dielectric spheres. Results for optical forces are presented as a function of three parameters: inter
particle separation, particle size, and respective polarizations. A comprehensive calculation based on
the generalized Mie scattering theory for the experiment has also been conducted. This paper will
present a comparison between experimental and theoretical results.