13 research outputs found
Floquet Topological Polaritons in Semiconductor Microcavities
We propose and model Floquet topological polaritons in semiconductor
microcavities, using the interference of frequency detuned coherent fields to
provide a time periodic potential. For arbitrarily weak field strength, where
the Floquet frequency is larger than the relevant bandwidth of the system, a
Chern insulator is obtained. As the field strength is increased, a topological
phase transition is observed with an unpaired Dirac cone proclaiming the
anomalous Floquet topological insulator. As the relevant bandwidth increases
even further, an exotic Chern insulator with flat band is observed with
unpaired Dirac cone at the second critical point. Considering the polariton
spin degree of freedom, we find that the choice of field polarization allows
oppositely polarized polaritons to either co-propagate or counter-propagate in
chiral edge states.Comment: Accepted by PR
Qualitative chirality effects on the Casimir-Lifshitz torque with liquid crystals
We model a cholesteric liquid crystal as a planar uniaxial multilayer system,
where the orientation of each layer differs slightly from that of the adjacent
one. This allows us to analytically simplify the otherwise acutely complicated
calculation of the Casimir-Lifshitz torque. Numerical results differ
appreciably from the case of nematic liquid crystals, which can be treated like
bloc birefringent media. In particular, we find that the torque deviates
considerably from its usual sinusoidal behavior as a function of the
misalignment angle. In the case of a birefringent crystal faced with a
cholesteric liquid one, the Casimir-Lifshitz torque decreases more slowly as a
function of distance than in the nematic case. In the case of two cholesteric
liquid crystals, either in the homochiral or in the heterochiral configuration,
the angular dependence changes qualitatively as a function of distance. In all
considered cases, finite pitch length effects are most pronounced at distances
of about 10 nm
Roughness correction to the Casimir force beyond perturbation theory
Up to now there has been no reliable method to calculate the Casimir force
when surface roughness becomes comparable with the separation between bodies.
Statistical analysis of rough Au films demonstrates rare peaks with heights
considerably larger than the root-mean-square (rms) roughness. These peaks
define the minimal distance between rough surfaces and can be described with
extreme value statistics. We show that the contributions of high peaks to the
force can be calculated independently of each other while the contribution of
normal roughness can be evaluated perturbatively beyond the proximity force
approximation. The developed method allows a reliable force estimation for
short separations. Our model explains the strong hitherto unexplained deviation
from the normal Casimir scaling observed experimentally at short separations.Comment: 6 pages, 2 figures, to be published in EP
Roughness correction to the Casimir force at short separations: Contact distance and extreme value statistics
So far there has been no reliable method to calculate the Casimir force at
separations comparable to the root-mean-square of the height fluctuations of
the surfaces. Statistical analysis of rough gold samples has revealed the
presence of peaks considerably higher than the root-mean-square roughness.
These peaks redefine the minimum separation distance between the bodies and can
be described by extreme value statistics. Here we show that the contribution of
the high peaks to the Casimir force can be calculated with a pairwise additive
summation, while the contribution of asperities with normal height can be
evaluated perturbatively. This method provides a reliable estimate of the
Casimir force at short distances, and it solves the significant, so far
unexplained discrepancy between measurements of the Casimir force between rough
surfaces and the results of perturbation theory. Furthermore, we illustrate the
importance of our results in a technologically relevant situation.Comment: 29 pages, 11 figures, to appear in Phys. Rev.
Nonlinear Actuation Dynamics of Driven Casimir Oscillators with Rough Surfaces
At separations below 100 nm, Casimir-Lifshitz forces strongly influence the actuation dynamics of microelectromechanical systems (MEMS) in dry vacuum conditions. For a micron-size plate oscillating near a surface, which mimics a frequently used setup in experiments with MEMS, we show that the roughness of the surfaces significantly influences the qualitative dynamics of the oscillator. Via a combination of analytical and numerical methods, it is shown that surface roughness leads to a clear increase of initial conditions associated with chaotic motion, that eventually lead to stiction between the surfaces. Since stiction leads to a malfunction of MEMS oscillators, our results are of central interest for the design of microdevices. Moreover, stiction is of significance for fundamentally motivated experiments performed with MEMS
The Casimir force and micro-electromechanical systems at submicron-scale separations
Quantum mechanics teaches us that vacuum is not empty. Rather, it contains all kinds of virtual particles. The energy of such particles is called zero point energy. If two surfaces come in close proximity of each other, they will create a difference between the zero point energies in between them and on the outside. Consequently the surfaces will be pushed toward each other. This phenomenon is known as the Casimir effect. It has turned out to be a generalization of the more familiar van der Waals force. Present technology has only recently enabled us to measure the Casimir force directly. Part of this thesis is about a complication that arises in such measurements: a real surface does not have a nice, smooth shape, but it is often rough. Surface roughness influences the Casimir force mainly at relatively short distances of one ten millionth meter or less. This influence is predominantly determined by statistically rare high asperities in the surfaces. This thesis introduces a model that reproduces measurements of the Casimir force between rough surfaces. The Casimir force is unavoidable: the existence of virtual particles cannot be shut down in any way. The smaller the distance between the surfaces, the larger the Casimir force. Hence at short distances it is of interest for technology of micro electromechanical systems (MEMS), such as micro switches or accelerometers. The Casmir force has a considerable influence on the motion of MEMS components at short distances. Surface roughness turns out to make this motion more predictable
Natural modes and resonances in a dispersive stratified N-layer medium
The properties of the natural modes in a dispersive stratified N-layer medium are investigated. The focus is on the (over) completeness properties of these modes. Also the distribution of the natural frequencies is considered. Both the degree of (over) completeness and the natural frequency distribution turn out to be totally different from what is known for the non-dispersive case
Significance of the Casimir force and surface roughness for actuation dynamics of MEMS
<p>Using the measured optical response and surface roughness topography as inputs, we perform realistic calculations of the combined effect of Casimir and electrostatic forces on the actuation dynamics of microelectromechanical systems (MEMS). In contrast with the expectations, roughness can influence MEMS dynamics, even at distances between bodies significantly larger than the root-mean-square roughness. This effect is associated with statistically rare high asperities that can be locally close to the point of contact. It is found that even though surface roughness appears to have a detrimental effect on the availability of stable equilibria, it ensures that those equilibria can be reached more easily than in the case of flat surfaces. Hence our findings play a principal role for the stability of microdevices such as vibration sensors, switches, and other related MEM architectures operating at distances below 100 nm.</p>