Vacuum-induced torque between corrugated metallic

plates Robson B. Rodrigues

Federal University of Rio de Janeiro, Brazil

QFEXT07University of Leipizig, Germany

Collaborators:Paulo A. Maia Neto

Federal University of Rio de Janeiro, Brazil

and

Astrid Lambrecht and Serge Reynaud

Laboratoire Kastler Brossel, CNRS, ENS, UPMC, Paris, France

Based on the paper:Europhys. Lett., 76 (5), pp. 822–828 (2006)

Motivations - Aplication in Mems/Nems

•The Casimir effect plays a major role in micro- and nano-electromechanical systems (MEMS and NEMS).

•The Casimir force could lead to stiction of Mems/Nems components that results in permanent adhesion of nearby surface elements…

•...but can also be used to actuate components of small devices without contact.

Examples of Actuation of Mems by the Casimir Force:

Motivations - Aplication in Mems/Nems

• Actuator powered by the normal Casimir force between a flat plate and a sphere :

Chan, Aksyuk, Kleiman, Bishop, Capasso,

Science 291, 1943 (2001).

Figure from www.seas.harvard.edu/capasso/

Examples of Actuation of Mems by the Casimir Force:

Motivations - Aplication in Mems/Nems

• “Non-contact rack and pinion powered by the lateral Casimir force”: Ashourvan, Miri, Golestanian, Phys. Rev. Lett. 98, 140801 (2007) [1]. (talk by Golestanian).

• “Casimir force driven ratchets”: T. Emig, Phys. Rev. Lett. 98, 160801 (2007) [2].

• Figures from [1] and [2].

New schemes for actuation: Lateral Casimir force between corrugated metallic plates : Chen, Mohideen, Klimchitskaya and Mostepanenko, Chen, Mohideen, Klimchitskaya and Mostepanenko,

Phys. Rev. Lett 88,88, 101801 (2002) 101801 (2002) :F ~ 0.3 pN for L ~ 200 nm, Corrugation amplitudes ~ 8 nm, 59 nm

F

FL

Courtesy of S. Reynaud

a

Motivations - Aplication in Mems/Nems

• When the corrugations are not aligned, the rotational symmetry is broken and a Casimir torque arise to minimize the system’s energy.

L

• Casimir torque : new mechanism of micro-mechanical control to be exploited in the design of MEMS and NEMS.

Courtesy of S. Reynaud

Motivations - Investigation of the non-trivial geometry dependence of the Casimir energy

• The Proximity Force Approximation (PFA) connect plane-plane (PP) and curved geometries. Basic idea: replace the curved surface by a set of differential planes, and add contributions of each plane. Valid for corrugation wavelengths>>plate separation.

• (Talk by Reynaud, Talk by Dalvit): Scattering approach for non-planar surfaces is a theoretical model beyond the PFA. Perturbative approach requires but allows arbitrary corrugation wavelengths with respect to plate separation.

• Others approachs beyond PFA: worldline numerics, Gies, Klingmuller, Phys.Rev.Lett. (2006); talk by Bordag.

Other proposal: torque between birefringent plates

• J N Munday, D Iannuzzi and F Capasso, New Journal of Physics 8 (2006) 244 [3].

• Y. Barash, Izv. Vyssh. Uchebn. Zaved., Radiofiz., 12, 1637 (1978).

•The Casimir energy depend on the relative orientation of the optical axes of the materials.•This leads to a torque that tends to align two of the principal axes of the materials in order to minimize the system’s energy.•Maximum torque per unit area (L=100nm) :

Figure from [3]

Scattering approach (flat plates)

C. Genet, A. Lambrecht and S. Reynaud, Phys. Rev. A67, 043811 (2003).

Applies for dissipative and/or magnetic media;Applies for dissipative and/or magnetic media; Lifshitz Lifshitz

(1956) in a particular case (Lifshitz formula)(1956) in a particular case (Lifshitz formula)

Scattering approach with rough mirrors (Stochastic roughness correction)

P. A. Maia Neto, A. Lambrecht and S. Reynaud, Phys. Rev. A72, 012115 (2005).

Casimir energy obtained from non-specular reflection coefficients; Perturbation expansion in powers of , up

to second order

Extension to rough mirrors (perturbative theory)

We follow the approach of :

Basic Formalism - Principal References

z

ZZ=0

Example: mirror 1

Lateral Casimir force beyond the PFA: R. B. Rodrigues, P. A. Maia Neto, A. Lambrecht and S. Reynaud Phys. Rev. Lett. 96, 100402 (2006); Phys. Rev. A75, 062108 (2007). (Talk by Reynaud)

Casimir energy:

Perturbation theory holds for :

Casimir energy between corrugated plates

Second-order correction to the Casimir energy

• is the Fourier transform of .•The response function is a function of the specular and non-specular reflection coefficients and does not depend on the direction of the corrugation wave vector K.

Casimir torque between corrugated plates

Corrugation wave vectors have the same modulus:

The corrugations have sinusoidal shapes:

: lateral displacements with respect to the configuration with a line of maximum height at the origin

: angle between and (angular mismatch between the two corrugations)

Conventions:

Casimir torque between corrugated plates

Corrugation is imprinted on a very large plate :

Corrugation is restricted to a section of area centered at

b is the relative lateral displacement along the direction of

The energy does not depend on displacements perpendicular to and is invariant under and

Limit of long corrugation lines: .

Let us take ( is negligible otherwise) and

Casimir torque between corrugated plates

The scale of variation of is set by

Casimir torque between corrugated plates

•The Casimir energy is minimum at and

and and (shallow wells)

•For , the plate is attracted back to without sliding laterally.•For , its motion will be a combination rotation and lateral displacement.

It is maximum at ,

where it is given by:

Casimir torque between corrugated plates

Torque = (optimum)

•We take , , .

•Dotted line : . At L = 100nm,

(3 orders of magnitude larger than the torque/area for anisotropic plates).

•Dashed line: optimum torque; occurs at (maximizing ).

•Solid line: optimum torque for

(in this case . ).

•PFA holds for smooth surfaces: or .•Response function satisfies

Casimir torque between corrugated plates

Perfect reflectors

PFA

Scaterringapproach•Solid line: Torque as a function of k for L = 1μm

•Dashed line: model with perfect reflectors. Overestimates the torque by 16% near thepeak region.•Dotted line: PFA. Overestimates the torque by 103% at the peak value k = 2.6/L

Under optimum conditions, the torque probes a non-trivial geometry dependence of the Casimir energy

Torque (PFA):

Conclusions

• Casimir torque between corrugated metallic plates: may provide a new mechanism of micro-mechanical control to be exploited in the design of MEMS and NEMS.

• The torque is up to three orders of magnitude larger than the torque between anisotropic dielectric plates for comparable distance and area.

• An experimental observation of the Casimir torque with seems feasible.

• The PFA grossly overestimates the optimum torque by a factor of the order of 2.