D. Iannuzzi

Casimir Forces and Quantum Electrodynamical Torques: Physics and Nanomechanics

This paper discusses recent developments on quantum electrodynamical (QED) phenomena, such as the Casimir effect, and their use in nanomechanics and nanotechnology in general. Casimir forces and torques arise from quantum fluctuations of vacuum or, more generally, from the zero-point energy of materials and their dependence on the boundary conditions of the electromagnetic fields. Because the latter can be tailored, this raises the interesting possibility of designing QED forces for specific applications. After a concise review of the field in an historical perspective, high precision measurements of the Casimir force using microelectromechanical systems (MEMS) technology and applications of the latter to nonlinear oscillators are presented, along with a discussion of its use in nanoscale position sensors. Then, experiments that have demonstrated the role of the skin-depth effect in reducing the Casimir force are presented. The dielectric response of materials enters in a nonintuitive way in the modification of the Casimir-Lifshitz force between dielectrics through the dielectric function at imaginary frequencies epsiv(ixi). The latter is illustrated in a dramatic way by experiments on materials that can be switched between a reflective and a transparent state (hydrogen switchable mirrors). Repulsive Casimir forces between solids separated by a fluid with epsiv(ixi) intermediate between those of the solids over a large frequency range is discussed, including ongoing experiments aimed at its observation. Such repulsive forces can be used to achieve quantum floatation in a virtually frictionless environment, a phenomenon that could be exploited in innovative applications to nanomechanics. The last part of the paper deals with the elusive QED torque between birefringent materials and efforts to observe it. We conclude by highlighting future important directions

Monolithic fiber-top sensor for critical environments and standard applications

We present a monolithic device obtained by carving a cantilever on the top of a single-mode optical fiber. We show that the vertical position of the cantilever can be determined with accuracy comparable to atomic force microscopes and other commonly used scientific instruments. The device does not require any alignment procedure and can be used in critical environments as well as in standard applications.

Halving the Casimir force with Conductive Oxides

The possibility to modify the strength of the Casimir effect by tailoring the dielectric functions of the interacting surfaces is regarded as a unique opportunity in the development of micro- and nanoelectromechanical systems. In air, however, one expects that, unless noble metals are used, the electrostatic force arising from trapped charges overcomes the Casimir attraction, leaving no room for exploitation of Casimir force engineering at ambient conditions. Here we show that, in the presence of a conductive oxide, the Casimir force can be the dominant interaction even in air, and that the use of conductive oxides allows one to reduce the Casimir force up to a factor of 2 when compared to noble metals.

Effect of hydrogen-switchable mirrors on the Casimir force

We present systematic measurements of the Casimir force between a gold-coated plate and a sphere coated with a hydrogen-switchable mirror. Hydrogen-switchable mirrors are shiny metals that can become transparent upon hydrogenation. Despite such a dramatic change of the optical properties of the sphere, we did not observe any significant decrease of the Casimir force after filling the experimental apparatus with hydrogen. This counterintuitive result can be explained by the Lifshitz theory that describes the Casimir attraction between metallic and dielectric materials.

Computation and Visualization of Casimir Forces in Arbitrary Geometries: Nonmonotonic Lateral-Wall Forces and the Failure of Proximity-Force Approximations

We present a method of computing Casimir forces for arbitrary geometries, with any desired accuracy, that can directly exploit the efficiency of standard numerical-electromagnetism techniques. Using the simplest possible finite-difference implementation of this approach, we obtain both agreement with past results for cylinder-plate geometries, and also present results for new geometries. In particular, we examine a pistonlike problem involving two dielectric and metallic squares sliding between two metallic walls, in two and three dimensions, respectively, and demonstrate nonadditive and nonmonotonic changes in the force due to these lateral walls.

Torque on birefringent plates induced by quantum fluctuations

We present detailed numerical calculations of the mechanical torque induced by quantum fluctuations on two parallel birefringent plates with in-plane optical anisotropy, separated by either vacuum or a liquid (ethanol). The torque is found to vary as sin(2{theta}), where {theta} represents the angle between the two optical axes, and its magnitude rapidly increases with decreasing plate separation d. For a 40 {mu}m diameter disk, made out of either quartz or calcite, kept parallel to a barium titanate plate at d{approx_equal}100 nm, the maximum torque (at {theta}={pi}/4) is of the order of {approx_equal}10{sup -19} N m. We propose an experiment to observe this torque when the barium titanate plate is immersed in ethanol and the other birefringent disk is placed on top of it. In this case the retarded van der Waals (or Casimir-Lifshitz) force between the two birefringent slabs is repulsive. The disk would float parallel to the plate at a distance where its net weight is counterbalanced by the retarded van der Waals repulsion, free to rotate in response to very small driving torques.

Carving fiber-top optomechanical transducers from an optical fiber

In a recent paper (Iannuzzi et al 2006 Monolithic fiber-top sensor for critical environments and standard applications Appl. Phys. Lett. 88 053501) we have presented the principle of the fiber-top position sensor, having a monolytical structure carved out of a single-mode optical fiber. The device alleviates sensing in a critical environment via interferometric readout, and because of its monolithic construction, facilitates plug-and-play utilization without alignment. In this paper we describe the fabrication method of a similar device which, however, was also equipped with a pyramidal tip on the top of the cantilever, an important detail for future implementation in scanning probe microscopy. A silicon surface was then periodically put in contact and moved out of contact with the device. The output signal resembles force curves that can be similarly obtained with atomic force microscopes.

Fiber-top atomic force microscope

We present the implementation of an atomic force microscope (AFM) based on fiber-top design. Our results demonstrate that the performances of fiber-top AFMs in contact mode are comparable to those of similar commercially available instruments. Our device thus represents an interesting alternative to existing AFMs, particularly for applications outside specialized research laboratories, where a compact, user-friendly, and versatile tool might often be preferred.

Observation of the skin-depth effect on the Casimir force between metallic surfaces

We have performed measurements of the Casimir force between a metallic plate and a transparent sphere coated with metallic films of different thicknesses. We have observed that, if the thickness of the coating is less than the skin-depth of the electromagnetic modes that mostly contribute to the interaction, the force is significantly smaller than that measured with a thick bulk-like film. Our results provide direct evidence of the skin-depth effect on the Casimir force between metallic surfaces.

No anomalous scaling in electrostatic calibrations for Casimir force measurements

In a recent paper [Phys. Rev. A 78, 020101(R) (2008)], Kim et al. have reported a large anomaly in the scaling law of the electrostatic interaction between a sphere and a plate, which was observed during the calibration of their Casimir force setup. Here we experimentally demonstrate that this behavior is not universal. Electrostatic calibrations obtained with our setup follow the scaling law expected from elementary electrostatic arguments, even when the electrostatic voltage that one must apply to minimize the force (typically ascribed to contact potentials) depends on the separation between the surfaces.