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Holographic detection of electromagnetic oscillations of the vacuum

Dynamic holography reveals the Casimir force between two metallic bodies, and can exploit light pressure to measure its absolute magnitude.
12 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200701.0500

Quantum mechanics states that there are so-called zero-point electromagnetic oscillations even in vacuum. These electromagnetic fluctuations are an intrinsic property of vacuum, and do not originate from any source. They cannot be absorbed and cannot be a source of energy. Nevertheless, the electromagnetic vacuum fluctuations can be detected, for instance by very precise optical measurements of the spectral lines of hydrogen.1

Further evidence of electromagnetic fluctuations is the so-called Casimir force,2 which is an attraction between two metallic plates separated by a short distance (of the order of a hundred nanometers). The attraction occurs because the the plates lower the energy density of the zero-point fluctuations between them, compared to the energy density outside. Of course, this force is not strong. For instance, if the distance between plates is 500nm, the pressure due to the Casimir force is of the order of 10−4g/cm2. Nevertheless, this tiny force can be detected by modern experimental techniques such as atomic-force microscopy.3

Such investigations verify fundamental theories concerning quantum mechanics and the nature of vacuum, and also have important practical implications. For example, during the last decade new types of mechanical device—microelectromechanical systems—have emerged, and the next step is nanomechanical devices. The Casimir force can be one factor limiting integration of the latter, but can also be the basis of new devices. This is a powerful stimulus for studies of the Casimir force.

A team of scientists from Russia (Ioffe Physical-Technical Institute of the Russian Academy of Sciences, St Petersburg) and Germany (Institute of Applied Physics, Darmstadt University of Technology) invented a new technique for Casimir-force detection.4 They studied how oscillations of one conducting body can generate mechanical oscillations of another conducting body if there are no other forces between them except the Casimir force. The experimental difficulty was that displacements of the second body were very small, of the order of 0.1nm (10−8 cm), so a very sensitive method for measuring surface vibrations had to be used.

To measure these small oscillations, the authors4 used a new principle, dynamic holography, in which a laser beam is directed onto the vibrating surface. The reflected beam acquires a phase modulation that can be transferred into amplitude modulation of the light by the holographic process occurring in a photorefractive BaTiO3 crystal. This method is very sensitive and eliminates low-frequency environmental fluctuations.

In the experiments, oscillations of the second body were observed at the fundamental and doubled frequencies of the oscillations of the first body. This is the consequence of the nonlinear dependence of the Casimir force on distance, which causes an anharmonic Casimir force and anharmonic oscillations of the second body. The data were in good agreement with theoretical predictions.

The remaining experimental challenge is measuring the absolute value of the Casimir force. We have proposed4 using light pressure as a natural and precise etalon. In the first experiments of this type, the accuracy was not very high, but the idea seems promising. Modifications of this technique could provide very detailed investigations of the Casimir force for different materials (not just highly conducting metals). The technique is flexible enough to explore the force between nonplanar components, such as those encountered in micro- and nano-electromechanical systems.

In conclusion, it should be emphasized that very weak physical effects, such as light pressure and the Casimir force, can have important practical implications.

Mikhail P. Petrov
Quantum Electronics, Ioffe Physico-Technical Institute
St. Petersburg, Russia