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Interferometry Measures a Bubble Without Bursting It

Photonics Spectra
Aug 2006
Hank Hogan

Guy Ropars, a researcher at Centre National de la Recherche Scientifique (CNRS) at Université de Rennes 1 in France, has been blowing soap bubbles. He does so to measure the thickness of the film that is the bubble’s skin, which varies over time and space as a result of gravity and capillary action.

Blowing bubbles may seem like child’s play, but studying them could pay serious dividends. Thin films play a part in the biology of cell membranes and in the action of industrial foams.

 Unfortunately, measuring the skin of a bubble is not easy. Various optical techniques, such as transmission interferometry and infrared absorption, offer detailed information about the thickness over time but not space. So Ropars and his colleagues at the center and at the University of Rochester in New York have developed a method as part of a collaboration of CNRS and the US National Science Foundation.

They turned to a multiwave dual interferometer, also known as a Jamin-Fabry-Perot interferometer. Developed by CNRS, the setup features birefringent YVO4 crystals placed between a pair of 99 percent reflective mirrors.

AASoap.jpg
Using a multiwave dual interferometer, researchers measured a soap bubble film at two spots and determined how rapidly its thickness changed. A typical soap film has two zones: color and black. The first has a large average value of thickness gradient: about 20 nm/mm for a 3-mm separation of vertical probe points. The color region sits above the silvery-gray zone, which has gradient near zero. Courtesy of Guy Ropars.


To study soap films, the investigators inject a 1064-nm beam from an Innolight GmbH Nd:YAG laser into the interferometer. The laser beam travels through the mirror and through one of the birefringent crystals. Because the polarization of the beam is at 45° to the neutral axis of the crystal, it splits into two beams with a 3-mm distance between them. These traverse the sample and then recombine in the second birefringent crystal, which is aligned at the same angle as the first. The single beam strikes the second mirror, with a small percentage of the light passing through to a detector and the rest bouncing back into the cavity.

By attaching a piezoelectric transducer to one of the mirrors, the investigators can change the cavity length of the interferometer. By rotating the crystals 90°, they can switch the separation from vertical to horizontal.

The measurement sensitivity of the instrument is related to the finesse of the cavity, which is about 30 and results in a resolution of about 1 nm. If the finesse were increased tenfold and other improvements were made, Ropars said, thickness variations of 0.1 nm could be measured. He noted that achieving the increase should not be difficult.

“With empty Fabry-Perot cavities, finesse up to 1000 can be routinely obtained,” he said. “Using shorter YVO4 crystals with high antireflection coatings, one can expect to obtain a finesse of 300.”

In a demonstration of the technique reported in the June 5 issue of Applied Physics Letters, the group looked at soap films draining from a frame. When that happens, two zones are visible under white light. One has colorful parallel fringes, and the second appears black because it does not reflect light. This black zone can be a relatively thick, and common, black film, or it can be a microscopically thin Newton black film.

The researchers placed a frame with a soap film in the sample area of the interferometer. As the soap drained, the separating line between the two zones moved downward and passed the dual beams in the interferometer, enabling the investigators to make gradient and thickness measurements.

They discovered that their soap films were about 600 nm thick in the color area and 50 nm thick in the black zone. The gradient was about 17 to 20 nm/mm, with large fluctuations at the end of the color zone and essentially no fluctuations in the black zone.

With the method proved and with its suitability for measurements in the nanometer range, Ropars said that the scientists will adapt the approach for the study of other thin films, including cell membranes.

Contact: Guy Ropars, Université de Rennes 1, Rennes, France; +33 2 2323 5574; e-mail: guy.ropars@univ-rennes1.fr.


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