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Immersion enhances holographic microscopy

08 May 2006

Researchers say that they have found a short-cut to achieving higher resolutions; one that could improve imaging resolutions by up to 80%.

Researchers in Canada have demonstrated a new way of improving the image resolution obtained from digital in-line holographic microscopy (DIHM) (Optics Letters 31, 1211).

"By using immersion oil as the filling medium, an improvement of 50% in resolution can be easily achieved," one of the researchers, Jorge Garcia-Sucerquia of Dalhousie University told optics.org.

One way to obtain higher resolutions is to use a shorter wavelength light source. The advantage the new technique, according to the researchers, is that it is cheaper and eliminates the need to provide the much smaller pinhole and high laser power associated with shorter wavelength sources.

The team borrowed its approach from optical microscopy. Here, in order to improve resolution, materials with higher refractive indices than air are used to fill the space between the sample and recording screen. "Immersion microscope objectives use oil to increase their numerical aperture and consequently increase their resolution capability. Immersion DIHM uses the same idea," added Garcia-Sucerquia.

In the experiment, the team shone a green laser past latex beads, 1micron in diameter, placed in immersion oil of refractive index 1.5 at 550 nm. At a distance of 15 mm from base of the tank, where the beads were sitting, the researchers placed a CCD chip, itself 2 mm clear of the oil's surface.

As a result of using the dielectric medium, the team found "a considerable increase" in numerical aperture and, consequently, the resolution. "Since the higher refractive index medium lead to shorter wavelengths and holography is an interference phenomenon, our fringe spacing was reduced, we captured more fringes and obtained a higher resolution," said Garcia-Sucerquia.

According to Garcia-Sucerquia, given how resolution improves with the refractive index of the dielectric material, there is "no theoretical limit" to the possible improvement. However, in practice there are limits, since at visible wavelengths most high refractive index materials are poisonous.

However, the researchers say they can improve resolution by 350%, with the longer wavelengths involved in near infra-red digital holography and by using silicon, which can provide a very high refractive index of 3.5 at 1300 nm.

The main challenge of the technique for the team is finding non-poisonous materials with high refractive index at working wavelengths. "Right now, we are limited to the materials available in the market and the highest refractive index of a friendly material is about 1.9 leading us to theoretical improvements in the resolution of about 80%," added Garcia-Sucerquia.


Author
Darius Nikbin is Science/Technology Reporter on Optics.org and Opto & Laser Europe magazine.

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