Reflectivity Yields Fresnel Lenses

Daniel S. Burgess

Continuing their work with photoinduced reflectivity, scientists at W.W. Hansen Experimental Physics Laboratory at Stanford University in California have produced transient diffractive optical elements in semiconductor wafers by optical stimulation. The technique promises applications in near-field infrared microscopy and may enable the fabrication of transient three-dimensional structures with characteristics similar to photonic crystals.

Photoinduced reflectivity in high-refractive-index semiconductors enables the production of transient diffractive optical elements for use in near-field infrared microscopy. Courtesy of Keith R. Cohn.

In the December 2001 issue of Photonics Spectra, we described the group's investigation of the apertures in its effort to dispense with the slow mechanical scanning in near-field microscopy. However, Keith R. Cohn, a graduate student in the department of physics and member of the team with Dmitrii Simanovskii, Todd Smith and Daniel Palanker, explained that the approach was limited to use with approximately 1-µm-thick substrates. "It is not so easy to obtain and work with crystalline semiconductors of this size," he said.

The team has turned to the production of transient diffractive optical elements in thick wafers of Si and GaP. The 400-nm second harmonic from a Ti:sapphire laser forms electron-hole plasmas in the semiconductor substrate, causing it to become opaque to a 6.25-µm imaging beam generated by an optical parametric amplifier.

Unlike the previous setup, which incorporated a shadow mask to leave simple 0.5- to 3-µm-diam-eter transmissive windows in the substrate, the new design employs a Nikon 50X microscope objective to project a demagnified image of a lithographic mask on the substrates.

For example, the researchers have demonstrated that the pump light can produce a series of opaque rings designed to act like a NA-2.1 Fresnel lens.

Biological applications

In practice, such a lens in a 28-µm-thick Si substrate, with which the researchers have resolved two contiguous 1.8-µm-diameter holes in a 200-nm-thick gold film, performed better than one in 300-µm-thick GaP. Cohn attributed this difference to the thickness of the latter wafer, which required a structure 7 1/2 times larger and with nearly five times as many zones.

"Not only is uniformly illuminating such a large zone structure with both visible and IR light more difficult, but the optical path length difference between light arriving at the focus from the inner and outer zones introduces a temporal delay on the order of the IR pulse duration," he said. "This inhibits IR light arriving at the focus from the outer zones from interfering constructively with light that arrives from the inner zones."

The scientists plan to work with thinner GaP wafers for improved resolution, he said. They also will seek to better understand the contrast mechanisms associated with imaging complex objects, with the goal of demonstrating that transient probes can be used to investigate biological samples.