Two-Photon Absorption Could Advance 3-D Data Storage
Michael D. Wheeler
TUCSON, Ariz. -- Molecules that can simultaneously absorb two photons efficiently may soon find uses in three-dimensional fluore scence imaging, optical data storage and lithographic microfabrication, according to University of Arizona researchers.
In the March 4 issue of Nature, they demonstrated that certain synthetic materials are sensitive enough to two-photon excitation with laser light to trigger chemical or physical changes with submicron resolution in three dimensions. When irradiated with laser light, these electron-rich molecules absorb enough energy to cause them to kick one of the electrons to a neighboring molecule. This photo-induced electron-transfer reaction begins a polymerization process, during which a network of chemical bonds forms.
Researchers at the University of Arizona produced this photonic bandgap structure by two-photon-initiated polymerization.
Since the two-photon absorption is confined to a tiny volume at the focus of the laser, 3-D polymer patterns can be produced by scanning the focus within the material. The researchers, led by Seth R. Marder and Joseph W. Perry, said they can manipulate this reaction in ways that could lead to 3-D data storage that permits fluorescent and refractive readout as well as the fabrication of micro-optical and micromechanical structures.
For efficient two-photon polymerization to occur, Marder and Perry had to overcome a long-standing problem: stimulating efficient two-photon absorption. Some photolithographic materials have demonstrated this capacity. But researchers had to expose the materials to very intense laser light, which often destroyed them.
In 1995, the researchers stumbled upon a discovery while performing a nonlinear optical experiment. They aimed an orange laser into a black-cloth-covered optical cell containing electron-donor dye molecules. When they examined the cell while firing the laser, they noticed a streak of blue light. The only explanation for this phenomenon was that the dye molecules were simultaneously absorbing two photons, losing some energy and then emitting a photon at a higher energy level.
"This discovery led us to consider the aspects of the molecule that control this process," Perry said. The researchers hypothesized that the electron-donating end groups of the molecules contributed to the molecule's ability to transfer electronics within.
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