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Molecular cage enhances dye laser

17 Jun 2002

Dye lasers based on a unique macromolecule promise to deliver disposable chemical sensors.

Courtesy of Opto & Laser Europe (OLE) magazine

Scientists at the Communications Research Laboratory (CRL) in Iwaoka, Japan, have developed high-gain mirrorless dye lasers based on macromolecules called dendrimers.

Capable of holding high dye concentrations with a narrow spectral linewidth, the dendrimer lasers could lead to compact, disposable and cheap sensors for chemical and biological uses (Appl. Phys. Lett. 80 7).

CRL researcher Shiyoshi Yokoyama and colleagues fabricated the dye laser by mixing a methanol solution of the dendrimer with some DCM laser dye. "The ball shape [of the dendrimer macromolecule] means that the dye fills the dendrimer when it is in a methanol solution," Yokoyama explained.

The researchers focused a 4 ns pulsed, stripe-shaped beam from a nitrogen laser onto a cuvette containing the dye-doped dendrimer. This induced polarized, coherent 630 nm lasing - without mirrors - parallel to the excitation stripe. The emitted linewidth was only 0.1 nm and the researchers found that as they increased the dye concentration to 9 millimoles per litre, the lasing threshold dropped.

"With dye concentrations of more than 2.5 millimoles per litre, we observed a lasing action," said Yokoyama. "As we increased the concentration, the excitation energy needed to reach the lasing threshold decreased."

Most dye lasers can only support dye concentrations of up to 1 millimole per litre before energy transfers in clustered molecules suppress fluorescence. However, Yokoyama says that the size and shape of the dendrimer's unique shell-like structure limits this effect, allowing the researchers to increase the dye concentration.

Yokoyama adds that while complicated chemical synthesis has limited the use of dendrimers for dye lasers in the past, the team managed to synthesize their simple dendrimer quite easily.

The researchers believe that mirrorless lasing was due to the dendrimer solution's high optical density. According to Yokoyama, this restricted the penetration depth of the excitation laser. "A distinct refractive-index boundary forms between excited and unexcited regions, and behaves like a cavity," he said.

Yokoyama says that the team is currently investigating a coupling effect of the gain guiding - the process by which an optical mode is confined within a maximum gain region - and side reflections, to explain the optical feedback for lasing.

Yokoyama concluded: "In principle the [DCM] dye molecule can be replaced by other dye molecules to achieve the desired range of wavelengths. The results can be extended to an optical system that could be fine-tuned for mirrorless optical devices."

Rebecca Pool is news editor of Optics.org and Opto & Laser Europe magazine.

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