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Photonic crystals tailor beam patterns

23 Jun 2006

Optical tweezers and high density optical storage are just two of the applications that could benefit from compact photonic crystal lasers capable of producing diverse beam patterns on demand.

Researchers in Japan are using photonic crystals to produce semiconductor lasers that emit a tailored beam pattern whilst maintaining single mode operation. The team believes its lasers are ideal for a number of applications such as optical tweezers, microfluidics and ultrahigh-density optical memory. (Nature 441 946)

"Lasers with single- or multiple-doughnut beams are important for the manipulation of both transparent and non-transparent materials," Susumu Noda from Kyoto University told Optics.org. "A single-doughnut beam with radial polarization is important for light sources with super-resolution. We can apply these to increase the capacity of DVD systems significantly."

The far-field beam pattern emitted by a semiconductor laser is determined by the Fourier transform of the electromagnetic field in the output plane. Noda explains that in conventional semiconductor lasers, it is difficult to modify the field distribution in the output plane without forcing a multimodal and unstable optical mode.

"It is important to find an alternative mechanism for controlling the field distribution, which maintains stable single-mode oscillation over a large area," said Noda. "A photonic crystal with a 2D periodic refractive-index distribution is a promising candidate. The output beam can be emitted in the direction normal to the 2D crystal plane, using the crystal itself as a diffraction grating."

Noda and colleagues used electron beam lithography and dry etching to fabricate a photonic crystal in the thin cladding layer grown on top of the laser's active layer. In its initial studies, the team etched into a GaAs cladding layer of a InGaAs/GaAs laser emitting at 980 nm.

By engineering the lattice points and/or lattice phases in the photonic crystal structure, Noda's team was able to produce surface-emitted beams with a variety of shapes, including single, twin and quadruplet doughnuts.

"The engineered photonic crystal lasers all display single-mode oscillation at room temperature; the maximum output power exceeds 45 mW under continuous wave conditions," say the authors in their paper.

Confident in the approach, Noda believes that some of the lasers - such as the devices emitting circular beams thanks to triangular lattice points - are almost ready for commercialization once reliability tests have been carried out.

"We would also like to extend the wavelength to the blue light range," said Noda. "We also are planning a detailed investigation of single-doughnut beams with radial polarization for increasing the capacity of DVDs. We are also very interested in using doughnut beams for microfluidics applications."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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