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Concave microlens offers shortest focal length

01 Sep 2008

A microlens that uses negative refraction to achieve a record focal length of 12 µm could benefit high-density imaging and optoelectronics systems.

US researchers have engineered a concave photonic crystal microlens that they claim provides the shortest focal length ever achieved for infrared light. The team from Northeastern University hopes to pursue commercial applications for its device subject to a patent application (Applied Physics Letters 93 053111).

"We have fabricated the first negative index concave lens at telecommunications frequencies and with an ultrashort focus," Srinivas Sridhar, a professor at Northeastern University, told optics.org. "The lens has an extremely short focus, a numerical aperture close to one and greatly reduced aberrations compared with a conventional positive index planoconvex lens."

The lens consists of an InP/InGaAsP semiconductor two-dimensional photonic crystal, which exhibits a negative index of refraction (–0.7). The lens focuses 1.5 µm light to a near diffraction-limited spot size of 1.05 µm just 12 µm away from the surface of the lens.

"The device is based on a semiconductor heterostructure photonic crystal, which means that it can be integrated into existing semiconductor platforms like CMOS," explained Sridhar. "The heterostructure platform is made up of a 400 nm InGaAsP core layer sandwiched between a 200 nm InP substrate layer and a 300 µm bottom cladding InP substrate layer."

The photonic crystal arrangement creates a negative-index medium, which is then fabricated in the form of a concave lens, with a 20 µm radius of curvature, using nanolithography. The photonic crystal consists of air holes with a diameter of 295 nm and lattice spacing of 470 nm, and behaves like an isotropic dielectric medium, where refraction can be described by Snell's law.

According to the team, the high level of flexibility offered by photonic crystals combined with the low-loss dielectric medium means that the microlens can be easily scaled to operate in any frequency region.

The next challenge facing the Northeastern group is to integrate the lens with other optical components.

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