Photonic Crystal Microlens Achieves Ultrashort Focal Distance
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Photonic Crystal Microlens Achieves Ultrashort Focal Distance Recent theoretical investigations of negative refractive index metamaterials opened new fields of possible optoelectronic applications. Now the group of B.D.F. Casse, W.T. Lu, Y.J. Huang, and S. Sridhar from Northeastern University has demonstrated an ultra-short focal length negative index lens at telecommunication frequencies. This could lead to immediate micro- and nano-optics applications, such as miniaturization of chargecoupled devices (CCDs) used in digital cameras and sensors. The researchers described in the August 7 issue of Applied Physics Letters (DOI:10.1063/1.2968873; 053111) how, using dispersion engineering principles, they designed and fabricated a twodimensional (2D) photonic crystal microlens with a negative refractive index of -0.7, and ultrashort focal distance of only 12 μm. The lens also possess a numerical aperture close to unity, and a near diffraction-limited spot size of 1.05 μm in the infrared range (λ = 1.5 μm). The photonic crystal array consisted of a 2D square lattice of air holes (each hole diameter was 295 nm and the lattice spacing was 480 nm) in an InP/InGaAsP rectangular wafer with a semicircular cutting (radius 20 μm), thus creating a planoconcave microlens. The use of a pure dielectric system reduced to a minimum the intrinsic material losses. Light originating from a continuous wave tunable semiconductor laser (λ = 1550–1580 nm) was propagated through a waveguide into the microlens, and near-field scanning optical microscopy measurements revealed the focusing of the plane wave on the optical axis of the microlens inside the air cavity. The experimental results, both with respect to the focal distance (12 μm) and to the spot size at full width at half-maximum (1.05 μm, or 0.68 λ) for the given frequency range, were in agreement with results of 2D finitedifference time-domain simulations. The use of a photonic crystal material in a planoconcave configuration with a negative index of refraction produced focusing of the incident laser light in contrast to materials with a positive index of refraction, which require convex lens geometry to produce a real focus. The researchers said that the photonic-crystal–based planoconcave lenses “can be superior to plano-convex microlenses as they possess a larger numerical aperture (close to unity), diffraction-limited spot size, display less spherical aberrations and have shorter focal lengths.” The low-loss medium will allow for the device to be scaled to
work in any frequency region through appropriate adjustment of the photonic crystal design. EUGEN PANAITESCU
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