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Nano-optical microlens with ultrashort focal length using negative refraction
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View: Figures


Image of FIG. 1.
FIG. 1.

(a) The top view depicts the 2D PhC consisting of a square lattice of air holes, with diameter and lattice spacing . (b) The side view shows the air holes etched in the InP/InGaAsP heterostructure, which consists of a 400 nm InGaAsP core layer sandwiched between a 200 nm InP top cladding layer and a bottom cladding InP substrate layer.

Image of FIG. 2.
FIG. 2.

Band structure diagram of 2D PhCs consisting of a square lattice of air holes in dielectric with dimensions shown in Fig. 1. Effective refractive indexes of 3.231 (TE modes) (red curve) and 3.216 (TM modes) (blue curve) are used for the InP/InGaAsP semiconductor. (b) Isofrequency diagram of the 2D PhCs showing the periodic curves of constant . The isofrequency contours in the second band move inward with increasing , indicating that . This implies that the phase refractive index is negative. From band structure calculations, .

Image of FIG. 3.
FIG. 3.

SEM pictures illustrating (a) a bird’s eye view of the planoconcave 2D PhC microlens. The waveguide is inversely tapered from the end, with a tapered core width varying from 5 to , and (b) a close view of the microlens. The radius of the concave face is , while the PhC consists of air holes in dielectric with dimensions shown in Fig. 1.

Image of FIG. 4.
FIG. 4.

(a) NSOM reconstructed image of the transmitted field in the air cavity at 1530 nm. Focusing is observed away from the concave face. The FWHM of the beam spot size is in the order of (shown in the inset picture). The white dashed lines indicate the beam path. Note that (a) is a composite picture consisting of a SEM image of the lens superposed on a NSOM scan. (b) Electric field intensity plot of FDTD simulations results of the planoconcave microlens. For better visual illustration the radius of the concave face in the plane has been truncated to , while the transverse size ( plane) is still maintained at . The focusing effect occurs away from the concave face matching the experimental results. The FWHM of the focused beam is .


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Scitation: Nano-optical microlens with ultrashort focal length using negative refraction