Graded flat lens with negative index for silicon photonics

Abstract

We have realized a graded flat lens with negative effective index operating at telecom wavelength for Silicon Photonics. We report on its design, fabrication, and experimental characterization. This photonic device behaves as a convergent lens; it has been designed so as to be fabricated on a Silicon on Insulator platform and to operate at 1.55 μm. It consists of a graded photonic crystal whose filling factor has been varied so as to fit the profile of index of the graded index lens. Since it operates in the second band of Photonic Crystals, the effective index is negative. It has been characterized in two steps: (i) first, the output beam has been scanned by a set of waveguides and a CCD camera. (ii) Second, the focusing has been investigated by Scanning Near-field Optical Microscopy. This characterization validates the design: the focal length is about 5 μm (≃ 3λ), while the width of the focal spot is 0.87 μm (0.53 λ). This compact photonic device is at the wavelength scale (thickness = 4.4λ). Our design, based on Graded Index Optics, may be adapted to design new devices perfectly suited for Photonics applications, such as a short taper or a mode converter.