Inhomogenous Dielectric Metamaterials with Space-Variant Polarizability

An alternative approach is the realization of a system that benefits from devices that exploit the advantages ofboth continuous free space and discrete guided wave modes. A common example is the propagation of light in a slab waveguide, which supports confinement in the vertical direction and free space propagation in the plane of the slab. We call such a configuration "free space optics on a chip" (FSOC). This configuration allows light to propagate freely in the slab, while interacting with discrete optical components that are located along the propagation direction. Towards the implementation of practical FSOC devices, one needs to realize typical free space functionalities, e.g., focusing, beam steering, bending, and wavelength selectivity on chip. We implement such functionalities by introducing inhomogeneous dielectric metamaterials with space-variant polarizability into the slab waveguide.

Fig. 1

The metamaterial-based focusing component, schematically depicted in Figure 1, is realized by lithographically defining and etching subwavelength features into a high refractive index slab waveguide, modifying its local effective index of refraction.

Fig. 2

Device layout consisting of input ridge waveguide, nonpatterned slab, slab lens, and output ridge waveguide. Finite difference time domain simulation shows the beam propagation, which is found in good agreement with the approximate design of device layout using effective medium theory. The simulation results (Fig. 2) clearly show the focusing of light towards the output ridge waveguide.

Fig. 3

Fabricated samples are characterized using our heterodyne near field scanning optical microscope (H-NSOM) [1], capable of measuring both amplitude and phase of the propagating optical field with a resolution of about 100 nm. The HNSOM is ideal for characterization of our component because it allows direct observation of the curvature of the spatial phase front of the field propagating in the slab lens. Figure 3 shows the measured amplitude and phase of the optical field propagating through the device at a wavelength of 1550 nm. Figure 3(a) shows the amplitude of the optical field in the region that includes the input waveguide, the nonpatterned slab (S), and large portion of the slab lens section (L). The dashed vertical lines mark the boundaries between the various sections of the device. Light propagates from left to right. Figure 3(b) shows the measured phase in the same region. Figure 3(c) shows several cross sections of the phase front calculated from Figure 3(b) at several planes along the z-axis. The difference between simulation (Figure 2) and experimental results (Figure 3) is due to fabrication defects – the obtained local duty cycle was slightly larger than the designed one. This was particularly noticeable around the central portion of the lens. For example, the central air gap was only 50–60 nm wide, as oppose to the designed value of 100 nm. We believe that this deviation can be attributed mostly to the proximity effect. Proximity correction [2] can be applied to further improve the fabrication process.

 

References:

1. A. Nesci and Y. Fainman, in Wave Optics and Photonic Devices for Optical Information Processing II, edited by

P. Ambs and F. R. Beyette, Jr., Proc. SPIE Int. Soc. Opt. Eng. Vol. 5181 (SPIE-Internation Society for Optical Engineering, Bellingham, WA, 2003), p. 62.

 

2. M. Parikh, J. Appl. Phys. 50, 4371 (1979).

 

3. U. Levy, M. Abashin, K. Ikeda, A. Krishnamoorthy, J. Cunningham,and Y. Fainman, "Inhomogenous Dielectric Metamaterials with Space-Variant Polarizability," Phys. Rev. Lett. 98, 243901 (2007)