Optical Near-Field Microscopy of Light Focusing through a Photonic Crystal Flat Lens

We report here the direct observation by using a scanning near-field microscopy technique of the light focusing through a photonic crystal flat lens designed and fabricated to operate at optical frequencies. The lens is fabricated using a III-V semiconductor slab, and we directly visualize the propagation of the electromagnetic waves by using a scanning near-field optical microscope. We directly evidence spatially, as well as spectrally, the focusing operating regime of the lens. At last, in light of the experimental scanning near-field optical microscope pictures, we discuss the lens ability to focus light at a subwavelength scale.

Figure 1

(a) Band structure of TE (the electric field lies perpendicular to the 2D plane) and TM modes for a 2D triangular $\mathrm{InP}/\mathrm{InGaAsP}$ air-hole triangular photonic crystal with an air filling factor of 38% and an effective index of 3.26. (b) Equifrequencies plot of the TE second band for the same photonic crystal.

Figure 2

Schematic view of (a) the structure with the rib waveguide and the 21st row $\Gamma \mathrm{M}$-oriented photonic crystal lens ($d_1=9\mu \mathrm{m}=2d$) and of (b) the InP-based heterostructure. (c) Schematic of the optical near-field experimental setup.

Figure 3

Three-dimensional FDTD simulation of the focusing regime at 1525 nm: electric field intensity (a) in the $xy$ plane ($z=1.6\mu \mathrm{m}$ from the substrate corresponding to the InGaAsP layer) and (b) in the $xz$ plane (middle of the ridge waveguide). The full device geometry is recalled in (c).

Figure 4

(a) Scanning electronic microscope picture of the fabricated photonic crystal lens and the input waveguide that mimics a source point in the vicinity of the lens. (b) Scanning near-field optical microscope picture of the lens recorded for an input wavelength of $\lambda =1525\mathrm{nm}$. For a better visualization of the small amplitude signals that evidence the electromagnetic wave propagation inside the nanostructure, an illuminated picture is plotted.

Figure 5

SNOM pictures of the lens for different input wavelengths: (a) $\lambda =1510\mathrm{nm}$, (b) $\lambda =1530\mathrm{nm}$, (c) $\lambda =1540\mathrm{nm}$, and (d) $\lambda =1570\mathrm{nm}$. In each picture, the position of the lens facets is superimposed as dotted lines.