Negative Refraction at Infrared Wavelengths in a Two-Dimensional Photonic Crystal

We report on the first experimental evidence of negative refraction at telecommunication wavelengths by a two-dimensional photonic crystal field. Samples were fabricated by chemically assisted ion beam etching in the InP-based low-index constrast system. Experiments of beam imaging and light collection show light focusing by the photonic crystal field. Finite-difference time-domain simulations confirm that the observed focusing is due to negative refraction in the photonic crystal area.

Figure 1

Band structure and wave-vector diagram for the PhC structure: (a) calculated dispersion diagram showing the photonic band structure for all in-plane wave vectors and (b) photonic band structure at the edges of the irreducible Brillouin zone. The indicated working area shows the experimentally available wavelength range.

Figure 2

Schematic wave-vector diagrams illustrate the propagation of the wave for the conventional case (a) and for the PhC case (b).

Figure 3

Scanning electron microscope image of the fabricated device (top view).

Figure 4

(a) Measured light intensity distribution of the output beam, without the PhC field at different positions from the facet. (b) Measured light intensity distribution of the output beam with the PhC field. The evolution of full width at half maximum is plotted on the same graph with dots, showing the focusing of light ($\lambda =1480\mathrm{n}\mathrm{m}$).

Figure 5

Snapshot of the simulated light intensity distribution at $\lambda =1480\mathrm{n}\mathrm{m}$.

Figure 6

Scanning electron microscope image of the device, with a collecting waveguide at $d^{\prime }=14\mu \mathrm{m}$ (top view).

Figure 7

(a) The experimental transmission spectra for different distances between input and collecting waveguides are $d^{\prime }=14\mu \mathrm{m}$, $d^{\prime }=23\mu \mathrm{m}$, and $d^{\prime }=33\mu \mathrm{m}$. The case without PhC is also plotted. Experimental data were obtained using two different broad-spectrum light sources, covering together the wavelength range 1480–1610 nm, performed in TM mode. (b) Simulated transmission spectra for the same values of $d^{\prime }$.