Research Activities of the Integrated Photonic Devices and Materials Group

Figure 1) Super collimator composed of Si rods.

Figure 2) Super collimator using holes.

Devices Utilizing Photonic Crystals

Super-collimation (SC) is the propagation of light without diffraction using the intrinsic properties of photonic crystals (PhCs). Successful fabrication and measurement of SC have been achieved for planar PhCs composed of silicon rods (Figure 1) as well as air holes etched into silicon (Figure 2). The super-collimating PhC is fabricated on a silicon-on-insulator (SOI) wafer. The low-index silicon dioxide layer is used to minimize radiation loss into the high-index silicon substrate. The rods are defined using interference lithography and pattern transfer is achieved using reactive ion etching (RIE). Infrared images of the light that is scattered normal to the plane of the super-collimator that is composed of Si-rods on a SOI wafer are shown in Figure 3. Super-collimation is observed at a wavelength of 1530nm.

In principle, creating a beam which does not diverge for long distances is possible by making the distribution of the beam's constituent eigenmodes sufficiently narrow in k-space, i.e. as the beam approaches a single Bloch mode or plane wave. On the other hand, a super-collimator allows for nearly divergent-less propagation for beam widths only a few times the lattice constant of the PhC. A method of exploring the design space for super-collimating devices has been developed; the bandwidth for super-collimation for the photonic crystal of holes is wider than the bandwidth for that of rods. Hence, depending on the application, a photonic crystal that is composed of air holes may be more suitable than a photonic that is composed of dielectric rods.

Figure 3) Plan-view infrared images showing the wavelength dependence of the propagating beam inside the two-dimensional-slab PhC that is composed of Si rods. The optimal wavelength of SC is close to 1530nm and the beam diverges for non-optimal wavelengths.

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