3D Defect Structures Embedded in Opal-Based Photonic Crystals
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RESEARCH/RESEARCHERS
Low-Refractive-Index Nanoporous Silica Improves Reflectivity of Dielectric Mirror For light-emitting diodes (LEDs) to become practical replacements for incandescent bulbs, the light must be extracted more efficiently than is currently possible. Since the active region emits light in all directions, the efficiency can be improved by adding a mirror to the back of the LED to redirect light out of the device. One such mirror is a triple-layer omnidirectional reflector (ODR), which uses a lowrefractive-index (low-n), transparent dielectric in a semiconductor–dielectric– metal stack. As reported in the June 15 issue of Optics Letters (p. 1518), J.-Q. Xi and co-workers at Rensselaer Polytechnic Institute fabricated an ODR using nanoporous SiO2 as the low-n dielectric. The researchers used a spin-on sol-gel process to deposit a 105-nm-thick layer of nanoporous SiO2 (pore size, ~4 nm) on a 300-μm-thick GaP wafer and then evaporated 500 nm of Ag onto the dielectric. They made two similar structures, one with an 89-nm-thick, dense SiO2 layer, and another without a dielectric layer. Ellipsometry revealed that the nanoporous SiO2 has a refractive index of only n = 1.23, and the dense SiO2 has a refractive index of n = 1.457. Using a 632.8 nm coherent He-Ne laser, the group measured the angle-dependent reflectivity of the structures with the laser light incident from the GaP-air side and used it to calculate the angle-integrated reflectivity Rint, a key figure of merit for an ODR. They coupled light into the device with a high-index fluid and prism, allowing for off-normal measurements up to 25.5° for transverse electric (TE) polarization and 27.3° for transverse magnetic (TM) polarization. Reflectivity measurements for the TE mode in the nanoporous SiO2 device yielded Rint = 99.9%, while the Rint of the dense SiO2 was 99.8%, and the Rint of the simple silver mirror was only 97.2%. For TM polarization, which demonstrated more angular variation than the TE mode, the integrated reflectivities followed a similar trend, with Rint values of 98.9%, 97.8%, and 94.4% for the nanoporous SiO2 ODR, dense SiO2 ODR, and silver mirror, respectively. The improved reflectivity of the nanoporous dielectric is consistent with its lower refractive index. The researchers said that such small increases in reflectivity translate into considerable reductions in mirror losses, especially when multiple reflections take place, thus suggesting that ODRs with nanoporous SiO2 could improve LED performance. AMANDA GIERMANN 572
3D Defect Structures Embedded in Opal-Based Photonic Crystals Due to their potential to control light emission, routing, and filtering, photonic crystals can be employed as a tool to manipulate light for optical devices. To fabricate these materials, one route is to utilize the self-assembly of microspheres into colloidal crystals. In order to trap or localize light within these opal-based photonic crystals, researchers want to create controlled microscopic defects inside the lattice structure. As reported in th
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