Nanowire Photonics
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Scientists at the AMOLF research group Nanowire Photonics in collaboration with Philips Research have fabricated a new type of optical materials based on aligned semiconductor nanowires. The nanowires were made from a very high refractive index material Gallium Phosphide (GaP), grown epitaxially on a crystalline GaP substrate. First, the substrate was covered with gold nanoparticles, which act as catalyst for decomposition of chemical precursor gases, followed by precipitation of the semiconductor material from the liquid nanoparticle in the form of a nanowire (vapor-liquid-solid growth).
This method yields dense layers consisting of hundreds of aligned nanowires per square micrometer. Optical experiments showed that the very good alignment of the nanowires results in a highly anisotropic artificial material, yielding the largest optical birefringence to date. This giant birefringence is more than 75 times larger than the natural birefringence of quartz and more than two times that of other artificial materials based on inverted porous networks. The researchers have published their results in Applied Physics Letters.
Semiconductor nanowires are grown vertically aligned on a crystalline surface using chemical vapour phase epitaxy. The wires are several micrometers long and only 20 nanometers in diameter, as can be seen on the Scanning Electron Microscopy image in the left image. The almost perfect alignment of the nanowires results in a strong variation of the light velocity along different polarization directions in illustrates the change of the polarization state of light propagating through a sample of nanowires due to the difference in light velocity or refractive index. The AMOLF-scientists measured giant birefringence, a difference of the refractive index, of up to 0.8.
Source: AMOLF
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