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We have demonstrated a novel class of optical metamaterials consisting of high densities of aligned semiconductor nanowires.
These nanowire materials hold promise for applications in photonics, optoelectronics, non-linear and quantum optics, microfluidics, sensing, and photovoltaics.
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Figure 1 Cross-sectional SEM image of GaP nanowires grown epitaxially on a (111) GaP substrate.
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 Figure 1.
Figure 2 Schematic overview of light propagation through an aligned nanowire material. Different refractive indices n^ and n// result in a rotation of the polarization vector.
The almost perfect alignment of the nanowires results in a strong variation of the light velocity along different polarization directions in the layer parallel and perpendicular to the wire axis. Figure 2 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. We measured a giant birefringence, with a difference of the refractive index of up to 0.8 (Figure 3), which is the largest value for a random material to date. This value is 75 times the birefringence of quartz.
Figure 3 Birefringence Dn measured for a series of aligned nanowire materials with increasing nanowire volume fraction f. The birefringence reaches a value of 0.8. Solid line is a model calculation using the Maxwell-Garnett effective medium approximation, while dashed line is the same model for the corresponding inverted medium (air pores in GaP).
Several other important aspects of the nanowire material optical properties are an extremely large polarization contrast exceeding 4x103, which is important for sensing, and tunability of the direction of giant birefringence via the orientation of the nanowires.
References:
O. L. Muskens, S. L. Diedenhofen, R. Algra, E. P. A. M. Bakkers, B. Kaas, A. Lagendijk, Large Photonic Strength of Highly Tunable Resonant Nanowire Materials,
Nano Lett. 9, 930:934 (2009)
S. L. Diedenhofen, G. Vecchi, R. E. Algra, A. Hartsuiker, O. L. Muskens, G. Immink, E.P.A.M. Bakkers, W. L. Vos, J. Gómez Rivas, Broad-band and omnidirectional antireflection coatings based on semiconductor nanorods,
Adv. Mater. 21, 973:978 (2009)
O. L. Muskens, J. Gómez Rivas, R. E. Algra, E. P. A. M. Bakkers, A. Lagendijk, Design of Light Scattering in Nanowire Materials for Photovoltaic Applications,
Nano Lett. 8(9), 2638:2642 (2008)
O. L. Muskens, S. L. Diedenhofen, M. H. M. van Weert, M. T. Borgström, E. P. A. M. Bakkers, J. Gómez Rivas, Epitaxial growth of semiconductor nanowire metamaterials for photonic applications, Adv. Func. Mater. 18(7), 1:8 (2008)
O. L. Muskens, M. T. Borgström, E. P. A. M. Bakkers, J. Gómez Rivas, Giant optical birefringence in ensembles of semiconductor nanowires,
Appl. Phys. Lett. 89, 233117:9 (2006)
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