Optical anisotropy of semiconductor nanowires

Semiconductor nanowires are novel nanostructures full of promise for optical applications. Nanowires have subwavelength diameters and large aspect ratios, which combined with the high permittivity of semiconductors lead to a strong optical anisotropy. We review in this chapter this optical anisotropy, focusing on the polarization anisotropy of the photoluminescence of individual nanowires and the propagation of light through birefringent ensembles of aligned nanowires. Recent developments in bottom-up nanofabrication techniques allow the growth of free-standing semiconductor nanowires with controlled composition, lateral dimensions of typically 10–100 nm, and lengths of several micrometers (see Fig. 6.1). The small lateral dimensions of nanowires enables to grow them heteroepitaxially onto different substrates [1–3] or even to design heterostructures with segments, shells, and/or quantum dots of different semiconductors in a single nanowire [4–8]. Nanowires are full of promise for monolithic integration of high-performance semiconductors with new functionality [8–11] into existing silicon technology [2, 3, 12]. These nanostructures will offer new possibilities as next generation of optical and optoelectronical components. Junctions in semiconductor nanowires and light emitting devices have been demonstrated [4, 13–17]. Although the quantum efficiency of these nano-LEDs is still low, fast progress is being made on the passivation of the nanowire surface and the increase of their efficiency [18, 19]. Also, optically and electrically driven nanowire lasing have been reported [9, 20, 21]. Nanowires have been proposed as polarization sensitive photodetectors [22, 23] and as a source for single photons [8, 24]. The encouraging perspectives for novel applications has lead to improved control over nanowire synthesis and materials composition [4, 5, 25–27]. However, little is known about how light is emitted by individual nanowires or how light is scattered by ensembles of these nanostructures. The large geometrical anisotropy of nanowires and the high refractive index of semiconductors give rise to a huge optical anisotropy, which has been reported as a strongly polarized photoluminescence of individual nanowires along their long axis [22, 28]. In this chapter we review the polarization anisotropy in the photoluminescence of individual nanowires. We also describe the propagation of light through ensembles of nanowires oriented perpendicularly to the surface of a substrate. The controlled growth and alignment of the nanowires leads to a medium with giant birefringence [29], i.e., a medium with a large difference in refractive indexes for different polarizations. The giant birefringence in ensembles of nanowires can be easily tuned by changing the semiconductor filling fraction and is not restricted to narrow frequency bands as in periodic structures [30]. Broadband and giant birefringence constitutes an elegant example of the extreme optical anisotropy of nanowires, which may lead to nanoscale polarization controlling media [31], the efficient generation of nonlinear signals [32], and the observation of novel surface electromagnetic modes on birefringent materials [33]