Abstract
Disordered optical media are an emerging class of materials that can strongly scatter light. These materials are useful to investigate light transport phenomena and for applications in imaging, sensing and energy storage. While coherent light can be generated using such materials, its directional emission is typically hampered by their strong scattering nature. Here, the authors directly image Rayleigh scattering, photoluminescence and weakly localized Raman light from a random network of silicon nanowires via real‐space microscopy and Fourier imaging. Direct imaging enables us to gain insight on the light transport mechanisms in the random material, to visualize its weak localization length and to demonstrate out‐of‐plane beaming of the scattered coherent Raman light. The direct visualization of coherent light beaming in such random networks of silicon nanowires offers novel opportunities for fundamental studies of light propagation in disordered media. It also opens venues for the development of next generation optical devices based on disordered structures, such as sensors, light sources, and optical switches.
Highlights
Maria Lo Faro, Giovanna Ruello, Antonio Leonardi, Dario Morganti, Alessia Irrera, et al
The random media are both made of crystalline silicon (c-Si) in the core of the nanowires, native silicon dioxide in their outer shell and air voids in the gaps separating the nanowires
We have further demonstrated the possibility of using Fourier imaging for tuning the directional beaming of coherent Raman light scattered by a random medium
Summary
We fabricated disordered arrays of vertically aligned Si nanowires on silicon wafers by metal-assisted chemical etching (see the Experimental Section).[35]. The Raman signal provides an advantage when compared to Rayleigh scattering, for which the coherence time is longer, and a localization length can only be extracted when averaging speckle patterns over hundreds different realization of disorder (i.e., over hundreds of images).[46] In these random materials the propagating coherent light interferes in reciprocal light paths at the backscattering direction, giving rise to a coherent backscattering cone.[43,46] This is the case for the multiply scattered Raman light.[42] Practically, this cone is the Fourier transform of the intensity distribution generated at the output sample surface by all the coherent light paths in the material. Error bars on the data points are included in the legend to the graphs and represent the experimental uncertainty of the normalized scattered intensity, taking into account the reliability of both the HCC and VH polarization configurations (see the Experimental Section for details)
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