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Expanding the scope of nonparaxial scalar diffraction theory

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2020
Günöven, Mete
The modelling of light scattering from rough textured surfaces is important to assess the light trapping performances of thin film solar cells. In this regard, Harvey-Shack scalar scattering theory is a method of choice established in the solar cell community. It can be used to calculate the angular intensity distribution both in reflection and in transmission, by using the Fourier transform of the optical phase light accumulates while traversing the rough surface texture to evaluate a far-field approximation of the Rayleigh-Sommerfeld scalar diffraction integral, observed on a hemisphere centered around the sample aperture. In this work, different versions of the Harvey-Shack scalar scattering theory are implemented, and their results are compared to actual angular intensity measurements, using a purpose-built high-resolution goniometric instrument. These comparisons generally show remarkable quantitave predictions, which validate the overall approach. However, differences with the measurements suggest that the optical phase accumulation could benefit from an additional correction factor for rough surfaces containing lateral feature sizes on the order of the wavelength, which can be attributed to effective medium effects. Moreover, secondary interactions within the surface topography are shown to be a mechanism that partly redistributes scattered power, affecting angular intensity distribution results. These mechanisms emerge as the two main limitations of the generalized nonparaxial Harvey-Shack theory in the far-field. When applied to the scattering into an optically denser medium, this model predicts polar angle regions where no scattering should accur, regardless of the angle of incidence or the roughness of the texture. This prediction points at a limitation of light trapping using rough textured interfaces. Furthermore, the near-aperture and near-field terms of the Rayleigh-Sommerfeld scalar diffraction integral were investigated using a modification to the generalized Harvey-Shack theory computational algorithm. Within the restrictions of its scalar nature, this novel method can be an important tool for the characterization of near-field diffraction for thin film solar cells and many other problems.