The nature of structural changes of nanosecond laser modification inside silicon is investigated. Raman spectroscopy and transmission electron microscopy measurements of cross sections of the modified channels reveal highly localized crystal deformation. Raman spectroscopy measurements prove the existence of amorphous silicon inside nanosecond laser induced modifications, and the percentage of amorphous silicon is calculated based on the Raman spectrum. For the first time, the high-resolution transmission electron microscopy images directly show the appearance of amorphous silicon inside nanosecond laser induced modifications, which corroborates the indirect measurements from Raman spectroscopy. The laser modified channel consists of a small amount of amorphous silicon embedded in a disturbed crystal structure accompanied by strain. This finding may explain the origin of the positive refractive index change associated with the written channels that may serve as optical waveguides.

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