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TitleNonlinear increase, invisibility, and sign inversion of a localized fs-laser-induced refractive index change in crystals and glasses.
Publication TypeJournal Article
Year of Publication2020
AuthorsLapointe, Jerome, Jean-Philippe Bérubé, Yannick Ledemi, Albert Dupont, Vincent Fortin, Younès Messaddeq, and Réal Vallée
JournalLight Sci Appl
Volume9
Pagination64
Date Published2020
ISSN2047-7538
Abstract

Multiphoton absorption via ultrafast laser focusing is the only technology that allows a three-dimensional structural modification of transparent materials. However, the magnitude of the refractive index change is rather limited, preventing the technology from being a tool of choice for the manufacture of compact photonic integrated circuits. We propose to address this issue by employing a femtosecond-laser-induced electronic band-gap shift (FLIBGS), which has an exponential impact on the refractive index change for propagating wavelengths approaching the material electronic resonance, as predicted by the Kramers-Kronig relations. Supported by theoretical calculations, based on a modified Sellmeier equation, the Tauc law, and waveguide bend loss calculations, we experimentally show that several applications could take advantage of this phenomenon. First, we demonstrate waveguide bends down to a submillimeter radius, which is of great interest for higher-density integration of fs-laser-written quantum and photonic circuits. We also demonstrate that the refractive index contrast can be switched from negative to positive, allowing direct waveguide inscription in crystals. Finally, the effect of the FLIBGS can compensate for the fs-laser-induced negative refractive index change, resulting in a zero refractive index change at specific wavelengths, paving the way for new invisibility applications.

DOI10.1038/s41377-020-0298-8
Alternate JournalLight Sci Appl
PubMed ID32351688
PubMed Central IDPMC7171118