Nonlinear increase, invisibility, and sign inversion of a localized fs-laser-induced refractive index change in crystals and glasses

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.

Record-high positive refractive index change in bismuth germanate crystals through ultrafast laser enhanced polarizability

“…A record refractive index change value of is observed for the waveguide inscribed at 1,000 mm/min. A value of being the previous best was reported very recently using femtosecond-laser-induced electronic band-gap shift (FLIBGS) technique 21 . But the same was limited to a particular operating wavelength in visible.…”

Designer Glasses—Future of Photonic Device Platforms

“…The values obtained are similar to the ones reported in Corning Eagle and Schott AF glasses. However, much larger index contrasts for instance have been reported in germanium sulphide glass (1.7 × 10 −2 ) at a wavelength close to its bandgap, [ 29 ] fused silica (3.4 × 10 −2 ) [ 30 ] and bismuth germanate crystal (4.25 × 10 −2 ). [ 18 ] While the refractive index contrast for the purpose designed sodium‐aluminosilicate glass of 1 × 10 −2 is similar to optical fibres, a further enhancement is possible by replacing lighter Na with heavier K as alkaline earth and alkali are the refractive index providers in these glass waveguides.…”

Femtosecond laser-inscribed optical waveguides in dielectric crystals: a concise review and recent advances

“…More than that a new phenomenon allowing a significant increase of the refractive index contrast has been recently reported, which is called the effect of femtosecond-laser-induced electronic bandgap shift (FLIBGS). 103 When FLIBGS occurs, the absorption edge of the transmission spectrum shifts. At a propagation wavelength near the transparent material electronic resonance, the FLIBGS greatly affects the absorption and thus the refractive index, as predicted by the Kramers-Kronig relations.…”