Index Changes In Crystals Research Articles

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

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.

The influence of shallow traps on the properties of LiNbO 3 waveguides

Optical damage effects in planar lithium niobate waveguides are discussed using a modified band-transport model of the photorefractive process in this material. Introducing a shallow trap makes it possible to explain the occurrence of strong light-induced refractive index changes in crystals containing less than 5 ppm Fe. The influence of this effect on the effective indices of the guided modes and on the cutoff is investigated theoretically.

THE INFLUENCE OF PHOTO-INDUCED REFRACTIVE INDEX CHANGE ON RAMAN SPECTRA OF LiNbO3: Fe CRYSTALS

Obvious anomalous phenomenon in Raman spectrum of LiNbO3:Fe crystals for some scattering geometry has been observed. The phenomenon is associated with the anomalous light scattering process occured after photo-induced refractive index change in crystals.