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.

Photonic properties of novel Yb3＋ doped germanium-lead oxyfluoride glass-ceramics for laser cooling applications

Abstract In recent years, our research group has developed and studied new rare-earth doped materials for the promising technology of solid-state laser cooling, which is based on anti-stokes fluorescence. To the best of our knowledge, our group is the only one in Canada leading the research into the properties of nanoparticles, glasses and glass-ceramics for optical refrigeration appli- cations. In the present work, optical properties of 50GeO2- 30PbF2-18PbO-2YbF3 glass-ceramics for laser cooling are presented and discussed as a function of crystallization temperature. Spectroscopic results show that samples have near infrared photoluminescence emission due to the 2F5/2 - 2F7/2 Yb3＋ transition, centered at -1016 nm with an excitation wavelength of 920 nm or 1011 nm, and the highest photoluminescence emission efficiency occurs for heat-treatment for 5 h at 350℃. The internal photolumi- nescence quantum yield varies between 99% and 80%, depending on the temperature of heat-treatment, being the most efficient under 1011 nm excitation. The 2F5/2 lifetime increases from 1.472 to 1.970 ms for heat treatments at 330℃ to 350℃, respectively, due to energy trapping and the low phonon energy of the nanocrystals. The sample temperature dependence was measured with a fiber Bragg grating sensor, as a function of input pump laser wavelength and processing temperature. These measure- ments show that the heating process approaches near zero for an excitation wavelength between 1020 and 1030 nm, which is an indication that phonons are removed effectivelly from the glass-ceramic materials, and they can be used for optical laser cooling applications. On theother hand, the temperature increase as a function of input laser power into samples remains constant between 920 and 980 nm wavelength excitation, a temperature variation of 36 K/W （temperature of 58℃/W） was attained under excitation at 950 nm, showing a possible use for biomedical applications to be explored.

Comment on"Reversible 3D optical data storage and information encryption in photo-modulated transparent glass medium"

Dear Editor,The development of reversible photochromic materials combining efficient writing/erasing properties, low-cost and environmentally friendly compositions is a breakthrough challenge and a subject of many research works in the last decades. In this perspective, transparent oxide glasses are promising since their production is ensured by a simple melt-quenching technique and a proper laser writing setup is able to give rise to a localized photochromic effect, opening opportunities for 3D optical data storage.