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

Abstract

Unlike other crystals, the counter intuitive response of bismuth germanate crystals ($${\text {Bi}}_4{\text {Ge}}_3{\text {O}}_{12}$$ Bi 4 Ge 3 O 12 , BGO) to form localized high refractive index contrast waveguides upon ultrafast laser irradiation is explained for the first time. While the waveguide formation is a result of a stoichiometric reorganization of germanium and oxygen, the origin of positive index stems from the formation of highly polarisable non-bridging oxygen complexes. Micro-reflectivity measurements revealed a record-high positive refractive index contrast of $$4.25\times 10^{-2}$$ 4.25 × 10 - 2 . The currently accepted view that index changes $$>1\times 10^{-2}$$ > 1 × 10 - 2 could be brought about only by engaging heavy metal elements is strongly challenged by this report. The combination of a nearly perfect step-index profile, record-high refractive index contrast, easily tunable waveguide dimensions, and the intrinsic high optical non-linearity, electro-optic activity and optical transparency up to $$5.5\,\upmu {\text {m}}$$ 5.5 μ m of BGO make these waveguides a highly attractive platform for compact 3D integrated optics.

Designer Glasses—Future of Photonic Device Platforms

“…We have tried to map oxygen migration using EPMA since these defects are oxygen excess defects as previously reported. [ 18,19 ] However, no substantial variation could be traced, a result we attribute to oxygen being a light element coupled with potentially only a small degree of migration as suggested by the relatively weak refractive index change obtained in Gorilla glass.…”

“…We have previously reported two different cases where the origin of positive index change was due to an increase in polarisability caused by i) migration of constituents having a higher polarizability [ 17 ] and ii) the formation of laser induced non‐bridged silicon defects that exhibit a highly distorted electron cloud compared to bridged silicon within an unmodified network. [ 18 ] Non briding oxygen atoms (NBO) are oxygen atoms that do not bridge two silicon atoms in a silica network, instead they are non‐bonded and carry a negative charge associated with them eg: Si‐O − . The low density zones in the investigated alkali‐rich display cover glasses produced a strong photoluminescence (PL) from their corresponding NBOs indicating that the origin of positive index change is due to the highly polarizable NBOs which were formed during laser inscription.…”

“…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.…”

Optical Waveguides Fabricated via Femtosecond Direct Laser Writing: Processes, Materials, and Devices

“…In 2020, Fernandez et al. [ 118 ] achieved a record‐high positive refractive index contrast of 4.25 × 10 −2 and explained the Type‐I waveguide formation via FsDLW for the first time in BGO. The inverted behavior of peak shift between external and internal vibrations in the µ‐Raman spectra reveals that migration of elements between the core and cladding instead of densification or change in molar volume.…”

“…l) The schematic illustration of chemical bond changes in BGO after Fs-laser irradiation. Reproduced with permission [118]. Copyright 2020, Springer Nature.…”