Ultrafast laser-induced refractive index changesin Ge15As15S70 chalcogenide glass
2016
Understanding and controlling laser-induced refractive index
modifications in bulk chalcogenide glasses is important for a range of
photonics applications targeting the mid-infrared spectral region. We focus
here on material engineering aspects and pulse spatio-temporal design
characteristics that are able to induce and maintain positive refractive index
changes in laser-irradiated Sulfur-based chalcogenide glass, mandatory for
3D photonic design. Specifically we study the photoinscription process of a
Ge-doped Sulfur-based chalcogenide glass, Ge15As15S70, irradiated by
focused ultrafast near-infrared laser pulses where Ge doping plays a
determinant role in generating high-contrast positive index changes. By
means of aposteriori and real-time in situ observations we show that
positive refractive index changes (type I) are the result of a restructuring of
the glass matrix and a photo-induced contraction process initiated by twophoton
electronic excitation leading to bond softening, molecular mobility,
structural changes and rearrangements. Oppositely, negative refractive
index changes (type II) could be associated with two different processes:
photo-expansion at higher intensities and hydrodynamic evolution initiated
by plasma generation and laser heating, with thermomechanical relaxation
and stress unload. Alongside the role of Ge in setting various degrees of the
matrix connectivity, the structural arrangement developed under different
thermal history schemes for glass preparation is equally important as it
defines to which extent further structural flexibility is possible. Thus we
indicate the role of glass matrix metastability in generated high-contrast
refractive index changes and we show that a higher degree of relaxation is
an impediment for contrasted positive index changes, while these are
developing in unrelaxed glasses, where several degrees of structural
flexibility exist. Alongside dynamic time-resolved imaging experiments
probing the development and relaxation of excitation, we also show, via
static Raman analysis of the modified regions, that significant structural
changes are induced by laser irradiation and we discuss the potential
processes involved.
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