Photocycle of point defects in highly- and weakly-germanium doped silica revealed by transient absorption measurements with femtosecond tunable pump

Abstract

We report pump-probe transient absorption measurements addressing the photocycle of the Germanium lone pair center (GLPC) point defect with an unprecedented time resolution. The GLPC is a model point defect with a simple and well-understood electronic structure, highly relevant for several applications. Therefore, a full explanation of its photocycle is fundamental to understand the relaxation mechanisms of such molecular-like systems in solid state. The experiment, carried out exciting the sample resonantly with the ultraviolet (UV) GLPC absorption band peaked at 5.1 eV, gave us the possibility to follow the defect excitation-relaxation dynamics from the femto-picosecond to the nanosecond timescale in the UV-visible range. Moreover, the transient absorption signal was studied as a function of the excitation photon energy and comparative experiments were conducted on highly- and weakly-germanium doped silica glasses. The results offer a comprehensive picture of the relaxation dynamics of GLPC and allow observing the interplay between electronic transitions localized on the defect and those related to bandgap transitions, providing a clear evidence that the role of dopant high concentration is not negligible in the earliest dynamics.

Figure 1

(a) Two-dimensional time-probing energy plot of the TA signal measured in the 6% Ge-doped preform sample after photo-excitation with 200 fs laser pulses at 5.1 eV and 50 nJ/pulse. (b) TA spectra recorded at different pump-probe delays: 1 ps in red, 10 ps in green, 100 ps in blue, 1 ns in orange, 3 ns in purple and 6 ns in magenta. The black dashed line is a guide to the eyes for the zero. (c) TA kinetics at different probing energies: in purple at 3.35 eV, in blue at 2.95 eV, in green at 2.75 eV and in magenta at 2.64 eV. The red line represents the decay time fitting with the best fit parameter τ = 1.46 ns of the negative component observed at 3.35 eV.

Figure 2

(a) TA spectra at different pump-probe delays measured in the 0.1% Ge-doped sample upon UV excitation at 5.1 eV and 50 nJ/pulse: 1 ps in red, 10 ps in green, 100 ps in blue, 1 ns in orange, 3 ns in purple and 5 ns in magenta. (b) TA kinetics at different probing energies: in purple at 3.30 eV, in blue at 2.95 eV, in green at 2.75 eV and in magenta at 2.64 eV.

Figure 3

The normalized TA probed at 2.8 eV and at 5 ps of pump-probe delay as function of the exciting energy, overlapped on the normalized calculated GLPC absorption band (red line): black dots are related to the 6% Ge-doped sample and the green ones to the 0.1% Ge-doped sample.

Figure 4

Global fitting of the different kinetics related to the GLPC’s relaxation under 5.1 eV pump excitation: in black at 3.35 eV, in purple at 3.10 eV, in blue at 2.95 eV, in magenta at 2.75 eV and in olive at 2.60 eV. The red line represents the best fitting function for each probing energy. (a) kinetics in the whole pump-probe delay investigated window, (b) zoom up to 50 ps.

Figure 5

TA spectra measured at negative times in the 6% doped Ge sample: the positive signal is a fingerprint of the T₁ → T₂ transition.

Figure 6

GLPC's photocycle in low Ge-content sample: the yellow spheres represent the electrons occupying the electronic levels with the corresponding spin states (black arrow up and down), dashed arrows indicate the different transitions characterizing the GLPC’s excitation/relaxation dynamics, purple and blue arrows are related to the incoming pump and probe pulse, respectively. In the figure are pictured respectively the GLPC in the ground states, the processes for pump-probes delays shorter than 1 ns and the dynamics between 1 ns and ~ 10² µs leading the system newly to the ground states.