This result can be explained by closer position of
SPR peak to the laser wavelength at 532 nm, so the
probability of laser absorption much higher in case
of Br-containing samples with SPR peak near 450
nm. Nevertheless, in case of laser irradiation at 355
nm there is no similar dependence, where the
photobleaching process of Ag-Br sample takes
10000 pulses, whereas Ag-Br-F and Ag-Cl samples
become colorless even after 6000 pulses. It is also
reasonable to suggest that the photobleaching by
laser pulses at 532 nm wavelength took less pulses
because of higher energy density (0.5 J/cm
2
) to
compare with laser pulses at 355 nm (0.2 J/cm
2
).
Table 2: SPR band shift.
Laser
wavelength,
nm
Ag-Br, nm Ag-Cl, nm Ag-Br-F, nm
532 2 1 1
355 27 32 35
According to the theoretical research (Nikonorov et
al., 2009) the position of SPR peak depends on the
permittivity of the nanoparticle itself, its shell and
surrounding dielectric. It is assumed that SNP in the
investigated glasses are surrounded by high
refractive index shell (AgBr, Na
0.8
Ag
0.2
Cl), which
causes the initial SPR peak red-shifted position. The
following laser irradiation results in the
photodestruction of SNP, which can occur in
different ways (Hashimoto et al., 2012). It seems
that two main mechanisms of the photodestruction
take place: photothermal evaporation and Coulomb
explosion. Near-field ablation hardly involved in the
photodestruction of considered glasses as long as it
requires high intensity density, which can be
achieved rather by femtosecond laser pulses.
Therefore, it reasonable to explain the red shift of
SPR peak after laser irradiation at 355 nm by
photodestruction of SNP through the photothermal
evaporation of silver ions that come from the
nanoparticles core to the surroundings. That process
causes a local increase of refractive index of SNP
shell and following red-shift of SPR peak. This
process occurs gradually because the laser pulse
wavelength slightly overlap with SPR band.
Coulomb explosion mechanism is likely involved in
the photodestruction of SNPs by laser radiation at
532 nm. In this case SNPs are breaking at smaller
ones and as long as it occurs in condensed material
like glass with high value of viscosity the smaller
parts of SNPs can not overcome the glass matrix and
they stay near each other surrounded by shell with
the same refractive index. The reason of involving of
such mechanisms of photodestruction for particular
wavelengths is not quite understood yet.
4 CONCLUSIONS
In conclusion, we have demonstrated a possibility of
reduction of the absorption band by bleaching
technology with the use of pulse (9 ns) laser
radiation with two wavelengths at 355 and 532 nm.
X-ray diffraction analysis have revealed the
existence of AgBr, Na
0.8
Ag
0.2
Cl and NaF crystalline
phase in the investigated glasses after the mercury
lamp irradiation and following thermal treatment.
Such a crystalline phase is located around SNPs and
affects on the SPR band position. During the
irradiation the SNPs absorption band decreases
depending on the exposure dose. This process
accompanies with a red shift of SPR band (35 nm)
after the laser pulses at 355 nm, whereas there is no
significant shift of the absorption band after the laser
pulses at 532 nm. The photothermal evaporation is
responsible for the photodestruction of SNPs in case
of laser pulses at 355 nm, whereas the Coulomb
explosion can explain the results of the action by the
nanosecond laser pulses at 532 nm. The technology
allowed us to control the size of the silver
nanoparticeles in PTR glasses and record the phase
holograms in visible range.
ACKNOWLEDGEMENTS
This work was financially supported by Russian
Scientific Foundation (Agreement # 14-23-00136).
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