PbSe Core, PbSe/PbS and PbSe/PbSe
x
S
1 x
Core–Shell Nanocrystal Quantum Dots 761
between 490 ns (for 4.2 nm NQDs with x 0) to 250 ns (for 4.9 nm NQDs with x 0.2). This
can be correlated with the behaviour of the core PbSe structures, suggesting a spreading of the
wavefunctions into the shell, leading to the reduction of the recombination rate. The reliance of
the emission lifetime on the excitation peak intensity for three kinds of NQD structures is shown
in Fig. 25.8d . This dependency seems to be stronger for the PbSe core NQDs, and weaker for the
PbSe/PbSe core–shell NQDs and the PbSe/PbSe
x
S
1
x
core-alloyed–shell NQDs, with dimensions
and compositions as indicated in the fi gure. The decay curves shown here should be further
investigated in the near future, clarifying the distribution of the wavefunction between the core
and the shell, and the origin of the short and long component of the decay processes.
25.5 Passive Q-switching, using PbSe NQDs, PbSe/PbS core–shell NQDs and PbSe/PbSe
x
S
1
x
core-alloyed–shell NQDs
The use of a saturable absorber inside a laser cavity may act as a passive Q-switching device,
contributing to the output beam quality and simplifying the laser system design. A few materials
have already been recognized as saturable absorbers operating in the near-IR regime. The sys-
tems described in past studies [42–47] provided Q-switching in well-defi ned wavelength ranges,
i.e. ⬃1.34, 1.44 and 1.54 μ m, with typical ground-state absorption cross-sections of σ
gs
0 . 8 –
5.7 10
1 9
cm
2
. The PbSe NQDs can function as Q-switches over wide spectral regions, and as
will be shown below, they exhibit exceptionally large σ
gs
.
The present work describes an attempt to use the PbSe NQDs, PbSe/PbS core–shell NQDs and
PbSe/PbSe
x
S
1
x
core-alloyed–shell NQDs as passive Q-switches in Er:glass lasers operating at
1.54 μ m. This laser is of special interest, due to its potential applications in light detection and
ranging, surgery, and telecommunications. This stems from the fact that its emission wavelength
coincides with the so-called eye-safe spectral region, corresponding also to one of the atmos-
pheric transmission windows, and to the third transmission window of silica fi bres.
The performance of these NQDs as passive Q-switches was initially investigated by characteri-
zation of their behaviour as saturable absorbers. This was done by following the change in optical
transmission versus the power of the pumping laser. For that purpose, we used 1.537 μ m signal
output pulses from a 1.064 μ m pumped KTiOPO
4
optical parametric oscillation laser. The output
(signal) beam, with energy up to 150 mJ/pulse and typical 10 ns duration (FWHM), was verti-
cally polarized with a near-TEM
00
transverse energy distribution. The laser pulse fl uency (energy
density) at the investigated sample was varied by moving the sample along the propagation axis
of the laser beam, which was focused by a focusing lens. The transverse beam energy distribution
at each point was measured using the knife-edge scanning method.
Figure 25.9 shows representative examples of the 1.54 μ m transmission through an NQD
solution versus the pumping pulse intensity (upper scale). In the case of PbSe NQDs (top curve),
the transmission starts at approximately T
0
62% in the low power range, rises with increasing
pulse power, and saturates at approximately T
max
77% in the high power range. This general
behaviour was also observed when the light was transmitted through the PbSe/PbS core–shell
and PbSe/PbSe
x
S
1
x
core-alloyed–shell NQDs samples (lower curves) as indicated in the fi g-
ure. The saturation occurs due to depletion of the ground state above a given laser fl uency. The
fact that the transmission never reaches 100% is explained by the existence of excited-state
absorption.
The 1S exciton singlets τ
f
are of the order of 70–1000 ns ( Fig. 25.8 ), which are considerably
longer than the pump laser duration of ⬃10 ns. Thus, initially we assumed that PbSe NQDs
may act as “ slow ” saturable absorbers, with respect to the pumping laser. It should, however, be
remembered that τ
f
was estimated from the PL spectrum observed under low-power illumina-
tion, ensuring that there was never more than a single exciton created in a single NQD. Then, the
ground-state absorption coeffi cient, σ
gs
(1/ NL )ln T
0
, where N is the NQD volume density and
L is the sample thickness, could be calculated to be σ
gs
艑 10
1 5
10
1 6
cm
2
. This preliminary
assumption, however, yielded a serious internal inconsistency. The calculated saturation fl uency,
J
S
⬅ hv / σ
gs
, is of the order of several hundred μ J /cm
2
, in contradiction with the experimental value
of ⬃1 J / cm
2
( Fig. 25.9 ). This inconsistency led to a second thought that actually under absorption
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