180
P. Bianchini et al.
7.2.3 State-of-the-Art of Pump–Probe Microscopy
The first pump–probe microscope was realized by Dong et al. in the 1990s to measure
the fluorescence lifetime through SE detection [24, 25]. Their approach relied on the
use of two ultrafast pulsed lasers characterized by slightly different repetition rates
and a consequent variable delay between their pulses. In this way, the probe pulse,
tuned to induce stimulated emission in the fluorophore, was repeatedly sampling the
excited population at different delays, providing dynamic information with superior
spatial and temporal resolution without any amplitude modulation and delay line.
In 2007, the Warren group proposed the first pump–probe microscope based on a
high-frequency modulation scheme to detect melanins in cells and differentiate them
by looking at the phase as well as at the temporal evolution of the pump–probe signal
[26, 27]. Since then, transient absorption microscopy has started to be explored more
in different research fields, from material to biological to art sciences.
Because of its non-fluorescent-based contrast, single-particle sensitivity, and high
temporal resolution, pump–probe microscopy is extensively used in imaging and
characterizing the carrier dynamics of metallic and semiconducting nanostructures
for studying their optoelectronic properties [17, 28–39]. Pump–probe techniques
have acquired importance also in mapping weakly or non-fluorescent nanostructures
(like single-walled carbon nanotubes, nanodiamonds, gold nanoparticles, etc.) used
as biorthogonal labels and for biomedical applications inside living cells and tissues
[40–44].
Among nanomaterials, recently graphene has started to be intensively studied for
the development of novel electronic and optoelectronic devices [45]. Owing to its particular linear electronic band structure, graphene shows a wavelength-independent,
broadband optical absorption (~2.3% per layer [46]) in the NIR part of the spectrum [47], together with a large third-order susceptibility χ
(3) [48]. This makes
pump–probe microscopy an ideal technique for studying its carrier dynamics and for
performing imaging, mainly using NIR wavelengths [49–62]. The carrier dynamics
were extensively explored for different graphene structures, like in few-layer epitaxial graphene [49], single- and multi-layer exfoliated graphene [50, 55], graphene
suspensions [56, 58], and graphene oxide [59]. Few experiments were performed on
single-layer graphene (SLG) [50, 57]. For all these graphene-based structures, the
relaxation dynamics results are characterized by two timescales. The faster one falls
in the range of 70–120 fs and it is attributed to carrier thermalization by electron—
electron interactions. The slower one is in the range of 0.4–1.7 ps; it is associated with
carrier cooling via electron–phonon interactions and its value is inversely proportional to the crystal disorder [45]. In Fig. 7.4a an example of time-resolved spectrum
of multi-layer graphene flakes is presented, fitted with a double exponential decay.
The fast component is sub-resolved due to the limited temporal resolution of the
system. The slow component falls in the range found in the literature. Images of a
graphene flake at different delays are also shown.
In 2010, Huang et al. [60] first obtained pump–probe microscopy imaging of
multi-layer epitaxial graphene grown on a silicon substrate, and showed the linear
P. Bianchini et al.
7.2.3 State-of-the-Art of Pump–Probe Microscopy
The first pump–probe microscope was realized by Dong et al. in the 1990s to measure
the fluorescence lifetime through SE detection [24, 25]. Their approach relied on the
use of two ultrafast pulsed lasers characterized by slightly different repetition rates
and a consequent variable delay between their pulses. In this way, the probe pulse,
tuned to induce stimulated emission in the fluorophore, was repeatedly sampling the
excited population at different delays, providing dynamic information with superior
spatial and temporal resolution without any amplitude modulation and delay line.
In 2007, the Warren group proposed the first pump–probe microscope based on a
high-frequency modulation scheme to detect melanins in cells and differentiate them
by looking at the phase as well as at the temporal evolution of the pump–probe signal
[26, 27]. Since then, transient absorption microscopy has started to be explored more
in different research fields, from material to biological to art sciences.
Because of its non-fluorescent-based contrast, single-particle sensitivity, and high
temporal resolution, pump–probe microscopy is extensively used in imaging and
characterizing the carrier dynamics of metallic and semiconducting nanostructures
for studying their optoelectronic properties [17, 28–39]. Pump–probe techniques
have acquired importance also in mapping weakly or non-fluorescent nanostructures
(like single-walled carbon nanotubes, nanodiamonds, gold nanoparticles, etc.) used
as biorthogonal labels and for biomedical applications inside living cells and tissues
[40–44].
Among nanomaterials, recently graphene has started to be intensively studied for
the development of novel electronic and optoelectronic devices [45]. Owing to its particular linear electronic band structure, graphene shows a wavelength-independent,
broadband optical absorption (~2.3% per layer [46]) in the NIR part of the spectrum [47], together with a large third-order susceptibility χ
(3) [48]. This makes
pump–probe microscopy an ideal technique for studying its carrier dynamics and for
performing imaging, mainly using NIR wavelengths [49–62]. The carrier dynamics
were extensively explored for different graphene structures, like in few-layer epitaxial graphene [49], single- and multi-layer exfoliated graphene [50, 55], graphene
suspensions [56, 58], and graphene oxide [59]. Few experiments were performed on
single-layer graphene (SLG) [50, 57]. For all these graphene-based structures, the
relaxation dynamics results are characterized by two timescales. The faster one falls
in the range of 70–120 fs and it is attributed to carrier thermalization by electron—
electron interactions. The slower one is in the range of 0.4–1.7 ps; it is associated with
carrier cooling via electron–phonon interactions and its value is inversely proportional to the crystal disorder [45]. In Fig. 7.4a an example of time-resolved spectrum
of multi-layer graphene flakes is presented, fitted with a double exponential decay.
The fast component is sub-resolved due to the limited temporal resolution of the
system. The slow component falls in the range found in the literature. Images of a
graphene flake at different delays are also shown.
In 2010, Huang et al. [60] first obtained pump–probe microscopy imaging of
multi-layer epitaxial graphene grown on a silicon substrate, and showed the linear