152
C. Li and J.-X. Cheng
especially in the biologically informative amide I and C=O carboxyl bands [120],
limited the broad interests of such strategy in practical applications since the mid-IR
pump wavelength has to be tuned off resonance with these bands. The nonlinear
PT effect has been observed in spectroscopy studies [89, 121], but super-resolution
mid-IR excited PTM has not been enabled via nonlinear PT effects. With this field
actively growing, those technical barriers that limit the pursuit of higher resolution
will eventually be eliminated.
Similar to the case of visible excited PTM, the pump laser-induced photodamage
is also considered in mid-IR excited PTM. Just like what have been discussed in
the visible excited PTM in Sect. 6.2.4, the photodamage in mid-IR excited PTM is
also induced by the thermal damage instead of direct photon excitation. However,
the mid-IR photodamage threshold for most samples is much higher than that in the
case of visible excitation since mid-IR photons only induce covalent bond vibrations which are more reversible processes compared to the electronic state excitation
induced in visible excited PTM. To avoid accumulated heat causing damage to the
sample, it is suggested to control the duty cycle of the mid-IR pump beam according to the heat dissipation rate of the sample and surroundings. Li et al. proposed
a model to simulate the PT relaxation time as a function of object size for a series
of polystyrene spheres in the air–glass surface [60]. Their results indicated that the
accumulated heat will vanish in 10
−6 s, which is apparently faster than experimental
observations in other reports [59, 116]. Therefore, such model needs further amendment to be more accurate in predicting the actual relaxation time in mid-IR excited
PT processes. So far, most of the reported mid-IR PTM studies adopted ~10 µs as a
single period of PT measurement. Note that some mediums with large heat capacity
like water (4.18 J/kg K) will accelerate the heat dissipation, which helps reduce the
photodamage from local overheating. Such phenomenon was found in the in vivo
mid-IR PT imaging of cells and organisms [59].
6.2.6 Applications of Super-Resolution Photothermal
Microscopy
Owing to the superb sensitivity in the detection of non-fluorescent species, both the
visible and mid-IR excited super-resolution photothermal microscopies (SR-PTMs)
have found numerous applications in the detection of trace analytes in materials
science and biomedical studies. As discussed in the preceding sections, compared to
mid-IR excitation, visible excited SR-PTM is able to take advantage of plasmonic
resonance of metallic analytes to achieve single-molecule detection limit, high spatial
resolution (~100 nm), and provides electronic absorption information. Therefore,
it has been widely applied in the label-free imaging of individual nanoparticles,
nanoclusters, and nanocrystals [80, 122]. Note that the size dependence of the PT
signal has been well studied, as shown in Fig. 6.9, which enables the discrimination
among different-sized particles that are much smaller than the diffraction limits of
C. Li and J.-X. Cheng
especially in the biologically informative amide I and C=O carboxyl bands [120],
limited the broad interests of such strategy in practical applications since the mid-IR
pump wavelength has to be tuned off resonance with these bands. The nonlinear
PT effect has been observed in spectroscopy studies [89, 121], but super-resolution
mid-IR excited PTM has not been enabled via nonlinear PT effects. With this field
actively growing, those technical barriers that limit the pursuit of higher resolution
will eventually be eliminated.
Similar to the case of visible excited PTM, the pump laser-induced photodamage
is also considered in mid-IR excited PTM. Just like what have been discussed in
the visible excited PTM in Sect. 6.2.4, the photodamage in mid-IR excited PTM is
also induced by the thermal damage instead of direct photon excitation. However,
the mid-IR photodamage threshold for most samples is much higher than that in the
case of visible excitation since mid-IR photons only induce covalent bond vibrations which are more reversible processes compared to the electronic state excitation
induced in visible excited PTM. To avoid accumulated heat causing damage to the
sample, it is suggested to control the duty cycle of the mid-IR pump beam according to the heat dissipation rate of the sample and surroundings. Li et al. proposed
a model to simulate the PT relaxation time as a function of object size for a series
of polystyrene spheres in the air–glass surface [60]. Their results indicated that the
accumulated heat will vanish in 10
−6 s, which is apparently faster than experimental
observations in other reports [59, 116]. Therefore, such model needs further amendment to be more accurate in predicting the actual relaxation time in mid-IR excited
PT processes. So far, most of the reported mid-IR PTM studies adopted ~10 µs as a
single period of PT measurement. Note that some mediums with large heat capacity
like water (4.18 J/kg K) will accelerate the heat dissipation, which helps reduce the
photodamage from local overheating. Such phenomenon was found in the in vivo
mid-IR PT imaging of cells and organisms [59].
6.2.6 Applications of Super-Resolution Photothermal
Microscopy
Owing to the superb sensitivity in the detection of non-fluorescent species, both the
visible and mid-IR excited super-resolution photothermal microscopies (SR-PTMs)
have found numerous applications in the detection of trace analytes in materials
science and biomedical studies. As discussed in the preceding sections, compared to
mid-IR excitation, visible excited SR-PTM is able to take advantage of plasmonic
resonance of metallic analytes to achieve single-molecule detection limit, high spatial
resolution (~100 nm), and provides electronic absorption information. Therefore,
it has been widely applied in the label-free imaging of individual nanoparticles,
nanoclusters, and nanocrystals [80, 122]. Note that the size dependence of the PT
signal has been well studied, as shown in Fig. 6.9, which enables the discrimination
among different-sized particles that are much smaller than the diffraction limits of