6 Absorption-Based Far-Field Label-Free Super-Resolution …
161
One of the major concerns of such approach is the issue of photodamage since the
molecules are repeatedly excited/depleted by the pump/depletion beam in the raster
scanning mode. As Bretschneider et al. discussed in their report on GSD microscopy,
care must be taken when determining the pixel dwell time since molecules excited
to the triplet state must relax to ground state before beam moves on [150]. In SCSTA
experiments, Wang et al. also investigated the photodamage threshold of the intense
depletion beam and recovery of TA signals after repeated excitations using graphene
nanoplate as a model. The photodamage was observed when power density exceeds
~2.4 MW cm
−2 , while the TA signal recovery rate could reach 100% below that
threshold. This approach can be applied to other materials that have saturable absorption properties, such as single-walled carbon nanotubes [154–156], iron oxides [157],
or zinc oxides [158].
6.3.3 Super-Resolution Stimulated Raman Microscopy
Inspired by the success of other super-resolution nonlinear optical microscopy [62,
79], Gong et al. proposed to combine the advantages of stimulated emission depletion
(STED) microscopy and SRS microscopy by developing the super-resolution saturated SRS microscopy in a theoretical study [159]. They proposed to split the Stokes
beam into two components, one of which is modulated as an intense doughnut-shape
by a phase plate and the other one remains a normal Gaussian beam. By recombining
the two Stokes components, an effective Stokes beam with narrower PSF is created at
the focus. However, according to their simulated results, the power density required
for the intense Stokes saturation beam is as high as a few TW cm
−2 , which is almost
impossible to reach in most laboratory conditions.
The super-resolution SRS microscopy was realized in 2015 by Silva et al. using
a triple-beam configuration [63]. Instead of splitting the Stokes beam to create a
doughnut-shape Stokes, Silva et al. deploy another beam as the decoherence beam
whose wavelength is close to that of Stokes beam and destroy the vibrational coherence at the ring of the doughnut-shape. According to some preliminary superresolution SRS imaging results, the spatial resolution was improved by a factor
of ~1.7. As measured in the experiment, the power density of the decoherence beam
required to induce the saturation SRS is ~10 W cm
−2 , which is close to the average power used in previously reported SRS experiments [15, 38, 160]. Note that
this approach uses the same idea as STED microscopy. Thus, an ultimate resolution
of ~50 nm is expected as such technique can be actively developed in the future.
Along with the ability of SRS microscopy providing the informative vibrational
spectroscopy of various biomolecules and inorganic materials, super-resolution SRS
microscopy will find broad applications in biological and materials sciences.
161
One of the major concerns of such approach is the issue of photodamage since the
molecules are repeatedly excited/depleted by the pump/depletion beam in the raster
scanning mode. As Bretschneider et al. discussed in their report on GSD microscopy,
care must be taken when determining the pixel dwell time since molecules excited
to the triplet state must relax to ground state before beam moves on [150]. In SCSTA
experiments, Wang et al. also investigated the photodamage threshold of the intense
depletion beam and recovery of TA signals after repeated excitations using graphene
nanoplate as a model. The photodamage was observed when power density exceeds
~2.4 MW cm
−2 , while the TA signal recovery rate could reach 100% below that
threshold. This approach can be applied to other materials that have saturable absorption properties, such as single-walled carbon nanotubes [154–156], iron oxides [157],
or zinc oxides [158].
6.3.3 Super-Resolution Stimulated Raman Microscopy
Inspired by the success of other super-resolution nonlinear optical microscopy [62,
79], Gong et al. proposed to combine the advantages of stimulated emission depletion
(STED) microscopy and SRS microscopy by developing the super-resolution saturated SRS microscopy in a theoretical study [159]. They proposed to split the Stokes
beam into two components, one of which is modulated as an intense doughnut-shape
by a phase plate and the other one remains a normal Gaussian beam. By recombining
the two Stokes components, an effective Stokes beam with narrower PSF is created at
the focus. However, according to their simulated results, the power density required
for the intense Stokes saturation beam is as high as a few TW cm
−2 , which is almost
impossible to reach in most laboratory conditions.
The super-resolution SRS microscopy was realized in 2015 by Silva et al. using
a triple-beam configuration [63]. Instead of splitting the Stokes beam to create a
doughnut-shape Stokes, Silva et al. deploy another beam as the decoherence beam
whose wavelength is close to that of Stokes beam and destroy the vibrational coherence at the ring of the doughnut-shape. According to some preliminary superresolution SRS imaging results, the spatial resolution was improved by a factor
of ~1.7. As measured in the experiment, the power density of the decoherence beam
required to induce the saturation SRS is ~10 W cm
−2 , which is close to the average power used in previously reported SRS experiments [15, 38, 160]. Note that
this approach uses the same idea as STED microscopy. Thus, an ultimate resolution
of ~50 nm is expected as such technique can be actively developed in the future.
Along with the ability of SRS microscopy providing the informative vibrational
spectroscopy of various biomolecules and inorganic materials, super-resolution SRS
microscopy will find broad applications in biological and materials sciences.