6 Absorption-Based Far-Field Label-Free Super-Resolution …
139
microscopy, on the other hand, is rising as a popular tool in biology, biomedicine,
and so on [47–49].
Although absorption-based spectroscopic imaging techniques have come a long
way to pursue fast and sensitive measurement, the limited spatial resolution remains
to be a problem for further applications that require sub-micron or nanoscale resolution. In particular, the most popular absorption-based spectroscopic imaging techniques, such as mid-IR microscopy, near-IR microscopy, and transient-absorption
microscopy, all use long wavelength light source (0.8–10 µm) whose spatial resolution stay in the micrometer scale. To achieve higher resolution, many methods have
been reported during the recent years. One way of improving the image resolution is
to deconvolve the captured image with the PSF of the system through image processing [50, 51]. Albeit widely applied and ready to use, deconvolution method could not
really break the diffraction limit in far-field imaging systems. Thus, other methods
using near-field configuration to achieve absorption-based super-resolution imaging
were proposed. In 2000s, scanning near-field microscopy was combined with IR
spectroscopy to achieve 20 nm resolution [52]. Later, atomic force microscope tips
were used to probe the thermal expansion of objects induced by the absorption of
IR, which obtained IR absorption spectroscopy from samples smaller than 10 nm
[53–56]. However, the low throughput and sample-probe contact of these near-field
methods limited their applications in biomedical sciences.
To address this problem, ideas of absorption-based far-field LFSRM were proposed in recent years. The first category is featured as photothermal (PT) microscopy,
which uses a probe beam to detect the subtle PT lens effect induced by the linear
absorption of the excitation (pump) beam. The PT lens measurement was first conducted in 1965 in a single-beam apparatus which uses the same beam for both pump
and detection [57]. The dual-beam apparatus was then exploited to further take advantage of the sensitivity of PT detection [58]. It was not until the twenty-first century
when PT detection was realized to be an approach to break the diffraction limit of
the pump beam. As an example, various mid-infrared PT microscopes achieve submicron IR spectroscopic imaging by confocally exciting the objects with a mid-IR
beam and probing with another tightly focused visible or near-IR beam [59]. The
improved spatial resolution (~700 nm) allows those methods to obtain the IR spectra
from the sub-cellular structures of living organisms or tissue slices for the first time.
Besides, recent attempts which deploy counter-propagation measures have pushed
the resolution up to 300 nm to obtain the IR spectrum of single Escherichia coli
and local cation heterogeneities in mixed cation perovskite [60]. The idea of detecting the nonlinear PT terms to further increase the spatial resolution has also been
demonstrated [61].
The second category of absorption-based far-field LFSRM, instead of probing
the linear absorption, uses an apparatus that is inspired by super-resolution GSD
microscopy and measures the nonlinear absorption of incident beams [62]. In the
saturated transient absorption, a doughnut-shaped saturation pump beam was applied
to confine the transient absorption signal within the very center of the focal spot to
achieve super-resolution pump–probe imaging. The same apparatus can be used
in other nonlinear optical processes to detect the materials absorption with sub-
139
microscopy, on the other hand, is rising as a popular tool in biology, biomedicine,
and so on [47–49].
Although absorption-based spectroscopic imaging techniques have come a long
way to pursue fast and sensitive measurement, the limited spatial resolution remains
to be a problem for further applications that require sub-micron or nanoscale resolution. In particular, the most popular absorption-based spectroscopic imaging techniques, such as mid-IR microscopy, near-IR microscopy, and transient-absorption
microscopy, all use long wavelength light source (0.8–10 µm) whose spatial resolution stay in the micrometer scale. To achieve higher resolution, many methods have
been reported during the recent years. One way of improving the image resolution is
to deconvolve the captured image with the PSF of the system through image processing [50, 51]. Albeit widely applied and ready to use, deconvolution method could not
really break the diffraction limit in far-field imaging systems. Thus, other methods
using near-field configuration to achieve absorption-based super-resolution imaging
were proposed. In 2000s, scanning near-field microscopy was combined with IR
spectroscopy to achieve 20 nm resolution [52]. Later, atomic force microscope tips
were used to probe the thermal expansion of objects induced by the absorption of
IR, which obtained IR absorption spectroscopy from samples smaller than 10 nm
[53–56]. However, the low throughput and sample-probe contact of these near-field
methods limited their applications in biomedical sciences.
To address this problem, ideas of absorption-based far-field LFSRM were proposed in recent years. The first category is featured as photothermal (PT) microscopy,
which uses a probe beam to detect the subtle PT lens effect induced by the linear
absorption of the excitation (pump) beam. The PT lens measurement was first conducted in 1965 in a single-beam apparatus which uses the same beam for both pump
and detection [57]. The dual-beam apparatus was then exploited to further take advantage of the sensitivity of PT detection [58]. It was not until the twenty-first century
when PT detection was realized to be an approach to break the diffraction limit of
the pump beam. As an example, various mid-infrared PT microscopes achieve submicron IR spectroscopic imaging by confocally exciting the objects with a mid-IR
beam and probing with another tightly focused visible or near-IR beam [59]. The
improved spatial resolution (~700 nm) allows those methods to obtain the IR spectra
from the sub-cellular structures of living organisms or tissue slices for the first time.
Besides, recent attempts which deploy counter-propagation measures have pushed
the resolution up to 300 nm to obtain the IR spectrum of single Escherichia coli
and local cation heterogeneities in mixed cation perovskite [60]. The idea of detecting the nonlinear PT terms to further increase the spatial resolution has also been
demonstrated [61].
The second category of absorption-based far-field LFSRM, instead of probing
the linear absorption, uses an apparatus that is inspired by super-resolution GSD
microscopy and measures the nonlinear absorption of incident beams [62]. In the
saturated transient absorption, a doughnut-shaped saturation pump beam was applied
to confine the transient absorption signal within the very center of the focal spot to
achieve super-resolution pump–probe imaging. The same apparatus can be used
in other nonlinear optical processes to detect the materials absorption with sub-