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C. Li and J.-X. Cheng
diffraction resolution. In one example, a doughnut-shaped decoherence beam was
combined with the confocal pump and probe beams to confine the stimulated Raman
gain within the center of the beams, which enables sub-diffraction Raman imaging
[63].
In this review, we review the history and fundamental principles of both linear
and nonlinear absorption-based LFSRM techniques, scrutinize the pros and cons of
different modalities, and present the latest applications and possible future directions
of the field.
6.2 Breaking the Diffraction Limit in IR Microscopy
Through Photothermal Detection
6.2.1 Laser-Induced Photothermal Lens Effect: The Origin
of the Photothermal Contrast
The PT lens effect was first reported and discussed by Gordon et al., whose analysis
built the basis of later works in this field [57]. In that experiment, a liquid sample
cell filled with various pure samples was placed in the beam path and heated by
the absorption of the laser power to produce a thermal gradient in the vicinity of the
incident beam, which resulted in long transients of intensity profiles of the transmitted
beam. It is believed that such observations were induced by the lens effect arising
from the refractive index change near the beam as a result of the thermal gradient. To
further explain it, Gordon et al. built a model to quantitatively depict the refractive
index gradient produced by the PT process as a function of the temperature coefficient
of the refractive index (∂n/∂ T ), through which the detected beam intensity profile
could be used to retrieve the unknown quantities related to the sample, such as
concentration and thermal constants. Since most liquids have a negative value of
∂n/∂ T , the thermal lens effect is usually divergent [58]. That very first experiment
achieved measurement of absorptivity as low as 10
−4 cm
−1 , indicating a promising
approach to probe weak absorptions indirectly with high sensitivity.
6.2.2 Improvement of Detection Sensitivity of Photothermal
Spectroscopy
Soon after Gordon’s experiment, many other works were reported to improve the
physical model of the PT lens effect [64–68]. On the other hand, more efforts were
dedicated to the enhancement of detection sensitivities of the PT measurements
[69–71]. Stone designed an interferometry to better resolve the phase shift induced by
the PT lens effect and achieved ~10
−5 cm
−1 sensitivity [72]. Boccara et al. exploited
an accurate position sensor and applied fast modulation to the pump beam to measure
C. Li and J.-X. Cheng
diffraction resolution. In one example, a doughnut-shaped decoherence beam was
combined with the confocal pump and probe beams to confine the stimulated Raman
gain within the center of the beams, which enables sub-diffraction Raman imaging
[63].
In this review, we review the history and fundamental principles of both linear
and nonlinear absorption-based LFSRM techniques, scrutinize the pros and cons of
different modalities, and present the latest applications and possible future directions
of the field.
6.2 Breaking the Diffraction Limit in IR Microscopy
Through Photothermal Detection
6.2.1 Laser-Induced Photothermal Lens Effect: The Origin
of the Photothermal Contrast
The PT lens effect was first reported and discussed by Gordon et al., whose analysis
built the basis of later works in this field [57]. In that experiment, a liquid sample
cell filled with various pure samples was placed in the beam path and heated by
the absorption of the laser power to produce a thermal gradient in the vicinity of the
incident beam, which resulted in long transients of intensity profiles of the transmitted
beam. It is believed that such observations were induced by the lens effect arising
from the refractive index change near the beam as a result of the thermal gradient. To
further explain it, Gordon et al. built a model to quantitatively depict the refractive
index gradient produced by the PT process as a function of the temperature coefficient
of the refractive index (∂n/∂ T ), through which the detected beam intensity profile
could be used to retrieve the unknown quantities related to the sample, such as
concentration and thermal constants. Since most liquids have a negative value of
∂n/∂ T , the thermal lens effect is usually divergent [58]. That very first experiment
achieved measurement of absorptivity as low as 10
−4 cm
−1 , indicating a promising
approach to probe weak absorptions indirectly with high sensitivity.
6.2.2 Improvement of Detection Sensitivity of Photothermal
Spectroscopy
Soon after Gordon’s experiment, many other works were reported to improve the
physical model of the PT lens effect [64–68]. On the other hand, more efforts were
dedicated to the enhancement of detection sensitivities of the PT measurements
[69–71]. Stone designed an interferometry to better resolve the phase shift induced by
the PT lens effect and achieved ~10
−5 cm
−1 sensitivity [72]. Boccara et al. exploited
an accurate position sensor and applied fast modulation to the pump beam to measure