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11.1 Introduction
The widely spread technology of microscopy can offer tremendous amount of information about the structural, electronic, optical, and chemical properties of materials, as well as a means to manipulate and trigger processes in materials and
devices. Regrettably, in its conventional implementation, the spatial resolution of
probing and manipulating materials is limited to about λ/2, due to the wave-like
nature of light. The quest to understand structure, dynamics, and function at the
nanoscale has inspired new ultra-high-resolution imaging techniques. In particular,
optical microscopy has succeeded in surpassing the Abbé resolution limit (~0.5λ),
either by near-field techniques [1] or by far-field super-resolution (SR) techniques,
such as stimulated emission depletion (STED) [2, 3], photo-activated localization
microscopy (PALM), stochastic optical reconstruction microscopy (STORM) [4, 5],
saturated absorption (SAX) [6], structured illumination [7], SR optical fluctuation
imaging (SOFI) [8], and quantum emitters microscopy [9].
These fluorescence-based techniques are very useful when functional groups can
be reliably and selectively labeled. However, in many instances, such as optoelectronic devices or untreated biological tissues, far-field, label-free microscopy is desirable. Near-field scanning optical microscopy (NSOM) indeed serves such purpose
by coupling light to nanostructures that are specifically designed to manipulate,
enhance, and/or extract optical signals down to sizes in the order of 20 nm. Despite
its diverse and sophisticated methods, such as absorption of light or tip-enhanced
Raman scattering, its usability is limited to a range of few tens of nanometers from
the surface.
Recently, far-field methods, free of fluorescent labeling, were introduced and many
of them are reviewed in this book. We name here a couple of approaches which are
not mentioned in the book. Wang and co-workers used ground-state depletion of
the charge carriers in graphene-like structures in transmission mode [10]. Photoacoustics microscopy has been recently developed to demonstrate label-free superresolution [11]. In this chapter, we will present our methodology for far-field labelfree SR that is based on the nonlinear response of the reflectance to photo-excitation.
11.1.1 Pump and Probe with Large Difference
in Wavelengths for SR
Implicit to our approach for SR is the use of pump–probe (P&P) configuration, where
the two beams coincide and are focused with diffraction-limited point spread function
(PSF) on the same spot of the sample. In the simple, linear case we assume that the
change in the probe intensity, as a consequence of the interaction of the sample with
the exciting pump, is linear with the exciting pump and the probe. A basic assumption
in our method is that the instantaneous temperature or charge-carriers’ distributions
mimic the three-dimensional energy absorption profile of the pump. For instance,
O. Tzang et al.
11.1 Introduction
The widely spread technology of microscopy can offer tremendous amount of information about the structural, electronic, optical, and chemical properties of materials, as well as a means to manipulate and trigger processes in materials and
devices. Regrettably, in its conventional implementation, the spatial resolution of
probing and manipulating materials is limited to about λ/2, due to the wave-like
nature of light. The quest to understand structure, dynamics, and function at the
nanoscale has inspired new ultra-high-resolution imaging techniques. In particular,
optical microscopy has succeeded in surpassing the Abbé resolution limit (~0.5λ),
either by near-field techniques [1] or by far-field super-resolution (SR) techniques,
such as stimulated emission depletion (STED) [2, 3], photo-activated localization
microscopy (PALM), stochastic optical reconstruction microscopy (STORM) [4, 5],
saturated absorption (SAX) [6], structured illumination [7], SR optical fluctuation
imaging (SOFI) [8], and quantum emitters microscopy [9].
These fluorescence-based techniques are very useful when functional groups can
be reliably and selectively labeled. However, in many instances, such as optoelectronic devices or untreated biological tissues, far-field, label-free microscopy is desirable. Near-field scanning optical microscopy (NSOM) indeed serves such purpose
by coupling light to nanostructures that are specifically designed to manipulate,
enhance, and/or extract optical signals down to sizes in the order of 20 nm. Despite
its diverse and sophisticated methods, such as absorption of light or tip-enhanced
Raman scattering, its usability is limited to a range of few tens of nanometers from
the surface.
Recently, far-field methods, free of fluorescent labeling, were introduced and many
of them are reviewed in this book. We name here a couple of approaches which are
not mentioned in the book. Wang and co-workers used ground-state depletion of
the charge carriers in graphene-like structures in transmission mode [10]. Photoacoustics microscopy has been recently developed to demonstrate label-free superresolution [11]. In this chapter, we will present our methodology for far-field labelfree SR that is based on the nonlinear response of the reflectance to photo-excitation.
11.1.1 Pump and Probe with Large Difference
in Wavelengths for SR
Implicit to our approach for SR is the use of pump–probe (P&P) configuration, where
the two beams coincide and are focused with diffraction-limited point spread function
(PSF) on the same spot of the sample. In the simple, linear case we assume that the
change in the probe intensity, as a consequence of the interaction of the sample with
the exciting pump, is linear with the exciting pump and the probe. A basic assumption
in our method is that the instantaneous temperature or charge-carriers’ distributions
mimic the three-dimensional energy absorption profile of the pump. For instance,