11 Label-Free Super-Resolution Microscopy by Nonlinear …
263
the PSF pp of the linear light-induced thermo-reflectance (TR) is the product of the
individual PSFs of the pump, PSF pump , and that of the probe, PSF probe .
PSF pp = PSF pump × PSF probe
(11.1)
By itself, the P&P configuration bears considerable improvement in resolution
over the single-beam PSF. In this respect, stimulated Raman scattering and coherent
anti-Stokes Raman scattering microscopies are SR techniques. In the case when the
P&P has similar wavelengths, the improvement amounts to ~
√
2. However, when
the interrogated property of the sample by the pump is in the mid-IR, while the
probe monitors the deposited energy at visible wavelengths, the effective resolution
surpasses the mid-IR PSF by almost an order of magnitude. The distinct fingerprints
of the mid-IR spectral feature add chemical recognition in this modality, while the
pump wavelength can be selected in much shorter wavelengths. This approach to
achieve IR SR has been demonstrated recently using photo-thermal technique. JiXin Cheng and co-workers [12] reported on label-free three-dimensional chemical
imaging of live cells and organisms. Hartland and co-workers [13] reported on SR
in polystyrene beads, thin polymer films, and single Escherichia coli bacterial cells.
11.2 Nonlinear Photo-modulated Reflectivity
11.2.1 Photo-Induced Reflectivity Changes
Upon ultra-fast photo-excitation, materials undergo fast dynamics of energy transfer between various degrees of freedom. Charge-carrier excitation (10–100 fs) is
followed by carrier–carrier and carrier–phonon scattering/thermalization processes
(10 fs–10 ps) [14, 15]. Eventually, on a timescale of a few picoseconds, the thermal transport can be treated classically. The instantaneous diffraction-limited photoexcited spatial profile diffuses quickly and blurs in time (see Fig. 11.1). The above
dynamics induce spatial and temporal changes in the reflectivity. Photo-induced
reflectance originates from numerous physical effects [16], mostly from changes
in carrier concentration and in temperature. Light-induced thermo-reflectance (or
time-domain thermo-reflectance), which records changes of reflectance upon photoexcitation, is extensively used to measure the thermal properties of materials using
linear models [17–19]. We are concerned with the nonlinear components of photomodulated reflectivity, in respect to photo-excitation, which allow the dramatic narrowing of the effective PSF.
263
the PSF pp of the linear light-induced thermo-reflectance (TR) is the product of the
individual PSFs of the pump, PSF pump , and that of the probe, PSF probe .
PSF pp = PSF pump × PSF probe
(11.1)
By itself, the P&P configuration bears considerable improvement in resolution
over the single-beam PSF. In this respect, stimulated Raman scattering and coherent
anti-Stokes Raman scattering microscopies are SR techniques. In the case when the
P&P has similar wavelengths, the improvement amounts to ~
√
2. However, when
the interrogated property of the sample by the pump is in the mid-IR, while the
probe monitors the deposited energy at visible wavelengths, the effective resolution
surpasses the mid-IR PSF by almost an order of magnitude. The distinct fingerprints
of the mid-IR spectral feature add chemical recognition in this modality, while the
pump wavelength can be selected in much shorter wavelengths. This approach to
achieve IR SR has been demonstrated recently using photo-thermal technique. JiXin Cheng and co-workers [12] reported on label-free three-dimensional chemical
imaging of live cells and organisms. Hartland and co-workers [13] reported on SR
in polystyrene beads, thin polymer films, and single Escherichia coli bacterial cells.
11.2 Nonlinear Photo-modulated Reflectivity
11.2.1 Photo-Induced Reflectivity Changes
Upon ultra-fast photo-excitation, materials undergo fast dynamics of energy transfer between various degrees of freedom. Charge-carrier excitation (10–100 fs) is
followed by carrier–carrier and carrier–phonon scattering/thermalization processes
(10 fs–10 ps) [14, 15]. Eventually, on a timescale of a few picoseconds, the thermal transport can be treated classically. The instantaneous diffraction-limited photoexcited spatial profile diffuses quickly and blurs in time (see Fig. 11.1). The above
dynamics induce spatial and temporal changes in the reflectivity. Photo-induced
reflectance originates from numerous physical effects [16], mostly from changes
in carrier concentration and in temperature. Light-induced thermo-reflectance (or
time-domain thermo-reflectance), which records changes of reflectance upon photoexcitation, is extensively used to measure the thermal properties of materials using
linear models [17–19]. We are concerned with the nonlinear components of photomodulated reflectivity, in respect to photo-excitation, which allow the dramatic narrowing of the effective PSF.