5.1 Conclusions
In conclusion, the combination of reverse saturable scattering and SAX is demonstrated. With exerting modulated excitation light, the nonlinear signals can be extracted by Fourier transform to realize SAX microscopy. Different from the SAX images of fluorescence, the nonlinear components of scattering show many dips at higher excitation intensity. Based on quantitative analysis of amplitude and phase of different frequency components in SAX, the phase change and resolution enhancement of signal are observed after every dip. The results suggest better resolution enhancement and higher signal contrast can be achieved by SAX microscopy in reverse saturable scattering region. The results can be used for achieving better resolution in imaging and investigating the nonlinear properties more precisely.
5.2 Future works
For the potential application from the experiment and calculation, we can get even higher slopes (in log-log scale) in 2fm and 3fm signals when the excitation intensity is higher compared with the case in lower one. With the aid of this different phenomenon of saturation in scattering, better resolution can be achieved in scattering than in fluorescence images by SAX microscopy. The 2fm signals may have the same resolution enhancement as other higher order SAX signals, but with much higher signal-to-noise ratio, which is also a critical issue in SAX microscopy. What’s more, if the excitation intensity is selected correctly, we can further get better resolution when the SAX signals arise from a drop, with extremely steep slope. The results can be applied for superresolution by the agents with high nonlinearity.
Another possible application by SAX analysis is to find the signal-excitation
relation more precisely. It is very common that the measured signal-excitation curve is not precise due to some noise or error, and the coefficients of the expanding polynomial are more incorrect when the order of the polynomial is higher. To get the signal-excitation curve, we can use the SAX method in reverse way. First we get the SAX signals of enough high order terms (ex. 6fm or 7fm) by experiment from low to high excitation intensity, and it is not difficult to know that we can use every acquired term to do inverse Fourier transform, and further get scattering curve. The method can get even more precise curve because the drops at some specific excitation intensity show more information about the saturation. However, in experiment, higher order SAX signals imply lower signal-to-noise ratio; therefore high pass filter when getting higher order terms is required.
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