Cross-Term Selective, Two-Pulse Correlation Measurements by Phase-Shifted Parallel Modulation for Analysis of a Multi-Photon Process

Abstract

We propose a new modulation technique for phase-sensitive detection to extract a cross-term signal appearing in multi-pulse experiments and demonstrate the selective detection of cross-term signal on two-pulse correlation measurements. This modulation technique can be widely applied to the selective detection in multi-beam laser experiments such as two-pulse correlation, auto-correlation, and double-resonance experiments, because the selectivity can be realized by using a conventional single-frequency mechanical chopper and 2f phase-sensitive detection. The effectiveness of the proposed technique was confirmed by two-pulse correlation measurements of two-photon-excited fluorescence from rhodamine 6G. In addition to the selective observation of a correlation peak, the technique succeeded in observing a weak correlation background. By combining asymmetric, two-pulse correlation measurements with the proposed technique, it was clarified that the background was produced by a three-photon process and assigned to fluorescence depletion caused by an up-conversion process. The results indicate that the cross-term selective, two-pulse correlation method that is based on the phase-shifted parallel modulation technique is powerful tool to find and analyze a high-order optical event buried in a low-order optical event.

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“…PI, on the other hand, cannot penetrate intact cell membranes; it can only enter cells when the cell membrane is damaged. PI then binds to the DNA of dead cells and emits red fluorescence . Bacteria treated with different samples were observed by high-resolution laser confocal microscopy after staining, and the results are shown in Figure .…”

“…PI then binds to the DNA of dead cells and emits red fluorescence. 61 Bacteria treated with different samples were observed by high-resolution laser confocal microscopy after staining, and the results are shown in Figure 7. We analyzed Figure 7 using ImageJ software and obtained the corresponding ratio of red to green fluorescence intensities, as shown in Figure S9.…”

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