Defocus-induced phase contrast enhancement in pattern illumination time-resolved phase microscopy

Photo-excited charge carrier dynamics in photocatalytic materials with rough surfaces have been studied via measurements using pattern-illumination time-resolved phase microscopy. Optimal defocusing is necessary for the phase-contrast detection of the refractive index change due to the photo-excited charge carriers. The signal enhancement of the phase-change was explained theoretically and experimentally. The optical phase variation due to the transmission of a rough surface is coupled with the quadratic phase term in Fresnel diffraction, and a slight defocusing can convert the phase image to the corresponding amplitude image. The phase-contrast image due to the photo-excited charge carriers is also enhanced by the defocusing. The explanation was supported by wave optics calculation, and the enhancement was demonstrated for two types of TiO2 substrates with different roughnesses.