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

Abstract

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.

Effect of CoOx and Rh Cocatalysts on Local Charge Carrier Dynamics of BiVO4 Particles by Pattern-Illumination Time-Resolved Phase Microscopy

“…By minor defocusing, the refractive index-changed image due to pump light illumination was converted into an intensity image and observed by the CMOS camera; thus, the image intensity change is proportional to the refractive index change, and we study the appropriate focus position from the theory, simulation, and the data in the previous paper. 28 We could recognize the absorption change by adjusting the focal position because the refractive index change is totally zero at the exact focal position based on the theory. We did not observe a detectable absorption change in the focal position under our experimental setup.…”

“…The time indicated in the time sequence of images and the time responses corresponds to the difference between the pump and probe pulses, and the pump and probe pulses overlapped in the time of less than a few nanoseconds. By minor defocusing, the refractive index-changed image due to pump light illumination was converted into an intensity image and observed by the CMOS camera; thus, the image intensity change is proportional to the refractive index change, and we study the appropriate focus position from the theory, simulation, and the data in the previous paper . We could recognize the absorption change by adjusting the focal position because the refractive index change is totally zero at the exact focal position based on the theory.…”

Microscopic Interfacial Charge Transfer at Perovskite/Hole Transport Layer Interfaces Clarified Using Pattern-Illumination Time-Resolved Phase Microscopy

“…The refractive index was imaged using the self-imaging technique by defocusing. 38,39 In this setup, the changes both in the refractive index change and absorption change are included in the images, but the absorption change was negligible for MAPbI 3 and HTLs at the probe illumination wavelength, 532 nm. 15,40−42 The refractive index change is explained by the carrier number change of the Drude carriers or the charge distribution change relevant to the optical transitions, 43,44 but it is not sensitive for the probe wavelength, compared with the absorption, and the selection of the probe wavelength is not strict in this method.…”

“…The distribution of the photoexcited charge carriers is observed via the refractive index change using the phase contrast imaging. The refractive index was imaged using the self-imaging technique by defocusing. , In this setup, the changes both in the refractive index change and absorption change are included in the images, but the absorption change was negligible for MAPbI 3 and HTLs at the probe illumination wavelength, 532 nm. ,− The refractive index change is explained by the carrier number change of the Drude carriers or the charge distribution change relevant to the optical transitions, , but it is not sensitive for the probe wavelength, compared with the absorption, and the selection of the probe wavelength is not strict in this method. , The pattern illumination is preferable for applying image recovery calculations, as described in a different paper . In this study, the image sequence was processed using the three-dimensional total variation regularization, where the signal-to-noise (SN) ratio is improved, while the structural features in images are not lost.…”

Distinction and Separation of Different Types of Charge Carriers from the Time-Resolved Local Charge Carrier Mapping for Photocatalytic Materials

“…However, no blurring of the boundaries was observed for particulate semiconductor samples used here, and the effect of diffusion can be neglected. The refractive index change is induced due to the photo-excited charge carriers, and the image was obtained by the phase-contrast imaging techniques. , Pattern-illumination is preferable for applying image recovery calculations. In this study, the image sequence was processed by the three-dimensional total variation regularization, where the signal-to-noise (SN) ratio is improved while the structural features in images are not lost.…”