Photothermal Fourier-plane Phase Synchronization for Interferometric Scattering Microscopy

Abstract

We introduce and experimentally implement Fourier-plane phase synchronization for optical microscopy, and demonstrate its performance with interferometric scattering microscopy. By combining a photothermal phase plate and laser beam scanning, we realize a synchronized phase for all scattering components on the Fourier plane of high numerical-aperture microscopes, where the evanescent waves and optical aberration normally produce highly inhomogeneous phase distributions. We achieve an almost perfect point spread function, exhibiting a tighter focus with 50\% enhancement of the signal and ideal circular symmetry. Particularly, by synchronizing the phase to $π/2$, we demonstrate the background speckles exhibit an anti-symmetric dependence on axial defocus, enabling the effective suppression of the speckles via defocus integration and thus the detection of 10 nm particles immobilized on the substrate. The concept and technique of seamless dynamic phase control on the Fourier plane constitute a key asset for modern optical microscopy.

Figures (4)

• Figure 1: (a) Schematic of the experimental setup. (b) Structure of the photothermal-phase plate (PT-PP) and the illustration of the light-absorption induced temperature change $\Delta T$. Calculated phase profiles on the Fourier plane ($\phi _ F$) for a typical iSCAT system with an aberration level of $0.4\pi$ under (c) linearly and (d) circularly polarized illuminations. (e) Phase profile with all scattering components synchronized ($\phi _ F = \pi$). The white-dashed and central green circles in (c), (d) and (e) denote NA = 1.0 and the reference, respectively. (f), (g) and (h) Calculated iSCAT PSFs based on the phase profiles in (c), (d) and (e), respectively. (i) and (j) are the PSFs with $\phi _ F = 0$ and $\pi/2$, respectively. $\Delta z$ in (f)$\sim$(j) are chosen to obtain the maximum contrast. (k) Contrast evolutions with the defocus for (f)$\sim$(j). PBS: polarized beam splitter, QWP: quarter-wave plate, DM: dichroic mirror, BFP: back focal plane, PDMS: polydimethylsiloxane, SPF: short pass filter, AOD: acoustic optical deflector, RF: radio frequency.
• Figure 2: (a) Measured normalized profile of the photothermal phase change due to a single heating laser spot. (b) Target profile of the phase change for achieving phase synchronization $\phi_F=\pi/2$ for a point scatterer on the coverslip surface. (c) The dwell-time distribution of the heating laser. (d) Measured profile of the photothermal phase change.
• Figure 3: Measured iSCAT contrast images for a 40 nm GNP on a glass coverslip when $\Delta z$ is (a) -1 $\rm{\mu m}$, (b) -0.6 $\rm{\mu m}$, (c) -0.2 $\rm{\mu m}$, (d) 0.2 $\rm{\mu m}$, and (e) 0.6 $\rm{\mu m}$ with no phase synchronization. (f) Position localization uncertainties of the GNP at varied defocus in the range of -1 $\rm{\mu m}$ to 1 $\rm{\mu m}$. (g)$\sim$(l) represent the same as in (a)$\sim$(f) with phase synchronization.
• Figure 4: (a) iSCAT contrast images at varied defocus values for a 40 nm GNP on a coverslip after phase synchronization. (b) iSCAT contrast as a function of $\Delta z$ at the positions denoted by the two arrows. Dashed and solid lines denote before and after phase synchronization, respectively. (c) Contrast image obtained by integrating the iSCAT contrast in (a) over $\Delta z$ = -0.5 $\rm{\mu m}$ to 0.5 $\rm{\mu m}$. (d) Histogram of the SBR enhancements by phase synchronization and defocus integration obtained from measuring 61 individual 40 nm GNPs. (e) iSCAT contrast images for an area of a coverslip spin-coated with 10 nm GNPs. (f) Contrast image obtained after integrating the contrast images in (e) over the defocus. The 10 nm GNPs become clearly visible.