Figure 3. Basic performance of ATOM.

(a, b) Two single-angle ATOM images of a MIHA cells fixed on a glass slide, which show the opposite phase-gradient contrasts, respectively. (c, d) The corresponding line profiles (yellow dotted lines) of the single-angle ATOM images in (a), (b), respectively. Each line scan of the image is captured within ~4 ns. (e) By calculating the sum of the two single-angle ATOM images (i.e. a + b), an image with absorption contrast can be revealed. (f) Dual-angle ATOM image with differential (enhanced) phase-gradient contrast can be obtained by calculating the difference of the two single-angle ATOM images (i.e. a − b). (g) White-light DIC image of the same MIHA cell for comparison. (h, i) Demonstration of the contrast enhancement in ATOM by comparing the single-angle asymmetric detection scheme (i.e. off-axis fiber coupling) and the BF scheme (i.e. on-axis fiber coupling), in the context of high-speed flow imaging (8 m/s) of the MIHA cells in a PDMS microfluidic channel. 2D images are here acquired by continuous 1D line-scan at a rate of 26 MHz (governed by the repetition rate of the laser), which is naturally provided by the unidirectional cell flow, without any laser beam or sample stage scanning. Two representative time-multiplexed ATOM images are shown in (h) and (i). In these time-multiplexed ATOM images, they show the image replica of the same cell, as if they are flowing in parallel: one is captured by asymmetric detection (left) whereas another is captured by on-axis detection, i.e. BF time-stretch images (right). The bottom insets are the corresponding line profiles (yellow dotted lines) of ATOM images and BF time-stretch images shown in (h), (i). Note that in one single-shot line-scan, both phase-gradient and BF contrasts of the same cell are captured within ~8 ns.