Continuous wave multi-pass imaging flow cytometry

Biological cells have weak optical properties that limit the sensitivity of measurements and imaging when the total illumination is limited by dynamics or throughput. Multi-pass microscopy enhances the sensitivity of classical light fields to these weak properties.We present a wide-field multi-pass microscope for label-free imaging flow cytometry. By passing the optical field through cells in a high-speed flow up to 4 times, we realize a ×4 enhancement in contrast and signal-to-noise ratio. We show that this technique approaches close to the quantum limit of measurement sensitivity, extending the range of optimal imaging to samples in theweakly absorbing regime. This allows for near optimal imaging sensitivity and throughput in practical scenarios involving dynamic biological samples and surpasses the sensitivity achieved with currently available quantum light sources.