Phase-retrieval in scanning digital holography for optofluidics

Abstract

Optofluidic microscopy has been an open challenge during past decade; it is also a well-established paradigm where precise control of microfluidic streams is smartly exploited. Digital holography (DH) has been proved as one of the optimal tools for flow-cytometry, cell sorting and classification, cell counting and study of cell mechanics. In this framework, Space-Time Digital Holography (STDH) is a convenient complement to conventional holographic cell imaging. Thanks to a spatiotemporal reassembling strategy, one single space-time hologram can efficiently store information of a series of timelapse holograms using a small subset of detecting elements, e.g. a linear sensor array. In this case, the modulated pattern of interference fringes is projected onto a new hybrid space-time domain and reassembled by time series. Here we propose a phase-retrieval process in STDH for optofluidics, which allows the quantitative phase information reconstruction for flowing cells in different focus planes simultaneously with extended field of view. For a space-time hologram storing information from flowing cells, a unique flow velocity meeting the matching condition of STDH would enable accurate space-time phase shifting. In the case of mismatches between cells speed and recording frame rate, an ad-hoc reconstruction algorithm is developed that compensates for the mismatch and retrieves the correct phase-contrast map of the sample by smartly adapting the method to the microfluidic speed. Based on the proposed strategy, we show the 4D mapping of flowing cells in space-time domain; in other words, the ASTDH is able to encode efficiently a 4D information in a 2D map, self-adapting to unexpected variations of the flow profiles.