High-resolution On-chip Imaging using Synthetic Aperture

Abstract

Fig. 1. (a) Optical setup of lensfree imaging using synthetic aperture. (b) and (c): schematic and close-up of sample and image sensor; spatial frequency passbands of lensfree imaging at 700 nm illumination using (d) only normal illumination and (e) synthetic aperture. Reconstructions of 250 nm grating lines using (f) normal illumination and (g) synthetic aperture. The object is imaged at a sample-to sensor distance of ~0.1 mm Abstract: We report a synthetic-aperture-based on-chip lensfree microscope. Having a wide fieldof-view of ~20.5 mm 2 , this technique sets the largest numerical aperture (1.4) for on-chip microscopy. Main textWide field-of-view (FOV), high-resolution microscopes are needed in various applications in biomedicine, engineering and applied sciences. Such tasks require imaging modalities with large space-bandwidth products (SBPs). Different from conventional lens-based microscopes, lensfree on-chip microscopy decouples the resolution and FOV and therefore provides significantly larger SBPs within compact and field-portable designs [1,2]. This is achieved by taking advantage of the state-of-the-art image sensors with large mega-pixels and ~1-2 μm pixel-pitch, where the specimen is placed directly above the image sensor, with sub-mm distance to its active area. Under unit magnification, our lensfree platform adopts source-shifting based pixel super resolution to capture the diffraction patterns with sub-pixel resolution [2]. In the pursuit of higher SBPs, however, the resolution of lensfree on-chip imaging is limited mainly by the signal-to-noise ratio (SNR) deterioration and aberrations that affect high spatial frequencies of the sample. The source of these challenges is related to the narrow angular response and large pixel size of image sensor chips, and it gets much worse at longer illumination wavelengths since the diffraction angles of a given high spatial frequency band of the sample increase with wavelength.