Abstract

Lensless imaging is an approach to microscopy in which a high-resolution image of an object is reconstructed from one or more measured diffraction patterns, providing a solution in situations where the use of imaging optics is not possible. However, current lensless imaging methods are typically limited by the need for a light source with a narrow, stable and accurately known spectrum. We have developed a general approach to lensless imaging without spectral bandwidth limitations or sample requirements. We use two time-delayed coherent light pulses and show that scanning the pulse-to-pulse time delay allows the reconstruction of diffraction-limited images for all the spectral components in the pulse. In addition, we introduce an iterative phase retrieval algorithm that uses these spectrally resolved Fresnel diffraction patterns to obtain high-resolution images of complex extended objects. We demonstrate this two-pulse imaging method with octave-spanning visible light sources, in both transmission and reflection geometries, and with broadband extreme-ultraviolet radiation from a high-harmonic generation source. Our approach enables effective use of low-flux ultra-broadband sources, such as table-top high-harmonic generation systems, for high-resolution imaging. Researchers in The Netherlands have overcome the restriction of monochromatic illumination when performing lensless imaging. Stefan Witte and co-workers from LaserLAB Amsterdam have developed a lensless scheme that employs two coherent time-delayed pulses and is compatible with broadband sources. Lensless imaging — whereby diffraction patterns are interpreted to reconstruct an image of a sample — is popular in the X-ray and extreme-ultraviolet regimes, where high-quality lenses for performing conventional imaging are not available. Because this approach has traditionally been limited to narrowband coherent radiation, scientists have been eager to make it compatible with broadband sources such as tabletop high-harmonic generation. This new broadband technique involves scanning the pulse-to-pulse time delay and then applying a phase-retrieval algorithm to produce high-resolution images of complex objects.

Highlights

  • Lensless imaging is an elegant approach to high-resolution microscopy that is rapidly gaining popularity in applications where the use of imaging optics is problematic

  • We introduce an iterative phase retrieval algorithm that uses these spectrally resolved Fresnel diffraction patterns to obtain high-resolution images of complex extended objects. We demonstrate this two-pulse imaging method with octave-spanning visible light sources, in both transmission and reflection geometries, and with broadband extreme-ultraviolet radiation from a high-harmonic generation source

  • In summary, we have presented two-pulse imaging as a method that enables lensless microscopy with ultra-broadband light sources

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Summary

Introduction

Lensless imaging is an elegant approach to high-resolution microscopy that is rapidly gaining popularity in applications where the use of imaging optics is problematic. As only the intensity is recorded in a typical diffraction measurement, the central issue in lensless imaging is the retrieval of phase information from a recorded diffraction pattern Solutions to this problem have been found both through numerical and optical means.[1,5,6] In coherent diffractive imaging, the missing phase information is numerically reconstructed by iterative algorithms,[7] a process that has been shown to have a unique solution as long as the oversampling condition is satisfied.[8] This condition can be satisfied by using a finite object support,[9,10] a localized illumination profile[11] and/or recording multiple partially overlapping diffraction patterns.[12,13] Holographic methods, which directly record the phase through interference with a separate reference wave, have been developed for a lensless imaging context.[6,14]

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