Lensless diffractive imaging with ultra-broadband table-top sources: from infrared to extreme-ultraviolet wavelengths

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.