Coherent Diffractive Imaging Using Randomly Coded Masks

Coherent diffractive imaging (CDI) provides new opportunities for high resolution X-ray imaging with simultaneous amplitude and phase contrast. Extensions to CDI broaden the scope of the technique for use in a wide variety of experimental geometries and physical systems. Here, we experimentally demonstrate a new extension to CDI that encodes additional information through the use of a series of randomly coded masks. The information gained from the few additional diffraction measurements removes the need for typical object-domain constraints; the algorithm uses prior information about the masks instead. The experiment is performed using a laser diode at 532.2 nm, enabling rapid prototyping for future X-ray synchrotron and even free electron laser experiments. Diffraction patterns are collected with up to 15 different masks placed between a CCD detector and a single sample. Phase retrieval is performed using a convex relaxation routine known as "PhaseCut" followed by a variation on Fienup's input-output algorithm. The reconstruction quality is judged via calculation of phase retrieval transfer functions as well as by an object-space comparison between reconstructions and a lens-based image of the sample. The results of this analysis indicate that with enough masks (in this case 3 or 4) the diffraction phases converge reliably, implying stability and uniqueness of the retrieved solution

Lensless wide-field fluorescent imaging on a chip using compressive decoding of sparse objects.

We demonstrate the use of a compressive sampling algorithm for on-chip fluorescent imaging of sparse objects over an ultra-large field-of-view (>8 cm(2)) without the need for any lenses or mechanical scanning. In this lensfree imaging technique, fluorescent samples placed on a chip are excited through a prism interface, where the pump light is filtered out by total internal reflection after exciting the entire sample volume. The emitted fluorescent light from the specimen is collected through an on-chip fiber-optic faceplate and is delivered to a wide field-of-view opto-electronic sensor array for lensless recording of fluorescent spots corresponding to the samples. A compressive sampling based optimization algorithm is then used to rapidly reconstruct the sparse distribution of fluorescent sources to achieve approximately 10 microm spatial resolution over the entire active region of the sensor-array, i.e., over an imaging field-of-view of >8 cm(2). Such a wide-field lensless fluorescent imaging platform could especially be significant for high-throughput imaging cytometry, rare cell analysis, as well as for micro-array research

Mid infrared digital holography and terahertz imaging

Mid IR and Far IR (THz) regions have been attracting a continuously growing interest, especially for imaging applications. Mid IR imaging systems are widespread in the military, security and medical fields and are, consequently, in continuous development. Even greater expectation is placed on THz imaging techniques, because of the well-known capacity of THz radiation to penetrate many common materials and to provide important spectroscopic information about various strategic stuffs. In this scenario Digital Holography, a quite recent interferometric imaging technique, is proving to be mature enough to play a key role among the other imaging techniques, both in the Mid IR and in the Far IR

Coherent lensless imaging techniques using terahertz radiation

Terahertz (THz) radiation denotes the portion of the electromagnetic spectrum lying between the infrared and microwave bands, corresponding to frequencies in the range 0.1-10 THz. The most intriguing feature of using THz waves is their ability to penetrate several non-conducting and optically opaque materials, such as plastics, textiles, paper, and some building materials as well as intrinsic semiconductors. While this property is also shared by microwaves, THz radiation provides a better spatial resolution thanks to the shorter wavelength, thereby imaging hidden objects with sub-millimeter resolution. The non-ionizing nature of THz radiation when it interacts with living tissues also makes THz imaging techniques promising for biomedical and biological applications. In this thesis, I focus on the development and implementation of THz imaging techniques. All the techniques presented here belong to the realm of coherent lensless imaging, aiming at reconstructing the amplitude and phase of the wavefront diffracted by an unknown object, illuminated with coherent radiation, based on measurements of the intensity of their diffraction pattern recorded with a camera. The fact that the imaging process is carried out fully computationally and without the need of lenses has a crucial impact on the experimental setup, which is therefore compact and can be better tailored to real-life applications. In particular, I am going to discuss both theoretical and experimental aspects of synthetic aperture THz off-axis digital holography, the first experimental demonstration of THz ptychography and how to image objects hidden behind weakly and strongly diffracting barriers. A potential biomedical application for such THz imaging techniques will also be suggested

Multi-Color Imaging of Magnetic Co/Pt Multilayers

We demonstrate for the first time the realization of a spatial resolved two color, element-specific imaging experiment at the free-electron laser facility FERMI. Coherent imaging using Fourier transform holography was used to achieve direct real space access to the nanometer length scale of magnetic domains of Co/Pt heterostructures via the element-specific magnetic dichroism in the extreme ultraviolet spectral range. As a first step to implement this technique for studies of ultrafast phenomena we present the spatially resolved response of magnetic domains upon femtosecond laser excitation

Quantum imaging and polarimetry with two-color photon pairs

Verschränkte Photonenpaare, gemeinhin als Signal- und Idler-Photonen bezeichnet, wurden als Grundlage für Quantum imaging with undetected photons (QIUP) und Quantum ghost imaging (QGI) verwendet. Mit QIUP können wir ein Objekt abbilden, indem wir nur die Signal-Photonen messen, die nie mit dem Objekt wechselwirken, während die Idler-Photonen, die das Objekt beleuchten, undetektiert bleiben. Bei QGI werden die beleuchtenden Idler-Photonen von einem räumlich nicht auflösenden Detektor gemessen, während die nicht wechselwirkenden Signal-Photonen von einer Kamera gemessen werden und das Bild dann nur aus den Koinzidenzen von Signal und Idler rekonstruiert wird. Nennenswert ist hier die Verwendung von zweifarbigen Photonenpaaren, welche es uns ermöglichen, Komplikationen bei der Bildgebung in Wellenlängenbereichen zu überwinden, in denen Kameras nur eine geringe Effizienz aufweisen. Daraus ergibt sich ein enormes Potenzial für die Biosensorik, bei der empfindliche Proben, die für Strahlungsschäden anfällig sind, mit herkömmlichen Einzelphotonen-Kameras abgebildet werden können, wie zum Beispiel im sichtbaren Spektralbereich, während die Probe von Photonen mit viel geringerer Energie beleuchtet wird. In dieser Arbeit wurden drei Lücken in der Literatur zur Quantenbildgebung und Polarimetrie geschlossen: (1) Die fundamentale transversale Auflösungsgrenze von QIUP und QGI, die zweifarbige Photonenpaare verwenden, wurde diskutiert. (2) Ein linsenloses QGI-Verfahren wurde vorgestellt, das sich speziell für die Abbildung in Wellenlängenbereichen eignet, für die weniger Linsen zur Verfügung stehen, wie zum Beispiel im Terahertz-Bereich. Wir haben es Pinhole QGI genannt, da wir gezeigt haben, dass es analog zur klassischen Lochkamera ist. (3) Ein Quantum ghost polarimetry (QGP) Schema wurde vorgeschlagen, bei dem dielektrische Metaoberflächen verwendet werden können, um den Einsatz rekonfigurierbarer optischer Elemente zu vermeiden

Holography in the invisible. From the thermal infrared to the terahertz waves: outstanding applications and fundamental limits

peer reviewedSince its invention, holography has been mostly applied at visible wavelengths in a variety of applications. Specifically, non-destructive testing of manufactured objects was a driver for developing holographic methods and all of their parents based on the speckle pattern recording. One substantial limitation of holography non-destructive testing is the setup stability requirements directly related to the laser wavelength. This observation has driven some works for 15 years: developing holography at wavelengths much longer than visible ones. In this paper, we will first review researches carried out in the infrared, mostly digital holography at thermal infrared wavelengths around 10 micrometers. We will discuss the advantages of using such wavelengths and show different examples of applications. In non-destructive testing, large wavelengths allow using digital holography in perturbed environments on large objects and measure large deformations, typical of the aerospace domain. Other astonishing applications such as reconstructing scenes through smoke and flames were proposed. Going further in the spectrum, digital holography with so-called Terahertz waves (up to 3 millimeters wavelength) has also been studied. The main advantage here is that these waves easily penetrate some materials. Therefore, one can envisage Terahertz digital holography to reconstruct the amplitude and phase of visually opaque objects. We review some cases in which Terahertz digital holography has shown potential in biomedical and industrial applications. We will also address some fundamental bottlenecks that prevent fully benefiting from the advantages of digital holography when increasing the wavelength

Single-scan multiplane phase retrieval with a radiation of terahertz quantum cascade laser

Terahertz phase retrieval from a set of axially separated diffractive intensity distributions is a promising single-beam computational imaging technique that ensures the obtention of high spatial resolutions and phase wavefronts, but remains restricted by time-consuming data acquisition processes. In this work, we have adopted an approach, relying on the radiation of a quantum cascade laser and the implementation of an express single-scan measurement of intensity distributions through the continuous on-the-go displacement of a high-sensitivity antenna-coupled microbolometer sensor array. In addition to the simplicity of this practical implementation and the minimization of measurement times, such an approach overcomes the problem of preliminary optimal selections of transverse intensity distributions used in the iterative phase retrieval algorithm and guarantees the required data diversity for high-quality wavefront reconstruction