Shot noise limited characterization of femtosecond light pulses

Probing the evolution of physical systems at the femto- or attosecond timescale with light requires accurate characterization of ultrashort optical pulses. The time profiles of such pulses are usually retrieved by methods utilizing optical nonlinearities, which require significant signal powers and operate in a limited spectral range\cite{Trebino_Review_of_Scientific_Instruments97,Walmsley_Review_09}. We present a linear self-referencing characterization technique based on time domain localization of the pulse spectral components, operated in the single-photon regime. Accurate timing of the spectral slices is achieved with standard single photon detectors, rendering the technique applicable in any spectral range from near infrared to deep UV. Using detection electronics with about $70$ ps response, we retrieve the temporal profile of a picowatt pulse train with $\sim10$ fs resolution, setting a new scale of sensitivity in ultrashort pulse characterization.Comment: Supplementary information contained in raw dat

Engineering of plasmonic excitations for hand-held and ultra-sensitive biosensors

Thesis (Ph.D.)--Boston UniversityEarly detection and effective diagnosis are important for disease screening and preventing epidemics. Recently, optical biosensors have attracted significant attention, as they are very powerful detection and analysis tools that have variety of applications in homeland security, public and global healthcare, biomedical research and pharmacology. However, most of these biosensors are time-consuming, require costly chemical procedures and bulky instrumentation, and need advanced medical infrastructures with trained laboratory professionals. In order to address these needs, recently lensfree computational on-chip imaging techniques have been introduced to eliminate the need for bulky and costly optical components. However, this technology is limited by the size of the analytes as it uses a lensfree computational technique insufficient for detecting biomolecules down to nm-scale. In order to provide highly sensitive and massively multiplexed detection of biomolecular binding events, fluorescent imaging and surface plasmon resonance (SPR) based platforms are the most favored. However, SPR sensors are limited due to the alignment sensitive prism coupling scheme and bulky instrumentation while the fluorescence imaging suffers from quantitative and qualitative drawbacks of the labeling steps. This thesis focuses on the unique integration of lensfree telemedicine technology and nanostructured plasmonic chip technology to realize ultra-sensitive and label-free biosensing in a high-throughput and massively multiplexed manner for field-settings. Toward this aim, we introduce a handheld on-chip biosensing technology that employs plasmonic microarrays coupled with a lensfree computational imaging system. Employing a sensitive plasmonic array design that is combined with lensfree computational imaging, we demonstrate label-free and quantitative detection of biomolecules with a protein layer thickness down to 3 nm. Integrating large-scale plasmonic microarrays, our platform enables the simultaneous detection of protein mono- and bilayers on the same platform over a wide range of biomolecule concentrations. In this plasmonic device, we also monitor binding dynamics of protein complexes as a function of time by integrating it with microfluidics. Plasmonic antennas utilized in our lensfree platform, supporting very sharp and sensitive spectral feature as well as easily accessible large local electromagnetic fields, are highly advantageous for biosensing applications as they enable stronger interaction between surface waves and biological molecules on the sensing chip

Improving performance of single-pass real-time holographic projection

© 2019 Elsevier B.V. This work describes a novel approach to time-multiplexed holographic projection on binary phase devices. Unlike other time-multiplexed algorithms where each frame is the inverse transform of independently modified target images, Single-Transform Time-Multiplexed (STTM) hologram generation produces multiple sub-frames from a single inverse transform. Uniformly spacing complex rotations on the diffraction field then allows the emulation of devices containing 2N modulation levels on binary devices by using N sub-frames. In comparison to One-Step Phase Retrieval (OSPR), STTM produces lower mean squared error for up to N=5 than the equivalent number of OSPR sub-frames with a generation time of [Formula presented] of the equivalent OSPR frame. A mathematical justification of the STTM approach is presented and a hybrid approach is introduced allowing STTM to be used in conjunction with OSPR in order to combine performance benefits.Engineering and Physical Sciences Research Council (EP/L016567/1 and EP/L015455/1

Spatially multiplexed interferometric microscopy: from basic principles to advanced arrangements

La posibilidad de visualizar y analizar objetos microscópicos transparentes de una manera no invasiva ha sido uno de los principales retos de la microscopía óptica a lo largo del siglo XX. Para ello, se desarrollaron diversas técnicas de microscopía que convertían las variaciones en el índice de refracción de los objetos en variaciones de intensidad, haciendo estos objetos visibles a simple vista, entre las que destacan la microscopía de contraste de fase de Zernike o de contraste diferencial de Nomarski. Sin embargo, estas técnicas solamente proporcionan información cualitativa del objeto, por lo que su análisis se limita a la simple visualización. Por otro lado, existen otras técnicas de microscopía basadas en la interferometría, que proporcionan información cuantitativa de fase de un modo sencillo y directo. A partir de esta información de fase es posible obtener, de una manera precisa, información sobre la morfología y el índice de refracción del objeto bajo análisis. Este hecho hace que este tipo de técnicas sean muy interesantes en diversas áreas de conocimiento como la medicina, la biofotónica, o la biología, entre otras. Quizás la técnica interferométrica por excelencia para la obtención de imágenes cuantitativas de fase sea la microscopía holográfica digital. La microscopía holográfica digital surge de la combinación de holografía digital y la microscopía óptica. En los últimos años, se han llevado a cabo numerosos avances en el campo de la microscopía holográfica digital con el fin de introducir mejoras en términos de robustez, simplicidad, precisión y coste. En la misma línea de estos avances, esta tesis está centrada en el desarrollo y la mejora de una técnica llamada “microscopía interferométrica por multiplexado espacial”. Esta técnica se basa en la introducción de una serie de modificaciones sencillas en el cuerpo de un microscopio estándar de campo claro, con el objetivo de convertirlo en uno holográfico de una manera muy robusta, sencilla y económica. Todas las modificaciones realizadas están encauzadas a la implementación de un interferómetro de camino común empleando estrategias de multiplexado espacial en el microscopio. Estas modificaciones son principalmente tres: 1) la sustitución de la fuente de iluminación de banda ancha del propio microscopio por una fuente luminosa coherente que permita interferencias; 2) el multiplexado espacial del campo de visión mediante su división en dos o tres regiones para la transmisión de un haz de referencia; y 3) la inserción de un elemento interferométrico, tal como una red de difracción o un cubo divisor de haz, que produzca el patrón interferencial a registrar. Así pues, todas las técnicas desarrolladas en esta tesis están encaminados a la mejora de esta técnica en términos de: 1) ruido coherente, 2) diseño del campo de visión, 3) resolución espacial, 4) capacidad de análisis de objetos no transparentes, 5) caracterización del índice de refracción, y 6) capacidad de análisis a tiempo real. Todas las validaciones experimentales realizadas durante esta tesis demuestran que la técnica de microscopía interferométrica por multiplexado espacial es una técnica muy versátil, potente y económica que permite la obtención de imágenes cuantitativas de fase a partir de un microscopio de campo claro convencional.The possibility of visualizing and analysing transparent microscopic objects in a non-invasively manner was one of the addressed challenges in the microscopy field during 20th century. Several microscopy techniques were created for that purpose, including quantitative phase imaging. Quantitative phase imaging provides numerical information about the morphology and the refractive index of such objects, so that it can be very appealing in diverse fields of knowledge such as medicine, biophotonics or life science, just to cite a few. One of the easiest ways of achieving quantitative phase imaging is employing digital holographic microscopy techniques. Digital holographic microscopy arises from the combination of digital holography and optical microscopy. In recent years, many novel digital holographic microscopy approaches have been successfully developed in order to improve their capabilities in terms of robustness, simplicity, usability, accuracy, and price. In line with that, this thesis is focused on the development and improvement of the technique named "Spatially Multiplexed Interferometric Microscopy". This technique introduces minimal modifications in the embodiment of a conventional bright field microscope in order to convert it into a holographic one in an extremely simple, low-cost and highly-stable way. The modifications are aimed to implement a common-path interferometer by a spatially multiplexed approach in the embodiment of the microscope and are mainly three: 1) the replacement of the broadband illumination source of the microscope by a coherent one; 2) the spatial multiplexed of the input plane by dividing it into two or three regions; 3) and the insertion of an interferometric component such as a diffraction grating or a beam splitter cube. All performed arrangements and phase retrieval procedures are focused on the enhancement of such a technique regarding: 1) coherent noise; 2) spatial multiplexed input plane; 3) spatial resolution; 4) ability for reflective samples analysis; 5) refractive index characterization; and 6) real-time analysis. Experimental validations carried out during the thesis demonstrate that spatially multiplexed interferometric microscopy is a powerful, versatile, and low-cost technique for achieving quantitative phase images from a commercially available standard microscope

Super-condenser enables labelfree nanoscopy.

Labelfree nanoscopy encompasses optical imaging with resolution in the 100 nm range using visible wavelengths. Here, we present a labelfree nanoscopy method that combines coherent imaging techniques with waveguide microscopy to realize a super-condenser featuring maximally inclined coherent darkfield illumination with artificially stretched wave vectors due to large refractive indices of the employed Si3N4 waveguide material. We produce the required coherent plane wave illumination for Fourier ptychography over imaging areas 400 μm2 in size via adiabatically tapered single-mode waveguides and tackle the overlap constraints of the Fourier ptychography phase retrieval algorithm two-fold: firstly, the directionality of the illumination wave vector is changed sequentially via a multiplexed input structure of the waveguide chip layout and secondly, the wave vector modulus is shortend via step-wise increases of the illumination light wavelength over the visible spectrum. We test the method in simulations and in experiments and provide details on the underlying image formation theory as well as the reconstruction algorithm. While the generated Fourier ptychography reconstructions are found to be prone to image artefacts, an alternative coherent imaging method, rotating coherent scattering microscopy (ROCS), is found to be more robust against artefacts but with less achievable resolution