Whispering gallery modes in quantum dot-embedded dielectric microspheres for tagless remote refractometric sensing

This thesis presents the development of a refractometric sensor based on quantum dot-embedded polystyrene microspheres. The technique uses optical resonances within a microsphere, known as Whispering-Gallery Modes (WGMs), which produce narrow spectral peaks. The basic theory of WGMs is reviewed and specifically discussed for biosensing application. The spectral shifts of WGM peaks are sensitive to changes in the local refractive index. In the experiments, two-photon excited luminescence from the quantum dots couples into several WGMs within the microresonator. By optimizing the detection area, the spectral visibility of the WGMs is improved. The spectral shifts are measured as the surrounding index of refraction changes. The experimental sensitivity is about five times greater than that predicted by Mie theory. The sensor element is based on commercially available dielectric microspheres with a diameter about 10 μm. Thus, the technique is more economic and suitable for sensing applications, compared to microspheres of 100 μm in size which can only be made in the laboratory

Fluorescence microscopy imaging with a Fresnel zone plate array based optofluidic microscope

We report the implementation of an on-chip microscope system, termed fluorescence optofluidic microscope (FOFM), which is capable of fluorescence microscopy imaging of samples in fluid media. The FOFM employs an array of Fresnel zone plates (FZP) to generate an array of focused light spots within a microfluidic channel. As a sample flows through the channel and across the array of focused light spots, the fluorescence emissions are collected by a filter-coated CMOS sensor, which serves as the channel’s floor. The collected data can then be processed to render fluorescence microscopy images at a resolution determined by the focused light spot size (experimentally measured as 0.65 mm FWHM). In our experiments, our established resolution was 1.0 mm due to Nyquist criterion consideration. As a demonstration, we show that such a system can be used to image the cell nuclei stained by Acridine Orange and cytoplasm labeled by Qtracker

Few-photon computed x-ray imaging

X-ray is a ubiquitous imaging modality in clinical diagnostics and industrial inspections, thanks to its high penetration power. Conventional x-ray imaging system, equipped with energy-integrating detectors, collects approximately 1000 to 10000 counts per pixel to ensure sufficient signal to noise ratio (SNR). The recent development of energy sensitive photon counting detectors opens new possibilities for x-ray imaging at low photon flux. In this letter, we report a novel photon-counting scheme that records the time stamp of individual photons, which follows a negative binomial distribution, and demonstrated the reconstruction based on the few-photon statistics. The projection and tomography reconstruction from measurements of roughly 10 photons shows the potential of using photon counting detectors for dose-efficient x-ray imaging systems.Comment: Revised manuscrip

Implementation of a color-capable optofluidic microscope on a RGB CMOS color sensor chip substrate

We report the implementation of a color-capable on-chip lensless microscope system, termed color optofluidic microscope (color OFM), and demonstrate imaging of double stained Caenorhabditis elegans with lacZ gene expression at a light intensity about 10 mW/cm^2