Ouverture numérique | EPFL Graph Search

L’ouverture numérique est une caractéristique d'un système optique, généralement notée O.N. (ou NA dans la littérature anglophone, pour Numerical Aperture). Elle est définie par \mathrm{O.N.} = n_{0} \cdot \sin i_{0} \ , où n0 est l'indice de réfraction dans le milieu d'observation, et i0 est l'angle entre l'axe optique et le rayon le plus écarté de l'axe optique qui entre dans la lentille. Cet angle est appelé demi-angle d'ouverture. L'ouverture numérique est caractéristique de l'espace dans lequel elle est définie : l'ouverture numérique objet est définie par l'indice de réfraction dans l'espace objet et l'angle d'ouverture des rayons entrant dans le système, tandis que l'ouverture numérique image dépend de l'indice de réfraction dans l'espace image et de l'angle d'ouverture des rayons sortant du système. En conception optique, l'ouverture numérique est liée plus généralement au concept d'ouverture d'un système optique, qui peut également se définir par le nombre d'ou

Advanced Techniques in Optical Diffraction Tomography

Ahmed Bassam Sayed Ayoub Mohamed Emam

In this thesis, we study the 3 challenges described above. First, we study different reconstruction techniques and assess the fidelity of each reconstruction results by means of structured illumination and phase conjugation. By reconstructing the 3D refractive index of the sample using different algorithms (i.e. Born, Rytov, and Radon) and then perform a numerical back-propagation of experimentally measured structured illumination pattern we are able to assess the fidelity of each reconstruction algorithms without prior information about the 3D RI distribution of the sample.The second part of the thesis is concerned with the 3D reconstruction of samples using intensity-only measurements which the need to holographically acquire them. We show that using intensity-only measurements, we could still be able to reconstruct the 3D volume of the sample with edge-enhanced effects which was proven useful for drug delivery applications in which nano-particles were identified on the cell membrane of immune T-cells in a drug delivery studies. Such reconstruction technique would result in more robust imaging system where the commercial imaging microscope systems can be incorporated with LEDs for high-quality speckle noise-free imaging systems. In addition, we show that under certain conditions, we can be able to reconstruct the 3D refractive index distribution of different samples.The third part of the thesis is contributing to high-speed complex wave-front shaping using DMDs. In that part, new modulation technique is demonstrated that can boost the speed of the current time-multiplexing techniques by a factor of 32. The modulation technique is based on amplitude modulation where an amplitude modulator is synchronized withvthe DMD to modulate the intensity of each bit-plane of an 8-bit image and then all the modulated bit-planes are linearly added on the detector. Such modulation technique can be used not only for structured illumination microscopy but also for high-speed 3D printing applications as well as projectors.The last part is concerned with using deep learning approaches to solve the missing cone problem usually accompanied with optical imaging due to the limited numerical aperture of the imaging system. Two techniques are discussed; the first is based on using a physical model to enhance the quality of the 3D RI reconstruction and the second is based on using deep neural network to solve the missing cone problem.

EPFL2022

High sensitivity, incoherent differential imaging

Chiara Bonati

Microscopic visualisation of optically transparent samples has been a topic of interest for several decades. Features such as density or chemical composition can influence the optical phase of transmitted light, and phase contrast can reveal these structures. Several methods of phase contrast have been developed, which can be categorised as either interferometric or non-interferometric, based on the type of coherence properties of the light used. In this work, I focus on incoherent based phase contrast, in particular Differential Phase Contrast (DPC). The choice of incoherent light brings benefits such as the absence of distortions like speckle patterns and ringing patterns, increased spatial resolution, and a simpler setup that can be used for in-vivo applications. Moreover, DPC allows reconstruction of quantitative phase maps of the samples. On the other hand, coherent-based techniques demonstrate superior performance in terms of phase sensitivity. The first part of this thesis offers a quantitative analysis of the phase sensitivity of DPC and investigates the influence of optical parameters and sample characteristics. With simulations and experiments, a relation between numerical aperture and phase sensitivity is demonstrated, and the concept of spectral matching is introduced to enhance the contrast. The methods can be generalised to any DPC setup, and allow a-priori investigation of the sensitivity of a DPC microscope at the design stage rather than through testing. Comparison between the best sensitivity that can be achieved in DPC and state-of-the-art interferometric techniques, shows that it is not possible to reach comparable single-shot performances. In this thesis, it is shown that the reason for this limitation is the strong background in DPC images, which degrades the dynamic range. DPC images are obtained with mirrored illuminations, for which the background is identical and the phase term switches in sign. The difference between these pairs of images is computed digitally, which does not improve the limited dynamic range. Lock-in DPC is proposed as a solution: instead of sampling different illumination states, lock-in DPC demodulates the phase signal when the illumination is switched, and the background is never encoded. This is enabled by periodical switching of sources coupled with a smart pixel detector, the so-called "lock-in camera". Part of this work is dedicated to the theoretical description of this method, and the analysis of the expected benefit. Experiments are also described, that demonstrate a factor of 8 improvement in single-shot sensitivity compared to standard DPC. DPC is not the only imaging technique to suffer from high intensity background: it is easy to see how the use of a lock-in camera for differential imaging can be generalised to any situation where weak modulations can be induced over a strong background. Here, an example is presented with lock-in Shifted Excitation Raman Difference Spectroscopy (SERDS). SERDS is an established Raman spectroscopy technique that takes advantage of the Raman emission spectrum being relative to the excitation wavelength to remove unwanted fluorescence emission. Two spectra with shifted excitation wavelengths are measured, their difference is computed, and the fluorescence is thus digitally removed. The parallel with DPC is immediately apparent. Both simulations and experiments are used to demonstrate the advantage of analog demodulation.

EPFL2022

A solvent- and vacuum-free route to large-area perovskite films for efficient solar modules

Yuhang Wang, Fei Ye

Recent advances in the use of organic-inorganic hybrid perovskites for optoelectronics have been rapid, with reported power conversion efficiencies of up to 22 per cent for perovskite solar cells1-9. Improvements in stability have also enabled testing over a timescale of thousands of hours(10-14). However, large-scale deployment of such cells will also require the ability to produce large-area, uniformly high-quality perovskite films. A key challenge is to overcome the substantial reduction in power conversion efficiency when a small device is scaled up: a reduction from over 20 per cent to about 10 per cent is found(15-21) when a common aperture area of about 0.1 square centimetres is increased to more than 25 square centimetres. Here we report a new deposition route for methyl ammonium lead halide perovskite films that does not rely on use of a common solvent(1,2,4-15) or vacuum3: rather, it relies on the rapid conversion of amine complex precursors to perovskite films, followed by a pressure application step. The deposited perovskite films were free of pin-holes and highly uniform. Importantly, the new deposition approach can be performed in air at low temperatures, facilitating fabrication of large-area perovskite devices. We reached a certified power conversion efficiency of 12.1 per cent with an aperture area of 36.1 square centimetres for a mesoporous TiO2-based perovskite solar module architecture.

Nature Publishing Group2017

Advanced Techniques in Optical Diffraction Tomography

Ahmed Bassam Sayed Ayoub Mohamed Emam

EPFL2022

High sensitivity, incoherent differential imaging

Chiara Bonati

EPFL2022