Rotationally symmetric formulation of the wave propagation method-application to the straylight analysis of diffractive lenses

Efficient and accurate intensity diffraction tomography of multiple-scattering samples

Optical Diffraction Tomography (ODT) is a label-free method to quantitatively estimate the 3D refractive index (RI) distributions of microscopic samples. Recently, significant efforts were directed towards methods to model multiple-scattering objects. The fidelity of reconstructions rely on accurately modelling light-matter interactions, but the efficient simulation of light propagation through high-RI structures over a large range of illumination angles is still challenging. Here we present a solution dealing with these problems, proposing a method that allows one to efficiently model the tomographic image formation for strongly scattering objects illuminated over a wide range of angles. Instead of propagating tilted plane waves we apply rotations on the illuminated object and optical field and formulate a new and robust multi-slice model suitable for high-RI contrast structures. We test reconstructions made by our approach against simulations and experiments, using rigorous solutions to Maxwell's equations as ground truth. We find the proposed method to produce reconstructions of higher fidelity compared to conventional multi-slice methods, especially for the challenging case of strongly scattering samples where conventional reconstruction methods fail.

Coupling light emission of single-photon sources into single-mode fibers: mode matching, coupling efficiencies, and thermo-optical effects

We discuss the coupling efficiency of single-photon sources into single-mode fibers using 3D printed micro-optical lens designs. Using the wave propagation method, we optimize lens systems for two different quantum light sources and assess the results in terms of maximum coupling efficiencies, misalignment effects, and thermo-optical influences. Thereby, we compare singlet lens designs with one lens printed onto the fiber with doublet lens designs with an additional lens printed onto the semiconductor substrate. The single-photon sources are quantum dots based on microlenses and circular Bragg grating cavities at 930 nm and 1550 nm, respectively.

Superconducting nanowire single-photon detector with 3D-printed free-form microlenses

We present an approach to increase the effective light-receiving area of superconducting nanowire single-photon detectors (SNSPD) by free-form microlenses. These lenses are printed in situ on top of the sensitive detector areas using high-resolution multi-photon lithography. We demonstrate a detector based on niobium-nitride (NbN) nanowires with a 4.5 µm × 4.5 µm sensitive area, supplemented with a lens of 60-µm-diameter. For a plane-wave-like free-space illumination at a wavelength of 1550 nm, the lensed sensor has a 100-fold increased effective collection area, which leads to a strongly enhanced system detection efficiency without the need for long nanowires. Our approach can be readily applied to a wide range of sensor types. It effectively overcomes the inherent design conflict between high count rate, high timing accuracy, and high fabrication yield on the one hand and high collection efficiency through a large effective detection area on the other hand.

3D printed stacked diffractive microlenses

Planar lenses such as metalenses and diffractive lenses exhibit severe field-dependent aberrations when imaging extended objects with high numerical aperture. This problem can be overcome by stacking at least two of such devices on top of each other. In this work, we present such stacked imaging systems, namely doublets and triplets of diffractive optical elements. They are fabricated by femtosecond direct laser writing in one single step without the need for alignment in sizes of below 200 µm in diameter and 100 µm in height. The lenses allow for efficient sub µm resolution imaging at visible wavelengths combined with a full field-of-view of up to 60°. As additional benefit, our approach dramatically reduces the writing times of 3D printed lens systems to below 15 minutes.

Noninvasive characterization of optical fibers

Capillary optical fibers with hole diameters of several micrometers are important for novel plasmonic applications and medical diagnosis. In order to ensure the optical functionality of these fibers, the diameter of the capillary hole needs to be realized with high accuracy. Here, we introduce a novel and noninvasive methodology to characterize optical fibers and discuss it for the assessment of capillaries. In this method, the fiber is side-illuminated by a coherent beam, and the resulting diffraction pattern is analyzed. This corresponds to an in-line holographic measurement in the presence of strong scattering. A numerical parameter retrieval allows us to characterize the capillary hole diameter with an accuracy of approximately 100 nm for radii between several hundreds of nanometers and several tens of micrometers.

Coherent field propagation between tilted planes

Propagating electromagnetic light fields between nonparallel planes is of special importance, e.g., within the design of novel computer-generated holograms or the simulation of optical systems. In contrast to the extensively discussed evaluation between parallel planes, the diffraction-based propagation of light onto a tilted plane is more burdensome, since discrete fast Fourier transforms cannot be applied directly. In this work, we propose a quasi-fast algorithm (O(N³ log N)) that deals with this problem. Based on a proper decomposition into three rotations, the vectorial field distribution is calculated on a tilted plane using the spectrum of plane waves. The algorithm works on equidistant grids, so neither nonuniform Fourier transforms nor an explicit complex interpolation is necessary. The proposed algorithm is discussed in detail and applied to several examples of practical interest.

Optical tomographic reconstruction based on multi-slice wave propagation method

In optical tomography, it is challenging to obtain high-quality results for complex-structured objects which induce multiple scattering. Nonlinear reconstruction methods outperform linear ones in these situations. A promising nonlinear method is the approach based on beam propagation method, but its accuracy may decrease for complicated structures. In this paper, we describe a novel tomographic reconstruction method using multi-slice wave propagation method (WPM) as the forward model, which simulates the scattering process more precisely but has not been introduced in tomographic reconstruction before. The computational model of WPM is presented. To tackle the computational complexity, we propose an efficient scheme to compute the transmitted field and its derivative. We then use an iterative optimization method to recover the quantitative refraction index distribution. We also discuss the influences of the parameters in the method and how to determine their values. The experimental results demonstrate that this method can address multiple scattering problems and provide high accuracy for complex-structured objects.