13 research outputs found
Multifunctional meta-mirror: polarization splitting and focusing
Metasurfaces are paving the way to improve traditional optical components by
integrating multiple functionalities into one optically flat metasurface
design. We demonstrate the implementation of a multifunctional gap surface
plasmon-based metasurface which, in reflection mode, splits orthogonal linear
light polarizations and focuses into different focal spots. The fabricated
configuration consists of 50 nm thick gold nanobricks with different lateral
dimensions, organized in an array of 240 nm x 240 nm unit cells on the top of a
50 nm thick silicon dioxide layer, which is deposited on an optically thick
reflecting gold substrate. Our device features high efficiency (up to ~65%) and
polarization extinction ratio (up to ~30 dB), exhibiting broadband response in
the near-infrared band (750-950 nm wavelength) with the focal length dependent
on the wavelength of incident light. The proposed optical component can be
forthrightly integrated into photonic circuits or fiber optic devices.Comment: 18 pages, including 5 figure
AmbientFlow: Invertible generative models from incomplete, noisy measurements
Generative models have gained popularity for their potential applications in
imaging science, such as image reconstruction, posterior sampling and data
sharing. Flow-based generative models are particularly attractive due to their
ability to tractably provide exact density estimates along with fast,
inexpensive and diverse samples. Training such models, however, requires a
large, high quality dataset of objects. In applications such as computed
imaging, it is often difficult to acquire such data due to requirements such as
long acquisition time or high radiation dose, while acquiring noisy or
partially observed measurements of these objects is more feasible. In this
work, we propose AmbientFlow, a framework for learning flow-based generative
models directly from noisy and incomplete data. Using variational Bayesian
methods, a novel framework for establishing flow-based generative models from
noisy, incomplete data is proposed. Extensive numerical studies demonstrate the
effectiveness of AmbientFlow in correctly learning the object distribution. The
utility of AmbientFlow in a downstream inference task of image reconstruction
is demonstrated
Investigating the robustness of a learning-based method for quantitative phase retrieval from propagation-based x-ray phase contrast measurements under laboratory conditions
Quantitative phase retrieval (QPR) in propagation-based x-ray phase contrast
imaging of heterogeneous and structurally complicated objects is challenging
under laboratory conditions due to partial spatial coherence and
polychromaticity. A learning-based method (LBM) provides a non-linear approach
to this problem while not being constrained by restrictive assumptions about
object properties and beam coherence. In this work, a LBM was assessed for its
applicability under practical scenarios by evaluating its robustness and
generalizability under typical experimental variations. Towards this end, an
end-to-end LBM was employed for QPR under laboratory conditions and its
robustness was investigated across various system and object conditions. The
robustness of the method was tested via varying propagation distances and its
generalizability with respect to object structure and experimental data was
also tested. Although the LBM was stable under the studied variations, its
successful deployment was found to be affected by choices pertaining to data
pre-processing, network training considerations and system modeling. To our
knowledge, we demonstrated for the first time, the potential applicability of
an end-to-end learning-based quantitative phase retrieval method, trained on
simulated data, to experimental propagation-based x-ray phase contrast
measurements acquired under laboratory conditions. We considered conditions of
polychromaticity, partial spatial coherence, and high noise levels, typical to
laboratory conditions. This work further explored the robustness of this method
to practical variations in propagation distances and object structure with the
goal of assessing its potential for experimental use. Such an exploration of
any LBM (irrespective of its network architecture) before practical deployment
provides an understanding of its potential behavior under experimental
settings.Comment: Under review as a journal submission. Early version with partial
results has been accepted for poster presentation at SPIE-MI 202
Assessing the capacity of a denoising diffusion probabilistic model to reproduce spatial context
Diffusion models have emerged as a popular family of deep generative models
(DGMs). In the literature, it has been claimed that one class of diffusion
models -- denoising diffusion probabilistic models (DDPMs) -- demonstrate
superior image synthesis performance as compared to generative adversarial
networks (GANs). To date, these claims have been evaluated using either
ensemble-based methods designed for natural images, or conventional measures of
image quality such as structural similarity. However, there remains an
important need to understand the extent to which DDPMs can reliably learn
medical imaging domain-relevant information, which is referred to as `spatial
context' in this work. To address this, a systematic assessment of the ability
of DDPMs to learn spatial context relevant to medical imaging applications is
reported for the first time. A key aspect of the studies is the use of
stochastic context models (SCMs) to produce training data. In this way, the
ability of the DDPMs to reliably reproduce spatial context can be
quantitatively assessed by use of post-hoc image analyses. Error-rates in
DDPM-generated ensembles are reported, and compared to those corresponding to a
modern GAN. The studies reveal new and important insights regarding the
capacity of DDPMs to learn spatial context. Notably, the results demonstrate
that DDPMs hold significant capacity for generating contextually correct images
that are `interpolated' between training samples, which may benefit
data-augmentation tasks in ways that GANs cannot.Comment: This paper is under consideration at IEEE TM
A Method for Evaluating the Capacity of Generative Adversarial Networks to Reproduce High-order Spatial Context
Generative adversarial networks are a kind of deep generative model with the
potential to revolutionize biomedical imaging. This is because GANs have a
learned capacity to draw whole-image variates from a lower-dimensional
representation of an unknown, high-dimensional distribution that fully
describes the input training images. The overarching problem with GANs in
clinical applications is that there is not adequate or automatic means of
assessing the diagnostic quality of images generated by GANs. In this work, we
demonstrate several tests of the statistical accuracy of images output by two
popular GAN architectures. We designed several stochastic object models (SOMs)
of distinct features that can be recovered after generation by a trained GAN.
Several of these features are high-order, algorithmic pixel-arrangement rules
which are not readily expressed in covariance matrices. We designed and
validated statistical classifiers to detect the known arrangement rules. We
then tested the rates at which the different GANs correctly reproduced the
rules under a variety of training scenarios and degrees of feature-class
similarity. We found that ensembles of generated images can appear accurate
visually, and correspond to low Frechet Inception Distance scores (FID), while
not exhibiting the known spatial arrangements. Furthermore, GANs trained on a
spectrum of distinct spatial orders did not respect the given prevalence of
those orders in the training data. The main conclusion is that while low-order
ensemble statistics are largely correct, there are numerous quantifiable errors
per image that plausibly can affect subsequent use of the GAN-generated images.Comment: Submitted to IEEE-TPAMI. Early version with partial results has been
accepted for poster presentation at SPIE-MI 202
A review of gap-surface plasmon metasurfaces: fundamentals and applications
Plasmonic metasurfaces, which can be considered as the two-dimensional analog of metal-based metamaterials, have attracted progressively increasing attention in recent years because of the ease of fabrication and unprecedented control over the reflected or transmitted light while featuring relatively low losses even at optical wavelengths. Among all the different design approaches, gap-surface plasmon metasurfaces – a specific branch of plasmonic metasurfaces – which consist of a subwavelength thin dielectric spacer sandwiched between an optically thick metal film and arrays of metal subwavelength elements arranged in a strictly or quasi-periodic fashion, have gained awareness from researchers working at practically any frequency regime as its realization only requires a single lithographic step, yet with the possibility to fully control the amplitude, phase, and polarization of the reflected light. In this paper, we review the fundamentals, recent developments, and opportunities of gap-surface plasmon metasurfaces. Starting with introducing the concept of gap-surface plasmon metasurfaces, we present three typical gap-surface plasmon resonators, introduce generalized Snell’s law, and explain the concept of Pancharatnam-Berry phase. We then overview the main applications of gap-surface plasmon metasurfaces, including beam-steerers, flat lenses, holograms, absorbers, color printing, polarization control, surface wave couplers, and dynamically reconfigurable metasurfaces. The review is ended with a short summary and outlook on possible future developments