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Demetri Psaltis

Publications and source records attributed to Demetri Psaltis.

At least 55 records · Page 3Linked to original sources
arXiv2017

Imaging thick samples with optical tomography

Optical diffraction tomography (ODT), initially described in the seminal paper of Emil Wolf [Opt. Comm., 1(4), 153-156 (1969)], has received renewed attention recently. The vast majority of ODT experiments reported to date have been 3D images of single cells or beads demonstrating excellent image quality and sectioning capability. The sample thickness in these experiments is 10 to 20 microns and the optical thickness is generally well below 2π. In this paper, we explore ODT when the sample consists of multiple layers of cells. We assess experimentally the impact of sample thickness for the different ODT reconstruction algorithms and we describe a strategy that allows us to image, for the first time, multi-cell clusters.

physics.optics↗
arXiv2017

Assessment of learning tomography using Mie theory

In Optical diffraction tomography, the multiply scattered field is a nonlinear function of the refractive index of the object. The Rytov method is a linear approximation of the forward model, and is commonly used to reconstruct images. Recently, we introduced a reconstruction method based on the Beam Propagation Method (BPM) that takes the nonlinearity into account. We refer to this method as Learning Tomography (LT). In this paper, we carry out simulations in order to assess the performance of LT over the linear iterative method. Each algorithm has been rigorously assessed for spherical objects, with synthetic data generated using the Mie theory. By varying the RI contrast and the size of the objects, we show that the LT reconstruction is more accurate and robust than the reconstruction based on the linear model. In addition, we show that LT is able to correct distortion that is evident in Rytov approximation due to limitations in phase unwrapping. More importantly, the capacity of LT in handling multiple scattering problem are demonstrated by simulations of multiple cylinders using the Mie theory and confirmed by experimental results of two spheres.

physics.optics↗
arXiv2016

Three-dimensional microfabrication through a multimode optical fiber

Additive manufacturing, also known as 3D printing, is an advanced manufacturing technique that allows the fabrication of arbitrary macroscopic and microscopic objects. All 3D printing systems require large optical elements or nozzles in proximity to the built structure. This prevents their use in applications in which there is no direct access to the area where the objects have to be printed. Here, we demonstrate three-dimensional microfabrication based on two-photon polymerization (TPP) with sub diffraction-limited resolution through an ultra-thin, 50 mm long printing nozzle of 560 micrometers in diameter. Using wavefront shaping, femtosecond infrared pulses are focused and scanned through a multimode optical fiber (MMF) inside a photoresist that polymerizes via two-photon absorption. We show the construction of arbitrary 3D structures of 500 nm resolution on the other side of the fiber. To our knowledge, this is the first demonstration of microfabrication through a multimode optical fiber. Our work represents a new area which we refer to as endofabrication.

physics.optics↗
arXiv2015

Confocal microscopy through a multimode fiber using optical correlation

We report on a method to obtain confocal imaging through multimode fibers using optical correlation. First, we measure the fiber's transmission matrix in a calibration step. This allows us to create focused spots at one end of the fiber by shaping the wavefront sent into it from the opposite end. These spots are scanned over a sample, and the light coming back from the sample via the fiber is optically correlated with the input pattern. We show that this achieves spatial selectivity in the detection. The technique is demonstrated on microbeads, a dried epithelial cell, and a cover glass.

physics.optics↗
arXiv2015

Two-photon imaging through a multimode fiber

In this work we demonstrate 3D imaging using two-photon excitation through a 20 cm long multimode optical fiber (MMF) of 350 micrometers diameter. The imaging principle is similar to single photon fluorescence through a MMF, except that a focused femtosecond pulse is delivered and scanned over the sample. In our approach, focusing and scanning through the fiber is accomplished by digital phase conjugation using mode selection by time gating with an ultra-fast reference pulse. The excited two-photon emission is collected through the same fiber. We demonstrate depth sectioning by scanning the focused pulse in a 3D volume over a sample consisting of fluorescent beads suspended in a polymer. The achieved resolution is 1 micrometer laterally and 15 micrometers axially. Scanning is performed over an 80x80 micrometers field of view. To our knowledge, this is the first demonstration of high-resolution three-dimensional imaging using two-photon fluorescence through a multimode fiber.

physics.optics↗
arXiv2015

Digital confocal microscopy through a multimode fiber

Acquiring high-contrast optical images deep inside biological tissues is still a challenging problem. Confocal microscopy is an important tool for biomedical imaging since it improves image quality by rejecting background signals. However, it suffers from low sensitivity in deep tissues due to light scattering. Recently, multimode fibers have provided a new paradigm for minimally invasive endoscopic imaging by controlling light propagation through them. Here we introduce a combined imaging technique where confocal images are acquired through a multimode fiber. We achieve this by digitally engineering the excitation wavefront and then applying a virtual digital pinhole on the collected signal. In this way, we are able to acquire images through the fiber with significantly increased contrast. With a fiber of numerical aperture 0.22, we achieve a lateral resolution of 1.5um, and an axial resolution of 12.7um. The point-scanning rate is currently limited by our spatial light modulator (20Hz).

physics.optics↗
arXiv2015

Isotropic inverse-problem approach for two-dimensional phase unwrapping

In this paper, we propose a new technique for two-dimensional phase unwrapping. The unwrapped phase is found as the solution of an inverse problem that consists in the minimization of an energy functional. The latter includes a weighted data-fidelity term that favors sparsity in the error between the true and wrapped phase differences, as well as a regularizer based on higher-order total-variation. One desirable feature of our method is its rotation invariance, which allows it to unwrap a much larger class of images compared to the state of the art. We demonstrate the effectiveness of our method through several experiments on simulated and real data obtained through the tomographic phase microscope. The proposed method can enhance the applicability and outreach of techniques that rely on quantitative phase evaluation.

physics.optics↗
arXiv2015

Delivery of focused short pulses through a multimode fiber

Light propagation through multimode fibers suffers from spatial distortions that lead to a scrambled intensity profile. In previous work, the correction of such distortions using various wavefront control methods has been demonstrated in the continuous wave case. However, in the ultra-fast pulse regime, modal dispersion temporally broadens a pulse after propagation. Here, we present a method that compensates for spatial distortions and mitigates temporal broadening due to modal dispersion by a selective phase conjugation process in which only modes of similar group velocities are excited. The selectively excited modes are forced to follow certain paths through the multimode fiber and interfere constructively at the distal tip to form a focused spot with minimal temporal broadening. We demonstrate the delivery of focused 500 fs pulses through a 30 cm long step-index multimode fiber. The achieved pulse duration corresponds to approximately 1/30th of the duration obtained if modal dispersion was not controlled. Moreover, we measured a detailed two-dimensional map of the pulse duration at the output of the fiber and confirmed that the focused spot produces a two-photon absorption effect. This work opens new possibilities for ultra-thin multiphoton imaging through multimode fibers.

physics.optics↗
arXiv2015

Optical-resolution photoacoustic imaging through thick tissue with a thin capillary as a dual optical-in acoustic-out waveguide

We demonstrate the ability to guide high-frequency photoacoustic waves through thick tissue with a water-filled silica-capillary (150 μm inner diameter and 30 mm long). An optical-resolution photoacoustic image of a 30 μm diameter absorbing nylon thread was obtained by guiding the acoustic waves in the capillary through a 3 cm thick fat layer. The transmission loss through the capillary was about -20 dB, much lower than the -120 dB acoustic attenuation through the fat layer. The overwhelming acoustic attenuation of high-frequency acoustic waves by biological tissue can therefore be avoided by the use of a small footprint capillary acoustic waveguide for remote detection. We finally demonstrate that the capillary can be used as a dual optical-in acoustic-out waveguide, paving the way for the development of minimally invasive optical-resolution photoacoustic endoscopes free of any acoustic or optical elements at their imaging tip.

physics.optics↗
arXiv2015

A Learning Approach to Optical Tomography

We describe a method for imaging 3D objects in a tomographic configuration implemented by training an artificial neural network to reproduce the complex amplitude of the experimentally measured scattered light. The network is designed such that the voxel values of the refractive index of the 3D object are the variables that are adapted during the training process. We demonstrate the method experimentally by forming images of the 3D refractive index distribution of cells.

physics.optics↗
arXiv2013

Increasing the imaging capabilities of multimode fibers by exploiting the properties of highly scattering media

We present a novel design that exploits the focusing properties of scattering media to increase the resolution and the working distance of multimode fiber based imaging devices. Placing a highly scattering medium in front of the distal tip of the multimode fiber enables the formation of smaller sized foci at increased working distances away from the fiber tip. We perform a parametric study of the effect of the working distance and the separation between the fiber and the scattering medium on the focus size. We experimentally demonstrate submicron focused spots as far away as 800μm with 532nm light.

physics.optics↗
arXiv2013

Optical-resolution photoacoustic microscopy by use of a multimode fiber

We demonstrate Optical-Resolution Photoacoustic Microscopy (OR-PAM), where the optical field is focused and scanned using Digital Phase Conjugation (DPC) through a multimode fiber. The focus is scanned across the field of view using digital means, and the acoustic signal induced is collected by a transducer. Optical-resolution photoacoustic images of a knot made by two absorptive wires are obtained and we report on resolution smaller than 1.5μm across a 201μm by 201μm field of view. The use of a multimode optical fiber for the optical excitation part can pave the way for miniature endoscopes that can provide optical-resolution photoacoustic images at large optical depth.

physics.optics↗
arXiv2008

Coupled-Resonator Optical Near-Field Lithography

The problem of pattern formation in resonantly-enhanced near-field lithography by the use of dielectric or plasmonic planar resonators is investigated. Sub-diffraction-limited bright or dark spots can be produced by taking advantage of the rotational invariance of planar resonators. To increase the spatial bandwidth of the resonant enhancement, an array of coupled planar resonators can open up a band of Bloch resonances, analogous to coupled-resonator optical waveguides.

physics.optics↗
arXiv2007

Magnifying perfect lens and superlens design by coordinate transformation

The coordinate transformation technique is applied to the design of perfect lenses and superlenses. In particular, anisotropic metamaterials that magnify two-dimensional planar images beyond the diffraction limit are designed by the use of oblate spheroidal coordinates. The oblate spheroidal perfect lens or superlens can naturally be used in reverse for lithography of planar subwavelength patterns.

physics.optics↗
arXiv2007

Modulational Instability in Nonlinearity-Managed Optical Media

We investigate analytically, numerically, and experimentally the modulational instability in a layered, cubically-nonlinear (Kerr) optical medium that consists of alternating layers of glass and air. We model this setting using a nonlinear Schrödinger (NLS) equation with a piecewise constant nonlinearity coefficient and conduct a theoretical analysis of its linear stability, obtaining a Kronig-Penney equation whose forbidden bands correspond to the modulationally unstable regimes. We find very good {\it quantitative} agreement between the theoretical analysis of the Kronig-Penney equation, numerical simulations of the NLS equation, and the experimental results for the modulational instability. Because of the periodicity in the evolution variable arising from the layered medium, we find multiple instability regions rather than just the one that would occur in uniform media.

nlin.PS↗
arXiv2006

Modulational instability in a layered Kerr medium: Theory and Experiment

We present the first experimental investigation of modulational instability in a layered Kerr medium. The particularly interesting and appealing feature of our configuration, consisting of alternating glass-air layers, is the piecewise-constant nature of the material properties, which allows a theoretical linear stability analysis leading to a Kronig-Penney equation whose forbidden bands correspond to the modulationally unstable regimes. We find very good {\it quantitative} agreement between theoretical, numerical, and experimental diagnostics of the modulational instability. Because of the periodicity in the evolution variable arising from the layered medium, there are multiple instability regions rather than just one as in the uniform medium.

nlin.PS↗
arXiv2006

Reflectionless evanescent-wave amplification by two dielectric planar waveguides

Utilizing the underlying physics of evanescent wave amplification by a negative-refractive-index slab, it is shown that evanescent waves with specific spatial frequencies can also be amplified without any reflection simply by two dielectric planar waveguides. The simple configuration allows one to take advantage of the high resolution limit of a high-refractive-index material without contact with the object.

physics.optics↗