Focal Plane Imaging Research Articles

Phase retrieval of large-scale time-varying aberrations using a non-linear Kalman filtering framework

This paper presents a computationally efficient framework in which a single focal-plane image is used to obtain a high-resolution reconstruction of dynamic aberrations. Assuming small-phase aberrations, a non-linear Kalman filter implementation is developed whose computational complexity scales close to linearly with the number of pixels of the focal-plane camera. The performance of the method is tested in a simulation of an adaptive optics system, where the small-phase assumption is enforced by considering a closed-loop system that uses a low-resolution wavefront sensor to control a deformable mirror. The results confirm the computational efficiency of the algorithm and show a large robustness against noise and model uncertainties.

Combined transmission, dark field and fluorescence microscopy for intact, 3D tissue analysis of biopsies.

.Significance: Currently, tissue biopsies are sectioned into 3- to -thick slices that are used for conventional pathology analysis. Previous work by confocal microscopy and light-sheet microscopy has shown that analyzing biopsies intact in three-dimensions (3D) is possible and may lead to a better understanding of cancer growth patterns. Although accurate, these methods require fluorescent staining of the tissue, in addition to tissue clearing. If the 3D biopsy analysis could be done sufficiently swiftly, this approach may be used for on-site assessment of the adequacy of a biopsy taken.Aim: We aim to show that, by transmission microscopy of optically cleared tissue punches, the tissue architecture can be determined without the need for fluorescent staining.Approach: Transmission microscopy is used by combining bright field microscopy with dark field and epifluorescent microscopy to compare samples that have also been analyzed by fluorescent confocal microscopy.Results: With increasing distance to the focal plane, the higher-frequency part of the spatial frequency spectrum of transmitted light is attenuated increasingly. This property is exploited for tissue segmentation, detecting whether tissue is present at a certain position in the focal plane image. Using this approach, we show that a 3D rendering of the internal cavity or tubules structure of punch biopsies, which are up to 1-mm thick, can be acquired in scan time per imaging modality. The images of the overall tissue architecture that are obtained are similar to those from the confocal microscopy benchmark, without requiring fluorescent staining.Conclusions: Images of the overall tissue architecture can be obtained from transmission microcopy; they are similar to those from the confocal microscopy benchmark without requiring fluorescent staining. Tissue clearing is still needed. The total scan time of the present method is significantly shorter at a fraction of the device costs.

A Micro-Polarizer Array Configuration Design Method for Division of Focal Plane Imaging Polarimeter

The division of focal plane (DoFP) imaging polarimeter operates by integrating a focal plane array (FPA) with a micro-polarizer array (MPA). While the instantaneous field of view (IFoV) error is an inherent problem in such imaging sensor, which negatively affects the imaging accuracy. The influence of IFoV error can be effectively mitigated by optimizing the MPA configuration. In this paper, we propose an MPA configuration design scheme with four basic polarization angles 0°, 45°, 90° and 135° for DoFP imaging polarimeter. First, we analyze the frequency spectrum characteristics of MPA-filtered images and polarization frequency structure is proposed to record the frequency components and their positions. Then we analyze the frequency components and the crosstalk between them. Finally, we propose our design method that formulates the MPA design as a multi-objective optimization problem. And we design MPAs with better reconstruction performance of various sizes by optimizing their spectrum distribution. Besides, the reconstruction performance of many existing MPA designs were tested on statistically estimated scenes or a single image. This paper builds a database of 500 groups of infrared images of four polarization angles to test the performance of various MPA designs. Experiment results demonstrate that the proposed MPA designs outperform the state-of-the-art MPA designs in both objective and visual evaluations. Thus, the effectiveness of our proposed design method is further confirmed.

Characterization of the angular-dependent emission of nitrogen-vacancy centers in nanodiamond

We report on the characterization of the angular-dependent emission of single-photon emitters based on single nitrogen-vacancy (NV-) centers in nanodiamond at room temperature. A theoretical model for the calculation of the angular emission patterns of such an NV-center at a dielectric interface will be presented. For the first time, the orientation of the NV-centers in nanodiamond was determined from back focal plane images of NV-centers and by comparison of the theoretical and experimental angular emission pattern. Furthermore, the orientation of the NV-centers was also obtained from measurements of the fluorescence intensity in dependence on the polarization angle of the linearly polarized excitation laser. The results of these measurements are in good agreement. Moreover, the collection efficiency in this setup was calculated to be higher than 80% using the model of the angular emission of the NV-centers.

Terahertz phase retrieval imaging in reflection

Terahertz phase retrieval is a promising technique able to assess the complex diffracted wave properties through an iterative processing algorithm. In this Letter, we demonstrate the implementation of this technique in reflection geometry with a continuous wave acquisition system working at 0.287 THz. To ensure a high signal-to-noise ratio in the measured dataset, we proposed a double parallel recording scheme with one detector and two lock-in amplifiers operating with the complimentary sensitivity setting. This provided a higher numerical aperture than conventional raster-scanning focal plane imaging. A specialized digital interferometric postprocessing procedure was applied to obtain a surface height map from the reconstructed phase distribution in the object's irradiated area.

Single-shot multispectral angle-resolved ellipsometry.

We describe an instantaneous measurement scheme of multispectral angle-resolved ellipsometry with a color camera. A back focal plane image captured by the color camera enables us to simultaneously measure various polarization states along incidence angles and a multispectral domain. Modified parameters based on the principle of micro-ellipsometry are derived for an adequate form in a red, green and blue (RGB) domain by considering the wideband multispectral acquisition. The proposed method is validated by the measurement of uniformly deposited films and comparing our results with a commercial ellipsometer. The comparison shows that our proposed method enables real-time inspection with high precision.

Advancement and application of terahertz pulsed focal-plane imaging technique

As an important composition of terahertz (THz) technology, THz pulsed focal-plane imaging has been paid widely attention since it was invented. Until now, researchers have introduced all kinds of methods to enhance the performance of this imaging technique. Simultaneously, this imaging technique has been tried to apply into various industrial and fundamental research fields. In this paper, recent technique improvements and application researches for THz pulsed focal-plane imaging are reviewed, including the spatial resolution enhancement, signal-to-noise ratio improvement, information acquiring ability as well as applications of this imaging technique in spectroscopic identification inspections, function demonstrations of meta-surface devices, measurements of THz special beams, observations of THz surface electromagnetic waves, and so on. The aim of this paper is to push the technique innovation and application exploration of THz pulsed focal-plane imaging.

Multiplexing lobster-eye optics: a concept for wide-field x-ray monitoring

We propose a concept of multiplexing lobster-eye (MuLE) optics to achieve significant reductions in the number of focal plane imagers in lobster-eye (LE) wide-field X-ray monitors. In the MuLE configuration, an LE mirror is divided into several segments and the X-rays reflected on each of these segments are focused on a single image sensor in a multiplexed configuration. If each LE segment assumes a different rotation angle, the azimuthal rotation angle of a cross-like image reconstructed from a point source by the LE optics identifies the specific segment that focuses the X-rays on the imager. With a focal length of 30 cm and LE segments with areas of 10 x 10 cm^2, ~1 sr of the sky can be covered with 36 LE segments and only four imagers (with total areas of 10 x 10 cm^2). A ray tracing simulation was performed to evaluate the nine-segment MuLE configuration. The simulation showed that the flux (0.5 to 2 keV) associated with the 5-sigma detection limit was ~2 x 10^-10 erg cm^-2 s^-1 (10 mCrab) for a transient with a duration of 100 s. The simulation also showed that the direction of the transient for flux in the range of 14 to 17 mCrab at 0.6 keV was determined correctly with 99.7% confidence limit. We conclude that the MuLE configuration can become an effective on-board device for small satellites for future X-ray wide-field transient monitoring.

Exact design of complex amplitude holograms for producing arbitrary scalar fields.

Typical methods to holographically encode arbitrary wavefronts assume the hologram medium only applies either phase shifts or amplitude attenuation to the wavefront. In many cases, phase cannot be introduced to the wavefront without also affecting the amplitude. Here we show how to encode an arbitrary wavefront into an off-axis transmission hologram that returns the exact desired arbitrary wavefunction in a diffracted beam for phase-only, amplitude-only, or mixed phase and amplitude holograms with any periodic groove profile. We apply this to design thin holograms for electrons in a TEM, but our results are generally applicable to light and X-ray optics. We employ a phase reconstruction from a series of focal plane images to qualitatively show the accuracy of this method to impart the expected amplitude and phase to a specific diffraction order.

Quantitative objective-based ring TIRFM system calibration through back focal plane imaging.

Being the established imaging tool for cell membrane-associated studies, total internal reflection fluorescence microscopy (TIRFM) still has some limitations. The most important one is the inhomogeneous evanescent excitation field mainly caused by the large-angle and fixed-azimuth illumination scheme, which can be eliminated by using ring-shaped illumination (ring TIRFM). However, it is challenging in assembling a ring TIRFM system with precise parameter control that works well. Here we emphasize the quantification of the ring TIRFM system and introduce a robust calibration routine to simultaneously rectify the asymmetry of the spinning light beam and determine the crucial experimental parameter, i.e., the incident angle. The calibration routine requires no specific sample preparation and is entirely based on the automatic back focal plane manipulation, avoiding possible errors caused by the sample difference and manual measurement. Its effectiveness is experimentally demonstrated by both the qualitative and quantitative comparisons of the images acquired using different samples, illumination schemes, and calibration approaches. These characteristics should enable our approach to greatly improve the practicability of TIRFM in life sciences.

A Linear-Array of 300-GHz Antenna Integrated GFET Detectors on a Flexible Substrate

Terahertz imaging has potential in a variety of applications, such as noninvasive inspection, medical examination, and security. Many of these applications call for flexible focal-plane arrays with large fields of view. Here, we demonstrate the implementation of a flexible, 300 GHz, 1 × 6 linear detector array based on graphene field-effect transistors and integrated bow-tie antennas. Conservative estimates based on room temperature measurements at 300 GHz show element voltage responsivities in the range from 20 to 70 V/W, and noise equivalent powers in the range from 0.06 to 0.2 nW/ √Hz. Measured radiation patterns, showing good agreement with simulations, reveal half-power beamwidths of 45° and 60° for H-and E-planes, respectively. Characterization of the antenna array in a curved configuration shows that the voltage response is reduced up to 3 dB compared to the flat configuration due to a decrease of the antenna directivity. We believe that our preliminary results could serve as an enabling platform for the future development of flexible antenna arrays based on graphene field-effect transistors for curved focal plane imaging, important for wearable sensors and many other applications.

Single‐Layer Aberration‐Compensated Flat Lens for Robust Wide‐Angle Imaging

AbstractMetasurfaces are planarized and miniaturized versions of conventional optical elements. Subwavelength‐thick single‐layer metalenses have diffraction limited resolution for on‐axis imaging but relatively low resolution for off‐axis imaging due to off‐axial aberrations. The aberrations of planar single‐layer metalenses have been corrected by patterning two metasurfaces on both sides of a substrate to form metalens doublets with a thickness of hundreds of micrometers to millimeters. The multilevel diffractive lenses are demonstrated to achieve wide angle imaging with a thickness of several micrometers, however, the off‐axial aberrations are not compensated. Here, an epsilon‐greedy algorithm‐based scheme for achieving a planar wavelength‐thick single‐layer aberration‐compensated (SLAC) flat lens consisting of dielectric nanoring structures fabricated by 3D printing are proposed. The scheme is experimentally validated via a SLAC flat lens. This SLAC flat lens has a thickness of 1 µm, a numerical aperture of 0.45, a focal length of 1 mm, a full field of view (FOV) of 32° that enables aberration‐compensated imaging along the focal plane and monochromatic microscopic imaging with resolution better than 2.2 µm at a wavelength of 633 nm. This scheme can lead to ultrathin wide‐FOV flat lens designs as well as low‐cost mass production, which has various applications in miniscopes, mobile camera modules, and machine vision.

Robust diffraction-limited near-infrared-to-near-ultraviolet wide-field imaging from stratospheric balloon-borne platforms—Super-pressure Balloon-borne Imaging Telescope performance

At a fraction of the total cost of an equivalent orbital mission, scientific balloon-borne platforms, operating above 99.7% of the Earth's atmosphere, offer attractive, competitive, and effective observational capabilities-namely, space-like seeing, transmission, and backgrounds-which are well suited for modern astronomy and cosmology. The Super-pressure Balloon-borne Imaging Telescope (SUPERBIT) is a diffraction-limited, wide-field, 0.5 m telescope capable of exploiting these observing conditions in order to provide exquisite imaging throughout the near-infrared to near-ultraviolet. It utilizes a robust active stabilization system that has consistently demonstrated a 48 mas 1σ sky-fixed pointing stability over multiple 1 h observations at float. This is achieved by actively tracking compound pendulations via a three-axis gimballed platform, which provides sky-fixed telescope stability at < 500 mas and corrects for field rotation, while employing high-bandwidth tip/tilt optics to remove residual disturbances across the science imaging focal plane. SUPERBIT's performance during the 2019 commissioning flight benefited from a customized high-fidelity science-capable telescope designed with an exceptional thermo- and opto-mechanical stability as well as a tightly constrained static and dynamic coupling between high-rate sensors and telescope optics. At the currently demonstrated level of flight performance, SUPERBIT capabilities now surpass the science requirements for a wide variety of experiments in cosmology, astrophysics, and stellar dynamics.

Focal-plane three-dimensional imaging method based on temporal ghost imaging: a proof of concept simulation.

A new focal-plane three-dimensional (3D) imaging method based on temporal ghost imaging is proposed and demonstrated. By exploiting the advantages of temporal ghost imaging, this method enables the utilization of slow integrating cameras and facilitates 3D surface imaging within the framework of sequential flood-illumination and focal-plane detection. The depth information is achieved by a temporal correlation between received and reference signals with multiple-shot, and the reflectivity information is achieved by flash imaging with a single-shot. The feasibility and performance of this focal-plane 3D imaging method have been verified through theoretical analysis and numerical experiments.

Patterned laser-induced graphene for terahertz wave modulation

Laser-induced graphene (LIG) has received a large amount of attention because of its superior properties of high electrical conductivity, high thermal stability and conductivity, simple synthesis, and the low fabrication cost of the patterned structures. However, most studies on LIG are focused on electrical applications. In this work, we first examine the effect of the substrate on LIG generated on polyimide and then fabricate the patterned LIG structures including gratings and Fresnel zone plates for terahertz (THz)-wave modulations. The functions of the structures are experimentally demonstrated by a THz focal plane imaging system. It is anticipated that LIG-based structures could widen the application of THz technology.

Efficient wavefront sensing for space-based adaptive optics

Future large space telescopes will be equipped with adaptive optics (AO) to overcome wavefront aberrations and achieve high contrast for imaging faint astronomical objects, such as earth-like exoplanets and debris disks. In contrast to AO that is widely used in ground telescopes, space-based AO systems will use focal plane wavefront sensing to measure the wavefront aberrations. Focal plane wavefront sensing is a class of techniques that reconstruct the light field based on multiple focal plane images distorted by deformable mirror (DM) probing perturbations. In this paper, we report an efficient focal plane wavefront sensing approach for space-based AO which optimizes the DM probing perturbation and thus also the integration time for each image. Simulation of the AO system equipped with a vortex coronagraph has demonstrated that our new approach enables efficient information acquisition and significantly reduces the time needed for achieving high contrast in space.

Birefringence characteristics of magnesium oxide crystal in terahertz frequency region by using terahertz focal plane imaging

Fabricating integratable and high-efficiency optical polarization devices is one of the fundamentally important challenges in the field of terahertz optics. Compared with the traditional polarization materials such as quartz crystal and liquid crystal, MgO crystal is one of the most important potential candidates for fabricating terahertz optical devices due to its high transmittance in terahertz frequency region. To determine the birefringence characteristics of MgO crystal in the terahertz frequency region, the modulation of the polartization state of a terahtertz wave through a &lt;inline-formula&gt;&lt;tex-math id="Z-20201014200839"&gt;\begin{document}$\left\langle {100} \right\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200766_Z-20201014200839.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200766_Z-20201014200839.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; crystalline MgO flake is studied using terahertz focal plane imaging method. Within this approach, the polarization of a terahertz wave can be intuitively identified from the imaging of the amplitude and the phase of the &lt;i&gt;z&lt;/i&gt;-direction component of terahertz electronic field. By measuring the imaging of both the amplitude and the phase of terahertz field with and without passing through the &lt;inline-formula&gt;&lt;tex-math id="Z-20201014200825"&gt;\begin{document}$\left\langle {100} \right\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200766_Z-20201014200825.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200766_Z-20201014200825.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; crystalline MgO flake, it is found that the left and right circularly polarized light are converted into perpendicular linearly-polarized light after passing through the MgO flake. The polarization direction of the linearly polarized light changes with the rotating of MgO flake along the direction perpendicular to the light propagation. The conversion between the linearly polarized light and the circularly polarized light is analyzed by using the Jones matrix approach. These properties indicate that the &lt;inline-formula&gt;&lt;tex-math id="Z-20201014200830"&gt;\begin{document}$\left\langle {100} \right\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200766_Z-20201014200830.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200766_Z-20201014200830.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; crystalline MgO flake acts as a quarter wave plate for terahertz waves. To further identify the character of terahertz quarter wave plate, the refractive index of the ordinary and extrordinary light within terahertz frequency region of crystalline MgO crystal are measured by using transmission terahertz time-domain spectroscopy system. By comparing the phase difference between the ordinary and extraordinary light after passing through the MgO flake, it is shown that a quarter of wavelength difference between the ordinary and extraordinary light is obtained. These results indicate that the &lt;inline-formula&gt;&lt;tex-math id="Z-20201014200904"&gt;\begin{document}$\left\langle {100} \right\rangle $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200766_Z-20201014200904.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20200766_Z-20201014200904.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; crystalline MgO crystals can be used to fabricate quarter wave plates and relevant polarization devices in the terahertz band.

Concept of scanning imaging device for optical telescopes

We propose an approach of a focal plane imaging device for a telescope, which is based on spatially-scanned solitary photodetector, and can be used as an alternative to matrix detectors (CMOS or CCD). This approach allows to set efficient sensor size, pixel resolution, sensitivity and recording time on customer demand. Technical feasibility of up to 200 pixel/mm (5000 ppi) spatial resolution for focal field of up to astronomical photographic plate size, with the detection noise-equivalent power of 3 x 1015 W⁄√(Hz ) and the measurement time of ≈ 1 minute per megapixel is justified.

High-efficiency subwavelength bridge-shaped photo-absorbers based on transverse cavity modes.

We study a frequency-selective photon absorber able to efficiently confine light within very thin semiconducting layers. The active material is appropriately embedded in a cavity resonator where lateral resonances occur, based on nonplasmonic modes, thus minimizing undesired losses in the metallic parts. Typical examples and configurations in the infrared and visible ranges are illustrated. Theoretical quantum efficiencies around 90% may be reached, with a broad angular tolerance. Also, multispectral photon sorters may be designed by combining resonators of different sizes. These results especially find direct applications for elementary ultracompact detectors and imaging focal plane arrays.

Predictive wavefront sensorless adaptive optics for time-varying aberrations.

A new wavefront sensorless adaptive optics method is presented that can accurately correct for time-varying aberrations using a single focal plane image at each sample instance. The linear relation between the mean square of the aberration gradient and the change in second moment of the image forms the basis of the presented method. The new algorithm results in significant improvements when an accurate model of the aberration's temporal dynamics is known, by applying a Kalman filter and optimal control. Moreover, where existing wavefront sensorless adaptive optics methods update all modes sequentially, the information of the Kalman filter is used to select and update the modes that are expected to give the greatest improvement in performance. The performance is analyzed in a simulation of an adaptive optics system for atmospheric turbulence. The results show that the new method is able to correct for the aberration more accurately for higher wind speeds and higher noise levels than existing algorithms.