Focal Plane Imaging Research Articles

Asymmetric Silicon Dimers Made by Single‐Shot Laser‐Induced Transfer Demultiplex Light of Different Wavelengths

AbstractNanofabrication technologies significantly influence the development of modern optical science. One of such technologies is laser‐induced transfer, which allows the creation of single Mie‐resonant spherical particles on a wide range of substrates. This study shows that this method can provide asymmetric dimers at the output: a single femtosecond pulse being focused on a silicon‐on‐insulator wafer results in appearing two nearly spherical particles of different sizes. The resulting dimers are characterized by scanning electron microscopy, elastic light scattering and Raman spectroscopy to gain insight into their structural properties. Back focal plane imaging and variable‐color evanescent‐wave illumination are then employed to measure the light scattering patterns from isolated dimers. Due to the interference of the excited resonances, the observed patterns are strongly asymmetric in a range of visible wavelengths, which is consistent with theoretical predictions. The results demonstrate the potential of asymmetric silicon dimers made by single‐shot laser‐induced transfer for color routing at visible light.

Optical Design of a Miniaturised Solar Magnetograph for Space Applications

Measuring the Sun’s magnetic field is a key component of monitoring solar activity and forecasting space weather. The main goal of the research presented in this paper is to investigate the possibility of reducing the dimensions and weight of a solar magnetograph while preserving its optical quality. This article presents a range of different designs, along with their advantages and disadvantages, and an analysis of the optical performance of each. All proposed designs are based on the magneto-optical filter (MOF) technique. As a result of the design study, a miniaturised solar magnetograph is proposed with an ultra-compact layout. The dimensions are 345 mm × 54 mm × 54 mm, and the optical quality is almost at the diffraction limit. The design has an entrance focal ratio of F/17.65, with a plate scale of 83.58 arcsec/mm at the telescope image focal plane, and produces a magnification of 0.79. The field of view is 1920 arcsec in diameter, equivalent to ±0.27 degrees, sufficient to cover the entire solar disk.

Conceptual Design of an On-Axis 6 m Space Telescope at the Diffraction Limit: Characteristics, Performance and Advantages

This paper presents the conceptual design of an on-axis 6 metre aperture space telescope designed to cover a field of view of ±0.2 degrees with an optical quality at the diffraction limit within a spectral range between 0.5 μm and 2.5 μm. The plate scale is 3 arcsec/mm, and the overall length is less than 12 m. A Korsch layout has been selected based on the superb aberration compensation offered by Three-Mirror Anastigmat systems. The proposed design presents some characteristics: an almost flat response in RMS wavefront error across the field and for the entire spectral range; a flat mirror has been included to reduce the overall volume, and this has been adjusted to be placed at an intermediate pupil position, acting as a baffle for stray light and as a Lyott to restrict background radiation. This mirror presents a central hole, defined to the aperture of the pupil, allowing the transmission of the beam towards the image focal plane, where it can be split for multiple payloads. It also allows the transmission of the central field, at 90 degrees with respect to the science beam, to be used for Active Optics monitoring and correction of the primary mirror in order to ensure optimum optical performance. This on-axis solution significantly reduces the technical complexity for manufacturing, metrology, integration, and tests and has an important impact in the cost of the telescope.

Human sperm rotate with a conserved direction during free swimming in four dimensions.

Head rotation in human spermatozoa is essential for different swimming modes and fertilisation, as it links the molecular workings of the flagellar beat with sperm motion in three-dimensional (3D) space over time. Determining the direction of head rotation has been hindered by the symmetry and translucent nature of the sperm head, and by the fast 3D motion driven by the helical flagellar beat. Analysis has been mostly restricted to two-dimensional (2D) single focal plane image analysis, which enables tracking of head centre position but not tracking of head rotation. Despite the conserved helical beating of the human sperm flagellum, human sperm head rotation has been reported to be uni- or bi-directional, and even to intermittently change direction in a given cell. Here, we directly measure the head rotation of freely swimming human sperm using multi-plane 4D (3D+t) microscopy and show that: (1) 2D microscopy is unable to distinguish head rotation direction in human spermatozoa; (2) head rotation direction in non-capacitating and capacitating solutions, for both aqueous and viscous media, is counterclockwise (CCW), as seen from head to tail, in all rotating spermatozoa, regardless of the experimental conditions; and (3) head rotation is suppressed in 36% of spermatozoa swimming in non-capacitating viscous medium, although CCW rotation is recovered after incubation in capacitating conditions within the same viscous medium, possibly unveiling an unexplored aspect of the essential need of capacitation for fertilisation. Our observations show that the CCW head rotation in human sperm is conserved. It constitutes a robust and persistent helical driving mechanism that influences sperm navigation in 3D space over time, and thus is of critical importance in cell motility, propulsion of flagellated microorganisms, sperm motility assessments, human reproduction research, and self-organisation of flagellar beating patterns and swimming in 3D space.

Challenges in Imaging Analyses of Biomolecular Condensates in Cells Infected with Influenza A Virus.

Biomolecular condensates are crucial compartments within cells, relying on their material properties for function. They form and persist through weak, transient interactions, often undetectable by classical biochemical approaches. Hence, microscopy-based techniques have been the most reliable methods to detail the molecular mechanisms controlling their formation, material properties, and alterations, including dissolution or phase transitions due to cellular manipulation and disease, and to search for novel therapeutic strategies targeting biomolecular condensates. However, technical challenges in microscopy-based analysis persist. This paper discusses imaging, data acquisition, and analytical methodologies' advantages, challenges, and limitations in determining biophysical parameters explaining biomolecular condensate formation, dissolution, and phase transitions. In addition, we mention how machine learning is increasingly important for efficient image analysis, teaching programs what a condensate should resemble, aiding in the correlation and interpretation of information from diverse data sources. Influenza A virus forms liquid viral inclusions in the infected cell cytosol that serve as model biomolecular condensates for this study. Our previous work showcased the possibility of hardening these liquid inclusions, potentially leading to novel antiviral strategies. This was established using a framework involving live cell imaging to measure dynamics, internal rearrangement capacity, coalescence, and relaxation time. Additionally, we integrated thermodynamic characteristics by analysing fixed images through Z-projections. The aforementioned paper laid the foundation for this subsequent technical paper, which explores how different modalities in data acquisition and processing impact the robustness of results to detect bona fide phase transitions by measuring thermodynamic traits in fixed cells. Using solely this approach would greatly simplify screening pipelines. For this, we tested how single focal plane images, Z-projections, or volumetric analyses of images stained with antibodies or live tagged proteins altered the quantification of thermodynamic measurements. Customizing methodologies for different biomolecular condensates through advanced bioimaging significantly contributes to biological research and potential therapeutic advancements.

Piston Error Measurement for Segmented Telescopes Based on a Hybrid Artificial Neural Network.

To address the difficulty and complexity of detecting piston errors for segmented telescopes, this paper proposes a new piston error measurement method based on a hybrid artificial neural network. First, we use the Resnet network to learn the mapping relationship between the focal plane degradation image and signs of the piston error. Then, based on the established theoretical relationship between the modulation transfer function and the piston error, a BP neural network is used to learn the mapping relationship between the MTF and the absolute value of the piston error. After the training of the hybrid network is completed, a wide-range and high-precision detection of the piston error of the sub-mirrors can be achieved using the combined output of the two networks, where only a focal plane image of the point source with broadband illumination is used as the input. The detection range can reach the entire coherent length of the input broadband light, and the detection accuracy can reach 10 nm. The method proposed in this paper has the advantages of high detection accuracy, a wide detection range, low hardware cost, a small network scale, and low training difficulty.

Emission Dipole and Pressure‐Driven Tunability of Second Harmonic Generation in vdWs Ferroelectric NbOI2

Abstract2D in‐plane ferroelectric NbOI2 exhibits strong second harmonic generation (SHG) and ultrahigh effective susceptibility. To push forward their applications in nonlinear photonics and optoelectronics, it is highly desirable to understand the emission dipole orientation and tunability of SHG, which is not achieved. Here, by integrating tight focusing from parabolic mirror with back focal plane (BFP) imaging technique, for the first time it is demonstrated that SHG emission of NbOI2 presents purely in‐plane dipole orientation in consistent with numerical simulations, suggesting the in‐plane components of the SHG susceptibility tensor in NbOI2 dominate the emission. Moreover, with the aid of ab‐initio calculations, it is found that the hydrostatic pressure can dramatically change the structure and resultant SHG intensity of NbOI2. Explicitly, SHG intensity endures a slight increase due to the distortion of octahedral at low pressure pressure, and then monotonously decreases due to the improvement of structural symmetry with further increasing pressure, and drastically quenching resulting from the ferroelectric to paraelectric phase transition. This work unambiguously demonstrates the dipole emission behavior of SHG and the relationship between structural evolution and SHG intensity, which provides an avenue for tunable nonlinear optics and optoelectronics.

Scene-adaptive spatially channeled imaging Mueller polarimeter.

A spatially adaptive Mueller matrix imaging polarimeter is described, simulated, and demonstrated with preliminary experiments. The system uses a spatial light modulator (SLM) in the polarization state generator (PSG) to create spatial carriers that controlled by the pattern written to the SLM. The polarization state analyzer (PSA) is a commercial division of focal plane imaging polarimeter. The PSG/PSA pair form a 9-channeled partial Mueller matrix polarimeter that measures a 3 × 3 sub-matrix of the Mueller matrix. We demonstrate that adapting the PSG modulation to the spatial frequency structure of the scene can reduce channel crosstalk and improve reconstruction accuracy. Initial experiments are performed that demonstrate the SLM's ability to produce sufficient modulation diversity to create the desired channel structure. Though there are several experimental challenges to obtain accurate Mueller matrix imagery, we demonstrate a system that adapts to the particular scene spatial frequency structure.

Back focal plane imaging for light emission from a tunneling junction in a low-temperature ultrahigh-vacuum scanning tunneling microscope.

We report the design and realization of the back focal plane (BFP) imaging for the light emission from a tunnel junction in a low-temperature ultrahigh-vacuum (UHV) scanning tunneling microscope (STM). To achieve the BFP imaging in a UHV environment, a compact "all-in-one" sample holder is designed and fabricated, which allows us to integrate the sample substrate with the photon collection units that include a hemisphere solid immersion lens and an aspherical collecting lens. Such a specially designed holder enables the characterization of light emission both within and beyond the critical angle and also facilitates the optical alignment inside a UHV chamber. To test the performance of the BFP imaging system, we first measure the photoluminescence from dye-doped polystyrene beads on a thin Ag film. A double-ring pattern is observed in the BFP image, arising from two kinds of emission channels: strong surface plasmon coupled emissions around the surface plasmon resonance angle and weak transmitted fluorescence maximized at the critical angle, respectively. Such an observation also helps to determine the emission angle for each image pixel in the BFP image and, more importantly, proves the feasibility of our BFP imaging system. Furthermore, as a proof-of-principle experiment, electrically driven plasmon emissions are used to demonstrate the capability of the constructed BFP imaging system for STM induced electroluminescence measurements. A single-ring pattern is obtained in the BFP image, which reveals the generation and detection of the leakage radiation from the surface plasmon propagating on the Ag surface. Further analyses of the BFP image provide valuable information on the emission angle of the leakage radiation, the orientation of the radiating dipole, and the plasmon wavevector. The UHV-BFP imaging technique demonstrated here opens new routes for future studies on the angular distributed emission and dipole orientation of individual quantum emitters in UHV.

Design and Implementation of Multi-Channel Readout Circuits for Low-Temperature Environments

Infrared sensors and focal plane imaging arrays are among the most important types of devices in the field of aerospace applications. To effectively amplify the small signals collected by infrared sensors and focal plane imaging arrays for subsequent processing, a new multi-channel preamplifier circuit based on ultra-low temperatures was designed in this study to read the acquisition signals of such devices. The technology of an SMIC 180 nm CMOS with 1.8 V power was adopted to realize the circuit. Meanwhile, an eight-level adjustable gain switch was used to increase the selectivity of signal processing. According to the simulation’s results, the single-channel power consumption of the circuit in the 77 K ultra-low temperature environment was only 5.17 mW. The circuit could drive a large load of 200–400 pF with an open-loop gain of 131.4 dB, which showed excellent performance in driving large loads, providing high gain and consuming less power. Additionally, the circuit exhibited good aspects for front-end signal reading and processing of infrared sensors and focal plane arrays in extreme environments.

Snapshot multispectral imaging using a diffractive optical network

Multispectral imaging has been used for numerous applications in e.g., environmental monitoring, aerospace, defense, and biomedicine. Here, we present a diffractive optical network-based multispectral imaging system trained using deep learning to create a virtual spectral filter array at the output image field-of-view. This diffractive multispectral imager performs spatially-coherent imaging over a large spectrum, and at the same time, routes a pre-determined set of spectral channels onto an array of pixels at the output plane, converting a monochrome focal-plane array or image sensor into a multispectral imaging device without any spectral filters or image recovery algorithms. Furthermore, the spectral responsivity of this diffractive multispectral imager is not sensitive to input polarization states. Through numerical simulations, we present different diffractive network designs that achieve snapshot multispectral imaging with 4, 9 and 16 unique spectral bands within the visible spectrum, based on passive spatially-structured diffractive surfaces, with a compact design that axially spans ~72λm, where λm is the mean wavelength of the spectral band of interest. Moreover, we experimentally demonstrate a diffractive multispectral imager based on a 3D-printed diffractive network that creates at its output image plane a spatially repeating virtual spectral filter array with 2 × 2 = 4 unique bands at terahertz spectrum. Due to their compact form factor and computation-free, power-efficient and polarization-insensitive forward operation, diffractive multispectral imagers can be transformative for various imaging and sensing applications and be used at different parts of the electromagnetic spectrum where high-density and wide-area multispectral pixel arrays are not widely available.

Far-Red Interlayer Excitons of Perovskite/Quantum-Dot Heterostructures.

Interlayer excitons (IXs) at the interface of heterostructures (HSs) with a staggered band alignment are fascinating quantum quasi-particles with light-emitting and long-lifetime characteristics. In this study, the energy band alignments (EBAs) of the HS of MAPbI3 perovskite thin sheets with CdSe-ZnS core-shell quantum dot (QD) layers are modulated by using different diameters of the QDs. Far-red IX emission is observed at 1.42eV from the HS of MAPbI3 /CdSe-ZnS-QD (λem =645nm) with type-II EBA owing to charge transfer. The lifetime of the far-red IXs is estimated to be 5.68µs, which is considerably longer than that (0.715ns) of the intralayer excitons from CdSe-ZnS-QD. With increasing incident excitation power, the PL peak and its intensity of IXs are blue-shifted and linearly increased, respectively, indicating a strong dipole alignment of far-red IXs at the heterojunction. Back focal plane imaging suggests that the directions of dipole moments of the IXs are relatively out-of-plane compared to those of the intralayer excitons (MAPbI3 and CdSe-ZnS-QD). Notably, the abnormal behavior of the optical characteristics is observed near the phase transition temperature (90K) of MAPbI3 . MAPbI3 /CdSe-ZnS-QD HS photodetectors show the increase in photocurrent and detectivity compared to MAPbI3 at IX excitation.

Quantum-Inspired Multi-Parameter Adaptive Bayesian Estimation for Sensing and Imaging

It is well known in Bayesian estimation theory that the conditional estimator attains the minimum mean squared error (MMSE) for estimating a scalar parameter of interest. In quantum, e.g., optical and atomic, imaging and sensing tasks the user has access to the quantum state that encodes the parameter. The choice of a measurement operator, i.e. a positive-operator valued measure (POVM), leads to a measurement outcome on which the aforesaid classical MMSE estimator is employed. Personick found the optimum POVM that attains the MMSE= over all possible physically allowable measurements and the resulting MMSE [1]. This result from 1971 is less-widely known than the quantum Fisher information (QFI), which lower bounds the variance of an unbiased estimator over all measurements without considering any prior probability. For multi-parameter estimation, in quantum Fisher estimation theory the inverse of the QFI matrix provides an operator lower bound on the covariance of an unbiased estimator, and this bound is understood in the positive semidefinite sense. However, there has been little work on quantifying the quantum limits and measurement designs, for multi-parameter quantum estimation in a Bayesian setting. In this work, we build upon Personick's result to construct a Bayesian adaptive (greedy) measurement scheme for multiparameter estimation. We illustrate our proposed measurement scheme with the application of localizing a cluster of point emitters in a highly sub-Rayleigh angular field-of-view, an important problem in fluorescence microscopy and astronomy. Our algorithm translates to a multi-spatial-mode transformation prior to a photon-detection array, with electro-optic feedback to adapt the mode sorter. We show that this receiver performs superior to quantum-noise-limited focal-plane direct imaging.

Laboratory demonstration of the prediction of wind-blown turbulence by adaptive optics at 8 kHz with use of LQG control.

The low-latency adaptive optical mirror system (LLAMAS) is designed to push the limits on achievable latencies and frame rates. It has 21 subapertures across its pupil. A reformulated version of the linear quadratic Gaussian (LQG) method predictive Fourier control is implemented in LLAMAS; for all modes, it takes just 30µs to compute. In the testbed, a turbulator mixes hot and ambient air to produce wind-blown turbulence. Wind prediction clearly improves correction when compared to an integral controller. Closed-loop telemetry shows that wind-predictive LQG removes the characteristic "butterfly" and reduces temporal error power by up to a factor of three for mid-spatial frequency modes. Strehl changes seen in focal plane images are consistent with telemetry and the system error budget.

End-to-End Implementation of a Convolutional Neural Network on a 3D-Integrated Image Sensor with Macropixel Array.

Three-dimensional-integrated focal-plane array image processor chips offer new opportunities to implement highly parallelised computer vision algorithms directly inside sensors. Neural networks in particular can perform highly complex machine vision tasks, and therefore their efficient implementation in such imagers are of significant interest. However, studies with existing pixel-processor array chips have focused on the implementation of a subset of neural network components-notably convolutional kernels-on pixel processor arrays. In this work, we implement a continuous end-to-end pipeline for a convolutional neural network from the digitisation of incoming photons to the output prediction vector on a macropixel processor array chip (where a single processor acts on group of pixels). Our implementation performs inference at a rate between 265 and 309 frames per second, directly inside of the sensor, by exploiting the different levels of parallelism available.

High-speed Focal Plane Wave Front Sensing with an Optical Chopper

Focal plane wave front sensing and control is a critical approach to reducing noncommon path errors between a conventional astronomical adaptive optics (AO) wave front sensor (WFS) detector and a science camera. However, in addition to mitigating noncommon path errors, recent focal plane wave front sensing techniques have been developed to operate at speeds fast enough to enable “multi-WFS” AO, where residual atmospheric errors are further corrected by a focal plane WFS. Although a number of such techniques have been recently developed for coronagraphic imaging, here we present one designed for noncoronagraphic imaging. Utilizing conventional AO system components, this concept additionally requires (1) a detector imaging the focal plane of the WFS light source and (2) a pupil plane optical chopper device that is the noncommon path to the first WFS and is synchronized to the focal plane imager readout. These minimal hardware requirements enable the temporal amplitude modulation to resolve the sine ambiguity of even wave-front modes for low, mid, and high wave front spatial frequencies. Similar capabilities have been demonstrated with classical phase diversity by defocusing the detector, but such techniques are incompatible with simultaneous science observations. This optical chopping technique, however, enables science imaging at up to 50% duty cycle. We present both simulations and laboratory validation of this concept on SEAL, the Santa Cruz Extreme AO Laboratory testbed.

Estimating non-common path aberrations with an adaptive coronagraph

Context. The focal-plane contrast of exoplanet imagers is affected by non-common path aberrations (NCPAs) that the adaptive optics system cannot correct for because they occur after the wavefront has been measured. NCPA estimation is commonly based on the long-exposure science image. Phase retrieval algorithms are often used, and they mostly assume that the residual phase error right after the adaptive optics system and averaged over the integration time is zero. This assumption is not always correct, for instance when controlling the adaptive optics to maximize the focal-plane contrast at the location of an exoplanet, that is to say in an adaptive coronagraph. For such cases, we present a method to calculate the NCPA using the phase information derived from the wavefront sensor (WFS) data and the science focal-plane image. Aims. We aim to accurately estimate the NCPA phase in the presence of (residual) atmospheric turbulence with a nonzero average wavefront. We then aim to take the NCPA into account in the adaptive coronagraph controller and achieve a higher contrast. Methods. The WFS measures the wavefront throughout the integration time of the science image. We combine information from the recorded WFS phases to remove the effects of the nonzero average phase from the Point Spread Function (PSF) and to remove the effects of the residual turbulence averaging over time. Then we estimate the NCPA by applying a phase-diversity-based algorithm to the resulting images. Our method is currently limited to imagers with pupil-plane coronagraphs. Results. We are able to recover the NCPA in an adaptive coronagraph setting with 0.01 radian RMS residuals and with a residual turbulence phase error of approximately 0.4 radian RMS. When accounted for in a contrast-control scheme, the NCPA correction leads to an order of magnitude improvement of contrast and a 50% increase in Strehl ratio, in numerical simulations.

Event-based imaging polarimeter simulation with a single DoFP image.

An event camera is a neuromorphic vision sensor with a high dynamic range (HDR), high temporal resolution (HTR), low latency, and low power consumption. A polarimeter is an instrument for measuring the state of polarization of light. Currently, most imaging polarimeters are limited in dynamic range and frame rate when used with frame-based cameras. We can establish an event-based imaging polarimeter using the principle of the event camera to obtain HDR and HTR polarized event streams for processing polarization information. However, because of the short history and high cost of event cameras, developing an event-based imaging polarimeter requires substantial resources. We propose an event-based imaging polarimeter simulation method with a single division of focal plane image based on the existing research on event simulation. This method can easily convert existing data into a polarized event stream. It is beneficial to lower the requirement of processing polarized event streams and to create large datasets for deep learning.

Voltage-dependent extended shortwave infrared (e-SWIR) photodetection-band tuning utilizing the Moss–Burstein effect

Extended shortwave infrared (e-SWIR) photodetectors and imaging focal plane arrays covering the wavelength beyond the conventional In0.53Ga0.47As cutoff wavelength of 1.65 micrometers (µm) can find numerous applications in infrared sensing and imaging. This paper reports voltage-tunable e-SWIR photodetectors based on the conventional gallium antimonide (GaSb) n–i–p and p–i–n homojunctions on GaSb substrates, which offer bias-dependent photodetection band tuning with a simple structure and high material crystal quality due to the perfect lattice matching on the substrates. Detection bands between the cutoff wavelengths of 1.7 µm and 1.9 µm can be tuned with a low reverse bias voltage of <0.1 volts (V). The mechanism of the voltage-dependent band-tuning was analyzed and attributed to the Moss–Burstein effect, which changes the electron and hole filling factors under different reverse bias voltages. This analysis agreed with the experimental data. The Moss–Burstein effect-induced voltage-dependent band-tuning mechanism can provide useful guidance for the designs of e-SWIR photodetectors.

A deep learning approach for focal-plane wavefront sensing using vortex phase diversity

Context. The performance of high-contrast imaging instruments is limited by wavefront errors, in particular by non-common path aberrations (NCPAs). Focal-plane wavefront sensing (FPWFS) is appropriate to handle NCPAs because it measures the aberration where it matters the most, that is to say at the science focal plane. Phase retrieval from focal-plane images results, nonetheless, in a sign ambiguity for even modes of the pupil-plane phase. Aims. The phase diversity methods currently used to solve the sign ambiguity tend to reduce the science duty cycle, that is, the fraction of observing time dedicated to science. In this work, we explore how we can combine the phase diversity provided by a vortex coronagraph with modern deep learning techniques to perform efficient FPWFS without losing observing time. Methods. We applied the state-of-the-art convolutional neural network EfficientNet-B4 to infer phase aberrations from simulated focal-plane images. The two cases of scalar and vector vortex coronagraphs (SVC and VVC) were considered using a single post-coronagraphic point spread function (PSF) or two PSFs obtained by splitting the circular polarization states, respectively. Results. The sign ambiguity has been properly lifted in both cases even at low signal-to-noise ratios (S/Ns). Using either the SVC or the VVC, we have reached a very similar performance compared to using phase diversity with a defocused PSF, except for high levels of aberrations where the SVC slightly underperforms compared to the other approaches. The models finally show great robustness when trained on data with a wide range of wavefront errors and noise levels. Conclusions. The proposed FPWFS technique provides a 100% science duty cycle for instruments using a vortex coronagraph and does not require any additional hardware in the case of the SVC.