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Inspired by the unique internal structures with inherent anisotropy in biological tissues and their dynamic scattering characteristic, a simple and easy-to-fabricate optical encryption technique using structural uncorrelated characteristics of biological tissues was proposed. It can generate unique keys with the random characteristics of the tissues and transforms multiple image plaintexts into speckle-like ciphertexts. The optical encryptions of grayscale images are demonstrated utilizing the uncorrelated characteristics of shallot and chicken breast respectively. Ciphertext-only-attack is resisted by phase retrieval due to the inherent dynamic randomness of biological tissues and the complexity of the plaintexts. Besides, once the unique PSF keys is recorded by the optical setup, the encryption can be accomplished, either online using the setup, or offline through performing convolutions on the plaintexts with pre-recorded uncorrelated PSFs to yield ciphertexts. Therefore, it is very secure, simple, and flexible to guarantee the promising potentials for information encryption of grayscale images and/or videos. [ABSTRACT FROM AUTHOR]

Organic photodetectors (OPDs) have aroused intensive attention for signal detection in industrial and scientific applications due to their advantages including low cost, mechanical flexibility, and large-area fabrication. As one of the most common organic light-emitting materials, 8-hydroxyquinolinato aluminum (Alq3) has an absorption wavelength edge of 460 nm. Here, through the introduction of Ag nanoparticles (Ag NPs), the spectral response range of the Alq3-based OPD was successfully extended to the near-infrared range. It was found that introducing Ag NPs can induce rich plasmonic resonances, generating plenty of hot electrons, which could be injected into Alq3 and then be collected. Moreover, as a by-product of introducing Ag NPs, the dark current was suppressed by around two orders of magnitude by forming a Schottky junction on the cathode side. These two effects in combination produced photoelectric signals with significant contrasts at wavelengths beyond the Alq3 absorption band. It was found that the OPD with Ag NPs can stably generate electric signals under illumination by pulsed 850 nm LED, while the output of the reference device included no signal. Our work contributes to the development of low-cost, broadband OPDs for applications in flexible electronics, bio-imaging sensors, etc. [ABSTRACT FROM AUTHOR]

• The phase detection of physical lenses based on single-pixel common-path off-axis wavefront imaging is demonstrated. • The Fourier fringe analysis can avoid multi-step phase-shifting interference and reconstruct the target object more quickly. • Compared with the previously proposed SPI-based lens phase detection, the measurement speed of our method is increased by at least three folds. • A simple and cost-effective way for lens detection is proposed, which further propels the SPI technology toward practical application. The lens phase provides useful information about the optical path length, from which other key parameters of a lens can be calculated. Yet, the traditional 2D detector-based phase measurement methods are always limited by the specific spectral response range. Here, phase detection of the physical lens is accomplished via a single-pixel detector. The digital micromirror device (DMD)-based common-path off-axis single-pixel (DCOS) wavefront imaging system is established. Benefiting from the advantages of single-pixel detectors, this method is anticipated to be applied in specialist bands. In this method, the wavefront of a target object is reconstructed quickly by Fourier fringe analysis (FFA) from a hologram, which is obtained by the normal SPI technique. Thus, the measuring speed is at least three times faster than that achieved by multi-step phase-shifting. The average relative errors between the measured value and the calibrated value of focal lengths for 1000-mm and 500-mm optical lenses are 0.254% and 0.982%, respectively. Our work provides a fast, simple, and cost-effective way for phase detection of optical lenses and further enlarges the application scope of single-pixel wavefront imaging. [ABSTRACT FROM AUTHOR]

The memory‐effect‐based speckle auto‐correlation method has gained significant attention for its ability to perform single‐shot noninvasive imaging through scattering media. Despite its advantages, however, not only its imaging field‐of‐view (FOV) is limited by the memory effect (ME), but also the relative positions and orientations of the reconstructed objects beyond ME are lost. Therefore, an approach that enables the reconstruction of multiple objects or a whole object behind scattering media beyond the ME region while accurately preserving the relative positions and orientations of its sub‐objects is proposed. The method achieves this by directly leveraging the correlation of the collected speckle patterns. Experimental results validate the effectiveness of the proposed method. It not only can reconstruct the image of multiple objects or a whole object beyond the ME region simulated by a digital micromirror device but also can reconstruct the image of a fluorescent object beyond the ME region noninvasively, which is well consistent with the theoretical predictions. The method provides a promising pathway for noninvasively visualizing fluorescent objects behind scattering media without being constrained by ME, thereby expanding the imaging FOV. [ABSTRACT FROM AUTHOR]

Single-pixel detecting is suitable for wavefront reconstruction in some special wavelengths where array detectors are immature or even unavailable and/or under low light conditions. However, most of the demonstrations are generally realized by multi-step phase-shifting interferometry with a liquid crystal spatial light modulator (LC-SLM) that has a slow modulation speed, which limits the reconstruction speed, thus restricting practical applications of the technique. Here, we propose to use a digital-micromirror-device-(DMD)-based single-pixel off-axis common-path interferometry (SOCI) for faster wavefront reconstruction. The method utilizes passive detection based on the DMD to realize SOCI for accelerating wavefront reconstruction. As compared to the LC-SLM-based phase-shifting techniques, since the modulation speed of the DMD is hundreds of times faster than that of the LC-SLM, the DMD-based SOCI accelerates several folds of reconstruction speed further, which, thus, makes the final wavefront reconstruction three orders of magnitude faster. The effectiveness and advantages of our method are experimentally demonstrated by quantitatively reconstructing the amplitude and phase images of a biological sample. [ABSTRACT FROM AUTHOR]

Single-pixel imaging (SPI) uses a single-pixel detector instead of a detector array with a lot of pixels in traditional imaging techniques to realize two-dimensional or even multi-dimensional imaging. For SPI using compressed sensing, the target to be imaged is illuminated by a series of patterns with spatial resolution, and then the reflected or transmitted intensity is compressively sampled by the single-pixel detector to reconstruct the target image while breaking the limitation of the Nyquist sampling theorem. Recently, in the area of signal processing using compressed sensing, many measurement matrices as well as reconstruction algorithms have been proposed. It is necessary to explore the application of these methods in SPI. Therefore, this paper reviews the concept of compressive sensing SPI and summarizes the main measurement matrices and reconstruction algorithms in compressive sensing. Further, the performance of their applications in SPI through simulations and experiments is explored in detail, and then their advantages and disadvantages are summarized. Finally, the prospect of compressive sensing with SPI is discussed. [ABSTRACT FROM AUTHOR]

Single-pixel imaging (SPI) recently has made significant progress, however, it is still an open issue to adaptively tradeoff its imaging fidelity and speed. Here, we present an adaptive sampling method for SPI to reduce the sampling rate for improving the imaging speed adaptively, sacrificing as little as possible of its imaging fidelity. The proposed method only requires setting an error band to realize adaptive under-Nyquist sampling, based upon the fact that the coefficient matrix consisted of the frequency components obtained by the orthogonal transforms is sparse. First, an appropriately elaborated sampling path, which can be divided into multiple segments, can be set according to this coefficient matrix measured by a single-pixel detector. Then the variance of each segment is calculated and expressed in logarithmic form for the real-time polynomial curve fitting. When the variation of the curve's slope is small enough and reaches an error band, the sampling is adaptively stopped. Finally, the object image can be reconstructed via the adaptive sampling method following the elaborated sampling path for improving the imaging speed adaptively. The effectiveness of the adaptive sampling SPIs based on the orthogonal transforms, such as Hadamard transform, discrete cosine transform, Fourier transform, and Krawtchouk moments transform, is theoretically and experimentally demonstrated. It should be noted the proposed method can be also applied in SPIs using other orthogonal transforms, in which only an error band is set by the users. • An adaptive under-Nyquist sampling method for single-pixel imaging. • The sparser the target, the higher the fidelity of the image. • The proposed method is not limited to a specific orthogonal transform. • The proposed method can be suitable for SPI with high-frequency noise. [ABSTRACT FROM AUTHOR]

• Simultaneously amplitude and phase (i.e. spatial wavefront) imaging of a biological sample via DMD-based single-pixel phase-shifting interferometric techniques is demonstrated. • The experimental performances of those techniques for wavefront imaging of a biological sample are quantitatively and comprehensively compared, when different orthogonal bases, different reference strategies, and different modulation methods (i.e. binary grating, Lee, and super-pixel methods) are employed there. • Experimental results suggest that the grating-phase-shifting-based SWI using Hadamard basis (G-HSWI) performs best, in which one can use the checkerboard reference for the maximum field of view or select the peripheral reference for a better imaging resolution. • 2fps dynamic wavefront imaging of the sample at a 10 % downsampling rate with a resolution of 128 × 128 pixels can be realized by G-HSWI. Single-pixel wavefront imaging (SWI) is an innovative technique for simultaneously measuring the amplitude and phase of an unknown field, which is particularly important for wavefront imaging in invisible wavelengths and under low light conditions. However, most of the existing SWI techniques are still in the technical development stage. Here, we provide a comprehensive analysis and comparison of the SWI methods using DMD-based single-pixel multi-step phase-shifting interferometric techniques. Through experimental demonstrations, we first compare the performances of the techniques using the Hadamard basis, Fourier basis, and discrete cosine transform (DCT) basis. Further, the performances of the Hadamard basis-based SWI (HSWI) technique using phase-shifting interferometry with the different reference strategies are investigated while the binary grating, Lee, and super-pixel methods are applied to control the DMD for realizing the necessary complex amplitude modulations there. Experimental results suggest the grating-phase-shifting-based HSWI performs best, in which one can use the checkerboard reference for the maximum field of view or select the peripheral reference for a better imaging resolution; and it can do 2 fps wavefront imaging of the sample at 10 % downsampling rate with a resolution of 128 × 128 pixels. Our work provides researchers with a guideline to choose a suitable SWI technique for their applications. [ABSTRACT FROM AUTHOR]

• An OAM inter-mode interferometry is proposed to achieve OAM-basis wavefront single-pixel imaging (OAM-WSPI), in which the 0 th -order OAM mode with the necessary phase-shiftings is used as the reference field, and the other-order OAM modes are used as the modulation patterns for sampling the target wavefront. • The proposed method does not require sacrificing the pixel of the SLM to introduce the reference light, which enhancement the resolution of existing common-path OAM-WSPI. • Experimental demonstrations suggest the feasibility and effectiveness of the proposed method and reconstruct the different target wavefronts with high resolution, employing the LG basis and Bessel basis, respectively. Wavefront single-pixel imaging (WSPI) allows the development of imaging systems where pixel-array detectors are immature or even unavailable. However, the state-of-the-art common-path interference-based WSPI methods generally sacrifice the pixels of the spatial light modulator (SLM) to introduce the reference light reducing the imaging resolution. Here, we propose to use inter-mode common-path interferometry to improve the spatial resolution of orbital angular momentum basis wavefront single-pixel imaging (OAM-WSPI), since the inter-mode interferometry does not sacrifice the pixels of the SLM to introduce reference light. Experimental demonstrations suggest the feasibility and effectiveness of the proposed method. Such a method has potential applications in the fields of high-resolution holography applications, in which OAM beams need to be used. [ABSTRACT FROM AUTHOR]

Metal–inorganic semiconductor–metal photodetectors (MSM‐PDs) have received great attention in many areas, such as optical fiber communication, sensing, missile guidance, etc., due to their inherent merits of high speed, high sensitivity, and easy integration. This review focuses on MSM‐PDs with the semiconductor layer made of inorganic materials including traditional semiconductors (such as GaAs and Si), the third‐generation wide bandgap semiconductors (such as GaN, ZnO, and SiC), as well as several emerging semiconductors (such as perovskites and 2D materials). First, the basic structures of MSM‐PDs, including the planar and vertical configurations, are presented. Then, their working principles of MSM‐PDs are discussed. Subsequently, the research progresses on MSM‐PDs consisting of different photosensitive semiconductor materials are described in detail. Additionally, the efforts to optimize MSM‐PDs from the aspects of dark current, response speed, responsivity, spectral adjustment, etc., are also introduced. Finally, the review is concluded with the perspectives of MSM‐PDs from the authors' vision. [ABSTRACT FROM AUTHOR]

• To guarantee almost the same wavefront imaging quality as Fourier single-pixel imaging (FSPI) does while the imaging speed can be accelerated several folds, a discrete cosine transform single-pixel imaging (DCT-SPI) technique for measuring the spatial wavefront of an unknown field was proposed. • The DCT-SPI technique for measuring different wavefronts as well as its application for quantitative phase imaging of a dragonfly wing was experimentally demonstrated, and its performance was compared with that of FSPI in detail. • Experimental results suggest that the imaging speed of DCT-SPI can be several times faster than that of FSPI, and it can obtain the phase and amplitude of a wavefront with a higher imaging resolution under the RAM capacity limit of the DMD, as compared to the FSPI technique. • The results also suggest that DCT-SPI can be used for measuring the phase and amplitude of a dragonfly wing, which suggests it can be applied in the field of quantitative phase imaging with the merits mentioned above. In this work, a checkerboard pattern is used to divide an unknown field into the signal and reference parts, and based on the interference between them, we propose and demonstrate a discrete cosine transform single-pixel imaging (DCT-SPI) technique for measuring the spatial wavefront of an unknown field. Here, the discrete cosine transform (DCT) basis was used for generating the modulation patterns to acquire the spectrum of the wavefront by the phase-shifting technique, and the wavefront to be measured can be retrieved by applying a second-order correlation calculation. We first demonstrate the proposed DCT-SPI can be used for measuring the spatial wavefronts simulated by an amplitude SLM(A-SLM). As compared to the phase-shifting Fourier single-pixel imaging (FSPI) technique, the imaging speed of DCT-SPI is four times faster. In addition, under the RAM capacity limit of a digital micromirror device (DMD), DCT-SPI can realize a higher resolution. Further, we experimentally demonstrate that DCT-SPI can also be used for measuring the phase and amplitude of a dragonfly wing, which suggests it can be applied in the field of quantitative phase imaging with a higher speed and imaging resolution as compared with FSPI. [ABSTRACT FROM AUTHOR]

Hot-electron photodetectors (HEPDs) are triggering a strong surge of interest in applications of image sensors and optics communication, since they can realize photoelectric responses when the incident photon energy is lower than the bandwidth of the semiconductor. In traditional HEPD systems, the metal layers are dressed with regular gratings, which can only excite plasmonic resonance over a narrow bandwidth, limiting the hot-electron photoelectric effect. To break this limitation, hybrid plasmonic nanostructures should be applied in HEPDs. Here, we propose a TiO2 based HEPD device incorporated with a hybrid plasmonic nanostructure, which consists of Au nanoparticles (Au NPs) and a conformal transparent Au film. With the assistance of the plasmonic resonances excited in this hybrid nanostructure, the spectrum of the photocurrent response is significantly broadened from the UV band to the visible and near-infrared (NIR) ranges. It is demonstrated that at the wavelengths of 660 nm and 850 nm, the photocurrent in the hybrid HEPD device is enhanced by 610% and 960%, respectively, compared with the counterparts without the addition of Au NPs. This work promotes the development of high performances HEPDs, offering an alternative strategy for realizing photodetection and image sensing in the NIR range. [ABSTRACT FROM AUTHOR]

Abstract: A new 3D shape measurement method based on non-integral twin-frequency grating projection is proposed. In this paper, the projected composite grating is composed of two sinusoidal gratings, and the quotient of whose frequencies is not an integer. By using appropriate phase-shifting algorithms, two wrapped phases can be obtained from sixteen frames of the distorted grating patterns. In aid of appropriate phase unwrapping method, the unwrapped phase of high frequency fringe is obtained only from the relation of the two wrapped phases by a pixel-to-pixel phase unwrapping technique. Therefore the unwrapped phase of high frequency fringe pattern is independent of the low frequency fringe pattern, different from traditional integral twin-frequency grating projection methods in which the phase unwrapping error of low frequency fringe pattern may be propagated onto the unwrapped phase of high frequency fringe pattern. The new theory proves that the proposed method is applicable to measure discontinuous object, and has considerable measurement accuracy. [Copyright &y& Elsevier]

• A passive-detection-based WSPI method using a flexible SLM-based common-path interferometer is proposed. The common-path phase-shift or off-axis technique can be flexibly realized by a "sandwich" structure consisting of a half-wave plate, a SLM, and a polarizer. • The proposed method does not require sacrificing the pixel resolution of the SLM to introduce the reference light, therefore, there does not exist a tradeoff between the wavefront imaging resolution and field of view. • The proposed method for measuring wavefront as well as its application for quantitative phase imaging of a dragonfly wing was experimentally demonstrated, which indicates it has potential applications in the field of biological quantitative phase imaging. • The proposed method paves the way for passively measuring spatial wavefronts of unknown fields with higher performances, only using a single detector and the SLM-based "sandwich" structure, which is compact and simple. Wavefront single-pixel imaging (WSPI) is an important tool for simultaneously measuring the amplitude and phase of an unknown field, especially suitable for wavefront detecting in some special wavelengths where array detectors are immature or even unavailable. Here, a passive-detection-based WSPI method using a flexible SLM-based common-path interferometer is proposed, in which by detecting the zero-order diffraction of the SLM directly, the common-path phase-shift or off-axis technique can be flexibly realized by a "sandwich" structure consisting of a half-wave plate, a SLM, and a polarizer. The reference light can be conveniently chosen as the direct reflected light from the SLM, the unmodulated light from the pixels of the SLM, or the light from both of them. We experimentally verify the feasibility and advantages of the proposed method. We further demonstrate the method for wavefront measurement of a dragonfly wing, which suggests it can be used for quantitative phase imaging. It provides a high-resolution spatial wavefront passive detection technique, with a compact, stable, and flexible setup, which has lots of applications involving three-dimensional imaging, aberration correction, digital holographic measurement, and digital microscopy. [ABSTRACT FROM AUTHOR]

Abstract: A methodology for measuring the telecentricity of illumination system in lithography tool based on the pinhole imaging is proposed. Illumination light goes through the pinhole located on the reticle plane and forms the pupil-fill image in CCD image sensor. Based on the Fresnel diffraction theory, the image intensity of source in CCD photosurface and the optimization of pinhole size are discussed. By simulating, effects of the stability of telecentricity measuring from non-uniformity of light field and pinhole defocus in different illumination patterns are analyzed respectively. Simulation results prove the effectiveness of this new method. [Copyright &y& Elsevier]

Abstract: In this paper, Arnold transformation and double random-phase encoding technique widely used in digital image information hiding are introduced to digital audio information hiding. The digital audio is transformed into a 2-D image called sound map and then the sound map will be divided into many windows and each window will be encrypted based on the Arnold transformation. Finally sound map will be re-encrypted based on double random-phase encoding technique. This method offers many advantages for digital audio information hiding: improve security and high attack immunity. [Copyright &y& Elsevier]