Monolayer transition metal dichalcogenides are considered to be promising candidates for flexible and transparent optoelectronics applications due to their direct bandgap and strong light-matter interactions. Although several monolayer-based photodetectors have been demonstrated, single-layered optical memory devices suitable for high-quality image sensing have received little attention. Here we report a concept for monolayer MoS2 optoelectronic memory devices using artificially-structured charge trap layers through the functionalization of the monolayer/dielectric interfaces, leading to localized electronic states that serve as a basis for electrically-induced charge trapping and optically-mediated charge release. Our devices exhibit excellent photo-responsive memory characteristics with a large linear dynamic range of ∼4,700 (73.4 dB) coupled with a low OFF-state current (<4 pA), and a long storage lifetime of over 104 s. In addition, the multi-level detection of up to 8 optical states is successfully demonstrated. These results represent a significant step toward the development of future monolayer optoelectronic memory devices. Memory devices are key building blocks of image sensing circuitry. Here, the authors demonstrate a MoS2monolayer optoelectronic memory device based on charge trapping and subsequent optically-induced charge release, capable of 12-bit operation.

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Monolayer optical memory cells based on artificial trap-mediated charge storage and release.

Typical high dynamic range HDR imaging approaches based on multiple images have difficulties in handling moving objects and camera shakes, suffering from the ghosting effect and the loss of sharpness in the output HDR image. While there exist a variety of solutions for resolving such limitations, most of the existing algorithms are susceptible to complex motions, saturation, and occlusions. In this paper, we propose an HDR imaging approach using the coded electronic shutter which can capture a scene with row-wise varying exposures in a single image. Our approach enables a direct extension of the dynamic range of the captured image without using multiple images, by photometrically calibrating rows with different exposures. Due to the concurrent capture of multiple exposures, misalignments of moving objects are naturally avoided with significant reduction in the ghosting effect. To handle the issues with under-/over-exposure, noise, and blurs, we present a coherent HDR imaging process where the problems are resolved one by one at each step. Experimental results with real photographs, captured using a coded electronic shutter, demonstrate that our method produces a high quality HDR images without the ghosting and blur artifacts.

Single-shot High Dynamic Range Imaging Using Coded Electronic Shutter

The rapidly emerging requirement for device miniaturization and structural flexibility make 2D semiconductors and their van der Waals (vdWs) heterostructures extremely attractive for nonvolatile optoelectronic memory (NOM) applications. Although several concepts for 2D NOM have been demonstrated, multi-heterojunction devices capable of further improving storage performance have received little attention. This work reports a concept for MoS2/black phosphorus (BP)/MoS2 multi-heterojunction NOM with artificial trap sites through the BP oxidation, in which the trapped holes at BP/POx interface intrigue a persistent photoconductivity that hardly recovers within the experimental time scales (exceeding 104 s). As a result of the interfacial trap-controlled charge injection, the device exhibits excellent photoresponsive memory characteristics, including a record high detectivity of ≈1.2 × 1016 Jones, a large light-to-dark switching ratio of ≈1.5 × 10, an ultralow off-state current of ≈1.2 pA, and an outstanding multi-bit storage capacity (11 storage states, 546 nC state–1). In addition, the middle BP layer in the multi-heterojunction enables broadband spectrum distinction (375–1064 nm), together with a high polarization ratio of 8.4. The obtained results represent the significant step toward the high-density integration of optoelectronic memories with 2D vdWs heterostructures.

Polarization‐Resolved Broadband MoS2/Black Phosphorus/MoS2 Optoelectronic Memory with Ultralong Retention Time and Ultrahigh Switching Ratio

Method of optical image coding by time integration is proposed. Coding in proposed method is accomplished by
shifting object image over photosensor area of digital camera during registration. It results in optically calculated
convolution of original image with shifts trajectory. As opposed to optical coding methods based on the use of diffractive
optical elements the described coding method is feasible for implementation in totally incoherent light. The method was
preliminary tested by using LC monitor for image displaying and shifting. Shifting of object image is realized by
displaying video consisting of frames with image to be encoded at different locations on screen of LC monitor while
registering it by camera. Optical encoding and numerical decoding of test images were performed successfully. Also
more practical experimental implementation of the method with use of LCOS SLM Holoeye PLUTO VIS was realized.
Objects images to be encoded were formed in monochromatic spatially incoherent light. Shifting of object image over
camera photosensor area was accomplished by displaying video consisting of frames with blazed gratings on LCOS
SLM. Each blazed grating deflects reflecting from SLM light at different angle. Results of image optical coding and
encoded images numerical restoration are presented. Obtained experimental results are compared with results of
numerical modeling. Optical image coding with time integration could be used for accessible quality estimation of
optical image coding using diffractive optical elements or as independent optical coding method which can be
implemented in incoherent light.

Method of optical image coding by time integration

At present time methods of optical encryption are actively developed. The majority of existing methods of optical encryption use not only light intensity distribution, easily registered with photosensors, but also its phase distribution which require application of complex holographic schemes in conjunction with spatially coherent light. This leads to complex optical schemes and low decryption quality. To eliminate these disadvantages it is possible to implement optical encryption using spatially incoherent illumination which requires registration of light intensity distribution only. However this applies new restrictions on encryption keys: Fourier spectrum amplitude distribution of encryption key should overlap Fourier spectrum amplitude distribution of image to be encrypted otherwise loss of information is unavoidable. Therefore it seems that best key should have white spectrum. On the other hand due to fact that only light intensity distribution is registered, spectra of image to be encrypted and encryption key always have peaks at zero frequency and their heights depend on corresponding total energy. Since encrypted image contains noise, ratio of its average spectrum energy to noise average energy determines signal to noise ratio of decrypted image. Therefore ratio of amplitude at zero frequency to average spectrum amplitude (RZA) of encryption key defines decrypted images quality. For generation of encryption keys with low RZA method of direct search with random trajectory (DSRT) was used. To estimate impact of key RZA on decrypted images error numerical experiments were conducted. For experiments keys with different RZA values but with same energy value were generated and used. Numerically simulated optical encryption and decryption of set of test images was conducted. Results of experiment demonstrate that application of keys with low RZA generated by DSRT method leads to up to 20% lower error in comparison to keys generated by means of uniform random distribution.

Generation of keys for image optical encryption in spatially incoherent light aimed at reduction of image decryption error

Diffraction image correlator based on commercial digital SLR photo camera was reported earlier. The correlator was
proposed for recognition of external scenes illuminated by quasimonochromatic spatially incoherent light. The
correlator hardware consists of digital camera with plugged in optical correlation filter unit and control computer. The
kinoform used as correlation filter is placed in a free space of the SLR camera body between the interchangeable camera
lens and the swing mirror. On the other hand, this correlator can be considered as a hybrid optical-digital imaging
system with wavefront coding. It allows not only to recognize objects in input scene but to restore, if needed, the whole
image of input scene from correlation signals distribution registered by SLR camera sensor. Linear methods for image
reconstruction in the correlator are discussed. The experimental setup of the correlator and experimental results on
images recognition and input scenes restoration are presented.

Input scene restoration in pattern recognition correlator based on digital photo camera

Performance capabilities of commercial digital cameras are demonstrated by the example of a Canon EOS 400D camera in measuring and detecting spatial distributions of laser radiation intensity. It is shown that software extraction of linear data expands the linear dynamic range of the camera by a factor greater than 10, up to 58 dB. Basic measurement characteristics of the camera are obtained in the regime of linear data extraction: the radiometric function, deviation from linearity, dynamic range, temporal and spatial noises (both dark and those depending on the signal value). The parameters obtained correspond to those of technical measuring cameras.

http://iopscience.iop.org/article/10.1070/QE2010v040n04ABEH014202/pdf

Extension of the possibilities of a commercial digital camera in detecting spatial intensity distribution of laser radiation

Diffraction image correlator based on commercial digital SLR photo camera is described. The correlator is proposed for recognition of external 2-D and 3-D scenes illuminated by quasimonochromatic spatially incoherent light. Principal optical scheme of the correlator is analogous to that of incoherent holographic correlator by Lohmann. The correlator hardware consists of digital camera with attached optical correlation filter unit and control computer. No modifications have been introduced in units of commercial digital SLR photo camera. Digital camera was connected via camera interface to computer for controlled camera shooting, transfer of detected correlation signals and post-processing. Two ways were used for correlation filter unit mounting. In the first case, correlation filter was attached to the front of the camera lens. In the second one, it was placed in a free space of the SLR camera body between the interchangeable camera lens and the swing mirror. Computer generated Fourier holograms and kinoforms were used as correlation filters 
in experiments. The experimental setup of the correlator and experimental results on images recognition are presented. The recognition of test objects of direct and reversed contrast with the same correlation filter was performed.

Pattern recognition correlator based on digital photo camera

In optical-digital correlators for pattern recognition, linear registration of correlation signals is significant for
both of recognition reliability and possible input image restoration. This usually achieves with scientific graduated
technical cameras, but most of commercial digital cameras now have an option of RAW data output.
With appropriate software and parameters of processing, it is possible to get linearized image data from photo
camera's RAW file. Application of such photo cameras makes optical-digital systems cheaper, more flexible and
brings along their wider propagation.
For linear registration of correlation signals, open-source Dave Coffins's RAW converter DCRAW was used in
this work. Data from photo camera were linearized by DCRAW converter in "totally RAW documental mode"
with 16-bit output.
Experimental results of comparison between linearized and non-linearized correlation signals and digitally restored
input scene images are presented. It is shown, that applied linearization allows to increase linear dynamic
range for used Canon EOS 400D camera more that 3 times.

Using commercial photo camera's RAW-based images in optical-digital correlator for pattern recognition

In this work the application of the spatially varying pixels exposure technique for obtaining linear high dynamic range (HDR) images of correlation signals by Bayer-covered photo sensors is presented. Bayer colour filters array is considered as an array of attenuating filters in the quasimonochromatic light. The procedure of HDR images reconstruction using data from neighbour pixels and preliminary obtained correction coefficients is described. Experimental results of HDR registration of correlation signals are provided. It is shown that reconstructed HDR correlation signals are linear as normal signals are. The increase of dynamic range of signal’s registration from 58 dB up to 73 dB is obtained. Results on recognition of test objects with normal and HDR registration are discussed.

Sergey N. Starikov

Papers

Input scene restoration in pattern recognition correlator based on digital photo camera

Extension of the possibilities of a commercial digital camera in detecting spatial intensity distribution of laser radiation

Pattern recognition correlator based on digital photo camera

Using commercial photo camera's RAW-based images in optical-digital correlator for pattern recognition

Optical-digital correlator with increased dynamic range using spatially varying pixels exposure technique