Frequency-domain fluorescence lifetime imaging for endoscopic clinical cancer photodetection: Apparatus design and preliminary results

We describe a new fluorescence imaging device for clinical cancer photodetection in hollow organs in which the tumor/normal tissue contrast is derived from the fluorescence lifetime of endogenous or exogenous fluorochromes. This fluorescence lifetime contrast gives information about the physicochemical properties of the environment which are different between normal and certain diseased tissues. The excitation light from a CW laser is modulated in amplitude at a radio frequency by an electrooptical modulator and delivered by an optical fiber through an endoscope to the hollow organ. The image of the tissue collected by the endoscope is separated in two spectral windows, one being the backscattered excitation light and the other the fluorescence of the fluorochrome. Each image is then focused on the photocathode of image intensifiers (II) whose optical gain is modulated at the same frequency as the excitation intensity, resulting in homodyne phase-sensitive images. By acquiring stationary phase-sensitive frames at different phases between the excitation and the detection, it is possible to calculate in quasi-real time the apparent fluorescence lifetime of the corresponding tissue region for each pixel. A result obtained by investigating the endogenous fluorochromes present in the mucous membrane of an excised human bladder is presented to illustrate this method and most of the optical parameters which are of major importance for this photodetection modality have been evaluated.     

Self-Powered Red/UV Narrowband Photodetector by Unbalanced Charge Carrier Transport Strategy

Narrowband photodetector (NB-PD) with selective light detection is critical for artificial vision and imaging. Intrinsic (optical-filter-free) NB-PDs using conjugated organics or halide perovskite materials have been developed for eliminating the current complex filtering systems in NB-PDs. However, the poor performance and external driving circuit of organic NB-PDs as well as complex doping and uncontrollable recombination reactions in typical perovskite NB-PDs have limited their applicational diversification. A p-type self-doped perovskite for intrinsic NB detection is reported which exhibits unique unbalanced electron–hole transfer kinetics. In conjunction with the optical field distribution, an unbalanced charge transport within the self-doped perovskite triggers a wavelength-dependent photo-carrier collection, resulting in a novel spontaneous internal quantum efficiency narrowing mechanism. As a result, by reverting the device architectural polarity, an NB detection at a monochromic light of either red or UV is observed. Using such a revertible asymmetric device design, self-powered NB-PDs are successfully achieved. Briefly, the corresponding NB-PDs exhibit excellent narrow response with a response window of ≈100 nm, high detectivity ≈10¹¹ Jones, and fast response speed (f_−3dB ≈ 60 kHz) at zero bias. These results demonstrate a new strategy of manipulating internal charge transport to realize power-free and filter-free intrinsic NB-PDs.     

Unconventional Image-Sensing and Light-Emitting Devices for Extended Reality

Extended reality (XR) refers to a space where physical and digital elements coexist and comprises three elements, namely, environment, human, and computer, which interact with each other. Image sensors and displays are the core elements of XR systems because visual information is important for recognizing and judging objects. Recently, new features of image sensors and displays that are useful for developing next-generation XR systems have been reported. For example, a miniaturized version of image sensors with the superb object detection and recognition capability offers new opportunities for machine vision technology. Furthermore, transparent and deformable displays are the key components of XR systems because they not only provide highly realistic virtual image information but also serve as efficient user interfaces. Herein, the recent progresses in such unconventional image sensors and display technologies are reviewed. First, image sensors with features of wavelength-selective photodetection for color discrimination, neuromorphic image acquisition for facile pattern recognition, and curved image sensor designs inspired by biological eyes for miniaturization and unconventional imaging performances are discussed. Then, light-emitting device technologies focusing on devices with transparency and deformable form factors are described. Finally, the review is concluded with a brief summary and a future outlook.     

High Performance Ternary Organic Phototransistors with Photoresponse up to 2600 nm at Room Temperature

Infrared (IR) photodetection is important for light communications, military, agriculture, and related fields. Organic transistors are investigated as photodetectors. However, due to their large band gap, most organic transistors can only respond to ultraviolet and visible light. Here high performance IR phototransistors with ternary semiconductors of organic donor/acceptor complex and semiconducting single-walled carbon nanotubes (SWCNTs), without deep cooling requirements are developed. Due to both the ultralow intermolecular electronic transition energy of the complex and charge transport properties of SWCNTs, the phototransistor realizes broadband photodetection with photoresponse up to 2600 nm. Moreover, it exhibits outstanding performance under 2000 nm light with photoresponsivity of 2.75 × 10⁶ A W⁻¹, detectivity of 3.12 × 10¹⁴ Jones, external quantum efficiency over 10⁸%, and high I_photo/I_dark ratio of 6.8 × 10⁵. The device exhibits decent photoresponse to IR light even under ultra-weak light intensity of 100 nW cm⁻². The response of the phototransistor to blackbody irradiation is demonstrated, which is rarely reported for organic phototransistors. Interestingly, under visible light, the device can also be employed as synaptic devices, and important basic functions are realized. This strategy provides a new guide for developing high performance IR optoelectronics based on organic transistors.     

Sodium‐Mediated Epitaxial Growth of 2D Ultrathin Sb2Se3 Flakes for Broadband Photodetection

As an important member of group VA–VIA semiconductors, 2D Sb₂Se₃ has drawn widespread attention thanks to its outstanding optoelectronic properties as compared to the bulk material. However, due to the intrinsic chain‐like crystal structure, the controllable synthesis of ultrathin 2D planar Sb₂Se₃ nanostructures still remains a huge challenge. Herein, for the first time, the crystal structure limitation is overcome and the successful structural evolution of 2D ultrathin Sb₂Se₃ flakes (as thin as 1.3 nm), by introducing a sodium‐mediated chemical vapor deposition (CVD) growth method, is realized. The formation of 2D planar geometry is mainly attributed to the preferential growth of (010) plane with the lowest formation energy. The thickness‐dependent band structure of 2D Sb₂Se₃ flakes shows a wide absorption band from UV to NIR region (300–1000 nm), suggesting its potential application in broadband photodetection. Strikingly, the Sb₂Se₃ flakes–based photodetector demonstrates excellent performance such as broadband response varying from UV to NIR region, high responsivity of 4320 mA W^?1, fast response time (τ_rise ≈ 13.16 ms and τ_decay ≈ 9.61 ms), and strong anisotropic ratio of 2.5@ 532 nm, implying promising potential application in optoelectronics.     

Van der Waals Heterostructure Devices with Dynamically Controlled Conduction Polarity and Multifunctionality

Controlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe₂/MoS₂ field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio >10⁷ and diode with rectification ratio >10⁶. Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices.     

Enhanced photogalvanic effect in the two-dimensional MgCl2/ZnBr2 vertical heterojunction by inhomogenous tensile stress

The photogalvanic effect (PGE) occurring in noncentrosymmetric materials enables the generation of a dc photocurrent at zero bias with a high polarization sensitivity, which makes it very attractive in photodetection. However, the magnitude of the PGE photocurrent is usually small, leading to a low photoresponsivity, and therefore hampers its practical application in photodetection. Here, we propose an approach to largely enhancing the PGE photocurrent by applying an inhomogenous mechanical stretch, based on quantum transport simulations. We model a two-dimensional photodetector consisting of the wide-bandgap MgCl₂/ZnBr₂ vertical van der Waals heterojunction with the noncentrosymmetric C_3v symmetry. Polarization-sensitive PGE photocurrent is generated under the vertical illumination of linearly polarized light. By applying inhomogenous mechanical stretch on the lattice, the photocurrent can be largely increased by up to 3 orders of magnitude due to the significantly increased device asymmetry. Our results propose an effective way to enhance the PGE by inhomogenous mechanical strain, showing the potential of the MgCl₂/ZnBr₂ vertical heterojunction in the low-power UV photodetection.     

Non-artificial Layered Heterostructure as Inch-size Single Crystal for Shortwave Polarized-Light Array Detector

Layered heterostructures of different 2D building blocks have invigorated the booming of 2D materials toward high-performance optoelectronic devices. However, contrary to the typical artificial multi-component form, the engineering of non-artificial layered heterostructure into single-phase crystals and resultant properties are largely overlooked. Here, for the first time, an inch-sized single crystal of a non-artificial layered heterostructure is exploited, (PbBr₂)₂(AMTP)₂PbBr₄ ( 1 , AMTP is 4-ammoniomethyltetrahydropyran), serving as polarization-sensitive candidate. Notably, it adopts an interleaved architecture of 2D perovskite slabs with the distinct non-perovskite lattice, thus forming a self-assembled perovskite-intergrowth layered heterostructure. This motif leads to new electronic transitions distributed across two sublattices and affords an inherent in-plane anisotropy ratio of ≈1.6, beyond some known inorganic materials (e.g., GeSe: 1.44; GeAs: 1.49). Combining this in-plane anisotropy and wide bandgap (≈2.9 eV), lateral crystal array of 1 enables shortwave polarized-light detection with ultrahigh responsivity and detectivity under weak illumination compared to some inorganic polarized detectors. As the first demonstration of inch-sized single crystals of non-artificial layered heterostructure, this study affords a new platform to explore candidates toward high-performance optoelectronic devices.