2021 Vol. 10, No. 10

News & Views
Research Highlights
Photonic-dispersion neural networks for inverse scattering problems
Tongyu Li, Ang Chen, Lingjie Fan, Minjia Zheng, Jiajun Wang, et al.
Published. 2021, 10(10) : 1819-1828 doi: 10.1038/s41377-021-00600-y
Inferring the properties of a scattering objective by analyzing the optical far-field responses within the framework of inverse problems is of great practical significance. However, it still faces major challenges when the parameter range is growing and involves inevitable experimental noises. Here, we propose a solving strategy containing robust neural-networks-based algorithms and informative photonic dispersions to overcome such challenges for a sort of inverse scattering problem—reconstructing grating profiles. Using two typical neural networks, forward-mapping type and inverse-mapping type, we reconstruct grating profiles whose geometric features span hundreds of nanometers with nanometric sensitivity and several seconds of time consumption. A forward-mapping neural network with a parameters-to-point architecture especially stands out in generating analytical photonic dispersions accurately, featured by sharp Fano-shaped spectra. Meanwhile, to implement the strategy experimentally, a Fourier-optics-based angle-resolved imaging spectroscopy with an all-fixed light path is developed to measure the dispersions by a single shot, acquiring adequate information. Our forward-mapping algorithm can enable real-time comparisons between robust predictions and experimental data with actual noises, showing an excellent linear correlation (R2 > 0.982) with the measurements of atomic force microscopy. Our work provides a new strategy for reconstructing grating profiles in inverse scattering problems.
MXene-GaN van der Waals metal-semiconductor junctions for high performance multiple quantum well photodetectors
Lingzhi Luo, Yixuan Huang, Keming Cheng, Abdullah Alhassan, Mahdi Alqahtani, et al.
Published. 2021, 10(10) : 1829-1839 doi: 10.1038/s41377-021-00619-1
A MXene-GaN-MXene based multiple quantum well photodetector was prepared on patterned sapphire substrate by facile drop casting. The use of MXene electrodes improves the responsivity and reduces dark current, compared with traditional Metal-Semiconductor-Metal (MSM) photodetectors using Cr/Au electrodes. Dark current of the device using MXene-GaN van der Waals junctions is reduced by three orders of magnitude and its noise spectral intensity shows distinct improvement compared with the traditional Cr/Au–GaN–Cr/Au MSM photodetector. The improved device performance is attributed to low-defect MXene-GaN van der Waals interfaces. Thanks to the high quality MXene-GaN interfaces, it is possible to verify that the patterned substrate can locally improve both light extraction and photocurrent collection. The measured responsivity and specific detectivity reach as high as 64.6 A/W and 1.93 × 1012 Jones, respectively, making it a potential candidate for underwater optical detection and communication. The simple fabrication of MXene-GaN-MXene photodetectors spearheaded the way to high performance photodetection by combining the advantages of emerging 2D MXene materials with the conventional III-V materials.
Single-step-fabricated disordered metasurfaces for enhanced light extraction from LEDs
Peng Mao, Changxu Liu, Xiyan Li, Mengxia Liu, Qiang Chen, et al.
Published. 2021, 10(10) : 1840-1848 doi: 10.1038/s41377-021-00621-7
While total internal reflection (TIR) lays the foundation for many important applications, foremost fibre optics that revolutionised information technologies, it is undesirable in some other applications such as light-emitting diodes (LEDs), which are a backbone for energy-efficient light sources. In the case of LEDs, TIR prevents photons from escaping the constituent high-index materials. Advances in material science have led to good efficiencies in generating photons from electron–hole pairs, making light extraction the bottleneck of the overall efficiency of LEDs. In recent years, the extraction efficiency has been improved, using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum. However, the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and time-consuming fabrication, which causes a gap between lab demonstration and industrial-level applications. Inspired by lightning bugs, we propose and realise a disordered metasurface for light extraction throughout the visible spectrum, achieved with single-step fabrication. By applying such a cost-effective light extraction layer, we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs, demonstrating a substantial potential for global energy-saving and sustainability.
Ultrasensitive detection of endocrine disruptors via superfine plasmonic spectral combs
Lanhua Liu, Xuejun Zhang, Qian Zhu, Kaiwei Li, Yun Lu, et al.
Published. 2021, (10) : 1849-1861 doi: 10.1038/s41377-021-00618-2
The apparent increase in hormone-induced cancers and disorders of the reproductive tract has led to a growing demand for new technologies capable of detecting endocrine disruptors. However, a long-lasting challenge unaddressed is how to achieve ultrahigh sensitive, continuous, and in situ measurement with a portable device for in-field and remote environmental monitoring. Here we demonstrate a simple-to-implement plasmonic optical fiber biosensing platform to achieve an improved light–matter interaction and advanced surface chemistry for ultrasensitive detection of endocrine disruptors. Our platform is based on a gold-coated highly tilted fiber Bragg grating that excites high-density narrow cladding mode spectral combs that overlap with the broad absorption of the surface plasmon for high accuracy interrogation, hence enabling the ultrasensitive monitoring of refractive index changes at the fiber surface. Through the use of estrogen receptors as the model, we design an estradiol–streptavidin conjugate with the assistance of molecular dynamics, converting the specific recognition of environmental estrogens (EEs) by estrogen receptor into surface-based affinity bioassay for protein. The ultrasensitive platform with conjugate-induced amplification biosensing approach enables the subsequent detection for EEs down to 1.5 × 10−3 ng ml−1 estradiol equivalent concentration level, which is one order lower than the defined maximal E2 level in drinking water set by the Japanese government. The capability to detect EEs down to nanogram per liter level is the lowest limit of detection for any estrogen receptor-based detection reported thus far. Its compact size, flexible shape, and remote operation capability open the way for detecting other endocrine disruptors with ultrahigh sensitivity and in various hard-to-reach spaces, thereby having the potential to revolutionize environment and health monitoring.
Microglia modulation with 1070-nm light attenuates Aβ burden and cognitive impairment in Alzheimer's disease mouse model
Lechan Tao, Qi Liu, Fuli Zhang, Yuting Fu, Xi Zhu, et al.
Published. 2021, 10(10) : 1863-1876 doi: 10.1038/s41377-021-00617-3
Photobiomodulation, by utilizing low-power light in the visible and near-infrared spectra to trigger biological responses in cells and tissues, has been considered as a possible therapeutic strategy for Alzheimer's disease (AD), while its specific mechanisms have remained elusive. Here, we demonstrate that cognitive and memory impairment in an AD mouse model can be ameliorated by 1070-nm light via reducing cerebral β-amyloid (Aβ) burden, the hallmark of AD. The glial cells, including microglia and astrocytes, play important roles in Aβ clearance. Our results show that 1070-nm light pulsed at 10 Hz triggers microglia rather than astrocyte responses in AD mice. The 1070-nm light-induced microglia responses with alteration in morphology and increased colocalization with Aβ are sufficient to reduce Aβ load in AD mice. Moreover, 1070-nm light pulsed at 10 Hz can reduce perivascular microglia and promote angiogenesis to further enhance Aβ clearance. Our study confirms the important roles of microglia and cerebral vessels in the use of 1070-nm light for the treatment of AD mice and provides a framework for developing a novel therapeutic approach for AD.
Hot-band absorption of indocyanine green for advanced anti-stokes fluorescence bioimaging
Jing Zhou, Xiaoxiao Fan, Di Wu, Jie Liu, Yuhuang Zhang, et al.
Published. 2021, 10(10) : 1877-1888 doi: 10.1038/s41377-021-00627-1
Bright anti-Stokes fluorescence (ASF) in the first near-infrared spectral region (NIR-I, 800 nm–900 nm) under the excitation of a 915 nm continuous wave (CW) laser, is observed in Indocyanine Green (ICG), a dye approved by the Food and Drug Administration for clinical use. The dependence of fluorescence intensity on excitation light power and temperature, together with fluorescence lifetime measurement, establish this ASF to be originated from absorption from a thermally excited vibrational level (hot-band absorption), as shown in our experiments, which is stronger than the upconversion fluorescence from widely-used rare-earth ion doped nanoparticles. To test the utility of this ASF NIR-I probe for advanced bioimaging, we successively apply it for biothermal sensing, cerebral blood vessel tomography and blood stream velocimetry. Moreover, in combination with L1057 nanoparticles, which absorb the ASF of ICG and emit beyond 1100 nm, these two probes generate multi-mode images in two fluorescent channels under the excitation of a single 915 nm CW laser. One channel is used to monitor two overlapping organs, urinary system & blood vessel of a live mouse, while the other shows urinary system only. Using in intraoperative real-time monitoring, such multi-mode imaging method can be beneficial for visual guiding in anatomy of the urinary system to avoid any accidental injury to the surrounding blood vessels during surgery.
Dehydration of plant cells shoves nuclei rotation allowing for 3D phase-contrast tomography
Zhe Wang, Vittorio Bianco, Daniele Pirone, Pasquale Memmolo, Massimiliano Maria Villone, et al.
Published. 2021, 10(10) : 1889-1904 doi: 10.1038/s41377-021-00626-2
Single-cell phase-contrast tomography promises to become decisive for studying 3D intracellular structures in biology. It involves probing cells with light at wide angles, which unfortunately requires complex systems. Here we show an intriguing concept based on an inherent natural process for plants biology, i.e., dehydration, allowing us to easily obtain 3D-tomography of onion-epidermal cells' nuclei. In fact, the loss of water reduces the turgor pressure and we recognize it induces significant rotation of cells' nuclei. Thanks to the holographic focusing flexibility and an ad-hoc angles' tracking algorithm, we combine different phase-contrast views of the nuclei to retrieve their 3D refractive index distribution. Nucleolus identification capability and a strategy for measuring morphology, dry mass, biovolume, and refractive index statistics are reported and discussed. This new concept could revolutionize the investigation in plant biology by enabling dynamic 3D quantitative and label-free analysis at sub-nuclear level using a conventional holographic setup.
Micro-scale opto-thermo-mechanical actuation in the dry adhesive regime
Weiwei Tang, Wei Lyu, Jinsheng Lu, Fengjiang Liu, Jiyong Wang, et al.
Published. 2021, 10(10) : 1905-1914 doi: 10.1038/s41377-021-00622-6
Realizing optical manipulation of microscopic objects is crucial in the research fields of life science, condensed matter physics, and physical chemistry. In non-liquid environments, this task is commonly regarded as difficult due to strong adhesive surface force (~µN) attached to solid interfaces that makes tiny optical driven force (~pN) insignificant. Here, by recognizing the microscopic interaction mechanism between friction force—the parallel component of surface force on a contact surface—and thermoelastic waves induced by pulsed optical absorption, we establish a general principle enabling the actuation of micro-objects on dry frictional surfaces based on the opto-thermo-mechanical effects. Theoretically, we predict that nanosecond pulsed optical absorption with mW-scale peak power is sufficient to tame µN-scale friction force. Experimentally, we demonstrate the two-dimensional spiral motion of gold plates on micro-fibers driven by nanosecond laser pulses, and reveal the rules of motion control. Our results pave the way for the future development of micro-scale actuators in non-liquid environments.
Nano-precision metrology of X-ray mirrors with laser speckle angular measurement
Hongchang Wang, Simone Moriconi, Kawal Sawhney
Published. 2021, 10(10) : 1915-1927 doi: 10.1038/s41377-021-00632-4
X-ray mirrors are widely used for synchrotron radiation, free-electron lasers, and astronomical telescopes. The short wavelength and grazing incidence impose strict limits on the permissible slope error. Advanced polishing techniques have already produced mirrors with slope errors below 50 nrad root mean square (rms), but existing metrology techniques struggle to measure them. Here, we describe a laser speckle angular measurement (SAM) approach to overcome such limitations. We also demonstrate that the angular precision of slope error measurements can be pushed down to 20nrad rms by utilizing an advanced sub-pixel tracking algorithm. Furthermore, SAM allows the measurement of mirrors in two dimensions with radii of curvature as low as a few hundred millimeters. Importantly, the instrument based on SAM is compact, low-cost, and easy to integrate with most other existing X-ray mirror metrology instruments, such as the long trace profiler (LTP) and nanometer optical metrology (NOM). The proposed nanometrology method represents an important milestone and potentially opens up new possibilities to develop next-generation super-polished X-ray mirrors, which will advance the development of X-ray nanoprobes, coherence preservation, and astronomical physics.
Perfecting and extending the near-infrared imaging window
Zhe Feng, Tao Tang, Tianxiang Wu, Xiaoming Yu, Yuhuang Zhang, et al.
Published. 2021, 10(10) : 1949-1966 doi: 10.1038/s41377-021-00628-0
In vivo fluorescence imaging in the second near-infrared window (NIR-Ⅱ) has been considered as a promising technique for visualizing mammals. However, the definition of the NIR-Ⅱ region and the mechanism accounting for the excellent performance still need to be perfected. Herein, we simulate the photon propagation in the NIR region (to 2340 nm), confirm the positive contribution of moderate light absorption by water in intravital imaging and perfect the NIR-Ⅱ window as 900-1880 nm, where 1400-1500 and 1700-1880 nm are defined as NIR-Ⅱx and NIR-Ⅱc regions, respectively. Moreover, 2080-2340 nm is newly proposed as the third near-infrared (NIR-Ⅲ) window, which is believed to provide the best imaging quality. The wide-field fluorescence microscopy in the brain is performed around the NIR-Ⅱx region, with excellent optical sectioning strength and the largest imaging depth of intravital NIR-Ⅱ fluorescence microscopy to date. We also propose 1400 nm long-pass detection in off-peak NIR-Ⅱ imaging whose performance exceeds that of NIR-Ⅱb imaging, using bright fluorophores with short emission wavelength.
Non-line-of-sight reconstruction with signal–object collaborative regularization
Xintong Liu, Jianyu Wang, Zhupeng Li, Zuoqiang Shi, Xing Fu, et al.
Published. 2021, 10(10) : 1967-1986 doi: 10.1038/s41377-021-00633-3
Non-line-of-sight imaging aims at recovering obscured objects from multiple scattered lights. It has recently received widespread attention due to its potential applications, such as autonomous driving, rescue operations, and remote sensing. However, in cases with high measurement noise, obtaining high-quality reconstructions remains a challenging task. In this work, we establish a unified regularization framework, which can be tailored for different scenarios, including indoor and outdoor scenes with substantial background noise under both confocal and non-confocal settings. The proposed regularization framework incorporates sparseness and non-local self-similarity of the hidden objects as well as the smoothness of the signals. We show that the estimated signals, albedo, and surface normal of the hidden objects can be reconstructed robustly even with high measurement noise under the proposed framework. Reconstruction results on synthetic and experimental data show that our approach recovers the hidden objects faithfully and outperforms state-of-the-art reconstruction algorithms in terms of both quantitative criteria and visual quality.
Rapid, continuous projection multi-photon 3D printing enabled by spatiotemporal focusing of femtosecond pulses
Paul Somers, Zihao Liang, Jason E. Johnson, Bryan W. Boudouris, Liang Pan, et al.
Published. 2021, 10(10) : 1987-1997 doi: 10.1038/s41377-021-00645-z
There is demand for scaling up 3D printing throughput, especially for the multi-photon 3D printing process that provides sub-micrometer structuring capabilities required in diverse fields. In this work, high-speed projection multi-photon printing is combined with spatiotemporal focusing for fabrication of 3D structures in a rapid, layer-by-layer, and continuous manner. Spatiotemporal focusing confines printing to thin layers, thereby achieving print thicknesses on the micron and sub-micron scale. Through projection of dynamically varying patterns with no pause between patterns, a continuous fabrication process is established. A numerical model for computing spatiotemporal focusing and imaging is also presented which is verified by optical imaging and printing results. Complex 3D structures with smooth features are fabricated, with millimeter scale printing realized at a rate above 10−3 mm3 s−1. This method is further scalable, indicating its potential to make fabrications of 3D structures with micro/nanoscale features in a practical time scale a reality.
All-optical synthesis of an arbitrary linear transformation using diffractive surfaces
Onur Kulce, Deniz Mengu, Yair Rivenson, Aydogan Ozcan
Published. 2021, 10(10) : 1928-1948 doi: 10.1038/s41377-021-00623-5
Spatially-engineered diffractive surfaces have emerged as a powerful framework to control light-matter interactions for statistical inference and the design of task-specific optical components. Here, we report the design of diffractive surfaces to all-optically perform arbitrary complex-valued linear transformations between an input (Ni) and output (No), where Ni and No represent the number of pixels at the input and output fields-of-view (FOVs), respectively. First, we consider a single diffractive surface and use a matrix pseudoinverse-based method to determine the complex-valued transmission coefficients of the diffractive features/neurons to all-optically perform a desired/target linear transformation. In addition to this data-free design approach, we also consider a deep learning-based design method to optimize the transmission coefficients of diffractive surfaces by using examples of input/output fields corresponding to the target transformation. We compared the all-optical transformation errors and diffraction efficiencies achieved using data-free designs as well as data-driven (deep learning-based) diffractive designs to all-optically perform (i) arbitrarily-chosen complex-valued transformations including unitary, nonunitary, and noninvertible transforms, (ii) 2D discrete Fourier transformation, (iii) arbitrary 2D permutation operations, and (iv) high-pass filtered coherent imaging. Our analyses reveal that if the total number (N) of spatially-engineered diffractive features/neurons is ≥Ni × No, both design methods succeed in all-optical implementation of the target transformation, achieving negligible error. However, compared to data-free designs, deep learning-based diffractive designs are found to achieve significantly larger diffraction efficiencies for a given N and their all-optical transformations are more accurate for N < Ni × No. These conclusions are generally applicable to various optical processors that employ spatially-engineered diffractive surfaces.