2022 Vol. 11, No. 2

Light People
Light People: Elizabeth Rogan
Hui Wang, Cun Yu
Published. 2022, 11(2) : 138-144 doi: 10.1038/s41377-021-00706-3
Throughout history, there have been many outstanding women whose achievements continue to impress and amaze us today. For example, in the field of science, Madame Marie Curie was the first woman Nobel Prize winner and the only person to be awarded a Nobel Prize in two scientific fields. From China, Tu Youyou is a Nobel laureate who discovered artemisinin and dihydroartemisinin, used to treat malaria, a breakthrough in twentieth-century tropical medicine, saving millions of lives around the globe. Businesswomen such as Angela Ahrendts, a former fashion executive who helped revitalize Apple, Inc., and Sheryl Sandberg, Chief Operating Officer of Meta Platforms (formerly Facebook), are recognized as two of the world's most influential business leaders.Now, more than ever, women are at the forefront of developments in optics and photonics research and business. One of those leaders is Elizabeth Rogan, CEO of Optica (formerly the Optical Society and the Optical Society of America.) As the executive in charge of an organization devoted to promoting the generation, application, archiving, and dissemination of knowledge in optics and photonics worldwide, Ms. Rogan has successfully expanded the depth and breadth of Optica's technical and global reach. Her education and expertise are in industry, finance, and strategy. She utilizes these skills in partnership with a large and technically diverse group of Ph.D. volunteers and staff specialists. Combining the efforts of these many talented people with a unity of purpose has proven to be a highly effective approach for Rogan and the association she has led for nearly two decades.Ms. Rogan is a strong advocate for women. For instance, the association's "Faces of Optica" campaign features a wide range of accomplished women in research and applications. And she was an enthusiastic participant in the "Rose in Science," which celebrates the extraordinary accomplishments of women scientists.Light Special Correspondents interviewed Elizabeth Rogan about Optica's legacy, culture, and personal experiences as its CEO in this issue. She also discussed the reasons behind the recent rebranding of the organization and the bonds of friendship the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, and Optica have built over the years.Please join us for an in-depth look at why this century-plus-year-old organization has a fresh new vision for the future.
News & Views
Nanophotonics shines light on hyperbolic metamaterials
Andreas Aigner, Judith M. Dawes, Stefan A. Maier, Haoran Ren
Published. 2022, 11(2) : 145-148 doi: 10.1038/s41377-021-00688-2
Hyperbolic metamaterials with a unique hyperbolic dispersion relation allow propagating waves with infinitely large wavevectors and a high density of states. Researchers from Korea and Singapore provide a comprehensive review of hyperbolic metamaterials, including artificially structured hyperbolic media and natural hyperbolic materials. They explain key nanophotonic concepts and describe a range of applications for these versatile materials.
Solar X-ray and Extreme Ultraviolet Imager (X-EUVI) loaded onto China's Fengyun-3E Satellite
Bo Chen, Guang-Xing Ding, Ling-Ping He
Published. 2022, 11(2) : 149-150 doi: 10.1038/s41377-022-00711-0
The solar X-ray and Extreme Ultraviolet Imager (X-EUVI), which was developed by CIOMP, is China's first space-based solar X-ray and Extreme Ultraviolet (EUV) imager; it has been loaded onto the Fengyun-3E Satellite, which is supported by the China Meteorological Administration (CMA), for solar observation. It commenced working on July 11, 2021, and was used to obtain the first X-ray and EUV images in China. X-EUVI employs an innovation dual band design to monitor a much larger temperature range across the Sun, covering the 0.6–8.0 nm wavelength band of the X-ray region and the 19.5 nm band of the EUV region.
Spatial nonlinear optics for securing information
Wen Chen
Published. 2022, 11(2) : 151-152 doi: 10.1038/s41377-021-00699-z
The high degrees of freedom of light, various optical structures and optical materials can be explored and applied to develop optical encryption for securing information. An exciting optical image encryption approach has been proposed based on spatial nonlinear optics.
Gas fiber lasers may represent a breakthrough in creating powerful radiation sources in the mid-IR
Andrey Pryamikov
Published. 2022, 11(2) : 153-155 doi: 10.1038/s41377-022-00728-5
Continuous wave fiber laser created on the basis of silica glass negative curvature hollow core fiber filled with HBr make it possible to obtain efficient narrow linewidth mid-IR emission with a maximum laser power of about 500 mW at wavelength of 4200 nm. It is for the first time that emission from a continuous wave fiber laser have been achieved at a wavelength of 4496 nm with the largest tuning range of 686 nm.
Research Highlights
Photonic matrix multiplication lights up photonic accelerator and beyond
Hailong Zhou, Jianji Dong, Junwei Cheng, Wenchan Dong, Chaoran Huang, et al.
Published. 2022, 11(2) : 158-178 doi: 10.1038/s41377-022-00717-8
Matrix computation, as a fundamental building block of information processing in science and technology, contributes most of the computational overheads in modern signal processing and artificial intelligence algorithms. Photonic accelerators are designed to accelerate specific categories of computing in the optical domain, especially matrix multiplication, to address the growing demand for computing resources and capacity. Photonic matrix multiplication has much potential to expand the domain of telecommunication, and artificial intelligence benefiting from its superior performance. Recent research in photonic matrix multiplication has flourished and may provide opportunities to develop applications that are unachievable at present by conventional electronic processors. In this review, we first introduce the methods of photonic matrix multiplication, mainly including the plane light conversion method, Mach–Zehnder interferometer method and wavelength division multiplexing method. We also summarize the developmental milestones of photonic matrix multiplication and the related applications. Then, we review their detailed advances in applications to optical signal processing and artificial neural networks in recent years. Finally, we comment on the challenges and perspectives of photonic matrix multiplication and photonic acceleration.
Van der Waals two-color infrared photodetector
Peisong Wu, Lei Ye, Lei Tong, Peng Wang, Yang Wang, et al.
Published. 2022, 11(2) : 179-187 doi: 10.1038/s41377-021-00694-4
With the increasing demand for multispectral information acquisition, infrared multispectral imaging technology that is inexpensive and can be miniaturized and integrated into other devices has received extensive attention. However, the widespread usage of such photodetectors is still limited by the high cost of epitaxial semiconductors and complex cryogenic cooling systems. Here, we demonstrate a noncooled two-color infrared photodetector that can provide temporal-spatial coexisting spectral blackbody detection at both near-infrared and mid-infrared wavelengths. This photodetector consists of vertically stacked back-to-back diode structures. The two-color signals can be effectively separated to achieve ultralow crosstalk of ~0.05% by controlling the built-in electric field depending on the intermediate layer, which acts as an electron-collecting layer and hole-blocking barrier. The impressive performance of the two-color photodetector is verified by the specific detectivity (D*) of 6.4 × 109 cm Hz1/2 W−1 at 3.5 μm and room temperature, as well as the promising NIR/MWIR two-color infrared imaging and absolute temperature detection.
Nanoscale mapping of optically inaccessible bound-states-in-the-continuum
Zhaogang Dong, Zackaria Mahfoud, Ramón Paniagua-Domínguez, Hongtao Wang, Antonio I. Fernández-Domínguez, et al.
Published. 2022, 11(2) : 188-196 doi: 10.1038/s41377-021-00707-2
Bound-states-in-the-continuum (BIC) is an emerging concept in nanophotonics with potential impact in applications, such as hyperspectral imaging, mirror-less lasing, and nonlinear harmonic generation. As true BIC modes are non-radiative, they cannot be excited by using propagating light to investigate their optical characteristics. In this paper, for the 1st time, we map out the strong near-field localization of the true BIC resonance on arrays of silicon nanoantennas, via electron energy loss spectroscopy with a sub-1-nm electron beam. By systematically breaking the designed antenna symmetry, emissive quasi-BIC resonances become visible. This gives a unique experimental tool to determine the coherent interaction length, which we show to require at least six neighboring antenna elements. More importantly, we demonstrate that quasi-BIC resonances are able to enhance localized light emission via the Purcell effect by at least 60 times, as compared to unpatterned silicon. This work is expected to enable practical applications of designed, ultra-compact BIC antennas such as for the controlled, localized excitation of quantum emitters.
Toward implantable devices for angle-sensitive, lens-less, multifluorescent, single-photon lifetime imaging in the brain using Fabry–Perot and absorptive color filters
Adriaan J. Taal, Changhyuk Lee, Jaebin Choi, Björn Hellenkamp, Kenneth L. Shepard
Published. 2022, 11(2) : 197-211 doi: 10.1038/s41377-022-00708-9
Implantable image sensors have the potential to revolutionize neuroscience. Due to their small form factor requirements; however, conventional filters and optics cannot be implemented. These limitations obstruct high-resolution imaging of large neural densities. Recent advances in angle-sensitive image sensors and single-photon avalanche diodes have provided a path toward ultrathin lens-less fluorescence imaging, enabling plenoptic sensing by extending sensing capabilities to include photon arrival time and incident angle, thereby providing the opportunity for separability of fluorescence point sources within the context of light-field microscopy (LFM). However, the addition of spectral sensitivity to angle-sensitive LFM reduces imager resolution because each wavelength requires a separate pixel subset. Here, we present a 1024-pixel, 50 µm thick implantable shank-based neural imager with color-filter-grating-based angle-sensitive pixels. This angular-spectral sensitive front end combines a metal–insulator–metal (MIM) Fabry–Perot color filter and diffractive optics to produce the measurement of orthogonal light-field information from two distinct colors within a single photodetector. The result is the ability to add independent color sensing to LFM while doubling the effective pixel density. The implantable imager combines angular-spectral and temporal information to demix and localize multispectral fluorescent targets. In this initial prototype, this is demonstrated with 45 μm diameter fluorescently labeled beads in scattering medium. Fluorescent lifetime imaging is exploited to further aid source separation, in addition to detecting pH through lifetime changes in fluorescent dyes. While these initial fluorescent targets are considerably brighter than fluorescently labeled neurons, further improvements will allow the application of these techniques to in-vivo multifluorescent structural and functional neural imaging.
MXene supported surface plasmons on telecommunications optical fibers
Victor Pacheco-Peña, Toby Hallam, Noel Healy
Published. 2022, 11(2) : 212-220 doi: 10.1038/s41377-022-00710-1
MXenes, an emerging class of two-dimensional materials, exhibit characteristics that promise significant potential for their use in next generation optoelectronic sensors. An interplay between interband transitions and boundary effects offer the potential to tune the plasma frequencies over a large spectral range from the near-infrared to the mid-infrared. This tuneability along with the 'layered' nature of the material not only offer the flexibility to produce plasmon resonances across a wide range of wavelengths, but also add a degree of freedom to the sensing mechanism by allowing the plasma frequency to be modulated. Here we show, numerically, that MXenes can support plasmons in the telecommunications frequency range and that surface plasmon resonances can be excited on a standard MXene coated side polished optical fiber. Thus, presenting the tantalising prospect of highly selective distributed optical fiber sensor networks.
Nonvolatile electrical switching of optical and valleytronic properties of interlayer excitons
Tong Ye, Yongzhuo Li, Junze Li, Hongzhi Shen, Junwen Ren, et al.
Published. 2022, 11(2) : 221-229 doi: 10.1038/s41377-022-00718-7
Long-lived interlayer excitons (IXs) in van der Waals heterostructures (HSs) stacked by monolayer transition metal dichalcogenides (TMDs) carry valley-polarized information and thus could find promising applications in valleytronic devices. Current manipulation approaches for valley polarization of IXs are mainly limited in electrical field/doping, magnetic field or twist-angle engineering. Here, we demonstrate an electrochemical-doping method, which is efficient, in-situ and nonvolatile. We find the emission characteristics of IXs in WS2/WSe2 HSs exhibit a large excitonic/valley-polarized hysteresis upon cyclic-voltage sweeping, which is ascribed to the chemical-doping of O2/H2O redox couple trapped between WSe2 and substrate. Taking advantage of the large hysteresis, a nonvolatile valley-addressable memory is successfully demonstrated. The valley-polarized information can be non-volatilely switched by electrical gating with retention time exceeding 60 min. These findings open up an avenue for nonvolatile valley-addressable memory and could stimulate more investigations on valleytronic devices.
Phase-matching-induced near-chirp-free solitons in normal-dispersion fiber lasers
Dong Mao, Zhiwen He, Yusong Zhang, Yueqing Du, Chao Zeng, et al.
Published. 2022, 11(2) : 230-241 doi: 10.1038/s41377-022-00713-y
Direct generation of chirp-free solitons without external compression in normal-dispersion fiber lasers is a long-term challenge in ultrafast optics. We demonstrate near-chirp-free solitons with distinct spectral sidebands in normal-dispersion hybrid-structure fiber lasers containing a few meters of polarization-maintaining fiber. The bandwidth and duration of the typical mode-locked pulse are 0.74 nm and 1.95 ps, respectively, giving the time-bandwidth product of 0.41 and confirming the near-chirp-free property. Numerical results and theoretical analyses fully reproduce and interpret the experimental observations, and show that the fiber birefringence, normal-dispersion, and nonlinear effect follow a phase-matching principle, enabling the formation of the near-chirp-free soliton. Specifically, the phase-matching effect confines the spectrum broadened by self-phase modulation and the saturable absorption effect slims the pulse stretched by normal dispersion. Such pulse is termed as birefringence-managed soliton because its two orthogonal-polarized components propagate in an unsymmetrical "X" manner inside the polarization-maintaining fiber, partially compensating the group delay difference induced by the chromatic dispersion and resulting in the self-consistent evolution. The property and formation mechanism of birefringence-managed soliton fundamentally differ from other types of pulses in mode-locked fiber lasers, which will open new research branches in laser physics, soliton mathematics, and their related applications.
First Chinese ultraviolet–visible hyperspectral satellite instrument implicating global air quality during the COVID-19 pandemic in early 2020
Cheng Liu, Qihou Hu, Chengxin Zhang, Congzi Xia, Hao Yin, et al.
Published. 2022, 11(2) : 242-253 doi: 10.1038/s41377-022-00722-x
In response to the COVID-19 pandemic, governments worldwide imposed lockdown measures in early 2020, resulting in notable reductions in air pollutant emissions. The changes in air quality during the pandemic have been investigated in numerous studies via satellite observations. Nevertheless, no relevant research has been gathered using Chinese satellite instruments, because the poor spectral quality makes it extremely difficult to retrieve data from the spectra of the Environmental Trace Gases Monitoring Instrument (EMI), the first Chinese satellite-based ultraviolet–visible spectrometer monitoring air pollutants. However, through a series of remote sensing algorithm optimizations from spectral calibration to retrieval, we successfully retrieved global gaseous pollutants, such as nitrogen dioxide (NO2), sulfur dioxide (SO2), and formaldehyde (HCHO), from EMI during the pandemic. The abrupt drop in NO2 successfully captured the time for each city when effective measures were implemented to prevent the spread of the pandemic, for example, in January 2020 in Chinese cities, February in Seoul, and March in Tokyo and various cities across Europe and America. Furthermore, significant decreases in HCHO in Wuhan, Shanghai, Guangzhou, and Seoul indicated that the majority of volatile organic compounds (VOCs) emissions were anthropogenic. Contrastingly, the lack of evident reduction in Beijing and New Delhi suggested dominant natural sources of VOCs. By comparing the relative variation of NO2 to gross domestic product (GDP), we found that the COVID-19 pandemic had more influence on the secondary industry in China, while on the primary and tertiary industries in Korea and the countries across Europe and America.
Angular engineering strategy of an additional periodic phase for widely tunable phase-matched deep-ultraviolet second harmonic generation
Mingchuan Shao, Fei Liang, Haohai Yu, Huaijin Zhang
Published. 2022, 11(2) : 254-261 doi: 10.1038/s41377-022-00715-w
Manipulation of the light phase lies at the heart of the investigation of light-matter interactions, especially for efficient nonlinear optical processes. Here, we originally propose the angular engineering strategy of the additional periodic phase (APP) for realization of tunable phase matching and experimentally demonstrate the widely tunable phase-matched second harmonic generation (SHG) which is expected for dozens of years. With an APP quartz crystal, the phase difference between the fundamental and frequency-doubled light is continuously angularly compensated under this strategy, which results the unprecedented and efficient frequency doubling at wavelengths almost covering the deep-UV spectral range from 221 to 332 nm. What's more, all the possible phase-matching types are originally realized simultaneously under the angular engineering phase-matching conditions. This work should not only provide a novel and practical nonlinear photonic device for tunable deep-UV radiation but also be helpful for further study of the light-matter interaction process.
Microsphere-assisted, nanospot, non-destructive metrology for semiconductor devices
Soonyang Kwon, Jangryul Park, Kwangrak Kim, Yunje Cho, Myungjun Lee
Published. 2022, 11(2) : 262-275 doi: 10.1038/s41377-022-00720-z
As smaller structures are being increasingly adopted in the semiconductor industry, the performance of memory and logic devices is being continuously improved with innovative 3D integration schemes as well as shrinking and stacking strategies. Owing to the increasing complexity of the design architectures, optical metrology techniques including spectroscopic ellipsometry (SE) and reflectometry have been widely used for efficient process development and yield ramp-up due to the capability of 3D structure measurements. However, there has been an increasing demand for a significant reduction in the physical spot diameter used in the SE technique; the spot diameter should be at least 10 times smaller than the cell dimension (~30 × 40 μm2) of typical dynamic random-access memory to be able to measure in-cell critical dimension (CD) variations. To this end, this study demonstrates a novel spectrum measurement system that utilizes the microsphere-assisted super-resolution effect, achieving extremely small spot spectral metrology by reducing the spot diameter to ~210 nm, while maintaining a sufficiently high signal-to-noise ratio. In addition, a geometric model is introduced for the microsphere-based spectral metrology system that can calculate the virtual image plane magnification and depth of focus, providing the optimal distance between the objective lens, microsphere, and sample to achieve the best possible imaging quality. The proof of concept was fully verified through both simulations and experiments for various samples. Thus, owing to its ultra-small spot metrology capability, this technique has great potential for solving the current metrology challenge of monitoring in-cell CD variations in advanced logic and memory devices.
Probing the orbital angular momentum of intense vortex pulses with strong-field ionization
Yiqi Fang, Zhenning Guo, Peipei Ge, Yankun Dou, Yongkai Deng, et al.
Published. 2022, 11(2) : 276-286 doi: 10.1038/s41377-022-00726-7
With the rapid development of femtosecond lasers, the generation and application of optical vortices have been extended to the regime of intense-light-matter interaction. The characterization of the orbital angular momentum (OAM) of intense vortex pulses is very critical. Here, we propose and demonstrate a novel photoelectron-based scheme that can in situ distinguish the OAM of the focused intense femtosecond optical vortices without the modification of light helical phase. We employ two-color co-rotating intense circular fields in the strong-field photoionization experiment, in which one color light field is a plane wave serving as the probing pulses and the other one is the vortex pulses whose OAM needs to be characterized. We show that by controlling the spatial profile of the probing pulses, the OAM of the vortex pulses can be clearly identified by measuring the corresponding photoelectron momentum distributions or angle-resolved yields. This work provides a novel in situ detection scenario for the light pulse vorticity and has implications for the studies of ultrafast and intense complex light fields with optical OAM.