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An Adjusted Two-Leaf Light Use Efficiency Model for Improving GPP Simulations Over Mountainous Areas 期刊论文
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 2020, 125 (13)
作者:  Xie, Xinyao;  Li, Ainong
收藏  |  浏览/下载:21/0  |  提交时间:2020/08/18
mountainous two-leaf light use efficiency model (MTL-LUE)  topographic effects  gross primary productivity (GPP)  direct radiation  diffuse radiation  sunlit canopy area  
On the Functional Relationship Between Fluorescence and Photochemical Yields in Complex Evergreen Needleleaf Canopies 期刊论文
GEOPHYSICAL RESEARCH LETTERS, 2020, 47 (9)
作者:  Maguire, Andrew J.;  Eitel, Jan U. H.;  Griffin, Kevin L.;  Magney, Troy S.;  Long, Ryan A.;  Vierling, Lee A.;  Schmiege, Stephanie C.;  Jennewein, Jyoti S.;  Weygint, William A.;  Boelman, Natalie T.;  Bruner, Sarah G.
收藏  |  浏览/下载:22/0  |  提交时间:2020/07/02
canopy irradiance  evergreen needleleaf forest  light use efficiency  nonphotochemical quenching  
Exploring dynamical phase transitions with cold atoms in an optical cavity 期刊论文
NATURE, 2020, 580 (7805) : 602-+
作者:  Halbach, Rebecca;  Miesen, Pascal;  Joosten, Joep;  Taskopru, Ezgi;  Rondeel, Inge;  Pennings, Bas;  Vogels, Chantal B. F.;  Merkling, Sarah H.;  Koenraadt, Constantianus J.;  Lambrechts, Louis;  van Rij, Ronald P.
收藏  |  浏览/下载:23/0  |  提交时间:2020/07/03

Interactions between light and an ensemble of strontium atoms in an optical cavity can serve as a testbed for studying dynamical phase transitions, which are currently not well understood.


Interactions between atoms and light in optical cavities provide a means of investigating collective (many-body) quantum physics in controlled environments. Such ensembles of atoms in cavities have been proposed for studying collective quantum spin models, where the atomic internal levels mimic a spin degree of freedom and interact through long-range interactions tunable by changing the cavity parameters(1-4). Non-classical steady-state phases arising from the interplay between atom-light interactions and dissipation of light from the cavity have previously been investigated(5-11). These systems also offer the opportunity to study dynamical phases of matter that are precluded from existence at equilibrium but can be stabilized by driving a system out of equilibrium(12-16), as demonstrated by recent experiments(17-22). These phases can also display universal behaviours akin to standard equilibrium phase transitions(8,23,24). Here, we use an ensemble of about a million strontium-88 atoms in an optical cavity to simulate a collective Lipkin-Meshkov-Glick model(25,26), an iconic model in quantum magnetism, and report the observation of distinct dynamical phases of matter in this system. Our system allows us to probe the dependence of dynamical phase transitions on system size, initial state and other parameters. These observations can be linked to similar dynamical phases in related systems, including the Josephson effect in superfluid helium(27), or coupled atomic(28) and solid-state polariton(29) condensates. The system itself offers potential for generation of metrologically useful entangled states in optical transitions, which could permit quantum enhancement in state-of-the-art atomic clocks(30,31).


  
Massively parallel coherent laser ranging using a soliton microcomb 期刊论文
NATURE, 2020, 581 (7807) : 164-+
作者:  Casanova, Emmanuelle;  Knowles, Timothy D. J.;  Bayliss, Alex;  Dunne, Julie;  Baranski, Marek Z.;  Denaire, Anthony;  Lefranc, Philippe;  di Lernia, Savino;  Roffet-Salque, Melanie;  Smyth, Jessica;  Barclay, Alistair;  Gillard, Toby;  Classen, Erich;  Coles, Bryony;  Ilett, Michael;  Jeunesse, Christian;  Krueger, Marta;  Marciniak, Arkadiusz;  Minnitt, Steve;  Rotunno, Rocco;  van de Velde, Pieter;  van Wijk, Ivo;  Cotton, Jonathan;  Daykin, Andy;  Evershed, Richard P.
收藏  |  浏览/下载:67/0  |  提交时间:2020/07/03

Coherent ranging, also known as frequency-modulated continuous-wave (FMCW) laser-based light detection and ranging (lidar)(1) is used for long-range three-dimensional distance and velocimetry in autonomous driving(2,3). FMCW lidar maps distance to frequency(4,5) using frequency-chirped waveforms and simultaneously measures the Doppler shift of the reflected laser light, similar to sonar or radar(6,7) and coherent detection prevents interference from sunlight and other lidar systems. However, coherent ranging has a lower acquisition speed and requires precisely chirped(8) and highly coherent(5) laser sources, hindering widespread use of the lidar system and impeding parallelization, compared to modern time-of-flight ranging systems that use arrays of individual lasers. Here we demonstrate a massively parallel coherent lidar scheme using an ultra-low-loss photonic chip-based soliton microcomb(9). By fast chirping of the pump laser in the soliton existence range(10) of a microcomb with amplitudes of up to several gigahertz and a sweep rate of up to ten megahertz, a rapid frequency change occurs in the underlying carrier waveform of the soliton pulse stream, but the pulse-to-pulse repetition rate of the soliton pulse stream is retained. As a result, the chirp from a single narrow-linewidth pump laser is transferred to all spectral comb teeth of the soliton at once, thus enabling parallelism in the FMCW lidar. Using this approach we generate 30 distinct channels, demonstrating both parallel distance and velocity measurements at an equivalent rate of three megapixels per second, with the potential to improve sampling rates beyond 150 megapixels per second and to increase the image refresh rate of the FMCW lidar by up to two orders of magnitude without deterioration of eye safety. This approach, when combined with photonic phase arrays(11) based on nanophotonic gratings(12), provides a technological basis for compact, massively parallel and ultrahigh-frame-rate coherent lidar systems.


  
A biomimetic eye with a hemispherical perovskite nanowire array retina 期刊论文
NATURE, 2020, 581 (7808) : 278-+
作者:  Hueckel, Theodore;  Hocky, Glen M.;  Palacci, Jeremie;  Sacanna, Stefano
收藏  |  浏览/下载:78/0  |  提交时间:2020/07/03

A biomimetic electrochemical eye is presented that has a hemispherical retina made from a high-density array of perovskite nanowires that are sensitive to light, mimicking the photoreceptors of a biological retina.


Human eyes possess exceptional image-sensing characteristics such as an extremely wide field of view, high resolution and sensitivity with low aberration(1). Biomimetic eyes with such characteristics are highly desirable, especially in robotics and visual prostheses. However, the spherical shape and the retina of the biological eye pose an enormous fabrication challenge for biomimetic devices(2,3). Here we present an electrochemical eye with a hemispherical retina made of a high-density array of nanowires mimicking the photoreceptors on a human retina. The device design has a high degree of structural similarity to a human eye with the potential to achieve high imaging resolution when individual nanowires are electrically addressed. Additionally, we demonstrate the image-sensing function of our biomimetic device by reconstructing the optical patterns projected onto the device. This work may lead to biomimetic photosensing devices that could find use in a wide spectrum of technological applications.


  
Observation of topologically enabled unidirectional guided resonances 期刊论文
NATURE, 2020, 580 (7804) : 467-+
作者:  Wang, Renjing;  Wang, Shengliu;  Dhar, Ankita;  Peralta, Christopher;  Pavletich, Nikola P.
收藏  |  浏览/下载:22/0  |  提交时间:2020/07/03

Unidirectional radiation is important for various optoelectronic applications, such as lasers, grating couplers and optical antennas. However, almost all existing unidirectional emitters rely on the use of materials or structures that forbid outgoing waves-that is, mirrors, which are often bulky, lossy and difficult to fabricate. Here we theoretically propose and experimentally demonstrate a class of resonances in photonic crystal slabs that radiate only towards one side of the slab, with no mirror placed on the other side. These resonances, which we name '  unidirectional guided resonances'  , are found to be topological in nature: they emerge when a pair of half-integer topological charges(1-3) in the polarization field bounce into each other in momentum space. We experimentally demonstrate unidirectional guided resonances in the telecommunication regime by achieving single-side radiative quality factors as high as 1.6 x 10(5). We further demonstrate their topological nature through far-field polarimetry measurements. Our work represents a characteristic example of applying topological principles(4,5) to control optical fields and could lead to energy-efficient grating couplers and antennas for light detection and ranging.


Unidirectional radiation is achieved in a photonic crystal slab without the use of mirrors by merging a pair of topological defects carrying half-integer charges.


  
Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites 期刊论文
NATURE, 2020, 580 (7803) : 360-+
作者:  van den Brink, Susanne C.;  Alemany, Anna;  van Batenburg, Vincent;  Moris, Naomi;  Blotenburg, Marloes;  Vivie, Judith;  Baillie-Johnson, Peter;  Nichols, Jennifer;  Sonnen, Katharina F.;  Arias, Alfonso;  van Oudenaarden, Alexander
收藏  |  浏览/下载:39/0  |  提交时间:2020/07/03

Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices(1,2). This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively(3)) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects(4). Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance(5), perovskite devices still have many states deep within the bandgap that trap charge carriers and cause them to recombine non-radiatively. These deep trap states thus induce local variations in photoluminescence and limit the device performance(6). The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions(7) and with local strain(8), both of which make devices less stable(9). Here we use photoemission electron microscopy to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor photoluminescence efficiency, we observe discrete, nanoscale trap clusters. By correlating microscopy measurements with scanning electron analytical techniques, we find that these trap clusters appear at the interfaces between crystallographically and compositionally distinct entities. Finally, by generating time-resolved photoemission sequences of the photo-excited carrier trapping process(10,11), we reveal a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters. Our approach shows that managing structure and composition on the nanoscale will be essential for optimal performance of halide perovskite devices.


  
Mott and generalized Wigner crystal states in WSe2/WS2 moire superlattices 期刊论文
NATURE, 2020, 579 (7799) : 359-+
作者:  Yuan, Jie;  Chang, Si-Yuan;  Yin, Shi-Gang;  Liu, Zhi-Yang;  Cheng, Xiu;  Liu, Xi-Juan;  Jiang, Qiang;  Gao, Ge;  Lin, De-Ying;  Kang, Xin-Lei;  Ye, Shi-Wei;  Chen, Zheng;  Yin, Jiang-An;  Hao, Pei;  Jiang, Lubin;  Cai, Shi-Qing
收藏  |  浏览/下载:79/0  |  提交时间:2020/07/03

Strongly correlated insulating Mott and generalized Wigner phases are detected in WSe2/WS2 moire superlattices, and their electrical properties and excited spin states are studied using an optical technique.


Moire superlattices can be used to engineer strongly correlated electronic states in two-dimensional van der Waals heterostructures, as recently demonstrated in the correlated insulating and superconducting states observed in magic-angle twisted-bilayer graphene and ABC trilayer graphene/boron nitride moire superlattices(1-4). Transition metal dichalcogenide moire heterostructures provide another model system for the study of correlated quantum phenomena(5) because of their strong light-matter interactions and large spin-orbit coupling. However, experimental observation of correlated insulating states in this system is challenging with traditional transport techniques. Here we report the optical detection of strongly correlated phases in semiconducting WSe2/WS2 moire superlattices. We use a sensitive optical detection technique and reveal a Mott insulator state at one hole per superlattice site and surprising insulating phases at 1/3 and 2/3 filling of the superlattice, which we assign to generalized Wigner crystallization on the underlying lattice(6-11). Furthermore, the spin-valley optical selection rules(12-14) of transition metal dichalcogenide heterostructures allow us to optically create and investigate low-energy excited spin states in the Mott insulator. We measure a very long spin relaxation lifetime of many microseconds in the Mott insulating state, orders of magnitude longer than that of charge excitations. Our studies highlight the value of using moire superlattices beyond graphene to explore correlated physics.


  
Electrically pumped topological laser with valley edge modes 期刊论文
NATURE, 2020, 578 (7794) : 246-+
作者:  Erickson, Peter;  van Asselt, Harro;  Koplow, Doug;  Lazarus, Michael;  Newell, Peter;  Oreskes, Naomi;  Supran, Geoffrey
收藏  |  浏览/下载:54/0  |  提交时间:2020/07/03

Quantum cascade lasers are compact, electrically pumped light sources in the technologically important mid-infrared and terahertz region of the electromagnetic spectrum(1,2). Recently, the concept of topology(3) has been expanded from condensed matter physics into photonics(4), giving rise to a new type of lasing(5-8) using topologically protected photonic modes that can efficiently bypass corners and defects(4). Previous demonstrations of topological lasers have required an external laser source for optical pumping and have operated in the conventional optical frequency regime(5-8). Here we demonstrate an electrically pumped terahertz quantum cascade laser based on topologically protected valley edge states(9-11). Unlike topological lasers that rely on large-scale features to impart topological protection, our compact design makes use of the valley degree of freedom in photonic crystals(10,11), analogous to two-dimensional gapped valleytronic materials(12). Lasing with regularly spaced emission peaks occurs in a sharp-cornered triangular cavity, even if perturbations are introduced into the underlying structure, owing to the existence of topologically protected valley edge states that circulate around the cavity without experiencing localization. We probe the properties of the topological lasing modes by adding different outcouplers to the topological cavity. The laser based on valley edge states may open routes to the practical use of topological protection in electrically driven laser sources.


  
Probing the core of the strong nuclear interaction 期刊论文
NATURE, 2020, 578 (7796) : 540-+
作者:  Bialas, Allison R.;  Presumey, Jessy;  Das, Abhishek;  van der Poel, Cees E.;  Lapchak, Peter H.;  Mesin, Luka;  Victora, Gabriel;  Tsokos, George C.;  Mawrin, Christian;  Herbst, Ronald;  Carroll, Michael C.
收藏  |  浏览/下载:31/0  |  提交时间:2020/07/03

High-energy electron scattering that can isolate pairs of nucleons in high-momentum configurations reveals a transition to spin-independent scalar forces at small separation distances, supporting the use of point-like nucleon models to describe dense nuclear systems.


The strong nuclear interaction between nucleons (protons and neutrons) is the effective force that holds the atomic nucleus together. This force stems from fundamental interactions between quarks and gluons (the constituents of nucleons) that are described by the equations of quantum chromodynamics. However, as these equations cannot be solved directly, nuclear interactions are described using simplified models, which are well constrained at typical inter-nucleon distances(1-5) but not at shorter distances. This limits our ability to describe high-density nuclear matter such as that in the cores of neutron stars(6). Here we use high-energy electron scattering measurements that isolate nucleon pairs in short-distance, high-momentum configurations(7-9), accessing a kinematical regime that has not been previously explored by experiments, corresponding to relative momenta between the pair above 400 megaelectronvolts per c (c, speed of light in vacuum). As the relative momentum between two nucleons increases and their separation thereby decreases, we observe a transition from a spin-dependent tensor force to a predominantly spin-independent scalar force. These results demonstrate the usefulness of using such measurements to study the nuclear interaction at short distances and also support the use of point-like nucleon models with two- and three-body effective interactions to describe nuclear systems up to densities several times higher than the central density of the nucleus.