Ferromagnetism emerged from non-ferromagnetic atomic crystals.

The recently emerged ferromagnetic two-dimensional (2D) materials provide unique platforms for compact spintronic devices down to the atomic-thin regime; however, the prospect is hindered by the limited number  of ferromagnetic 2D materials discovered with limited choices of magnetic properties. If 2D antiferromagnetism could be converted to 2D ferromagnetism, the range of 2D magnets and their potential applications would be significantly broadened. Here, we discovered emergent ferromagnetism by interfacing non-magnetic WS2 layers with the antiferromagnetic FePS3. The WS2 exhibits an order of magnitude enhanced Zeeman effect with a saturated interfacial exchange field ~38 Tesla. Given the pristine FePS3 is an intralayer antiferromagnet, the prominent interfacial exchange field suggests the formation of ferromagnetic FePS3 at interface. Furthermore, the enhanced Zeeman effect in WS2 is found to exhibit a strong WS2-thickness dependence, highlighting the layer-tailorable interfacial exchange coupling in WS2-FePS3 heterostructures, which is potentially attributed to the thickness-dependent interfacial hybridization.

Coherent momentum control of forbidden excitons.

A double-edged sword in two-dimensional material science and technology is optically forbidden dark exciton. On the one hand, it is fascinating for condensed matter physics, quantum information processing, and optoelectronics due to its long lifetime. On the other hand, it is notorious for being optically inaccessible from both excitation and detection standpoints. Here, we provide an efficient and low-loss solution to the dilemma by reintroducing photonics bound states in the continuum (BICs) to manipulate dark excitons in the momentum space. In a monolayer tungsten diselenide under normal incidence, we demonstrated a giant enhancement (~1400) for dark excitons enabled by transverse magnetic BICs with intrinsic out-of-plane electric fields. By further employing widely tunable Friedrich-Wintgen BICs, we demonstrated highly directional emission from the dark excitons with a divergence angle of merely 7°. We found that the directional emission is coherent at room temperature, unambiguously shown in polarization analyses and interference measurements. Therefore, the BICs reintroduced as a momentum-space photonic environment could be an intriguing platform to reshape and redefine light-matter interactions in nearby quantum materials, such as low-dimensional materials, otherwise challenging or even impossible to achieve.

Nanotexture Shape and Surface Energy Impact on Electroadhesive Human-Machine Interface Performance.

With the ubiquity of touch screens and the commercialization of electroadhesion-based surface haptic devices, modeling tools that capture the multiphysical phenomena within the finger-device interface and their interaction are critical to design devices that achieve higher performance and reliability at lower cost. While electroadhesion has successfully demonstrated the capability to change tactile perception through friction modulation, the mechanism of electroadhesion in the finger-device interface is still unclear, partly due to the complex interfacial physics including contact deformation, capillary formation, electric field, and their complicated coupling effects that have not been addressed comprehensively. A multiphysics model is presented here to predict the friction force for finger-surface tactile interactions at the nanoscale. The nanoscopic multiphysical phenomena are coupled to study the impacts of nanotexture and surface energy in the touch interface. With macroscopic friction force measurements as verification, the model is further used to propose textures that have maximum electroadhesion effect and minimum sensitivity to relative humidity and user perspiration rate. This model can guide the performance improvement of future electroadhesion-based surface haptic devices and other touch-based human-machine interfaces.

Observation of strong excitonic magneto-chiral anisotropy in twisted bilayer van der Waals crystals.

The interplay between chirality and magnetism generates a distinct physical process, the magneto-chiral effect, which enables one to develop functionalities that cannot be achieved solely by any of the two. Such a process is universal with the breaking of parity-inversion and time-reversal symmetry simultaneously. However, the magneto-chiral effect observed so far is weak when the matter responds to photons, electrons, or phonons. Here we report the first observation of strong magneto-chiral response to excitons in a twisted bilayer tungsten disulfide with the amplitude of excitonic magneto-chiral (ExMCh) anisotropy reaches a value of ~4%. We further found the ExMCh anisotropy features with a spectral splitting of ~7 nm, precisely the full-width at half maximum of the excitonic chirality spectrum. Without an externally applied strong magnetic field, the observed ExMCh effect with a spontaneous magnetic moment from the ferromagnetic substrate of thulium iron garnet at room temperature is favorable for device applications. The unique ExMCh processes provide a new pathway to actively control magneto-chiral applications in photochemical reactions, asymmetric synthesis, and drug delivery.

Monolithic Full-Stokes Near-Infrared Polarimetry with Chiral Plasmonic Metasurface Integrated Graphene-Silicon Photodetector.

The ability to detect the full-Stokes polarization of light is vital for a variety of applications that often require complex and bulky optical systems. Here, we report an on-chip polarimeter comprising four metasurface-integrated graphene-silicon photodetectors. The geometric chirality and anisotropy of the metasurfaces result in circular and linear polarization-resolved photoresponses, from which the full-Stokes parameters, including the intensity, orientation, and ellipticity of arbitrarily polarized incident infrared light (1550 nm), can be obtained. The design presents an ultracompact architecture while excluding the standard bulky optical components and structural redundancy. Computational extraction of full-Stokes parameters from mutual information among four detectors eliminates the need for a large absorption contrast between different polarization states. Our monolithic plasmonic metasurface integrated polarimeter is ideal for a variety of polarization-based applications including biological sensing, quantum information processing, and polarization photography.

Nonlinear Optics at Excited States of Exciton Polaritons in Two-Dimensional Atomic Crystals.

Exciton polaritons (EPs) are partial-light partial-matter quasiparticles in semiconductors demonstrating striking quantum phenomena such as Bose-Einstein condensation and single-photon nonlinearity. In these phenomena, the governing process is the EP relaxation into the ground states upon excitation, where various mechanisms are extensively investigated with thermodynamic limits. However, the relaxation process becomes drastically different and could significantly advance the understanding of EP dynamics for these quantum phenomena, when excited states of EPs are involved. Here, for the first time, we observe nonlinear optical responses at the EP excited states in a monolayer tungsten disulfide (WS2) microcavity, including dark excited states and dynamically metastable upper polariton bands. The nonlinear optics leads to unique emissions of ground states with prominent valley degree of freedom (DOF) via an anomalous relaxation process, which is applicable to a wide range of semiconductors from monolayer transition metal dichalcogenides (TMDs) to emerging halide perovskites. This work promises possible approaches to challenging experiments such as valley polariton condensation. Moreover, it also constructs a valley-dependent solid-state three-level system for terahertz photonics and stimulated Raman adiabatic passage.

Publisher Correction: Intensity-dependent modulation of optically active signals in a chiral metamaterial.

This corrects the article DOI: 10.1038/ncomms14602.

Circular RNA expression profiles reveal that hsa_circ_0018289 is up-regulated in cervical cancer and promotes the tumorigenesis.

Circular RNAs (circRNAs) are a type of non-coding RNAs that have been identified as critical regulators in various diseases, especially in cancers. However, the expression profiles and functions of circRNAs in cervical cancer are still unclear. In present study, human circRNAs microarray were performed to screen the circRNAs expression in cervical cancer tissue. Microarray analysis revealed 45 significantly expressed circRNAs with 4 fold change. Among these up-regulated circRNAs, hsa_circ_0018289 was validated to be significantly up-regulated in 35 pairs of cervical cancer tissue compared with adjacent normal tissue and cell lines. Loss-of-function experiments revealed that, in vitro and in vivo, hsa_circ_0018289 knockdown inhibited the proliferation, migration and invasion of cervical cancer cells. Via bioinformatics prediction program and luciferase reporter assays, hsa_circ_0018289 was observed to directly bind to miR-497. Taken together, the results indicate that hsa_circ_0018289 plays important role in cervical cancer proliferation, migration and invasion, suggesting the miRNA 'sponge' of hsa_circ_0018289 and its oncogenic role on cervical cancer tumorigenesis.

Preserving Spin States upon Reflection: Linear and Nonlinear Responses of a Chiral Meta-Mirror.

Conventional metallic mirrors flip the spin of a circularly polarized wave upon normal incidence by inverting the direction of the propagation vector. Altering or maintaining the spin state of light waves carrying data is a critical need to be met at the brink of photonic information processing. In this work, we report a chiral metamaterial mirror that strongly absorbs a circularly polarized wave of one spin state and reflects that of the opposite spin in a manner conserving the circular polarization. A circular dichroic response in reflection as large as ∼0.5 is experimentally observed in a near-infrared wavelength band. By imaging a fabricated pattern composed of the enantiomeric unit cells, we directly visualize the two key features of our engineered meta-mirrors, namely the chiral-selective absorption and the polarization preservation upon reflection. Beyond the linear regime, the chiral resonances enhance light-matter interaction under circularly polarized excitation, greatly boosting the ability of the metamaterial to perform chiral-selective signal generation and optical imaging in the nonlinear regime. Chiral meta-mirrors, exhibiting giant chiroptical responses and spin-selective near-field enhancement, hold great promise for applications in polarization sensitive electro-optical information processing and biosensing.

Intensity-dependent modulation of optically active signals in a chiral metamaterial.

Chiral media exhibit optical phenomena that provide distinctive responses from opposite circular polarizations. The disparity between these responses can be optimized by structurally engineering absorptive materials into chiral nanopatterns to form metamaterials that provide gigantic chiroptical resonances. To fully leverage the innate duality of chiral metamaterials for future optical technologies, it is essential to make such chiroptical responses tunable via external means. Here we report an optical metamaterial with tailored chiroptical effects in the nonlinear regime, which exhibits a pronounced shift in its circular dichroism spectrum under a modest level of excitation power. Strong nonlinear optical rotation is observed at key spectral locations, with an intensity-induced change of 14° in the polarization rotation from a metamaterial thickness of less than λ/7. The modulation of chiroptical responses by manipulation of input powers incident on chiral metamaterials offers potential for active optics such as all-optical switching and light modulation.

Electrically Tunable Harmonic Generation of Light from Plasmonic Structures in Electrolytes.

An emerging trend in plasmonics is to exploit nanostructured metals as a self-contained electrooptic platform with simultaneously supported electrical and optical functions. When it comes to nonlinear optics, this dual electrical and optical functionality offers an exciting potential to enable electrically controlled wave mixing processes in various nanometallic systems. Here we demonstrate tunable nonlinear generation of light enabled by an electrically active plasmonic crystal in aqueous electrolytic solutions. A modulation depth of ∼150%/V is observed in the second-harmonic signal, thanks to the light concentrating ability of the resonant plasmonic structure as well as the voltage-assisted charge accumulation on the metallic surfaces. The hybrid plasmonic-electrolyte system demonstrated in this work offers the exciting new potential to use plasmonic nanostructures for voltage-controlled nonlinear signal generation and in situ biochemical sensing in an aqueous environment.

An Active Metamaterial Platform for Chiral Responsive Optoelectronics.

Chiral-selective non-linear optics and optoelectronic signal generation are demonstrated in an electrically active photonic metamaterial. The metamaterial reveals significant chiroptical responses in both harmonic generation and the photon drag effect, correlated to the resonance behavior in the linear regime. The multifunctional chiral metamaterial with dual electrical and optical functionality enables transduction of chiroptical responses to electrical signals for integrated photonics.

Backward phase-matching for nonlinear optical generation in negative-index materials.

Metamaterials have enabled the realization of unconventional electromagnetic properties not found in nature, which provokes us to rethink the established rules of optics in both the linear and nonlinear regimes. One of the most intriguing phenomena in nonlinear metamaterials is 'backward phase-matching', which describes counter-propagating fundamental and harmonic waves in a negative-index medium. Predicted nearly a decade ago, this process is still awaiting a definitive experimental confirmation at optical frequencies. Here, we report optical measurements showing backward phase-matching by exploiting two distinct modes in a nonlinear plasmonic waveguide, where the real parts of the mode refractive indices are 3.4 and -3.4 for the fundamental and the harmonic waves respectively. The observed peak conversion efficiency at the excitation wavelength of ∼780 nm indicates the fulfilment of the phase-matching condition of k(2ω) = 2k(ω) and n(2ω) = -n(ω), where the coherent harmonic wave emerges along a direction opposite to that of the incoming fundamental light.

Metamaterials enable chiral-selective enhancement of two-photon luminescence from quantum emitters.

The amplification of chirally modified, non-linear signals from quantum emitters is demonstrated by manipulating the geometric chirality of resonant plasmonic nanostructures. The chiral center of the metamaterial is opened and emitters occupy this light-confining and chirally sensitive region. Non-linear emission signals are enhanced by 40× that of the emitters not embedded in the metamaterial and display a 3× contrast for the opposite circular polarization.

Electrifying photonic metamaterials for tunable nonlinear optics.

Metamaterials have not only enabled unprecedented flexibility in producing unconventional optical properties that are not found in nature, they have also provided exciting potential to create customized nonlinear media with high-order properties correlated to linear behaviour. Two particularly compelling directions are active metamaterials, whose optical properties can be purposely tailored by external stimuli in a reversible manner, and nonlinear metamaterials, which enable intensity-dependent frequency conversion of light waves. Here, by exploring the interaction of these two directions, we leverage the electrical and optical functions simultaneously supported in nanostructured metals and demonstrate electrically controlled nonlinear optical processes from a metamaterial. Both second harmonic generation and optical rectification, enhanced by the resonance behaviour in the metamaterial absorber, are modulated externally with applied voltage signals. Our results reveal an opportunity to exploit optical metamaterials as self-contained, dynamic electro-optic systems with intrinsically embedded electrical functions and optical nonlinearities.

Nonlinear imaging and spectroscopy of chiral metamaterials.

A chiral metamaterial produces both distinguishable linear and non-linear resonant features when probed with left and right circularly polarized light. The material demonstrates a linear transmission contrast of 0.5 between left and right circular polarizations and a 20× contrast between second-harmonic responses from the two incident polarizations. Non-linear and linear response images probed with circularly polarized light show strongly defined contrast.

Giant chiral optical response from a twisted-arc metamaterial.

We demonstrate enormously strong chiral effects from a photonic metamaterial consisting of an array of dual-layer twisted-arcs with a total thickness of ∼ λ/6. Experimental results reveal a circular dichroism of ∼ 0.35 in the absolute value and a maximum polarization rotation of ∼ 305°/λ in a near-infrared wavelength region. A transmission of greater than 50% is achieved at the frequency where the polarization rotation peaks. Retrieved parameters from measured quantities further indicate an actual optical activity of 76° per λ and a difference of 0.42 in the indices of refraction for the two circularly polarized waves of opposite handedness.

Application of dynamic line narrowing in resonant optical sensing.

We propose a dynamic operational mode and the resulting dynamic line narrowing as a method for enhancing the resolution and the detection limit of high-quality (high-Q) resonant optical sensors. Using a silica microtoroid as an experimental platform, we demonstrate that dynamic line narrowing through the thermo-optic effect can significantly improve the detection limit in both resonant shift and resonance splitting operating modes.

Measurement of the refractive index of human teeth by optical coherence tomography.

We describe a novel method based on optical coherence tomography (OCT) for the accurate measurement of the refractive index of in vitro human teeth. We obtain the refractive indices of enamel, dentin, and cementum to be 1.631+/-0.007, 1.540+/-0.013, and 1.582+/-0.010, respectively. The profile of the refractive index is readily obtained via an OCT B scan across a tooth. This method can be used to study the refractive index changes caused by dental decay and therefore has great potential for the clinical diagnosis of early dental caries.