Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI3.

Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI3) has been shown to be a layered antiferromagnetic insulator in its few-layer form1, opening up opportunities for various functionalities2-7 in electronic and optical devices. Here we report an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI3. The observed second-harmonic generation (SHG; a nonlinear optical process that converts two photons of the same frequency into one photon of twice the fundamental frequency) is several orders of magnitude larger than known magnetization-induced SHG8-11 and comparable to the SHG of the best (in terms of nonlinear susceptibility) two-dimensional nonlinear optical materials studied so far12,13 (for example, molybdenum disulfide). We show that although the parent lattice of bilayer CrI3 is centrosymmetric, and thus does not contribute to the SHG signal, the observed giant nonreciprocal SHG originates only from the layered antiferromagnetic order, which breaks both the spatial-inversion symmetry and the time-reversal symmetry. Furthermore, polarization-resolved measurements reveal underlying C2h crystallographic symmetry-and thus monoclinic stacking order-in bilayer CrI3, providing key structural information for the microscopic origin of layered antiferromagnetism14-18. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices.

Contrast-enhanced phase-resolved second harmonic generation microscopy.

The characterization of inverted structures (crystallographic, ferroelectric, or magnetic domains) is crucial in the development and application of novel multi-state devices. However, determining these inverted structures needs a sensitive probe capable of revealing their phase correlation. Here a contrast-enhanced phase-resolved second harmonic generation (SHG) microscopy is presented, which utilizes a phase-tunable Soleil-Babinet compensator and the interference between the SHG fields from the inverted structures and a homogeneous reference. By this means, such inverted structures are correlated through the π-phase difference of SHG, and the phase difference is ultimately converted into the intensity contrast. As a demonstration, we have applied this microscopy in two scenarios to determine the inverted crystallographic domains in two-dimensional van der Waals material MoS2. Our method is particularly suitable for applying in vacuum and cryogenic environments while providing optical diffraction-limited resolution and arbitrarily adjustable contrast. Without loss of generality, this contrast-enhanced phase-resolved SHG microscopy can also be used to resolve other non-centrosymmetric inverted structures, e.g. ferroelectric, magnetic, or multiferroic phases.

Indirect spectrum measurement via random phase modulation and detection in temporal domain.

Spectroscopy continues to provide possibilities for a deeper understanding of fundamental physical phenomena. Traditional spectral measurement method, dispersive Fourier transformation, is always limited by its realization condition (detection in the temporal far-field). Inspired by Fourier ghost imaging, we put forward an indirect spectrum measurement to overcome the limitation. The spectrum information is reconstructed via random phase modulation and near-field detection in the time domain. Since all operations are realized in the near-field region, the required length of dispersion fiber and optical loss are greatly reduced. Considering the application in spectroscopy, the length of required dispersion fiber, the spectrum resolution, the range of spectrum measurement and the requirement on bandwidth of photodetector are investigated.

Morphology and statistics of wide-spectrum speckles.

Although the theory of scattered speckles was initially established via idealization of treating the incident light as monochromatic, phenomenon and regulations of wide-spectrum speckles are yet urgent to be studied, with immense growing applications of broadband source such as femtosecond laser, light-emitting-diode and sunlight illumination. Here we quantitatively analyze the morphology and statistics of speckles produced by a point-like source with wide-spectrum, using a phase plate model to describe the scattering layer. Due to differences in induced phase related to wavelength, wide-spectrum speckle patterns appear radial divergence in intensity distribution, as well as in visibility of both speckles and that of the second-order coherence. This is significantly different from the translation-invariance of monochromatic speckles. The spatially-varying morphology and statistics of the speckles contain spatial and spectral information of the incidence, thus can be used as an indicator to achieve optical metrology or sensing with a wide-spectrum source in the scattering environment.

Selective excitation of four-wave mixing by helicity in gated graphene.

Gapless Dirac fermions in monolayer graphene give rise to an abundance of peculiar physical properties, including exceptional broadband nonlinear optical responses. By tuning the chemical potential, stacking order, and photonic structures, the effective modulation of nonlinear optical phenomena in graphene has been demonstrated in recent years. Here, we demonstrate that optical helicity can be used as an extra tuning knob for four-wave mixing in gated graphene. Our results reveal the helicity selection rule for four-wave mixing in monolayer graphene, revealing nearly perfect circular polarization. Corresponding theoretical interpretations of the helicity selection rule that are also applicable to other nonlinear optical processes and materials are presented.

Probing the Chiral Domains and Excitonic States in Individual WS2 Tubes by Second-Harmonic Generation.

Distinct from carbon nanotubes, transition-metal dichalcogenide (TMD) nanotubes are noncentrosymmetric and polar and can exhibit some intriguing phenomena such as nonreciprocal superconductivity, chiral shift current, bulk photovoltaic effect, and exciton-polaritons. However, basic characterizations of individual TMD nanotubes are still quite limited, and much remains unclear about their structural chirality and electronic properties. Here we report an optical second-harmonic generation (SHG) study on multiwalled WS2 nanotubes on a single-tube level. As it is highly sensitive to the crystallographic symmetry, SHG microscopy unveiled multiple structural domains within a single WS2 nanotube, which are otherwise hidden under conventional white-light optical microscopy. Moreover, the polarization-resolved SHG anisotropy patterns revealed that different domains on the same tube can be of different chirality. In addition, we observed the excitonic states of individual WS2 nanotubes via SHG excitation spectroscopy, which were otherwise difficult to acquire due to the indirect band gap of the material.

Enhancing robustness of ghost imaging against environment noise via cross-correlation in time domain.

Research towards practical applications of ghost imaging attracts more and more attention in recent years. Signal-to-noise ratio (SNR) of bucket results thus quality of images can be greatly affected by environmental noise, such as strong background light. We introduce temporal cross-correlation into typical ghost imaging to improve SNR of bucket value, taking temporal profile of illumination pulses as a prior information. Experimental results at sunny noontime verified our method, with the imaging quality greatly improved for the object at a distance of 1.3km. We also show the possibility of 3-dimensional imaging, experimentally.

Narrowband nonlinear optical spectroscopy with spatially chirped broadband pulses.

Nonlinear optical vibrational spectroscopies are powerful experimental tools for inspecting material properties that are difficult to acquire otherwise. As ultrafast lasers used in such experiments are typically of much broader bandwidth than vibrational modes, narrowband filtering is usually essential, and the utility of laser energy is often highly inefficient. Here we introduce an experimental scheme to break this trade-off. A broadband beam is spatially chirped as it reaches the sample, and generates sum-frequency signals upon overlapping with another broadband, unchirped beam. A narrowband spectrum can then be retrieved from the spatially dispersed image of signals, with both broadband pulses fully utilized. The scheme is also readily employed as a spatially resolved spectroscopy technique without scanning, and can be easily extended to other wave-mixing experiments.

Influence of pulse characteristics on ghost imaging lidar system.

A pulsed pseudo-thermal light source obtained using a rotating ground glass disk, spatial light modulator, or digital micromirror device is widely used in a ghost imaging (GI) lidar system. The property of the pulsed pseudothermal light field determines the reconstruction quality of the image in the GI lidar system, which depends on the pulse extinction ratio (PER) and pulse duty ratio. In this paper, pseudo-thermal light fields obtained at different pulse characteristics are given, taking into account the influence of the exposure time of the charge-coupled device (CCD) camera. The statistical distribution, contrast, and normalized intensity correlated function of the pseudo-thermal light field at different pulse characteristics are analyzed quantitatively for what we believe is the first time. Then the peak signal-to-noise ratio of the reconstructed image using a GI algorithm and a differential ghost imaging (DGI) algorithm is numerically simulated. The simulation results demonstrate that the PSNR decreases as the PER decreases, which is affected by the pulse duty ratio and the CCD exposure time. The deterioration of the reconstruction quality can be reduced by using a DGI algorithm or by shorting the exposure time of the CCD in the GI lidar system.

Facet-specific interaction between methanol and TiO2 probed by sum-frequency vibrational spectroscopy.

The facet-specific interaction between molecules and crystalline catalysts, such as titanium dioxides (TiO2), has attracted much attention due to possible facet-dependent reactivity. Using surface-sensitive sum-frequency vibrational spectroscopy, we have studied how methanol interacts with different common facets of crystalline TiO2, including rutile(110), (001), (100), and anatase(101), under ambient temperature and pressure. We found that methanol adsorbs predominantly in the molecular form on all of the four surfaces, while spontaneous dissociation into methoxy occurs preferentially when these surfaces become defective. Extraction of Fermi resonance coupling between stretch and bending modes of the methyl group in analyzing adsorbed methanol spectra allows determination of the methanol adsorption isotherm. The isotherms obtained for the four surfaces are nearly the same, yielding two adsorbed Gibbs free energies associated with two different adsorption configurations singled out by ab initio calculations. They are (i) ∼-20 kJ/mol for methanol with its oxygen attached to a low-coordinated surface titanium, and (ii) ∼-5 kJ/mol for methanol hydrogen-bonded to a surface oxygen and a neighboring methanol molecule. Despite similar adsorption energetics, the Fermi resonance coupling strength for adsorbed methanol appears to depend sensitively on the surface facet and coverage.

Denoising ghost imaging under a small sampling rate via deep learning for tracking and imaging moving objects.

Ghost imaging (GI) usually requires a large number of samplings, which limit the performance especially when dealing with moving objects. We investigated a deep learning method for GI, and the results show that it can enhance the quality of images with the sampling rate even down to 3.7%. With a convolutional denoising auto-encoder network trained with numerical data, blurry images from few samplings can be denoised. Then those outputs are used to reconstruct both the trajectory and clear image of the moving object via cross-correlation based GI, with the number of required samplings reduced by two-thirds.

Tuning the optical nonlinearity of graphene.

Tuning of nonlinear optical responses is the essence to many photonics and optoelectronics applications. Due to the low-dimensionality and dispersion of massless Dirac Fermions, the nonlinear optical susceptibilities of graphene can be readily controlled via electrical gating. Based on the quantum interference between multi-photon transition pathways, the tuning mechanism of graphene nonlinearity is intrinsically different from most other systems. The phenomenon enables investigations into some nonlinear optical processes from fundamental regards. It also exhibits appealing features contrasting conventional materials, which can be desirable for novel device applications.

Ghost imaging utilizing experimentally acquired degree of linear polarization with no prior information.

Ghost imaging is developed to obtain images of the objects based on intensity correlation of illumination patterns. However, it can be hard to distinguish between objects if the difference between their reflectivities is small. Considering the difference between degrees of polarization in the reflected light from different points, we put forward a method to retrieve distribution of the degree of linear polarization, and obtain high quality image of the objects. With the illumination source being linearly polarized, two orthogonal polarization components of the reflected intensities are measured, from which we can get the distribution of the degree of linear polarization. Furthermore, for the case that the degree of linear polarization can be approximately described with two different values within the field of view, we demonstrate retrieving of the image with high contrast. Our method can be widely applied in different situations, such as extracting the image of target hidden behind disguise or getting higher contrast in bio-imaging.

Gradual ghost imaging of moving objects by tracking based on cross correlation.

The requirement of a large number of samplings limits the performance of ghost imaging for moving objects. Conventionally, tracking and imaging of the moving objects are done independently; thus, sequential clear images of the moving target during its evolution are required. In this Letter, we propose to obtain the displacement of the object via cross correlation between sequential unclear rough images. Then, a high-quality image of the moving object can be reconstructed gradually during its evolution. Our method works well for translating and rotating objects.

Chiral selection rules for multi-photon processes in two-dimensional honeycomb materials.

We examine the chirality-dependent optical selection rules in two-dimensional monolayer materials with honeycomb lattices, and, based on symmetry argument, we generalize these rules to multi-photon transitions of arbitrary orders. We also present the phase relations between incident and outgoing photons in such processes. The results agree nicely with our experimental observations of second- and third-harmonic generation. In particular, we demonstrate that the phase relation of chiral second-harmonic generation can serve as a handy tool for imaging domains and domain boundaries of these monolayers. Our results can benefit future studies on chirality-related optical phenomena and opto-electronic applications of such materials.

Ghost imaging normalized by second-order coherence.

Towards improvements in the quality of reconstructed images, the errors in the point spread function of a ghost imaging system caused by a limited number of samplings and imperfect illumination are discussed. We propose an algorithm by normalizing with the second-order coherence of the illumination field, with which the errors caused by imperfect illumination can be reduced, such as non-uniform spatial distribution of the average intensity, spatially varying profile of the second-order degree of coherence, or power fluctuation.

Doping-Induced Second-Harmonic Generation in Centrosymmetric Graphene from Quadrupole Response.

For centrosymmetric materials such as monolayer graphene, no optical second-harmonic generation (SHG) is generally expected, because it is forbidden under the electric-dipole approximation. Yet we observe a strong, doping-induced SHG from graphene, with its highest strength comparable to the electric-dipole-allowed SHG in noncentrosymmetric 2D materials. This novel SHG has the nature of an electric-quadrupole response, arising from the effective breaking of inversion symmetry by optical dressing with an in-plane photon wave vector. More remarkably, the SHG is widely tuned by carrier doping or chemical potential, being sharply enhanced at Fermi-edge resonances but vanishing at the charge neutral point that manifests the electron-hole symmetry of massless Dirac fermions. This striking behavior in graphene, which should also arise in graphenelike Dirac materials, expands the scope of nonlinear optics and holds the promise of novel optoelectronic and photonic applications.

Structural variation of anatase (101) under near infrared irradiations monitored by sum-frequency surface phonon spectroscopy.

We probed the anatase (101) surface irradiated by near-infrared and infrared photons in different ambient gases by monitoring the surface lattice phonon mode using sum-frequency spectroscopy. We found that even under the irradiation of such low energy photons, the stability of surface oxygen vacancies, in comparison to sub-surface oxygen vacancies, can increase sensibly. The variation of this surface phonon mode is also in accordance with the photo-induced hydrophilicity of titanium oxide surfaces, which may provide the microscopic insight into this phenomenon.

Probing Phonon Dynamics in Individual Single-Walled Carbon Nanotubes.

Interactions between elementary excitations, such as carriers, phonons, and plasmons, are critical for understanding the optical and electronic properties of materials. The significance of these interactions is more prominent in low-dimensional materials and can dominate their physical properties due to the enhanced interactions between these excitations. One-dimensional single-walled carbon nanotubes provide an ideal system for studying such interactions due to their perfect physical structures and rich electronic properties. Here we investigated G-mode phonon dynamics in individual suspended chirality-resolved single-walled carbon nanotubes by time-resolved anti-Stokes Raman spectroscopy. The improved technique allowed us to probe the intrinsic phonon information on a single-tube level and exclude the influences of tube-tube and tube-substrate interactions. We found that the G-mode phonon lifetime ranges from 0.75-2.25 ps and critically depends on whether the tube is metallic or semiconducting. In comparison with the phonon lifetimes in graphene and graphite, we revealed structure-dependent carrier-phonon and phonon-phonon interactions in nanotubes. Our results provide new information for optimizing the design of nanotube electronic/optoelectronic devices by better understanding and utilizing their phonon decay channels.

Gate Switching of Ultrafast Photoluminescence in Graphene.

The control of optical properties by electric means is the key to optoelectronic applications. For atomically thin two-dimensional (2D) materials, the natural advantage lies in that the carrier doping could be readily controlled through the electric gating effect, possibly affecting the optical properties. Exploiting this advantage, here we report the gate switching of the ultrafast upconverted photoluminescence from monolayer graphene. The luminescence can be completely switched off by the Pauli-blocking of one-photon interband transition in graphene with an on/off ratio exceeding 100, which is remarkable compared to other 2D semiconductors and 3D bulk counterparts. The chemical potential and pump fluence dependences of the luminescence are nicely described by a two-temperature model, including both the hot carrier dynamics and carrier-optical phonon interaction. This gate switchable and background-free photoluminescence can open up new opportunities for graphene-based ultrafast optoelectronic applications.