Full description of dipole orientation in organic light-emitting diodes

Considerable progress has been made in organic light-emitting diodes(OLEDs)to achieve high external quantum efficiency,among which dipole orientation has a remarkable effect.In most cases,the radiation of the dipoles in OLEDs is theoretically predicted with only one orientation parameter to match with corresponding experiments.Here,we develop a new theory with three orientation parameters to fully describe the relationship between dipole orientation and power density.Furthermore,we design an optimal test structure for measuring all three orientation parameters.All three orientation parameters could be retrieved from non-polarized spectra.Our theory provides a universal plot of dipole orientations in OLEDs,paving the way for designing more complicated OLED devices.

Phase characterisation of metalenses

Metalenses have emerged as a new optical element or system in recent years,showing superior performance and abundant applications.However,the phase distribution of a metalens has not been measured directly up to now,hindering further quantitative evaluation of its performance.We have developed an interferometric imaging phase measurement system to measure the phase distribution of a metalens by taking only one photo of the interference pattern.Based on the measured phase distribution,we analyse the negative chromatic aberration effect of monochromatic metalenses and propose a feature size of metalenses.Different sensitivities of the phase response to wavelength between the Pancharatnam-Berry phase-based metalens and propagation phase-reliant metalens are directly observed in the experiment.Furthermore,through phase distribution analysis,it is found that the distance between the measured metalens and the brightest spot of focusing will deviate from the focal length when the metalens has a low nominal numerical aperture,even though the metalens is ideal without any fabrication error.We also use the measured phase distribution to quantitatively characterise the imaging performance of the metalens.Our phase measurement system will help not only designers optimise the designs of metalenses but also fabricants distinguish defects to improve the fabrication process,which will pave the way for metalenses in industrial applications.

Routing valley exciton emission of a WS_(2) monolayer via delocalized Bloch modes of in-plane inversion-symmetry-broken photonic crystal slabs

The valleys of two-dimensional transition metal dichalcogenides(TMDCs)offer a new degree of freedom for information processing.To take advantage of this valley degree of freedom,on the one hand,it is feasible to control valleys by utilizing different external stimuli,such as optical and electric fields.On the other hand,nanostructures are also used to separate the valleys by near-field coupling.However,for both of the above methods,either the required low-temperature environment or low degree of coherence properties limit their further applications.Here,we demonstrate that all-dielectric photonic crystal(PhC)slabs without in-plane inversion symmetry(C_(2) symmetry)can separate and route valley exciton emission of a WS2 monolayer at room temperature.Coupling with circularly polarized photonic Bloch modes of such PhC slabs,valley photons emitted by a WS_(2) monolayer are routed directionally and are efficiently separated in the far field.In addition,far-field emissions are directionally enhanced and have long-distance spatial coherence properties.

Momentum-space imaging spectroscopy for the study of nanophotonic materials

The novel phenomena in nanophotonic materials, such as the angle-dependent reflection and negative refraction effect, are closely related to the photonic dispersions EepT. EepT describes the relation between energy E and momentum p of photonic eigenmodes, and essentially determines the optical properties of materials. As EepT is defined in momentum space(k-space), the experimental method to detect the energy distribution, that is the spectrum, in a momentum-resolved manner is highly required. In this review, the momentum-space imaging spectroscopy(MSIS) system is presented, which can directly study the spectral information in momentum space. Using the MSIS system, the photonic dispersion can be captured in one shot with high energy and momentum resolution. From the experimental momentumresolved spectrum data, other key features of photonic eigenmodes, such as quality factors and polarization states, can also be extracted through the post-processing algorithm based on the coupled mode theory. In addition, the interference configurations of the MSIS system enable the measurement of coherence properties and phase information of nanophotonic materials, which is important for the study of light-matter interaction and beam shaping with nanostructures. The MSIS system can give the comprehensive information of nanophotonic materials, and is greatly useful for the study of novel photonic phenomena and the development of nanophotonic technologies.

Photonic-dispersion neural networks for inverse scattering problems

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 neuralnetworks-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.

Thin-film neural networks for optical inverse problem

The thin-film optical inverse problem has attracted a great deal of attention in science and industry,and is widely applied to optical coatings.However,as the number of layers increases,the time it takes to extract the parameters of thin films drastically increases.Here,we introduce the idea of exploiting the structural similarity of all-optical neural networks and applied it to the optical inverse problem.We propose thin-film neural networks(TFNNs)to efficiently adjust all the parameters of multilayer thin films.To test the performance of TFNNs,we implemented a TFNN algorithm,and a reflectometer at normal incidence was built.Operating on multilayer thin films with 232 layers,it is shown that TFNNs can reduce the time consumed by parameter extraction,which barely increased with the number of layers compared with the conventional method.TFNNs were also used to design multilayer thin films to mimic the optical response of three types of cone cells in the human retina.The light passing through these multilayer thin films was then recorded as a colored photo.

Metalenses phase characterization by multidistance phase retrieval

Metalens,characterized by their unique functions and distinctive physical properties,have gained significant attention for their potential applications.To further optimize the performance of metalens,it is necessary to characterize the phase modulation of the metalens.In this study,we present a multi-distance phase retrieval system based on optical field scanning and discuss its convergence and robustness.Our findings indicate that the system is capable of retrieving the phase distribution of the metalens as long as the measurement noise is low and the total length of the scanned light field is sufficiently long.This system enables the analysis of focal length and aberration by utilizing the computed phase distribution.We extend our investigation to measure the phase distribution of the metalens operating in the near-infrared(NIR)spectrum and identify the impact of defects in the sample on the phase.Additionally,we conduct a comparative analysis of the phase distribution of the metalens in air and ethanol and observe the variations in the phase modulation of the metalens in different working mediums.Our system provides a straightforward method for the phase characterization of metalens,aiding in optimizing the metalens design and functionality.