Metasurfaces are engineered interfaces that contain a thin layer of plasmonic or dielectric nanostructures capable of manipulating light in a desirable manner. Advances in metasurfaces have led to various practical applications ranging from lensing to holography. Metasurface holograms that can be switched by the polarization state of incident light have been demonstrated for achieving polarization multiplexed functionalities. However, practical application of these devices has been limited by their capability for achieving high efficiency and high image quality. Here we experimentally demonstrate a helicity multiplexed metasurface hologram with high efficiency and good image fidelity over a broad range of frequencies. The metasurface hologram features the combination of two sets of hologram patterns operating with opposite incident helicities. Two symmetrically distributed off-axis images are interchangeable by controlling the helicity of the input light. The demonstrated helicity multiplexed metasurface hologram with its high performance opens avenues for future applications with functionality switchable optical devices.

The capability of locally engineering the nonlinear optical properties of media is crucial in nonlinear optics. Although poling is the most widely employed technique for achieving locally controlled nonlinearity, it leads only to a binary nonlinear state, which is equivalent to a discrete phase change of π in the nonlinear polarizability. Here, inspired by the concept of spin-rotation coupling, we experimentally demonstrate nonlinear metasurfaces with homogeneous linear optical properties but spatially varying effective nonlinear polarizability with continuously controllable phase. The continuous phase control over the local nonlinearity is demonstrated for second and third harmonic generation by using nonlinear metasurfaces consisting of nanoantennas of C3 and C4 rotational symmetries, respectively. The continuous phase engineering of the effective nonlinear polarizability enables complete control over the propagation of harmonic generation signals. Therefore, this method seamlessly combines the generation and manipulation of harmonic waves, paving the way for highly compact nonlinear nanophotonic devices.

To construct PSOEs, two anisotropic substrates [ 7,8 ] or a structure with form-birefringent [ 9,10 ] have been traditionally used to impose the desired phase delay for the two cross-polarizations of incident light. Although these devices prove to be effective, it is challenging for the etching depth control especially when some extreme phase profi les are needed. Besides, these devices suffer from the limited phase levels and the large pixel size due to the current fabrication techniques.Benefi ting from the easy fabrication and the fl exible control of the light propagation, metasurfaces have provided new opportunities to realize virtually fl at optics. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] In comparison with the traditional PSOEs, which realize the birefringence by carefully designing the structure of each pixel, the metasurface provides a much easier way by directly merging different optical elements together with each one working for a particular incident polarization. Refl ective-type metasurface holograms have been demonstrated to reconstruct two interchangeable images [ 27,28 ] by controlling the linear polarization state of the incident light. Although the previously proposed metasurfaces have shown their unique advantages such as broadband [ 27 ] and wide fi eld of view, [ 28 ] further application is hindered by the limited phase levels [ 27 ] and binary modulation [ 28 ] of the holograms since most optical elements such as a lens or an axicon are required to precisely manipulate the phase of the wavefront.Driven by miniaturization and system integration, ultrathin, multifunction optical elements are urgently needed. Traditional polarization-selective optical elements are mainly based on birefringence, which is realized by using the well-designed structure of each phase pixel. However, further reduction of the pixel size and improvement of the phase levels are hindered by the complicated fabrication process. An approach is proposed to realize a metasurface device that possesses two distinct functionalities. The designed metasurface device, consisting of gold nanorods with spatially varying orientation, has been experimentally demonstrated to function as either a lens or a hologram, depending on the helicity of the incident light. As the phase of the scattered light is controlled by the orientation of the nanorods, arbitrary phase levels and dispersionless phase profi le can be realized through a much simpler fabrication process than the conventional device. This approach provides an unconventional alternative to realize multifunction optical element, dramatically increasing the functionality density of the optical systems.

Studies of elastohydrodynamic lubrication (EH L) frequently utilize the phenomenon of optical interference which is exhibited when an illuminated steel ball is in contact with a partially Crcoated glass disc in conjunction with a lubricant layer of known refractive index. The use of the steel and chromium surfaces enables lubricants having simila r refractive indices to glass to be examined. The EH L contact is composed of two microscopically thin layers: the Cr coating and the lubricant ®lm, bounded by the glass disc and the steel ball. That the two-beam interference method frequently used in optical EH L research may induce signi®cant errors in the measurement of ®lm thickness is reported in this paper. A theoretical analysis which includes consideration of the effects of both multibeam interference and optical absorption within the Cr ®lm and the steel surface is presented. The analysis is used to devise a multi-b eam intensity-based test technique which has been validated for a wide range of lubricant ®lm thickness measurements at a resolution of 1 nm and offers a practical minimum detectable ®lm thickness as thin as 1 nm.

This paper presents a deliberately designed elastohydrodynamical lubrication (EHL) experiment for the study of the individual effect of the limiting shear stress and wall slippage. Very slow entrainment speeds were employed to avoid influential shear heating and oils of high viscosities were chosen to ensure that the conjunction was under typical EHL. An anomalous EHL film, characterized by a dimple at the inlet region, was obtained. Literature revealed that this inlet dimple was reported in some numerical studies taking into consideration the limiting-shear-stress characteristics of the lubricant and wall slippage. It was found that even under the same kinematic conditions, different types of film shape would be generated by simple disc sliding and simple ball sliding. Simple disc sliding produces an inlet dimple with a comparatively thick inlet film thickness, which droops rapidly toward the outlet region. For simple ball sliding, there is also an inlet dimple but the central film thickness is rather uniform. However, by prerunning the conjunction at a zero entrainment velocity (at the same linear speeds but in opposite directions) before the sliding experiment, the slope of the central film of simple disc sliding becomes smaller. It is probably due to the modification of solid-liquid interface, i.e., the slippage level, by the highly pressurized and stressed prerunning conditions. With a prescribed prerunning, which can produce very similar films at simple disc sliding and simple ball sliding, variation of film thickness was studied and it was found that the inlet dimple film has obvious dependence on entrainment speeds, but was not sensitive to loads. The present experimental results can be considered as direct evidence for those numerical findings of the inlet dimple. Tentatively, an effective viscosity wedge is proposed to account for the formation of the inlet dimple.