Circular Dammann gratings for enhanced control of the ring profile of perfect optical vortices

Perfect optical vortices(POVs)provide a solution to address the challenge induced by strong dependence of classical optical vortices on their carried topological charges.However,traditional POVs are all shaped into bright rings with a single main lobe along the radial direction.Here we propose a method for enhanced control on the ring profile(the radial intensity profile of circular rings)of POVs based on modulated circular sine/cosine radial functions,which is realized by a circular Dammann grating embedded with a spiral phase.Specifically,a type of"absolute"dark POVs surrounded by two bright lobe rings in each side is presented,which provides a perfect annular potential well along those dark impulse rings for trapping low-index particles,cells,or quantum gases.In addition,several POVs with different ring profiles,including conventional POVs with bright rings,the dark POVs mentioned above,and also POVs with tunable ring profiles,are demonstrated.This work opens up new possibilities to controllably tune the ring profile of perfect vortices,and this type of generalized POVs will enrich the content of singular optics and expand the application scope of perfect vortices in a range of areas including optical manipulation,both quantum and classical optical communications,enhanced optical imaging,and also novel structured pumping lasers.

Imaging method of single layer graphene on metal substrate based on imaging ellipsometer with large field of view

Single layer lattice graphene deposited on the metal substrate can hardly be imaged by the optical microscope. In this Letter, a large field-of-view imaging ellipsometer is introduced to image single layer graphene which is deposited on a metal substrate. By adjusting the polarizer and the analyzer of imaging ellipsometer, the light reflected from surfaces of either single layer graphene or a Au film substrate can be extinguished, respectively.Thus, single layer graphene can be imaged correspondingly under brightfield or darkfield imaging modes. The method can be applied to imaging large-area graphene on a metal substrate.

Simplified mode analysis of guided mode resonance gratings with asymmetric coatings

A simplified modal method to explain the resonance phenomenon in guided mode resonance （GMR） grat- ings with asymmetric coatings is presented. The resonance observed is due to the interaction of two propagation modes inside the grating. The reflectivity spectra and electric field distributions calculated from the simplified modal method are compared using rigorous coupled-wave analysis （RCWA）. The in- fluences of high-order evanescent modes on the resonance peak are analyzed. A matrix Fabry-Perot （FP） resonance condition is developed to evaluate the resonance wavelength. An explanation for the resonance phenomenon observed based on the FP resonance phase condition is also proposed and demonstrated. The simplified method provides clear physical insights into CMR gratings that are useful for the analysis of a variety of other resonance gratings.

Modal analysis of the 0th order nulled phase masks

A simple modal analysis （MA） method to explain the diffraction process of 0th order nulled phase mask is presented. In MA, multiple reflections of the grating modes at grating interfaces are considered by introducing equivalent Fresnel coefficients. Analytical expressions of the diffraction efficiencies and modal guidelines for the 0th order hulled phase grating design are also presented. The phase mask structure, which comprises a high-index contrast HfO2 grating and a fused-silica substrate, is optimized using rigorous coupled-wave analysis around the 800-nm wavelength, after which the modal guideline for cancellation of the 0th order in a phase mask is verified. The proposed MA method illustrates the inherent physical mechanism of multiple reflections of the grating modes in the diffraction process, which can help to analyze and design both low-contrast and high-contrast gratings.

Two-wavelength sinusoidal phase-modulating interferometer for nanometer accuracy measurement

A two-wavelength sinusoidal phase-modulating （SPM） laser diode （LD） interferometer for nanometer accu- racy measurement is proposed. To eliminate the error caused by the intensity modulation, the SPM depth of the interference signal is chosen appropriately by varying the amplitude of the modulation current pe- riodically. Then, the refine theory is induced to the measurement, and the two-wavelength interferometer （TWI） is combined with the single-wavelength LD interferometric technique to realize static displacement measurement with nanometer accuracy. Experimental results indicate that a static displacement measure- ment accuracy of 5 nm can be achieved over a range of 200 μm.