Yaoyu Cao

Diatomic Metasurface for Vectorial Holography

The emerging metasurfaces with the exceptional capability of manipulating an arbitrary wavefront have revived the holography with unprecedented prospects. However, most of the reported metaholograms suffer from limited polarization controls for a restrained bandwidth in addition to their complicated meta-atom designs with spatially variant dimensions. Here, we demonstrate a new concept of vectorial holography based on diatomic metasurfaces consisting of metamolecules formed by two orthogonal meta-atoms. On the basis of a simply linear relationship between phase and polarization modulations with displacements and orientations of identical meta-atoms, active diffraction of multiple polarization states and reconstruction of holographic images are simultaneously achieved, which is robust against both incident angles and wavelengths. Leveraging this appealing feature, broadband vectorial holographic images with spatially varying polarization states and dual-way polarization switching functionalities have been demonstrated, suggesting a new route to achromatic diffractive elements, polarization optics, and ultrasecure anticounterfeiting.

Improved lateral resolution with an annular vortex depletion beam in STED microscopy

We report on the experimental demonstration of improved lateral resolution in stimulated emission depletion (STED) microscopy using an annular depletion beam configuration. A tight and finely tuned doughnut focal spot can be created by annular vortex illumination. Its application in STED microscopy for enhanced lateral resolution is systematically investigated by imaging 40xa0nm fluorescent beads. An improved resolution with more than 20% reduced effective point spread function of the imaging system determined by the full width at half-maximum compared to that of the conventional STED is achieved. The proposed scheme also demonstrates its resolving capability for biological samples. The principle holds great potential in the research fields of biological superresolution imaging as well as STED-based nanolithography and high-density optical data storage.

Generation of uniformly oriented in-plane magnetization with near-unity purity in 4π microscopy

In this Letter, we numerically demonstrate the all-optical generation of uniformly oriented in-plane magnetization with near-unity purity (more than 99%) under a 4π microscopic configuration. This is achieved through focusing two counter-propagating vector beams consisting of coherently configured linear and radial components. Based on the Debye diffraction theory, constructive and destructive interferences of the focal field components can be tailored under the 4π configuration to generate high-purity uniformly polarized transverse and longitudinal electric-field components in the center of the focal region. Consequently, near-unity purity in-plane magnetization with a uniform orientation within the focal volume defined by the full width at half-maximum can be created through the inverse Faraday effect. In addition, it reveals that the purity of the in-plane magnetization is robust against the numerical aperture of the focal lens. This result expands the flexibility of magnetization manipulations through light and holds great potential in all-optical magnetic recording and spintronics.

Bidirectional Plasmonic Coloration with Gold Nanorods by Wavelength-Switched Photoredox

Reversible tuning of localized plasmon resonance that is supported by nanometric-sized metal particles holds potentially huge benefits in terms of manipulating light for widespread photonic applications. Although the practice of altering the frequency and the amplitude of plasmon resonance on gold nanoparticles is relatively well established, dynamic tuning by all-optical approaches for coloration has long been hindered due to limited implementation approaches with which gold nanomaterials can be photosynthetically manipulated. Here, we develop a wavelength-switched photoredox approach and demonstrate bidirectional tuning of the plasmonic resonance of crystalline gold nanoparticles for reversible surface-plasmon-resonance-based coloration. The reversible plasmonic resonance control is achieved by a combination of photoreduction of gold ions and photooxidation of gold nanorods by switching the illumination between UV and near-UV-Vis light, respectively. As one example, the plasmon resonance peak of gold nanorods is reversibly tuned between 630 and 660 nm by switching the light wavelengths. Utilizing wavelength-switchable photoredox reactions, we demonstrate reversible color patterning by mask illuminating a gold nanorod sample solution. This approach offers not only an easy-to-implement method for realizing non-contact modulating plasmon-resonance based colors, but also new opportunities for reversibly tuning local plasmon resonance by all-optically shaping single nanoparticles. This holds great potential for a wide range of applications, including active-substrate-based surface-enhanced Raman scattering (SERS), erasable optical data storage and dynamic laser color printing, among others.

Facile metagrating holograms with broadband and extreme angle tolerance

The emerging meta-holograms rely on arrays of intractable meta-atoms with various geometries and sizes for customized phase profiles that can precisely modulate the phase of a wavefront at an optimal incident angle for given wavelengths. The stringent and band-limited angle tolerance remains a fundamental obstacle for their practical application, in addition to high fabrication precision demands. Utilizing a different design principle, we determined that facile metagrating holograms based on extraordinary optical diffraction can allow the molding of arbitrary wavefronts with extreme angle tolerances (near-grazing incidence) in the visible–near-infrared regime. By modulating the displacements between uniformly sized meta-atoms rather than the geometrical parameters, the metagratings produce a robust detour phase profile that is irrespective of the wavelength or incident angle. The demonstration of high-fidelity meta-holograms and in-site polarization multiplexing significantly simplifies the metasurface design and lowers the fabrication demand, thereby opening new routes for flat optics with high performances and improved practicality.Metasurfaces: Wide-Angle HologramsThe use of plasmonic metasurfaces has enabled the realization of broadband holograms that can operate at extreme angles approaching grazing incidence. Zi-Lan Deng and coworkers from Jinan University, Southern University of Science and Technology, Shenzhen University and Nankai University fabricated the holograms from an array of closely spaced, identical silver nanorods (90nm wide, 200nm long) on top of a silicon substrate coated with a thin layer of silver and then a layer of SiO2. The phenomenon of extraordinary optical diffraction allows highly efficient diffraction at very large angles and customizing the spacing of the nanorods allows the desired phase profile to be programmed, into the surface. The design, which operates in the visible and near-infrared range, may prove useful for various holographic applications including data encryption, anti-counterfeiting and 3D displays.

Invited Article: Saturation scattering competition for non-fluorescence single-wavelength super-resolution imaging

Stimulated emission depletion nanoscopy and its derivatives based on saturation induced competition effects have become an indispensable tool for studying cellular events and their dynamics in living conditions. The successful implementation of these techniques heavily relies on the competition between excitation induced spontaneous emission and stimulated emission from fluorescent dyes. The use of two laser beams at different wavelengths perplexes the optical system and the high intensity saturation beam inevitably introduces detrimental photobleaching effects. Harnessing the emerging saturation scattering of plasmonic nanoparticles, here, we demonstrate a novel fluorescence-free single-wavelength super-resolution imaging technique using gold nanoparticles. A lateral resolution of 101.2 nm (<λ/5) is achieved through introducing saturation scattering competition (SSC) of 60 nm gold nanospheres between dual beams at the same wavelength. In addition, the SSC drastically reduces the saturation intensity by three orders of magnitude than the conventional stimulated emission depletion process at comparable resolutions. As a proof of concept, we realized robust single-wavelength super-resolved imaging in dMG-63 cells with a simplified system. The current technique provides a new modality of biosample-friendly technology for optical super-resolution imaging.

Resolution-improved refractive index sensing system based on microwave photonics filter and microfiber gratting

A novel refractive index (RI) sensing system with improved resolution based on a microwave photonics filter (MPF) is proposed and experimentally demonstrated. The sensing probe used for RI measurement is a microfiber Bragg grating. We use the frequency interrogation scheme (different from the traditional wavelength demodulation method) for resolution improvement of RI detection. The frequency shift of MPF notch point shows a linear relationship with the surrounding RI change over the range of 1.33 to 1.38 and a RI sensing resolution of 2.63×10-5 RIU has been achieved experimentally. The proposed MPF based RI sensing system provides a new interrogation method over the frequency domain with improved RI sensing resolution, as a good candidate for rapid and highly sensitive detection in chemical and environmental monitoring.