Second-harmonic generation and manipulation in lithium niobate slab waveguides by grating metasurfaces

Nonlinear optical processes in waveguides play important roles in compact integrated photonics,while efficient coupling and manipulations inside the waveguides still remain challenging.In this work,we propose a new scheme for second-harmonic generation as well as beam shaping in lithium niobate slab waveguides with the assistance of well-designed grating metasurfaces atλ=1064 nm.By encoding the amplitude and phase into the holographic gratings,we further demonstrate strong functionalities of nonlinear beam shaping by the metasurface design,including dual focusing and Airy beam generation.Our approach would inspire new designs in the miniaturization and integration of compact multifunctional nonlinear light sources on chip.

Spectral tomographic imaging with aplanatic metalens

Tomography is an informative imaging modality that is usually implemented by mechanical scanning,owing to the limited depth-of-field(DOF)in conventional systems.However,recent imaging systems are working towards more compact and stable architectures;therefore,developing nonmotion tomography is highly desirable.Here,we propose a metalens-based spectral imaging system with an aplanatic GaN metalens(NA=0.78),in which large chromatic dispersion is used to access spectral focus tuning and optical zooming in the visible spectrum.After the function of wavelength-switched tomography was confirmed on cascaded samples,this aplanatic metalens is utilized to image microscopic frog egg cells and shows excellent tomographic images with distinct DOF features of the cell membrane and nucleus.Our approach makes good use of the large diffractive dispersion of the metalens and develops a new imaging technique that advances recent informative optical devices.

Metalens-integrated compact imaging devices for wide-field microscopy

Metasurfaces have demonstrated unprecedented capabilities in manipulating light with ultrathin and flat architectures.Although great progress has been made in the metasurface designs and function demonstrations,most metalenses still only work as a substitution of conventional lenses in optical settings,whose integration advantage is rarely manifested.We propose a highly integrated imaging device with silicon metalenses directly mounted on a complementary metal oxide semiconductor image sensor,whose working distance is in hundreds of micrometers.The imaging performances including resolution,signal-to-noise ratio,and field of view(FOV)are investigated.Moreover,we develop a metalens array with polarization-multiplexed dual-phase design for a wide-field microscopic imaging.This approach remarkably expands the FOV without reducing the resolution,which promises a non-limited space-bandwidth product imaging for wide-field microscopy.As a result,we demonstrate a centimeter-scale prototype for microscopic imaging,showing uniqueness of meta-design for compact integration.

Bandpass-filter-integrated multiwavelength achromatic metalens

The design of large-scale,high-numerical-aperture,and broadband achromatism is a big challenge in metalens research. In fact,many colorful imaging systems have RGB color filters,which means the achromatism only for RGB lights would be sufficient. Avoiding broadband achromatism is expected to greatly improve the working efficiency of metalenses. Nevertheless,a proper bandpass filter is necessary under a white light illumination in the metalens integrated imaging system. Here we propose a bandpass-filter-integrated multiwavelength achromatic metalens (NA=0.2),which is designed using a searching optimization algorithm to achieve the achromatism of RGB lights with high efficiencies. The bandpass filter is implemented by composite DBRs and defect layers,by which three desired wavelengths are selected out. The simulations and experiments on the filter-integrated metalens definitely show a good RGB achromatism. Further imaging experiments demonstrate a higher signal-to-noise ratio and resolution compared with the one without the filter. Our approach provides not only an RGB achromatic meta-imaging device but also a new route to access a highly efficient spectrum tailoring metasystem by incorporating bandpass filter designs.

Subwavelength self-imaging in cascaded waveguide arrays

Self-imaging is an important function for signal transport,distribution,and processing in integrated optics,which is usually implemented by multimode interference or diffractive imaging process.However,these processes suffer from the resolution limit due to classical wave propagation dynamics.We propose and demonstrate subwavelength optical imaging in one-dimensional silicon waveguide arrays,which is implemented by cascading straight and curved waveguides in sequence.The coupling coefficient between the curved waveguides is tuned to be negative to reach a negative dispersion,which is an analog to a hyperbolic metamaterial with a negative refractive index.Therefore,it endows the waveguide array with a superlens function as it is connected with a traditional straight waveguide array with positive dispersion.With a judiciously engineered cascading silicon waveguide array,we successfully show the subwavelength self-imaging process of each input port of the waveguide array as the single point source.Our approach provides a strategy for dealing with optical signals at the subwavelength scale and indicates functional designs in high-density waveguide integrations.