| The rapid development of information technology has led to an enormous increase in the complexity and volume of data.Owing to the restriction including processing speed and power consumption,it becomes a challenge for the existing electronic digital system to realize real-time and high-throughput signal processing.Analog optical computing can circumvent the conversion between optical and electronic signals and manipulate the signal in the optical domain directly,offering a potential path to ultrafast,low-power consumption,and parallel data processing.Furthermore,advances in nanophotonics provide a platform for ultracompact and integrated all-optical computing devices.By engineering the transfer function of the structure,it is possible to perform a range of mathematical operations on optical signals,such as differentiation,integration and Hilbert transforms.Among them,the differentiation has gained considerable attention for various applications,such as pulses shaping in the time domain and image processing in the spatial domain.The main objective of this work is to realize optical differentiation utilizing nanophotonic structures in free space.Here,a theoretical analysis of the temporal and spatial responses of the computing device is carried out.Based on surface plasmon,the numerical simulation and experiment of different approaches to compact and multifunctional optical differentiator are present.The research results are as follows:(1)Optical spatiotemporal differentiators are proposed based on surface plasmon polariton excitation with metal gratings.Firstly,light normally impinges on a onedimensional symmetric grating and excites the surface plasmon polaritons propagating along the ± x direction.Under critical coupling conditions,it performs spatial second-order differentiation and temporal first-order differentiation at the same time.Secondly,by suitably engineering the relative displacement of bilayer plasmonic gratings,the horizontal and vertical symmetries of the structure have been broken,which can couple normally incident light into unidirectional surface plasmon polaritons.Thus,spatial and temporal first-order differentiation can be simultaneously achieved.The spatial and temporal resolution is evaluated numerically to be 2 μm and 50 fs,respectively.The calculated results confirm the combination of temporal and spatial operations with parallel operation capability,which may be applied in ultra-fast and high-resolution optical detection.(2)Optical spatial two-dimensional differentiation under different polarizations is proposed based on gap-surface plasmons supported by metal-insulator-metal metasurfaces.At first,by optimizing the geometry parameters of the circular resonators,an ideal transfer function corresponding to second-order differential operation can be achieved in normal incidence.Two-dimensional second-order differentiattion is performed in reflected mode,with a large numerical aperture and high efficiency.Then,an anisotropic metasurface consisting of supercells with four nanorods is proposed.The coupling between the adjacent resonantors can excite cross polarized reflected field.By tailoring the asymmetric angular responses,the metasurface reflects the first-order differentiation of two-dimensional input field in cross polarization.The polarization state is considered in this work,which can realize polarization division multiplexing in signal processing.Compared with the previous studies of orthogonal polarization differentiation,the structure is optimized along different directions and extends the differential operation to spatial two-dimensional scenarios.(3)Optical spatial two-dimensional differentiation is proposed and experimentally demonstrated using a metasurface consisting of metal nanorods.The nanorods are arranged in a hexagonal lattice,which can improve the symmetry of the structure and reduce the polarization dependency of the transfer function,providing a more homogeneous response.By engineering the wavevector-dependent responses of the resonators,the spatial secondorder differentiation is realized in the transmission mode.As proof of concept,the metasurface is fabricated using electron beam lithography.The transmittance spectrum as a function of angle of incidence and Fourier plane images are utilized to verify the spatial spectral transfer function of the device.Moreover,the two-dimensional edge detection for image processing is experimentally demonstrated with the metasurface under unpolarized light illumination.The plasmonic differentiator consisting of a single-layer metal nanorods is easy to fabricate,with a large operating bandwidth and improved response uniformity.The device works in transmission mode,which can be conveniently integrated with standard imaging systems and realize real-time image processing. |
