Engineering Bi-Functional Wavefronts of Spoof Surface Plasmon Polaritons via Photonic Spin Decoupling
Objective In the realm of nanophotonics,the discovery of a geometric phase solely dependent on the rotation angle of metasurfaces has catalyzed a flurry of research activity.The breakthrough has facilitated the development of avant-garde scientific and technological applications,such as metasurface microscopy and compact spectrometers.However,a pivotal challenge lies in the inherent conjugate relationship between the geometric phases of different chiral circularly polarized lights.The relationship manifests as phase values that are equal in magnitude but opposite in sign,thus precluding independent and unrestricted manipulation of phase profiles for each chiral polarization state.Addressing the limitation requires transcending traditional paradigms of geometric phase control.Recent advancements propose a suite of innovative control methodologies,integrating phase mechanisms that are independent of structural rotation.These include the resonance phase,transmission phase,and roundabout phase.The paradigm shift paves the way for a burgeoning research field focusing on multi-degree-of-freedom light field control.Our core aspiration is to achieve independent phase control for each circularly polarized light and ensure efficient coupling of these controlled light fields with on-chip photonic structures.By tackling these challenges,we aim to unlock new dimensions in light manipulation at the nanoscale,potentially revolutionizing applications in optical computing,advanced imaging,and beyond.Methods Based on the Jones matrix of the unit structure,our analysis elucidates the phenomenon of spin locking in the photonic Berry(PB)phase.This is attributed to the conjugate relationship between the PB phases carried by cross-polarized circularly polarized light,which is pivotal in manipulating light phase properties at the nanoscale.A key factor in the efficient coupling of the on-chip light field is the unique behavior of polarization coefficients.Specifically,we observe that the cross-polarization coefficient is effectively zero,while the co-polarization coefficient exhibits an inverse sign.These properties are instrumental in directing the light field's behavior.Furthermore,our metasurface design leverages the phase gradient at the interface to match the wave vector of surface plasmons.The approach facilitates efficient coupling of the on-chip light field,a critical factor in advanced photonic applications.Meanwhile,we introduce a novel strategy to break the PB phase conjugation relationship inherent in cross-polarized circularly polarized light.By integrating a chirality-independent resonance phase with the PB phase,we can exert distinct phase controls over the two circularly polarized lights.The innovation marks a significant advancement in phase manipulation techniques.Additionally,our structural design adheres to the mirror symmetry principles,which ensures that the cross-polarization coefficient remains zero,an essential condition for our intended phase control.By meticulously selecting parameters from our structure library,we tailor the co-polarization coefficients to differ by a π phase.The precision engineering is the key to yielding our desired light manipulation outcomes at the nanoscale.Results and Discussions We report a significant advancement in the efficient coupling of on-chip light fields.Our approach enables the propagation mode of electromagnetic waves,which typically traverse in free space,to couple with on-chip surface plasmons.The coupling is achieved with remarkable efficiency,reaching up to 80％within 60 to 100 GHz frequency band.The high efficiency represents a noteworthy result in on-chip photonic systems,potentially paving the way for more compact and efficient photonic devices.Furthermore,our findings include groundbreaking development in the PB phase manipulation.We have successfully"unlocked"the spin of the PB phase,a significant stride in light manipulation at the nanoscale which allows for the directional propagation of left-hand and right-hand spins.Notably,they propagate to the left and right sides of the coupling structure respectively.The left-hand spins culminate as a Bessel beam channeling 37％of the energy,while the right-hand spins form a focused beam carrying 41％of the energy.Conclusions We present a novel coupler design that capitalizes on the dual degrees of freedom offered by the resonance phase and the geometric phase.A key innovation of our design is the precise setting of the turning and opening angles of each unit cell.The meticulous configuration tackles a fundamental challenge in wavefront shaping,or the issue of varying circular shapes due to the geometric phase in dealing with circularly polarized light of different chiralities.Our approach effectively overcomes the limitations imposed by the conjugate phase under polarized light excitation.The advancement enables the wavefronts at both ends of the spectrum to be shaped independently,allowing for unprecedented control between different chiral incident polarized lights.By leveraging the methodology,we have successfully designed a dual-function wavefront-controlled coupler.The device exhibits remarkable capabilities in simultaneously focusing and generating Bessel beams.The multifunctionality is a significant stride forward in the wavefront manipulation field.Additionally,the developed coupling device is characterized by compact size and multifunctional nature.These attributes make it a promising candidate for functional design in integrated photonic integration.Finally,our study not only puts forward a practical solution to a complex challenge in photonics but also opens new avenues for the advancement of integrated photonic devices.The broad potential applications of the technology range from optical computing to advanced imaging systems,heralding a new era in integrated photonics.
photonic spin Hall effectpolarization conversionmetasurfacehigh-efficiency couplersurface wave
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