片上集成人工微结构光场调控(特邀)

Light Field Manipulation Based on On-Chip Integrated Artificial Microstructures(Invited)

光学元件的小型化与集成化一直是光场调控和集成光学领域的研究重点与难点之一。光学人工微结构具有在亚波长尺度上灵活调控光场振幅、偏振、相位、频率等属性的能力。通过与片上光波导或微腔集成,人工微结构可以为实现更紧凑的片上集成光子学器件以及更精确、更丰富的光场调控提供新的解决方案和更多的可能性。本文依据片上集成人工微结构在不同环节中调控的光场类型的差异,将其分成三类进行了讨论。介绍了基于不同设计原理的片上集成人工微结构在自由空间光入射耦合、波导模式面内调控以及离片辐射场调控方向的研究进展,并对该领域的部分新兴方向进行了展望。

Significance Light is an indispensable carrier of energy and information in humans daily life.The main information of light fields can be described by a few attributes such as amplitude,phase,frequency,and polarization.How to flexibly and effectively manipulate these light field dimensions has been a key research focus in optics and photonics.Meanwhile,with the development of technology,Moore s Law is gradually losing its effectiveness,and traditional electronic chips are facing increasing performance improvement challenges.Compared with electrons,photons have fast transmission velocity,high information-carrying capacity,and unique parallel processing capability.Therefore,replacing electronic components partially or completely with optical components is expected to solve many problems facing traditional electronic chips.However,traditional optical components are generally large in size and heavy.Therefore,the miniaturization and integration of multiple optical components into the same chip is an important trend in the future development of photonic chips.Optical artificial microstructure(also called metaatom)is a kind of artificial structure with subwavelength size in one or more dimensions,which can resonate with light fields to achieve functions beyond traditional natural materials.A metasurface can be formed by ordering optical artificial microstructures on a two-dimensional surface.It provides not only practical and effective solutions for the miniaturization and integration of traditional optical components but also more diverse means of controlling light fields and richer light-matter interactions.However,most current metasurfaces are focused on the manipulation of free-space light fields,and a metasurface can only achieve a single or a few functions.To further achieve more compact and versatile photonic chips,researchers have begun to integrate optical artificial microstructures with on-chip optical waveguides or optical microcavities in recent years.The research on on-chip integrated artificial microstructures injects new vitality into light field manipulation and nano-photonics devices.Thanks to their subwavelength sizes and unique resonance characteristics,artificial microstructures can serve as a bridge connecting free-space light fields with on-chip waveguide modes,thus opening up new opportunities for fully manipulating light in integrated optical systems and free space.Even though various novel optical devices have been proposed based on on-chip integrated artificial microstructures in the past few years,they still face a series of challenges in large-scale ultra-compact integration and performance improvement.Therefore,a review of light field manipulation based on on-chip integrated artificial microstructures is necessary to provide helpful guidance for researchers to design novel on-chip optical devices.Progress Based on different types of light fields manipulated by on-chip integrated artificial microstructures,we categorize them into three classes for discussions(Fig.1).The first category involves meta-couplers that can couple free-space optical modes into waveguides or microcavities and convert them into specific guided modes.The artificial microstructure-based meta-couplers can achieve more diverse and complex functions than traditional grating couplers,such as wavelength-and polarization-demultiplexing,or the excitation of specific guided modes(Fig.2).The second category involves in-plane modulators that enable on-chip manipulation of confined light fields within the chip plane.Artificial microstructures can be either partially-or fully-etched aperture antennas,or they can be directly integrated onto the waveguide surface.By adopting the refractive index perturbation or phase gradient provided by the microstructures,in-plane focusing of waveguide modes,filters,mode conversions between different guided modes,and on-chip nonlinear harmonic generations can be achieved(Figs.3 and 4).Additionally,on-chip integrated artificial microstructures can be combined with dynamic control schemes such as electro-optic modulators to further optimize the modulator s footprint and bandwidth performance.The third category involves guided wave-driven metasurfaces that can convert guided waves into free-space waves.By employing one-dimensional(Fig.5)and multi-dimensional(Figs.6 and 7)manipulation of far-field radiation,guided wave-driven metasurfaces can achieve various applications,such as holographic imaging,vortex beam generation,beam focusing,and beam deflection.Theoretically,the polarization,amplitude,phase,and orbital angular momentum of the emitted light field can be manipulated arbitrarily to provide new solutions for applications such as virtual reality,augmented reality,and information encryption and multiplexing.Conclusions and Prospects We systematically introduce the research progress of on-chip integrated artificial microstructures in the areas of free-space light coupling,in-plane manipulation of guided modes,and the manipulation of off-chip radiated fields.Additionally,we provide an outlook on some emerging directions in this field.By cascading multiple optical metasurfaces on waveguides,on-chip optical components can be more compact,and multifunctional devices beyond traditional metasurfaces can be realized.Some novel physical effects such as bound states in the continuum,parity-time symmetry,and exceptional points can provide a richer range of physical processes for on-chip integrated artificial microstructures.The introduction of new materials such as two-dimensional materials and laser gain materials can provide a new platform for studying excitons,valley spin,nonlinear effects,on-chip lasing,and other phenomena.The utilization of inverse design methods such as deep learning and topological optimization can serve as powerful tools for designing on-chip integrated artificial microstructures.In summary,with the advancement of technology,the application scope of on-chip integrated artificial microstructures will become more widespread.The operating wavelength range can expand from visible and near-infrared light to terahertz,microwave,and ultraviolet wavebands.Additionally,numerous miniaturized and integrated on-chip photonic devices will continue to emerge with the help of artificial microstructures.