基于光敏硅的多功能可重构超表面

Multifunctional Reconfigurable Metasurface Based on Photosensitive Silicon

提出一种双C环光敏硅可重构超表面,该超表面通过改变光敏硅电导率可以实现在太赫兹波段的多功能切换。当大小C环的电导率分别为5.0×10^(5)S/m和0 S/m时,所设计超表面表现为线-线极化转换器,在2.10~3.15 THz频率范围内极化转换率大于90%;当大小C环的电导率分别为0 S/m和5.0×10^(5)S/m,该结构在2.33~2.47 THz和2.78~4.40 THz频率范围内表现为线-圆极化转换器;当大小C环的电导率同时变化为2.5×10^(5)S/m时,该结构转化为吸收器,在2.40~4.60 THz频率范围内吸收率大于90%。将大小C环电导率都为0 S/m的单元与大小环电导率都为2.5×10^(5)S/m的单元进行编码,该结构在2.80~3.00 THz范围内实现近场成像。将大小C环电导率分别为5.0×10^(5)S/m和0 S/m的单元与大小环电导率都为0 S/m的单元进行周期性编码,该结构可实现对太赫兹波二分束和四分束。结果表明,通过改变外部光照条件,可以实现对所设计超表面重构,获得多种太赫兹调控功能。

Objective Tunability is an important requirement for metasurface application.In recent years,adjustable materials have been gradually used in the composite design of metasurface structures to achieve multifunctional switching.However,most of the existing relevant studies still have some shortcomings.First,the adjustable material is usually embedded in the metal pattern,which undoubtedly increases the complexity and manufacturing difficulty of the structure.Second,commonly used adjustable materials such as vanadium dioxide and graphene require particularly sensitive temperature environments or feeding conditions,and the temperature and bias voltage should be considered in the design and application,which not only complicates the production process but also brings a certain degree of difficulty to the practical application.Third,most of the reported metasurfaces only discuss the insulating and metallic states of adjustable materials,which limits the diversity of functions.Photosensitive silicon is a kind of lightadjustable material,and its conductivity changes with the change in pump light energy.It has attracted wide attention because of its simple regulation mode.Moreover,photosensitive silicon can continuously adjust the conductivity to generate a variety of coding states,so as to expand its functions.In this paper,a reconfigurable metasurface based on a photosensitive silicon pattern is designed.The metasurface does not need to change the shape,size,or direction of the unit but uses the optical control to continuously adjust the conductivity of photosensitive silicon,so as to realize several functions in the terahertz band,such as lineartolinear polarization conversion,lineartocircular polarization conversion,broadband absorption,nearfield imaging,and beam splitting,which makes the regulation mode of multifunctional terahertz devices more convenient.Methods In this study,the pattern of the unit structure is completely composed of photosensitive silicon,and the processing technology of siliconbased metasurface is very mature,which will greatly reduce the production difficulty.The conductivity of photosensitive silicon varies with the light energy pumped.When the light energy increases,the carrier concentration in the semiconductor also increases.By adjusting the conductivity of the two photosensitive silicon rings,five coding states can be obtained,so as to encode metasurfaces with different functions.In this paper,the polarization conversion and absorption function can be realized by using the resonance between the photosensitive silicon and the metal plate or between the double rings.The amplitude difference of different state units can be used for imaging,and the phase difference can be used for beam splitting.Results and Discussions The designed metasurface can generate multiple coding states by continuously adjusting the conductivity of two photosensitive silicon rings(Table 1),so as to realize multifunctional switching in the terahertz band.When the conductivity of the large and small Crings is 5.0×10^(5) S/m and 0 S/m,respectively,the designed metasurface is presented as a lineartolinear polarization converter(Fig.2),and the polarization conversion ratio(PCR)in the range of 2.10-3.15 THz is greater than 90%.When the conductivity of the large and small Crings is changed to 0 S/m and 5.0×10^(5) S/m,respectively,the structure behaves as a lineartocircular polarization converter(Fig.4)in the range of 2.33-2.47 THz and 2.78-4.40 THz.When the conductivity of the large and small Crings changes to 2.5×10^(5) S/m at the same time,the structure is transformed into an absorber(Fig.7)with an absorption rate of more than 90%in the range of 2.40-4.60 THz.By encoding the cells with the conductivity of both large and small Crings of 0 S/m and 2.5×10^(5) S/m,the structure achieves nearfield imaging(Fig.12)in the range of 2.80-3.00 THz.The cells with the conductivity of 5.0×10^(5) S/m and 0 S/m for the large and small Crings and those with the conductivity of 0 S/m for the large and small Crings are periodically coded,and this structure can realize two beam splitting(Fig.14)and four beam splitting(Fig.15)of the terahertz wave.The results show that the metasurface can be reconstructed by changing the external illumination conditions,and a variety of terahertz control functions can be obtained.Conclusions In this paper,the reconfigurable metasurface designed by the photosensitive silicon double Crings structure realizes the switching of several functions,such as lineartolinear polarization conversion,lineartocircular polarization conversion,broadband absorption,nearfield imaging,two beam splitting,and four beam splitting.Compared with the existing reports,the structure pattern designed in this paper is completely composed of photosensitive silicon,which greatly reduces the complexity of the pattern and the difficulty of device manufacturing.At the same time,this paper makes use of the continuously adjustable conductivity of photosensitive silicon to generate a variety of coding states,making the function of the metasurface more abundant.Compared with the singlefunction metasurface,it has greater advantages in integration and other aspects.In a word,the metasurface proposed in this paper is more flexible in switching functions and can realize a wider range of functions.It has excellent application prospects in terahertz modulation,stealth technology,communication system,and so on.