Flexible tuning of multifocal holographic imaging based on electronically controlled metasurfaces

Programmable hyper-coded holography has the advantage of being programmable as well as being flexibly modifiable.Digitally coded metamaterials with excellent electromagnetic modulation capability and the ability to control the phase to modulate the spatial radiation field through external excitation in the form of switching can be used to realize low-cost digital arrays.We design a 1-bit encoded programmable metasurface,which is electrically connected to control the PIN diode in the switching state and to switch the condition of each metasurface cell between“0”and“1.”Using the designed programmable metasurface,we can randomly encode the cell structure to realize single-focus focusing,multi-focusing,and simple holographic letter imaging.Based on the nonlinear holographic model,we employ the Gerchberg-Saxton improvement algorithm to modulate the energy distribution at the focus by adjusting the phase distribution.Importantly,we introduce the Fourier convolution principle to regulate the holographic imaging focus flexibly.

High-Q resonances in terahertz all-silicon metasurface with imperforated air-hole array

We propose and experimentally demonstrate a high quality(Q)-factor all-silicon bound state in the continuum(BIC)metasurface with an imperforated air-hole array.The metasurface supports two polarization-insensitive BICs originated from guided mode resonances(GMRs)in the frequency range of 0.4 to 0.6 THz,and the measured Q-factors of the two GMRs are as high as 334 and 152,respectively.In addition,the influence of the thickness of the silicon substrate on the two resonances is analyzed in detail.The proposed all-silicon THz metasurface has great potential in the design and application of high-Q metasurfaces.

Improvement of terahertz beam modulation efficiency for baseless all-dielectric coded gratings

Optical metasurfaces are two-dimensional ultrathin devices based on single-layer or multilayer arrays of subwave-length nanostructures. They can achieve precise control of phase, amplitude, and polarization on the subwave-length scale. In this paper, a substrate-free all-silicon coded grating is designed, which can realize the phase control of the outgoing beam after the y-polarized plane wave is vertically incident on the metasurface at 0.1 THz.Through a single-layer silicon nanoarray structure, a low-reflection anomalous transmission metasurface is realized, and a variety of different beam deflectors are designed based on these encoded gratings. We propose a coded grating addition principle, which adds and subtracts two traditional coded grating sequences to obtain a new coded grating sequence. The encoded supergrating can flexibly control the scattering angle, and the designed substrate-free all-silicon encoded grating can achieve a deflection angle of 48.59°. In order to verify the principle of coded grating addition, we experimented with cascade operation of two coded sequence gratings to obtain the flexible control of the terahertz beam of the composite supergrating. The principle of grating addition provides a new degree of freedom for the flexible regulation of the terahertz wavefront. At the same time, this method can be extended to the optical band or microwave band, opening up new ways for electromagnetic wave manipulation and beam scanning.

Focus control of wide-angle metalens based on digitally encoded metasurface

Based on the principle of super-symmetric lens with quadratic phase gradient transformation, combined with the principle of digital coding of metasurface, we propose a wide-angle coded metalens for focusing control in two-dimensional space. This metalens achieves focus shift in the x-direction by changing the oblique incidence angle of the incident wave,and focus control in the y-direction by combining with the convolution principle of the digitally coded metasurface to achieve flexible control of light focusing in the two-dimensional plane. The metasurface unit is mainly composed of threelayer of metal structure and two layers of medium, and the transmission phase is obtained by changing the middle layer of metal structure, which in turn obtains the required phase distribution of the metalens. The design of the metalens realizes the function of the lens with a large viewing angle at the x-polarized incidence, and realizes two-dimensional focus control. Experimentally, we prepared the designed coding metalens and tested the focus control function of the wide-angle coding metalens. The experimental results are in good agreement with the design results.

Ultra-high-Q resonances in terahertz all-silicon metasurfaces based on bound states in the continuum

High-Q metasurfaces have important applications in high-sensitivity sensing,low-threshold lasers,and nonlinear optics due to the strong local electromagnetic field enhancements.Although ultra-high-Q resonances of bound states in the continuum(BIC)metasurfaces have been rapidly developed in the optical regime,it is still a challenging task in the terahertz band for long years because of absorption loss of dielectric materials,design,and fabrication of nanostructures,and the need for high-signal-to-noise ratio and high-resolution spectral measurements.Here,a polarization-insensitive quasi-BIC resonance with a high-Q factor of 1049 in a terahertz all-silicon metasurface is experimentally achieved,exceeding the current highest record by 3 times of magnitude.And by using this ultra-high-Q metasurface,a terahertz intensity modulation with very low optical pump power is demonstrated.The proposed all-silicon metasurface can pave the way for the research and development of high-Q terahertz metasurfaces.

Resonant transparency of a planar anapole metamaterial at terahertz frequencies

Anapole metamaterials have attracted growing attention in recent years due to their unique nonradiating and nontrivial properties. Although anapole modes have been demonstrated in metamaterials with threedimensional structures, the design and realization of planar anapole metamaterials in a wide frequency range is still a big challenge. Here we propose and experimentally demonstrate a planar anapole metamaterial consisting of dumbbell-shaped apertures on a stainless-steel sheet at terahertz frequencies. The planar metamaterial can generate a resonant transparency in the terahertz spectrum due to the excitation of the anapole mode.Particularly, the frequency of anapole-induced resonant transparency can be tuned easily in the range of 0.15–0.93 THz by simply varying one geometric parameter of the dumbbell apertures. We anticipate that the resonant transparency in planar anapole metamaterials can be potentially used in filters, sensors, or other photonic devices.

Broadband cross-circular polarization carpet cloaking based on a phase change material metasurface in the mid-infrared region

In view of the fact that most invisibility devices focus on linear polarization cloaking and that the characteristics of mid-infrared cloaking are rarely studied,we propose a cross-circularly polarized invisibility carpet cloaking device in the mid-infrared band.Based on the Pancharatnam–Berry phase principle,the unit cells with the cross-circular polarization gradient phase were carefully designed and constructed into a metasurface.In order to achieve tunable cross-circular polarization carpet cloaks,a phase change material is introduced into the design of the unit structure.When the phase change material is in amorphous and crystalline states,the proposed metasurface unit cells can achieve highefficiency cross-polarization conversion,and reflection intensity can be tuned.According to the phase compensation principle of carpet cloaking,we construct a metasurface cloaking device with a phase gradient using the designed unit structure.From the near-and far-field distributions,the cross-circular polarization cloaking property is confirmed in the broadband wavelength range of 9.3–11.4μm.The proposed cloaking device can effectively resist detection of cross-circular polarization.

Broadband silicon-based tunable metamaterial microfluidic sensor

Tunable metamaterial absorbers play an important role in terahertz imaging and detection.We propose a multifunctional metamaterial absorber based on doped silicon.By introducing resonance and impedance matching into the absorber,a broadband absorption greater than 90%in the range of 0.8–10 THz is achieved.At the same time,the light regulation characteristics of the doped semiconductor are introduced into the absorber,and the precise amplitude control can be achieved in the range of 0.1–1.2 THz by changing the pump luminous flux.In addition,based on the principle of light-regulating the concentration of doped silicon carriers,the medium-doped silicon material is replaced by a highly doped silicon material,and a sensor with a sensitivity of up to 500 GHz/RIU is realized by combining the wave absorber with the microfluidic control.Finally,the broadband absorption characteristics and sensing performance of alcohol and water on the prepared device are verified by experiments,indicating that the absorber may have great potential in the field of sensor detection.

Free control of far-field scattering angle of transmission terahertz wave using multilayer split-ring resonators'metasurfaces

To enhance transmission eficiency of Pancharatnam Berry(PB)phase metasurfaces,multilayer split-ring resonators were proposed to develop encoding sequences.As per the generalized Snell's law,the deflection angle of the PB phase encoding metasurfaces depends on the metasurface period's size.Therefore,it is impossible to design an infinitesimal metasurface unit;consequently,the continuous transmission scattering angle cannot be obtained.In digital signal processing,this study introduces the Fourier convolution principle on encoding metasurface sequences to freely control the trausmitted scattering angles.Both addition and subtraction operations between two diferent encoding sequences were then performed to achieve the continuous variation of the scattering angle.Furthermore,we established that the Fourier convolution principle can be applied to the checkerboard coded metasur-faces.

Thermal and luminescent properties of 2 μm emission in thulium-sensitized holmium-doped silicate-germanate glass

In this paper, we present the luminescent properties of Tm^(3+)∕Ho^(3+)co-doped new glass. A series of silicategermanate glass was prepared by the conventional melt-quenching method. In the Tm^(3+)∕Ho^(3+)co-doped silicategermanate glass, a strong emission of 2 μm originating from the Ho^(3+):~5I_7→~5I_8transition can be observed under conventional 808 nm pumping. The characteristic temperatures, structure, and absorption spectra have been measured. The radiative properties of Ho^(3+)in the prepared glass were calculated. The emission cross section of Ho^(3+)ions transition can reach 4.78 × 10^(-21)cm^2 around 2 μm, and the FWHM is as high as 153 nm. The energy transfer efficiency between Ho^(3+)and Tm^(3+)has a large value(52%), which indicates the Tm^(3+)∕Ho^(3+)co-doped silicategermanate glass is a suitable candidate for the 2 μm laser. Moreover, the energy transfer mechanism between Tm^(3+)and Ho^(3+)ions was investigated.

Broadening and enhancing 2.7 μm emission spectra in Er/Ho co-doped oxyfluoride germanosilicate glass ceramics by imparting multiple local structures to rare earth ions

Er/Ho co-doped oxyfluoride germanosilicate glass and glass ceramics are prepared and compared. The results indicate that the glass consists of SiO_4 and GeO_4 structural units, while the network of the glass ceramics consists of SiO_4, GeO_4, and GeO_6 units together with NaYF_4 nanocrystals. The presence of multiple local structures in glass ceramics creates a range of dipole environments, which is beneficial to the broadening of 2.7 μm emission.Two other reasons are attributed to the broadening of 2.7 μm emission in glass ceramics: the energy-level splitting of Er^(3+) and the enhancement of the Ho^(3+):~5I_6→~5I_7 transition in NaYF_4 nanocrystals.