Tunable dielectric metasurfaces by structuring the phase-change material.

Metasurfaces have made great progress in the last decade for generating miniature and integrated optical devices. The optical properties of metasurfaces can be tuned dynamically by integrating with phase-change materials. However, the efficiency of tunable metasurfaces remains a bit low, which is a disadvantage for the realistic applications of metasurfaces. Here, we demonstrate the tunable dielectric metasurfaces by structuring the phase-change material Ge2Sb2Te5. The unit cell of metasurface is composed of several Ge2Sb2Te5 nanopillars with different geometric parameters, and the incident light interacts with different nanopillars at diverse phases of Ge2Sb2Te5, leading to various functions. By elaborately arranging the Ge2Sb2Te5 nanopillars, various tunable optical devices have been realized, including tunable beam steering, reconfigurable metalens and switchable wave plate. The refractive direction, focal length and polarization state can be tuned through the phase transition of Ge2Sb2Te5. The phase-change metasurfaces based on Ge2Sb2Te5 nanostructures could be used in cameras, optical microscopy and adaptive optics.

A Dual-Mode Optical Thermometer with High Sensitivity Based on BaAl12O19:Sm2+/SrAl12O19:Sm3+ Solid Solution Phosphors.

A series of BaAl12O19:Sm2+/SrAl12O19:Sm3+ mixed-phase phosphors were produced in one step using the traditional high-temperature solid-phase process. Because Sm is divalent in BaAl12O19 and trivalent in SrAl12O19, the coexistence of Sm2+ and Sm3+ is realized in the mixed-phase host. Since the temperature sensitivity of Sm2+ and Sm3+ in the solid solution host is significantly different, this makes it possible for the sample to measure temperature based on the fluorescence intensity ratio (FIR). The crystal model, ion emission spectrum, and temperature sensitivity of these phosphors are studied in detail. Under the co-excitation of a 410 nm excitation source, this sample has excellent temperature measurement performance in the range of 313-513 K. Based on the FIR method, the maximum absolute temperature sensitivity (Sa) is 0.55 K-1 at 513 K, and the maximum relative temperature sensitivity (Sr) is 2.47%K-1 at 453 K. Moreover, based on the photoluminescence lifetime temperature measurement mode, the largest value of Sa at 413 K is 0.046 K-1, and the maximum value of Sr at 473 K is 3.10%K-1. In short, the BaAl12O19:Sm2+/SrAl12O19:Sm3+ solid solution is a kind of phosphor with nice temperature measurement ability, and it has very strong potential in the application of noncontact optical thermometers.

Improvement of beam shaping on a metasurface by eliminating the interaction between coding units.

The interaction between subwavelength elements must be considered when constructing a metasurface. Generally, the interaction between cell structures is ignored when metasurface optoelectronic devices are designed, which results in a significant decrease in the design performance and efficiency of the overall metasurface structure. To reduce or further eliminate the interaction between cell structures, we propose a cell structure with borders to construct coded metasurface sequences. At the same time, we design a common frameless cell structure to construct a traditional coding metasurface. By numerical simulation of the near-field distribution and far-field scattering characteristics of these two types of coded metasurface sequences, we find that the element structure with a medium frame can attenuate the interaction between adjacent encoded particles. In the process of transmission on the encoding metasurface with a frame, different encoded particles can independently express their transmission phase and are not affected by adjacent structures, thus realizing a low coupling coding metasurface.

Highly sensitive biosensor based on an all-dielectric asymmetric ring metasurface.

We propose an all-dielectric asymmetric ring-cylindrical metasurface. Based on the analysis of transmission characteristics and the calculation of electromagnetic field distribution of the metasurface with this element structure, it is found that the high Q resonance of the ultra-narrowband can be realized when the symmetry of the ring-cylindrical structure is broken. Meanwhile, it is found that the degree of asymmetry of the ring, the refractive index of the material, the radius of the ring, and the substrate have great influence on the Q value and resonant frequency of the metasurface. Our proposed metasurface structure is applied to the detection of biological molecules based on the change in refractive index of biomolecular solutions. The designed metasurface with high sensitivity to detect biomolecules with different refractive indices, the Q value can reach 365.03, and the sensitivity is increased by 90.36 GHz/RIU compared to that without substrate, while the figure of merit value is as high as 100.56, providing label-free detection of biomolecules.

Transmission terahertz power beam splitter based on a single-layer metal metasurface.

A periodic metasurface composed of a single layer of copper structure is proposed. The general transmission power beam splitter is composed of a multilayer structure, which is difficult to fabricate. The proposed single-layer terahertz wave power beam splitter contains only a single-layer circular hole cell structure, and it can control the transmission angle by controlling the arrangement mode of the coding cells. At the same time, we can control the transmission angle and the transmitted energy distribution of each beam based on different incident angles. A simple monolayer round-hole metasurface was prepared and its transmission characteristics were analyzed based on a terahertz time domain spectrometer. Compared with traditional splitter devices, our coding metasurface beam splitters with a single layer have the potential to promote the development of integrated optical systems.

Broadband stealth devices based on encoded metamaterials.

Based on the generalized Snell's law, the relationship between the phase gradient of the metasurface and the incident frequency is demonstrated, and the principle of the achromatic metasurface is developed. By adjusting the phase gradient and linear dispersion simultaneously, the function of achromatic aberration is realized, and the influence of chromatic aberration on the metasurface is reduced. We propose a metasurface stealth device with achromatic multilayer frame metasurfaces with beam deflection, steering, and collection functions so that the incident electromagnetic beam is transmitted around the stealth object without scattering. In the range of 0.45-0.9 THz, the stealth function can be achieved. We have shown that the achromatic principle, design method, and stealth structure provide a guide for achieving transmissive cloaking.

Analogue of electromagnetically induced transparency in a metal-dielectric bilayer terahertz metamaterial.

We realize and numerically demonstrate the analogue of electromagnetically induced transparency (EIT) with a high-Q factor in a metal-dielectric bilayer terahertz metamaterial (MM) via bright-bright mode coupling and bright-dark mode coupling. The dielectric MM with silicon dimer rectangular-ring-resonator (Si-DRR) supports either a bright high-Q toroidal dipole resonance (TD) or a dark TD with infinite Q value, while plasmonic MM with metallic rectangular-ring-resonator (M-RR) supports a low-Q electric dipole resonance (ED). The results show that the near-field coupling between the dark TD and bright ED behaves just as that between the two bright modes, which is dependent on the Q factor of the TD resonance. Further, due to the greatly enhanced near-field coupling between the bright ED and dark TD, the coupling distance is significantly extended to about 1.9 times of the wavelength (in media), and robust EIT with large peak value over 0.9 and high Q-factor is achieved. The proposed bilayer MM provides a new EIT platform for design and applications in high-Q cavities, sensing, and slow-light based devices.

Transmission and reflection cloaking by using a zero-refractive-index photonic crystal in the microwave region.

We propose a rectangular column two-dimensional square lattice photonic crystal to realize zero refractive index. Through analysis of the energy band structure of the photonic crystal structure, the lattice constant and side length of the rectangular columns can be optimized, and the Dirac cone dispersion appears at the center of the Brillouin zone. The Dirac cone is formed by the interaction of a monopolar eigenstate and a dipolar eigenstate to form a triple accidental degenerate state. The effective medium theory is used to invert the effective electromagnetic parameters of the photonic crystal with a double zero refractive index. The zero-phase change and the focusing characteristic of the concave lens of this kind of zero-refractive-index material are verified. Importantly, we have achieved transmission and reflection cloaking with this zero-index medium. Through the analysis of the amplitude and phase distribution characteristics of the electromagnetic field, it is proved that the designed cloaking devices have obvious cloaking effect.

Enhancement of efficiency on the Pancharatnam-Berry geometric phase metalens in the terahertz region.

Traditional terahertz lenses face high thickness, low transmittance, difficult processing, and other problems that are not conducive to mass production and integration. Here, we propose a wideband all-dielectric Pancharatnam-Berry geometric phase cell structure to construct a metasurface flat lens. However, when the geometrical phase element structure rotates, the transmission efficiency of the periodic element structure obviously decreases, which will lead to the decrease of the efficiency of the designed flat lens. In order to improve the efficiency, we propose to add a layer of tapered microstructure on the flat substrate to greatly improve the transmission efficiency of the element structure, thus leading to the improvement of the efficiency of the metasurface lens. By comparing the metasurface lens with conical and planar substrates, the metasurfaces with conical structure can greatly improve the transmission efficiency at broadband and wide angle ranges.

High-efficiency transmissive invisibility cloaking based on all-dielectric multilayer frame structure metasurfaces.

Metasurfaces provide a completely new path to realize the cloaking effect due to their excellent electromagnetic wavefront manipulation. However, most previous metasurfaces realized cloaking by using phase compensation, which is limited by the reflection phase formula and can be used only for reflection mode. We use the generalized Snell's law to propose a free-space transmission stealth device, consisting of multilayer all-dielectric metasurfaces. We utilize three phase gradient all-dielectric silicon metasurfaces that, respectively, play the role of beam splitting, steering, and collection to guide incident waves around the object, thereby forming an ideal stealth area in free space. All-dielectric metasurfaces can greatly reduce transmission loss and enhance efficiency to a large extent. The advantage of choosing an all-dielectric material is that it is easy to process and more suitable in practice. Simulation results of the near field and far field prove that this cloak has a cloaking effect at 1 THz. Our work opens up a new path for transmissive stealth.

Manipulation of the arbitrary scattering angle based on all-dielectric transmissive Pancharatnam Berry phase coding metasurfaces in the visible range.

In order to improve the transmitted efficiency of the metasurface in the visible range, an all-dielectric Pancharatnam-Berry phase unit structure was proposed. Using these Pancharatnam-Berry phase element particles with different rotation angles, all-dielectric encoding metasurfaces can be constructed. The encoding metasurface connects the physical coding particles with digital coding in digital signal processing. The manipulation of the continuous transmission angle requires the continuous change of the encoding metasurface period. Since the size of encoding particles on the coded metasurfaces cannot be designed to be infinitesimally small, it is impossible to obtain the continuously changing period of the coded metasurfaces. To manipulate effectively and freely the angle of scattering in the visible range, Fourier convolution principle in digital signal processing was introduced on all-dielectric encoding metasurfaces with Pancharatnam-Berry phase meta-atoms. The addition and subtraction operations on two initial encoding sequences can be implemented to obtain a new encoding sequence. The manipulation of the arbitrary scattering pattern after Fourier convolution operations on different encoding sequences can be realized, especially for larger abnormal deflection angles. The checkerboard encoding metasurface was also designed to further prove the applicability of the Fourier convolution principle. Moreover, by using the proposed all-dielectric highly efficient Pancharatnam-Berry phase encoding meta-atoms, these coded particles with different rotation angles can be precisely arranged to build the generators of the orbital angular momentum beam with different topological charges.

Design of two invisibility cloaks using transmissive and reflective metamaterial-based multilayer frame microstructures.

Ultrathin metamaterials provide new possibilities for the realization of cloaking devices because of their ability to control electromagnetic waves. However, applications of metamaterials in cloaking devices have been limited primarily to reflection-type carpet cloaks. Hence, a transmissive free-space cloak was developed using a multilayer frame structure, wherein highly transparent metamaterials were used to guide incident waves into propagating around an object. The cloaking effect was quantitatively verified using near-field and far-field distributions. Metamaterials allow for the cloaking shells of transmissive cloaks to be developed without spatially varying extreme parameters. Moreover, a transmissive invisible cloak with metamaterial-based mirrors was designed. The design principle of this cloak with a frame structure consists of four metamaterial-based mirrors and two metal mirrors. After covered with the designed metamaterials-based mirrors cloak, the outgoing electromagnetic wave is restored greatly as if the wave passes directly through the obstacle without distortion. This cloak used the metamaterials mirrors to adjust the reflected angle, so that the outgoing electromagnetic wave does not change direction, thereby achieving the cloaking effect.

Analogue of electromagnetically induced transparency with high-Q factor in metal-dielectric metamaterials based on bright-bright mode coupling.

Based on bright-bright mode coupling, we numerically demonstrated the analogue of electromagnetically induced transparency (EIT) with a high quality factor (Q) in a stacked metal-dielectric metamaterial (MM) in the near-infrared regime. The optical coupling between a high-Q toroidal dipole mode supported by a silicon rod array and a low-Q dipole mode supported by a silver strip array was investigated from the near-field to the far-field regimes. We realized and significantly enhanced the long-range coupling between the two resonance modes through the MM-induced Fabry-Pérot (FP) effect. EIT with a Q factor greater than 1×104 could be achieved even when the two resonant structures were approximately a wavelength apart. These findings may open new avenues for realizing high-Q EIT, which is useful for photonic devices and biosensing applications. The proposed method can be extended to microwaves and terahertz waves.

Enhancement of bandwidth and angle response of metasurface cloaking through adding antireflective moth-eye-like microstructure.

Ultrathin metasurface provides a completely new path to realize cloaking devices on account of their fascinating ability to control electromagnetic wave. However, the conventional cloaking devices are limited by their narrow bandwidth. To overcome this challenge, we present the realization of ultrabroadband and wide angle metasurface cloaking through high refractive index dielectric layer and antireflective "moth-eye-like" microstructure in this work. Two options are proposed and demonstrated numerically in terahertz region. By using local phase compensation, the proposed carpet cloaks can suppress significantly the unexpected scattering and reconstruct wavefront. The cloaking effects of the proposed design are verified from 0.65THz to 0.9THz with a wide range of angles. Moreover, the proposed metasurface cloaking is probable to extend to the optical and microwave domains and can be applied in stealth, illusion optic, radar and antenna systems.

Flexible control of transmitting terahertz beams based on multilayer encoding metasurfaces.

The concept of encoding metasurfaces makes a connection between the physical metamaterial particles and the digital code so the digital coding can be performed on the coding surface for signal processing to achieve unusual physical phenomena. Here, a complete 2π transmission phase with high efficiency at the target frequency can be manipulated by a multilayer metasurface structure. Different period sequence codes are designed to obtain the deflection angles, and then a new sequence is obtained through a four-bit system operation sequence according to the Fourier convolution principle. This study shows that Fourier calculation provides an efficient way to optimize the coding to achieve a pre-designed transmitted beam. The advantage of this method over previous solutions in generating an anomalous single transmission beam is its flexibility and continuous control of arbitrary directions, and it is possible to transmit a normally incident terahertz beam to an abnormal arbitrary direction with cross-polarization. This work opens up a new digital perspective on the new approach to metamaterials, predicting the possibility of combining electromagnetic wave operations with the convolution theorems in digital signal processing and the transcoding surface.

Different dominant transitions in holmium and ytterbium codoped oxyfluoride glass and glass ceramics originating from varying phonon energy environments.

Transparent oxyfluoride glass and glass ceramics doped with 0.5% Ho3+ and 1.0% Yb3+ ions have been prepared. X-ray diffraction and transmission electron microscopy demonstrated the formation of NaYF4 nanocrystals during the heat treatment process. Raman spectra indicated the variation of glass structure brought about by the formation of NaYF4 nanocrystals. XRD curves and Judd-Ofelt intensity parameters confirmed the incorporation of Ho3+ into NaYF4 nanocrystals. Significantly enhanced visible upconversion and 2.85 µm emissions were achieved in glass ceramic under 980 nm laser diode pumping. A broadband spectrum with a full-width at half-maximum close to 132 nm was obtained in the glass ceramic. Besides, the calculated peak emission cross section was 0.6 × 10-20 cm2, suggesting that the glass ceramic is a promising gain material that can be applied to broadband amplifiers in the mid-infrared region. Furthermore, energy transfer mechanisms in glass and glass ceramics were proposed based on visible to mid-infrared emission spectra. It was found that the change in the photon energy environment around rare earth ions induced different dominant transitions in glass and glass ceramic. Finally, the influence of phonon energy on the transition processes was further quantitatively investigated, which may provide useful guidance for obtaining highly efficient 2.85 µm emission of holmium.

Determination of the effective constitutive parameters of bianisotropic planar metamaterials in the terahertz region.

We propose analytical expressions to determine the effective constitutive parameters of a planar bianisotropic metamaterial from scattering parameters in the terahertz region. In our retrieval method, the transmission and reflection coefficients in only one wave propagation direction are applied. Considering the nonsymmetry of planar metamaterials in the wave propagation direction, the effective refractive index and the impedance should be obtained by a modified S parameters retrieval process. The effective parameters of the permittivity, permeability, and magnetoelectric coupling coefficient of planar bianisotropic metamaterials can be retrieved by derived equations. Specifically, the constitutive parameters for different planar metamaterials, among which two are isotropic and the other two are bianisotropic metamaterials, are determined. The intrinsic differences between the normal planar metamaterials and the bianisotropic metamaterials are evidently illustrated. The phenomenon including electric coupling to magnetic resonance and only electric response in the transmission spectrum is confirmed by retrieval effective permittivity and permeability.

Manipulation of dual band ultrahigh index metamaterials in the terahertz region.

By drastically decreasing the diamagnetic effect with a thin metallic structure in the unit cell and increasing the effective permittivity through strong capacitive coupling, we designed the crossed I-shaped metallic patches metamaterial with an extremely high refractive index in the dual band terahertz region. The peak index of refraction of near 80 at about 0.75 THz is predicted, along with another peak index of about 25 at 2.85 THz. Based on the careful analysis of the high index on the dependence of the electric field coupling effect, the magnetic field diamagnetic response, and the geometric parameters in the unit cell, it is found that both of the high index bands respectively correspond to different components in the metamaterial structure. To realize a higher effective refractive index for the second band as well as for the first one, we proposed the triple I-shaped metallic metamaterial structure.

Quantitative Analysis of Energy Transfer and Origin of Quenching in Er(3+)/Ho(3+) Codoped Germanosilicate Glasses.

The energy transfer mechanism between Ho(3+) and Er(3+) ions has been investigated in germanosilicate glass excited by 980 nm laser diode. A rate equation model was developed to demonstrate the energy transfer from Er(3+) to Ho(3+) ions, quantitatively. Energy transfer efficiency from the Er(3+):(4)I13/2 to the Ho(3+):(5)I7 level can reach as high as 75%. Such a high efficiency was attributed to the excellent matching of the host phonon energy with the energy gap between Er(3+):(4)I13/2 and Ho(3+):(5)I7 levels. In addition, the energy transfer microparameter (CDA) from Er(3+):(4)I13/2 to Ho(3+):(5)I7 level was estimated to (4.16 ± 0.03) × 10(-40) cm(6)·s(-1) via the host-assisted spectral overlap function, coinciding with the CDA (2,88 ± 0.04) × 10(-40) cm(6)·s(-1) from decay analysis of the Er(3+):(4)I13/2 level which also indicated hopping migration-assisted energy transfer. Furthermore, the concentration quenching of Ho(3+):(5)I7 → (5)I8 transition was the dipole-dipole interaction in the diffusion-limited regime, and the quenching concentration of Ho(3+) reached 4.13 × 10(20) cm(-3).

Tunable mid-infrared luminescence from Er3+ -doped germanate glass.

Er(3+) -doped germanate glasses with superior thermal stability were prepared. Judd-Ofelt intensity parameters and important spectroscopic properties were discussed in detail. Upon 800 nm and 980 nm LD pumping, 2.7 µm fluorescence characteristics were investigated and it was found that the effective 2.7 µm emission bandwidth can reach to 101.79 nm in prepared glasses. The tunability of the 2.7 µm emission band can be realized by adjusting the Er(3+) content. Moreover, a high-emission cross-section (11.09 × 10(-21) cm(2) ), large gain bandwidth (772.30 × 10(-28) cm(3) ) and gain coefficient (6.72 cm(-1) ) were obtained in the prepared sample. Hence, Er(3+) -doped germanate glass might be a promising mid-infrared material for tunable amplifiers or lasers.