High focusing efficiency metalens with large numerical aperture at terahertz frequency.

Metalenses have been widely investigated for their features of high design freedom. For practical applications, it is necessary to maximize the efficiency of the metalens. However, it is a great challenge to realize both a high numerical aperture (NA) and high-efficiency metalens in the community. Here, we introduce a method to design a hybrid metalens with a large numerical aperture and high focusing efficiency at terahertz frequency. The hybrid metalens consists of gradient metasurfaces in the central area and metagrating in the peripheral area to achieve high-efficiency beam focusing. To verify this concept, a hybrid metalens with a numerical aperture of 0.95 was designed at λ = 118.8 µm. The simulation results demonstrate that the focusing efficiency of the hybrid metalens is 65.8%. The experimental results show that the designed metalens is able to increase the focusing efficiency from 22.8% to 41.7%. The full widths at half maxima (FWHMs) of the focused spots of the hybrid metalens in the x direction and y direction are 0.72λ and 0.45λ, respectively. The proposed high-efficiency hybrid metalens has promising application prospects in various applications of a complex optical system.

Superoscillation focusing with suppressed sidebands by destructive interference.

Optical superoscillation, a phenomenon that the local optical field can oscillate much faster than that allowed by its highest harmonic, can significantly overcome the Abbe diffraction limit. However, as the spot size is compressed below the superoscillation criteria of 0.38λ/NA, huge sidebands will inevitably appear around the central lobe with intensity hundreds of times higher than that of the central lobe. Here, we propose an approach to realize superoscillation by using destructive interference. The central lobe size can be compressed beyond the superoscillation criteria without formation of strong sidebands by destructive interference between focused fields. Such a super-resolution metalens can find its application in label-free far-field super-resolution microscopy.

Dynamically tunable coherent perfect absorption in topological insulators at oblique incidence.

The effective engineering of light absorption has been the focus of intensive research to realize the novel optoelectronic devices based on a topological insulator, a unique topologically protected surface Dirac-state quantum material with excellent prospects in electronics and photonics. Here, we theoretically proposed a versatile platform for manipulating the light-matter interaction employing the dynamically tunable coherent perfect absorption (CPA) in the topological insulator Bi1.5Sb0.5Te1.8Se1.2(BSTS). By simply varying the phase difference between two coherent counter-propagating beams, the BSTS-based CPA device can be continuously switched from the high transparency state to the strong absorption state, leading to the modulation of absorption ranging from 0.2% to 99.998%. Under the illumination of TE-polarized wave, the high absorption (>90%) can be implemented within a broad range from 0.47 to 1.51 µm through a proper incident angle alteration. In addition, the quasi-CPA wavelength can be flexibly selected by tuning the bulk thickness of BSTS film while maintaining high modulation depth of 104. Such BSTS-based CPA device with flexible tunability, wide absorption modulation range, and high modulation depth is expected to be utilized in a wide range of potential applications such as in next-generation coherent detectors, coherent modulators, all-optical switches, and signal processors.

Subdiffraction focusing lens based on quadrangular-frustum pyramid-shaped metasurface.

In order to realize the function of subdiffraction focusing using a single flat lens, a special metalens is proposed to modulate the circularly polarized light. Initially, the analyses indicate that the phase shift can cover [0,2π] by changing the rotation angle of a quadrangular-frustum pyramid-shaped structure from 0° to 180°, while the average amplitude transmittance can reach 96% at the wavelength of λ=1550 nm. Then, a flat metalens is designed by carefully arranging the quadrangular-frustum pyramid-shaped structures. The simulated results show that a focal spot could be obtained at the focal length of about f=2.87λ, and the focusing efficiency is ∼14.9%. Meanwhile, the full width at half maximum (FWHM) of the focusing spot is only 0.48λ, which is smaller than the diffraction limit. Furthermore, this designed metalens can function comparatively with a conventional lens in one-to-one imaging.

Broadband quarter-wave birefringent meta-mirrors for generating sub-diffraction vector fields.

Independent manipulation of phase and polarization of optical fields is of great interest in various applications, including vector-field generation, optical tweezers, and nanolithography. The integration of phase and polarization manipulation on a single optical device may greatly simplify optical systems and eases optical alignment. In this Letter, a family of reflective cross-shaped quarter-wave birefringent metasurfaces is proposed to achieve full control of polarization and phase of reflected waves. Based on the proposed metasurfaces, two meta-mirrors are designed with integrated functions of polarization conversion and sub-diffraction focusing. Numerical investigations also reveal the achromatic focusing performance of the two meta-mirrors. The proposed metasurfaces with independent manipulation of polarization and phase provide flexible building blocks for constructing complicated vector optical fields.

Sub-wavelength tight-focusing of terahertz waves by polarization-independent high-numerical-aperture dielectric metalens.

A focusing device is one of the key elements for terahertz applications, including homeland security, medicine, industrial inspection, and other fields. Sub-wavelength tight-focusing of terahertz waves is attractive for microscopy and spectroscopy. Flat optical lenses based on metasurfaces have shown potential in diffraction-limit focusing and advantages of ultrathin thickness and lightweight for large-aperture optics. However previously reported THz metalenses suffered from either polarization-dependency or small numerical aperture (NA), which greatly limits their focusing performance. In this paper, to achieve high-NA and polarization-free operation, we proposed a polarization-independent dielectric metasurface with a sub-wavelength period of 0.4λ. A planar terahertz lens based on such metasurface was designed for a wavelength of λ = 118.8 µm with a focal length of 100λ, a radius of 300λ, and a high NA of 0.95, which was fabricated with a silicon-on-insulator wafer. The experimental results demonstrate a tight focal spot with sub-wavelength full widths at half-maxima of 0.45λ and 0.61λ in the x and y directions, respectively, on the focal plane. In the x direction, the size of 0.45λ is even smaller than the diffraction limit 0.526λ (0.5λ/NA). Such a metalens is favorable for sub-wavelength tight-focusing terahertz waves with different polarizations, due to its polarization independence. The metalens has potential applications in THz imaging, spectroscopy, information processing, and communications, among others.

Influence of interface inhomogeneity on the electrical transport mechanism of CdSe nanowire/Au Schottky junctions.

Schottky junctions based on one-dimensional semiconductor nanomaterials, such as nanowires (NWs) and nanobelts (NBs), have been widely used in building high-performance nano-electric and nano-optoelectric devices during the past 15 years. Meanwhile, with considerable development in diverse application fields, more and more interests are turning to the investigation of the fundamental physics inside the junctions. The inhomogeneity of the interface between semiconductor NWs/NBs and metal electrodes has significant influence on the electrical transport mechanism of Schottky junctions. However, few researchers are involved in such studies and the physical mechanism here is far from fully understood. In this work, we fabricated Schottky junctions based on single CdSe NWs, in which Au was used as a Schottky contact with CdSe NW. The temperature dependence of the electrical transport characteristics of typical CdSe NW/Au Schottky junctions were characterized. The ideality factor was found to decrease and the zero-bias Schottky barrier height (SBH) increased monotonously as the temperature was increased from 140 to 320 K, and this relationship was ascribed to SBH inhomogeneity. The electrical transport mechanism was analyzed quantitatively with a spatial potential fluctuation model, in which SBHs obey the Gaussian distribution. The standard deviation of the SBH distribution was determined to be as high as 13.54% and 13.94% of the zero-bias mean SBH in the temperature ranges 140-200 K and 200-320 K, respectively. Our work revealed the barrier inhomogeneity at CdSe NW/Au interfaces and its influence on the electrical transport mechanism of NW-based Schottky junctions.

Mode energy of graphene plasmons and its role in determining the local field magnitudes.

We theoretically study the mode energy of graphene plasmons and its fundamental role in determining the local field magnitudes. While neglecting the magnetic field energy of the mode, we derive a concise expression for the total mode energy, which is independent on the details of the mode field distributions and valid for both propagating and localized modes. We find that the mean square of the local electric fields of a graphene plasmonic mode scales linearly with the light absorption rate of the mode and the electron relaxation time of graphene. The possible strategies for improving the local field magnitudes of graphene plasmons are also discussed. Our theoretical analysis presented here may benefit the design of various graphene-based optical and optoelectronic devices for light-harvesting or energy conversion.

Strong coherent coupling between graphene surface plasmons and anisotropic black phosphorus localized surface plasmons.

The anisotropic plasmons properties of black phosphorus allow for realizing direction-dependent plasmonics devices. Here, we theoretically investigated the hybridization between graphene surface plasmons (GSP) and anisotropic black phosphorus localized surface plasmons (BPLSP) in the strong coupling regime. By dynamically adjusting the Fermi level of graphene, we show that the strong coherent GSP-BPLSP coupling can be achieved in both armchair and zigzag directions, which is attributed to the anisotropic black phosphorus with different in-plane effective electron masses along the two crystal axes. The strong coupling is quantitatively described by calculating the dispersion of the hybrid modes using a coupled oscillator model. Mode splitting energy of 26.5 meV and 19 meV are determined for the GSP-BPLSP hybridization along armchair and zigzag direction, respectively. We also find that the coupling strength can be strongly affected by the distance between graphene sheet and black phosphorus nanoribbons. Our work may provide the building blocks to construct future highly compact anisotropic plasmonics devices based on two-dimensional materials at infrared and terahertz frequencies.

Electric Field Stiffening Effect in c-Oriented Aluminum Nitride Piezoelectric Thin Films.

Aluminum nitride offers unique material advantages for the realization of ultrahigh-frequency acoustic devices attributed to its high ratio of stiffness to density, compatibility with harsh environments, and superior thermal properties. Although, to date, aluminum nitride thin films have been widely investigated regarding their electrical and mechanical characteristics under alternating small signal excitation, their ultrathin nature under large bias may also provide novel and useful properties. Here, we present a comprehensive investigation of electric field stiffening effect in c-oriented aluminum nitride piezoelectric thin films. By analyzing resonance characteristics in a 2.5 GHz aluminum nitride-based film bulk acoustic resonator, we demonstrate an up to 10% linear variation in the equivalent stiffness of aluminum nitride piezoelectric thin films when an electric field was applied from -150 to 150 MV/m along the c-axis. Moreover, for the first time, an atomic interaction mechanism is proposed to reveal the nature of electric field stiffening effect, suggesting that the nonlinear variation of the interatomic force induced by electric field modulation is the intrinsic reason for this phenomenon in aluminum nitride piezoelectric thin films. Our work provides vital experimental data and effective theoretical foundation for electric field stiffening effect in aluminum nitride piezoelectric thin films, indicating the huge potential in tunable ultrahigh-frequency microwave devices.

A Low-G Silicon Inertial Micro-Switch with Enhanced Contact Effect Using Squeeze-Film Damping.

Contact time is one of the most important properties for inertial micro-switches. However, it is usually less than 20 µs for the switch with rigid electrode, which is difficult for the external circuit to recognize. This issue is traditionally addressed by designing the switch with a keep-close function or flexible electrode. However, the switch with keep-close function requires an additional operation to re-open itself, causing inconvenience for some applications wherein repeated monitoring is needed. The switch with a flexible electrode is usually fabricated by electroplating technology, and it is difficult to realize low-g switches (<50 g) due to inherent fabrication errors. This paper reports a contact enhancement using squeeze-film damping effect for low-g switches. A vertically driven switch with large proof mass and flexible springs was designed based on silicon micromachining, in order to achieve a damping ratio of 2 and a threshold value of 10 g. The proposed contact enhancement was investigated by theoretical and experimental studies. The results show that the damping effect can not only prolong the contact time for the dynamic acceleration load, but also reduce the contact bounce for the quasi-static acceleration load. The contact time under dynamic and quasi-static loads was 40 µs and 570 µs, respectively.

Simulation Study of Inertial Micro-Switch as Influenced by Squeeze-Film Damping and Applied Acceleration Load.

Squeeze-film damping and acceleration load are two major issues in the design of inertial micro-switches. In order to deeply and systematically study these two issues, this paper proposes a typical vertically-driven inertial micro-switch, wherein the air and electrode gaps were chosen to design the required damping ratio and threshold value, respectively. The switch was modeled by ANSYS Workbench, and the simulation program was optimized for computational accuracy and speed. Transient analysis was employed to investigate the relationship between the damping ratio, acceleration load, and the natural frequency, and the dynamic properties (including contact bounce, contact time, response time, and threshold acceleration) of the switch. The results can be used as a guide in the design of inertial micro-switches to meet various application requirements. For example, increasing the damping ratio can prolong the contact time of the switch activated by short acceleration duration or reduce the contact bounce of the switch activated by long acceleration duration; the threshold value is immune to variations in the damping effect and acceleration duration when the switch is quasi-statically operated; the anti-jamming capability of the switch can be improved by designing the sensing frequency of the switch to be higher than the acceleration duration but much lower than the other order frequencies of the switch.

Synthesis of the system modeling and signal detecting circuit of a novel vacuum microelectronic accelerometer.

A novel high-precision vacuum microelectronic accelerometer has been successfully fabricated and tested in our laboratory. This accelerometer has unique advantages of high sensitivity, fast response, and anti-radiation stability. It is a prototype intended for navigation applications and is required to feature micro-g resolution. This paper briefly describes the structure and working principle of our vacuum microelectronic accelerometer, and the mathematical model is also established. The performances of the accelerometer system are discussed after Matlab modeling. The results show that, the dynamic response of the accelerometer system is significantly improved by choosing appropriate parameters of signal detecting circuit, and the signal detecting circuit is designed. In order to attain good linearity and performance, the closed-loop control mode is adopted. Weak current detection technology is studied, and integral T-style feedback network is used in I/V conversion, which will eliminate high-frequency noise at the front of the circuit. According to the modeling parameters, the low-pass filter is designed. This circuit is simple, reliable, and has high precision. Experiments are done and the results show that the vacuum microelectronic accelerometer exhibits good linearity over -1 g to +1 g, an output sensitivity of 543 mV/g, and a nonlinearity of 0.94 %.