[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

비만아 부모의 돌봄 경험: q 방법론

Metasurfaces enable a new paradigm to control electromagnetic waves by manipulating subwavelength artificial structures within just a fraction of wavelength. Despite the rapid growth, simultaneously achieving low-dimensionality, high transmission efficiency, real-time continuous reconfigurability, and a wide variety of reprogrammable functions is still very challenging, forcing researchers to realize just one or few of the aforementioned features in one design. This study reports a subwavelength reconfigurable Huygens' metasurface realized by loading it with controllable active elements. The proposed design provides a unified solution to the aforementioned challenges of real-time local reconfigurability of efficient Huygens' metasurfaces. As one exemplary demonstration, a reconfigurable metalens at the microwave frequencies is experimentally realized, which, to the best of the knowledge, demonstrates for the first time that multiple and complex focal spots can be controlled simultaneously at distinct spatial positions and reprogrammable in any desired fashion, with fast response time and high efficiency. The presented active Huygens' metalens may offer unprecedented potentials for real-time, fast, and sophisticated electromagnetic wave manipulation such as dynamic holography, focusing, beam shaping/steering, imaging, and active emission control.

A Reconfigurable Active Huygens' Metalens.

Control over the dispersion of the refractive index is essential to the performance of most modern optical systems. These range from laboratory microscopes to optical fibres and even consumer products, such as photography cameras. Conventional methods of engineering optical dispersion are based on altering material composition, but this process is time-consuming and difficult, and the resulting optical performance is often limited to a certain bandwidth. Recent advances in nanofabrication have led to high-quality metasurfaces with the potential to perform at a level comparable to their state-of-the-art refractive counterparts. In this Review, we introduce the underlying physical principles of metasurface optical elements (with a focus on metalenses) and, drawing on various works in the literature, discuss how their constituent nanostructures can be designed with a highly customizable effective index of refraction that incorporates both phase and dispersion engineering. These metasurfaces can serve as an essential component for achromatic optics with unprecedented levels of performance across a broad bandwidth or provide highly customized, engineered chromatic behaviour in instruments such as miniature aberration-corrected spectrometers. We identify some key areas in which these achromatic or dispersion-engineered metasurface optical elements could be useful and highlight some future challenges, as well as promising ways to overcome them. Flat metasurface optics provides an emerging platform for combining semiconductor foundry methods of manufacturing and assembling with nanophotonics to produce high-end and multifunctional optical elements. This Review highlights the design of metasurfaces, recent advances in the field and initial promising applications.

Flat optics with dispersion-engineered metasurfaces

Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.

https://iopscience.iop.org/article/10.1088/0022-3727/47/37/374001/pdf

A review of terahertz sources

A novel slow-wave structure called sine waveguide has been proposed to develop a wideband high-power terahertz radiation source. The sine waveguide evolves from a rectangular waveguide oscillating with sinusoid along its longitudinal direction. This letter reports the electromagnetic characteristics of the sine waveguide and its effective surface plasmon amplification mechanism. From our calculation, this circuit structure possesses low ohmic losses and reflection and can be applied to produce terahertz waves ranging from 0.2 to 0.25 THz with several hundreds of watts. Moreover, the maximum gain and interaction efficiency may reach 37.7 dB and 9.6%, respectively.

Sine Waveguide for 0.22-THz Traveling-Wave Tube

In the free space optical communication, the information can be encoded as the orbital angular momentum (OAM) state of light, which is called OAM shift keying (OAM-SK). This paper has proposed a communication system with OAM-SK, in which an image has been delivered from the transmitter to the receiver successfully in the simulation environment. Specifically, we have carefully designed and implemented the phase holograms used at the transmitter and the receiver for multiplexing and de-multiplexing the OAM states, respectively. At the transmitter, the multiplexing phase hologram designed by the modified Lin's algorithm is loaded on the spatial light modulator 1 (SLM1) to generate the multiplexing vortex beam, which is a superposition of multiple vortex beams with different OAM states. Correspondingly, at the receiver, a novel phase hologram is designed and loaded on the SLM2 to effectively de-multiplex the multiplexing vortex beam in different directions. In our phase hologram used at the receiver, the detected power of each OAM state can be controlled by adjusting the weight coefficient by the modified Lin's algorithm. This way, the incident power can be concentrated to the target OAM states, from which the target OAM states can be detected more effectively than conventional fork grating.

Orbital Angular Momentum Shift Keying Based Optical Communication System

In this paper, a novel V-shaped microstrip meander-line slow-wave structure (SWS) is proposed for use in a low-voltage high-efficiency wide-bandwidth miniature millimeter-wave traveling-wave tube (TWT). The electromagnetic characteristics and the interaction between the sheet electron beam and slow wave in this SWS are obtained by utilizing the CST Microwave Studio and Particle Studio codes, respectively. From our calculations, it is predicted that, at a beam voltage of 3.7 kV and a beam current of 100 mA, an output power greater than 30 W can be obtained ranging from 75 to 100 GHz, and this V-shaped microstrip meander-line TWT will be helpful for a W-band millimeter-wave power module.

A Novel V-Shaped Microstrip Meander-Line Slow-Wave Structure for W-band MMPM

In this paper, a novel polarization splitter has been designed at the telecommunication wavelength of 1500 nm successfully based on the dielectric metasurface consisting of a silicon nanobrick array, which can generate two different wavefronts for two orthogonal input polarizations with well over 90% transmitted efficiency by exactly selecting the sizes of the used nanobricks. The splitting mechanism is attributed to the fact that the used nanobricks can supply two different incremental transmission phases for the X-linear-polarization (XLP) incidence and Y-linear-polarization (YLP) incidence respectively. A polarization-dependent beam splitter which can effectively deflect the X- and Y-linear-polarization incidences in the opposite direction with 90% transmitted efficiency have been realized based on the same metasurface, where the refracted angle of the co-polarized refracted light can be arbitrarily controlled by changing the lattice constant of the nanobricks. In addition, a polarization-independent beam deflector which can make the linear-polarization (XLP and YLP) incident lights refract in the same direction has also been designed and presented. However, it is very difficult to obtain concrete XLP and YLP compositions in the transmitted fields in real experiments. Thus we have designed a novel metalens to focus the two orthogonal polarization compositions in different positions. The capacities to completely manipulate the phase and wavefronts demonstrate the potential of the designed metasurfaces in realizing most free-space transmissive optical devices such as phase plates, polarizers, as well as vector beam generators and so on.

/pdf/dielectric-metasurface-based-high-efficiency-polarization-3tovpukatx.pdf

Dielectric metasurface based high-efficiency polarization splitters

A novel multi-focusing metalens in the longitudinal direction has been proposed and investigated based on the equal optical path principle, which is independent on the incident polarizations and can be suitable for both of the linear and circular polarization incidences simultaneously. Here, three novel designing principles: partitioned mode, radial alternating mode and angular alternating mode, have been proposed firstly for constructing different types of the longitudinal multi-focusing metalenses. The performances of the designed metalenses based on the different designed methods have also been analyzed and investigated in detail, and the intensity ratio of the focusing spots can be tuned easily by modulating the numbers of the relative type of nanoantennas, which is significant for the micro-manipulating optics and the multi-imaging technology in the integrated optics.

Fei Shen

Papers

Sine Waveguide for 0.22-THz Traveling-Wave Tube

Orbital Angular Momentum Shift Keying Based Optical Communication System

A Novel V-Shaped Microstrip Meander-Line Slow-Wave Structure for W-band MMPM

Dielectric metasurface based high-efficiency polarization splitters

Polarization-independent longitudinal multi-focusing metalens.