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The direct bandgap semiconductor serves as a fundamental terahertz (THz) source, generating terahertz waves through ultrafast femtosecond light excitation. While the direct bandgap semiconductor THz source does not rely on an external bias voltage, its limited radiation power poses a challenge to its implementation in compact THz systems. Here, nanometasurfaces etched on a semi‐insulating gallium arsenide (GaAs) substrate for enhanced THz emission are reported. The proposed semiconductor terahertz emitting nanometasurface (STEN) device based on GaAs direct bandgap semiconductor shows a 35‐fold increase in THz power and enables precise control of the emitted bandwidth. Giant emission is attributed to the enhanced absorption of pump light by the GaAs nano‐pillar array and the localized effect of the pump field. Experimental results confirm the effectiveness of the structures, demonstrating a negative correlation between the radiation power enhancement factor and pump power, indicating suitability for low‐power sources. This design offers broad prospects for semiconductor THz emitting nanometasurfaces, ideal for integration with compact femtosecond lasers. [ABSTRACT FROM AUTHOR]

Terahertz (THz) vortex beams with helical wavefronts promise many important applications ranging from high‐speed wireless communications to super‐resolution imaging. Conventional methods to generate THz beams mostly rely on manipulating and converting the incident THz beams. Herein is proposed and experimentally demonstrated how the patterned indium tin oxide (ITO) film, i.e., ITO Fresnel zone plate (ITO‐FZP), enables direct emission of focused THz vortex beams with desired orbital angular momentums. Such THz vortex‐beam emitters are based on the straightforward nonlinear THz emission through the patterned ITO film under femtosecond laser excitation. The proposed approach greatly integrates THz emission and vortex‐beam generation processes, which is low cost, robust, and easy to fabricate. [ABSTRACT FROM AUTHOR]

The achievable functionalities of metasurfaces are mainly determined by the capability of their unit cells to control the local electromagnetic responses. To realize full control over light, it is highly desired to find a kind of unit cell that can achieve complete control of the phase, amplitude, and polarization, which remains a big challenge and cannot be fully replaced with iteration algorithms. Here, such control is achieved by exploiting interference in a dielectric unit cell composed of multiple meta‐atoms, namely, a meta‐molecule. This strategy provides sufficient effective geometric degrees of freedom within the meta‐molecule to enable full control over its optical properties, as is demonstrated here with a complete meta‐molecule database. As a proof‐of‐concept demonstration, polarization‐independent, polarization‐dependent, and vectorial meta‐holograms are proposed and experimentally characterized. High imaging quality is achieved in both the measured intensity and phase distributions. This is allowed by the ability of the meta‐molecules to fully record the information of the target images. This strategy opens a direct way toward optical devices with various desirable functionalities. [ABSTRACT FROM AUTHOR]

Recently, chiral metasurfaces have drawn enormous attentions due to their flexibility in realizing nearly arbitrary chiral effect with designable strength. One intriguing direction in this field is chiral‐selective absorption. However, previous studies mostly require multilayer design and meanwhile often work in a passive way. Here, for the first time to the authors' knowledge, a chiral‐selective metasurface absorber is experimentally demonstrated with a single‐layer free‐standing metasurface using coherent control method, where the strength of the chiral‐selective behavior can be actively tuned with a 100% modulation depth by simply changing the phase difference between the two coherent inputs. Specifically, under coherent symmetric illumination, near unity absorption is achieved for one circularly polarized incidence, while high‐efficiency cross‐polarization conversion is obtained for the other circularly polarized incidence. Such chiral‐selective absorption property is quite suitable for achieving coherent chiral‐selective wavefront control for the unabsorbed circularly polarized incidence. Three coherent chiral metagratings are designed and experimentally characterized by further employing Pancharatnam–Berry phase method and multiplexed method, where very good performances are achieved. The proposed approach on coherent control can enrich the functionalities and also improve the performances of the conventional single‐layer metasurfaces, which can find extending applications in various practical cases. [ABSTRACT FROM AUTHOR]

The notion of metasurface has inspired the innovation of various functional devices in the terahertz band, but the intrisinc dispersion restricts their application in broadband scenarios. Here, two terahertz achromatic linear‐phase‐gradient metasurface devices are demonstrated, which are a beam deflector and a beam splitter, respectively. The phase and dispersion of the metasurfaces are simultaneously engineered by changing the geometric parameters of the unit cells made of silicon gratings and pillars. The simulated and experimental results demonstrate the achromatic feasibility of the beam deflector from 0.6 to 1.2 THz and of the beam splitter from 0.6 to 1.1 THz. The transmittances and the splitting ratios of the achromatic beam splitter are also analyzed. The metasurface based achromatic beam deflector and splitter presented here not only enrich the terahertz functional devices, but the methods and structures may also promote the research of broadband terahertz metasurfaces. [ABSTRACT FROM AUTHOR]

Fano resonances in metasurfaces are important due to their low loss subradiant behavior that allows excitation of high‐quality (Q) factor resonances extending from the microwave to the optical regime. High‐Q Fano resonances have recently enabled applications in the areas of sensing, modulation, filtering, and efficient cavities for lasing spasers. Highly conducting metals are the most commonly used materials for fabricating the metasurfaces, especially at the low‐frequency terahertz region where the DC, Drude, and perfect electric conductivity show similar resonant behavior of the subwavelength meta‐atoms. Here, it is experimentally and theoretically demontrated that the Q factor of a low asymmetry Fano resonance is extremely sensitive to the conducting properties of the metal at terahertz frequencies. Large differences in the Q factor and figure of merit of the Fano resonance is observed for perfect electric conductors, Drude metal, and a DC‐conducting metal, which is in sharp contrast to the behavior of the inductive–capacitive resonance of meta‐atoms at terahertz frequency. Identification of such a low asymmetry regime in Fano resonances is the key to engineer the radiative and nonradiative losses in plasmonic and metamaterial‐based devices that have potential applications in the microwave, terahertz, infrared, and the optical regimes. [ABSTRACT FROM AUTHOR]

Graphene, as a popular 2D semiconductor, with a carrier concentration that can be extensively tailored by external stimulants, such as electric bias and photoexcitation, is a promising candidate for active optoelectronics. An electrically tunable perfect terahertz absorber is presented by integrating a metallic grating into classical graphene Salisbury screen. The measurement shows that perfect absorption can be achieved for even lightly‐doped graphene and a modulation depth up to 25% is realized. Numerical simulation and analytical model based on the transmission line theory are developed to validate the design, showing a good agreement with the measured results. In addition, dependence of the terahertz response on physical dimensions and polarization of the incident terahertz field is also experimentally explored. It indicates that the hybrid terahertz device is able to function as tunable perfect absorber or tunable polarizer with an extinction ratio up to 23 dB. These results demonstrate the feasibility of using low carrier mobility chemical vapor deposition (CVD) grown graphene for tunable perfect terahertz absorption, which is suitable for potential applications, such as terahertz switching, and time domain and spatial domain modulation. [ABSTRACT FROM AUTHOR]

Abstract: Recently, metasurfaces made up of dielectric structures have drawn enormous attentions in the optical and infrared regimes due to their high efficiency and designing freedom in manipulating light propagation. Such advantages can also be introduced to terahertz frequencies where efficient functional devices are still lacking. Here, polarization‐dependent all‐silicon terahertz dielectric metasurfaces are proposed and experimentally demonstrated. The metasurfaces are composed of anisotropic rectangular‐shaped silicon pillars on silicon substrate. Each metasurface holds dual different functions depending on the incident polarizations. Furthermore, to suppress the reflection loss and multireflection effect in practical applications, a high‐performance polarization‐independent antireflection silicon pillar array is also proposed, which can be patterned at the other side of the silicon substrate. Such all‐silicon dielectric metasurfaces are easy to fabricate and can be very promising in developing next‐generation efficient, compact, and low‐cost terahertz functional devices. [ABSTRACT FROM AUTHOR]