Polarization-independent all-silicon dielectric metasurfaces in the terahertz regime

105

on distinctive spectral signatures resulting from vibrational modes of covalent and hydrogen bonds. [8,[13][14][15][16] The focus on this spectrally rich region is attributed to the highly coherent and nonionizing nature of terahertz radiation, wide unallocated frequency bands, distinctive wavelengths, and their penetration through a significant depth of dielectric materials. Terahertz technology is geared toward realizing devices that can efficiently manipulate the phase, amplitude, and polarization of terahertz radiations for the above-mentioned myriad of applications.However, progress in this domain is held back by low terahertz power available from compact sources, high free-space path loss, and limited choice of materials that exhibit less absorption of terahertz waves. [17] Owing to the fact that natural materials demonstrate weak wave-matter interaction at terahertz frequencies, terahertz devices with engineered subwavelength resonant metallic inclusions on dielectric spacers have been realized, to interact strongly with an incident electromagnetic wave.In the past, metamaterials have been a promising route to building terahertz devices. These devices have in essence been engineered as 3D resonating elements that effectively manipulate both permittivity and permeability of an effective medium to couple to free space. The underlining disadvantage of these structures is the difficulty involved in the fabrication of 3D geometrical structures using standard semiconductor fabrication techniques. Standard fabrication techniques are generally amenable to 2D design with extrusion of these structures into thickness (the third, vertical dimension). Moreover, the choice and availability of dielectric materials and metals that provide sufficient interaction while retaining device efficiency has proved challenging. [13,[18][19][20] Recently, effective manipulation of electromagnetic wave has been widely demonstrated with 2D metasurfaces, which were originally employed as building blocks for 3D metamaterials. In these lower-dimension designs, effective manipulation of terahertz waves for various applications is achieved by carefully designing sub-wavelength resonant structures as is evident in published review articles. [21,22] Metasurfaces present high degree of compactness that enhances radiation efficiency. Additionally, the planar form factor of metasurfaces enables Manipulation of terahertz radiation opens new opportunities that underpin application areas in communication, security, material sensing, and characterization. Metasurfaces employed for terahertz manipulation of phase, amplitude, or polarization of terahertz waves have limitations in radiation efficiency which is attributed to losses in the materials constituting the devices. Metallic resonators-based terahertz devices suffer from high ohmic losses, while dielectric substrates and spacers with high relative permittivity and loss tangent also reduce bandwidth and efficiency. To overcome these issues, a proper choice of low loss and low relative permittivi...

Section: Dielectric Materials As Resonant Elements and Structuresmentioning

confidence: 99%

Section: Lossless Dielectric Materialsmentioning

confidence: 99%

Section: Lossless Dielectric Materialsmentioning

confidence: 99%

Section: Lossless Dielectric Materialsmentioning

confidence: 99%

Section: Lossless Dielectric Materialsmentioning

confidence: 99%

See 3 more Smart Citations

Dielectrics for Terahertz Metasurfaces: Material Selection and Fabrication Techniques

Ako

Upadhyay

Withayachumnankul

et al. 2019

105

Dielectric Metasurfaces for Complete Control of Phase, Amplitude, and Polarization

Zhang

et al. 2021

Self Cite

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.

Direct Emission of Focused Terahertz Vortex Beams Using Indium‐Tin‐Oxide‐Based Fresnel Zone Plates

Feng

Chen

et al. 2022

Self Cite

For example, on account of the diffraction limit, the resolutions of the conventional THz imaging systems are usually limited to the sub-millimeter scale, [11,12] however, the hollow feature of the THz vortex beams provides a way to break through the limit and help achieve super-resolution imaging. [8,13] In addition, THz vortex beams are also promising in high-capacity communications as they can increase spatially multiplexing channels based on the orthogonality of the vortex beams with different orbital angular momenta (OAMs). [14,15] Up to now, many methods have been developed to generate THz vortex beams by introducing wavefront control devices into the THz beam path, such as phase plates [13,16,17] and metasurfaces. [18][19][20] However, these were all implemented under the condition of an existing THz beam. Namely, two separate devices, i.e., one for THz generation and the other for vortex wavefront manipulation are required. If the two functionalities can be integrated into a single THz device, then a higher degree of integration is achieved. Currently, several methods have been reported that can directly emit THz vortex beams by designing the nonlinear effects in nonlinear crystals [21,22] and metasurfaces. [7] However, dual-pump condition and the state-of-the-art nanofabrication are respectively required, which increase the complexity and cost in device implementation. Thus, seeking an easy-to-fabrication, low-cost and high-level integration method for THz vortex beam emission is urgently demanded.In our previous work, we demonstrated that an ultrathin indium tin oxide (ITO) film, a very common electrode material, could emit broadband THz waves under an oblique pump of infrared femtosecond laser (IR-Pump). [23] By combing it with an axicon, we demonstrated a direct THz vector Bessel beam emitter. [24] In this work, we experimentally demonstrate that such an ITO film can also be applied to directly emit focused THz vortex beams when it is patterned following the designing concept of a Fresnel zone plate, denoted as ITO-FZP. Three ITO-FZPs that can emit focused THz vortex beams with OAMs of l = 0, +1, and +2 are proposed, as schematically illustrated in Figure 1a. The calculated and measured results show very good consistence with the expectation. The proposed ITO-FZPs are low-cost and easy to fabricate, which is promising in building compact THz systems for vortex beam generation and applications. Terahertz (THz) vortex beams with helical wavefronts promise many important applications ranging from high-speed wireless communications to superresolution 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 fi...