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

Abstract

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...

Simultaneous Terahertz Pulse Generation and Manipulation with Spintronic Coding Surface

“…The effectiveness of this approach was exhibited by using a metasurface [23] or a patterned ITO (indium tin oxide) film. [24] Figure 3a depicts the photograph of spintronic coding surfaces composed of binary-phase zones that produce THz pulses with opposite polarities. It can be treated as a Fresnel zone plate when an unfocused optical beam illuminates the coding surface.…”

“…The slight distortion in the y-z section is due to the limited detecting area and an equivalent number of pixels. Notably, this technique relies solely on pure phase modulation, which theoretically provides a higher radiation efficiency than the patterned ITO film [24] (based on amplitude modulation) and does not require oblique incident conditions. In addition, unlike existing metasurface THz emitters, [12,13,23] it does not require nanofabrication and possibly has a higher damage threshold.…”

Broadband Achromatic Metalens for Tunable Focused Vortex Beam Generation in the Near-Infrared Range

“…Specifically, their spiral phase can be identified using the Hilbert factor exp(lθ), where l and θ are the topological charge and angular coordinate, respectively. Benefiting from the unique characteristics as well as the additional degree of freedom to control optical waves, the use of vortex beams has been demonstrated in various practical implantations that may significantly benefit current industrial technologies, for instance, high-dimensional information processing [3,4], super-resolution imaging [5,6], the manipulation of nanoparticles [7,8], etc. Although current techniques for generating vortex beams are quite mature by employing spiral phase plate [9], spatial light modulator [10], and Qwaveplate [11], these conventional optical devices are normally with complex geometrical structures to impart a helical phase that inevitably cause bulk size, high cost and confined performances.…”

Multi‐Dimensional Light‐Emitting Meta‐Display: Photoluminescence and Pumping Light Multiplexing

“…The nano-scale emitters act as the photon source, while the meta-atoms manipulate the properties of near-field and far-field emissions. [10][11][12] To enhance the versatility and efficiency, recent studies have utilized the principles of grating effect, [13,14] localized resonances, [14,15] bound states in continuum [16][17][18][19] to realize emission enhancement, [20,21] directional emission, [22][23][24][25] chiral emission, [26][27][28][29] and fluorescent lens, [30][31][32] etc. However, these studies prioritize luminescence functions without fully exploring the light-manipulating capabilities of pump light simultaneously.…”