The terahertz range possesses significant untapped potential for applications including high-volume wireless communications, noninvasive medical imaging, sensing, and safe security screening. However, due to the unique characteristics and constraints of terahertz waves, the vast majority of these applications are entirely dependent upon the availability of beam control techniques. Thus, the development of advanced terahertz-range beam control techniques yields a range of useful and unparalleled applications. This article provides an overview and tutorial on terahertz beam control. The underlying principles of wavefront engineering include array antenna theory and diffraction optics, which are drawn from the neighboring microwave and optical regimes, respectively. As both principles are applicable across the electromagnetic spectrum, they are reconciled in this overview. This provides a useful foundation for investigations into beam control in the terahertz range, which lies between microwaves and infrared light. Thereafter, noteworthy experimental demonstrations of beam control in the terahertz range are discussed, and these include geometric optics, phased array devices, leaky-wave antennas, reflectarrays, and transmitarrays. These techniques are compared and contrasted for their suitability in applications of terahertz waves.

We design and experimentally demonstrate an ultrathin, ultrabroadband, and highly efficient reflective linear polarization convertor or half-wave retarder operating at terahertz frequencies. The metamaterial-inspired convertor is composed of metallic disks and split-ring resonators placed over a ground plane. The structure exhibits three neighboring resonances, by which the linear polarization of incident waves can be converted to its orthogonal counterpart upon reflection. For an optimal design, a measured polarization conversion ratio for normal incidence is greater than 80% in the range of 0.65-1.45 THz, equivalent to 76% relative bandwidth. The mechanism for polarization conversion is explained via decomposed electric field components that couple with different resonance modes of the structure. The proposed metamaterial design for enhancing efficiency of polarization conversion has potential applications in the area of terahertz spectroscopy, imaging, and communications. V C 2014 AIP Publishing LLC. [http://dx.Terahertz science and technology have seen rapid development, underpinned by many promising applications in imaging, sensing, and communications. 1 Towards these applications, high-performance terahertz components become essential for manipulating terahertz waves. One important group of components is related to polarization manipulation, including polarizers, wave retarders, and polarization rotators. In particular, conventional wave retarders can be achieved by using waveplates made of natural birefringent materials with a retardation effect. 2,3 Those wave plates require a relatively long propagation distance to obtain sufficient phase accumulation, despite the limited operation bandwidth and availability. Thus, more convenient and flexible approaches are desirable to fully manipulate the polarization state of electromagnetic waves.Over the past decade, metamaterials as artificial composite materials have attracted great attention due to their exotic electromagnetic properties unavailable to natural materials. 4 Such unique properties open up significant opportunities, including an alternative approach to manipulating the polarization of electromagnetic waves. 5-9 Several high-efficiency wave retarders have been demonstrated through different metamaterial microstructure designs, and these polarization wave retarders were demonstrated for conversion between different polarization states, such as linear to linear, 10-14 linear to circular, 15,16 and circular to circular polarization. 17 Compared with the traditional wave plates, these metamaterial-based wave retarders have advantages including subwavelength thickness, high conversion efficiency, angular tolerance, and scalability. In most of the existing wave retarders, the polarization states are manipulated in the transmission mode with a limited number of designs operating in the reflection mode. 11,12,15,18 For most retarders in the reflection mode, the undesirable high co-polarization reflection severely limits the polarization conversion efficienc...

in high-resolution terahertz imaging and detection for security and biomedicine.By defi nition, perfect absorbers can absorb EM waves with near-unity absorbance, which are promising for applications in terahertz imaging and detection via enhanced contrast and sensitivity. Metamaterials are candidates for creating perfect absorbers owing to the possibility of tailoring the response of the structure with great fl exibility. [ 5,6 ] Landy et al. [ 7 ] fi rst demonstrated the perfect metamaterial absorber concept in the microwave range, and since then great interest in EM absorbers has extended toward optical frequencies in recent years. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Metamaterial absorbers typically consist of two coupled metallic layers separated by a dielectric spacer to create electric and magnetic responses for impedance matching with free space. [ 23 ] The electric response can be obtained from excitation of the top metal layer readily coupled to an external electric fi eld, while the magnetic response is provided by pairing the top layer with a metal ground plane or metal wire for an external magnetic fi eld. In the microwave and terahertz regions, these metamaterial absorbers obtain high absorption through dielectric loss and impedance matching at resonance. [ 23 ] The absorption frequency range and amplitude can be tuned by adjusting the shape, size, thickness, and properties of the metallic structure and dielectric spacer. Due to the nature of resonance response, these metamaterial absorbers usually exhibit narrowband absorption that has advantages in applications such as fi ltering, sensing, and modulation. [ 24 ] Broadband perfect absorbers are desirable for other applications such as high-effi ciency signal detection and communications. This has necessitated signifi cant research effort toward extending the absorption bandwidth. A straightforward approach is to cluster multiple resonating structures with different sizes in each unit cell to create a number of absorption bands. [ 9,22 ] Graphene has been introduced to construct broadband terahertz absorbers due to its exceptional properties, such as optical transparency, fl exibility, and tunability. [25][26][27] However, the structure is demanding in terms of cost and complexity. Another alternative promising material for terahertz absorption is a moderately doped semiconductor, which can be readily fabricated using conventional micro-fabrication techniques. At terahertz frequencies, doped semiconductors have desirable conductive loss, enabling them to sustain surface plasmon polaritons (SPPs) and correspondingly localized surface plasmon resonances (LSPRs) via periodic structures. [28][29][30] Recently, we have demonstrated that doped silicon can be engineered to attain highly Perfect absorbers that exhibit broadband absorption of terahertz radiation are promising for applications in imaging and detection due to enhanced contrast and sensitivity in this relatively untapped frequency regime. Here, terahertz plasmonics is used ...

Vanadium has 11 oxide phases, with the binary VO2 presenting stimuli-dependent phase transitions that manifest as switchable electronic and optical features. An elevated temperature induces an insulator–to–metal transition (IMT) as the crystal reorients from a monoclinic state (insulator) to a tetragonal arrangement (metallic). This transition is accompanied by a simultaneous change in optical properties making VO2 a versatile optoelectronic material. However, its deployment in scalable devices suffers because of the requirement of specialised substrates to retain the functionality of the material. Sensitivity to oxygen concentration and larger-scale VO2 synthesis have also been standing issues in VO2 fabrication. Here, we address these major challenges in harnessing the functionality in VO2 by demonstrating an approach that enables crystalline, switchable VO2 on any substrate. Glass, silicon, and quartz are used as model platforms to show the effectiveness of the process. Temperature-dependent electrical and optical characterisation is used demonstrating three to four orders of magnitude in resistive switching, >60% chromic discrimination at infrared wavelengths, and terahertz property extraction. This capability will significantly broaden the horizon of applications that have been envisioned but remained unrealised due to the lack of ability to realise VO2 on any substrate, thereby exploiting its untapped potential.