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 ...

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...

A reflectarray is designed and demonstrated experimentally for polarization-dependent beam splitting at 1 THz. This reflective component is composed of two sets of orthogonal strip dipoles arranged into interlaced triangular lattices over a ground plane. By varying the length and width of the dipoles a polarization-dependent localized phase change is achieved on reflection, allowing periodic subarrays with a desired progressive phase distribution. Both the simulated field distributions and the measurement results from a fabricated sample verify the validity of the proposed concept. The designed terahertz reflectarray can efficiently separate the two polarization components of a normally incident wave towards different predesigned directions of ±30°. Furthermore, the measured radiation patterns show excellent polarization purity, with a cross-polarization level below -27 dB. The designed reflectarray could be applied as a polarizing beam splitter for polarization-sensitive terahertz imaging or for emerging terahertz communications.

A thin-film polarization-dependent reflectarray based on patterned metallic wire grids is realized at 1 THz. Unlike conventional reflectarrays with resonant elements and a solid metal ground, parallel narrow metal strips with uniform spacing are employed in this design to construct both the radiation elements and the ground plane. For each radiation element, a certain number of thin strips with an identical length are grouped to effectively form a patch resonator with equivalent performance. The ground plane is made of continuous metallic strips, similar to conventional wiregrid polarizers. The structure can deflect incident waves with the polarization parallel to the strips into a designed direction and transmit the orthogonal polarization component. Measured radiation patterns show reasonable deflection efficiency and high polarization discrimination. Utilizing this flexible device approach, similar reflectarray designs can be realized for conformal mounting onto surfaces of cylindrical or spherical devices for terahertz imaging and communications. V C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4927386]Owing to the advantages of a flat profile and high efficiency, reflectarrays have been widely implemented across the electromagnetic spectrum, 1 from microwave, 2,3 millimeter-wave, 4,5 terahertz 6,7 to optics. 10-13 A typical reflectarray is composed of three layers: a top layer with resonant elements arranged periodically with a beam-forming phase distribution, a dielectric spacer, and a metal ground plane. As one of the possible functions of a reflectarray, waves incident on its surface can be deflected into an offspecular direction. Because of the presence of a full metal ground plane, the incident electric field can only be reradiated backwards into free space after interacting with the reflectarray. In this letter, a terahertz reflectarray using patterned double-layer metallic wire-grid geometry is proposed to realize two polarization-dependent functions: (i) reflective deflection for the incident polarization parallel to the grid and (ii) normal transmission for the orthogonal polarization-therefore, it can be considered as a combination of a reflectarray and a wire-grid polarizer. 8 This reflectarray structure is fabricated on a free-standing flexible polymer substrate. Hence, the structure is flexible and stretchable.A schematic diagram of the unit cell and the layout for the proposed reflectarray is given in Fig. 1. The array is composed of periodically arranged identical subarrays. Each subarray contains a certain number of subwavelength unit cells, and each cell is made of a resonant element on the ground plane with a flexible polymer polydimethylsiloxane (PDMS) 14 as a dielectric spacer. This individual resonant element is formed by grouping thin gold strips together in a square shape, while the ground plane is made of continuous gold strips. 9 All the gold strips for both the top and bottom layers have a width of 5 lm and are uniformly arranged with an inter-strip spacing of 5 lm. The siz...