Flexible terahertz wire grid polarizer with high extinction ratio and low loss

Fabrication of freestanding photonic devices combining polymer films with microfabrication techniques and 3D printing

We report a technological concept for freestanding photonic elements based on metamaterials fabricated on polymer films by clean-room processes and framed using 3D printing. A spin-coated cyclic olefin copolymer (TOPAS) of variable thickness down to one micrometer was used as the substrate onto which metamaterials were fabricated using optical lithography. We demonstrate the possibility of applying a second TOPAS layer to protect the device or to allow for stacking another metamaterial layer. To obtain freestanding elements, frames were 3D printed directly on top of the metamaterial before lift-off from the carrier wafer. This ensured maintaining the flatness of the elements. Both the cleanroom process and the 3D printing enabled the design and manufacturing of elements in different sizes and shapes, e.g., to adapt to specific experimental set-ups and holder geometries or to be compatible with standard optical mounts. While TOPAS is transparent for wavelengths from UV to the far infrared, except for a few infrared absorption lines, we illustrate the concept with the simulation and manufacturing of THz band-pass filters. The performance of the fabricated filters was assessed using THz time-domain spectroscopy. The process is scalable to other wavelength ranges and has the potential for upscaling in manufacturing.

Fluorinated graphene grating metasurface for terahertz dark state excitation

We propose an original technique for the grating metasurfaces fabrication by low-power ultraviolet laser treatment of fluorinated graphene (FG) films with the focus on terahertz applications. The laser treatment reduces dielectric FG to its conductive counterparts, increasing DC conductivity to 170 S·m^-1for treated areas. The electromagnetic response of the grating metasurfaces studied by THz time-domain spectroscopy in the 100 GHz-1 THz frequency range demonstrates enhanced resonant transmittance through metasurfaces. The intensity and position of transmittance peak could be tuned by changing the metasurface geometry, i.e. the period of the structure and width of the reduced and unreduced areas. In particular, the decrease of the reduced FG area width from 400 to 170μm leads to the shift of the resonance peak from 0.45 THz to the higher frequencies, 0.85 THz. Theoretical description based on the multipole theory supported by finite element numerical calculations confirms the excitation of the dark state in the metasurface unit cells comprising reduced and unreduced FG areas at resonance frequency determined by the structure geometrical features. Fabricated metasurfaces have been proved to be efficient narrowband polarizers being rotated by 50° about the incident THz field vector.

Nanoengineered Spintronic-Metasurface Terahertz Emitters Enable Beam Steering and Full Polarization Control

The demand for emerging applications at the terahertz frequencies motivates the development of novel and multifunctional devices for the generation and manipulation of terahertz waves. In this work, we report the realization of multifunctional spintronic-metasurface emitters, which allow simultaneous beam-steering and full polarization control over a broadband terahertz beam. This is achieved through engineering individual meta-atoms with nanoscale magnetic heterostructures and, thus, implementing microscopical control over the laser-induced spin and charge dynamics. By arranging the spintronic meta-atoms in the metagrating geometry, the generated terahertz beam can be flexibly steered in space between different orders of diffraction. Furthermore, we demonstrate a simultaneous control over the terahertz polarization states at different emission angles and show that the two control capabilities are mutually independent of each other. The nanoengineered multifunctional terahertz emitter demonstrated in this work can provide a solution to the challenge associated with a growing variety of applications of terahertz technology.

Unidirectional spintronic terahertz emitters with high efficiency

Due to the high performance and low cost, spintronic terahertz emitters (STEs) have been a hot topic in the field of terahertz sources. However, most of the research focuses on the THz generation process and little attention has been paid to the control and modulation of the THz wave generated by the STE. In this Letter, a unidirectional spintronic terahertz emitter (USTE) integrating a common STE with a metal grating is proposed to manipulate the THz emission process. The dyadic Green's function method and finite element method are adopted to survey the characteristics of the USTE. Simulations show that the metal grating not only has a transmission larger than 97% in the optical band, but also has a higher reflectivity larger than 99% in the THz band. As a result, the USTE has a unidirectional THz emission along the direction of the pump beam with a larger than 4-fold enhancement in intensity. Moreover, the USTE has the capability of tuning the central frequency and THz wave steering by adjusting the distance and angle between the STE and the metal grating. We believe that this USTE can be used in THz wireless communications and holographic imaging, especially in the field of THz bio-sensing, which needs some resonance frequencies to sense.

Broadband terahertz time-domain polarimetry based on air plasma filament emissions and spinning electro-optic sampling in GaP

We report on a time-domain polarimetry (TDP) system for generating and detecting broadband terahertz (THz) waves of different polarization angles. We generate THz waves from two-color laser filaments and determine their polarization states with a detection bandwidth of up to 8 THz using a spinning gallium phosphide crystal. The polarization of THz emission can be controlled by adjusting the position and tilt angle of the β-barium borate crystal. We characterize the precision of this system for polarimetric measurements at fixed time delay to be 1.6 ° and 1.9 ° for complete time-domain waveforms. We also demonstrate the feasibility of our TDP system by measuring broadband optical properties of anisotropic samples in both transmission and reflection geometries. The THz-TDP technique can be easily integrated in conventional THz time-domain spectroscopy setups using nonlinear crystal detectors.

Low-reflective wire-grid polariser sheet in the visible region fabricated by a nanoprinting process

For the construction of next-generation optical products and systems, the evolution of polariser sheets is a necessary requirement. To this end, a low-reflective wire-grid polariser (WGP) sheet for the visible light region is demonstrated, the nanowires of which consist of a sintered body of silver nanoparticle ink. The nanowires are formed by a nanoprinting process using a thermal nanoimprint method and ink filling. This process makes it easier to achieve multiple wafer-scale productions without using sophisticated equipment compared to conventional WGP nanofabrication techniques, which typically employ lithography and elaborate etching processes. The optical characteristics are controlled by the shape of the printed nanowires. A WGP sheet with a luminous degree of polarisation of 99.0%, a total luminous transmittance of 13.6%, and a luminous reflectance of 3.6% is produced. Its low reflectance is achieved through the uneven surface derived from the sintered body of the nanoparticle ink, and the shape of the bottom of the nanowire is derived from the tip shape of the mould structure. Furthermore, the printed WGP sheet has the durability required for the manufacturing of curved products, including sunglasses. The optical structures made of nanoparticle ink using this nanoprinting process have the potential to significantly contribute to the development of fine-structured optical elements with unprecedented functionality.

Dual-band passband terahertz filter based on multilayer metamaterial

We have theoretically verified the design of dual-band passband terahertz filter using multilayer metamaterial. The top and bottom layers are composed of periodically branching structure metallic patterns, which are exactly the same. The middle layer consists of a periodically metallic square aperture. The three metallic layers are separated by two thin dielectric layers. An equivalent circuit model is used to simplify the design procedure of the proposed structure. The performance of the terahertz filter is analyzed via equivalent circuit model together with the full wave CST method. The results show that the proposed structure provides two passbands. One is the center frequency of 0.32 THz with -3dB bandwidth of 164 GHz, and the other is the center frequency of 0.74 THz with -3dB bandwidth of 176 GHz.

Dispersive effects in imaging polarimetry based on a wire-grid polarizer

We explore effects of light dispersion by a wire-grid polarizer (WGP) in imaging polarimetry. The dispersive characteristics of a WGP, combined with off-axis scene incidence, cause significant non-uniformity. The normalized performance measure of contrast due to dispersion of WGP exceeded 0.84 for transmittance and 0.90 for extinction ratio (maximum non-uniformity at 1 and 0 for uniform performance). Dispersion also produces a lateral spread in the imaging plane, which may induce spectral image misregistration. Without higher-order excitation, the misregistration can be at the least a few pixels long in the detector. In the presence of higher-order modes, the dispersive misregistration can be severe and critical for polarized scene extraction. The results emphasize the need for an imaging polarimeter to be designed to manage the dispersive effects.

Fabrication and spectroscopic characterization of graphene transparent electrodes on flexible cyclo-olefin substrates for terahertz electro-optic applications

We demonstrate graphene on flexible, low-loss, cyclo-olefin polymer films as transparent electrodes for terahertz electro-optic devices and applications. Graphene was grown by chemical vapor deposition and transferred to cyclo-olefin polymer substrates by the thermal release tape method as layers on an approximate area of 4 cm2. The structural and electromagnetic properties of the graphene samples as well as their spatial variation were systematically mapped by means of µRaman, terahertz time-domain and mid-infrared spectroscopy. Thanks to the small thickness and very low intrinsic absorption of the employed substrates, both high transmittance and conductivity were recorded, demonstrating the suitability of the technique for the fabrication of a new class of transparent and flexible electrodes working in the terahertz spectrum.

Polarization of Magnetoplasmons in Grating Metamaterials Based on CdTe/CdMgTe Quantum Wells

Grating metamaterials were fabricated with electron beam lithography on CdTe/CdMgTe modulation doped structures with two non-interacting quantum wells. Two types of samples were studied: with etched gratings and with gratings formed by deposition of Au stripes. The polarization properties at THz frequencies of the gratings were determined at room temperature. It was shown that Au gratings formed a linear polarizer, while etched gratings did not polarize THz radiation. Transmission of circularly polarized THz radiation at low temperatures through a sample with no grating showed a strongly circularly polarized cyclotron resonance transition. Transmission of this radiation through a sample with an etched grating showed a magnetoplasmon transition that was almost perfectly linearly polarized. We concluded that magnetoplasmons in metamaterials with etched gratings are linearly polarized excitations, possibly with a small contribution of a circular component. This work opens the possibility of the detailed study of the polarization of magnetoplasmons, which has not been explored in the past.

Tunable broadband terahertz polarizer using graphene-metal hybrid metasurface

We demonstrate an electrically tunable polarizer for terahertz (THz) frequency electromagnetic waves formed from a hybrid graphene-metal metasurface. Broadband (>3 THz) polarization-dependent modulation of THz transmission is demonstrated as a function of the graphene conductivity for various wire grid geometries, each tuned by gating using an overlaid ion gel. We show a strong enhancement of modulation (up to ∼17 times) compared to graphene wire grids in the frequency range of 0.2-2.5 THz upon introduction of the metallic elements. Theoretical calculations, considering both plasmonic coupling and Drude absorption, are in good agreement with our experimental findings.

Proposal for a Quad-Elliptical Photonic Crystal Fiber for Terahertz Wave Guidance and Sensing Chemical Warfare Liquids

A porous-core photonic crystal fiber based on a cyclic olefin homopolymer (Zeonex) is proposed; it shows high birefringence, high core power fraction, low losses, and near-zero flat dispersion. The fiber’s core was designed with quad-elliptical (QE) air holes with its center occupied by bulk background material. The superiority of the QE design over the commonly adopted tri- and penta-elliptical (TE and PE) core designs is demonstrated. The presence of the bulk material at the core center and the geometrical configuration cause a broad contrast in phase refractive indices, thereby producing high birefringence and low transmission losses. A high birefringence of 0.096 was obtained at 1.2 THz, corresponding to a total loss of 0.027 cm−1 and core power fraction of approximately 51%. The chromatic dispersion and effective area of the reported fiber were also characterized within a frequency range of 0.4–1.6 THz. The QE air holes were then filled with chemical warfare agents, namely, tabun and sarin liquids. Then, the relative sensitivity, confinement loss, fractional power flow, and effective material loss (EML) of the sensor were calculated. Nearly the same relative sensitivity (r = 64%) was obtained when the QE core was filled with either liquid. Although the obtained EML for tabun was 0.033 cm−1 and that for sarin was 0.028 cm−1, the confinement loss of the fiber when it was immersed in either liquid was negligible. The proposed fiber can be fabricated using existing fabrication technologies. Moreover, it can be applied and utilized as a THz radiation conveyor in a terahertz time domain spectroscopy system for remote sensing of chemical liquids in the security and defense industries.

Guided-mode resonant narrowband terahertz filtering by periodic metallic stripe and patch arrays on cyclo-olefin substrates

We experimentally and theoretically demonstrate a class of narrowband transmissive filters in the terahertz spectrum. Their operation is based on the excitation of guided-mode resonances in thin films of the low-loss cyclo-olefin polymer Zeonor, upon which aluminum stripe and patch arrays are patterned via standard photolithography. The filters are engineered to operate in low atmospheric loss THz spectral windows, they exhibit very high transmittance and quality factors, compact thickness, and mechanical stability. The dependence of their filtering properties on the geometrical parameters, the substrate thickness and the angle of incidence is investigated, discussing the physical limitations in their performance. This class of filters provides a cost-effective solution for broadband source or channel filtering in view of emerging terahertz wireless communication systems.

Microscale heat transfer and thermal extinction of a wire-grid polarizer

We explore heat transfer and thermal characteristics of a wire-grid polarizer (WGP) on a microscale by investigating the effect of various geometrical parameters such as wire-grid period, height, and a fill factor. The thermal properties arise from heat transfer by light absorption and conduction in wire-grids. Fill factor was found to be the most dominant geometrical parameter. For TM polarized light, a higher fill factor with thicker wire-grids increased the temperature. The local temperature was found to rise up to T_max = 354.5 K. TE polarization tended to produce lower temperature. Thermal extinction due to polarimetric extinction by a WGP was also evaluated and highest extinction was observed to be 4.78 dB, which represents a temperature difference ΔT = 54.3 °C. We expect the results to be useful for WGPs in polarization-sensitive thermal switching applications.

A Fabry-Pérot interferometer with wire-grid polarizers as beamsplitters at terahertz frequencies

The design of a compact Fabry-Pérot interferometer (FPi) and results of the experimental studies carried out using the device are presented. Our FPi uses freestanding wire-grid polarizers (WGPs) as beamsplitters and is suitable for use at terahertz (THz) frequencies. The FPi was studied at the LUCX facility, KEK, Japan, and an 8 MeV linear electron accelerator was used to generate coherent Smith-Purcell radiation. The FPi was designed to be easy to align and reposition for experiments at linear accelerator facilities. All of the components used were required to have a flat or well understood frequency response in the THz range. The performance of the FPi with WGPs was compared to that of a Michelson interferometer and the FPi is seen to perform well. The effectiveness of the beamsplitters used in the FPi is also investigated. Measurements made with the FPi using WGPs, the preferred beamsplitters, are compared to measurements made with the FPi using silicon wafers as alternative beamsplitters. The FPi performs well with both types of beamsplitter in the frequency range used (0.3-0.5 THz). The successful measurements taken with the FPi demonstrate a compact and adaptable interferometer that is capable of analyzing THz radiation over a broad frequency range. The scheme is particularly well suited for polarization studies of THz radiation produced in an accelerator environment.

Polarization-maintaining reflection-mode THz time-domain spectroscopy of a polyimide based ultra-thin narrow-band metamaterial absorber

This paper reports the design, the microfabrication and the experimental characterization of an ultra-thin narrow-band metamaterial absorber at terahertz frequencies. The metamaterial device is composed of a highly flexible polyimide spacer included between a top electric ring resonator with a four-fold rotational symmetry and a bottom ground plane that avoids misalignment problems. Its performance has been experimentally demonstrated by a custom polarization-maintaining reflection-mode terahertz time-domain spectroscopy system properly designed in order to reach a collimated configuration of the terahertz beam. The dependence of the spectral characteristics of this metamaterial absorber has been evaluated on the azimuthal angle under oblique incidence. The obtained absorbance levels are comprised between 67% and 74% at 1.092 THz and the polarization insensitivity has been verified in transverse electric polarization. This offers potential prospects in terahertz imaging, in terahertz stealth technology, in substance identification, and in non-planar applications. The proposed compact experimental set-up can be applied to investigate arbitrary polarization-sensitive terahertz devices under oblique incidence, allowing for a wide reproducibility of the measurements.

Robust terahertz polarizers with high transmittance at selected frequencies through Si wafer bonding technologies

Terahertz (THz) polarizers with robust structure and high transmittance are demonstrated using 3D-integrated circuit (IC) technologies. A Cu wire-grid polarizer is sealed and well protected by Si-bonded wafers through a low-temperature eutectic bonding method. Deep reactive-ion etching is used to fabricate the anti-reflection (AR) layers on outward surfaces of bonded wafers. The extinction ratio and transmittance of polarizers are between 20 dB and 33 dB, and 13 dB and 27 dB for 10 μm and 20 μm pitch wire-grids, respectively, and 100% at central frequency, depending on frequency and AR layer thickness. The process of polarizer fabrication is simple from mature semiconductor manufacturing techniques that lead to high yield, low cost, and potential for THz applications.

Blazed wire-grid polarizer for plasmon-enhanced polarization extinction: design and analysis

We explore plasmon-enhanced wire-gird polarizers (WGPs) to achieve improved polarimetric performance with more relaxed fabrication parameters compared to conventional WGP. A WGP designed with a blazed wire-grid profile was considered for plasmonic enhancement. The results show that a blazed WGP can achieve extremely high polarimetric extinction at a longer wire-grid period (Λ) compared to conventional WGP structure. Under the optimum geometrical parameters, a blazed WGP may attain an extinction ratio of over 40 dB at Λ = 800 nm, which may allow photolithography for fabrication. In contrast, conventional WGPs obtained comparable performance at Λ = 200 nm, requiring more difficult lithographic techniques. The study can therefore be of significant importance for WGPs to be more widely available for diverse applications.

Electrically tunable terahertz polarization converter based on overcoupled metal-isolator-metal metamaterials infiltrated with liquid crystals

Large birefringence and its electrical modulation by means of Fréedericksz transition makes nematic liquid crystals (LCs) a promising platform for tunable terahertz (THz) devices. The thickness of standard LC cells is in the order of the wavelength, requiring high driving voltages and allowing only a very slow modulation at THz frequencies. Here, we first present the concept of overcoupled metal-isolator-metal (MIM) cavities that allow for achieving simultaneously both very high phase difference between orthogonal electric field components and large reflectance. We then apply this concept to LC-infiltrated MIM-based metamaterials aiming at the design of electrically tunable THz polarization converters. The optimal operation in the overcoupled regime is provided by properly selecting the thickness of the LC cell. Instead of the LC natural birefringence, the polarization-dependent functionality stems from the optical anisotropy of ultrathin and deeply subwavelength MIM structures. The dynamic electro-optic control of the LC refractive index enables the spectral shift of the resonant mode and, consequently, the tuning of the phase difference between the two orthogonal field components. This tunability is further enhanced by the large confinement of the resonant electromagnetic fields within the MIM cavity. We show that for an appropriately chosen linearly polarized incident field, the polarization state of the reflected field at the target operation frequency can be continuously swept between the north and south pole of the Poincaré sphere. Using a rigorous Q-tensor model to simulate the LC electro-optic switching, we demonstrate that the enhanced light-matter interaction in the MIM resonant cavity allows the polarization converter to operate at driving voltages below 10 Volt and with millisecond switching times.

High extinction ratio terahertz wire-grid polarizers with connecting bridges on quartz substrates

A terahertz (THz) wire-grid polarizer with metallic bridges on a quartz substrate was simulated, fabricated, and tested. The device functions as a wide-band polarizer to incident THz radiation. In addition, the metallic bridges permit the device to function as a transparent electrode when a DC bias is applied to it. Three design variations of the polarizer with bridges and a polarizer without bridges were studied. Results show the devices with bridges have average s-polarization transmittance of less than -3  dB and average extinction ratios of approximately 40 dB across a frequency range of 220-990 GHz and thus are comparable to a polarizer without bridges.

Ultrathin Terahertz Quarter-wave plate based on Split Ring Resonator and Wire Grating hybrid Metasurface

Planar metasurface based quarter-wave plates offer various advantages over conventional waveplates in terms of compactness, flexibility and simple fabrication; however they offer very narrow bandwidth of operation. Here, we demonstrate a planar terahertz (THz) metasurface capable of linear to circular polarization conversion and vice versa in a wide frequency range. The proposed metasurface is based on horizontally connected split ring resonators and is realized on an ultrathin (0.05λ) zeonor substrate. The fabricated quarter waveplate realizes linear to circular polarization conversion in two broad frequency bands comprising 0.64–0.82 THz and 0.96–1.3 THz with an insertion loss ranging from −3.9 to −10 dB. By virtue of ultrathin sub wavelength thickness, the proposed waveplate design is well suited for application in near field THz optical systems. Additionally, the proposed metasurface design offers novel transmission phase characteristics that present further opportunities to realize dynamic polarization control of incident waves.