Terahertz Waveband Research Articles

Sensitivity analysis of steering-wheel gas sensor against diverse core air hole sizes and core materials in terahertz wave band

We design a photonic crystal fiber (PCF) based gas sensor, which works based on evanescent field, by introducing a steering-wheel shape of large noncircular air-hole structure in the cladding region. Further, using the full-vectorial finite element method (FEM), we compute the relative sensitivities of the proposed sensor as 83% and 91% when the operating frequencies are 1THz and 0.5THz, respectively. The proposed sensor is suitable for detecting any kind of chemical and biological gases.

Waveguide-based terahertz metamaterial functional components

We suggest a flexible platform based on waveguides for constructing metamaterial functional components which work in the terahertz waveband. The properties of the components can be changed by selecting the specified metamaterial resonance structures on the waveguide’s narrow wall. In the paper, metallic circular patches are used as resonance structures to design the functional component which can be configured as a filter or absorber. The simulation and experimental results demonstrate that the component’s attenuation and absorption bandwidth can be configured by changing the quantities and sizes of the resonance structures.

Metamaterial Demonstrates Both a High Refractive Index and Extremely Low Reflection in the 0.3-THz Band

Communication and imaging in the terahertz waveband have advanced rapidly in offering industrial applications. However, optical elements such as collimated lenses in the terahertz waveband are bulky compared with the wavelength due to the lack of naturally occurring substances with a high refractive index and low loss. It is essential to miniaturize optical elements in the terahertz waveband for industrial application. Metamaterials consisting of subwavelength structures can arbitrarily control permittivity and permeability and provide a range of refractive indices. Here, we demonstrate a metamaterial with both a high refractive index and extremely low reflection consisting of symmetrically aligned paired cut metal wires with 18,800 units on the front and back surfaces of a dielectric substrate. Measurements by terahertz time-domain spectroscopy (THz-TDS) confirm a high effective refractive index of 6.66 + j0.123, extremely low reflection power of 1.16%, and the unprecedented high figure of merit (FOM = |n real/n imag|) of above 300 in the 0.3-THz band. Components with such specifications would enable miniature, high-performance optical elements in the terahertz waveband such as ultrathin flat antennas with high directivity. Further, the concept of the metamaterial with both a high refractive index and extremely low reflection potentially offers a wide range of attractive applications such as solid immersion lenses and cloaking devices.

Reconfigurable terahertz grating with enhanced transmission of TE polarized light

We demonstrate an optically reconfigurable grating with enhanced transmission of TE-polarized waves in the terahertz (THz) waveband. This kind of grating is realized by projecting a grating image onto a thin Si wafer with a digital micromirror device (DMD). The enhanced transmission is caused by a resonance of the electromagnetic fields between the photoexcited strips. The position of the transmission peak shifts with the variation of the period and duty cycle of the photoinduced grating, which can be readily controlled by the DMD. Furthermore, a flattened Gaussian model was applied to describe the distribution of the photoexcited free carriers in the Si wafer, and the simulated transmittance spectra are shown to be in good agreement with the experimental results. In future, the photoexcited carriers could also be used to produce THz diffractive elements with reconfigurable functionality.

Terahertz epsilon-near-zero cut-through metal-slit array antenna

Metamaterials can give rise to unprecedented refractive indices and drive the rapid development of metadevices with on-demand electromagnetic properties. Recent advances in terahertz science demand high-performance optical elements beyond conventional designs of naturally occurring materials in the terahertz wave band. However, how an epsilon-near-zero (ENZ) structure can exploit terahertz metadevices is still not fully demonstrated based on a physical analysis. Here, inspired by the ENZ concept, we demonstrate a design guideline of a terahertz ENZ cut-through metal-slit array antenna. Measurements by a terahertz imager visualize the beam profile of a terahertz wave, and the measured permittivity of 0.26 agrees well with that of 0.27 obtained by simulation and theory. The terahertz ENZ antenna provides a wide range of potential applications such as high-directivity antennas, beam dividers, beam-steering elements, phase-control devices, and novel filters.

300 GHz帯チューナブルフィルタの設計

To solve increase of the mobile traffic, radio communication systems using millimeter wave band or terahertz wave band are required. Therefore an accurate spectrum analyzer is necessary in these bands. A tunable pre-selection filter is a key device in building such a spectrum analyzer. Hence, we proposed FPW (Fabry-Perot resonator inside Waveguide) filter for millimeter-wave band tunable filter. The FPW filter was realized widely tunable band and small size. However, this filter was not developed over 140 GHz yet. In this paper, we describe a design of 300 GHz band FPW filer and simulation results.

多壁碳纳米管薄膜在THz波段的传输与偏振特性

多壁碳纳米管薄膜在THz波段的传输与偏振特性

メタマテリアルでテラヘルツ波帯の光学素子を作る

メタマテリアルでテラヘルツ波帯の光学素子を作る

Seamless Fiber-Wireless Bridge in the Millimeter- and Terahertz-Wave Bands

Fiber-wireless transmission in the terahertz-wave band as well as in the millimeter-wave band is the key for a seamless integration of radio and optical networks in a physical layer. Advanced optical fiber communication technology enables a high precision direct waveform transfer between optical and radio domains. For realization of a seamless conversion, an optical-terahertz converter is configured with optical heterodyning for an optical-to-terahertz conversion. Analog optical modulation techniques by an input millimeter-wave signal and an intermediate frequency component after a frequency downconversion are utilized for a terahertz-to-optical conversion. A quadrature phase-shift keying signal transmission in an optical-fiber-to-radio-to-optical-fiber link (fiber-wireless bridge) is successfully demonstrated with the 90 and 300-GHz signals, simultaneously, using an optical frequency comb source. The possible transmission distance in severe weather conditions in these frequency bands is also discussed.

Investigation on Sheet Beam Folded V-Shape Groove Waveguide for Millimeter-Wave TWT

In this paper, a novel slow-wave structure (SWS) called folded V-shape groove waveguide SWS is proposed. The high frequency characteristics of this SWS is studied. An input and output waveguide transition structure, which is appropriate for this new kind of SWS is put forward. The transmission loss of the folded V-shape groove waveguide is lower than that of folded rectangular groove waveguide. The dimension of the folded V-shape groove waveguide is larger than that of the folded rectangular groove waveguide at the millimeter waveband, even at terahertz waveband. Based on this kind of SWS, a W-band sheet beam traveling-wave tube is studied, whose beam-wave interaction is calculated by means of the simulation software. The calculation results show that the output power can be over 260 W within 3-dB bandwidth from 84 to 98 GHz, the gain can reach up to 36 dB, and the electron efficiency is 12.33% at the center frequency.

Abruptly autofocusing terahertz waves with meta-hologram.

An abruptly autofocusing ring-Airy beam is demonstrated in the terahertz (THz) waveband with a meta-hologram. The designed meta-hologram is composed of gold C-shaped slot antennas, which can realize both phase and amplitude modulation of the incident THz wave. A THz holographic imaging system is utilized to measure the generated ring-Airy beam; an abrupt focus following a parabolic trajectory is subsequently observed. THz ring-Airy beams with different parameters are also generated and investigated. This method can be expanded to other wavebands, such as the visible band, for which the meta-hologram can replace traditional computer-generated holography to avoid undesirable multiple diffraction orders.

THz electromagnetic radiation driven by intense relativistic electron beam based on ion focus regime

The simulation study finds that the relativistic electron beam propagating through the plasma background can produce electromagnetic (EM) radiation. With the propagation of the electron beam, the oscillations of the beam electrons in transverse and longitudinal directions have been observed simultaneously, which provides the basis for the electromagnetic radiation. The simulation results clearly show that the electromagnetic radiation frequency can reach up to terahertz (THz) wave band which may result from the filter-like property of plasma background, and the electromagnetic radiation frequency closely depends on the plasma density. To understand the above simulation results physically, the dispersion relation of the beam-plasma system has been derived using the field-matching method, and the dispersion curves show that the slow wave modes can couple with the electron beam effectively in THz wave band, which is an important theoretical evidence of the EM radiation.

Effect of in-situ annealing on the structural and optical properties of GeSn films grown by MBE

This paper reports the use of in-situ thermal treatments to modify the properties of GeSn films and their performance in the infrared and Terahertz wave bands. The GeSn films were grown by molecular beam epitaxy under various conditions. X-ray analysis indicated the presence of amorphous phases and oxides in the GeSn films when they were epitaxially grown under low substrate temperature. Raman spectroscopy showed that the GeSn characteristic peak moved toward the larger wavenumber of 3.711 cm−1, and the intensity increased by 22.2% for the annealed GeSn (3.28% Sn) film. Moreover, it was determined from the full-wave reflection spectra that the direct band-gap is around 0.843 eV (1.47 μm). In-situ annealing accelerated the diffusion and ion substitution process of Sn atoms in the Ge lattice, which promoted the stability of the GeSn crystal cell and their optical performances in the infrared and terahertz range.

Rain Attenuation in the Microwave-to-Terahertz Waveband

In recent years, there has been increased interest in the terahertz waveband for application to ultra-high-speed wireless communications and remote sensing systems. However, atmospheric propagation at these wavelengths has a significant effect on the operational stability of systems using the terahertz waveband, so elucidating the effects of rain on propagation is a topic of high interest. We demonstrate various methods for calculating attenuation due to rain and evaluate these methods through comparison with calculated and experimental values. We find that in the 90 - 225 GHz microwave band, values calculated according to Mie scattering theory using the Best and P-S sleet raindrop size distributions best agree with experimental values. At 313 and 355 GHz terahertz-waveband frequencies, values calculated according to Mie scattering theory using the Weibull distribution and a prediction model following ITU-R recommendations best agree with experimental values. We furthermore find that attenuation due to rain increases in proportion to frequency for microwave-band frequencies below approximately 50 GHz, but that there is a peak at around 100 GHz, above which the degree of attenuation remains steady or decreases. Rain-induced attenuation increases in proportion to the rainfall intensity.

Investigation of enhancement of near-field probing for sensing at terahertz waveband

Enhancement of localized surface plasmon resonance (LSPR) for near-field probing working at terahertz waveband is an important issue. Optimization of an apertureless metallic probe is presented for the purpose of sensing and detection in high resolution. Numerical calculation is carried out for the purpose of design and optimization of the probe. Corresponding results demonstrate that performance enhancement of the probe at terahertz waveband is different from that of the visible regime. Unlike the visible case that the smaller size of the probe apex results in stronger LSPR, actually, it is opposite to the case of visible for the terahertz regime while the size of probe apex d is ranging from 0 to 20nm. The enhancement of the probe working at terahertz regime is mainly dominated by surface effect. In addition, the numerical results give a proof again that exciting SPP enhanced LSPR signal at near-field for the terahertz wavelength is surely difficult than that of the visible case.

Quasi-three-dimensional post-array for propagation and focusing of a terahertz spoof surface plasmon polariton

This paper presents a quasi-three-dimensional post-array designed to propagate a terahertz spoof surface plasmon polariton (terahertz spoof SPP) with confinement. A transmission line making use of a terahertz spoof SPP is a promising device in the terahertz wave band, and there are many previous reports of two-dimensional structures. Three-dimensional structures provide sophisticated designs for transmission lines propagating a terahertz spoof SPP. Eigenmode analysis is used to derive a dispersion diagram for one post with boundary conditions extracted from the full model. The propagation frequency of the terahertz spoof SPP increases with lower heights or smaller diameters, and that remains virtually unchanged for post-spacing and thickness of a substrate. The analysis of the full model confirms the confinement of a terahertz spoof SPP vertically on the post-array. The magnitude of the electric field is strong around the top and bottom and weak at approximately one-third height. The terahertz spoof SPP is confined in the space around the post-array as well as a substrate, while it is confined only on substrates in conventional two-dimensional structures. The designed post-array can control the three-dimensional focusing of a terahertz spoof SPP in an arbitrary volume of space.

Passively Q-switched tri-wavelength Yb^3+:GdAl_3(BO_3)_4 solid-state laser with topological insulator Bi_2Te_3 as saturable absorber

In this paper, we reported a multiwavelength passively Q-switched Yb3+:GdAl3(BO3)4 solid-state laser with topological insulator Bi2Te3 as a saturable absorber (SA) for the first time, to the best of our knowledge. Bi2Te3 nanosheets were prepared by the facile solvothermal method. The influence of three Bi2Te3 densities on the laser operation was compared. The maximum average output power was up to 57 mW with a pulse energy of 511.7 nJ. The shortest pulsewidth was measured to be 370 ns with 110 kHz pulse repetition rate and 40 mW average power. The laser operated at three wavelengths simultaneously at 1043.7, 1045.3, and 1046.2 nm, of which the frequency differences were within the terahertz wave band. Our work suggests that solvothermal synthesized Bi2Te3 is a promising SA for simultaneously multiwavelength laser operation.

A Tunable Terahertz Photonic Crystal Narrow-Band Filter

We theoretically propose and investigate a magnetically tunable narrow-band terahertz filter based on a triangular lattice silicon photonic crystal with a point and two line defects. The optical properties of the filter have been analyzed in detail. It is found that a single resonant peak with the central frequency of $\sim 1$ THz is existed in the transmission spectrum, which has a narrow full width at half maximum of <2 GHz. Moreover, under the control of an external magnetic field, transmission frequency and width of passband are adjustable, which reveals that the 2-D silicon photonic crystal waveguide with point and line defects can serve as a continuously tunable bandpass filter at the terahertz waveband.

Infrared plasmonic filters integrated with an optical and terahertz multi-spectral material

Infrared plasmonic filters, consisting of subwavelength hole arrays etched into metal films, exhibit characteristics that are typically associated with the formation of surface plasmon polaritons, namely enhanced transmission and wavelength filtering of the incident light. In this article, the properties that dictate the plasmonic response of a material from the optical to the infrared regime are investigated, followed by the design, simulation, fabrication and characterisation of an infrared plasmonic filter set. Infrared plasmonic filters have also been integrated with optical plasmonic filters and a terahertz metamaterial to create a new hybrid multi-spectral material that can filter blue, green, red, near infrared, short wave infrared and two mid infrared wavelengths whilst simultaneously absorbing a single terahertz frequency. The multi-spectral material could be integrated with appropriate image sensors to create a multi-spectral camera capable of operating at optical, infrared and terahertz wavebands simultaneously.

A Transmission-Type Electrically Tunable Polarizer Based on Graphene Ribbons at Terahertz Wave Band

We theoretically and numerically demonstrate that a transmission-type electrically tunable polarizer can be realized by using graphene ribbons supported on a dielectric film with a graphene sheet behind. The polarization mechanism originates from the antenna plasmon resonance of graphene stripes. The results of full-wave numerical simulations reveal that transmittance of 0.70 for one polarization and 0.0073 for another polarization can be obtained at normal incidence. The transmission-type electrically tunable polarizer provides and facilitates a variety of applications, including filtering, detecting, and imaging.