Showing papers in "IEEE Transactions on Terahertz Science and Technology in 2015"

High-Power Terahertz Generation Using Large-Area Plasmonic Photoconductive Emitters

Abstract: In this paper, we present a novel design of large-area photoconductive emitters which incorporates plasmonic contact electrodes to offer significantly higher optical-to-terahertz conversion efficiencies compared with conventional designs. Use of plasmonic contact electrodes enables a more efficient separation and acceleration of photocarriers, enhancing the effective dipole moment induced within the device active area in response to an incident optical pump. At an optical pump power level of 240 mW, we demonstrate broadband, pulsed terahertz radiation with radiation power levels as high as 3.8 mW over the 0.1–5-THz frequency range, exhibiting an order of magnitude higher optical-to-terahertz conversion efficiency compared with conventional designs.

Novosibirsk Free Electron Laser—Facility Description and Recent Experiments

Abstract: The design and operational characteristics of the Novosibirsk free electron laser facility are described. Selected experiments in the terahertz range carried out recently at the user stations are surveyed in brief.

Advanced Photoconductive Terahertz Optoelectronics Based on Nano-Antennas and Nano-Plasmonic Light Concentrators

Abstract: High power sources and high sensitivity detectors are highly in demand for terahertz imaging and sensing systems. Use of nano-antennas and nano-plasmonic light concentrators in photoconductive terahertz sources and detectors has proven to offer significantly higher terahertz radiation powers and detection sensitivities by enhancing photoconductor quantum efficiency while maintaining its ultrafast operation. This is because of the unique capability of nano-antennas and nano-plasmonic structures in manipulating the concentration of photo-generated carriers within the device active area, allowing a larger number of photocarriers to efficiently contribute to terahertz radiation and detection. An overview of some of the recent advancements in terahertz optoelectronic devices through use of various types of nano-antennas and nano-plasmonic light concentrators is presented in this article.

Numerical Analysis and Characterization of THz Propagation Channel for Body-Centric Nano-Communications

Abstract: This paper presents the characteristics of electromagnetic waves propagating inside human body at Terahertz frequencies and an initial study of the system performance of nano-network. It has been observed that the path loss is not only the function of distance and frequency but also related to the dielectric loss of human tissues. Numerical results have been compared with analytical studies and a good match has been found which validates the proposed numerical model. Based on the calculation of path losses and noise level for THz wave propagation, the channel capacity is studied to give an insight of future nano-communications within the human body. Results show that at the distance of millimeters, the capacity can reach as high as 100 Terabits per second (Tbps) depending on the environment and exciting pulse types.

THz and mm-Wave Sensing of Corneal Tissue Water Content: In Vivo Sensing and Imaging Results

Abstract: A pulsed terahertz (THz) imaging system and millimeter-wave reflectometer were used to acquire images and point measurements, respectively, of five rabbit cornea in vivo. These imaging results are the first ever produced of in vivo cornea. A modified version of a standard protocol using a gentle stream of air and a Mylar window was employed to slightly dehydrate healthy cornea. The sensor data and companion central corneal thickness (CCT) measurements were acquired every 10–15 min over the course of two hours using ultrasound pachymmetry.. Statistically significant positive correlations were established between CCT measurements and millimeter wave reflectivity. Local shifts in reflectivity contrast were observed in the THz imagery; however, the THz reflectivity did not display a significant correlation with thickness in the region probed by the 100 GHz and CCT measurements. This is explained in part by a thickness sensitivity at least 10 $\times$ higher in the mm-wave than the THz systems. Stratified media and effective media modeling suggest that the protocol perturbed the thickness and not the corneal tissue water content (CTWC). To further explore possible etalon effects, an additional rabbit was euthanized and millimeter wave measurements were obtained during death induced edema. These observations represent the first time that the uncoupled sensing of CTWC and CCT have been achieved in vivo.

340 GHz On-Chip 3-D Antenna With 10 dBi Gain and 80% Radiation Efficiency

Abstract: This paper discusses the design methodologies of a 340 GHz on-chip 3-D antenna. Firstly, a high-gain and high-radiation efficiency substrate integrated waveguide (SIW) cavity backed on-chip antenna is designed using a standard 0.13- $\mu{\hbox{m}}$ SiGe BiCMOS technology. Then, a low-permittivity supporter and a dielectric resonator (DR) are vertically stacked on the proposed on-chip antenna, forming a 3-D Yagi-like antenna to further enhance the gain and radiation efficiency. The measurements showed that the proposed antenna achieved a peak gain of ${\sim}$ 10 dBi and radiation efficiency of ${\sim}$ 80% at 340 GHz; the impedance bandwidth is ${\sim}$ 12% with the use of dielectric resonator antenna (DRA) and the Yagi-like structure. The antenna size is ${\sim} {\hbox{0.7}}\times {\hbox{0.7}}\ {\hbox{mm}}^{2}$ .

Electrically and Magnetically Biased Graphene-Based Cylindrical Waveguides: Analysis and Applications as Reconfigurable Antennas

Abstract: The propagation of surface waves along electrically and magnetically biased graphene-based cylindrical waveguides (GCWs) is investigated in detail. Analytical dispersion equations are derived for several GCW geometries, considering the presence of an inner metallic core and multiple (coaxial-like) graphene layers. The proposed formulation reveals a fundamental connection between surface plasmons found in GCWs/carbon nanotubes and planar graphene structures. Numerical results confirm the higher confinement of modes supported by GCWs compared with their planar counterparts, while keeping a similar level of losses. The proposed structure is applied to develop plasmonic reconfigurable dipole antennas in the low THz band, which provide higher radiation efficiency than current graphene-based radiators, without requiring the presence of bulky lenses. We envision that the proposed GCWs may find application in reconfigurable THz transceivers, near-field application, wireless interconnects, and sensing systems.

Terahertz Conductivity of Copper Surfaces

Abstract: Terahertz (THz) radiation holds great promise for applications in communications, molecular detection, and imaging. Effective THz system design requires accurate models for the frequency-dependent conductivity of metals and the effect of surface roughness on conduction loss. However, predictive methods are currently unverified in the region between 0.3 and 0.9 THz because few experimental data exist in this regime. In order to address this problem, we have measured the conductivity of copper, of various surface roughnesses, using a semi-confocal open resonator system. This paper describes our measurements of the THz conductivity, dc conductivity, roughness, and microstructure of copper. We show that the classical Drude theory is sufficient for predicting the THz conductivity of copper at room temperature and that dissipation loss enhancement caused by surface roughness is modeled better by the Hammerstad–Bekkadal formula than second-order small perturbation theory.

Second-Order Terahertz Bandpass Frequency Selective Surface With Miniaturized Elements

Abstract: In this paper, a second-order frequency selective surface (FSS) made of miniaturized elements is proposed and designed for terahertz applications. The FSS is composed of two layers of metallic arrays separated from each other by a polymer dielectric spacer. The unit cells on the front and back layers are smaller than $\lambda _{0}/5$ , where $\lambda _{0}$ is the free space wavelength. The operation principle of the proposed FSS is described through a circuit model, and a synthesis procedure is presented for designing a desired filtering response. A prototype of the FSS is synthesized to operate at a center frequency of 0.42 THz with 45% fractional bandwidth. The designed FSS is fabricated by using microfabrication process. The performance is evaluated by using terahertz time-domain spectroscopy. Measurement results show a low sensitivity of the FSS response to oblique angles of incidence for both of the TE and TM polarizations.

4.7-THz Local Oscillator for the GREAT Heterodyne Spectrometer on SOFIA

Abstract: The design and the performance of a 4.7-THz local oscillator (LO) for the GREAT (German REceiver for Astronomy at Terahertz frequencies) heterodyne spectrometer on SOFIA, the Stratospheric Observatory for Infrared Astronomy, are presented. The LO is based on a quantum-cascade laser, which is mounted in a compact mechanical cryocooler. The LO provides up to ${\hbox{150}}~\mu{\hbox{W}}$ output power into a nearly Gaussian shaped beam. It covers the frequency range from approximately $+{\hbox{2}}$ to $-{\hbox{4}}~{\hbox{GHz}}$ around the fine structure line of neutral atomic oxygen, OI, at 4.7448 THz. The LO has been successfully operated on SOFIA during six observation flights in May 2014 and January 2015.

Non-Destructive Evaluation of Polymer Composite Materials at the Manufacturing Stage Using Terahertz Pulsed Spectroscopy

Abstract: Nowadays, composite materials are widely used in building and construction industry, in motor-vehicles, spacecrafts and aircrafts, and in biomedical science due to the ability of combining various consistent components for manufacturing composite materials with physical and chemical properties significantly different from the properties of each component. Polymer composite materials (PCMs) appear to be the most common type of composites. PCMs consist mainly of polymer binder reinforced with the glass-fiber-fabric. Although PCMs are widely applied, PCM manufacturing technology lacks the methods of non- destructive testing. In this paper we demonstrate that terahertz (THz) pulsed spectroscopy (TPS) appears to be a unique instrument for solving important problems of PCM manufacturing control. We experimentally demonstrate the efficiency of TPS for non-destructive control of PCM binder polymerization, since THz radiation is sensitive to changes of picosecond dynamics in media. Furthermore, we show the ability to detect the internal non-impregnated voids inside the PCM structure by means of THz time-of-flight tomography. These results highlight the potentials of TPS applications for non-destructive control of PCM manufacturing process.

A Silicon Micromachined Eight-Pixel Transceiver Array for Submillimeter-Wave Radar

Abstract: An eight-pixel transceiver array for operation in a 340 GHz imaging radar is presented. Silicon micromachining is applied to fabricate the submillimeter-wave front-end components to increase the density and uniformity of the array while lowering the cost compared to metal machining. Performance comparable with discrete metal machined housings was achieved with the 340 GHz transmitter nominally producing 0.5 mW and the mixers having a DSB noise temperature of 2000 K with a conversion loss of 8 dB. Radar performance is primarily limited by the isolation of the hybrid coupler, which is typically 28 dB, but excellent imaging performance is still achieved and improvements in penetration compared to higher frequency imaging radars is demonstrated.

Development of THz Gyrotrons at IAP RAS and FIR UF and Their Applications in Physical Research and High-Power THz Technologies

Abstract: In this paper, we present some of the most significant recent results that characterize the state of the art in the development of sub-THz and THz gyrotrons at IAP-RAS and FIR-UF after 15 years of collaboration as well as their applications in various novel and prospective research fields and advanced technologies.

Highly Accurate in Vivo Terahertz Spectroscopy of Healthy Skin: Variation of Refractive Index and Absorption Coefficient Along the Human Body

Abstract: A method to reconstruct the terahertz (THz) refractive index and absorption coefficient of in vivo tissue using THz pulsed spectroscopy (TPS) has been proposed. The method utilizes a reference THz window to fix the sample of interest during the TPS reflection mode measurements. Satellite pulses caused by multiple THz-wave reflections in the reference window are taken into account to accurately solve the inverse problem. The stability of the proposed method in the presence of various factors, including digital noise in the TPS waveforms and fluctuations of the reference THz window position, has been accurately analyzed. The method has been implemented to study in vivo the THz refractive index and absorption coefficient of the human skin. The skin from three persons has been measured, and the results agree with the well-known data on healthy skin spectroscopy in general, except for several regions of the skin. Thus, for the elbow, the hand, the knee, and the heel the THz refractive index and absorption coefficient considerably differ from the average values. The observed results are of principle importance for further development of novel approaches to skin diagnosis based on THz technologies.

Silicon Integrated 280 GHz Imaging Chipset With 4 $\times$ 4 SiGe Receiver Array and CMOS Source

Abstract: In this paper, we report an integrated silicon-based active imaging chipset with a detector array in 0.13 $\mu{\hbox{m}}$ SiGe process and a CMOS-based source array operating in the 240–290 GHz range. The chipset operates at room-temperature with no external RF or optical sources, high-resistivity silicon lenses (HRSi) or waveguides or any custom fabrication options, such as high-resistivity substrates or substrate thinning. The receiver chip consists of a 2-D array of 16 pixels, measuring 2.5 mm $\times$ 2.5 mm with integrated antennas. An electromagnetic-active circuit co-design approach is carried out to ensure high-efficiency interface with detectors operating above cut-off frequencies with good impedance matching, near-optimal noise performance, while simultaneously suppressing the dominant surface-wave modes in a lensless lossy bulk silicon substrate. The array performance is characterized in the WR-3 band between 220–320 GHz. At the designed frequency of 260 GHz, the NEP of all pixels stays between 7.9 ${\hbox{pW}}/\sqrt{\hbox{Hz}}$ –8.8 ${\hbox{pW}}/\sqrt{\hbox{Hz}}$ . The imaging chipset consists of this 2D detector array chip and a CMOS-based source array chip measuring 0.8 mm $\times$ 0.8 mm. The entire system dissipates less than 180 mW of DC power, representing a truly integrated solution.

THz and mm-Wave Sensing of Corneal Tissue Water Content: Electromagnetic Modeling and Analysis

Abstract: Terahertz (THz) spectral properties of human cornea are explored as a function of central corneal thickness (CCT) and corneal water content, and the clinical utility of THz-based corneal water content sensing is discussed. Three candidate corneal tissue water content (CTWC) perturbations, based on corneal physiology, are investigated that affect the axial water distribution and total thickness. The THz frequency reflectivity properties of the three CTWC perturbations were simulated and explored with varying system center frequency and bandwidths (Q-factors). The modeling showed that at effective optical path lengths on the order of a wavelength the cornea presents a lossy etalon bordered by air at the anterior and the aqueous humor at the posterior. The simulated standing wave peak-to-valley ratio is pronounced at lower frequencies and its effect on acquired data can be modulated by adjusting the bandwidth of the sensing system. These observations are supported with experimental spectroscopic data. The results suggest that a priori knowledge of corneal thickness can be utilized for accurate assessments of corneal tissue water content. The physiologic variation of corneal thickness with respect to the wavelengths spanned by the THz band is extremely limited compared to all other structures in the body making CTWC sensing unique amongst all proposed applications of THz medical imaging.

Subwavelength Graphene-Based Plasmonic THz Switches and Logic Gates

Abstract: In this paper, we report on the design procedure for developing subwavelength graphene-based plasmonic waveguide, performing as a THz switch or an AND/OR logic gate. The propagation length of the surface plasmons (SPs), stimulated by a 6 THz TM polarized incident wave along this waveguide with a top graphene layer whose chemical potential is held at $\mu_{\rm C}=300~{\hbox{meV}}$ (ON state) is more than 35 times larger than that in the waveguide with $\mu_{\rm C}= 0~{\hbox{eV}}$ (OFF state). Numerical results, obtained from full wave simulations using a finite element method, also show that the modulation depth density obtained for the straight plasmonic switching waveguide, whose length is just about 20% of the incident wavelength, is larger than those reported to date. Moreover, we also designed a logic AND gate composed of a straight waveguide, a Y-branch switch, and a logic OR gate composed of two face to face Y-branches, whose total lengths are ${\sim} {\hbox{37\%}}$ , ${\sim} {\hbox{45\%}}$ , and ${\sim} {\hbox{53\%}}$ of the incident wavelength, respectively. Simulations show that the maximum ON/OFF ratios for these subwavelength plasmonic waveguides that occur between their ‘1 1’ and ‘0 0’ logical states are ${\sim} {\hbox{41.37}}$ , ${\sim} {\hbox{39.87}}$ , and ${\sim} {\hbox{40.76}}~{\hbox{dB}}$ , respectively. These numerical data also show that the modulation depth densities obtained for these devices are also greater than those reported to date. The proposed graphene-based plasmonic switches and gates offer potential building blocks for the future digital plasmonic circuits operating around 6 THz.

Broadband Terahertz Metamaterial Absorber Based on Asymmetric Resonators With Perfect Absorption

Abstract: In this paper, a broadband terahertz (THz) metamaterial absorber using asymmetric split ring resonator (ASR) was designed, fabricated, and characterized. By breaking the symmetry of a split ring resonator, two asymmetric resonances are excited from a dipole resonance, which enhance both the absorption and $Q$ -factor. With the integration of four different ASRs into one unit cell, a broadband absorber experimentally obtained a 0.82-THz bandwidth with absorptivity of more than 0.9, which is 3.4 times as wide as the 0.24-THz bandwidth of the symmetric dipole peak. The proposed broadband absorber has great application potentials in the THz spectroscopy, imaging, and sensing.

Development of Complex Relative Permittivity Measurement System Based on Free-Space in 220–330-GHz Range

Abstract: For measuring complex relative permittivity of more than 300 GHz in the THz region, time-domain spectroscopy (TDS) is usually used. On the other hand, a free-space method using a vector network analyzer (VNA) is used below 300 GHz. However, these methods have not been compared and continuity of complex relative permittivity measurement around 300 GHz has not been evaluated. We developed a system for measuring complex relative permittivity that can be operated at 220–330 GHz. This system is based on the free-space method. We compared the complex relative permittivity using a VNA and by TDS to evaluate our system and the continuity of complex relative permittivity measurement in the frequency domain. We first compared relative permittivity and dielectric loss between both methods. We then evaluated the measurement uncertainty in consideration of the thickness of materials under test (MUT), time span of the time-domain gating, linearity, stability, aperture alignment, and measurement repeatability. The dispersion in MUT thickness measurement was found to be the dominant source of uncertainty in measuring complex relative permittivity measurement. The maximum difference in relative permittivity and dielectric loss between both methods was less than 0.22 and 0.17 when MUT E was measured. For the measurement result with expanded uncertainty, the relative permittivity of VNA was $3.92\pm 0.44$ and that of TDS was $3.70\pm 0.16$ , the dielectric loss of VNA was $0.30\pm 0.52$ and that of TDS was $0.13\pm 0.02$ , respectively. The measured complex relative permittivity by using the VNA and by TDS were observed within the expanded uncertainty. We verified the availability of the measured complex relative permittivity by using our measurement system and the continuity of complex relative permittivity measurement at 300-GHz band.

Silicon-on-Glass Dielectric Waveguide—Part II: For THz Applications

Abstract: A low-loss sub-millimeter-wave/THz integrated dielectric waveguide is presented. The proposed waveguide consists of a highly resistive Silicon (Si) guiding channel bonded to a glass substrate. To reduce the waveguide insertion loss due to the glass substrate, part of the substrate below the Si guiding channel is etched. A periodic configuration of supporting beams is used to hold the Si guiding channel over the glass substrate. A low-cost and high-precision fabrication process, which is fully compatible with current Si-based fabrication technologies, is developed for the proposed waveguide. Numerical simulations and experiments are conducted to investigate the performance of the proposed waveguide. Measured attenuation constant of the proposed waveguide is 0.0346 dB/ $\lambda_{0}$ (average value) over the 440–500 GHz band.

THz Absorption in Fabric and Its Impact on Body Scanning for Security Application

Abstract: In recent years, body scanner technologies based on millimeter-wave and terahertz technologies have been shown to improve security in areas sensitive to terrorist attacks by detecting hazardous objects hidden underneath the clothing of people. Inevitably, useful devices have to provide an adequate compromise between spatial resolution limited due to diffraction and penetration through clothing. Within this context, the spectral absorption of a wide range of fabric, interference caused by the texture, and the effect of moisture within the fabric have to be considered. We have studied these effects by time-domain and Fourier transformation spectroscopy. The experimental findings are verified using a passive THz security camera operating in two frequency bands at 0.85 and 0.35 THz. The latter band was proven to be of superior use for security checks as it provides useful results even in cases of wet clothes.

300 GHz Integrated Heterodyne Receiver and Transmitter With On-Chip Fundamental Local Oscillator and Mixers

Abstract: A 300 GHz integrated heterodyne receiver and transmitter for wideband communication and imaging applications have been developed in a 250 nm InP double-heterojunction bipolar transistor (DHBT) process. The receiver integrates a 300 GHz RF amplifier with a balun, a down-conversion mixer with an IF amplifier, and a local oscillator, all on a single chip. The transmitter is composed of the identical circuit blocks of RF amplifier and oscillator in addition to an up-conversion mixer. Compared to previous integrated receivers and transmitters reported at above 200 GHz, the proposed work includes the on-chip local oscillator and mixers operating at a fundamental mode. This simplifies the system architecture, thus not only reducing the chip area and DC consumption but also improving the RF performance such as high conversion gain, low spurious levels, and low noise figure. The receiver exhibits a peak conversion gain of 26 dB at 298 GHz, 3-dB bandwidth of 20 GHz, and noise figure of 12.0–16.3 dB at IF frequency from 1.1 to 7.7 GHz. The transmitter exhibits peak conversion gain of 25 dB, 3 dB bandwidth of 18 GHz, and output power of ${-}{\hbox{2.3}}$ dBm. The DC power consumption of the receiver and transmitter are 482 and 452 mW, respectively.

A 210 GHz Fully-Integrated OOK Transceiver for Short-Range Wireless Chip-to-Chip Communication in 40 nm CMOS Technology

Abstract: This paper presents a fully-integrated 210 GHz transceiver (TRX) with non-coherent on–off-keying (OOK) modulation for short-range multi-Gb/s wireless chip-to-chip communication in 40 nm CMOS technology. The direct detection-based receiver (RX) part comprises an on-die ${-}{\hbox{0.1}}$ dBi gain modified dipole antenna, an 8-stage 210-GHz 21.7-dB peak gain low noise amplifier (LNA) along with differential-pair neutralization technique and a 2.83-kV/W peak responsivity simple demodulator (push detector) in order to operate efficiently in THz regime. The phased-locked transmitter (TX) consists of a 210 GHz ${-}{\hbox{1.3}}$ dBm output power 3.8% locking-range phased-locked loop (PLL), two modified cascode transistors as a simple OOK modulator, a 4-stage 210-GHz 19.4-dB peak power gain power amplifier (PA) with neutralization and especially layout enhancement techniques and a similar modified on-die dipole antenna. The implemented PLL employs a triple-push circular-geometry voltage-controlled oscillator (VCO) followed by a divider chain $({\div}{\hbox{1024}})$ . The proposed TRX achieved error-free operation $({\rm BER} with ${+}{\hbox{6.8}}$ dBm of EIRP at center frequency of 210 GHz with ${\hbox{2}}^{31}{\hbox{--}}1$ PRBS of 10.7 Gb/s data-rate over 1 cm distance while consuming 421 mW of DC power which results to energy efficiency of 39.3 pJ/bit.

Long-Path THz-TDS Atmospheric Measurements Between Buildings

Abstract: We have measured THz pulse propagation through a 186 m distance for which the THz pulses propagated in the atmosphere between two buildings separated by 79.3 m, with different relative humidity (RH) and weather conditions such as clouds, rain, and snow. The THz pulses propagated through rain falling at 3.5 mm/h and snow falling at 2 cm/h equivalent to 2.0 mm/h rainfall. For calm weather, the transmitted THz pulseshapes and relative transit times were measured to a precision of 0.1 ps. These broadband measurements demonstrate the potential of line-of-sight THz communications, THz sensing, and THz imaging through fog and smoke.

4.7-THz Superconducting Hot Electron Bolometer Waveguide Mixer

Abstract: We present the first superconducting hot electron bolometer (HEB) waveguide mixer operating at 4.7 THz. The 5.5-nm-thick, 300-nm-long, and 3600-nm-wide NbN HEB microbridge is integrated into a normal metal (Au) planar circuit on a 2 $\mu$ m thick silicon substrate. This circuit is integrated in a 24 $\mu$ m $\,\times\,$ 48 $\mu$ m $\,\times\,$ 21 $\mu$ m waveguide cavity and a 14 $\mu$ m $\, \times \,$ 7 $\mu$ m $\, \times \,$ 200 $\mu$ m substrate channel, which is directly machined into a CuTe alloy block. The power spectrum of the HEB mixer, measured with a Fourier transform spectrometer, is in good agreement with the results of 3-D EM circuit simulation. Measured mixer performance shows a state-of-the-art double sideband noise temperature of 1100 K, averaged over the IF bandwidth of 0.2–3.5 GHz. The 3-dB noise roll-off is 3.5 GHz. This mixer is used in the German REceiver for Astronomy at Terahertz frequencies (GREAT) at the airborne Stratospheric Observatory for Far Infrared Astronomy (SOFIA).

Silicon-on-Glass Dielectric Waveguide—Part I: For Millimeter-Wave Integrated Circuits

Abstract: A low-cost Silicon-on-Glass (SOG) integrated circuit technology is proposed for millimeter-wave (mmW) applications, for the first time. In the proposed technology, all mmW passive components are made of high-resistivity Silicon (Si) on a glass substrate. The proposed technique leads to a high-precision and low-cost fabrication process, which eliminates the need for costly assembly of the complex structures. This is achieved by photolithography and dry etching of the entire integrated passive circuit through the Si layer of the SOG wafer. Silicon-on-Glass dielectric waveguide, as the basic component of the SOG integrated circuit, is theoretically and experimentally investigated. A test setup is designed to measure propagation characteristics of the proposed SOG waveguide. Measured dispersion diagrams of the SOG dielectric waveguide show average attenuation constants of 0.63 dB/cm, 0.28 dB/cm, and 0.53 dB/cm over 55–65 GHz, 90–110 GHz, and 140–170 GHz, respectively.

Narrow-Linewidth and High-Transmission Terahertz Bandpass Filtering by Metallic Gratings

Abstract: Accurate spectral resolving capability is important to THz spectroscopy. So far, the THz passband filters based on metamaterial and surface plasmon suffer from the broad passband. In this paper, we propose a THz filter with calculated pass band of $(\Delta f/f) together with a peak transmission above 70% and demonstrate the experimental results with $(\Delta f/f) limited by the measurement system. The calculated value of $\Delta f/f$ is at least one order of magnitude smaller than the previous results. The unique filtering capability was attributed to the guided-mode resonance that is generated by metallic gratings and a waveguide. Compared to the dielectric gratings, the metallic gratings with a narrow air slit are proved to effectively suppress off-resonance wavelength transmission resulting in a bandpass filtering response. In addition, a weak waveguide is found to benefit a narrow-linewidth filtering. Fabrication method of metallic gratings on membrane in a size of several ${\hbox{cm}}^{2}$ was developed and the experimental results show reasonably good agreement with the simulation for a filter with $(\Delta f/f) limited by the spectral resolution and detectability of our measurement system.

Trends in the Research of Modern Terahertz Detectors: Plasmon Detectors

Abstract: Terahertz (THz) wave detectors include Golay cells, bolometers, pyroelectric detectors, superconductor tunnel junction detectors (STJs), and Schottky-barrier diodes (SBDs). Of these, most (except STJs and SBDs) are of the thermoelectric type; they are highly sensitive, but the response speed is low. On the other hand, in SBDs operating at room temperature, the response is fast, but the sensitivity is rather low at high frequencies; this is due to the carrier transit-time effect. Therefore, a nonlinearity of the two-dimensional plasmons, quanta of collective excitation of electronic charge density waves, has attracted attention as a mechanism of fast and sensitive THz-wave rectification and detection. In this paper, technological trends in the research of modern THz detectors are reviewed, with an emphasis on plasmon-type detectors.

Metamaterial-Based Terahertz Imaging

Abstract: This paper presents the design of an innovative, low-cost, uncooled, metamaterial-based terahertz (THz) focal plane array (FPA). A single pixel is composed of a resonant metamaterial absorber and micro-bolometer sensor integrated in a standard 180 nm CMOS process. The metamaterial is made directly in the metallic and insulating layers available in the six metal layer CMOS foundry process. THz absorption is determined by the geometry of the metamaterial absorber which can be customized for different frequencies. The initial prototype consists of a 5 $\times$ 5 pixel array with a pixel size of 30 $\mu$ m $\times$ 30 $\mu$ m and is readily scalable to more commercially viable array sizes. The FPA imaging capability is demonstrated in a transmission and reflection mode experiment by scanning a metallic object hidden in a manila envelope.

Improved Hybrid FDTD Method for Studying Tunable Graphene Frequency-Selective Surfaces (GFSS) for THz-Wave Applications

Abstract: One hybrid finite-difference time-domain (FDTD) method is proposed for studying transmission characteristics of a THz wave through some tunable dispersive graphene frequency-selective surfaces (GFSSs) biased by an electrostatic or a magnetostatic field, respectively. It integrates auxiliary differential equation (ADE)-FDTD with high-order conformal FDTD (2, 4) so as to handle atomically thin periodic structures on an electrically anisotropic substrate with high flexibility and accuracy. The dispersion error and computational efficiency of the developed algorithm is validated in comparison with the piecewise linear recursive convolution (PLRC)-FDTD and commercial software HFSS, respectively. Numerical studies are further performed to demonstrate its capability for characterizing the effects of chemical potential of graphene, biasing electrostatic and magnetostatic fields, and geometrical parameters of different GFSSs on the central frequencies and 3-dB bandwidths of their passbands and stopbands. In particular, both frequency selectivity and tunability of GFSSs made of single- and double-layer periodic split-ring resonators are predicted, which can be exploited for developing some novel tunable THz structures.