sub-THz Region Research Articles

Design and Characterization of CMOS Transmission Lines in Sub-THz Region

ABSTRACTMicrostrip line, coplanar waveguide, ground coplanar waveguide, and two kinds of slow-wave transmission lines (T-Line) are designed and realized based on a 0.13 μm Complementary Metal-Oxide-Semiconductor (CMOS) process. The radiation loss in the sub-THz region is evaluated by E-field distribution and attenuation coefficient versus the dimensions. Characterizations of these T-Lines up to sub-THz region is performed by sub artificial neural network (ANN) based on T-Line models and measurement results. The sub ANN-based model is capable of capturing the frequency-dependent effect of the resistance, inductance, conductance, and capacitance (RLGC) model up to sub-THz region, promising a compact and wideband modeling approach. The effect of dummy-filling on degrade of T-Lines performance in sub-THz region is also analyzed and evaluated by measurement and simulation results. Good agreement is obtained between the simulation, modeling, and measurement results.

Comparison of Free Space Measurement Using a Vector Network Analyzer and Low-Cost-Type THz-TDS Measurement Methods Between 75 and 325 GHz

Specifications of two measurement systems, free space measurement using a vector network analyzer and low-cost-type terahertz time-domain spectroscopy using a multimode laser diode, have been compared in the frequency region of millimeter/sub-THz waves. In the comparison, accuracy, cost, measurement time, calculation time, etc. were considered. Four samples (Rexolite, RO3003, Ultralam 3850HT-design, and L1000HF) were selected for the comparison of the specifications of the two methods. The acquired data was used to compute the complex permittivity of measured materials. The extracted results by free space measurement agreed well to the ones obtained by low-cost-type terahertz time-domain spectroscopy. This result proves free space measurement that can be assessed as a new method of material characterization in the sub-THz region successfully worked. Furthermore, free space measurement was proved to be suitable for a measurement in a narrow frequency range. On the other hand, low-cost-type terahertz time-domain spectroscopy has features not only low cost but also measurement capability in wide frequency range.

Sub-terahertz imaging using time-domain signals obtained with photoconductive spiral antennas

A sub-terahertz time-domain spectroscopic system with spiral antennas, which have high efficiency in the sub-THz region, was applied to imaging. In the time-domain signals, there were several peaks with various widths. Images were obtained by translating samples at the focus of the sub-THz beam with the time delay fixed at certain positions while monitoring the signal. At the edge of the metal patterns, the signal basically showed step-like variations, which were sometimes accompanied by overshoot structures (peaks and dips) depending on the time-delay position. The overshoot can be utilized to make the images sharper. The time-delay position can be adjusted to make the step height zero and produce images that contain only the overshoot, which can be transformed into images of the edge lines of the metal pattern. In addition, an image of the watermark on a 1000-yen bill was obtained. The image contrast caused by the small variation of the bill paper thickness was enhanced by locating the time delay at the position where the signal shows maximum sensitivity to the phase shift.

Asymmetric devices based on carbon nanotubes as detectors of sub-THz radiation

Demand for efficient terahertz (THz) radiation detectors resulted in intensive study of the asymmetric carbon nanostructures as a possible solution for that problem. In this work, we systematically investigate the response of asymmetric carbon nanodevices to sub-terahertz radiation using different sensing elements: from dense carbon nanotube (CNT) network to individual CNT. We conclude that the detectors based on individual CNTs both semiconducting and quasi-metallic demonstrate much stronger response in sub-THz region than detectors based on disordered CNT networks at room temperature. We also demonstrate the possibility of using asymmetric detectors based on CNT for imaging in the THz range at room temperature. Further optimization of the device configuration may result in appearance of novel terahertz radiation detectors.

Microbolometer Terahertz Focal Plane Array and Camera with Improved Sensitivity in the Sub-Terahertz Region

A pixel in an uncooled microbolometer terahertz (THz) focal plane array (FPA) has a suspended structure above read-out integrated circuit (ROIC) substrate. An optical cavity structure is formed between a thin metallic layer deposited on the suspended structure and a thick metallic layer deposited on the ROIC surface. The geometrical optical cavity length for our previous pixel structure, 3–4 μm, is extended three times, so that responsivity can be increased in the sub-THz region. This modification is carried out by depositing a thick SiN layer on the thick metallic layer. The modified pixel structure is applied to 640 × 480 and 320 × 240 THz-FPAs with 23.5 μm pixel pitch. Minimum detectable powers per pixel (MDP) are evaluated for these FPAs at 4.3, 2.5, 0.6, and 0.5 THz, and the MDP values are found to be improved by a factor of ten at 0.6 and 0.5 THz. The MDP values of the THz-FPAs developed in this work are compared with those of other THz detectors, such as uncooled antenna-coupled CMOS (complimentary metal-oxide semiconductor) THz-FPAs and cooled bolometer arrays. It is found that our THz-FPAs are more sensitive in the sub-THz region than the CMOS THz-FPAs, while they are much less sensitive than the cooled bolometer arrays. These THz-FPAs are incorporated into a 640 × 480 THz camera and 320 × 240 THz camera, and imaging equipment is developed. The equipment consists of a linearly polarized sub-THz source, a collimator lens, a beam homogenizer, two wire grids, a quarter-wave plate, and two THz cameras, and sub-THz images are demonstrated. It should be mentioned for the equipment that imaging of transmission and reflection is realized by moving only the quarter-wave plate, and the reflection image is taken along a direction normal to a sample surface so that the reflection image is hardly deformed.

Pilot clinical study to investigate the human whole blood spectrum characteristics in the sub-THz region.

We have conducted a pilot clinical study to not only investigate the sub-THz spectra of ex-vivo fresh human whole blood of 28 patients following 8-hours fasting guideline, but also to find out the critical blood ingredients of which the concentration dominantly affects those sub-THz spectra. A great difference between the sub-THz absorption properties of human blood among different people was observed, while the difference can be up to ~15% of the averaged absorption coefficient of the 28 samples. Our pilot clinical study indicates that triglycerides and the number of red blood cells were two dominant factors to have significant negative correlation to the sub-THz absorption coefficients.

Design and Experiment Results of High-Speed Wireless Link Using Sub-terahertz Wave Generated by Photonics-Based Technology

Using a sub-terahertz (sub-THz) wave generated using a photonics-based technology, a high-speed wireless link operating at up to 10 Gbps is designed and demonstrated for realization of seamless connectivity between wireless and wired networks. The sub-THz region is focused upon because of the possibility to obtain sufficient bandwidth without interference with the allocated RF bands. To verify the high-speed wireless link, such dynamic characteristics as the eye diagrams and bit error rate (BER) are measured at up to 10 Gbps for non-return-to-zero pseudorandom binary sequence data. From the measurement results, a receiver sensitivity of –23.5 dBm at is observed without any error corrections when the link distance between the transmitter and receiver is 3 m. Consequently, we hope that our design and experiment results will be helpful in implementing a high-speed wireless link using a sub-THz wave.

Acoustic dynamics of network-forming glasses at mesoscopic wavelengths

The lack of long-range structural order in amorphous solids induces well known thermodynamic anomalies, which are the manifestation of distinct peculiarities in the vibrational spectrum. Although the impact of such anomalies vanishes in the long wavelength, elastic continuum limit, it dominates at length scales comparable to interatomic distances, implying an intermediate transition regime still poorly understood. Here we report a study of such mesoscopic domains by means of a broadband version of picosecond photo-acoustics, developed to coherently generate and detect hypersonic sound waves in the sub-THz region with unprecedented sampling efficiency. We identify a temperature-dependent fractal v3/2 frequency behaviour of the sound attenuation, pointing to the presence of marginally stable regions and a transition between the two above mentioned limits. The essential features of this behaviour are captured by a theoretical approach based on random spatial variation of the shear modulus, including anharmonic interactions.

Development of Kinetic Inductance Stationary-Wave Integrated Fourier-Transform Spectrometry (SWIFTS)

We present millimeter-wave Stationary-Waves Integrated Fourier Transform Spectrometry (SWIFTS) using the nascent Kinetic Inductance Detector (KID) technology. SWIFTS operation consists in converting a stationary-wave spatial sampling into the frequency domain; our SWIFTS devices are designed to operate in the sub-THz region. Millimeter wave power is probed using KIDs, high-quality superconducting resonators deemed to be the next generation millimetric photon detectors for large array astronomy cameras. We expect KIDs to be sensitive enough to sense the stationary wave without altering its properties. Moreover, KID multiplexing capabilities will allow the use of many detectors on a single transmission line, facilitating cryogenic measurements.The SWIFTS concept, already validated in the optical and microwave (<20 GHz) bands, will be useful in any applications where integrated and broadband spectral analysis is needed. We discuss SWIFTS device structure, its measurement operation and some preliminary results.

Sub-terahertz spectroscopic system using a continuous-wave broad-area laser diode and a spatial filter

We have proposed a low-cost type spectroscopic system in the sub-THz region similar to the THz-TDS, in which we use a continuous-wave broad-area laser diode (BLD). The Fourier spectrum of the signal waveform is discrete and is very weak at frequencies between the discrete peaks when the BLD beam is used in free space. In order to make the spectrum suitable for spectroscopy, the BLD beam is used through a spatial filter (SF) and as a result, the amplitude of the continuous spectral components between the discrete lines increases considerably. This change is explained by considering the filtering effect of the beam patterns of the BLD by the SF. Further, we show that the signal and then the signal-to-noise ratio of the transmission spectra of a sample are enhanced by using the SF compared with our previous work.

Sub-THz dielectric resonance in single crystal yttrium iron garnet and magnetic field tuning of the modes

The observation of dielectric resonance over the frequency range 40-110 GHz in single crystal yttrium iron garnet (YIG) and its magnetic field tuning characteristics are reported. The dimensions of YIG are appropriately chosen in order to have the dielectric resonance occur at a much higher frequency than the ferromagnetic resonance and avoid any hybrid spin-electromagnetic modes. The dielectric modes are magnetically tunable by 1 GHz with a magnetic field of ∼1.75 kOe. The tuning range and required bias magnetic fields, however, can be controlled with the sample dimensions (or the demagnetization factor Nzz). Theoretical calculations on magnetic field tuning characteristics for the dielectric modes are in reasonable agreement with the data. The theory also predicts a similar magnetic tuning of the dielectric modes in the sub-THz frequency range as well. The dielectric modes that can be tuned with a magnetic field are of importance for the realization of low-loss tunable devices, including resonators, isolators, and phase shifters operating in the sub-THz region.

Sub-Terahertz Excitations in Manganites with Commensurate Charge Order

The optical conductivity of La–Ca, Nd–Sr, and Bi–Sr manganites with commensurate charge order (CO) has been first observed in the sub-THz region. Below, the ordering temperature TCO well-defined peaks are observed, associated with side bands. They are assigned to pinned phasons, and to combinations of phasons and amplitudons, respectively. In La1−n/8Can/8MnO3 with n=5, 6, one can thus determine the electron-phonon coupling, the effective mass of the CO system, and its contribution to the dielectric constant. Their values support a description of the CO in La–Ca manganites in terms of a CDW approach. In Bi0.5Sr0.5MnO3, where TCO> 500 K and the coupling is stronger, only the combination bands are clearly detected.

Technical Report: THz Experiments with Coherent Synchrotron Radiation from BESSY II

The terahertz (THz) and sub-THz region of the electromagnetic spectrum bridges the infrared and the microwave. This boundary region is beyond the normal reach of optical and electronic measurement techniques normally associated with these better-known neighbors. It is only within a decade that the THz region has become scientifically accessible with broadband sources of moderate intensity, produced by ultra-fast laser pulses incident on biased semiconductors or non-linear crystals [1, 2]. Recently, a much higher power source of THz radiation was demonstrated: coherent synchrotron radiation (CSR) from short, relativistic electron bunches [3–6].

Generation of sub-terahertz waves using a semiconductor photomixer

A low-temperature-grown GaAs (LT-GaAs) photoconductive switch has been utilized as a photomixer in the sub-terahertz (sub-THz) region. The usefulness of the switch has been demonstrated by detecting the radiated sub-THz wave with a high-TC Josephson junction detector consisting a series of nine junctions and a gold-coated log-periodic antenna. The sub-THz waves are radiated from the photoconductive switch by introducing the optical heterodyne signal of two semiconductor lasers. Although the radiation power of the photoconductive switch is three orders of magnitude lower than a conventional photodiode, the sub-THz waves from the switch were successfully observed in the difference voltage response of the junctions. This result suggests that the LT-GaAs switch can be used as the photomixer in the optical-microwave converter for superconductive electronics. We have also studied the properties of a radiated sub-THz frequency spectrum using a Martin–Puplett interferometer.

Sub-THz spectroscopic system using a multimode laser diode and photoconductive antenna

A system for measuring transmittance of electromagnetic waves in the sub-THz region is proposed. The electromagnetic radiation is generated by the excitation of a photoconductive antenna with a commercially available multimode laser diode. The spectral coverage of the radiation is increased by defocusing the light spot on the photoconductive antenna. Transmitted radiation is detected by a hot-electron bolometer through a Martin–Puplett-type interferometer. Transmittance is measured for n-type Si wafers with various doping levels. The carrier densities calculated from the transmittance agree well with those obtained from the dc conductivity measurement.

Numerical characterization of impedance transformers consisting of vertically stacked superconducting transmission lines

A novel coupling configuration between an SIS mixer, an antenna, and a local oscillator has been numerically characterized for an integrated receiver operated in the sub-THz region. The configuration consists of two vertically stacked superconducting striplines, where the thickness of their common electrode t/sub 2/ is of the order of its magnetic penetration depth /spl lambda//sub 2/. It has been found that the coupling efficiency between the antenna and the mixer decreases with decrease of t/sub 2///spl lambda//sub 2/, while that between the mixer and the local oscillator has a local maximum. It is also shown that the coupling efficiencies increase with decrease of Q/sub A/, the Q-factor due to the tunneling loss in the mixer. For Q/sub A/=5.0 and 16, 75-85% of the available signal power and sufficient power from the local oscillator can be absorbed in the tunnel conductance of the mixer at 1 THz. This coupling efficiency is equal to or larger than that of a conventional planar coupler.