In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being geared toward the implementation of THz imaging systems in real operational conditions. The article will touch upon the advanced solid-state-based THz imaging systems, including room temperature THz sensors and arrays, as well as their on-chip integration with diffractive THz optical components. We will cover the current-state of compact room temperature THz emission sources, both optolectronic and electrically driven; particular emphasis is attributed to the beam-forming role in THz imaging, THz holography and spatial filtering, THz nano-imaging, and computational imaging. A number of advanced THz techniques, such as light-field THz imaging, homodyne spectroscopy, and phase sensitive spectrometry, THz modulated continuous wave imaging, room temperature THz frequency combs, and passive THz imaging, as well as the use of artificial intelligence in THz data processing and optics development, will be reviewed. This roadmap presents a structured snapshot of current advances in THz imaging as of 2021 and provides an opinion on contemporary scientific and technological challenges in this field, as well as extrapolations of possible further evolution in THz imaging.

Roadmap of Terahertz Imaging 2021.

A compact wideband four-way filtering PD is presented. The structure of the proposed device includes a pair of looped coupled-line structures to provide the needed power division to four output ports. Moreover, a pair of short-ended coupled-line stubs is used to introduce multiple transmission poles and transmission zeros for a sharp cut-off of the passband and high upper-stopband rejection. A detailed design procedure is shown to determine the initial design parameters. A prototype is designed, simulated and measured experimentally. The measured results show 56.5% bandwidth centered at 1.5 GHz with more than 15 dB upper-stopband rejection up to 4.15 GHz, more than 13 dB in-band isolation, and 15 dB return loss at all output ports.

Wideband Four-Way Filtering Power Divider With Sharp Selectivity and Wide Stopband Using Looped Coupled-Line Structures

A new class of in-phase and out-of-phase power dividers with constant equal-ripple frequency response and wide operating bandwidth is presented in this paper. The proposed design is based on microstrip-to-slotline transitions and slotline resonators. A slotted T-junction is adopted to split the power into two parts and obtain wideband isolation between the two output signals at the same time. The characteristic impedance of the transitions and resonators determines the operating bandwidth and in-band magnitude response. By reversing the placement direction of the slotline-to-microstrip transition, the electrical field is reversed, thus resulting in out-of-phase responses between output ports. A thorough analysis of the relations between the structure and the characteristic functions is provided to guide the selection of parameters of the structure in order to meet the design objectives. In the structure, simulation and measurement are conducted to verify the design method. For both in-phase and out-of-phase cases, more than 110% bandwidth has been achieved with excellent matching at all ports and isolation of output signals. Constant in-band ripple is obtained within the operating band of the power dividers, indicating that the proposed design can realise minimal power deviations, which is extremely desired in wireless systems.

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Design of Wideband In-Phase and Out-of-Phase Power Dividers Using Microstrip-to-Slotline Transitions and Slotline Resonators

Terahertz (THz) links will play a major role in high data rate communication over a distance of few meters. In order to achieve this task, antenna designs with high gain and wideband characteristics will spearhead these links. In this contribution, we present different antenna designs that offer characteristics better suited to THz communication over short distances. Firstly, a single-element antenna having a dipole and reflector is designed to operate at 300 GHz, which is considered as a sub-terahertz band. That antenna achieves a wide impedance bandwidth of 38.6% from 294 GHz to 410 GHz with a gain of 5.14 dBi. Secondly, two designs based on the same dipole structure but with added directors are introduced to increase the gain while maintaining almost the same bandwidth. The gains achieved are 8.01 dBi and 9.6 dBi, respectively. Finally, an array of elements is used to achieve the highest possible gain of 13.6 dBi with good efficiency about 89% and with limited director elements for a planar compact structure to state-of-the-art literature. All the results achieved make the proposed designs viable candidates for high-speed and short-distance wireless communication systems.

Sub-THz Antenna for High-Speed Wireless Communication Systems

This paper presents a novel and simple design approach of multiport power divider with arbitrary power splitting ratio as well as filtering response. The relationship between the external Q -factors ( Q es) and power splitting ratio is theoretically analyzed, and it is found that the power allocation for the multiple outputs is determined by their Q e's ratio. By altering the Q es of the splitting ports, the power ratio can be arbitrarily controlled while the output total Q e is equal to the input Q e. Based on this, the two functions of power division and bandpass response of the proposed filtering divider can be designed independently. Accordingly, two practical three-way filtering power dividers with different division ratios (1:1:1 and 3:1:1) using the same topology based on the dielectric resonator (DR) are constructed for the first time. Both of them can be easily built by properly changing the lengths of feeding probes for the output DR without increasing the circuit size, while the phase difference between the splitting ports mainly depends on the direction of the feeding probes. To verify the proposed design concept, the two filtering power dividers with different division ratios mentioned above are implemented and measured. Comparisons of the measured and simulated results show good accordance and verify the theoretical predications.

Theory and Experiment of Multiport Filtering Power Divider With Arbitrary Division Ratio Based on Dielectric Resonator

In this letter, a compact modified n-way Wilkinson power divider that allows physical isolation between output ports is presented. In the proposed power divider, the isolation impedances are positioned within quarter-wave transmission lines so that part of the lines can be used for interconnection and impedance transformation. Theoretical analysis is performed to derive the design equations of the proposed n-way power divider. Further, measurements show that the proposed power divider can reduce the insertion loss and circuit size when compared with a conventional Wilkinson power divider.

Compact Modified Wilkinson Power Divider With Physical Output Port Isolation

In this letter, we propose an $N$ -way unequal power division Wilkinson power divider (PD) with physical output port separation. These PDs employ isolation impedances at arbitrary positions within quarter-wave-long transmission lines, and hence both electrical isolation and physical separation can be obtained between output ports. Design equations are derived, meeting the impedance matching and isolation requirements of the PD. The fabricated 2-way (1:2) and 3-way (1:2:1) PDs operating at 2.0 GHz exhibit excellent insertion loss, return loss, and isolation performance with the output ports physically separated.

$N$ -Way Unequal Wilkinson Power Divider With Physical Output Port Separation

Microstrip transitions for wide circuits/wafers are proposed using stub arrays (SAs) and indented waveguides (IWs) Full-wave analysis is performed to show that the proposed techniques can effectively suppress the resonances excited in the slit region which is needed to accommodate wide substrates H -band waveguide-to-microstrip transitions consisting of two directors, dipole antenna, and balun were fabricated including the proposed techniques, accommodating a 1220-μm-wide substrate which is about three times the width of WR-03 standard waveguide (430 μm) The slit region is 1300 μm wide and 90 μm high The comparison of measurement results between four transitions proves that the proposed SAs and IWs can successfully eliminate in-band resonances and greatly enhance the bandwidth of the transitions The transition with both SAs and IW exhibits the best performance, a back-to-back loss of 23 dB across 234–314 GHz, where the variation in the insertion loss was less than ±05 dB This insertion loss includes the losses in the microstrip line and 3-cm-long waveguide section The return loss is better than 137 dB in this frequency range Bandwidth for 10-dB return loss is as large as 97 GHz from 227 to 324 GHz

Submillimeter-Wave Waveguide-to-Microstrip Transitions for Wide Circuits/Wafers

In this paper, we present a broadband THz CMOS on-chip antenna. The bandwidth of the on-chip antenna is improved by stacking four resonators on the top of a standard patch antenna. The proposed antenna was fabricated using 65-nm CMOS process, and the performance was verified by simulation and measurement. The simulation and measurement results show that the antenna can exhibit the fractional bandwidth for 10-dB return loss greater than 10%, and radiation efficiency greater than 15% at 300 GHz.

Broadband THz CMOS on-chip antenna using stacked resonators

In this paper, a V-shaped patch antenna with defected ground structure is proposed at terahertz to overcome the limited performance of a standard complementary metal-oxide semiconductor (CMOS) patch antenna consisting of several metal layers and very thin interdielectric layers. The proposed V-shaped patch with slots allows the increased radiation resistance and broadband performance. In addition, the patch resonating at different frequency from the V-shaped patch is stacked on the top to broaden the impedance-matching bandwidth. More importantly, the slots are formed in the ground plane, which is called the defected ground structure, to further increase the radiation resistance and thus improve the bandwidth and efficiency. It is verified from electromagnetic simulations that the leakage waves from the defected ground can enhance the antenna directivity and gain by coherently interfering with the topside radiation. The proposed on-chip antenna is fabricated using a standard 65 nm CMOS process. The on-wafer measurement shows very wide bandwidth in input reflection coefficient ( 320 GHz. The measured peak gain was as high as 5.48 dBi at 295 GHz. To the best of the authors’ knowledge, these results belong to the best performance among the terahertz CMOS on-chip antennas without using additional components or processes such as dielectric resonators, lens, or substrate thinning.

Wonseok Choe

Papers

Compact Modified Wilkinson Power Divider With Physical Output Port Isolation

$N$ -Way Unequal Wilkinson Power Divider With Physical Output Port Separation

Submillimeter-Wave Waveguide-to-Microstrip Transitions for Wide Circuits/Wafers

Broadband THz CMOS on-chip antenna using stacked resonators

A Terahertz CMOS V-Shaped Patch Antenna with Defected Ground Structure