Abstract-Terahertz subwavelength edge detection based on dispersion-induced plasmons

 

Tie-Jun Huang, Jin Zhao, Li-Zheng Yin,  Pu-Kun Liu, 

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-46-11-2746

Terahertz imaging has recently attracted great attention owing to the abilities of high penetration and low ionizing damages. However, the low resolution and low contrast resulting from the diffraction limit and unwanted background illumination significantly hinder the extensive usage. In this Letter, we propose and numerically demonstrate a terahertz subwavelength imaging method capable of extracting only the edges and fine features of the targets. The underlying physics is the efficient transmission of the scattering evanescent waves related to key geometric information while blocking the propagating components. By exploiting the structurally induced plasmons in a bounded metallic waveguide, the transmission channel for evanescent waves is realized by hyperbolic metamaterials through periodically stacking dielectric layers. On this basis, high-contrast edge detection with a resolution up to 0.1𝜆$0.1\lambda$ is demonstrated at terahertz wavelengths. The proposed terahertz imaging method may find important applications in non-destructive testing, weak scattering object detection, and high-contrast microscopy.

© 2021 Optical Society of America

Abstract-High-efficiency terahertz spin-decoupled meta-coupler for spoof surface plasmon excitation and beam steering

Li-Zheng Yin, Tie-Jun Huang, Feng-Yuan Han, Jiang-Yu Liu, Pu-Kun Liu

https://assert.pub/papers/1904.06133

Spoof surface plasmon (SSP) meta-couplers that efficiently integrate other diversified functionalities into a single ultrathin device are highly desirable in the modern microwave and terahertz fields. However, the diversified functionalities, to the best of our knowledge, have not been applied to circular polarization meta-couplers because of the spin coupling between the orthogonal incident waves. In this paper, we propose and numerically demonstrate a terahertz spin-decoupled bifunctional meta-coupler for SSP excitation and beam steering. The designed meta-coupler is composed of a coupling metasurface and a propagating metasurface. The former aims at realizing anomalous reflection or converting the incident waves into SSP under the illumination of the left or right circular polarization waves, respectively, and the latter are used to guide out the excited SSP. The respective converting efficiency can reach 82% and 70% at 0.3THz for the right and left circular polarization incident waves. Besides, by appropriately adjusting the reflection phase distribution, many other diversified functionalities can also be integrated into the meta-coupler. Our study may open up new routes for polarization-related SSP couplers, detectors, and other practical terahertz devices.

Abstract-Regenerated amplification of terahertz spoof surface plasmon radiation

Juan-Feng Zhu, Chao-Hai Du, Lu-Yao Bao,  Pu-Kun Liu,

https://iopscience.iop.org/article/10.1088/1367-2630/ab0aa4/meta

Regenerated amplification induced by a Fabry–Perot (F–P) cavity is introduced to enhance the interaction efficiency of the free-electron-driven spoof surface plasmon. A direct-current electron beam flies above the meta-grating surface and seeds noise-level plasmonic waves. This weak signal experiences multiple back-and-forth regenerated amplifications in the F–P cavity loaded grating system, and the system outputs a pulsed radiation when the signal leaves the cavity. When compared with the condition without the F–P cavity, the equivalent beam-wave interaction length is effectively extended, and the interaction efficiency is improved by orders of magnitude. The proof-of-principle scheme is verified in both backward-wave and forward-wave modes using the particle-in-cell simulation. This scheme is promising for developing high-efficient on-chip terahertz free-electron radiation sources.

Abstract-Investigation of high-order mode excitation in a terahertz second-harmonic gyro-BWO

Physics of Plasmas

Shi Pan, Chao-Hai Dua), Zi-Chao Gao, Fan-Hong Li, Hui-Qi Bian, Pu-Kun Liu

FIG. 1.A quarter of a full sectional view of the cathode-end output circuit in a THz gyro-BWO.

https://aip.scitation.org/doi/10.1063/1.5089598

As to gyrotron oscillators, operation at high harmonics is an effective solution to decrease the required magnetic field strengths and improve radiation frequencies towards the terahertz (THz) band. Unfortunately, significant challenges related to a harmonic gyrotron include weak interaction strengths and serious mode competition conditions. In this paper, we report on the excitation strategy and stable-state property of a THz second-harmonic (SH) gyro-BWO with the TE24,3 mode. Such an extremely high-order mode interaction system is inherently capable of providing high power capacity and advanced mode selectivity. A competition-free parameter space is created by increasing the Doppler sensitivity of one fundamental-harmonic (FH) competing mode at low magnetic fields and simultaneously suppressing the Q factor of another FH competing mode in the near-cutoff region at high magnetic fields. The SH quasi-whispering-gallery mode can be stimulated with a medium output power at around 0.5 THz during the FH mode switching process. This work contributes to further exploiting high frequency steps in the high-order multi-mode frequency-tuning gyro-BWO.

Abstract-Terahertz multichannel metasurfaces with sparse unit cells

Li-Zheng Yin, Tie-Jun Huang, Feng-Yuan Han, Jiang-Yu Liu, and Pu-Kun Liu

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-7-1556

Reflective multichannel metasurfaces are flat reflectors that can control incident and reflected waves in a number of propagating directions simultaneously. However, they are always densely discretized with a high spatial resolution, which increases the manufacturing complexity. In this Letter, to the best of our knowledge, a new method that combines the array antenna theory with the metagratings theory is proposed. We demonstrate that the unit cells with a linear gradient phase in each period of the metasurfaces can eliminate specific space harmonics. With this method, multichannel metasurfaces can be designed with sparse unit cells, and high efficiency is maintained simultaneously. As proofs of the method, we design three different terahertz multichannel metasurfaces with no more than three unit cells per period. The simplification of structures can efficiently reduce the manufacturing complexity. This work may open up new routes in designing multichannel metasurfaces.

© 2019 Optical Society of America

Abstract-Terahertz Frequency- and Mode-Insensitive Broadband Quasi-optical Converter Antenna System

Hui-Qi Bian, Chao-Hai Du, Shi Pan, Pu-Kun Liu

https://link.springer.com/article/10.1007%2Fs10762-018-0523-1

Recently emerged multimode gyrotron, a high-power broadband terahertz radiator, encounters the challenge of efficiently converting a series of operating whispering-gallery modes (WGMs) into free-space Gaussian beams. To this demand, we propose a frequency- and mode-insensitive antenna capable of broadband multimode converting. For a single mode, to achieve broadband operation, special reflector configuration and large-radius launcher guarantee the system high robustness to frequency-induced wave number variation. Furthermore, for a series of operating WGMs, in order to achieve multimode operation, high-order mode indices guarantees familiar field patterns and ray trajectories. In particular, high-purity Gaussian beams are simultaneously achieved in different WGMs of broad continuous bands, including 351–361 GHz for TE11,2 mode, 375–385 GHz for TE12,2 mode, and 398–410 GHz for TE13,2mode. The results are verified by both the vector diffraction theory and the method of momentum. This kind of mode converter will promote the development of multimode gyrotrons and other antenna-feeder systems for high-power terahertz applications.

Abstract-Terahertz super-resolution imaging using four-wave mixing in graphene

Jiang-Yu Liu, Tie-Jun Huang, and Pu-Kun Liu

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-9-2102

A perfect lens made from negative refraction (NR) materials is utilized to overcome the diffraction limit. However, these NR lenses are realized by metamaterials, which suffer from high losses, and the volume is bulky. In this Letter, we propose a terahertz NR lens by using a four-wave mixing (FWM) process in graphene. NR is demonstrated because of the phase matching along the surface of graphene. Evanescent waves that store high spatial frequency information can be converted into propagating waves in the nonlinear NR process. An image with subwavelength resolution is reconstructed at the FWM wavelength. Theoretical analysis and numerical simulations are performed to demonstrate the capability of such imaging. The lens has a subwavelength resolution of around 𝜆/5$\lambda /5$. The lens needs low field intensity due to the strong nonlinear response of graphene in the terahertz frequency. This Letter may have applications in terahertz microscopy.

© 2018 Optical Society of America

Abstract-Frequency pulling in a low-voltage medium-power gyrotron

Li Luo, Chao-Hai Du, Ming-Guang Huang, Pu-Kun Liu,

https://aip.scitation.org/doi/abs/10.1063/1.5027639

Many recent biomedical applications use medium-power frequency-tunable terahertz (THz) sources, such as sensitivity-enhanced nuclear magnetic resonance, THz imaging, and biomedical treatment. As a promising candidate, a low-voltage gyrotron can generate watt-level, continuous THz-wave radiation. In particular, the frequency-pulling effect in a gyrotron, namely, the effect of the electron beam parameters on the oscillation frequency, can be used to tune the operating frequency. Most previous investigations used complicated and time-consuming gyrotron nonlinear theory to study the influence of many beam parameters on the interaction performance. While gyrotron linear theory investigation demonstrates the advantages of rapidly and clearly revealing the physical influence of individual key beam parameters on the overall system performance, this paper demonstrates systematically the use of gyrotron linear theory to study the frequency-pulling effect in a low-voltage gyrotron with either a Gaussian or a sinusoidal axial-field profile. Furthermore, simulations of a gyrotron operating in the first axial mode are carried out in the framework of nonlinear theory as a contrast. Close agreement is achieved between the two theories. Besides, some interesting results are obtained. In a low-current sinusoidal-profile cavity, the ranges of frequency variation for different axial modes are isolated from each other, and the frequency tuning bandwidth for each axial mode increases by increasing either the beam voltage or pitch factor. Lowering the voltage, the total tuning ranges are squeezed and become concentrated. However, the isolated frequency regions of each axial mode cannot be linked up unless the beam current is increased, meaning that higher current operation is the key to achieving a wider and continuous tuning frequency range. The results presented in this paper can provide a reference for designing a broadband low-voltage gyrotron.

Abstract- Time-Domain Multimode Analysis of a Terahertz Gyro-TWT Amplifier

 Chao-Hai Du, Shi Pan, Hui-Qi Bian, Pu-Kun Liu

http://ieeexplore.ieee.org/document/8306118/

 High-order mode employed as the operating mode of a terahertz (THz) gyrotron traveling-wave tube (gyro-TWT) amplifier shows strong dispersion, which deteriorates the device performance in amplifying picosecond THz pulses. Theoretical investigation of a THz gyro-TWT amplifier with a lossy cylindrical circuit is carried out to study the problems, such as lossy structure design, parameters selection, frequency-domain performance, and time-domain dynamics. The robust high growth rate low-kz amplification and the broadband high-kz amplification are two preferred operation conditions. The revealed characteristics are also useful for developing THz gyro-TWTs with other kinds of over-moded circuits

Abstract-Controllable Thermal-Frequency Tuning of a Terahertz Gyrotron

 Li Luo, Chao-Hai Du, Xiang-Bo Qi,  Zheng-Di Li, Shi Pan,  Ming-Guang Huang, Pu-Kun Liu

http://ieeexplore.ieee.org/document/8240959/

Gyrotron performance is sensitive to cavity structure parameters, and the cavity shape is temperature dependent due to thermal deformation induced by temperature rise from ohmic loss power on finite-conductivity cavity wall. Accordingly, this paper studies a frequency-tuning scheme for terahertz gyrotron by properly controlling the cavity thermal deformation. By combining gyrotron nonlinear theory and finite-element method software, controllable thermal-frequency-tuning capability of a continuous-wave 263-GHz gyrotron is systematically investigated, toward maintaining gyrotron operating under gyromonotron condition in frequency-tuning band, and achieving high efficiency in broadband frequency-tuning range. After studying cavity thermal distribution, structure deformation, and electron beam-wave interaction, an optimized cavity structure with transition sections on both ends is proposed. Simulation predicts that with the two-transition-section cavity, via additional thermal tuning, the continuous-frequency-tuning band is capable of reaching 1.75 GHz, which is 5 times of the initial bandwidth. Furthermore, using the thermal-frequency-tuning technology, impressive high efficiency above 17% is obtainable in the whole frequency-tuning range.

Abstract-Tunable Terahertz Deep Subwavelength Imaging Based on a Graphene Monolayer

• Heng-He Tang
• , Tie-Jun Huang
• , Jiang-Yu Liu
• , Yunhua Tan
•  & Pu-Kun Liu

https://www.nature.com/articles/srep46283

The resolution of conventional terahertz (THz) imaging techniques is limited to about half wavelength, which is not fine enough for applications of biomedical sensing and nondestructive testing. To improve the resolution, a new superlens, constructed by a monolayer graphene sheet combining with a grating voltage gate, are proposed in this paper to achieve deep super-resolution imaging in the THz frequency range. The main idea is based on the Fabry-Perot resonance of graphene edge plasmon waves. By shaping the voltage gate into a radial pattern, magnified images of subwavelength targets can be obtained. With this approach, the finest resolution can achieve up to λ/150. Besides, the superlens can be conveniently tuned to work in a large frequency band ranging from 4.3 THz to 9 THz. The proposal could find potential applications in THz near-field imaging systems.

Abstract-Experimental demonstration of ultra-wideband and high-efficiency terahertz spoof surface plasmon polaritons coupler

Heng-He Tang1, Tian-Jun Ma2 and Pu-Kun Liu1,a)


1 School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China
2 Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China
a) Author to whom correspondence should be addressed. Electronic mail: pkliu@pku.edu.cn.
Appl. Phys. Lett. 108, 191903 (2016); http://dx.doi.org/10.1063/1.4948928

Spoof surface plasmonpolaritons (SSPPs) are promising for subwavelength waveguiding in the terahertz (THz) frequency range. However, they cannot be efficiently excited from spatial propagating or guided waves due to the mismatched momenta. In this paper, a THz coupler is designed to smoothly bridge SSPPs and guided (or propagating) waves. By using a tapered parallel-plate waveguide, the incident energies are efficiently compressed and coupled into a subwavelength gap. Then, the momenta differences are mitigated with a graded grating. The numerical simulations show that the relative bandwidth of the coupler reaches up to 127%, and the maximum coupling efficiency is 99%. More importantly, experiment results in the 0.22 THz–0.33 THz frequency range are also presented to verify the good performance of the coupler. The work provides a technical support for terahertz waveguiding.

Abstract-Terahertz far-field superresolution imaging through spoof surface plasmons illumination

Heng-He Tang and Pu-Kun Liu

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-40-24-5822

The applications of terahertz (THz) imaging are always restricted by the low resolution. We introduce here a new way to realize far-field superresolution imaging at THz frequency. Assisted by a new spoof surface plasmons (SSP) probe illumination, the images of two subwavelength separated slits can be reconstructed by a single shot. Although only 0.06𝜆resolution is numerically demonstrated at 0.3 THz, the resolution potentially can be further enhanced by scaling down the size of the SSP probe. Deep subwavelength focusing is also achieved by the SSP probe. Our work may open a new avenue for SSP-based superresolution at longer wavelengths.

© 2015 Optical Society of America

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