Terahertz Wave Source Research Articles

Resolution enhancement in terahertz imaging with multi-wavelength information

Thanks to the unique characteristics of terahertz waves, terahertz imaging has become one of the promising imaging technologies. However, due to the weak signal source and strong diffraction of terahertz waves, terahertz imaging has a significant amount of noise, which makes it is challenge to achieve satisfactory clarity in images. In this work, we propose an algorithm that uses multi-wavelength information to improve the resolution of terahertz imaging. The resolvability of the images has been improved by at least 1.4 times, and the noise has been effectively filtered out. This algorithm enhances the image resolution without requiring any hardware upgrades, benefiting the terahertz imaging system with multi-wavelength imaging capabilities.

Analysis of multimode radiation from 0.4 THz backward wave oscillator

Abstract A terahertz backward wave oscillator (BWO) is a high-power terahertz wave source based on the Cherenkov interaction between an electron beam and a slow backward wave. Excitation of slow wave modes with different mode numbers degrades the performance of BWO due to mode competition. Analyzing the mode spectrum is a difficult challenge to confirm single mode operation. In this study, the excitation of the slow waves in a 0.4 THz cylindrical corrugated waveguide is examined. In the corrugated waveguide, azimuthal modes become slow backward waves with evanescent fields. We found the superposition of the modes with different azimuthal indices has an asymmetric field intensity and the excitation of the superposed modes produces the terahertz radiation with the asymmetric pattern. We conduct a BWO experiment using a 0.4 THz corrugated waveguide. The experimental radiation pattern is obtained and corresponds well to the calculated asymmetric pattern. The symmetry of the radiation can be regarded as an indicator of the mode control.

Proposal and numerical verification of an ultrasmall terahertz source using integrated photonic crystal waveguides for highly efficient differential frequency generation

We propose and numerically investigate integrated photonic crystal waveguides (PhC-WGs) formed in a semiconductor slab to realize an ultrasmall and highly efficient terahertz (THz) wave source. The structure consists of a straight PhC-WG with low-group-velocity and low-dispersion (LVLD) for efficient difference frequency generation (DFG) connected to two PhC-WGs to introduce two fundamental lights into the LVLD PhC-WG. The fundamental light propagating through each PhC-WG designed to enhance their electric fields by the slow-light effect is efficiently coupled to the LVLD PhC-WG owing to the reduced refractive index differences at the boundaries of the heterostructures. The DFG from the two fundamental lights was numerically simulated, and a temporal intensity oscillation corresponding to the difference in frequency was clearly observed. By comparing the DFG intensities of the integrated structures with an LVLD PhC-WG and a strip WG, the estimated DFG intensity from the LVLD PhC-WG was more than 100 times higher than that from the strip WG. These results indicate the effectiveness of the proposed heterostructure in the application of a highly efficient THz source with an ultrasmall footprint compared with conventional materials.

Dichlorinated Organic‐Salt Terahertz Sources for THz Spectroscopy

AbstractAlthough in terahertz (THz) source materials molecular anions significantly influence the performance of THz generation, only limited classes of molecular counter anions have been reported. Here, utilizing dichlorinated molecular anions in THz generators is reported for the first time, to the best of our knowledge. In these new crystals, two dichlorinated molecular anions with different molecular symmetries, asymmetric 3,4‐dichlorobenzenesulfonate (34DCS) and symmetric 3,5‐dichlorobenzenesulfonate (35DCS), are incorporated with a 2‐(4‐hydroxystyryl)‐1‐methylquinolinium (OHQ) cation possessing top‐level molecular optical nonlinearity. OHQ‐34DCS exhibits a strong nonlinear optical response, in contrast to OHQ‐35DCS. In OHQ‐34DCS crystals, the dichlorinated groups form strong halogen bonds (XBs) and hydrogen bonds (HBs), which are beneficial for suppressing molecular (phonon) vibrations. The optical‐to‐THz conversion efficiency of the OHQ‐34DCS crystals is extremely high, comparable to that of the benchmark organic THz generators. Moreover, the THz emission spectra from the OHQ‐34DCS crystals, compared to those of previously reported benchmark analogous crystals, are stronger modulated toward a flatter shape, but possess substantially reduced spectral dimples. Therefore, the introduction of dichlorinated molecular anions is an efficient approach for the design of highly efficient electro‐optic salt crystals as efficient broadband THz wave sources.

Design and simulation of widely tunable picosecond synchronously pumped terahertz parametric oscillator based on silicon waveguide platform

AbstractA picosecond synchronously pumped terahertz optical parametric oscillator based on silicon membrane waveguide (Si‐based WTPO) is proposed and numerically simulated. Through designing the structure parameters of the silicon membrane waveguide, a broadband terahertz frequency range is obtained by satisfying the phase matching condition. The parametric oscillation process and output characteristics of Si‐based WTPO are numerically analyzed by solving the four‐wave mixing coupled‐wave equations, and the result has indicated that the parametric oscillation within the resonant cavity can apparently enhance the output terahertz wave power. Meanwhile, high bandwidth and efficiency output terahertz wave can be obtained, and too high pump power will lead to a reaction of reverse conversion. Finally, the threshold pump power is calculated. The Si‐based WTPO system we proposed is expected to make some contributions to the study of compact and efficient terahertz wave sources.

Dispersion relation of spoof surface plasmon resonant mode on concentric annular metal corrugation

In this study, we demonstrate near-field measurement of a spoof surface plasmon (SSP) on a corrugated metal disk with concentric annular corrugation. The SSP is excited at the center of the disk and is probed by a needle antenna on the corrugated surface. In the measurement, SSP resonant modes are observed around the upper cut-off frequency. The dispersion relation and group velocity are extracted from the measured parameters and correspond well to the calculated dispersion relation. Our results help in designing SSP resonators to improve the device performance of an intense terahertz wave source.

Incident-Angle-Dependent Extraordinary Transmission of the Terahertz Bull’s-Eye Structure

The bull's-eye structure in the terahertz (THz) frequency region has ample applications owing to its ability to focus free-propagating waves into subwavelength apertures, resulting in enhanced transmission, that is, extraordinary transmission. However, its coupling properties have been primarily discussed in terms of the normal plane-wave incidence to the structure. In this study, we investigate the multiple resonances in extraordinary transmission with normal and oblique incident waves. The experiment using a widely tunable and high-power THz wave source revealed two types of resonances. The main resonance split depends on the incident angle, and the other corresponds to the side lobe of the main resonances. The results are explained by a simple analytical model using a finite number of scattering media. The analysis is supported by the full-wave simulation using the finite-element method, which agrees with the experimental results. The coupling mechanisms will be applicable to design devices, such as THz biosensing devices or THz antennas for rapid communication systems.

Distance Measurement of a Frequency-Shifted Sub-Terahertz Wave Source

In this paper, we report the development of a frequency-shifted (FS) terahertz (THz) wave source for the non-destructive inspection of buildings. Currently, terahertz-time domain spectroscopy (THz-TDS) is the mainstream method for non-destructive inspection using THz waves. However, THz-TDS is limited by its measurement range and difficulties encountered when there is a strong frequency dependence in the absorption characteristics and refractive index of the measurement target. To address these issues, we developed a novel non-destructive approach for inspection applications using frequency-shifted THz waves. Our system uses a frequency-shifted feedback (FSF) laser as the pump light source to generate FS-THz waves; this allowed us to obtain precise distance measurements of objects over a broad range of distances. We tested a prototype FS-THz system and confirmed successful measurement of spatial distances inside a building material.

Experimental study on performance change in F-band surface-wave oscillator due to slow-wave structure length

A surface-wave oscillator (SWO) is a high-power terahertz-wave source based on a slow-wave structure (SWS). A cylindrical corrugated waveguide is used as the SWS. The length of SWS is an important factor in the oscillation. Experiments of the SWO with various length structures are conducted. An instability responsible for the SWO changes depending on the SWS length. For the 200-period SWS, the radiation is due to the absolute instability and depends on the beam current like the temporal growth rate. By contrast, for the 80-period SWS, the radiation is due to the global instability having the current dependence like the spatial growth rate. Compared to the 80-period SWS, the 200-period SWS can improve the radiation power more than seven times in the backward wave oscillation (BWO) region. The maximum BWO power reaches up to about 10 kW and the maximum electric efficiency of 1.5% is obtained.

Terahertz waves generation using the isomorphs of PPKTP crystal: a theoretical investigation

Highly efficient terahertz (THz) wave sources based on the difference frequency generation (DFG) process in nonlinear optical crystals plays an important role for the applications of THz waves. In order to find more novel nonlinear crystals, here, we theoretically investigate the generation of THz waves using the isomorphs of periodically poled KTiOPO4 (PPKTP), including periodically poled RTP, KTA, RTA and CTA. By solving the cascaded difference frequency coupled wave equations, it is found that the intensities of the THz wave generated from the cascaded difference frequency processes are improved by 5.27, 2.87, 2.82, 3.03, and 2.76 times from the non-cascaded cases for KTP, RTP, KTA, RTA and CTA, respectively. The effects of the crystal absorption, the phase mismatch, and the pump intensity are also analyzed in detail. This study might help to provide a stronger THz radiation source based on nonlinear crystals.

Design and Construction of an Intense Terahertz-Wave Source Based on Coherent Cherenkov Radiation Matched to Circle Plane Wave

Design and Construction of an Intense Terahertz-Wave Source Based on Coherent Cherenkov Radiation Matched to Circle Plane Wave

Increase in terahertz-wave generation by difference frequency mixing by the overlap of exciton states in different GaAs/AlAs quantum wells and spectroscopic measurements.

Intense terahertz-wave emission in the higher frequency region can result in various applications such as terahertz spectroscopy and ultrafast data communication. In this study, an increase in terahertz waves by the overlap of exciton states in different quantum wells and spectroscopic demonstration are reported. The excitation energy dependence of signal intensity shows the effect of the overlap. The signals measured under the condition of square dependence of intensity on the excitation power indicate interference with the periods corresponding to the laser energy difference. Furthermore, the absorption coefficient of the transparent sheet is obtained at specific frequency. These results indicate that the generation of intense terahertz waves at various frequencies using excitons is possible and that difference frequency mixing is a useful terahertz-wave source.

Linearly Polarized High-Purity Gaussian Beam Shaping and Coupling for 330 GHz/500 MHz DNP-NMR Application

Terahertz waves generated by vacuum electron devices have been successfully applied in dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR) technology to significantly enhance the sensitivity of high-field NMR. To reduce the magnetic field interference, the high-power terahertz wave source and the NMR spectrometer need to be separated by a few meters apart. Corrugated horns and directional couplers are key components for shaping high linearly polarized terahertz Gaussian beam and accurately coupling electromagnetic power in the transmission system. In this paper, a corrugated TE11-HE11 mode converter and a three-port directional coupler realized by its inner cylindrical wire array are proposed for a 330 GHz/500 MHz DNP-NMR system. The output mode of the mode converter presents a characteristic of highly linear polarization, which is 98.8% at 330 GHz for subsequent low loss transmission. The designed three-port directional coupler can produce approximately −33 dB electromagnetic wave power on port 3 in the frequency range between 300–360 GHz stably, which can be used to measure the electromagnetic wave power of the transmission line in real-time. The designed mode converter and direction coupler can be installed and replaced easily in the corrugated waveguide transmission system.

Measuring material parameters using a tunable monochromatic terahertz wave source.

We measured complex refractive indices of materials that transmitted well in the terahertz (THz) range using a tunable, monochromatic pulsed THz wave source and a high-sensitivity semiconductor detector. This approach is based on simple monochromatic transmission power measurements. Contrast enhancement by refractive index cladding was employed to measure the complex refractive indices. The experimental results were in good agreement with previously reported values. These results suggested that our approach could be used to quickly analyze material properties, such as surface roughness, and to determine the complex refractive indices of liquids or gases. In addition, as an application of the findings obtained here, we applied an anti-reflection polymer film to the silicon prism that served as an output coupler of the THz wave source. The film reduced surface reflection by 25%.

Flowing cryogenic liquid target for terahertz wave generation

Terahertz wave emission from liquids excited by intense laser pulses not only reflects the details in laser–matter interaction but also offers bright terahertz wave sources. Flowing liquid targets possess the advantage of providing a fresh area for each laser pulse. To demonstrate a debris-free target under laser excitation, we investigate the use of liquid nitrogen as a target. By creating a flowing liquid nitrogen line in an ambient environment, we successfully observe broadband terahertz wave emission under short pulse excitation. Our cryogenic line is able to sustain the excitation of a high-repetition-rate (1 kHz) laser. The terahertz peak field emitted from liquid nitrogen is comparable to that from liquid water, yet a broader bandwidth is observed. This demonstration prompts opportunities in choosing potential materials for studying terahertz wave generation processes and in understanding laser-induced ionization of different liquids.

Amplitude-modulated continuous-wave radar in the terahertz range using lock-in phase measurement

We present the principle and the experimental verification of a distance measurement method based on the propagation of terahertz waves. The method relies on modulating the amplitude of a resonant-tunneling-diode (RTD) oscillator used as terahertz-wave source and on measuring the phase of the detected wave by applying a quadrature mixing technique. The distance measurement is found to have a residual error as small as 0.063 mm (standard deviation), which is a record for an RTD-based terahertz-wave radar system. This is almost five times better than our previous record of 0.29 mm, when an oscilloscope was used for phase measurements; additionally, the quadrature mixing brings about numerous practical benefits, such as greatly reduced cost, size, weight, complexity, and power consumption.

Design and Fabrication of 200-GHz Oversized Surface-Wave Oscillator

A surface-wave oscillator (SWO) is one of the promising terahertz-wave sources based on excitation of a surface wave using an electron beam. The surface wave is formed by a metal periodic structure, and the frequency is determined by the size parameter of the structure and the parameters of the electron beam. In this article, we design and fabricate a 200-GHz cylindrical corrugated waveguide which has the periodic structure on the inner wall. The field intensity of the surface wave rapidly decreases to zero from the structured wall. Due to a manufacturing error of the corrugated waveguide, the frequency of the surface wave in the fabricated structure decreases to 189 GHz at $\pi $ -point in comparison with the frequency of 204 GHz in the designed one. The effect of the error is evaluated by calculating the intensity distribution of the electric field of the surface wave and its growth rate due to the beam interaction. We also conduct the SWO experiment using the fabricated waveguide. Intense radiation with a total power is roughly estimated to 0.2 kW with a 45-keV and 60-A electron beam.

Improved terahertz Smith–Purcell radiation using side-opening cylindrical gratings

The Smith–Purcell radiation (SPR) is a promising way for generating terahertz emission, which has tremendous application prospects. Yet, the conventional SPR schemes generally utilize the Gaussian electron beam/bunch with round cross section (also called a pencil beam) to interact with a planar grating. The shape mismatching between the electron beam and the grating essentially restricts the interaction efficiency together with the radiation intensity. To counteract this effect, we propose to use side-opening cylindrical gratings (SOCGs) to replace planar gratings. Detailed theoretical analyses using the surface-current model and mode-matching method are performed for the first time. A train of premodulated electron bunches is proposed to drive the SOCG, on which the electron bunches are further micro-bunched by interacting with the surface waves. The multi-bunched electron beam then generates multi-color coherent SPR at the superimposed harmonics of the train of bunches. Compared with that from a planar grating, the radiation intensity from the SOCG can be significantly enhanced, indicating that it is a promising way of developing high-power terahertz wave sources.

Research of coherent edge radiation generated by electron beams oscillating free-electron lasers

We have studied far-infrared coherent radiation with S-band linacs at Laboratory for Electron Beam Re-search and Application (LEBRA) at Nihon University and at Kyoto University Free Electron Laser. We have already developed a couple of terahertz-wave sources based on coherent synchrotron radiation and coherent transition radiation, which have been applied to spectroscopic research. Moreover, we developed coherent edge radiation (CER) at the downstream bending magnets in the FEL sections. Because the edge radiation has an annular shape distribution characterized by the asymmetric first-order Laguerre-Gaussian mode, the CER can be extracted from an optical cavity of the FEL system without a diffraction loss of the FEL beam. The root-mean-squared bunch length of the electron beam was evaluated by measuring the CER spectra, which was about the same level as the FEL micropulse width. Although the infrared FELs at LEBRA had a long slippage length, the FEL energy became higher as the bunch length was shorter. The CER intensity can be a guidepost enhancing the FEL power because of the existence of their correlation.

Some Advances in Theory and Experiment of High-Frequency Vacuum Electron Devices in China

The advances of high-frequency vacuum electron devices (VEDs) in China are reviewed in this paper, including traveling-wave tubes (TWTs) based on novel slow-wave structures (SWSs), high harmonic TWT amplifier, gyrotrons, Smith-Purcell (SP) radiation sources, surface plasmon polaritons (SPPs) sources, and terahertz (THz) radiation sources in ion-focused beam-plasma system. As one of the most widely used VEDs, TWTs based on folded type SWSs, planar SWSs, and grating type SWSs are studied theoretically and experimentally from Ka-band to R-band. Using folded type SWSs, two TWT amplifiers are developed. As high-power millimeter wave and THz wave sources, gyrotron TWTs, gyrotron oscillators, and gyroklystrons are presented. Moreover, this paper shows SP radiation sources, SPPs sources, and THz radiation sources in ion-focused beam-plasma system open good prospects for developing high-frequency VEDs with new mechanisms, which undoubtedly brings about innovative changes for the development of VEDs.