Diwei Liu

Coherent and tunable terahertz radiation from graphene surface plasmon polarirons excited by cyclotron electron beam

Terahertz (THz) radiation can revolutionize modern science and technology. To this date, it remains big challenges to develop intense, coherent and tunable THz radiation sources that can cover the whole THz frequency region either by means of only electronics (both vacuum electronics and semiconductor electronics) or of only photonics (lasers, for example, quantum cascade laser). Here we present a mechanism which can overcome these difficulties in THz radiation generation. Due to the natural periodicity of 2π of both the circular cylindrical graphene structure and cyclotron electron beam (CEB), the surface plasmon polaritions (SPPs) dispersion can cross the light line of dielectric, making transformation of SPPs into radiation immediately possible. The dual natural periodicity also brings significant excellences to the excitation and the transformation. The fundamental and hybrid SPPs modes can be excited and transformed into radiation. The excited SPPs propagate along the cyclotron trajectory together with the beam and gain energy from the beam continuously. The radiation density is enhanced over 300 times, up to 105 W/cm2. The radiation frequency can be widely tuned by adjusting the beam energy or chemical potential. This mechanism opens a way for developing desired THz radiation sources to cover the whole THz frequency regime.

Study on the Terahertz Denisov Quasi-Optical Mode Convertor

Considering the backward waves, the coupled-mode theory for the dimpled-wall Denisov launcher has been developed. The 140-GHz TE28,8 mode was recalculated with this method and the results are consistent with those reported by Thumms team. With the coupled-mode theory, the Denisov-type quasi-optical mode convertor for 220-GHz TE14,4 mode gyrotron has been investigated, of which the maximal relative power of the backward waves at the launcher cut is just about 0.7%, the power-conversion efficiency is about 95%, and the scalar Gaussian mode contents are 98% at the output window.

Characteristics Analysis of a Coaxial Cavity With a Misaligned Inner Rod

The effects of a small misalignment of the inner rod in a coaxial cavity on the cutoff frequencies and the ohmic quality factors of TE modes are investigated. For axially symmetric modes, the cutoff frequencies decrease and the ohmic quality factors increase when the misalignment of the inner rod increases. The effects are more noticeable for axially symmetric modes with a larger radial index and coaxial cavities with a larger ratio of the inner and outer radii. For axially asymmetric modes, there exists a critical value near the caustic radius. When the inner rod radius is smaller than the critical value, the cutoff frequencies are slightly affected by the misalignment, and the ohmic quality factors decrease with the increase of the misalignment. When the inner rod radius is greater than the critical value, the cutoff frequencies and the ohmic quality factors increase with the increase of the misalignment, and the effects are more noticeable for a larger inner rod radius.

Theoretical and Experimental Investigations on the Quasi-Optical Mode Converter for a Pulsed Terahertz Gyrotron

Theoretical and experimental investigations on the quasi-optical (QO) mode converter for a 0.42 THz, TE26 mode pulsed gyrotron have been carried out. The vector diffraction theory and the multiple iterative algorithms are used to design and optimize the QO mode converter, and the theoretical value for the conversion efficiency is ~88%. The actual conversion efficiency is ~86% by measuring the input and output power of the QO mode converter indirectly with a calorimeter. The field distribution on the output window of the QO mode converter is measured with an infrared video camera, and a well-focused Gaussian-like beam has been observed on the output window. The experimental results agree well with the theoretical predictions.

A Method to Separate Radiations From a Dual-Frequency Operation Gyrotron

A method was proposed to efficiently separate radiations from a dual-frequency operation gyrotron. The operation modes TE02 and TE04 are converted into linearly polarized Gaussian beams, and the theoretical predictions of the power conversion efficiency are 85% for TE02 mode and 90% for TE04 mode, respectively. The linearly polarized beams with different frequencies are oblique incident toward a spatial filter based on the frequency-selective surface, the beam at 0.11 THz is totally reflected, the beam at 0.22 THz is transmitted, and the transmittance is ~95%. The transmission characteristics of the spatial filter change a little when the incident angle ranges from 0° to 30°. The preliminary experiment for the transmission coefficient of the spatial filter is carried out when the incident angle is 0°, the trend of the transmission coefficient from the theoretical predictions agrees well with the experimental results, but there is a little difference at 0.11 and 0.22 THz due to the fabrication error.

Quasi-Optical Mode Converter for a 0.42 THz TE 17,4 Mode Pulsed Gyrotron Oscillator

A quasi-optical mode converter (QOMC) for a 0.42 THz, TE17,4 mode pulsed gyrotron oscillator is designed, fabricated and tested. Based on the coupled-wave theory, geometric optics theory and vector diffraction theory, a computer code is developed to design and optimize the QOMC which converts the high order TE17,4 mode into a fundamental Gaussian distribution. The numerical results show that the scalar fundamental Gaussian mode content of the wave beam at the output window is about 98.1%. The experiment result shows that a well-focused Gaussian-like beam with 95.2% scalar Gaussian mode content has been obtained by using an infrared video camera. The measured results agree well with the theoretical predictions.

Quasi-Optical Mode Converter for a 0.42-THz TE 26 Mode Pulsed Gyrotron Oscillator

A quasi-optical (QO) mode converter for a 0.42-THz, TE26 mode, pulsed gyrotron is designed, fabricated, and tested. Based on the geometric optics principle, vector diffraction theory, and the iterative Stratton-Chu method, a computer code is developed to design and optimize the QO mode converter. The scalar Gaussian mode content is about 93.7%, which agrees well with that obtained by the electromagnetic simulation software. The preliminary experiment of the 0.42-THz, TE26 mode QO mode converter shows that a well-focused Gaussian-like beam with 90.4% scalar Gaussian mode content has been observed on the output window by using an infrared video camera. The experimental results agree well with the theoretical prediction.

Effects of the backward waves on the denisov-type quasi-optical mode converter

Taking the backward waves into account, the coupled-mode theory for the dimpled-wall denisov launcher has been developed. The 140GHz, TE28,8 mode was recalculated with this method. The maximal relative power of the backward waves at the cut of the denisov launcher is just about 0.3%.

THz gyroklystron with fourth harmonic

three-cavity 225GHz fourth harmonic gyroklystron with successive frequency-doubling in each cavity is presented and analyzed with linear and nonlinear theories. The input coupler for the TE11 mode was designed and simulated with CST code. Starting currents and nonlinear wave-beam interactions were calculated with linear and self-consistent nonlinear theories. When the 225GHz fourth harmonic gyroklystron is driven by a 40kV, 3A electron beam, and 1 Watts input power, the optimal output efficiency is about 1.8%.

Calculation and design of an experimental verification of coaxial gyrotron with two beams

Theoretical studies and computer simulation show that the coaxial gyrotron with two electron beams (CGTB) can increase the output power of gyrotron and even can operate at dual frequency. In this paper, calculation and design of a coaxial gyrotron with two beams is presented for the experimental verification.