In order to demonstrate the modulation of terahertz wave propagation in atmospheric pressure microplasmas, in this work, the band structure and the transmission characteristics of a one-dimensional collisional microplasma photonic crystal are investigated, using the transfer matrix method For a lattice constant of 150 μm and a plasma width of 100 μm, three stopbands of microplasma photonic crystal are observed, in a frequency range of 01–5 THz Firstly, an increase in gas pressure leads to a decrease in the central frequency of the stopband When the gas pressure increases from 505 kPa to 202 kPa, the transmission coefficient of the THz wave first increases and then decreases at high frequency, where the wave frequency is much greater than both the plasma frequency and the collision frequency Secondly, it is interesting to find that the central frequency and the bandwidth of the first THz stopband remain almost unchanged for electron densities of less than 1015 cm–3, increasing significantly when the electron density increases up to 1016 cm–3 A central frequency shift of 110 GHz, and a bandgap broadening of 200 GHz in the first stopband are observed In addition, an atmospheric pressure microplasma with the electron density of 1 × 1015–6 × 1015 cm–3 is recommended for the modulation of THz wave propagation by plasma photonic crystals

Numerical study on the modulation of THz wave propagation by collisional microplasma photonic crystal

The invention provides a terahertz wave tunable mode converter, which comprises a hollow cylindrical metal inner-wall waveguide formed by rectangular periodically fluctuating structures. According to the invention, resonance interaction among different transverse standing wave modes in the waveguide is utilized to realize conversion of terahertz waves from a fundamental mode to a single high-order mode. Furthermore, tuning for output frequency of the terahertz wave mode converter can be realized through changing the thickness of a metal coating in the rectangular periodically fluctuating pipe wall, and the tunable mode converter can realize that the output center frequency is linearly tunable in broadband. The terahertz wave tunable mode converter is reasonable in design, has the characteristics of simple structure, wide tunable range, low loss, high conversion efficiency, convenient integration and the like, can acquire wide applications in fields which have special requirements for terahertz wave mode field distribution and the operating frequency, and provides a new choice for designing and manufacturing functional devices in a terahertz wave application system.

Terahertz wave tunable mode converter

The invention discloses a terahertz narrow bandpass filter for narrowband frequency selection and frequency tuning and a method thereof. A filter plate is employed, and electromagnetic waves are subjected to multiple reflection and transmission in the filter plate. When the thickness of the filter plate matches with specific frequency of the electromagnetic waves, constructive interference occurs to the transmitted electromagnetic waves, and the transmitted electromagnetic waves are the strongest to realize narrowband filtering. An air gap between a filtering tuning cylinder and the filter plate changes the resonant frequency of the transmitted electromagnetic waves, thereby realizing offsetting and adjusting the central frequency of a filter, increasing the flexibility and application scope of the filter, and satisfying the requirements of narrowband filtering in several GHz bandwidth within a terahertz frequency range. The structure is simple, the preparation process flow is short, and the preparation cost is reduced, and the performance is improved.

Terahertz narrow bandpass filter for narrowband frequency selection and frequency tuning and method thereof

Terahertz (THz) devices, especially for narrowband filters, have been widely investigated over the past decades, but mostly based on semiconductor materials or metamaterials. Here, we experimentally and numerically demonstrate a waveguide-type narrowband THz filter, which is realized by two parallel plates with corrugations. The corrugated plates are fabricated by sputtering a layer of gold after laser ablation of polymer sheets. The laser periodically etches the groove and produces a wide groove in the middle, which destroys the periodicity of the structure and forms a defect. The periodicity provides a forbidden band, in which THz waves cannot propagate, and the defect leads to an additional transmission in the forbidden band. This additional transmission occurs in the middle of the gap and has a very narrow linewidth, which is a good candidate for THz filtering. The achieved linewidth and the extinction ratio are about 1.5 GHz and 29 dB, respectively, limited by our experimental system, which are further optimized in numerical simulations. The proposed fabrication method is very efficient in THz waveguide devices, which can be easily integrated in applicable THz systems. 

Narrow Linewidth Terahertz Filter Based on Gold-Coated Polymer Plates With Corrugations

The invention belongs to the technical field of terahertz wave science, and in particular relates to a terahertz wave filter based on interaction of forbidden bands. The terahertz wave filter based on the interaction of the forbidden bands comprises two cylindrical metal waveguides, namely a waveguide A and a waveguide B, wherein the waveguide A and the waveguide B both are based on Bragg resonance, and the positions of zero points of second-order modes of the waveguide A and the waveguide B are different. The terahertz wave filter is based on a novel filtering principle, and no complicated optical system is involved; there is no direction requirement for THz wave incidence, and the filter can be used bidirectionally; the filter is high in out-of-band rejection; the center frequency of the filter can be changed through parameter design; and the filter is simple in structure and is easy to process.

Terahertz wave filter based on interaction of forbidden bands

The invention relates to the terahertz functional device, especially to a composite-waveguide-structure-based tunable terahertz narrow-band filter comprising two periodic-structure cylindrical hollow metal waveguides a and b; and the two waveguides different dimensions are connected and combined at an axial direction. The composite waveguide tube wall is made of a low-loss metal material; and structural parameters of the periodic-structure waveguides are obtained by dispersion curves. The filter has advantages of small size, simple structure, easy production, and simple and convenient operation. The frequency band is narrow and the bandwidth of the filtered terahertz electromagnetic wave can reach 0.24MHz and the transmittance is larger than 99%. The filter having no direction requirement can be used bidirectionally. The central wavelength can be controlled by adjusting dimensions of the two periodic-structure waveguides and the transmittance can be controlled by changing the number of periods of the two waveguides. The filter has a complete metal closing structure, so that the electromagnetic radiation and dielectric loss can be avoided and no packaging problem needs to be considered.

Composite-waveguide-structure-based tunable terahertz narrow-band filter

We propose a thermally tunable terahertz narrow band filter by combining two different periodic waveguides. When the terahertz tube wall is corrugated periodically, the transparent spectrum presents a very complex structure as various pass and stopbands emerging. The terahertz wave cannot propagate through the waveguide when its frequency falls into the stopbands. When we connected two tubes with different types of stopbands, it is very intriguing that we have found an unexpected ultra-narrow-band transparency in the frequency gap. Our findings provide a more feasible and effective terahertz waveguide filter beyond these existing ones. By using the finite-element method, we have obtained the filtering characteristics of the proposed structures. The filter bandwidth could be narrowed from 2.3 GHz to 0.29 MHz by increasing the number of waveguide segments and the highest $Q$ -factor achieved is about $3.5 \times 10^{6}$ . Moreover, the tunability on temperature is obtained in a waveguide device of Au based on the combination of opaque structures.

Thermally Tunable Narrow Band Filter Achieved by Connecting Two Opaque Terahertz Waveguides

The invention provides a composite waveguiding structure broadband unidirectional silencer. A cuboid wave guide is formed by upper and lower walls, and front and back walls; the upper and lower walls are of a rectangular concave-convex structure with a fixed cycle; the front and back walls are flat; the size of each concave-convex structure at the left and right sides of the upper and lower walls is given by a dispersion curve; the waveguides with different functions at the left and right sides of the waveguide are named as the wave guide 1 and the wave guide 2; the concave-convex structure of the upper and lower walls can be produced on the selected flat material by a digital controlled lathe according to the size; the concave-convex structure of the waveguide 2 is to be carved when the cycle of the waveguide 1 is carved; the combination of the two waveguides can realize unidirectional conduction of the sound wave; and when incidence of the sound wave occurs, the waveguide 2 generates a frequency domain band gap for Bragg resonance of fundamental modes, the interaction among horizontal standing wave modes occurs in the waveguide 1, a single higher-order mode is generated and will not be influenced by the band gap generated by the waveguide 2, and the higher-order mode can be penetrated. The composite waveguiding structure broadband unidirectional silencer of the invention has the advantages of being high in noise abatement rate, being simple in structure, being high in adaptability, being lower in cost, and being easy to realize.

Composite waveguiding structure broadband unidirectional silencer

Transparency associated with a single second mode in a frequency band-gap, caused by orthogonality-breaking-induced multimode interactions, is investigated in a TM-polarized terahertz waveguide with an arbitrary wall profile. It is found that a pure single mode can penetrate through the Bragg gap originating from the lower-mode resonances. We propose a method of achieving bandwidth-agile transparency in a periodic cylindrical waveguide by means of a suitable wall profile. Perturbation theory with respect to the wall corrugation amplitude reveals the underlying physics of the single-mode transparency, in that the major Fourier component of the wall profile affects the width of the stopband proportionally, whereas the transparent passband within the stopband varies in inverse proportion. Numerical simulations of four different types of wall profile confirm that the width of the transparent band can be manipulated as proposed. When other Fourier components are present, simulations demonstrate only a small variation of the frequency band structure around 1 THz, indicating the validity of the proposed method. More extensive investigations of single-mode transparency will provide significant benefits in future mode-control engineering of terahertz devices.

Bandwidth-Agile Transparency of the Second Mode in a Periodically Corrugated Waveguide With an Arbitrary Wall Profile

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