We propose a terahertz (THz) vortex emitter that utilizes a high-resistance silicon resonator to generate vortex beams with various topological charges. Addressing the challenge of double circular polarization superposition resulting from the high refractive index contrast, we regulate the transverse spin state through a newly designed second-order grating partially etched on the waveguide’s top side. The reflected wave can be received directly by a linearly polarized antenna, simplifying the process. Benefiting from the tuning feature, a joint detection method involving positive and negative topological charges identifies and detects rotational Doppler effects amid robust micro-Doppler interference signals. This emitter can be used for the rotational velocity measurement of an on-axis spinning object, achieving an impressive maximum speed error rate of ∼2 % . This approach holds promise for the future development of THz vortex beam applications in radar target detection and countermeasure systems, given its low cost and potential for mass production.
Liquid targets are capable of offering rapid refreshment for per shot, which makes it viable of using high repetition rate lasers. Here, we push the liquid targets to a low temperature condition. By generating a stable flowing line of liquid nitrogen in ambient environment, we successfully observe broadband terahertz wave generation under the excitation of subpicosecond laser pulses. This demonstration provides more possibilities in choosing potential materials for studying the THz wave generation process, and in understanding the laser-induced ionization from a new perspective.
The influence of the inner disk radius r, the filling ratio α, numbers of sectors N, and the gap g on transmission response for corrugated metallic disk (CMD) with single C-shaped resonator(CSR) has been fully studied. The results indicate that varying parameters r can efficiently excite the higher order spoof localized surface plasmon modes in corrugated metallic disk. The relationship between the bright dipole and dark multipolar resonances presents the possibility of high Q dark resonances excitation. All results may be of great interest for diverse applications.
In this paper, several organic-inorganic composites were prepared for Terahertz (THz) devices fabrication. First, a two-layer structure was designed for femtosecond (fs) laser/THz radiation separation. The top layer was made by sintered 20-40 nm hollow quartz particles which can diffuse the incident fs laser thus decrease the power intensity. The bottom layer comprised of silicon 100 nm particles and cycle-olefine polymer (COP), by which the fs laser light can be greatly scattered and absorbed but THz radiation can propagate insusceptibly. With this two-layer structure a high efficient fs-laser/THz filter was fabricated successfully. Second, titania–polymer composites with a very high refractiveindex tunability and high transparency in the THz region were prepared. By controlling the blending ratio of the titania particle, a broad refractive-index tuning range from 1.5 to 3.1 was realized. Then, the composites were used to fabricate antireflective (AR) layers on a high-resistivity silicon (HR-Si) substrate. By utilizing the thermoplasticity of the titania– polymer composite, a graded-index structure was fabricated via a hot-embossing method. Because of the good refractive-index matching between the composite and the HR-Si substrate, a broadband AR layer was fabricated.
We fabricated a terahertz anti-reflective structure on a polystylene by using a hot-embossing method. Polystylene was spin-coated onto a silicon substrate and then transformed by using a metallic mould comprising a bunch of Chinese acupuncture needles. The transformation layer yielded gradient refractive index profiles on the substrate which can reduce the surface reflection effectively. The samples were evaluated by a terahertz time-domain spectroscope. Compared with a bare silicon substrate, we observed an increase of ~30% in the transmittance. We also observed broader bandwidth properties compared with a single-layer antireflective structure. The process imposes no substrate limiting; i.e., it has great potential to be applied onto various THz devices.
The influence of the waveguide spacing on transmission response for parallel-plate waveguide with single deep groove has been experimentally studied. The results indicate that the decrease of spacing can efficiently excite the higher order cavity modes in very deep groove. The relationship between the spacing and the channel number presents the possibility of a notch filter to mechanically select the channel number. Experimental data and simulations results verify this channel number tunibility, which may be of great interest for communication applications.
A novel invisible media based on transformation optics consisting of three regions to investigate invisibility was proposed here. What the media does is to compress the light to a small region so that the light can avoid interacting with objects, and then it would be recovered in the latter. Comparing with the traditional cylinder cloak, which require the light propagating around the concealed region, the media would help us to avoid singularity of material parameters. Finite element simulations for two-dimensional cases have been performed to prove these ideas.
FSS is a two-dimensional periodic array of resonating metallic-dielectric structures, When FSS device steps into Terahertz range from microwave range, it is studied as THz functional components (such as Terahertz filter, Terahertz biochemical sensor, etc.) to promote the functionality of the THz spectroscopy/imaging system. When the device requires a narrow band transmission window for frequency selecting or a high electric field concentration in certain area to improve its sensitivity for sensing, normally, a high quality (Q) resonant structure can give helps. Recently, high-Q resonance induced by trapped mode resonance i studied widely in FSS research areas. To induce trapped mode resonance, one can simply break the symmetric of the unit structure of FSS. In this paper, several asymmetric X-shaped resonators for FSS working in terahertz range have been studied numerically. To compare the behaviour of X-shape resonator under different conditions (with additional part: Heart lines, Shoulder lines, Wrap or Shoes squares), a common platform (θ=60, θis angle of X shape) which is suitable for most of cases was used to make the study more meaningful. As the field enhancement behaviour is related to the trapped mode introduced by the asymmetric structure, we propose such kind of device to be used as a high quality filter or as a sensing element for biochemical samples.
This paper puts forward a newer sensor which is symmetrical two rectangular resonant cavities based on parallel-plate
waveguide, using the lowest-order transverse-electric mode in terahertz region . From the simulation result we found when
we place the material in one of the resonant cavity, the original symmetric state has changed, not only a new resonance peak
appear ,but it also every resonant peak has obvious relative frequency shift with different material. So the structure can be used
as the sensor, which can detect the material.
A metal hole arrays terahertz filter based on surface plasmon polaritons and fabricated by aluminum slab of different holes scales have been experimentally investigated by using THz time-domain spectroscopy system from 0.1 to 2.7 THz. The experiment results indicated there is a transmission peak at 0.26 THz, approximately. The results in simulation by finite element method agree well with the experimental one for the big scale sample. The mismatch of experimental and simulated results for small scale sample can be attributed to boundary condition and insufficient periodical extension. Further, the theoretical analyses about extraordinary optical transmission and filter phenomena are also discussed.
Based on transformation optics and complementary media (folded geometry), levitative carpet cloak and overlapping effects (anti-mirror effects) can be realized. In common case, the levitative carpet cloak is limited by the shape of the hidden object, and the early folded geometry method can just make two objects with different shapes appear as only one. Here, based on transformation optics and finite element simulations, we propose an anisotropic but homogeneous shifting media that can shift an arbitrary-shaped object from original place to another place in terahertz (THz) region. As a result, an arbitrary shaped object levitated on a flat perfect electric conductor ground and covered with the shifting media cannot be detected, leading to the levitative and shape-independent carpet cloak. Furthermore, optical overlapping effects such as making two separated objects with the same shape appear as only one, can be also realized by using this kind of shifting media.
Theoretical and experimental works were carried out on a double channel mechanically tunable terahertz filter integrated with parallel plate waveguide cavities. The filter includes two rectangular grooves on upper and bottom plates of waveguide, respectively. The filter frequencies can be quasilinearly tuned by altering the overlap length between two rectangle grooves on metal plates. From the experiment, we found low (high) resonant frequency can be adjusted from 0.417(0.346) THz to 0.399(0.374) THz when the overlap length is altered from 0 to 500 μm, respectively. The Q values can reach 46 with the resonant frequency (0.41THz), overlap length (220 μm) and waveguide spacing (650 μm). Theoretical results show good agreement with experiment.
We fabricated micro-spikes on the surface of silicon by using femtosecond laser pulses. By changing the
fabrication condition, i.e., the power of laser, the number of laser pulses, the wavelength of laser, and the proportional
relation between laser power and pulse number under the same laser fluence, we found many interesting phenomena,
which proved there was a special relation between laser parameters and the surface morphology. All these results are
important for the optimal fabrication of surface-microstructured photovoltaic material with high absorptance and good
photoelectric properties, for the practical applications of solar cell, et al.
By using the free-space terahertz (THz) electro-optic (EO) sampling technique, the THz waveforms emitted from intrinsic bulk GaAs photoexcited by femtosecond laser pulses under strong bias electric fields up to 300 kV/cm were recorded. The initial acceleration signal in THz emission waveforms, which are corresponding to the acceleration of electrons in the bottom of Γ-valley, has been found to start decreasing with applied electric field above 50 kV/cm, in contrast to the simple picture of electron acceleration in the Γ-valley. This result suggests that the effective acceleration
mass of electrons in the Γ-valley significantly increases with increasing F, most likely due to strong band mixing under
very high fields.
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