The wave manipulation in the waveguide is of great importance for integrated terahertz (THz) signal processing and communication. Here, a design of metallic waveguide with meta-holes (MWMH) is proposed to realize focusing in the waveguide with unlimited focal length. When the MWMH is used to realize focusing with a focal-length of 45 mm, the gain at the focus can reach 26.8 dB and the focusing efficiency is 52.77% of the incident power. The full width at half maximum (FWHM) is 3 mm, and the depth of focus (DOF) is 22 mm. Furthermore, the MWMH for focusing with a focal-length of 10 mm and 0 mm are demonstrated, as illustrations of its flexible wave manipulation functions. The proposed MWMH with flexible wavefront manipulation functions opens up new possibilities for the development of THz integrated photonics devices, and has great potential in non-destructive testing, spectral imaging, and 6G wireless communication.
In this paper, the broadband absorption and narrowband absorption conversion can be achieved by introducing vanadium dioxide from the insulator to metal phase transition. The absorption tunable terahertz metamaterial device is composed of vanadium dioxide metasurface, silica spacer, vanadium dioxide film, patterned metal layer, silica spacer and metal substrate form the top to the bottom. The simulations demonstrate that the device can efficiently absorb more than 90% of the energy in the frequency range of 1.20-2.29 THz when the vanadium dioxide is in its fully metal state. The broadband performance is sustained over a wide range of incident angles, and is also insensitive to polarization due to the symmetry. However, as the vanadium dioxide is in its insulate state, the device behaves as a multi-narrowband absorber and absorbs over 95% of the incident power at 0.506, 1.022, 1.703, 1.967 THz, respectively. The narrowband performance is maintained over a wide range of incident angles. Furthermore, the influence of the different geometrical parameters on the absorptance performance is discussed. The proposed tunable absorber can be used in various applications, such as modulating, sensing and imaging technology.
Tin disulfide (SnS2) quantum dots (QDs) have been used in the fields of sensors. However, the reported SnS2 QDs were fabricated by the wet chemical method which was complicated. In this paper, we synthesized SnS2 QDs by a facile ultrasonic probe sonication process. The average size of SnS2 QDs was 3 nm which was observed in the transmission electron microscope (TEM) images. Two vibrational modes of SnS2 QDs were observed at 203 cm-1 (Eg) and 310 cm-1 (A1g) in the Raman spectrum. There was an absorption peak at 320 nm in the UV-Vis spectrum. Excitation wavelength dependent photoluminescence (PL) was measured. The maximum PL intensity of SnS2 QDs was observed at 450 nm under the excitation wavelength of 370 nm. This indicates that the SnS2 QDs have potential applications in optical devices.
Both the strong mode confinement and the low propagation loss are longed for designing highly integrated terahertz (THz) devices, but they are difficult to be achieved at the same time. Here, a graphene-coated nanowire with a dropshaped cross section (GNDCS) is proposed with the long-range propagation and strong confinement. We found this waveguide can support two kinds of graphene surface plasmon polaritons (GSPPs), outside-dominant and insidedominant modes, with distinctly different energy distributions. Interestingly, both modes can achieve low-loss propagation with strong mode confinement. In particular, the outside-dominant mode can attain an extremely long propagation length (1mm) and the inside-dominant mode has a very high energy utilization rate. These excellent characteristics make the waveguide very useful in the nanophotonics, bio-photonics and highly integrated THz circuits.
For a good THz waveguide, both low propagating loss and small mode width are usually very important. However, the high ohmic loss of metals and the high absorption loss of dielectric materials result in that it still remains a challenge to obtain the two capabilities at the same time. In this paper, planar dielectric-gap-metal (DGM) waveguides are presented to guide THz wave. According to the dispersion equations of the waveguides, we calculate their mode characteristics by numerical calculation, and we find that THz wave can propagate in the waveguides with low loss and simultaneously subwavelength mode width. When compared with the parallel-plate waveguide, the mode losses of the DGM waveguide can be 1-3 orders of magnitude lower, but the mode widths do not increase. The combination of low propagating loss and subwavelength mode width makes the DGM waveguides particularly useful for many THz applications such as sensing, communication, and imaging.
To enhance continuous wave terahertz (CW-THz) scanning images contrast and denoising, a method based on wavelet transform and Retinex theory was proposed. In this paper, the factors affecting the quality of CW-THz images were analysed. Second, an approach of combination of the discrete wavelet transform (DWT) and a designed nonlinear function in wavelet domain for the purpose of contrast enhancing was applied. Then, we combine the Retinex algorithm for further contrast enhancement. To evaluate the effectiveness of the proposed method in qualitative and quantitative, it was compared with the adaptive histogram equalization method, the homomorphic filtering method and the SSR(Single-Scale-Retinex) method. Experimental results demonstrated that the presented algorithm can effectively enhance the contrast of CW-THZ image and obtain better visual effect.
A novel multi-wavelength fiber laser was achieved simultaneously tunable and switchable outputs of wavelength spacing with an Erbium-doped fiber amplifiers (EDFA) based on nonlinear polarization rotation (NPR) effect. We described and proved the tunable of wavelength spacing in detail by using different length of polarization maintaining fiber (PMF). The switchable outputs was simply realized an interchange among multi-wavelength operations by adjusting the polarization controller (PC) in the ring cavity. The tunable and switchable multi-wavelength fiber ring laser could stably operate in room temperature, and the peak power variations in each wavelength were measured to be less than 1dB.
Real-time terahertz imaging is demonstrated using a 1.63 THz (184.31μm) optically-pumped terahertz
laser (Coherent Sifir-50 FPL) and a 124×124 element room-temperature pyroelectric camera (Pyrocam
III). Transmission-mode THz images are presented with samples hidden in various wrapping materials.
Experimental results show that this THz imaging system has the potential for the application in
real-time mail inspection and non-destructive inspection.
In this report, a required aperiodically poled structure is designed, where two quasi-phase-matched conditions of optical
parametric oscillator and frequency difference are required to be satisfied simultaneously. By numerical simulation, we
analyze the effects of crystal structure, the cavity parameter of optical parametric oscillator, and terahertz absorption
coefficient on terahertz generation. And, the terahertz wave is generated via cascaded processes, which is of great
interest in fundamental and applied sciences, and is a great demand for many applications in spectroscopy, sensing,
communication, medial diagnoses and biomedical imaging. In addition, the design method for poled ferroelectric crystal
is universal and applicable to many other fields, particularly in those processes where multiple quasi-phase-matched
conditions are required to be satisfied simultaneously.
Raman scattering is an inelastic process, in which we can obtain information about material lattice vibration frequencies. If the wavelength of the excitation laser is within the electronic spectrum of the material, the intensity of some Raman-active vibrations will increase enormously. This resonant Raman effect can be quite useful to explore the electron-phonon coupling in the substance. In this article, we report the investigations on the electron-phonon coupling effect in Se nanoclusters using resonant Raman technique. Up to 10 different laser lines are used in the experiment. The Raman intensity strongly depends on the energy of the laser lines used for excitation. The one-phonon symmetric A1 modes for Se single helix and Se8 rings are enhanced in the vicinity of their absorption bands. Detailed analysis shows that the Raman intensity in the high frequency range 450 - 550 cm-1 is a sum of individual second-order Raman scattering intensities for the confined Se species. These two-phonon Raman shifts occur at twice the frequency shift of the first-order Raman lines, and their intensities are also enhanced when the excitation laser energy matches an electronic transition in Se nanoclusters.
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