In this study, a novel surface plasmon resonance (SPR) sensor based on a single-mode polymer-tipped optical fiber (PTOF) is designed and optimized. The refractive index (RI) measurement is achieved ...by exciting SPR at the interface between gold film and TiO2 layer coated on the polymer-tip. Finite-difference time-domain (FDTD) method based commercial optiFDTD software is used to optimize the design of the proposed sensor for a better performance related to the materials of the metal film, the shape of polymer-tips, and the thickness of TiO2 layer. The results show that the sensitivity of optimal single-mode PTOF SPR sensor reaches 3798 nm/RIU when analyte RI is 1.333, and reaches 5708 nm/RIU when analyte RI is 1.383. Compared with conventional optical fiber SPR sensors, the single-mode PTOF SPR sensor proposed in the paper is simpler in fabrication, smaller in size, and higher in sensitivity, which has broad application prospect in biomedical sensing.
Pt-doped WO3 materials have good hydrogen-sensing properties, which are widely used in the field of hydrogen sensing. In this paper, Sol-Gel method is used to prepare WO3/Pt sensing material and ...sensing characteristic to hydrogen is characterized by transmission optical fiber hydrogen sensing system fabricated at home. By optimizing the fabrication parameters including sol-gel viscosity, aging time, Pt:W moral ratio and layers of the sensing film, the sensitivity and repeatability of the film were greatly improved. The repeatability of the film is further improved by irradiating the film with an UV lamp. Experimental results show that by increasing the number of film layers from one to five (the thickness of the film is about 222 nm, and the content of Pt element is 13.47% by SEM test), the sensitivity increased by 1.666 times. The repeatability of the film can be improved by 3% through the irradiation of the ultraviolet lamp. The optimized WO3/Pt film was tested continuously for 12 times at 5% hydrogen concentration, and the zero drift was only 0.723%.
A high-gain and tri-band terahertz microstrip antenna with a photonic band gap (PBG) substrate is presented in this paper for terahertz communications. Polyimide dielectric columns are inserted into ...the silicon substrate to form the PBG substrate to improve the gain of the antenna. The PBG substrate and polyimide substrate constituted a multilayer substrate structure and enabled the multi-band operation of the antenna. The PBG substrate antenna achieves gains of 6.28 dB, 4.84 dB, and 7.66 dB at resonant frequencies of 0.360 THz, 0.580 THz, and 0.692 THz, respectively, outperforming the homogeneous substrate THz microstrip antenna (H antenna) by 1.18 dB, 1.74 dB, and 1.82 dB, respectively. The radiation efficiencies at three operating bands are over 93%, 92%, and 88%, respectively, which are slightly higher than that of the H antenna and greater than that of the standard multi-band antenna.
With the working frequency of wireless communication systems moving to a higher terahertz (THz) band, the design of high-performance THz wave modulators has become a pivotal issue to be tackled ...urgently in THz communication. In this paper, we design a high-performance dual-channel photonic crystal modulator to enable ON–OFF regulation of the THz wave based on the defect mode disappearance of combined microcavities. The modulator introduces Y-type line defects into silicon rod photonic crystals as a dual-channel waveguide and the point defects and ring resonator form a combined microcavity. Due to the refractive index of the ring resonator filler, gallium arsenide, it is tunable with pump light excitation, and the defect mode frequency of the combined microcavity can be dynamically changed. Under pump excitation with a wavelength of 810 nm and an intensity of 0.4 μJ/cm2, 1.34 THz and 1.83 THz dual-channel waves can be OFF due to the defect mode disappearance of the combined microcavity. This is simulated by the time-domain steady-state response and steady-state THz wave field intensity distribution of the modulator by the finite-difference time-domain method. The results indicate that the dual-channel modulator has large modulation depths of 100% and 99.7%, high modulation rates of 4.05 GHz and 4.17 GHz, and low insertion losses of 0.31 dB and 0.18 dB, which lays foundation for the development of high-speed and low-loss THz communication technology.
Three sizes of CuO nanosheets were synthesized by hydrothermal method. The structure and morphology of CuO nanosheets were characterized by X-ray diffraction and scanning electron microscopy. ...Dielectrophoresis nano-manipulation technique was employed to arrange the materials on pre-designed Ti/Au electrodes to fabricate the three humidity sensors, and the sensing properties were then tested. The experimental results show that the sensitivity greatly increases with the decreasing size of CuO nanosheets, the sensitivity of sensor a, b, c are 369%, 3278%, 22,611% in 97.3% RH, respectively. The smaller sized CuO nanomaterials have better response characteristic, the response time of sensor a, b, c under 11.3–97.3% RH are 53 s, 49 s, 32 s, respectively. And correspondingly, hysteresis properties and the repeatability are also a little influenced. In addition, based on complex impedance spectroscopy and multilayer adsorption theory, the impact of size on humidity sensing property was discussed. The results indicated the feasibility to obtain higher performance of humidity sensor, especially the higher sensitivity, via employment the smaller size sensing nanomaterials.
Graphic Abstract
Ultrafast carrier dynamics in intrinsic and nitrogen-doped (N-doped) 6H-SiC crystals is studied by time-resolved two-color pump-probe transmission spectroscopy with temperature ranging from 295 to ...12 K. With photoexcitation at 400 nm, and a pump fluence of 5.3 µJ/cm
2
, both samples show ultrafast photoinduced absorption at 800 nm, and the magnitudes of the photoinduced absorption and recovery process have different temperature dependences for both samples, in which the intrinsic 6H-SiC shows almost no temperature dependence, while the magnitudes of the photoinduced absorption and recovery process in N-doped 6H-SiC are temperature dependent. The different temperature dependences of photoinduced absorption and recovery process are due to the different band-gap changes for intrinsic and N-doped 6H-SiC.
Ultrafast carrier dynamics and terahertz conductivity of intrinsic 6H-SiC are investigated by optical-pump terahertz-probe spectroscopy at room temperature. The amplitude of photoinduced absorption ...increases gradually and followed by a biexponential recovery with photogenerated carrier densities from 7.43 × 1017 to 2.81 × 1018 cm −3 at 400 nm. With the increase of photogenerated carrier density, the fast decay time decreases may be due to higher order recombination (i.e., Auger recombination), and the slow one increases is related to the band-filling effect. The evolution of complex conductivity with photogenerated carrier density at the delay time of about 50 ps can be better fitted with Drude–Smith model than Drude–Lorenz model. The Drude–Smith scattering time τDS and Smith parameter c1 increase with photogenerated carrier density implies carriers backscattering is dominant. Our analysis provides important basic data for research and improvement ultrafast semiconductor devices of intrinsic 6H-SiC.
•Photogenerated carrier density dependence of ultrafast carrier dynamics in intrinsic 6H-SiC were investigated.•It was analyzed the dependence of photoinduced absorption and carrier recovery process of intrinsic 6H-SiC with photogenerated carrier density.•It was analyzed the time-resolved photoconductivity and transient complex photoconductivity of intrinsic 6H-SiC with photogenerated carrier density.
•The temperature dependence of absorption coefficient and phase matching angle of Potassium titanyl arsenate (KTiOAsO4) with temperatures from 5.8 to 300 K was determined by terahertz time-domain ...spectroscopy.•It is demonstrated that the absorption coefficient of KTA crystal exhibits a decrease with decreasing temperature, particularly the absorption coefficient along the Z axis changes more pronounced.•It is calculated the relationship between the phase matching angle and the output wavelength with temperatures of 300 K and 5.8 K, and the phase matching angle changes by less than 1° at the same output wave.•The results demonstrate that KTiOAsO4 has low absorption coefficient and can achieve phase matching with output wavelengths in the terahertz range at low temperatures.
The temperature dependence of absorption coefficient and phase matching (PM) angle of Potassium titanyl arsenate (KTiOAsO4, KTA) with temperatures from 5.8 to 300 K was determined by terahertz time-domain spectroscopy. It is demonstrated that the absorption coefficient of KTA crystal exhibits a decrease with decreasing temperature, particularly the absorption coefficient along the Z axis changes more pronounced, which can be attributed to the reduction in the mobility of K+ at low temperatures. The Sellmeier equation was employed to fit the refractive indices along the X,Y, and Z axes with temperatures from 300 to 5.8 K to obtain the refractive index dispersion equation. The refractive index dispersion equation was substituted into the PM equation to calculate the relationship between the PM angle and the output wavelength with temperatures of 300 K and 5.8 K, and the PM angle changes by less than 1° with temperature at the same output wavelength. The results demonstrate that KTA can achieve PM at low temperatures with output wavelengths in the THz range. Moreover, the THz absorption coefficient decreases and the PM angle remains relatively stable at low temperatures provide a foundation for efficient THz wave generation using KTA under low temperature conditions.
Zn
1−x
Ni
x
O (X = 0%, 1%, 3%, 5%, 10%) nanowires were synthesized by the hydrothermal method, and the prepared materials were characterized and analyzed by XRD, SEM and XPS. Then, the materials were ...positioned on the pre-designed Ti/Au interdigital electrode to fabricate humidity sensors by the dielectrophoresis method. The humidity sensitive characteristics of the sensors were studied by combining complex impedance spectroscopy and theory of multilayer adsorption. The results show that the surface oxygen vacancies concentration can be regulated by Ni doping process. Under the combined action of Ni ions and oxygen vacancies, the performance of humidity sensors has been improved significantly. Especially, 5% Ni doped ZnO humidity sensor shows a high capacitance sensitivity, which is varied by more than four orders of magnitude with increasing the relative humidity from 11 to 95%, respectively. Moreover, the response time and recovery time are reduced to 27 s and 2 s, which is much better than undoped ZnO humidity sensor. The results indicate that the doping of Ni elements play an important role in the sensing property improvement of ZnO humidity sensor.
Size dependence of spin dynamics in colloidal CdSe quantum dots (QDs) are investigated with circularly polarized pump–probe transmission spectroscopy at room temperature. The excitation energy is ...tuned to resonance with the lowest exciton (1Sh1Se) energy of the CdSe QDs. The exciton spin dynamics of CdSe QD with the diameter of 5.2 nm shows monoexponential decay with a typical time constant of about 1–3 ps depending on the excitation energy. For the cases of CdSe QDs with smaller size (with the diameter of 4.0 and 2.4 nm), the exciton spin relaxation shows biexponential decay, a fast component with time constant of several ps and a slow one with time constant of hundreds of ps to nanosecond time scale. The fast spin relaxation arises from the bright-dark transition, i.e., J = ±1 ↔ ∓2 transition. This process is dominated by the hole spin flips, while the electron spin conserves. The slow spin relaxation is attributed to the intralevel exciton transitions (J = ±1 ↔ ∓1 transition), which is relevant to the electron spin flip. Our results indicate that the exciton spin relaxation pathways in CdSe QD are controllable by monitoring the particle size, and polarized pump–probe spectroscopy is proved to be a sensitive method to probe the exciton transition among the fine structures.
