Active control of terahertz waves based on hybrid VO2 periodic corrugated waveguides.

We describe a method for the active control of terahertz (THz) waves using hybrid vanadium dioxide (VO2) periodic corrugated waveguide. Unlike liquid crystals, graphene and semiconductors and other active materials, VO2 exhibits a unique insulator-metal transition characteristic by the electric fields, optical, and thermal pumps, resulting in five orders of magnitude changes in its conductivity. Our waveguide consists of two gold coated plates with the VO2-embedded periodic grooves, which are placed in parallel with the grooves face to face. Simulations show that this waveguide can realize mode switching by changing the conductivity of the embedded VO2 pads, whose mechanism is attributed to the local resonance induced by defect mode. Such a VO2-embedded hybrid THz waveguide is favorable in practical applications such as THz modulators, sensors and optical switches, and provides an innovative technique for manipulating THz waves.

Internal Cylinder Identification Based on Different Transmission of Longitudinal and Shear Ultrasonic Waves.

We have built a Fizeau fiber interferometer to investigate the internal cylindrical defects in an aluminum plate based on laser ultrasonic techniques. The ultrasound is excited in the plate by a Q-switched Nd:YAG laser. When the ultrasonic waves interact with the internal defects, the transmitted amplitudes of longitudinal and shear waves are different. The experimental results show that the difference in transmission amplitudes can be attributed to the high frequency damping of internal cylinders. When the scanning point is close to the internal defect, the longitudinal waves attenuate significantly in the whole defect area, and their amplitude is always smaller than that of shear waves. By comparing the transmitted amplitudes of longitudinal and shear waves at different scanning points, we can achieve a C scan image of the sample to realize the visual inspection of internal defects. Our system exhibits outstanding performance in detecting internal cylinders, which could be used not only in evaluating structure cracks but also in exploring ultrasonic transmission characteristics.

Hybrid structure of PbS QDs and vertically-few-layer MoS2 nanosheets array for broadband photodetector.

A novel three-dimensional (3D) vertically-few-layer MoS2 (V-MoS2) nanosheets- zero-dimensional PbS quantum dots (QDs) hybrid structure based broadband photodetector was fabricated, and its photoelectric performance was investigated in detail. We synthesized the V-MoS2 nanosheets by chemical vapor deposition, using the TiO2 layer as the induced layer, and proposed a possible growth mechanism. The use of the TiO2 induction layer successfully changed the growth direction of MoS2 from parallel to vertical. The prepared V-MoS2 nanosheets have a large specific surface area, abundantly exposed edges and excellent light absorption capacity. The V-MoS2 nanosheets detector was then fabricated and investigated, which exhibits a high sensitivity for 635 nm light, a fast response time and an excellent photoelectric response. The V-MoS2 nanosheets with a height of approximately 1 µm successfully broke the light absorption limit caused by the atomic thickness. Finally, we fabricated the PbS QDs/V-MoS2 nanosheets hybrid detector and demonstrated their potential for high-performance broadband photodetectors. The response wavelength of the hybrid detector extends from the visible band to the near-infrared band. The responsivity of the hybrid detector reaches 1.46 A W-1 under 1450 nm illumination. The combination of 3D MoS2 nanosheets and QDs further improves the performance of MoS2-based photodetector devices. We believe that the proposed zero-dimensional QDs and 3D vertical nanosheets hybrid structure broadband photodetector provides a promising way for the next-generation optoelectronic devices.

Self-catalyst ß-Ga2O3 semiconductor lateral nanowire networks synthesis on the insulating substrate for deep ultraviolet photodetectors.

Monoclinic gallium oxide (ß-Ga2O3) is a super-wide bandgap semiconductor with excellent chemical and thermal stability, which is an ideal candidate for detecting deep ultraviolet (DUV) radiation (100-280 nm). The growth of ß-Ga2O3 is challenging and most methods require Au as the catalyst and a long reacting time (more than 1 hour). In this work, the self-catalyst ß-Ga2O3 lateral nanowire networks were synthesized on an insulating substrate rapidly by a simple low-cost Chemical Vapor Deposition (CVD) method. A thin film of ß-Ga2O3 nanowire networks was synthesized within a reacting time of 15 minutes, which possesses a huge possibility for the rapid growth of ß-Ga2O3 metal oxide nanowires networks and application in the future solar-blind photodetector. MSM (metal-semiconductor-metal) photodetectors based on the ß-Ga2O3 nanowire networks revealed fast response (on-off ratios is about 103), which is attributed to the unique cross-junction barrier-dominated conductance of the nanowire networks. In addition, the self-catalyst ß-Ga2O3 nanowires grown on insulating SiO2 are achieved and could be expected to find important applications in a bottom-up way of fabricating the next generation semiconductor nanoelectronics.

Wave packet interactions in a thin aluminum plate partially immersed in water.

When investigating the wave propagation and mode conversions in a thin aluminum plate partially immersed in water, a kind of wave packet interaction was observed. It was found that the transmitted ultrasonic signal consists of different wave packets, which contain essential information of different wave types. When the incident angle is very small, the signals can be identified as the major wave packet followed by its tail. The major packet includes the information of the incident wave while the tail is related to the mode conversion and propagation in the plate. When the incident angle increased, the major packet was literally engulfed by its tail, indicating that the directly transmitted incident ultrasound disappeared and more energy was coupled into the plate. The interactions between different wave packets found here reveal the excitation and propagation mechanisms of Lamb waves in plates, which would benefit applications in ultrasonic imaging, signal recognition, underwater acoustic communication, and so on.

High-Performance Ultrafast Humidity Sensor Based on Microknot Resonator-Assisted Mach-Zehnder for Monitoring Human Breath.

Monitoring the dynamic humidity requires sensors with fast response and anti-electromagnetic interference, especially for human respiration. Here, an ultrafast fiber-optic breath sensor based on the humidity-sensitive characteristics of gelatin film is proposed and experimentally demonstrated. The sensor consists of a microknot resonator superimposed on a Mach-Zehnder (MZ) interferometer produced by a tapered single-mode fiber, which has an ultrafast response (84 ms) and recovery time (29 ms) and a large dynamic transmission range. The humidity in dynamic ambient causes changes in the refractive index of gelatin coating, which could trigger spectral intensity transients that can be explicitly distinguished between the two states. The sensing principle is analyzed using the traditional transfer-matrix analysis method. The influence of coating thickness on the sensor's trigger threshold is further investigated. Experiments on monitoring breath patterns indicate that the proposed breath sensor has high repeatability, reliability, and validity, which enable many other potential applications such as food processing, health monitoring, and other biomedical applications.

Intense mid-infrared emission at 3.9 µm in Ho3+-doped ZBYA glasses for potential use as a fiber laser.

Intense mid-infrared emission at 3.9 µm in Ho3+-doped ZBYA glasses with direct upper laser level (Ho3+:5I5) pumping at a wavelength of 888 nm is reported for the first time, to the best of our knowledge. Spectroscopic parameters were determined using the Judd-Ofelt theory and the measured absorption spectrum. The maximum emission cross section of the Ho3+-doped ZBYA glass is estimated to be 2.7×10-21cm2 at 3906 nm. Additionally, fluorescence spectra and lifetimes of ZBYA glasses with different Ho3+ ion doping concentrations were measured. The results provide theoretical and experimental basis for better selection of rare-earth-doped matrix glasses to achieve a fluorescence output centered on a wavelength of 3.9 µm.

Tm3+-doped fluorotellurite glass microsphere resonator laser at 2.3 µm.

In this Letter, we report lasing at 2.3 µm in Tm3+-single-doped and Tm3+/Ho3+-codoped fluorotellurite glass microsphere resonators. By employing a 793 nm diode laser as a pump and exploiting whispering gallery mode microresonators (WGMRs), dual-wavelength lasing at 1.9 and 2.3 µm and triple-wavelength lasing at 1.9, 2.07, and 2.3 µm are achieved in Tm3+-doped and Tm3+/Ho3+-codoped microspheres, respectively. The introduction of Ho3+ ions significantly reduces the lasing threshold of Tm3+ at 2.3 µm because of energy transfer.

Vanadium dioxide-assisted broadband tunable terahertz metamaterial absorber.

Tunable terahertz (THz) functional devices have exhibited superior performances due to the use of active materials, such as liquid crystals, graphene, and semiconductors. However, the tunable range of constitutive parameters of materials is still limited, which leads to the low modulation depth of THz devices. Here, we demonstrate a broadband tunable THz absorber based on hybrid vanadium dioxide (VO2) metamaterials. Unlike other phase change materials, VO2 exhibits an insulator-to-metal transition characteristic and the conductivity can be increased by 4-5 orders of magnitude under external stimulus including electric fields, optical, and thermal pumps. Based on the unique transition character of VO2, the maximum tunable range of the proposed absorber can be realized from 5% to 100% by an external thermal excitation. Meanwhile, an absorption greater than 80% in a continuous range with a bandwidth about 2.0 THz can be obtained when VO2 is in its metal phase at high temperature. Furthermore, the absorber is insensitive to the incident angle up to 50° and such a broadband THz absorber can be used in applications including imaging, modulating, cloaking, and so on.

Acoustic extraordinary transmission manipulation based on proximity effects of heterojunctions.

Heterojunctions between two crystalline semiconductor layers or regions can always lead to engineering the electronic energy bands in various devices, including transistors, solar cells, lasers, and organic electronic devices. The performance of these heterojunction devices depends crucially on the band alignments and their bending at the interfaces, which have been investigated for years according to Anderson's rule, Schottky-Mott rule, Lindhard theory, quantum capacitance, and so on. Here, we demonstrate that by engineering two different acoustic waveguides with forbidden bands, one can achieve an acoustic heterojunction with an extraordinary transmission peak arising in the middle of the former gaps. We experimentally reveal that such a transmission is spatially dependent and disappears for a special junction structure. The junction proximity effect has been realized by manipulating the acoustic impedance ratios, which have been proven to be related to the geometrical (Zak) phases of the bulk bands. Acoustic heterojunctions bring the concepts of quantum physics into the classical waves and the macroscopic scale, opening up the investigations of phononic, photonic, and microwave innovation devices.

Synthesis of ternary oxide Zn2GeO4 nanowire networks and their deep ultraviolet detection properties.

Ternary oxide Zn2GeO4 with a wide bandgap of 4.84 eV, as a candidate for fourth generation semiconductors, has attracted a great deal of attention for deep ultraviolet (DUV) photodetector applications, because it is expected to be blind to the UV-A/B band (290-400 nm) and only responsive to the UV-C band (200-290 nm). Here, we report on the synthesis of Zn2GeO4 nanowire (NW) networks by lower pressure chemical vapor deposition and investigate their corresponding DUV detection properties. We find that pure Zn2GeO4 NWs could be obtained at a growth pressure of 1 kPa. The DUV detection tests reveal that growth pressure exerts a significant effect on DUV detection performance. The Zn2GeO4 NW networks produced under 1 kPa show an excellent solar-blind photoresponsivity with fast rise and decay times (t rise ≈ 0.17 s and t decay ≈ 0.14 s).

Self-adaptive terahertz spectroscopy from atmospheric vapor based on Hilbert-Huang transform.

Absorption lines of atmospheric vapor commonly appear in terahertz (THz) spectra measured in a humid air environment. However, these effects are generally undesirable because they may mask critical spectroscopic information. Here, a self-adaptive method is demonstrated for effectively identifying and eliminating atmospheric vapor noise from THz spectra of an all-fiber THz system with the Hilbert-Huang transform. The THz signal was decomposed into eight components in different time scales called the intrinsic mode functions and the interference of atmospheric vapor was accurately isolated. A series of experiments confirmed the effectiveness and strong self-adaptiveness of the proposed system in vapor noise elimination.

Single-mode interface states in heterostructure waveguides with Bragg and non-Bragg gaps.

Interface states can always arise in heterostructures that consist of two or more (artificial) materials with topologically different energy bands. The gapped band structure can be classified by the Chern number (a topological invariant) generally or the Zak phase in one-dimensional periodic systems. Recently, topological properties have been employed to investigate the interface states occurring at the connecting regions of the heterostructures of mechanical isostatic lattices and acoustical waveguides. Here, we study this heterostructure phenomenon by carefully connecting two corrugated stainless steel waveguides with Bragg and non-Bragg gaps at approximately the same frequency. These two waveguide structures can be achieved by continuously varying their geometry parameters when a topological transition exists in the forbidden bands, in which the reflection impedance changes the sign. Furthermore, a localized single high-order mode has been observed at the interface because of the transverse mode interactions, which relate to the non-Bragg gaps created by the different transverse mode resonances. Such a localized acoustic single mode with very large enhanced intensity could find its applications in sound detection, biomedical imaging, and underwater sound control, and could also enrich our means of wave front manipulations in various engineering fields.

Passive photonic integrated ratiometric wavelength monitor with resolution better than 15 pm.

This paper presents a compact and low-loss photonic integrated device consisting of a Y-branch and a pair of multimode interferometers (MMI) for a ratiometric wavelength monitoring around 1550 nm on silicon-on-insulator (SOI) technique. Two MMIs are designed in terms of width and length to achieve overlapping but opposite slope spectral responses used as two edge filters over a wavelength measurement range from 1500 nm to 1600 nm. The developed integrated photonic ratiometric structure demonstrates a suitable discrimination range for a high-speed passive wavelength measurement, with a high resolution better than 15 pm over a 100 nm wavelength range.

Orthogonality breaking induces extraordinary single-mode transparency in an elaborate waveguide with wall corrugations.

Orthogonality plays a fundamental role in various mathematical theorems and in physics. The orthogonal eigenfunctions that represent the intrinsic motions of various physical systems can also be regarded as transverse wave modes in a straight waveguide. Because of their orthogonality, these modes propagate independently, without mutual interference. When the wall separation fluctuates, the former mode orthogonality is destroyed because of the change in the Euclidean space of the system. Here, we experimentally demonstrate the extraordinary single-mode transparency that arises as a result of the intense mode interference induced by orthogonality breaking in a waveguide with a varying cross section. A mode diagram is also introduced to illuminate these mode interactions. In particular, measurements of the transverse field distributions indicate that a three-mode interaction leads to a single high-order mode that penetrates through the lower-mode bandgaps when the wall period is carefully selected. The observation of Bessel-like transverse distributions is promising for applications in wave-control engineering.

Unidirectional optical transmission in dual-metal gratings in the absence of anisotropic and nonlinear materials.

We predict the unidirectional optical transmission in dual-metal grating structures composed of two gratings with different structures in the absence of anisotropy and nonlinearity. The zero-order unidirectional transmission is achieved. Based on the unique property and by modulating the structural parameters, the transmittance approaches to 0% and 60% in the two opposite directions, respectively.

Flat-plateau supercontinuum generation in liquid absorptive medium by femtosecond filamentation.

We have studied filamentation and supercontinuum generation by focusing the intense femtosecond laser pulses into an absorptive medium (CuSO(4) aqueous solution). A broad spectrum from 350 to 950 nm with a flat plateau spanning approximately from 450 to 700 nm with a flatness of 9% is obtained without any additional filters. The results indicate that the absorptive medium not only suppresses the strong surplus femtosecond laser signal but also flattens the supercontinuum spectrum efficiently.

Femtosecond second-harmonic generation in periodically poled lithium niobate waveguides written by femtosecond laser pulses.

We present in this Letter the second-harmonic generation of femtosecond pulses in double-line-written waveguides fabricated in periodically poled lithium niobate (PPLN) with femtosecond laser pulses. In a 10-mm-long sample, a normalized conversion efficiency of 12.6% W(-1) cm(-2) has been achieved for 40 fs pump pulses with the wavelengths centered at 1550 nm. Simulation results show that in PPLN waveguides the FWHM of wavelength tuning curve for 40 fs pump pulses is 42 nm, which is 15 times of that for 40 ps pump pulses.

High-efficiency continuous-wave Raman conversion with a BaWO(4) Raman crystal.

We report a high-efficiency cw Raman conversion with a BaWO(4) Raman crystal in a diode-end-pumped Nd:YVO(4) laser. The Raman threshold is as low as 3.6 W of diode power at 808 nm. The highest output power obtained at the 1,180 nm first-order Stokes line is 3.36 W under the diode power of 25.5 W, corresponding to a slope efficiency of 15.3% and a diode-to-Stokes optical conversion efficiency of 13.2%. The intracavity Raman conversion efficiency is 21.5% with respect to the available output of the 1,064 nm fundamental.

FDTD approach to optical forces of tightly focused vector beams on metal particles.

We propose an improved FDTD method to calculate the optical forces of tightly focused beams on microscopic metal particles. Comparison study on different kinds of tightly focused beams indicates that trapping efficiency can be altered by adjusting the polarization of the incident field. The results also show the size-dependence of trapping forces exerted on metal particles. Transverse tapping forces produced by different illumination wavelengths are also evaluated. The numeric simulation demonstrates the possibility of trapping moderate-sized metal particles whose radii are comparable to wavelength.