Short Wavelength Direction Research Articles

Coupling package and WGM resonator filled with Terfenol-D for magnetic field sensing and tuning

In this work, we proposed a whispering gallery mode (WGM) resonator filled with Terfenol-D (WGMRFT), which could be applied to magnetic field sensing and tuning. We used a hollow quartz rod to fabricate the resonator, and then filled it with Terfenol-D to create a magnetic field-sensitive device. The WGM of the resonator were excited using a tapered fiber evanescent field. As the external magnetic field increased, the WGM resonant spectrum of the resonator shifted toward the short wavelength direction. The sensitivity of the resonator is −3.05 pm/mT. The structure has many advantages, such as simple structure, low cost and easy operation. In addition, we have demonstrated a resonator coupling and packaging device, using this device to adjust the coupling state of the resonator within a certain range. The package demonstrated good repeatability and reversibility in magnetic field measurement experiments. This lays the foundation for the application of millimeter-scale large WGM resonators.

Size Quantization of the Electron Energy Spectrum in a Microscopic Semiconductor Crystal

The interband absorption spectrum of microscopic CdS crystals ranging in size from ~30 to 800 Å and dispersed in a transparent insulating matrix has been studied. There is a significant (~0.8-eV) shift of the fundamental absorption edge in the short-wavelength direction, and there are oscillations in the interband absorption spectrum caused by quantum size effect.

Strong Anti-Stokes and Flat Supercontinuum in Specified Band Based on Non-Degenerate Raman Four-Wave Mixing Modulation

We proposed a method for flat supercontinuum (SC) in specified band based on non-degenerate Raman four-wave mixing (NRFWM) modulation. The dispersion of the photonic crystal fiber (PCF) and peak power of the pump light, are controlled such that the gain of the NRFWM falls in our desired band. This provides a new method for realizing SC with broadband hyperspectral intensities in the short-wavelength direction. By this method we developed a near-infrared flat SC. Its spectral intensity of 3 dB corresponds to a bandwidth of 420 nm at a central wavelength of 800 nm. In addition, the relative intensity of the anti-Stokes light is 75.2%, and 49.6 % of the total spectral energy is concentrated in the 610–1030 nm band, which provides a more effective light source for optical coherence tomography (OCT). Using this SC as the light source of OCT can improve the axial resolution to 0.8μm.

Sideband-free tunable and switchable dual-wavelength mode-locked fiber laser based on the Lyot filter and spontaneous radiation peaks.

We demonstrate a compact tunable and switchable dual-wavelength fiber laser based on the Lyot filtering effect and the spontaneous radiation peaks of gain fiber. By introducing a period of polarization-maintain Er-doped fiber (PM-EDF), stable dual-wavelength pulses can operate in both the anomalous dispersion region and the normal dispersion region. The corresponding repetition frequency difference of the dual wavelengths has excellent stability while the relative center wavelength can be adjusted in the range of 5 nm to 13 nm. There is no existence of significant sidebands in the optical spectrum during the whole tuning process. This dual-wavelength laser based on two spontaneous radiation peaks in the shorter wavelength direction has great application potential. Our work provides a new design solution for dual-comb sources (DCSs).

Melioration of Electrical and Optical Properties of Al and B Co-Doped ZnO Transparent Semiconductor Thin Films

Undoped, Al-doped and Al-B co-doped ZnO transparent semiconductor thin films were deposited on glass substrates by sol-gel method and spin coating technique. This study investigated the influence of Al (2 at.%) doping and Al (2 at.%)-B (1 or 2 at.%) co-doping on the microstructural, surface morphological, electrical and optical properties of the ZnO-based thin films. XRD analysis indicated that all as-prepared ZnO-based thin films were polycrystalline with a single-phase hexagonal wurtzite structure. The substitution of extrinsic dopants (Al or Al-B) into ZnO thin films can significantly degrade the crystallinity, refine the microstructures, improve surface flatness, enhance the optical transparency in the visible spectrum and lead to a shift in the absorption edge toward the short-wavelength direction. Experimental results showed that the Al-doped and Al-B co-doped ZnO thin films exhibited high average transmittance (>91.3%) and low average reflectance (<10%) in the visible region compared with the ZnO thin film. The optical parameters, including the optical bandgap, Urbach energy, extinction coefficient and refractive index, changed with the extrinsic doping level. Measured results of electrical properties revealed that the singly doped and co-doped samples exhibited higher electron concentrations and lower resistivities than those of the undoped sample and suggested that 2 at.% Al and 1 at.% B were the optimum dopant concentrations for achieving the best electrical properties in this study.

Preparation and Characterization of (Al, Fe) Codoped ZnO Films Prepared by Sol–Gel

In this research, Al-doped and (Al, Fe) codoped ZnO films were prepared on glass substrate by the sol–gel method. The surface morphology, structure, and optical property were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Ultraviolet-Visible-Near-infrared (UV-Vis-NIR) spectroscopy. The film surface morphologies all exhibited granular characteristics. With the Fe doping concentration increasing, the codoped films had smaller grain size and tended to be smoother. XRD analysis revealed that all films had a hexagonal wurtzite structure. The codoping can contribute to more Al and Fe ions entering the ZnO crystal structure, but result in the crystalline degree of the films decreasing. XPS results showed that the Al and Fe irons in the films exist in the form of trivalent. Moreover, the doped films had higher transmission, especially for codoped (Fe:Al = 3) film, but their absorption edge shifted to the short-wavelength direction.

Enhanced temperature characteristics of strain-assisted fiber Bragg gratings inscription by femtosecond laser pulses

A strain assisted fiber Bragg grating (S-FBG) has been fabricated by using femtosecond laser mask phase method. The spectral characteristics of the FBG with different pre-stress has been investigated. It was found that the wavelengths of the S-FBGs shift linearity towards shorter wavelength direction as the applied strain increases and show a sensitivity of 0.1596 pm/με and 0.8796 pm/με before and after the stress released, respectively. Moreover, the S-FBGs show better temperature characteristics than the normal FBG, including smaller decay rate of the reflection intensity, smaller wavelength hysteresis effect and better high temperature stability. Meanwhile, the temperature test has demonstrated that the S-FBG with larger pre-stress shows better temperature characteristics. In addition, the S-FBG has advantages of compact size, easy fabrication and high reflection, which is of high interest in developing high temperature sensing platforms.

Rapid Growth of Colloidal Crystal Films from the Concentrated Aqueous Ethanol Suspension

Developing a rapid fabrication of colloidal crystal film is one of the technical issues to apply to wide and various fields. We have been investigating a drying process of colloidal aqueous ethanol (EtOH) suspension formed by electrophoretic deposition (EPD). Here, the detailed formation mechanism of the colloidal crystal films with the closest packing structure was investigated by optical microscope and spectroscopy. The growth mechanism from the colloidal suspension to the colloidal crystal film was found to consist of four stages. In the first stage, concentrated colloidal suspension changed to order structure, i.e., nonclosely packed colloidal crystal by Alder phase transition. After this crystallization, we observed Bragg's diffraction peak and structural color. In the second stage, the diffraction peak shifts toward the shorter-wavelength direction (blue shift) due to the reduction of the interparticle distance of the nonclosely packed colloidal crystal. Finally, this peak shift continued until the closely packed colloidal crystal film was formed. In the third stage, the diffraction peak kept almost a similar wavelength due to the liquid film of aqueous EtOH covering on the colloidal crystal film. In the fourth stage, the colloidal crystal film changed from wet to dry condition. The structural color changes from green to blue by the evaporation of the solvent from the interspace of the colloidal crystal film. This color change is explained by the change in the refractive index of the interparticle medium from solvent to air. One of the key findings in our process is a rapid crystal growth using concentrated colloid aqueous EtOH suspension. Drying the concentrated suspension formed a closely packed colloidal crystal film within 55 s. This process has the potential for high-speed deposition of the colloidal crystalline thin films.

Degradation of Bisphenol A by CeCu Oxide Catalyst in Catalytic Wet Peroxide Oxidation: Efficiency, Stability, and Mechanism.

The CeCu oxide catalyst CC450 was prepared by citric acid complex method and the catalytic wet peroxide oxidation (CWPO) reaction system was established with bisphenol A (BPA) as the target pollutant. By means of characterization, this research investigated the phase structure, surface morphology, reducibility, surface element composition, and valence of the catalyst before and after reuse. The effects of catalyst dosage and pH on the removal efficiency of BPA were also investigated. Five reuse experiments were carried out to investigate the reusability of the catalyst. In addition, this research delved into the changes of pH value, hydroxyl radical concentration, and ultraviolet-visible spectra of BPA in CWPO reaction system. The possible intermediate products were analyzed by gas chromatography-mass spectrometry (GC-MS). The catalytic mechanism and degradation pathway were also discussed. The results showed that after reaction of 65 min, the removal of BPA and total organic carbon (TOC) could reach 87.6% and 77.9%, respectively. The catalyst showed strong pH adaptability and had high removal efficiency of BPA in the range of pH 1.6–7.9. After five reuses, the removal of BPA remained above 86.7%, with the structure of the catalyst remaining stable to a large extent. With the reaction proceeding, the pH value of the reaction solution increased, the concentration of OH radicals decreased, and the ultraviolet-visible spectrum of BPA shifted to the short wavelength direction, that is, the blue shift direction. The catalysts degraded BPA rapidly in CWPO reaction system and the C–C bond or O–H bond in BPA could be destroyed in a very short time. Also, there may have been two main degradation paths of phenol and ketone.

Preparation, properties, and photodoping behavior of GeS2-, Ga2S3-, and Sb2S3-based glasses with excess sulfur and CsCl

Abstract

Preparation and characterization of Mg, Al and Ga co-doped ZnO transparent conductive films deposited by magnetron sputtering

Mg, Al and Ga co-doped ZnO (AGMZO) thin films were deposited on flexible substrates by rf magnetron sputtering using ceramic targets at room temperature. The effect of Mg content on structural, morphological, electrical and optical properties of the films was investigated in detail. X-ray photoelectron spectroscopy measurements indicated that the Mg content in the films was substantially consistent with that of corresponding targets. X-ray diffraction (XRD) results revealed that the films had the hexagonal wurtzite structure. The (0 0 2) preferred orientation of the crystallites detected by XRD suggested that the grains grew perpendicular to the surface of the film, which was confirmed by the cross-section image. The average transmittance in the visible range was above 82% for all the films. The lowest resistivity of 2.8 × 10−3 Ω cm with carrier concentration of 1.54 × 1020 cm−3 and Hall mobility of 14.6 cm2/Vs can be obtained. The optical transmission spectra revealed that when the Mg concentration increased, the UV absorption edge moved toward the shorter wavelength direction and the optical band gap increased from 3.4 to 3.5 eV.

Influence of SnO on bismuth emission centers in germanate glass

It was found that SnO addition widens the emission spectrum of Bi:GeO2 glass and shifts it in the short-wavelength direction. This emission band overlaps the spectral gap at 1540–1600 nm where no laser action in Bi-activated materials has been obtained yet. The amplifiers covering this gap may be of great interest for telecommunication systems of the next generation. The changes in the spectra and in lifetimes give a reason to assume that a variety of new types of bismuth emission centers are formed in these glasses.

A study of interaction surface acoustic wave with localized surface plasmon resonance

A surface acoustic wave (SAW) has the ability to manipulate droplets, and a localized surface plasmon resonance (LSPR) is expected to be utilized for biosensors. By integrating both of them on a single substrate, it is possible to create a new microfluidic system. In order to achieve this goal, the interaction the SAW with the LSPR was investigated by fabricating the substrate and measuring. As a result, it was found that the reflectance characteristics of the LSPR decreased and the spectrum shift in the short wavelength direction was observed during the SAW generated. The reason was experimentally considered and found that the LSPR was influenced from the temperature distribution on the SAW device. On the other hand, the increase of the SAW amplitude was observed by the LSPR excited. The results obtained indicate that control of the temperature is necessary to integrate the SAW and LSPR on the same substrate.

Characterization of the microstructure and the optical and electrical properties of the direct-current magnetron sputtered CuO films at different substrate temperatures

Single-phased monoclinic CuO films with direct energy gap were deposited on the clean glass substrates by direct-current magnetron sputtering method. Effect of substrate temperature was in particular studied on the microstructure, and the optical and electrical properties of the films. With increase in substrate temperature, the crystallization of the polycrystalline CuO films is enhanced and thus the CuO film at substrate temperature of 600 °C has a clear <111> preferred orientation. Substrate temperature has different effect on the stress along the crystal axis a, b, and c, indicating an anisotropic growth along different crystal axis. The absorption edge of the films in the vicinity of 1.88 eV first moves towards short wavelength direction and then towards long wavelength direction with substrate temperature, which can be explained by the quantum size effect of the crystallites. The conduction transition of the films from p to n type takes places at substrate temperature of 500 °C. The p-type conduction is attributed to the copper vacancies and interstice oxygen, while n-type conduction is to the oxygen vacancies and interstice copper. The CuO film deposited at high substrate temperature has a larger carrier concentration and a lower electrical resistivity.

Simultaneous Measurement of Hydrogen Concentration and Temperature Based on Fiber Loop Mirror Combined With PCF

Simultaneous measurement of hydrogen concentration and temperature with high sensitivity and high precision was realized by integrating a photonic crystal fiber (PCF) modal interferometer and a high-birefringence fiber loop mirror (HBFLM). The PCF was coated with sol-gel Pd/WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> coating, whose refractive index and volume would be changed along with the variation of surrounding hydrogen concentration or temperature, and then the resonant dip of PCF modal interferometer would shift accordingly. Meanwhile, the resonant dip of the HBFLM has a high sensitivity to surrounding temperature. By integrating PCF modal interferometer and HBFLM together, the two independent resonant dips that can be simultaneously monitored at the output spectrum of the integrated interferometers would all shift with the change of external hydrogen concentration or temperature. Finally, combined with the dual-wavelength matrix method, the hydrogen concentration and temperature could be simultaneously measured. Experimental results showed that the resonant dip of the PCF modal interferometer moved in the short wavelength direction with the increase of hydrogen concentration, and the sensitivity of the hydrogen concentration was -1.12 nm/% within the concentration range from 0% to 1%. In the process of temperature change, the HBFLM realized a higher temperature sensitivity of -1.84 nm/°C. The sensor had high sensitivity, low cost, and simple structure.

Supercontinuum Generation in an Ytterbium-Doped Fiber Amplifier With Cascaded Double-Clad Passive Fiber Tapers

Supercontinuum (SC) generation directly from an Ytterbium-doped fiber amplifier (YDFA) has many advantages such as high output power, low splicing loss, and high optical to optical conversion efficiency. However, the output spectrum can only extend to longer wavelength. We report a method to further extend the spectral range of SC generated in an YDFA at short wavelength direction by using cascaded double-clad passive fiber tapers. A 750 ps seed pulse at 1060 nm is amplified to 15.9 W and spectrally broadened from 1000 to 1600 nm in an YDFA system. Using this broadband source to pump cascaded double-clad passive fiber tapers, a 14.1 W SC source is obtained, covering 630 to 2000 nm, which is in an all-fiber and low-cost structure. It is found that, in fiber tapers, especially at the taper waist, the frequency shift rate of solitons increases significantly, and those red-shifted solitons are group-velocity matched to dispersive waves even in the red wavelength region. To our best knowledge, this is the first time there has been generation in tapered double-clad passive fiber.

Dimmable sunlight-like organic light emitting diodes with ultra-high color rendering index

We propose novel dimmable sunlight-like white organic light-emitting diodes that were fabricated using three luminophores to form an emitting spectrum similar to black body radiation at 2250K with ultra-high color rendering index (CRI) value of 91, which nearly remained the constant at various luminance values ranging from 100 to more than 2500cd/m2 at Commission Internationale de l’Eclairage chromaticity coordinates of (0.51,0.41). Introducing charge modification layers suppressed the energy transfer between the emitting material layers and increased the probability of carrier recombination. Moreover, we reveal that covering long-wavelength ranges played a vital role in achieving high CRI values; the CRI values of a spectrum artificially shifted toward a long-wavelength direction (from 610 to 620nm) remained constant, whereas those of a spectrum shifted toward a short-wavelength direction (from 610 to 600nm) dropped to 79.

Plasmon-induced resonance energy transfer for solar energy conversion

In Forster resonance energy transfer (FRET), energy non-radiatively transfers from a blue-shifted emitter to a red-shifted absorber by dipole–dipole coupling. This study shows that plasmonics enables the opposite transfer direction, transferring the plasmonic energy towards the short-wavelength direction to induce charge separation in a semiconductor. Plasmon-induced resonance energy transfer (PIRET) differs from FRET because of the lack of a Stoke's shift, non-local absorption effects and a strong dependence on the plasmon's dephasing rate and dipole moment. PIRET non-radiatively transfers energy through an insulating spacer layer, which prevents interfacial charge recombination losses and dephasing of the plasmon from hot-electron transfer. The distance dependence of dipole–dipole coupling is mapped out for a range of detuning across the plasmon resonance. PIRET can efficiently harvest visible and near-infrared sunlight with energy below the semiconductor band edge to help overcome the constraints of band-edge energetics for single semiconductors in photoelectrochemical cells, photocatalysts and photovoltaics. Plasmon-induced resonance energy transfer is revealed and explored for solar energy harvesting from visible and near-infrared light.

Active control of chirality in nonlinear metamaterials

An all-optical tunabe chirality is realized in a photonic metamaterial, the metamolecule of which consists of a nonlinear nano-Au:polycrystalline indium-tin oxide layer sandwiched between two L-shaped gold nano-antennas twisted 90° with each other. The maximum circular dichroism reached 30%. Under excitation of a 40 kW/cm2 weak pump light, the peak in the circular dichroism shifts 45 nm in the short-wavelength direction. An ultrafast response time of 35 ps is maintained. This work not only opens up the possibility for the realization of ultralow-power and ultrafast all-optical tunable chirality but also offers a way to construct ultrahigh-speed on-chip biochemical sensors.

Luminescent Performance of the Long-Persistence Sr2MgSi2O7: Eu2+, Dy3+ Inorganic Nanofibers

Long-persistence silicate luminescent nanofibers doped with rare earths (Sr2MgSi2O7: Eu2+, Dy3+) were successfully prepared via electrospinning technique combined with calcination method, and long-lasting silicate phosphors (Sr2MgSi2O7: Eu2+, Dy3+) with the same stoichiometric proportion as the nanofibers were prepared by sol-gel method. The morphology, crystal structure, photoluminescence and thermoluminescence properties of the nanofibers compared with the phosphors were investigated. The average diameter of the Sr2MgSi2O7: Eu2+, Dy3+ luminescent nanofibers was 350 nm, and the whole shape of the nanofibers was consecutive which seemed like pearl necklace. The synthesis temperature of the Sr2MgSi2O7: Eu2+, Dy3+ luminescent nanofibers was 1100°C which lower 200°C than phosphors. The excitation spectrum of the nanofibers moved to short wavelength direction compared with the phosphors, and the nanofibers could reveal a bright blue light after been excited by UV light at 365 nm. The initial afterglow intensity of the nanofibers was 1.26 cd·m−2, and the afterglow decay curve of the nanofibers was similar to phosphors. Even though the depth of the traps of the luminescent nanofibers was shallower than phosphors, the nanofibers still presented strong afterglow intensity and long decay time.