Continuous-wave and SESAM mode-locked operation of a Yb:YSr3(PO4)3 laser.

We report on the continuous-wave (CW) and, for what we believe to be the first time, passively mode-locked (ML) laser operation of an Yb3+-doped YSr3(PO4)3 crystal. Utilizing a 976-nm spatially single-mode, fiber-coupled laser diode as pump source, the Yb:YSr3(PO4)3 laser delivers a maximum CW output power of 333 mW at 1045.8 nm with an optical efficiency of 55.7% and a slope efficiency of 60.9%. Employing a quartz-based Lyot filter, an impressive wavelength tuning range of 97 nm at the zero level was achieved in the CW regime, spanning from 1007 nm to 1104 nm. In the ML regime, incorporating a commercially available semiconductor saturable absorber mirror (SESAM) to initiate and maintain soliton-like pulse shaping, the Yb:YSr3(PO4)3 laser generated pulses as short as 61 fs at 1062.7 nm, with an average output power of 38 mW at a repetition rate of ∼66.7 MHz.

Preparation and Optical Properties of Nd:Bi2Ti2O7 Laser Crystal with Disordered Structure and Attractive Multiwavelength Emission Characteristics.

Laser crystals with multiwavelength emission characteristics are potential light sources for terahertz radiation. Herein, the pure and Nd-doped Bi2Ti2O7 (BTO) laser crystals with sizes up to 16 × 13 × 5 mm3 were successfully grown using the flux method in the KF-B2O3-CaBi4Ti4O15 growth system. The crystal structure, ideal morphology, chemical, mechanical, and thermal properties, optical transmission and Raman spectra, refractive index, absorption, and fluorescence spectra, as well as fluorescence lifetimes, were systematically studied. Besides, the spectral parameters of Nd3+ ions in the BTO crystal were systematically calculated based on the Judd-Ofelt theory. The Nd:BTO crystal has a wide transmittance range (0.44-7.30 µm), a small coefficient of thermal expansion (5.80 × 10-6 K-1), and a large absorption full width at half-maximum (fwhm) (31.2 nm) at around ∼804 nm, making it more potential for use in high-power laser systems. Moreover, fluorescence spectra show four emission peaks at 1054, 1062, 1104, and 1112 nm. The strong multiwavelength emission property makes Nd:BTO a promising laser crystal, serving as a potential light source for terahertz radiation.

PVDF Membrane-Based Dual-Channel Acoustic Sensor Integrating the Fabry-Pérot and Piezoelectric Effects.

A resonant acoustic wave detector combined with Fabry-Pérot interference (FPI) and piezoelectric (PE) effects based on a polyvinylidene fluoride (PVDF) piezoelectric film was proposed to enhance the ability of the sensor to detect acoustic signals in a specific frequency band. The deformation of circular thin films was indicated by the interference and piezoelectric effects simultaneously, and the noise level was decreased by the real-time convolution of the two-way parallel signal. This study reveals that, at the film's resonance frequency, the minimum detection limits for the FPI and piezoelectric impacts on acoustic waves are 3.39 µPa/Hz1/2 and 20.8 µPa/Hz1/2, respectively. The convolution result shows that the background noise was reduced by 98.81% concerning the piezoelectric signal, and by 85.21% concerning the FPI signal. The convolution's signal-to-noise ratio (SNR) was several times greater than the other two signals at 10 mPa. Therefore, this resonance sensor, which the FPI and the piezoelectric effect synergistically enhance, can be applied to scenarios of acoustic wave detection in a specific frequency band and with ultrahigh sensitivity requirements.

Intrinsic Defects and the Inducing Conduction Mechanism of Langasite-Type High-Temperature Piezoelectric Crystals.

Increasing the crystal resistivity is critically important for enhancing the signal-to-noise ratio and improving the sensing capability of high-temperature piezoelectric sensors based on langasite-type crystals. The resistivity of structural ordered langasite-type crystals is much higher compared to that of the disordered crystals. Here, we selected structural ordered Ca3TaGa3Si2O14 (CTGS) and disordered La3Ga5SiO14 (LGS) as representatives to investigate the microscopic conduction mechanism and further reveal the origin of the different resistivities of the ordered and disordered langasite-type crystals at elevated temperatures. By combining first-principles calculations and experimental investigations, we found that the different conductivity behaviors of the ordered and disordered crystals originate from different types of point defects formed in the crystal and their different contributions to the conductivity. For the disordered LGS crystal, the oxygen vacancies are apt to be formed at high temperatures, promoting the transition of valence electrons and yielding high conductivity. For the ordered CTGS crystal, the dominant TaGa antisite defects can introduce an electron-hole recombination center in the electronic band gap, significantly shortening the carrier lifetime and thus reducing the conductivity. This provides effective guidance to improve the resistivity performance of langasite-type crystals at high temperatures by optimizing the experimental conditions, such as oxygen atmosphere treatment, antisite defect modification, etc.

Regulating the Electronic Structure of Freestanding Graphene on SiC by Ge/Sn Intercalation: A Theoretical Study.

The intrinsic n-type of epitaxial graphene on SiC substrate limits its applications in microelectronic devices, and it is thus vital to modulate and achieve p-type and charge-neutral graphene. The main groups of metal intercalations, such as Ge and Sn, are found to be excellent candidates to achieve this goal based on the first-principle calculation results. They can modulate the conduction type of graphene via intercalation coverages and bring out interesting magnetic properties to the entire intercalation structures without inducing magnetism to graphene, which is superior to the transition metal intercalations, such as Fe and Mn. It is found that the Ge intercalation leads to ambipolar doping of graphene, and the p-type graphene can only be obtained when forming the Ge adatom between Ge layer and graphene. Charge-neutral graphene can be achieved under high Sn intercalation coverage (7/8 bilayer) owing to the significantly increased distance between graphene and deformed Sn intercalation. These findings would open up an avenue for developing novel graphene-based spintronic and electric devices on SiC substrate.

Growth, Structure and Optical Characterization of Rb3Ti3P5O20 Single Crystal.

Phosphate crystals attract much attention on account of their rich crystal structures and excellent physical and chemical properties. Herein, Rb3Ti3P5O20 single crystals were grown by the high temperature solution method using Rb2CO3 and NH4H2PO4 as the fluxes. This crystal, with non-centrosymmetric Pca21 space group, presents a three-dimensional framework structure composed of [TiO6] octahedron, [PO4] tetrahedra, and [P2O7] dimers. The electronic structure was measured via X-ray photoelectron spectroscopy. The measurements found that Rb3Ti3P5O20 has stronger Ti-O ionic bonding properties and weaker P-O covalent bonding properties compared to RbTiOPO4. Optical measurements indicated that Rb3Ti3P5O20 has a 3.54 eV band gap and a wide transmission range (0.33-4.5 µm). Theoretical calculations showed that Rb3Ti3P5O20 crystals have a moderate birefringence of 0.079 at 1064 nm. In addition, the relationship of the structure-property was studied using first-principles method. The results demonstrated that TiO6 octahedron played a significant role for the optical properties.

Broadband second-harmonic-generation in GdCOB crystals.

Broadband second-harmonic-generation (SHG) in GdCOB crystals was demonstrated for the first time. Theoretical calculation and experiments for the type-I frequency doubling of GdCOB crystal was performed. The result revealed that the spectral retracing point of phase-matching angle was at around 1.65 µm. For broadband fundamental laser source tuning in the range of 1.55-1.7 µm, efficient SHG was realized, the highest conversion efficiency was 56%, and the output bandwidth reached 16 nm.

Pinning and Anharmonic Phonon Effect of Quasi-Free-Standing Bilayer Epitaxial Graphene on SiC.

Epitaxial graphene on SiC without substrate interaction is viewed as one of the most promising two-dimensional (2D) materials in the microelectronics field. In this study, quasi-free-standing bilayer epitaxial graphene (QFSBEG) on SiC was fabricated by H2 intercalation under different time periods, and the temperature-dependent Raman spectra were recorded to evaluate the intrinsic structural difference generated by H2 time duration. The G peak thermal lineshift rates dω/dT showed that the H2 intercalation significantly weakened the pinning effect in epitaxial graphene. Furthermore, the G peak dω/dT value showed a perspicuous pinning effect disparity of QFSBEG samples. Additionally, the anharmonic phonon effect was investigated from the Raman lineshift of peaks. The physical mechanism responsible for dominating the G-mode temperature-dependent behavior among samples with different substrate coupling effects was elucidated. The phonon decay process of different samples was compared as the temperature increased. The evolution from in situ grown graphene to QFSBEG was determined. This study will expand the understanding of QFSBEG and pave a new way for its fabrication.

Growth, Optical, and Spectroscopic Properties of Pure and Nd3+-Doped GdSr3(PO4)3 Crystals with Disordered Structure.

Disordered crystals have attracted immense attention for the generation of ultrashort laser pulses due to their good thermomechanical characteristics and wide emission bandwidths. In this work, a Gd-based orthophosphate crystal, GdSr3(PO4)3, (GSP), and a Nd3+-doped GdSr3(PO4)3 crystal, (Nd:GSP), were obtained by the Czochralski method. The crystal structure, optical properties, electronic band structure, laser damage threshold, and hardness of the GSP crystal were comprehensively investigated. It exhibited a disordered structure due to the random distribution of Sr and Gd atoms in the same Wyckoff site, which caused inhomogeneous spectral broadening. Additionally, it exhibited a short UV absorption cutoff edge (<200 nm), a large band gap (5.81 eV), and a high laser damage threshold (∼1850 MW/cm2). The spectral properties and Judd-Ofelt calculations of the Nd:GSP crystals were analyzed. A wide absorption band at 803 nm, with a full width at half-maximum value of 20 nm, makes the Nd:GSP crystal suitable for the efficient pumping of AlGaAs laser diodes. Sub-100-fs pulses could be supported by its 25 nm emission bandwidth. Hence, the GSP crystal could be a promising disordered crystal laser matrix.

A Miniature Fabry-Pérot Fiber Interference Sensor Based on Polyvinyl Chloride Membrane for Acoustic Pressure Sensing in Mid-High-Frequency Band.

In this paper, a Fabry-Pérot interference fiber sensor was fabricated by using a Polyvinyl chloride membrane (20 µm in thickness) attached at the end of a ferrule with an inner diameter of 1.1 mm. In consideration of the vibration response of the membrane, the feature of the first-order natural frequency of membrane was analyzed by COMSOL Multiphysics. The acoustic sensing performance of the Fabry-Pérot fiber interference sensor was studied in air. The results reveal that the sensor possessed good acoustic pressure sensitivity, in the order of 33.26 mV/Pa. In addition, the noise-limited minimum detectable pressure level was determined to be 58.9 µPa/Hz1/2 and the pressure-induced deflection obtained was 105 nm/Pa at the frequency of 1 kHz. The response of the sensor was approximately consistent with the reference sensor from 1 to 7 kHz. All these results support that the fabricated Fabry-Pérot fiber interference sensor may be applied for ultra-sensitive pressure sensing applications.

All-solid-state widely wavelength-tunable and high-efficiency Yb:YSr3(PO4)3 laser.

We demonstrate an all-solid-state widely wavelength-tunable Yb:YSr3(PO4)3 (Yb:YSP) laser with high efficiency. The free-running Yb:YSP laser oscillating at multiple wavelengths in the range of 1024-1054 nm is realized with different crystal lengths and output coupler transmittances. The maximum output power of 2.72 W is obtained under the absorption pump power of 7.30 W. The highest slope efficiency is 66.9%, using the crystal of 6.5-mm-length. Simultaneous dual-wavelength operation can be realized as well. Furthermore, the widely wavelength-tunable Yb:YSP laser with a range of more than 60 nm (from 1004 to 1066 nm) is achieved using a birefringent filter. The experimental results indicate that the Yb:YSP crystal can be a promising candidate for ultrafast lasers in the 1 µm region.

Synthesis, Electronic Structure, and Electrochemical Properties of the Cubic Mg2MnO4 Spinel with Porous-Spongy Structure.

Mg2MnO4 nanoparticles with cubic spinel structure were synthesized by the sol-gel method using polyvinyl alcohol (PVA) as a chelating agent. X-ray powder diffraction, infrared spectrum (IR), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to characterize the crystalline phase and particle size of as-synthesized nanoparticles. The electronic structure of Mg2MnO4 spinel was studied by X-ray photoelectron spectroscopy (XPS). The results showed that pure cubic Mg2MnO4 spinel nanoparticles were obtained when the annealing temperature was 500-700 °C. The samples had a porous-spongy structure assembled by nanoparticles. XPS studies indicated that Mg2MnO4 nanoparticles were mixed spinel structures and the degree of cation inversion decreased with increasing annealing temperature. Furthermore, the performance of Mg2MnO4 as lithium anode material was studied. The results showed that Mg2MnO4 samples had good cycle stability except for the slight decay in the capacity at 50 cycles. The coulombic efficiency (ratio of discharge and charge capacity) in most cycles was near 100%. The sample annealed at 600 °C exhibited good electrochemical properties, the first discharge capacity was 771.5 mAh/g, and the capacity remained 340 mAh/g after 100 cycles. The effect of calcination temperature on the charge-discharge performance of the samples was studied and discussed.

Remarkable temperature-dependent second-harmonic-generation performance of a YCOB crystal.

For the first time, the temperature stability of second-harmonic-generation (SHG) is reported for the entire space of a YCa4O(BO3)3 (YCOB) crystal for a temperature range of -10 - 520 °C. Both theoretical calculations and experimental data indicate an optimum phase-matching (PM) direction of (θ = 149.2°, ϕ = 0°), which is located in the XZ principle plane (90° < θ < 180°). A special regression phenomenon of the PM angle was found in this direction, which further increased the SHG output at high temperature (> 200 °C). As a result, for SHG of the Nd:YAG laser, the measured temperature bandwidth of a YCOB crystal cut along the optimum PM direction is larger than 490 °C·cm. As demonstrated in this study, among all nonlinear optical crystals, this cut-type is currently the best choice when temperature-insensitive SHG is required.

Direct Synthesis of Graphene Dendrites on SiO2/Si Substrates by Chemical Vapor Deposition.

The long-standing interest in graphene has recently brought graphene-derived materials including graphene hydrogel, graphene fiber and graphene paper into sharp focus. These graphene-derived materials show outstanding properties in mechanics and physics. In this paper, for the first time, we demonstrate the novel synthesis of graphene dendrites on SiO2/Si substrates by chemical vapor deposition. The tree-like graphene dendrites with well-controlled morphology can be directly grown on both the Si and the SiO2 surfaces of the substrates by using methane and hydrogen as precursors. The graphene dendrites on SiO2/Si substrates can be directly used in the fabrication of the electronic device. The conductivity and the Hall mobility of graphene dendrites are ~ 286 Scm-1 and ~ 574 cm2(Vs)-1, respectively. Young's modulus of graphene dendrites is up to 2.26 GPa. The developed method avoids the need for a metal substrate and is scalable and compatible with the existing semiconductor technology, making graphene dendrites be very promising in nanoelectronic applications.

Damage mechanism and electro-elastic stability of LiNbO3 crystals irradiated with 6 MeV Xe23.

The trigonal lithium niobate crystal (LiNbO3, LN) is a multi-functional material that possesses excellent nonlinear optical, pyroelectric and piezoelectric properties. In this work, the irradiation damage mechanism and stability of the electro-elastic properties of LN crystals irradiated with different doses (1013-1016 ions per cm2) of 6 MeV Xe23+ ions were evaluated for potential piezoelectric applications under irradiation conditions below 650 °C. The vacancy formation energies for Li, O, and Nb atoms are much lower than the irradiation energy of 6 MeV, with the lowest vacancy formation energy being obtained for Li, so that a high concentration of vacancies will be generated in LN upon irradiation. The vacancies narrow the band gap and decrease the electrical resistivity after irradiation. In contrast to the electrical resistivity, the relative dielectric permittivity of the LN crystal was found to increase with increasing irradiation dose, due to the weakened chemical bonds and distorted crystal structure, as confirmed by X-ray photoelectron spectroscopy. Despite the irradiation, the effective piezoelectric coefficients of bulk LN crystal remain nearly unchanged, indicating the favorable properties of LN for use under irradiation conditions at temperatures up to 650 °C.

A Strategy To Prepare High-Quality Monocrystalline Graphene: Inducing Graphene Growth with Seeding Chemical Vapor Deposition and Its Mechanism.

High-quality monocrystalline graphene has gained considerable attention in fundamental physics, materials science, and nanoelectronics. However, the performance of the graphene obtained by chemical synthesis methods is currently significantly restricted by the crystal quality. Herein, a seeding chemical vapor deposition (SCVD) method is designed to cultivate high-quality monocrystalline graphene on a Cu(111) substrate with hexagonal boron nitride (h-BN) as the seed crystal. Combining the experimental and theoretical research, the nucleation behavior of the growth-induced graphene on the h-BN seed crystal is investigated, and the induced growth mechanism on the Cu(111) substrate is studied. The results show that the h-BN seed crystal can dramatically reduce the adsorption energy of active carbon atoms and the energy barrier for C-C aggregation at the BN/Cu(111) step, thus promoting graphene growth around the h-BN seed. Large monocrystalline graphene domains are obtained by the proposed SCVD method. Further study shows that the growth-induced graphene has good crystal quality and could maintain high structural integrity. This new strategy can be applied for growing high-quality graphene and other two-dimensional materials.

Temperature bandwidth of second-harmonic-generation in GdCOB crystal.

The temperature bandwidth of the second harmonic generation (SHG) phase-matching process was investigated for the GdCa4O(BO3)3 (GdCOB) crystal. GdCOB exhibits a much broader temperature bandwidth in comparison with many familiar nonlinear optical (NLO) crystals. For a fundamental wave of 1,064 nm, the maximum temperature bandwidth appeared at (θ=135°, ϕ=47.3°), as predicted by the theoretical calculation and demonstrated by the SHG experiments. The GdCOB crystal is a good candidate for NLO frequency conversions under extreme temperatures.

Induced growth of quasi-free-standing graphene on SiC substrates.

Free-standing graphene grown on SiC substrates is desirable for micro- and nano-electronic device applications. In this work, an induced growth method to fabricate quasi-free-standing graphene on SiC was proposed, where graphene nucleation sites were generated on the SiC substrate and active carbon sources were subsequently introduced to grow graphene centered along the established nucleation sites. The structure and morphology of the cultivated graphene were characterized by using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). Compared to the traditional epitaxial growth methods on SiC substrates, this approach shows a significant reduction of the buffer layer. This study provides an efficient method for growing quasi-free-standing graphene on SiC substrates and is believed to be able to broaden the application of graphene in electronic devices as SiC is an intrinsically outstanding wide bandgap semiconductor.

Graphene Nucleation Preference at CuO Defects Rather Than Cu2O on Cu(111): A Combination of DFT Calculation and Experiment.

It is well-known that reducing the nucleation density is an effective way to enhance the growth quality of graphene. In this work, we explore the mechanism of graphene nucleation and growth around CuO defects on a Cu(111) substrate by using density functional theory combined with the nudged elastic band method. The defect formation mechanism at the initial nucleation stage is also studied. Our calculation results of the C adsorption energy and the reaction barrier of C-C dimer formation illustrate that the initial nucleation of graphene could be promoted by artificially introducing CuO defects on a Cu(111) surface and the nucleation on the clean Cu(111) substrate could thus be suppressed. These conclusions have been verified by graphene growth experiments using a chemical vapor deposition method. Further studies showed that graphene grown around CuO "seed crystals" could maintain its structural integrity without significantly producing defective carbon rings. This work provides a fundamental understanding and theoretical guidance for the controllable preparation of large-dimension and high-quality graphene by artificially introducing CuO seeds.

Double-mode lateral-field-excitation bulk acoustic wave characteristics of Ca3TaGa3Si2O14 crystals.

The double-mode lateral-field-excitation (LFE) bulk acoustic wave characteristics of Ca3TaGa3Si2O14 (CTGS) crystals are investigated. It is found that LFE devices based on (yxl)-57° CTGS crystals can work on both pure-LFE and pseudo-LFE modes when the driving electric field direction is normal to the crystallographic x axis of the piezoelectric substrate. Several double-mode LFE bulk acoustic wave devices based on CTGS crystals are designed and tested. The experimental results conform to the theoretical prediction well. Being able to operate in pure-LFE and pseudo-LFE modes, the double-mode LFE sensors show high sensitivity to both mechanical and electrical property changes of analytes. The results provide the crystal cut for double-mode LFE sensors, which is a critical basis of designing high-performance chemical and biological sensors by using double-mode LFE devices.