Glass transition in binary mixture of colloidal ellipsoids and spheres
The nature of glass and glass transition are considered to be one of the most fundamental research problems in condensed matter physics. Colloidal suspension provides a novel model system for studying glass and glass transition, since the structures and dynamics of a colloidal system can be quantitatively probed by video microscopy. Traditional systems for studying glass transition typically are single-component systems composed of either isotropic or anisotropic colloidal particles. Recently, glass transition of mixture of isotropic and anisotropic colloids has attracted great attention, such as the observation of rotational glass and translational glass, and the establishment of the two-step glass transition. Similarly, computer simulations have also shown that mixture of isotropic and anisotropic colloidal particles could manifest interesting, new glassy behaviors. However, the experimental study of the glass transition in such a colloidal mixture is still rare. In this paper, we experimentally investigate the glass transition of a binary mixture of colloidal ellipsoids and spheres. The colloidal spheres are polystyrene microspheres with a diameter of 1.6 μm, and the ellipsoids are prepared by physically stretching from polystyrene microspheres of 2.5 μm in diameter. The major and minor axes of the as-prepared ellipsoid are 2.0 μm and 1.2 μm, respectively. The mixture is confined between two glass slides to make a quasi-two-dimensional sample. To prevent the mixture from crystallizing, the mixing ratio of ellipsoids and spheres is chosen to be 1/4 in number, which is similar to the mixing ratio used in the classical Kob-Anderson model of binary sphere mixture. We systemically increase the area fraction of colloidal mixture to drive the glass transition. We then employ bright-field video microscopy to record the motion of the particles in the colloidal suspension at a single particle level, and the trajectories of individual particles are obtained by standard particle tracking algorithm. Through the analysis of radial distribution function, Voronoi diagram and local order parameter, we find that the ellipsoids can effectively inhibit the spheres from crystalizing, and the structure of the system remains disordered when increasing the area fraction. For dynamics, mean square displacement and self-intermediate scattering function are calculated. We find that the dynamic process of the system slows down substantially when increasing the area fraction, and the relaxation time of the system increases rapidly and diverges close to the glass transition point predicted by the mode coupling theory. Moreover, we analyze the fast particles that participate in cooperative rearrangement regions (CRRs) in the system, and find that the shapes, sizes and positions of CRRs are closely related to the locations of the ellipsoids in the system.
Abnormal viscosity changes in high-temperature metallic melts
Effects of organic cations on performance of halide perovskite solar cell
The halide perovskite solar cells employing CH3NH3PbX3 (X=Cl-, Br-, I-) and CH3NH3PbI3-xClx as light absorbers each have shown a rapid rise in power conversion efficiency (PCE) from 3.8% to 22.1% in recent years. The excellent photovoltaic performance is attributed to good optical and electrical properties such as appropriate bandgap, large absorption coefficient, high carrier mobility, long carrier lifetime and long carrier diffusion length. However, the physical mechanism of high PCE for halide perovskite solar cells is still unclear. The Gaussian 09 software is utilized to optimize the geometries of isolated CH3NH3+ and CH3NH3 at a B3 LYP/6-311++G(d, p) level, and the Multiwfn software is used to visualize the electrostatic potentials (ESPs) of CH3NH3+ and CH3NH3. Based on the ESPs of CH3NH3+ and CH3NH3, it is found that the CH3NH3+ has a strong electrophilic character, however, the NH3- side and CH3- side of CH3NH3 have weak nucleophilic and electrophilic character, respectively. So the electrostatic characteristics of CH3NH3+ and CH3NH3 are significantly different. The strong electrostatic repulsive interaction between two neighboring CH3NH3+ radicals plays an important role in structural phase transition of CH3NH3PbI3 material. At room temperature, the CH3NH3+ in the inorganic cage is activated and disordered, and has a strong electrophilic character. Due to these characteristics of CH3NH3+, the interfacial electrons at TiO2/CH3NH3PbI3 heterojunction are combined with CH3NH3+ to form CH3NH3 in the inorganic[PbI3]- framework. The CH3NH3 at the heterojunction under the built-in electric field is more easily oriented than CH3NH3+. Two initial geometrical configurations for CH3NH3+:CH3NH3 and CH3NH3:CH3NH3 dimers are optimized by using Gaussian 09 at an MP2/Aug-cc-PVTZ level. On the basis of the electrostatic characteristic of CH3NH3+:CH3NH3 dimer, the interfacial electrons at TiO2/CH3NH3PbI3 heterojunction are easily injected into the CH3NH3PbI3 material, which leads to the strong polarization of CH3NH3PbI3 material at the heterojunction. From the ESP of optimized CH3NH3:CH3NH3 dimer, it is found that the weak electrostatic field of the inorganic framework, parallel to C-N axis, is induced by the CH3NH3 orientational order, which is made for improving the photogenerated electron-hole pair separation and carrier transport. The TiO2/CH3NH3PbI3 heterojunction has more advantage than traditional p-n junction because of no consumption of carrier for CH3NH3PbI3 material in the process of forming built-in electric field. The physical mechanism is the origin of high PCE for CH3NH3PbI3 solar cells. According to the experimental results and first-principle calculations, we can draw an important conclusion that the electrostatic characteristics of organic CH3NH3+ cations in the inorganic[PbI3]- framework result in the high performances of halide perovskite solar cells.
Electronic structure and spin/valley transport properties of monolayer MoS2 under the irradiation of the off-resonant circularly polarized light
First-principles study on the optical properties of Fe-doped GaN
First-principles study of hydrogen storage properties of silicene under different Li adsorption components
Enhanced optical transmission by exciting hybrid states of Tamm and surface plasmon polaritons in single slit with multi-pair groove nanostructure
Electronic structures and ferroelectric properties of Ba-doped ZnO
Piezo-electrochemical coupling of AgNbO3 piezoelectric nanomaterials
In this work, the AgNbO3 piezoelectric nanomaterials are hydrothermally synthesized, and they have an average particle size of~1 μm, which is obtained from scanning electron microscopy pattern. The AgNbO3 nanomaterial possesses an orthorhombic crystal structure with an mm2 point group symmetry, indicated by the X-ray powder diffraction analysis result. The piezo-electrochemical coupling of AgNbO3 is characterized, and its physical mechanism is discussed. Under an external mechanical vibration, the surfaces of the piezoelectric AgNbO3 nanomaterials will generate a large number of positive and negative electric charges. Due to the existence of spontaneous polarization, these positive and negative electrical carriers are respectively distributed on the top surface and bottom surface of AgNbO3 and can further induce the generation of some strong oxidation middle active species such as hydroxyl radicals in solution on the basis of some special chemical redox reactions, realizing the piezo-electrochemical coupling. Therefore, we can consider the piezo-electrochemical coupling as the product of the piezoelectric effect and the electrochemical redox effect. Utilizing the strong piezo-electrochemical coupling, a practical application in mechano-catalysis is further developed to decompose dye solution under a driven vibration. After experiencing~60 min vibration with AgNbO3 nanomaterial as mechano-catalyst,~70% rhodamine B (~5 mg/L) is decomposed. Prior to the vibration, the rhodamine B solution with the addition of AgNbO3 catalyst is slowly stirred for 30 min to ensure the establishment of the physical adsorptiondesorption equilibrium between catalyst and dye. It is difficult to directly exert a mechanical stress on the micro/nanoparticles. Here, an ultrasonic source with a vibration frequency of~40 kHz is employed to exert a stress to compress and stretch the AgNbO3 particles through utilizing micro-bubble collapse forces during ultrasonic cavitations, which needs the AgNbO3 particle size to be roughly identical with the diameter (~μm) of micro-bubble. Our mechanocatalytic dye decomposition experiment is conducted at room-temperature and in a dark environment to avoid the influence of photocatalysis. The slight increase of temperature of the dye solution in the ultrasonic vibration process has no obvious influence on the dye decomposition efficiency, which has been confirmed from our experiment. Through a technology of fluorescence spectrum trapping, the intermediate active product in the piezo-electrochemical coupling process-the strongly oxidized hydroxyl radicals, is successfully observed. With the increase of vibration time, the number of hydroxyl radicals obviously increases, which proves that the piezo-electrochemical coupling plays a key role in our mechano-catalytic process. After using AgNbO3 catalyst in cyclic decomposition of rhodamine B 5 times, no obvious reduction in the piezo-electrochemical coupling performance occurs. The AgNbO3 nanomaterial possesses an efficient piezo-electrochemical coupling for mechano-catalysis, and it has the advantages of high decomposition efficiency and reusability, and potential applications in vibration decomposing dye.
Polarization-controlled dual-band broadband infrared absorber
Modes characteristics analysis of THz waveguides based on three graphene-coated dielectric nanowires
Development of quantum voltage noise source chip for precision measurement of Boltzmann constant
Molecular dynamics simulations on DNA flexibility: a comparative study of Amber bsc1 and bsc0 force fields
The structural flexibility of DNA plays a key role in many biological processes of DNA, such as protein-DNA interactions, DNA packaging in viruses and nucleosome positioning on genomic DNA. Some experimental techniques have been employed to investigate the structural flexibility of DNA with the combination of elastic models, but these experiments could only provide the macroscopic properties of DNA, and thus, it is still difficult to understand the corresponding microscopic mechanisms. Recently, all-atom molecular dynamics (MD) simulation has emerged as a useful tool to investigate not only the macroscopic properties of DNA, but also the microscopic description of the flexibility of DNA at an atomic level. The most important issue in all-atom MD simulations of DNA is to choose an appropriate force field for simulating DNA. Very recently, a new force field for DNA has been developed based on the last generation force field of Amber bsc0, which was named Amber bsc1. In this work, all-atom MD simulations are employed to study the flexibility of a 30-bp DNA with the force fields of Amber bsc1 and Amber bsc0 in a comparative way. Our aim of the research is to examine the improvement of the new development of force field (Amber bsc1) in the macroscopic and microscopic properties of DNA, in comparison with the corresponding experimental measurements. All the MD simulations are performed with Gromacs 4.6 and lasted with a simulation time of 600 ns. The MD trajectories are analyzed with Curves+ for the last 500 ns, since the system reaches equilibrium approximately after ~100 ns. Our results show that the new force field (Amber bsc1) can lead to the improvements in the macroscopic parameters of DNA flexibility, i.e., stretch modulus S and twist-stretch coupling D become closer to experimental measurements, while bending persistence lengths lp and torsional persistence lengths C from the two force fields (bsc1 and bsc0) are both in good agreement with experimental data. Our microscopic analyses show that the microscopic structure parameters of DNA from the MD simulation with the Amber bsc1 force field are closer to the experimental values than those with the Amber bsc0 force field, except for slide, and the obvious improvements are observed in some microscopic parameters such as twist and inclination. Our further analyses show that the improvements in macroscopic flexibility from the Amber bsc1 force field are tightly related to the microscopic parameters and their fluctuations. This study would be helpful in understanding the performances of Amber bsc1 and bsc0 force fields in the description of DNA flexibility at both macroscopic and microscopic level.
Objective assessment method of image quality based on visual perception of image content
Concentrating characteristics of Fresnel lens with prism secondary concentrator and optimization of high concentrating photovoltaic module with triple-junction cell
Compressed sensing based fast method of solving the electromagnetic scattering problems for threedimensional conductor targets
The method of moments is one of the most commonly used algorithms for analyzing the electromagnetic scattering problems of conductor targets. However, it is difficult to solve the matrix equation when analyzing the electromagnetic scattering problem of the electric large target. In recent years, the theory of the compressed sensing was introduced into the method of moments to improve the computation efficiency. The random selected impedance matrix is used as a measurement matrix, and the excitation voltage is used as a measurement value when using compressed sensing theory. The recovery algorithm is used to solve the induced current of target. The method can avoid the inverse problem of matrix equation and improve the computational efficiency of the method of moments, but it can be applied only to 2-dimensional or 2.5-dimensional target. The application of compressed sensing needs to know the sparse basis of the current in advance, but the induced current of three-dimensional target which is expressed by an Rao-Wilton-Glisson basis function is not sparse on the commonly used sparse basis, such as discrete cosine transform basis and discrete wavelet basis. To solve this problem, a method of combining compressed sensing with characteristic basis functions is proposed to analyze the electromagnetic scattering problem of three-dimensional conductor target in this paper. The characteristic basis function method is an improved method of moments. The target is divided into several subdomains, the main characteristic basis functions are comprised of current bases arising from the self-interactions within the subdomain, and the secondary characteristic basis functions are obtained from the mutual coupling effects of the rest of the subdomains. Then a reduction matrix is constructed to reduce the order of matrix equation, and the current can be expressed by the characteristic basis function and its weighting coefficient. In the method presented in this paper, the weighting coefficient is considered as a sparse vector to be solved when the characteristic basis function is used as sparse basis. The number of weighting coefficients is less than the number of unknown ones, so it can be obtained from the compressed sensing recovery algorithm. At the same time, the generalized orthogonal matching pursuit algorithm is used as the recovery algorithm to speed up the recovery process. Finally, the proposed method is used to calculate the radar cross sections of a PEC sphere, nine discrete PEC targets and a simple missile model. The numerical results validate the accuracy and efficiency of the method.
Novel dual channel polarization interference imaging system
The interference images with fixed spectral resolution can be obtained by using the existing static polarization-difference imaging system because the optical path of the system cannot be changed flexibly. However, for different detection targets, the spectral resolution of the system determined by the optical path difference must be appropriate. To satisfy a variety of application requirements, a novel dual channel polarization-difference interference imaging system (DPDⅡS), based on the lateral shear of the wide-field-of-view Savart polariscope (WSP) and the modulated Savart polariscope (MSP), is presented. The two-dimensional space images of a target and orthogonal interference images can be obtained by adjusting the MSP under different lateral displacements simultaneously. In addition, the remarkable characteristics of the system avoid spilling over rays and optimizing the system optical path effectively. In this paper, by using the Jones matrix, the system structure is demonstrated and the theoretical principle of DPDⅡS is analyzed in detail. The amplitudes of the four beams from the MSP and the interference intensity expressions of the coherent light are derived. Then the splitting characteristics of the Savart polariscope (SP) and WSP are presented. It is concluded that the WSP has better shear ability than SP and the WSP can optimize the optical path effectively compared with Wollaston prism in the DPDⅡS. The change ranges of the optical path difference and lateral displacement produced by the MSP for structure angles α=π/3, π/4, π/6 are analyzed in detail. The reconstructed orthogonal interferograms and the experimental interferograms under 632.8 nm monochromatic light for dMSP=1.00, 1.10, 1.20, 1.30 mm are obtained. A comparison between the experimental interference images and the simulated images proves that the interference fringes with different resolutions can be obtained simultaneously by adjusting the MSP. Meanwhile, the light intensities of the double optical paths are approximately equal and the same optical path difference is generated for the dual channel with the movement of MSP. The experimental results are consistent with the theoretical analyses. The spatial images of parallel and vertical components are detected under 632.8 nm polychromatic light. Then the total intensity image and the polarization-difference image are obtained through data processing. The conclusion that the polarization difference intensity image has a high resolution compared with the polarization intensity image is presented. The study has reference significance and practical value for the dual channel polarization interference imaging system.
Multiconfiguration time-dependent Hartree-Fock treatment of electron correlation in strong-field ionization of H2 molecules
H/D + Li2 → LiH/LiD + Li reactions studied by quantum time-dependent wave packet approach
The isotopic effect is a significant way to further understand the reaction mechanism without greatly changing the system. However, the isotopic effect of the H + Li2 reaction has received little attention in previous theoretical studies. Furthermore, as a deep potential well exists on the reaction path, obtaining convergent result is very time-consuming. So some approximate methods were used in previous theoretical calculations. However the Coriolis coupling effect plays an important role in the reaction, and thus whether these approximate methods are reasonable needs further testing. Based on the potential energy surface (PES) reported by Song et al., the dynamical calculations of H/D + Li2→ LiH/LiD + Li reactions are carried out by time dependent quantum wave packet method with second order split operator in a collision energy range from 0 to 0.4 eV. In order to obtain the convergent results, lots of convergence tests are carried out and because the Coriolis coupling effect plays an important role in the reaction, all the number of projections of total angular momentum J are included in the present calculation. The dynamical properties such as reaction probability, integral cross section, differential cross section are calculated and compared with previous theoretical values. Large discrepancies are found between present results and the values obtained from Gao et al. especially at high collision energies. Owing to the fact that the same PES is applied to the calculation and Gao's results of total angular momentum J=0 accord well with the present values, we suppose that the parameters used in the calculation have little influence on the final results and the main discrepancies are attributed to the number of projections of total angular momentum which are cut off in Gao et al.'s calculation. In order to verify our speculation, the numbers of projections of total angular momentum which are 1, 5, 10, 15, 20, and 25, are considered in the calculation, respectively. The results indicate that the main discrepancy between present values and the results obtained from Gao et al. can be attributed to the number of projections of total angular momentum used in Gao et al.'s calculation that is not convergent, and that the present values are more accurate than previous theoretical studies for all the numbers of projections of total angular momentum which are included in the calculation. Furthermore, when the H atom is substituted by the heavy isotope D atom, the reaction probability and integral cross section become large. However, it does not generate large effect on the reaction mechanism. The forward and backward symmetry differential cross section signals indicate that the complex forming reaction mechanism dominates the reaction.
Focusing properties of Lucas sieves
A kind of optical diffractive element named photon sieve, which is essentially Fresnel zone plate in which the transmissive rings are replaced with a large number of randomly distributed isolated pinholes, can be used to focus X-ray and extreme ultraviolet lithography spectrum into spots with sizes smaller than the diameter of the smallest circular pinhole. However, both the traditional photon sieves and Fibonacci sieves have no more than two axial foci. In order to break this limitation, the Lucas sequence is introduced into the design of photon sieves, and thus producing four axial foci. With respect to the previous Fibonacci sequence, Lucas sequence has the same recursion relation as well as the same eigenvalue of golden mean γ=(1 + √5)/2. The only difference between them is the first two initial seeds. Based on Fresnel-Kichhoff diffraction theory, the simulation results show that there exist four focal spots with approximately equal intensity along the optical axis on condition that the hole diameters are set to be 1.16 times the underlying Fresnel zone width. Then in order to verify the validity of our proposed model, a Lucas sieve of diameter 12.11 mm and referred focal length 180 mm is fabricated by photolithography and its focusing properties are precisely measured by the in-line phase-shifting digital holography. In experiment, a quarter wave plate is used to realize two-step phase-shift interferences, and obtain the quad-focal length by auto-focusing algorithm in holography. Meanwhile, the quad-focal spots can also be calculated through the diffraction propagation of reconstructed object wave. Compared with the theoretical values, the measurement results indicate that the maximum deviation of quad-focal lengths is less than 0.9%, and the relative errors of the full width at half maximum of four Airy spots are all less than 5%. The experimental results agree well with the theoretical analysis results. Owing to the advantages of small volume, little weight and easy processing, Lucas sieve has great potential in X-ray microscopy, array imaging for living biological cell and especially in the next generation of synchrotron light sources.
Transmission matrix optimization based on singular value decomposition in strong scattering process
In the last decade, the scattering medium has been gradually attacking attention from researchers. Among the proposed approaches, the transmission matrix (TM) is considered as an effect way to describe the scattering properties which relate to input optical and output optical fields. However, the acquired transmission matrix and its eigenvalues strongly depend on the experimental conditions, such as the numbers of input channels (limited numerical aperture and illumination area, or the pixel number of the spatial light modulator) and output channels. In other words, the actual transmission matrix of the scattering medium is the acquired transmission matrix with infinite numbers of the input and output channels. We propose an approach to obtaining the actual matrix by evaluating its eigenvalues. First, the matrix is expressed by the singular value decomposition to obtain its inverse matrix. Then first level optimization is to dispose of some extreme singular values to remove the ill-conditioned problem of the matrix, and then, as a second level optimization, the genetic algorithm is to remove the eigenvalues which have the negative contributions to the intensity of the selected focal point. Our experiments show that the gray value of the intensity and the signal-to-noise ratio (SNR) of the focal point after employing the phase pattern are 129 and 7.54, respectively. After the first level optimization, the gray value of the intensity and the SNR rise to 172 and 9.73, respectively. Then, they reach to 192 and 10.29, respectively, after adopting the genetic algorithm. After the second level optimizations, the intensity at the focal point increases 48.8% compared with the case with just the optimized phase pattern from the acquired TM, and the SNR increases by nearly 36.5%. The reason behind the increase of the intensity after the optimizations, we believe, is that the transmission matrix of the scattering medium reaches its actual matrix in certain conditions. The proposed approach opens the way to obtaining the actual transmission matrix by mathematic optimizations without increasing the experimental levels.
Entanglement characteristics of output optical fields in double-cavity optomechanics
Radiation pressure in an optomechanical system can be used to generate various quantum entanglements between the subsystems. Recently, one paid more attention to the study of quantum entanglement in an optomechanical system. Here in this work, we study the properties of output entanglement between two filtered output optical fields by the logarithmic negativity method in a double-cavity optomechanical system. Our calculations show that the decay rate of the mechanical resonator, the bandwidth of filter function, and non-equal-coupling will evidently affect the value of the output entanglement. In particular, under the parameters of equal-coupling and zero filter bandwidth, the output entanglement in the vicinity of resonant frequency (ω=0 in the rotating frame) will decease with mechanical decay rate increasing. But under the parameters of equal-coupling and non-zero filter bandwidth, the output entanglement will be suppressed if the center frequency of output field is in the vicinity of the resonant frequency. However, the output entanglement can be enhanced if we adopt a non-equal-coupling to counteract the suppression effect of the filter bandwidth. Furthermore, we find that there are three peaks in the whole center frequency domain of the output field if we adopt strong non-equal-coupling. This is because the normal mode of Hamiltonian Hint will split into three normal modes in this case. Our results can also be used in other parametrically coupled three-mode bosonic systems and may be applied to realizing the state transfer process and quantum teleportation in an optomechanical system.
Supperssion of higher order modes in gain-guided index-antiguided planar waveguide laser
In order to suppress the higher order modes and improve beam quality in high power waveguide laser, based on gainguided index-antiguided theory, a new symmetric layered waveguide structure is designed, and an interval layer is proposed to be sandwiched between waveguide layer and cladding layer in traditional symmetric GG-IAG waveguide structure. As a result, while reducing the leakage loss of fundamental mode, the threshold gain coefficient differences between fundamental mode and higher order modes will be further increased. When the gain in waveguide layer is between threshold gain coefficient of fundamental mode and that of higher order mode, the fundamental mode will have a greater advantage in mode competition than others, so higher order modes can be suppressed and the laser can obtain a single mode output. In the meantime, the guided-mode principle of this waveguide structure is explained with the theory of wave optics in this paper, the eigen equation of each mode is derived from the wave equation, and the field distributions of fundamental mode and higher order mode are also given. Additionally, in this paper we give the solution process of the threshold gain coefficient of each mode in this waveguide structure. The mode leakage losses of fundamental mode and higher order mode, after adding the interval layer, are numerically calculated, and the parameter optimization process of the interval layer is also given in this paper. In addition, the field distributions of fundamental mode and higher order mode are numerically simulated. The calculation results show that comparing with the traditional symmetric GG-IAG planar waveguide, after adding the interval layer, the loss of fundamental mode can be greatly reduced, while ensuring that the leakage loss of higher order mode reaches a maximum value by reasonably controlling the parameters of interval layer. In this way, we can suppress higher order modes and improve laser efficiency. This paper provides a new idea for improving the beam quality of high power waveguide laser with a large mode area.
Optimized design and epitaxy growth of high speed 850 nm vertical-cavity surface-emitting lasers
Using transfer matrix method and TFcalc thin film design software,the reflectance spectrum of distributed Bragg reflector (DBR) and vertical cavity surface emitting laser (VCSEL) are simulated.The reflectance spectra from the cavity and surface are compared with each other,thus providing the basis for white light source (WLS) optical reflectance spectrum of the VCSEL epitaxial wafer.When using WLS to characterize VCSEL wafer,it is necessary to combine the simulation results and the shape of optical reflectance spectrum to speculate the reflectance seen from the cavity.The influences of different cap layers on the reflectance of DBRs are discussed theoretically and experimentally.With a 1/4λ GaAs cap layer,the reflectance reaches up to 97.8% seen from the cavity.This design can make the wavelength of the VCSEL etalon picked easily because of avoiding the influence of test noise.
The active region has higher heat accumulation due to the small area and poor thermal conductivity.The characteristics of the gain spectrum of InGaAs/AlGaAs strained quantum well (QW) under different temperatures and the temperature distribution in VCSEL are simulated by Crosslight software.The gain-to-cavity wavelength detuning is used to improve the slope efficiency and the temperature stability.The temperature in active region ranges from 360 K to 370 K.The gain peak wavelength and the Fabry-Perot cavity wavelength are designed in the ranges of 829-832 nm and 845-847 nm,respectively.Epitaxial wafer with top-emitting VCSEL structure grown by metal-organic chemical vapor deposition is characterized.The room temperature photoluminescence peak is at 827.5 nm and the etalon cavity wavelength measured by optical reflectance is 847.7 nm,which are consistent with designed values.
The oxide restricted VCSELs with 7.5 μm oxide aperture are fabricated.The image of the infrared light source CCD shows that the oxide aperture is circular.A passivation layer of 120 nm SiO2 is finally deposited to insulate water vapor.The threshold current is 0.8 mA,and the maximum output power reaches up to 9 mW at 13.5 mA.The optical spectrum at 6.0 mA reveals multiple transverse modes.The center wavelength is 852.3 nm and the root mean square (RMS) spectrum width is 0.6 nm,meeting the high-speed Datacom standards.Shannon theory indicates that the maximum data rate is not only proportional to bandwidth but also related to signal-to-noise ratio (SNR).It is effective to reduce relative intensity noise and enhance the SNR by increasing output power.From the eye diagram of 25 Gbit/s on-off key VCSEL,it is demonstrated that fall time is 38.66 ps,rise time is 41.54 ps,SNR is 5.6,and jitter RMS is 1.57 ps.Clear eye opening is observed from eye diagram of 25 GBaud/s PAM-4 VCSEL,which indicates the qualified 50 Gbit/s high speed performance.
Measurement of molecular absorption spectrum with a laser locked on a high-finesse cavity
High-resolution and high-sensitivity molecular spectroscopy is widely used in fundamental molecular physics, atmospheric studies, remote sensing, industrial process monitoring, and medical diagnostics. Accurate determination of the parameters of molecule absorption lines, such as line positions, line strengths, line widths and profiles, is essential to support these studies and applications. For example, in order to retrieve the column density of carbon dioxide with a precision of one part per million (ppm), we need laboratory data of line positions with a uncertainty lower than 0.3 MHz and line intensities with a relative accuracy better than 0.5%.
Here we present precision spectroscopy of molecules using a laser locked with a high-finesse cavity. The cavity made of invar is thermo-stabilized to reduce the drifts of its length and the cavity mode frequencies. The frequency of the probe laser is locked on a longitudinal mode of the cavity by using the Pound-Drever-Hall method. Another beam from the probe laser, which is frequency shifted and on resonance with a nearby longitudinal mode of the cavity, is used for cavity ring-down spectrum (CRDS) measurement. The CRDS absorption spectrum is recorded by stepping the modulation frequency of a fiber electro-optic modulator in increment of the mode spacing of the cavity. Note that the cavity mode frequencies are shifted due to the dispersion introduced by the absorption lines. Prior to the CRDS measurements, the transmittance spectra of the cavity modes are recorded by scanning the probe laser frequencies over the resonance, which allows the determination of the cavity mode frequencies with an accuracy at a Hz level. Therefore, a dispersion spectrum is also obtained using the same setup by measuring the frequency shifts of cavity modes of the samples with and without absorption. The absolute frequency of the probe laser is determined by an optical frequency comb referring to a GPS-disciplined rubidium clock. The long term drift of beat frequency between the optical frequency comb and the probe laser is measured to be about 1.8 MHz per hour, which is consistent with the thermal expansion of the cavity under a temperature drift of 50 mK.
The performance of the spectrometer is demonstrated by measuring the Doppler-broadened spectra of CO2 around 6470.42 cm-1. Precise spectroscopic parameters are derived from both the absorption and dispersion spectra recorded by the same spectrometer. The line position is determined with an accuracy of 0.18 MHz, which is over one order of magnitude better than those given in previous studies and spectral databases.
Frequency locking of fiber laser to 1530.58 nm NH3 sub-Doppler saturation spectrum based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy technique
Optimized linear wavenumber spectrometer based spectral-domain optical coherence tomography system
Principle analysis of snapshot Mueller matrix imaging polarimeter using birefringent crystal
Characteristics of granular sheet of dense granular jet oblique impact
Elastic behavior of glass-rubber mixed particles system