Effect of thickness variations of lithium niobate on insulator waveguide on the frequency spectrum of spontaneous parametric down-conversion

We study the effect of waveguide thickness variations on the frequency spectrum of spontaneous parametric down-conversion in the periodically-poled lithium niobate on insulator (LNOI) waveguide. We analyze several variation models and our simulation results show that thickness variations in several nanometers can induce distinct effects on the central peak of the spectrum, such as narrowing, broadening, and splitting. We also prove that the effects of positive and negative variations can be canceled and thus lead to a variation-robust feature and an ultra-broad bandwidth. Our study may promote the development of on-chip photon sources in the LNOI platform, as well as opens up a way to engineer photon frequency state.     

宽带泵浦激光的聚焦位置对SRS发生过程的影响

 结合SRS实验,重点研究了聚焦位置对Raman介质中的物理过程、Stokes光的输出特性的影响,比较了SRS阈值和转换效率的实验结果与理论预期。     

Classical and quantum self amplified spontaneous emission in a high gain free electron laser

We describe the quantum theory and the photon statistics of self amplified spontaneous emission (SASE) in a high gain free electron laser (FEL) using Glauber's quantum theory of coherence. We generalize a previous theory by taking into account many-mode effects and the initiation process resulting from classical shot noise, quantum noise, an injected coherent field and coherent bunching. In particular, we define the concept of quantum SASE which is appropriate when the initial quantum fluctuations dominate over the classical shot noise. We also discuss the conditions for the observation. Quantum SASE is a new quantum phenomenon in which the single electron uncertainty fluctuations of the conjugate variables position and momentum produce exponential amplification of the vacuum field.     

Nonstoichiometric laser materials: Designer wavelengths in neodymium-doped garnets

The tunable nature of lasers provides for a wide range of applications. Most applications rely on finding available laser wavelengths to meet the needs of the research. This article presents the concept of compositional tuning, whereby the laser wavelength is designed by exploiting nonstoichiometry. For research where precise wavelengths are required, such as remote sensing, this is highly advantageous. A theoretical basis for the concept is presented and experimental results in spectroscopic measurements support the theoretical basis. Laser operation nicely demonstrates the validity of the concept of designer lasers.     

Neutron Star Structure in a Nonlinear Realization of Chiral SU(3) Spontaneous Breaking Model

We investigate neutron star properties by constructing a chiral SU(3) spontaneous breaking Lagrangian and using relativistic mean-field approximation. The results show that π- condensate appears at some baryon densities,and hyperons ∑- and A exist in neutron star matter at high density. In this model, neutron star's maximum mass is1.12Ms with corresponding radius about 8 km.     

Spontaneous magnetization of the Ising model on a layered square lattice

We have studied the Ising model on a layered square lattice with four different coupling constants and two different magnetic moments. The partition function at zero magnetic field is derived exactly. We propose a formula for the spontaneous magnetization which agrees with the exact low-temperature series expansion up to the 16th order and reduces to the exact result of Au-Yang and McCoy in a special case.     

自发形核生长的AlN单晶湿法腐蚀研究

采用湿法腐蚀工艺,使用熔融态KOH和NaOH作为腐蚀剂,对一种物理气相传输(PVT)自发形核新工艺在2100~2250 ℃条件下生长的AlN单晶进行了腐蚀实验.通过实验及扫描电子显微镜(SEM)结果分析,得到了典型的AlN单晶c面、r系列面及m面最佳的腐蚀工艺参数及腐蚀形貌.另外,基于腐蚀形貌分析,发现了采用该自发形核新工艺生长的AlN晶体某些独特习性并计算出AlN单晶腐蚀坑密度(EPD).     

Critical behaviour of the order parameters at the SmC* to SmA phase transition in a ferroelectric liquid crystal mixture

Critical behaviour of the order parameters has been investigated in the ferroelectric liquid crystal mixture ZLI‐3654 in a 7.5 µm thick planar cell. The temperature dependence of the primary (tilt angle) and secondary (spontaneous polarisation) order parameters is considered. The critical exponent (β) has been evaluated from the fitting of the temperature dependence of the experimental data for both tilt angle and spontaneous polarisation. Experimental results are compared with the predictions of the de Gennes and Landau models.     

X-ray diffraction and dielectric spectroscopy studies on a partially fluorinated ferroelectric liquid crystal from the family of terphenyl esters

The fluorinated compound, (S)-4′′-(6-perfluoropentanoyoxyhexyl-1-oxy)-2′,3′-difluoro-4-(1-methylheptyloxycarbonyl)-[1,1′:4′,1′′]-terphenyl, which exhibits antiferroelectric SmC_A*, ferroelectric SmC* and paraelectric SmA* phases, has been investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and frequency-dependent dielectric spectroscopy methods. X-ray studies have revealed that the layer thickness remains almost constant in the SmA* phase but within the SmC* and SmC_A* phases it decreases with decreasing temperature, a step jump being observed only at the SmA*–SmC* transition. The tilt angle in the SmC_A* phase decreases from 22.2° to 19.5°, and in the SmC* phase it decreases from 18.8° to 5.5°. Spontaneous polarisation is found to be quite high and varies between 74.1 and 118.7 nC cm^?2. The variation in ε′ and ε′′ with temperature shows a discontinuous change at the transition temperatures. Goldstone mode relaxation is only observed in the ferroelectric and antiferroelectric phases and is found to be of the Cole–Cole type. The soft mode is observed on application of a bias field near the SmC*–SmA* transition. Neither the soft mode nor the anti-phase azimuthal angle fluctuation mode is observed in SmC_A*. Rotational viscosity decreases quite rapidly with temperature but in a different manner in the ferroelectric and antiferroelectric phases. Activation energy for this process is found to be 48.14 kJ mol^?1 in the SmC* phase.     

A spontaneous combustion reaction for synthesizing Pt hollow capsules using colloidal carbon spheres as templates

Here we report a spontaneous combustion reaction in synthesizing Pt hollow capsules. In brief, Pt nanoparticles were loaded on the surface of colloidal carbon spheres by wet-chemical impregnation. When Pt-loaded carbon spheres were taken out of an argon-filled tube furnace at room temperature and exposed to air, they underwent spontaneous combustion. The internal carbon spheres templates were removed to leave nanostructured Pt hollow capsules. There are at least two critical conditions for the occurrence of the spontaneous combustion: the Pt particle size is below 5.8 nm, and the hydrogen content in the carbon spheres is above 2.570 wt %. Such a reaction is interesting for the preparation of metal hollow spheres and is also relevant with respect to removal of accumulated carbon on catalysts and for soot oxidation at room temperature.