Intersubband transitions in Al0.82In0.18N/GaN single quantum well

The influence of the width of a lattice-matched Al_0.82In_0.18N/GaN single quantum well (SQW) on the absorption coefficients and wavelength of the intersubband transition (ISBT) has been investigated by solving the Schrödinger and Poisson equations self-consistently. The wavelength of 1—2 ISBT increases with L, the thickness of the single quantum well, ranging from 2.88 μm to 3.59 μm. The absorption coefficients of 1—2 ISBT increase with L at first and then decrease with L, with a maximum when L is equal to 2.6 nm. The wavelength of 1—3 ISBT decreases with L at first and then increases with L, with a minimum when L is equal to 4 nm, ranging from approximately 2.03 μm to near 2.11 μm. The absorption coefficients of 1—3 ISBT decrease with L. The results indicate that mid-infrared can be realized by the Al_0.82In_0.18N/GaN SQW. In addition, the wavelength and absorption coefficients of ISBT can be adjusted by changing the width of the SQW.     

Tunnett

Conclusion The p⁺–n⁺–i(v)–n⁺ GaAs Tunnett diodes have been fabricated with control of the growing diffusion technique in TDM CVP liquid phase epitaxial growth. The threshold current density (J_th) of 2.8×10⁴ A/cm², the threshold voltage (V_th) of 5.3V for the oscillation and the 1% duty cycle operation have been succeeded. The more reduction of J_th and V_th will be able to realize the CW operation.     

Fabrication and low threshold current density CW operation of GaInAsP/InP multiple-reflector microcavity laser

We report CW operation of a GaInAsP/InP multiple-reflector microcavity (MRMC) laser operated at fairly low threshold current density. The threshold current density with broad contact (stripe widthW=240 m, cavity lengthL=60 m) under pulsed operation was 180 A cm^–2 (l _th=20 mA), and was 230 A cm^–2 under CW operation at room temperature operating at 1.52 m wavelength.     

WnNim(n+m=8)团簇的极性和光谱性质的理论研究

采用密度泛函理论(DFT)中的B3LYP方法,在LANL2DZ基组水平上对WnNim(n+m=8)团簇的各种可能构型进行了几何参数全优化,得到了它们的基态构型;并对基态构型的偶极距、极化率、红外光谱和拉曼光谱性质进行了分析,结果表明:团簇WnNim(n+m=8)都具有极性,富W团簇非线性光学效应强,容易被外加场极化;振动频率主要分布在0-350 cm-1范围内,团簇W4Ni4因其振动方式的特殊性,红外光谱和拉曼光谱在频率421.971 cm-1处,都有明显强峰;团簇W5Ni3因其结构的高对称性在振动光谱中出现多处共振现象.     

Overshoot inversion of a Nd laser system

An important parameter of pulsed solid state lasers is the maximum inversion n_max which has to exceed the threshold inversion n_th to start the laser. The overshoot Δn=n_max-n_th is determined experimentally for a Nd-glass laser, operating in the TEM₀₀ mode and compared with theoretical predictions. Both investigations indicate a relation between normalized pump rate and inversion overshoot of the type Δn ∝ (W/W_th-1)^{$\text{1}\text{2}$}.     

Evaluating the Quantum Confinement Effect of Isolated ZnO Nanorod Single-Quantum-Well Structures Using Near-Field Ultraviolet Photoluminescence Spectroscopy

Using low-temperature near-field spectroscopy, we obtained spatially and spectrally resolved photoluminescence (PL) images of individual ZnO nanorod single-quantum-well structures (SQWs) with a spatial resolution of 20 nm. We observed the dependence of the quantum confinement effect of the PL peak on the well width (L _aw), from which the linewidths of near-field PL spectra of ZnO nanorod SQWs (L _aw = 2.5 and 3.75 nm) were determined to be as narrow as 3 meV. However, near-field PL spectra of individual SQWs with L _aw = 5.0 nm exhibited two PL peaks, presumably due to strains or defects in the ZnMgO in the nanorod SQWs. Since the exciton in a quantum structure is an ideal two-level system with long coherence times, our results provide criteria for designing nanophotonic devices.     

Modifying the KP,then th constrained KP hierarchies and their Hamiltonian structures

The Kadomtsev-Petviashvili (KP) hierarchy has infinitely many Hamiltonian pairs, then ^th pair of them is associated withL ⁿ , whereL is the pseudodifferential operator (PDO) [3,4]. In this paper, by the factorizationL ⁿ =L _n ...L ₁ withL _j ,j=1,...,n being the independent PDOs, we construct the Miura transformation for the KP, which leads to a decomposition of the second Hamiltonian structure in then ^th pair to a direct sum. Each term in the sum is the second structure in the initial pair associated withL _j . When we impose a constraint (1.9) (i.e a new type of reduction) to the KP hierarchy, we obtain the similar results for the constrained KP hierarchy. In particular the second Hamiltonian structure for this hierarchy is transformed to a vastly simpler one.     

LP MOVPE growth of AlGalnP/GalnP and its application to visible laser diodes

High-quality and uniform bulk layers of (Al _x Ga_1–x )_0.5In_0.5P (x=0–0.7) and AlGalnP/GainP quantum wells (QWs) are grown on 2°-off (100) GaAs substrates by low-pressure metal organic vapour phase epitaxy at a low growth rate of 0.3 nm s^-1. The amount of lattice mismatch and the variation of PL peak energy of (Al_0.5Ga_0.5)_0.5In_0.5P on the 50-mm substrate are less than 6×10^-4 and 2 meV, respectively. (Al_0.5Ga_0.5)_0.5In_0.5P/Ga_0.5In_0.5P SQWs show narrow PL spectra even from a 0.6 nm well measured at 20 K. The variation of PL peak energy from (Al_0.5Ga_0.5)_0.5In_0.5P/Ga_0.5In_0.5P MQWs is less than 10 meV. Also, as-cleaved AlGalnP/GalnP lasers fabricated by a three-step MOVPE show a pulsed threshold current of 82 mA at room temperature, output power of 12 mW, and the lasing wavelength at 668.2 nm.     

Radiation characteristics of injection lasers based on vertically coupled quantum dots

We have studied injection lasers based on InGaAs/GaAs vertically coupled quantum dots (QD) grown by molecular beam epitaxy. The threshold current density decreases by one order of magnitude down to 90 A cm⁻²(300 K) with an increase of the number of QD stacks (N) up to 10. ForN≥ 3 lasing occurs via the QD ground state up to room temperature. Differential efficiency increases withNup to 50%. No change in range of high temperature stability of threshold current density (J_th) was observed, while the characteristic temperature (T₀) measured at 300 K increases from 60 to 120 K. Using InGaAs-AlGaAs QD with higher localization energy allowed us to decreaseJ_thdown to 60 A cm⁻²and to increase the differential efficiency up to 70%.     

Electronic structure and infrared spectrum of a WnC0＇± （n= 1-6） cluster

W_nC^0,± (n=1-6) clusters are investigated by using the density functional theory (DFT) at the B3LYP/LANL2DZ level. We find that the neutral, anionic and cationic ground state structures are similar within the same size, and constituted by substituting a C atom for one W atom in the structures of W_n+1 clusters. The natural bond orbital (NBO) charge analyses indicate that the direction of electron transfer is from the W atom to the 2p orbital of the C atom. In addition, the calculated infrared spectra of the W_nC^0,± (n=2-6) clusters manifest that the vibrational frequencies of neutral, anionic and cationic clusters are similar in a range of 80 cm^-1-864 cm^-1. The high frequency, strong peak modes are found to be an almost stretched deformation of the carbide atom. Finally, the polarizabilities of W_nC^0,± (n=1-6) clusters are also discussed.     

Design considerations and scaling laws for high power convective cooled CW CO2 lasers

Various criteria for designing high power convective cooled CO₂ lasers have been discussed. Considering the saturation intensity, optical damage threshold of the optical resonator components and the small-signal gain, the scaling laws for designing high power CW CO₂ lasers have been established. In transverse flow CO₂ lasers having discharge of square cross-section, the discharge lengthL and its widthW for a specific laser powerP (Watt) and gas flow velocityV (cm/s) can be given byL = 1.4 x 10⁴ p ^1/2 V ^-1 cms andW = 0.04P ^1/2 cms. The optimum transmitivity of the output coupler is found to be almost constant (about 60%), independent of the small signal gain and laser power. In fast axial flow CO₂ lasers the gas flow should be divided into several discharge tubes to maintain the flow velocity within sonic limit. The discharge length in this type of laser does not depend explicitly on the laser power, instead it depends on the input power density in the discharge and the gas flow velocity. Various considerations for ensuring better laser beam quality are also discussed.     

Free-field realization of theWKP(N) algebra

TheW _KP ^(N) algebra has been identified with the second Hamiltonian structure in theNth Hamiltonian pair of the KP hierarchy. In this Letter, by constructing the Miura map that decomposes the second Hamiltonian structure in theNth pair of the KP hierarchy, we show thatW _KP ^(N) can also be decomposed toN independent copies ofW _KP ⁽¹⁾ algebras, therefore its free-field realization can be worked out by constructing free fields for each copy ofW _KP ⁽¹⁾ . In this way, the free fields may consist ofN + 2n number of bosons, among them, 2n are in pairs, wheren is an arbitrary integer between 1 andN. We also express the currents ofW _KP ^(N) in terms of the currents ofN —n copies of U(1) andn copies of SL(2,R) _k algebras with levelk = 1. By reductions, we give similar results forW ^(N) andW ₃ ⁽²⁾ algebra.     

对以并五苯和酞菁铜为不同有源层的有机薄膜晶体管特性研究

通过扫描电镜和X射线衍射对SiO₂衬底上生长并五苯和酞菁铜薄膜的表面形貌进行表征,并得到在SiO₂衬底上生长的并五苯薄膜是以岛状结构生长,其大小约为100nm,且薄膜有较好的结晶取向,呈多晶态存在. 酞菁铜薄膜则没有表现出明显的生长机理,其呈非晶态存在. 还对通过掩膜的方法制作得以酞菁铜和并五苯为有源层的顶栅极有机薄膜晶体管的特性进行了研究. 有源层的厚度为40nm,绝缘层SiO₂的厚度为250nm,器件的沟道宽长比(W/关键词： 有机薄膜晶体管 并五苯薄膜 酞菁铜薄膜 μ_EF)')" href="#">场效应迁移率(μ_EF)     

The integrable hierarchy constructed from a pair of KdV-type hierarchies and its associatedW algebra

For any two arbitrary positive integers n and m, using them ^th KdV hierarchy and the (n+m)^th KdV hierarchy as building blocks, we are able to construct another integrable hierarchy (referred to as the (n, m)^th KdV hierarchy). TheW-algebra associated to the second Hamiltonian structure of the (n, m)^th KdV hierarchy (calledW(n, m) algebra) is isomorphic via a Miura map to the direct sum of aW _m -algebra, aW _n+m -algebra and an additionalU(1) current algebra. In turn, from the latter, we can always construct a representation of aW -algebra.     

Response times for a storage electrooptic effect in liquid crystals

By the optical storage effect in large pitch cholesterics, a relation of the type t_W (U_W-U_th)^-2 is established between the writing time t_W and the writing voltage U_W, revealing the existence of a threshold voltage U_th. Interpretations are given for the different erasure times observed at two selected scattering angles.     

W_n Ni_m(n+m≤7;m=1,2)团簇电子结构与光谱性质的理论研究

采用密度泛函理论(DFT)中的B3PW91方法在LANL2DZ基组水平上对WnNim(n+m≤7;m=1,2)团簇的各种可能构型进行了几何结构优化,得出了它们的基态构型,并对其NBO、振动频率、光谱和极化率进行了理论研究.研究结果表明:W,Ni原子内部杂化现象较强,而在W-Ni原子之间杂化较弱;在W和Ni相互作用形成合金团簇的过程中,发生原子间的电荷转移,使得合金团簇中大多数Ni原子带正电荷W原子带负电荷;从光学上分析显示,W6Ni团簇的IR和Raman谱中的振动峰最多,W5Ni2的IR和Raman谱中的振动峰最强,W2Ni的IR谱中只有一个较强峰值;WnNim(n+m≤7;m=1,2)团簇中原子间的成键相互作用随W成分的增加而增强.     

The optical properties of ZnO/ZnMgO single quantum well grown by P-MBE

In this paper, ZnO/Zn_0.9Mg_0.1O single quantum well (SQW) structures were fabricated on c-plane sapphire (Al₂O₃) substrate by plasma-assisted molecular beam epitaxy (P-MBE). The photoluminescence (PL) peak of the SQW shifted from 3.31 to 3.37 eV as the well layer thickness was decreased from 6 to 2 nm. The spectral linewidth increases with temperature due to the scattering of excitons with acoustic and optical phonons. The transition energy of the localized exciton in the ZnO/Mg_0.1Zn_0.9O SQW with well width of 3 nm was found to be about 3.407 eV at 80 K, consistent with theoretical calculation. The first subband energies in the conduction and valence band were calculated to be 49 and 11 meV, respectively.     

Effect of pressure on the properties of DH diode lasers in AlxGa1-xAsySb1-y/GaSb system

Abstract

A helium pressure appparatus for diode laser studies up to 1.4 GPa at 77–300 K has been developed. DH lasers with Al_xGa_1-xAs_ySb_1-y active layers (x=0-0.05) lattice-matched to GaSb substrates have been investigated. It has been shown that in lasers with x,y=0 pressure dependences of the threshold current density (J_th) and the average electron lifetime at the threshold (τ) measured at 80 K depend strongly on the quadratic recombination of L^c ₆ electrons, the characteristic coefficient being 1.5×10^?11 cm³s^?1. The pressure-composition equivalence coefficient dx/dP=2.2×10^?10 Pa^?1 has been obtained for the lowest temperatures used.     

Amplification in one-dimensional random active medium near the lasing threshold

We studied the transmittance of a random amplifying medium near the lasing threshold by using the convergence criterion proposed by Nam and Zhang [Phys. Rev. B 66 73101, 2002] that allows separating the physical solutions of the time-independent Maxwell equations from the unphysical ones and describing critical size L _c of a random system in statistical terms. We found that the dependence of the critical gain ∈ ^″ _c (at which the lasing threshold occurs) as a function of number of layers is configuration-dependent and thus the lasing condition for random media is described by means of the probability of finding of physical solutions and evaluated by averaging over the ensemble of random configurations. By employing this approach we inspect the validity of the two-parameter scaling model by Zhang [Phys. Rev. B 52 7960, 1995], according to which the behavior of the random system with gain is described by relation 1/ξ = 1/ξ ₀ + 1/l _g, where ξ and ξ ₀ are localization length with and without gain, respectively, and l _g = 2/ω∈ ^″, is gain length, where ∈ ^″ is imaginary part of the dielectric constant that represents gain. We show that the range of validity of this relation depends on the ratio of both lengths and also affects the slope of the ln Λ_c(q) (where Λ_c ≡ L _c/ξ ₀ is normalized critical size and q ^?1 ≡ l _g/ξ ₀ is dimensionless gain length). We extend the study of the relation for the critical size L _c by inspecting the dependence of the slope of the ln Λ_c(q) on the strength of the randomness. We interpret its behavior in terms of the statistical properties of the localized states. Namely, by studying of the variance of the Lyapunov exponent λ (the inverse of the localization length ξ ₀) we have found that the slope of the ln Λ_c(q)) reflects the transition between two different regimes of localization with Anderson and Lifshits-like behavior that is known to be indicated by peak in var(λ). We discuss the generalization of two-parameter scaling model by implementing revisited single parameter scaling (SPS) theory by Deych et al. [Phys. Rev. Lett. 84 2678, 2000] which allows describing non-SPS regime in terms of a new scale l _s.

     

Threshold current density in ZnS/MgBeZnS quantum well ultraviolet lasers

We calculate optical gain coefficient and threshold current density in ZnS/MgBeZnS quantum wells (QWs) because ZnS/MgBeZnS QWs are useful for the fabrication of an ultraviolet laser on zinc-blende substrates. The threshold current density in a ZnS/MgBeZnS QW laser diode (LD) with a 10 nm ZnS active layer is calculated to be 1.63 kA/cm². By comparing the measured J _th in a CdZnSe/ZnSSe/ZnMgSSe QW LD with that calculated by us, it is expected that the threshold current density in ZnS/MgBeZnS QW LDs measured by experiment is larger than that calculated by our calculation method.