Characteristics of GaAs?AlGaAs (SCH) lasers grown by low-temperature LPE technique

In this paper we report the performance and study of GaAs AlGaAs (SCH) laser. The structures were grown by a new variant of the LPE-technique with temperatures regimes lower than 600 °C. The Low Temperature LPE-technique, recently propoused by ANDREEV and coworkers at the IOFFE-Physico-Technical Institute exhibit growth rates of the growing layers as low as that corresponding to MBE or MOCVD techniques. In this way, it is possible to obtain and control layer thicknesses in the structure of the order of tens to hundreds of Ångströms. Laser heterostructures with active layer thickness of 30 nm, were performed and the lowest threshold current density of 437 A/cm² for a cavity length of 1.33 mm were obtained. A typical growth process is described and a SEM structure microphotograph is shown. The dependences of the threshold current density J_th and the differential quantum efficiency η_d with the cavity length L are reported. From these data, other laser parameters are evaluated such as internal optical losses α₁, internal quantum efficiency η_i and the dependence of the maximum of the gain factor against the nominal current density J_nom at threshold. This dependence was compared with the theoretical predictions and a good agreement was found.     

Double-Heterostructure Indium-Tin Oxide/InGaAsP/AlGaAs Lasers

The fabrication and characterization of double-heterostructure (DH) laser that utilizes an indium-tin oxide (Ito) transparent cladding and top contact layer. The first room-temperature lasing operation has been obtained form ITO/InGaAsP/AlGaAs DH laser diode at threshold current density of 12.1 kA/cm². The interfacial recombination velocity of ITO/InGaAsP Interface was estimated to be 4.9 × 10⁴ cm/s from a simple model to account for high threshold current, and optical measurements of spontaneous emission and lasing spectra from top stripe window and facet were done.     

Nonlinear optical properties of ZnS nanocrystals incorporated within the polyvinyl alcohole photopolymer matrices

Photoinduced nonlinear optical effects in ZnS nanocrystallites incorporated within the photopolymer polyvinyl alcohol matrices were studied. As a basic method we have used photoinduced optical second harmonic generation (SHG). The Q‐switched HF‐laser (λ = 2.64.μm; pulse duration 30‐40 ps) was used as a source of probing (fundamental) light beam and the pulsed nitrogen (λ = 337.7 nm) was applied as a source of the photoinducing pump light. With increasing photoinducing beam power, the output SHG signal (at λ = 1.32 μm) increases. The maximal second‐order non‐linear optical susceptibility corresponds to tensor component d₁₂₃ = d₁₄ = 3.8 ± 0.2 pm/V at a photoinducing power density equal to about 1.25 GW/cm². With decreasing temperature, the SHG signal increases within the temperature range 25‐30 K. Time‐dependent pump‐probe measurements of the SHG indicate an existence of the SHG maximum for a pump‐probe delaying time 20 ps. The ZnS hexagonal structural components play a crucial role in the observed photoinduced second‐order non‐linear optical effects. Large values of the non‐linear optical constants as well the good technological parameters open a possibility to enhance the non‐linear optical susceptibilities of the investigated ZnS nanocrystallites.     

MOVPE Growth of Wide Band-Gap II—VI Compounds for Near-UV and Deep-Blue Light Emitting Devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity ^tBuSH, ^tBu₂Se and the adduct Me₂Zn:Et₃N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MQWs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm².     

Laser Annealing of Chemical Bath Deposited CdS Films

Chemical Bath Deposited (CBD) CdS films have been exposed to a cw Ar⁺ laser annealing. Measurementes on the dark electrical resistivity have shown a two decade decrement in the resistivity. A trend of improving this resistivity is shown upon atmospheric pressure increase. The annealing mechanisms of the samples show a threshold laser power density of 50 W/cm². This power threshold accounts for some heating mechanisms and Cd ion evaporation from the samples; thus a gaseous Cd atmosphere annealing process is suggested. This laser annealing mechanism parallels thermal annealing of polycrystalline CdS films. Assessment of this mechanisms is supported by the fact that the mobility ratio, b = μ_n/μ_p, (greatly influenced by crystallite size and intergranular potential barrier in thin film semiconductors) does not show major changes to account for a two decade decrement of CBD laser annealed samples.     

Device grade hydrogenated polymorphous silicon deposited at high rates

Hydrogenated polymorphous silicon (pm-Si:H) thin films have been deposited by plasma-enhanced chemical vapor deposition at high rate (8–10 Å/s), and a set of complementary techniques have been used to study transport, localized state distribution, and optical properties of these films, as well as the stability of these properties during light-soaking. We demonstrate that these high deposition rate pm-Si:H films have outstanding electronic properties, with, for example, ambipolar diffusion length (L_d) values up to 290 nm, and density of states at the Fermi level well below 10¹⁵ cm^?3 eV^?1. Consistent with these material studies, results on pm-Si:H PIN modules show no dependence of their initial efficiency on the increase of the deposition rate from 1 to 10 Å/s. Although there is some degradation after light-soaking, the electronic quality of the films is better than for degraded standard hydrogenated amorphous silicon (values of L_d up to 200 nm). This result is reflected in the light-soaked device characteristics.     

Magnetic field dependence of the relative critical current density in γ‐irradiated polycrystalline YBa2Cu3O7

The effect of different doses of γ‐rays on the behavior of the critical current density, J_c in an YBa₂Cu₃O₇ polycrystalline sample has been investigated at high temperatures. The samples were irradiated at room temperature by a ⁶⁰Co γ‐ray source at a dose rate of 0.5 MR/h. J_c was found to increase significantly with after irradiation dose of 10 MR. Further irradiation dose of 50 MR produced a slight and field dependent enhancement of J_c above its values at 10 MR. The most interesting result is that the relative change in the critical current density was found to have a non‐monotonic behavior with the applied magnetic field. These results are discussed in terms of the roles of several mechanisms created by γ‐rays in the regions of the grain boundaries combined with the effect of the magnetic field on these mechanisms. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis,Crystal Structure and Hirshfeld Surfaces of the N-(bis(benzylamino)phosphoryl)-4-Methylbenzenesulfonamide Polymeric tetrakis-Complex {Na[Yb(L)4(iPrOH)]}n

Lanthanide containing polymer chain of title compound {Na[Yb(L)₄(ⁱPrOH)]}_n is formed by alternating complex anions and sodium cations. The anion is composed by four deprotonated bidentate chelate ligands of sulfonylamidophosphate type N-(bis(benzylamino)phosphoryl)-4-methylbenzenesulfonamide (L^?) coordinated to the Yb(III) ion via O atoms belonging to the sulfonyl and phosphoryl groups. The coordination polyhedron of the ytterbium has a slightly distorted triangular dodecahedron conformation, TDD-8, D_2d. The Na⁺ cation is coordinated by four sulfonyl O atoms, the NaO₄ polyhedra shape is intermediate between axially vacant trigonal bipyramid C_3v and tetrahedron T_d. Single-crystal X-ray diffraction studies show that one 2-propanol molecule is hydrogen-bonded to the sulphonyl oxygen of one of the chelate ligands. Also the extensive network of hydrogen bonding interactions between the aromatic rings both in the structural unit and between the polymer chains were detected. They were investigated by the analysis of Hirshfeld surfaces and fingerprint plots. The correlations between structure parameters obtained and the physico-chemical properties of the {Na[Yb(L)₄(ⁱPrOH)]}_n were investigated.

Graphic Abstract

New tetrakis-complex {Na[Yb(L)₄(ⁱPrOH)]}_n was synthesized and characterized by X-ray crystallography. Structural packaging characteristics were investigated by the analysis of Hirshfeld surfaces.

     

Lasing Spectra of Dye Doped Cholesteric Liquid Crystals Under Reversible Phototuning of the Frequency

We investigate the lasing frequency phototuning in a distributed feedback (DFB) laser based on cholesteric liquid crystals (CLC). To improve the CLC planar texture quality, we use new materials such as nematic ZhK-654 and an analog of cholesteryl oleate as a twisting additive. The selective excitation of the cis-form molecules of the azoxy-nematic is carried out with the use of a combination of an interference filter with maximum transmission at 436 nm and a cut-off filter. Using the orienting substrates with SnO₂ allows us to reduce the threshold pump power by more than an order of magnitude. The proposed combination of filters for the selective excitation enables us to align the ranges of lasing wavelengths in both directions.     

A new equation for the temperature dependence of density of saturated aqueous solutions of electrolytes

A new empirical three-parameter equation d = d₀ exp[β′(T – T₀)²] (where β′, d₀, and T₀ are constants for a salt) has been proposed to describe the dependence of density d (g/ml³) of saturated aqueous solutions of electrolytes on temperature T (K) in the whole range of temperature in which solubility increases or decreases with an increase in temperature.     

Epitaxial growth on optical gratings for distributed feedback GaAs injection lasers

Good quality epitaxial overgrowth of Al_0.3Ga_0.7As on corrugated GaAs and Al_0.12Ga_0.88As surfaces has been achieved by molecular-beam epitaxy. The electrical properties of the interface appear to be equivalent to those prepared by LPE without growth interruption. The corrugations were third order Bragg gratings of ～0.37 μm period and ～0.20 μm depth and were formed by ion milling. Separate confinement heterostructure injection lasers with these periodic corrugations in the optical cavity have demonstrated lasing behavior characteristic of distributed feedback. They have operated with room-temperature threshold current densities as low as 2.2 kA/cm². These results suggest that the MBE overgrowth is a useful technique for the fabrication of integrated optoelectronic structures which include active devices.     

The emission and structure of a new laser crystal Nd3+: NaY(WO4)2

A new laser Nd³⁺ : NaY(WO₄)₂ crystal rod of Φ 2.5 × 5 mm was pumped by a xenon flash lamp, and a 1.06 μm laser output was for the first time obtained in our laboratory, which has a threshold lower than 1 J. The crystal structure of NaY(WO₄)₂ was determined. The crystal is tetragonal, with the space group 14₁/a, a = 5.205(2) Å, c = 11.251(3) Å, V = 304.8(2) ų, Z = 2, D_calc = 6.62 g/cm³, D_obs = 6.51 g/cm³.     

Comparative characterization of boron-implanted silicon after pulse laser annealing along single traces by SEM (SE,EBIC) and TEM studies

(111) oriented n-type silicon wafers which were implanted with 2 × 10¹⁵ B⁺/cm² at 77 K and at an energy of 60 keV are laser annealed (Nd:YAG pulse laser) in air for mean laser powers between P = 1.8 W and P = 4.0 W. The comparative application of scanning electron microscopy (SE, EBIC) and high voltage transmission electron microscopy (HVTEM, 1000 keV) allows an estimation of the annealing quality in separated lines without overlapping. Values specifically ascertained by SEM in the SE contrast mode for the annealing threshold power P _th, the power range for optimal annealing ΔP ₀ and for the geometric width of the monocrystalline resolidification perpendicular to the direction of the specimen movement (and related to the spot diameter) at different laser pulse powers P agree completely with the results found in EBIC. At an optimal laser pulse power of P ₀ = 3.2 W HVTEM enables monocrystalline epitaxially resolidified areas free of defects to be identified in the same geometric width as that ascertained by SE and EBIC.     

Lattice Vibration and Transmissivity in Nd-Doped Yttrium Orthovanadate Crystals

Neodymium-doped yttrium orthovanadate Nd_xY_1—xVO₄(NYV), with x = 0.025, crystallizes with a zircon-type structure in the tetragonal system conforming to the space group I4₁/amd. The lattice constants are a = b = 0.7123 ± 0.0005 and c = 0.6292 ± 0.0005 nm at room temperature. There are the four formulas per unit cell. The lattice-vibration and transmissivity spectra show that the NYV crystal is a good laser host crystal, which provides a suitable crystal field for the active ions. A strong absorption at 808 nm and the stimulated emission at 1.06 μm have provided the practical application as a laser crystal. Also, a strong fluorescent radiation at 0.408 μm, corresponding to a transition ⁴D_3/2 → ⁴I_13/2, is observed.     

Growth and characterization of photorefractive Bi12TiO20 single crystals

Photorefractive Bi₁₂TiO₂₀ single crystals of high optical quality were grown in a resistive heating furnace from high temperature nonstoichiometric (10:1) solutions of Bi₂O₃ and TiO₂ at pulling rates 0.3–0.8 mm/h at rotation 20–30 rpm along the <001> and <011> axis. Powder X-ray analysis, Laue method, and electron-probe microanalysis were used for characterization. BTO crystals have the bcc structure of sillenite type with a₀ = 10.178(8) Å. The chemical composition of the crystals can be written down as Bi_{12.1 ± 0.2}Tio_{0.96 ± 0.09}O_20.1. Natural optical activity ρ of BTO crystals is 6.3 ± 0.2 deg/mm at λ = 0.633 μm and 11.9 ± 0.2 deg/mm at λ = 0.5145 μm, optical absorption coefficient α = 0.42 ± 0.04 cm⁻¹ at λ = 0.633 μm and linear electro-optic coefficient r₄₁ = r₅₂ = r₆₃ = 5.3 pm/V. Fanning effect in the “fiber-like” BTO sample was studied and double phase conjugation with conversion efficiency up to 8% was observed in a wide range of incidence angles of the pump at λ = 0.633 μm for 2 × 3 mW input light power.     

III–V alloys and their potential for visible emitter applications

Atmospheric pressure organometallic vapor phase epitaxy is used for the growth of GaInP-AlGaInP lattice matched to GaAs substrate. The investigation of the GaInP/GaAs heterostructure has been carried out and mobility as high as 75 000 cm² V^-1s^-1 for a sheet carrier concentration of 7.5 10¹¹ cm^-2 has been measured by Shubnikov-de Haas at 4 K. Red and yellow LED's have been obtained with emission wavelength of 670 nm and 576 nm respectively. Oxide stripe laser emitting at 660 nm in pulsed mode have been performed and present a threshold current density of 5 kA cm^-2. Three ridge waveguide laser diode arrays with maximum output power of 108 mW have also been achieved.     

An X-ray diffraction refinement of the structure of natural thomsonite

Thomsonite Na₂Ca₄Al₁₀Si₁₀O₄₀, orthorhombic Pncn, a = 1.3124 nm, b = 1.3078 nm, c = 0.662 nm, V = 1.1369 nm³, Z = 2, D_m = 2.30, D_c = 2.26, μ = 12.4 cm⁻¹. The average length of the bond T–O = 0.1685 nm. The final R-value for 456 independent observed reflections is 0.085.     

Refractive index and dispersion measurements in prismatic thin film of phasmidic columnar liquid crystals

In this work, we explore the refractive index of the phasmidic columnar liquid crystal for homeotropic oriented samples (column perpendicular to the plates). To measure the ordinary index n_o of the columnar liquid crystal C₁₀, a simple and accurate method is presented using the deflection by reflection of a laser beam (λ = 633nm) through a prismatic phasmidic thin film placed between two glass plates in the homeotropic orientation. For the C₁₀ samples, we found n_o = 1.5512 ± 0.0002 at T = 100°C. We used the same method to determine the isotropic index n_l of the isotropic phase and found n_l = 1.5301 ± 0.0002 at T = 120°C. Using the precision of the same method in polychromatic laser beam, we measured the variation of the ordinary index with the wavelength λ and characterize the dispersion by the known Cauchy's dispersion relationship n_o = A + B/λ². We deduce an estimation of the birefringence for several samples.     

Systematical changes in diffraction efficiencies through Ag-photodoping into amorphous GeSe thin films

It is well known that the phenomenon of Ag-photodoping into amorphous (a-) chalcogenide film depends on film thickness (d). Herein, we report that the effects of Ag-photodoping into a-GeSe thin films exhibit a systematical change dependent on d using a holographic exposure (HeCd laser) and the measurement of diffraction efficiency (η) in real time. The η-kinetics of Ag-photodoping in the sample structure of an a-GeSe/Ag/p-type Si substrate is divided into two steps. A photodarkening-dominant process related to the generation of valence-alternation pairs (VAPs) in the chalcogenide explains the first step while a doping-dominant process accounts for the second step. In particular, we confirmed that the d-dependencies of the η-kinetics are classified into 3 groups: (A) d < 2d_P; (B) 2d_P ? d < 4d_P; (C) d > 4d_P, where d_P indicates the penetration depth of the HeCd laser. In group A, the Ag-doping process occurs simultaneously with the darkening process. In group C, the η-kinetics results from the darkening process only. Contrarily, in group B, the η-kinetics exhibits a sharp distinction between the photodarkening process and the Ag-doping process, and in particular, η_M increases almost linearly with an increase in d.     

Interaction between a dislocation and monovalent anion in various alkali halide crystals

It was investigated from (L₀/L)² versus ϕ₀ curve that the Friedel relation between the effective stress and the average length of dislocation segments, L, is appropriate for the interaction between a dislocation and the monovalent anion in various alkali halides single crystals (NaCl: Br^‐, NaBr: Cl^‐ or I^‐, KCl: Br^‐or I^‐, and RbCl: Br^‐ or I^‐). Here, L₀ represents the average spacing of monovalent anions on a slip plane and ϕ₀ is the bending angle at which the dislocation breaks away from the anion at the temperature of 0 K. This is because the anions are the weak obstacles such as impede the dislocation at ϕ₀ above about 150 degrees, where the Friedel relation agrees with the Fleischer one (L₀² = L²(π–ϕ₀)/2). Furthermore, the values of (L /L₀) were found to be within 4.05 to 5.87 for the crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)