Size dependence of ground-state energy of the excitons in spherical Y2O3

The exciton energies of rare earth oxides (Ln₂O₃) have rarely been calculated by the theory. Experimentally, the blue-shift of exciton energy in nanocrystals deviates from the traditional size confinement effect. Herein, the dependence of the ground-state energy of an exciton in Y₂O₃ spheres on particle radius was calculated by using a variational method. In the model, an exciton confined in a sphere surrounded by a dielectric continuum shell was considered. The ground-state energy of exciton comprises kinetic energy, coulomb energy, polarization energy and exciton–phonon interaction energy. The kinetic and coulomb energy were considered by the effective mass and the dielectric continuum and the exciton–phonon interaction energy was given by the intermediate coupling method. The numerical results demonstrate that the present model is roughly consistent with the experimental results. The confinement effect of the kinetic energy is dominant of the blue-shift of the exciton energy in the region of R < 5 nm, while confinement effect of the coulomb energy is dominant of the blue-shift of the exciton energy in the region of R > 5 nm. The polarization energy contributes largely to the exciton energy as the particle size is smaller than ~ 10 nm, while the exciton–phonon interaction energy takes only a little contribution in all the range.     

The diamagnetic susceptibility of hydrogenic donor in two-dimensional semiconductors with anisotropic effective mass of carriers

In this study, the effects of quantum confinement and effective mass anisotropy parameter on the diamagnetic susceptibility of a hydrogenic donor placed in GaAs, Si, and Ge quantum wells with infinite confinement potential are investigated in the effective mass and parabolic band approximations by using two and one parameter trial wave functions. It is observed that the diamagnetic susceptibility of a hydrogenic donor in anisotropic quantum wells is essentially equal to the transverse diamagnetic susceptibility part when well widths are larger than L > 100 Å, and the impurity is located at center. Moreover, a two parameter trial wave function model gives higher values of diamagnetic susceptibility, except for χ_z (GaAs).     

Synthesis,growth and characterization of a semiorganic nonlinear optical single crystal: L-Valine cadmium bromide

A semiorganic nonlinear optical material L-valine cadmium bromide was synthesized and single crystal grown from an aqueous solution by the method of slow evaporation technique at room temperature. The grown crystal was characterized by Powder X-ray diffraction and FT IR studies. The LVCB crystallizes in monoclinic system with cell parameters a = 10.144(2) Å, b = 5.54(1) Å, c = 12.07(2) Å, β = 109.115(2)° with space group P₂₁. Thermal behavior and stability of crystal were studied using thermogravimetric analyses (TGA) and differential thermal analysis (DTA) techniques. The suitability of this material for NLO application was studied by optical absorption studies and second harmonic generation (SHG) efficiency measurement by Kurtz–Perry powder method.     

Structure,optical spectroscopy and dispersion parameters of ZnGa2Se4 thin films at different annealing temperatures

Thin films of ZnGa₂Se₄ were deposited by thermal evaporation method of pre-synthesized ingot material onto highly cleaned microscopic glass substrates. The chemical composition of the investigated compound thin film form was determined by means of energy-dispersive X-ray spectroscopy. X-ray diffraction XRD analysis revealed that the powder compound is polycrystalline and the as-deposited and the annealed films at T_a = 623 and 673 K have amorphous phase, while that annealed at T_a = 700 K is polycrystalline with a single phase of a defective chalcopyrite structure similar to that of the synthesized material. The unit-cell lattice parameters were determined and compared with the reported data. Also, the crystallite size L, the dislocation density δ and the main internal strain ε were calculated. Analyses of the AFM images confirm the nanostructure of the prepared annealed film at 700 K. The refractive index n and the film thickness d were determined from optical transmittance data using Swanepoel's method. It was found that the refractive index dispersion data obeys the single oscillator model from which the dispersion parameters were determined. The electric susceptibility of free carriers and the carrier concentration to the effective mass ratio were determined according to the model of Spitzer and Fan. The analysis of the optical absorption revealed both the indirect and direct energy gaps. The indirect optical gaps are presented in the amorphous films (as-deposited, annealed at 623 and 673 K), while the direct energy gap characterized the polycrystalline film at 700 K. Graphical representations of ε₁, ε₂, tan δ, ? Im[1/ε*] and ? Im[(1/ε* + 1)] are also presented. ZnGa₂Se₄ is a good candidate for optoelectronic and solar cell devices.     

First principle study of structural,phase stabilization and oxygen-ion diffusion properties of β-La2 − xLxMo2O9 (L = Gd,Sm, Nd and Bi) and β-La2Mo2 − yMyO9 (M = Cr,W)

We have performed a first principle study of structural and phase stabilization of β-La_2 ? xL_xMo₂O₉ (L = Gd, Sm, Nd and Bi) and β-La₂Mo_2 ? yM_yO₉ (M = Cr, W). The substitutional-site properties were discussed in terms of the empirical parameter, bond valence sums (BVS), which characterizes the interactions between atoms and its nearest-neighbor atoms and correlates well with the stability of the structure. We found that Gd, Sm and Nd atoms prefer the crystallographic sites with largest BVS values. The nonlinear dependence of cell parameter on W content in W-doped systems results from the nonlinear change in Mo/W–O bond length with W content. The decrease of cohesive energy and the deviation of BVS values from the expected values upon the Gd, Sm, Nd and W-doped concentration help us understand the experimentally observed stabilization of the β phase to lower temperatures in these doped system. The O ion diffusion properties in W-doped systems have been studied using the nudged elastic band method and the dimer method. We found that, W-doping leads to the obvious increase in the energy barriers of O ion concerted diffusion. In addition, there is a remarkable decrease in the difference of energy barriers between two diffusion channels involving O(1) ion, which sheds light on only one relaxation peak in the mechanical relaxation measurement in W-doped system, compared to undoped system.     

Highly nonlinear layered spiral microstructured optical fiber

A layered spiral microstructured optical fiber (LS-MOF) is presented, which offers the possibility of a good control of both the dispersion and the nonlinear properties. The proposed design is analyzed using a finite element method considering silica and air as the materials. Zero dispersion, low confinement loss, and a record value of γ = 70.0 W^?1/km for the LS-MOF nonlinear parameter are simultaneously obtained at 1.55 μm, whereas a higher value γ = 169.4 W^?1/km can be achieved at 1.06 μm. Our results demonstrate the great potential of the LS-MOF for several nonlinear applications, namely for an efficient generation of the supercontinuum.     

Two and four photon absorption and nonlinear refraction in undoped,chromium doped and copper doped ZnS quantum dots

The ZnS quantum dots (QDs) with Cr and Cu doping were synthesized by chemical co-precipitation method. The nanostructures of the prepared undoped and doped ZnS QDs were characterized by UV–vis spectroscopy, Transmission electron microscopy (TEM) and X-ray diffraction (XRD). The sizes of QDs were found to be within 3–5 nm range. The nonlinear parameters viz. Two photon absorption coefficient (β₂), nonlinear refractive index (n₂), third order nonlinear susceptibility (χ³) at wavelength 532 nm and Four photon absorption coefficient (β₄) at wavelength 1064 nm have been calculated by Z-scan technique using nanosecond Nd:YAG laser in undoped, Cr doped and Cu doped ZnS QDs. Higher values of nonlinear parameters for doped ZnS infer that they are potential material for the development of photonics devices and sensor protection applications.     

Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300 K, which consisted of two emissions at 3.338 and 3.279 eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004 eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12 K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279 eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9 meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10^?3 eV/K and β=4.9×10³ K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.     

Exciton-polariton state in nanocrystalline SiC films

We studied the features of optical absorption in the films of nanocrystalline SiC (nc-SiC) obtained on the sapphire substrates by the method of direct ion deposition. The optical absorption spectra of the films with a thickness less than ~500 nm contain a maximum which position and intensity depend on the structure and thickness of the nc-SiC films. The most intense peak at 2.36 eV is observed in the nc-SiC film with predominant 3C-SiC polytype structure and a thickness of 392 nm. Proposed is a resonance absorption model based on excitation of exciton polaritons in a microcavity. In the latter, under the conditions of resonance, there occurs strong interaction between photon modes of light with λ_ph=521 nm and exciton of the 3С polytype with an excitation energy of 2.36 eV that results in the formation of polariton. A mismatch of the frequencies of photon modes of the cavity and exciton explains the dependence of the maximum of the optical absorption on the film thickness.     

Raman scattering from confined phonons in GaAs/AlGaAs quantum wires

We report on photoluminescence and Raman scattering performed at low temperature (T =  10 K) on GaAs/Al_0.3Ga_0.7As quantum-well wires with effective wire widths ofL =  100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ο_L10 =  285.6 cm⁻¹forL =  11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderleinas applied to the GaAs/Al_0.3Ga_0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques.     

Enhanced nonlinear optical response of linear carbon chain colloid mixed with silver nanoparticles

Nanostructured silver and linear carbon chain (LCC) particle water colloids were prepared by a pulsed laser ablation procedure. Sample's optical transmission response was investigated in the 190–900 nm range, the third-order nonlinear optical properties were studied using the z-scan method and a nanosecond laser. The silver nanoparticles induce a structural change in the LCC colloids: the mixed Ag and LCC optical absorption looses the signature of the short carbon chain maintaining the features attributable to the longer ones. The stability of LCC colloids and their nonlinear response are remarkably improved by the Ag nanoparticles addition to the carbon water colloids. The Ag nanoparticles induce a limiting threshold reduction, an increased nonlinear absorption coefficient β and a marked asymmetrical peak/valley profile of the (Ag:LCC)_mix when compared to the LCC. All these nonlinear contributions determine the increase of the third order susceptibility, while maintaining a significantly high linear transmission value (75%) at 532 nm and high photostability. The magnitude of the nonlinear optical response of these nanohybrids makes them promising candidates for potential optoelectronic applications.     

Supersymmetric quantum-well shape optimization for intersubband bound–continuum second harmonic generation

A method is described for the optimized design of quantum-well structures, with respect to maximizing the second-order susceptibilities relevant for second harmonic generation. The possibility is explored of obtaining resonantly enhanced nonlinear optical susceptibilities in quantum wells with two bound states and a continuum resonance state as the dominant third state. The method relies on applying the isospectral (energy structure preserving) transformations to an initial Hamiltonian in order to generate a parameter-controlled family of Hamiltonians. By changing the values of control parameters one changes the potential shape and thus the values of matrix elements relevant to susceptibility to be maximized. The method was used for the design of Al_xGa_1 − xAs -based QWs. The results indicate the possibility of employing continuum states in resonant second harmonic generation at higher photon energies,ℏω =  200–300 meV.     

Interaction of dioxouranium(VI) ion with EDTA at different ionic strengths

The formation of complex species of dioxouranium(VI) ion with EDTA was studied in the pH range of 1–3.5 and at 25 °C using a combination of potentiometric and spectrophotometric techniques. Results showed evidence for formation of the following species: [UO₂H₄EDTA]²⁺, [UO₂H₃EDTA]⁺, and [UO₂H₂EDTA]. Investigations were performed in sodium perchlorate as background electrolyte at 0.1, 0.3, 0.5, 0.7, and 1.0 mol dm^? 3. The parameters based on the formation constants were calculated, and the dependences of protonation and the stability constants on ionic strength are described. The dependence on ionic strength of the formation constants was analyzed using the specific ion interaction theory (SIT) model. The stability constant values at infinite dilution, obtained using SIT model, are log β°₁₄₁ = 6.77, log β°₁₃₁ = 5.99 and log β°₁₂₁ = 9.29, where indexes for the overall stability constant, β_pqr, refer to the equilibrium pUO₂²⁺ + qH⁺ + rL^4? ? M_pH_qL_r^{(2p + q ? 4r)}. The specific interaction coefficients are also reported.     

Nonlinear optical study of palladium Schiff base complex using femtosecond differential optical Kerr gate and Z-scan techniques

A femtosecond differential optical Kerr gate (DOKG) and Z-scan techniques, have been applied to investigate the third-order optical nonlinearity of composite film of the coordination complex [PdLPPh₃] (L=N-(2-pyridyl)-N′-(salicylidene)hydrazine, PPh₃=triphenylphosphine). Film exhibits superior nonlinear optical properties in the near-infrared spectral region. The nonlinear response time and third-order nonlinear optical susceptibility of complex were found to be≤90 fs and 3.9×10^?10 esu, respectively. The Z-scan result shows that saturable absorption property of the film and its nonlinear absorption coefficient of the sample was found to be ?23 cm/GW.     

Optical and dielectric studies of solution grown glycinium maleate single crystal

Bulk single crystals of glycinium maleate have been grown from aqueous solution by slow evaporation method by optimizing the growth parameters within a period of 15 days. From X-ray diffraction analysis, the crystal was found to crystallize in monoclinic structure (space group C_2/c) with a = 17.866 Å, b = 5.684 Å, c = 17.408 Å and β = 112.65°. Presence of characteristic functional groups was confirmed in FTIR analysis. UV–Vis spectral analysis has revealed the absence of any high absorbance region between the wavelengths ranging from 300 to 900 nm. The optical band gap was calculated and found to be 3.91 eV. The activation energy for conduction at different frequencies was calculated and found to decrease from 0.54 eV to 0.41 eV as frequency increased from 100 Hz to 2 MHz. The dielectric behavior, conduction mechanism and the optical characterization of the glycinium maleate single crystals are being reported for the first time.     

Ion conduction in proton- and related defect (super) ionic conductors: Mechanical,electronic and structure parameters

We find the relationships among optical dielectric constant ε_∞, activation energy E_ac, averaged atomic mass per a formula unit, ∑_jm_j / N, volume V and transition temperature T_c for various type ion conductors with forms of E_ac = α / (ε_∞ ? β), E_ac = A₀ + δ / [(∑_jm_j / N) ? d], E_ac = A_v / V^2/3, and ln(T_c) = g ? hln(∑_jm_j), where α, β, δ, A₀, d, A_v, g and h are constants depending on the kinds of conduction elements. We derive those proportional forms from a simple equation of motion under the assumption of ion hopping assisted by enhanced vibration displacement of host lattice. The enhancement is induced from the large fourth-order term of the host lattice potential originating from the electronic shielding effect of Coulomb force, heavy atomic mass of constituent ion, and volume expansion under the long-range periodicity of crystal structure. This mechanism is ascertained from characteristic phenomena of various kinds of conduction elements. For impurity-type H⁺-ion or defect conductor, the proportional form is shifted from that of superionic conductor because of weakened effect of host lattice vibration mode on H⁺-ion or O-ion defect. Photo-induced spectra of mobile ion in AgCl are understood, and a small quantum effect of H⁺ -ion is suggested.     

The generalized Fresnel–Hadamard complementary transformation for asymmetric beamsplitter

Based on the newly developed parameterized coherent-entangled state representation we propose so-called the generalized Fresnel–Hadamard complementary transformation for asymmetric beamsplitter, which is unitary. The new unitary operator plays the role of both Fresnel transformation for a₁ sin θ ? a₂ cos θ and Hadamard transformation for a₁ cos θ + a₂ sin θ, respectively. Physically, a₁ sin θ ? a₂ cos θ and a₁ cos θ + a₂ sin θ could be a asymmetric beamsplitter’s two output fields. We show that the two transformations are concisely expressed in the parameterized coherent-entangled state representation as a projective operator in integration form.     

Synthesis,crystal growth and characterization of a semiorganic material: Calcium dibromide bis(glycine) tetrahydrate

Single crystals of semiorganic material calcium dibromide bis(glycine) tetrahydrate were grown from aqueous solution. The crystal belongs to monoclinic system, with a = 13.261(5) Å, b = 6.792(2) Å, c = 15.671(9) Å and β = 91.68(4)°. The presence of the elements in the title compound was confirmed by energy dispersive X-ray analysis. The solubility and metastable zone width were found. The grown crystals were tested by powder XRD, FTIR, Thermo Gravimetric and Differential Thermal Analysis, UV–vis–NIR analysis, dielectrical and mechanical studies. The transmittance of calcium dibromide bis(glycine) tetrahydrate crystal has been used to calculate the refractive index n, the extinction coefficient K and both the real ɛ_r and imaginary ɛ_i components of the dielectric constant as functions of wavelength. The optical band gap of calcium dibromide bis(glycine) tetrahydrate is 3.23 eV.     

Nonlinear optical properties of erbium doped zinc oxide (EZO) thin films

Undoped and Erbium (Er) doped zinc oxide (EZO) thin films were deposited on glass substrate by sol–gel method using spin coating technique with different doping concentration. EZO films were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), UV–VIS-NIR transmission and single beam z scan method under illumination of frequency doubled Nd:YAG laser. The deposited films were found to be well crystallized with hexagonal wurtzite structure having a preferential growth orientation along the ZnO (002) plane. A blue-shift was observed in the band gap of EZO films with increasing Er concentration. All the films exhibited a negative value of nonlinear refractive index (n₂) at 532 nm which is attributed to the two photon absorption and weak free carrier absorption. Third order nonlinear optical susceptibility, χ⁽³⁾ values of EZO films were observed in the remarkable range of 10^? 6 esu. EZO (0.4 at.%) sample was found to be the best optical limiter with limiting threshold of 1.95 KJ/cm².     

Exciton binding energy in a double quantum well: effect of the barrier shift

We have calculated the exciton binding energy in an Al _xGa_1 − x As  / GaAs double quantum well by a variational envelope function procedure using a simple two-band model. The influence of the shift of the AlAs separating barrier, introducing an asymmetry into the system, on the value of the exciton binding energy has been analysed. It has been observed that this shift induces significant changes of the exciton binding energy—even several meVs in the case of very thin barriers.