Low-temperature LPE technique for the performance of visible AlGaAs (SC) lasers

The possibilities of the low-temperature (LT)-LPE technique in the performance of visible AlGaAs laser diodes are studied. Low current density visible lasers λ ˜ 746 nm has been fabricated and characterized. The laser diode structures are of the separate confinement type (SC) to obtain low threshold current (J_th = 190 A/cm² at λ = 848 nm for a cavity length L = 1.15 mm). The threshold current density J_th versus lasing wavelength λ characteristic, as well as the dependences of the threshold current density J_th and the reciprocal of differential quantum efficiency 1/η_d with the cavity length L are reported. The expected linear behaviour of the J_th and 1/η_d with the cavity length characteristics appears to break down for the shortest cavity length which is larger for shorter lasing wavelengths.     

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.     

Selective crystallization of amorphous silicon thin film by a CW green laser

A simple and effective method for selective CW laser crystallization of a-Si (CLC) without pre-patterning of a-Si has been reported. By using a metallic shadow mask instead of a photolithographic process, we can reduce the process steps and time compared with a conventional CLC process. It shows very high performance – mobility of 173 cm²/s, I_off of ∼10⁻¹³ A @ V_d = −5 V, I_on/I_off of >10⁸ – as a p-channel poly-Si TFT even without any pre-/post-treatment to improve TFT characteristics such as plasma hydrogenation.     

Growth of bulk zinc chalcogenide single crystals from the vapour phase and their use for laser screens of projection colour television

Results on ZnSe, ZnSe_xS_1−x and ZnS crystal growing from the vapour phase up to 7.5 cm³ in volume are described. Crystals were grown on sapphire, ZnS, ZnSe_xS_1−x and quartz glass substrates without a contact of the growing crystal with a growth ampoule and using the molten tin as a heating medium. Large high-purity crystals with a density of etch pits of 10³ cm⁻² were obtained They exhibited an effective exciton luminescence and rather high radiation efficiency (30 ± 10% for ZnSe at T = 77 K). This made it possible to use these crystals for fabricating laser screens for a cathode ray tube. The main laser parameters obtained on a ZnSe screen at T = 80 and 300 K using a 75 keV electron beam excitation are presented. The light power output reached 0.8 W at T = 80 K; this allowed to obtain a 10 cd · m⁻² TV image of 1.5 × 2 m² in area.     

Synthesis,Characterization, Photo-physical Evaluation and Crystal Structure of 2-Methoxy-1-Methylquinolinium Tetrafluoroborate,a Potentially Selective Agent for the Quantitative Measurement of Biologically Relevant Thiols

Abstract  

We report the quantitative and facile conversion of 2-chloro-1-methylquinolinium tetrafluoroborate to 2-methoxy-1-methylquinolinium tetrafluoroborate via stirring in methanol at room temperature. The resulting compound was evaluated for use as a selective tag for biologically relevant thiols. The results displayed a fivefold increase in fluorescent quantum yield (Ф_FLUOR = 0.20) and an increased stability in pH when compared to 2-chloro-1-methylquinolinium tetrafluoroborate (Ф_FLUOR = 0.04), however, results demonstrated that it did not offer an improvement to 2-chloro-1-methylquinolinium tetrafluoroborate as a thiol specific tag. The compound was fully characterised by ¹HNMR, ESI mass spectroscopy and by single-crystal X-ray diffraction. The title compound crystallizes in the monoclinic space group P2₁/m with a = 8.2420(3) ?, b = 6.6751(2) ?, c = 10.8415 ? and β = 105.164(2)°, Z = 2, V = 575.69(3) ?³. Work is on-going to synthesise and evaluate other such analogues for their potential use as thiol tags.     

The Orientational Optical Non-Linearity of Liquid Crystals

Abstract

The cubic optical non-linearity of a cholesteric liquid crystal (CLC) having a planar helical texture is considered. A study was made of an effect which has a minimal establishment time and is due to the change in the director distribution along the helix under the orientational influence of the light field. Polarisation peculiarities of the non-linearity, when the light wave propagates along the helical axis, are discussed. Self-focusing should not occur for circulary polarised light, but has the constant ?₂ ～ 8.10^?8 cm³/erg for linearly polarised light. The additional rotation of the axes of elliptical polarisation is predicted to be proportional to the light intensity and determined by the same constant ?₂; this rotation is identical for both right-and left-handed CLC. Non-linear optical activity is absent in the approximation under consideration.

Analogous effects for light propagating through an oriented nematic are discussed.     

Temperature and excitation intensity tuned photoluminescence in Tl4GaIn3S8 layered single crystals

Photoluminescence spectra of Tl₄GaIn₃S₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 500–780 nm and in the temperature range of 26–130 K with extrinsic excitation source (λ_exc = 532 nm), and at T = 26 K with intrinsic excitation source (λ_exc = 406 nm). Three emission bands A, B and C centered at 514 nm (2.41 eV), 588 nm (2.11 eV) and 686 nm (1.81 eV), respectively, were observed for extrinsic excitation process. Variations in emission spectra have been studied as a function of excitation laser intensity in the 0.9‐183.0 mW cm^–2 range for extrinsic excitation at T = 26 and 50 K. Radiative transitions from the donor levels located at 0.03 and 0.01 eV below the bottom of the conduction band to the acceptor levels located at 0.81 and 0.19 eV above the top of the valence band were proposed to be responsible for the observed A‐ and C‐bands. The anomalous temperature dependence of the B‐band peak energy was explained by configurational coordinate model. From X‐ray powder diffraction and energy dispersive spectroscopic analysis, the monoclinic unit cell parameters and compositional parameters of Tl₄GaIn₃S₈ crystals were determined, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Suitable Stress Waveforms for the Deformation-Induced Electronic Excitation in Crystals

A quantum mechanical approach is made to the deformation-induced electronic excitation in crystals. The ML intensity is assumed to be related to the transition probability P(m, n) from n^th to m^th state, and it may be expressed as I_ML = βP(m, n) where P(m, n) = 4π²/hα² |σ_mn (ω_mn)|² ·( n · μ )_mn·². It is shown that the ML excitation can take place only by the time-dependent applied stress. The ML excitation probability is analysed for different waveforms of the applied stress such as constant stress, δ-function, rectangular stress, single sided and double sided exponential stress, unit step function, external sinusoidal signals cos ω₀t and sin ω₀t, external exponential function, a periodic function and the stress of sequence of equidistant impulses of unit strength separated by a particular time. It is found that when the stress is a δ-function, a rectangular waveform, sinusoidal singals cos ω₀t and sin ω₀t or a sequence of equidistant impulses of unit strength and separated by a particular time, then there is a considerable probability of electronic excitation by the resonance transfer mechanism between the Fourier transform of the stress waveform and the frequency of the emitting electronic system i.e. (E₂ – E₁)/h.     

Comparison of UV optical absorption and UV excited luminescence behaviours in Ge doped silica under H2 loading or CW UV laser irradiation

We compare the CW 244 nm laser induced absorption and photoluminescence excitation changes in Ge and Ge-P doped silica glass according to H₂ loading. Under H₂ loading only, the absorption does not change but the luminescence excitation intensity decreases by 60%. For samples not H₂ loaded, luminescence excited in the UV range is annihilated by the laser irradiation but not the absorption in the same UV range. During UV irradiation of H₂ loaded samples most of the luminescence excited in the UV range disappears whereas UV absorption behaves differently. In that way, we show that defects giving rise to luminescence exhibit a weak contribution to the absorption spectrum. In particular, there are two different oxygen deficient centres: one absorbing at 5 eV and another one giving rise to luminescence under 5 eV excitation. Due to some correlations existing sometimes between absorption and luminescence, we suggest that luminescence is quenched by trapping of electrons produced by 5 eV absorbing defects. In addition, the use of a CW laser allows us to observe two successive one-photon absorption steps for accounting for the absorption change: a singlet to singlet transition and then an ionisation.     

Site-selective excitation of europium-doped lanthanum oxycarbonates

The excitation at 77 K of the fluorescence of europium in two europiumdoped polymorphs of lanthanum oxycarbonate (LaO)₂CO₃, type Ia and type II, by UV light yielded several⁵ D ₀ ⁷ F ₀ transitions around 5800 Å for both compounds. Selective excitation at 77 K of these⁵ D ₀ levels with a Rhodamine 6G dye laser permitted the identification of La₂O₃ as a trace impurity in both compounds, and demonstrated the existence in both of two nearly axial sites for the europium atoms. Second-rank crystal field parameters are large (1000 cm^–1) for one of the two sites. In the case of type Ia the spectrum is very close to that found for other oxysalts with the quadratic (LnO) _n ⁿ⁺ layer such as the oxyhalides.     

Electrical and Photoelectrical Properties of n-CuInS2 Single Crystals

The temperature dependences of free electron density and mobility in n-CuInS₂ single crystals have been determined from the Hall effect measurements. The ionization energy of donors and their densities have been estimated. The results of measurements show that the crystals are strongly compensated. The spectral distribution of photoconductivity, the dependence of photoconductivity on excitation intensity, and the photoconductivity decay with time have been measured in the n-CuInS₂ crystals at different temperatures. The results of photo-conductivity measurements suggest linear recombination of photo-electrons at weak excitation and quadratic recombination at high excitation intensity. The compensated deep acceptor probably acts as the recombination centre.     

Crystallo-optical properties of the SmxTb1−xFeO3 orthoferrites

The paper reports the results of X-ray diffraction studies as well as the angular dependences of rotation of the plane of polarization and birefringes of the Sm_xTb_1−xFeO₃ crystals (x = 0; 0.2; 0.4; 0.55; 0.8; 1). The polar diagrams presented for the plane (001) have the symmetry mm2, while for (110), the symmetry lowers up to the axis of a second order. The gyrotropic measurements made have allowed us to refer the Sm_xTb_1−xFeO₃ to a planar class of an orthorhombic crystal system with the space group C_2v⁹–Pn2a, class mm2. – The magnitude of birefringence for the Sm_xTb_1−xFeO₃ is found to be sensitive both to a crystallographic direction and a composition, the greatest values stand for (110). For (001), a non-monotonous birefringence variation with the composition is observed. The angles between optic axes, calculated by the birefringence magnitudes, are in fair agreement with gyrotropic data for the compositions x = 0.55 – 1.     

The impurity ion off-center behavior induced by a localized vibronic Frenkel exciton in a soft-mode crystal

The 3d-4p Frenkel exciton localized at a Mn⁴⁺ impurity ion in BaTiO₃ is considered. The electron and hole of this exciton are correlated, degenerate, and involved in the pair Jahn-Teller effect. Such an exciton is characterized by the significant contribution of indirect electron-hole attraction through lattice (in particular, soft) vibrations to the coupling energy, which is comparable with the Coulomb attraction. Thus, the exciton under consideration is both Coulomb and vibronic excitation. The vibronic contribution increases when approaching the point of soft phonon condensation. It is shown that a vibronic Frenkel exciton allows the off-center behavior of an impurity ion, which is significantly enhanced when approaching the point of soft phonon condensation. As a result, the vibronic Frenkel exciton is a low-lying excitation.

     

New data on the physical properties of Y3Al5O12-based nanocrystalline laser ceramics

Microhardness and fracture toughness of highly transparent Y₃Al₅O₁₂-and Y₃Al₅O₁₂: Nd³⁺-based nanocrystalline ceramics are measured for the first time. For the Y₃Al₅O₁₂: Nd³⁺ laser ceramics, the use of a longitudinal scheme with a diode-laser pumping at a wavelength of 1.3186 mm (⁴ F _3/2 → ⁴ I _13/2 channel) enabled one to attain an output power of continuous-wave lasing of ～3.7 W with 35% efficiency.     

Refractive indices of laser nanocrystalline ceramics based on Y3Al5O12

Refractive indices of the nanocrystalline Y₃Al₅O₁₂ ceramic and a garnet single crystal of the same composition have been measured. In the visible and near IR range (0.4–1.064 μm), the prism method was used; in the medium IR range (2–6.2 μm), the interference method with the use of thin plates was applied. The refractive indices of these crystalline materials are practically the same over the entire spectral range studied and are described by the approximate formula proposed earlier for a single crystal. The parameters of the continuos-wave lasing in the nanocrystalline Y₃Al₅O₁₂ ceramic doped with Nd³⁺ and Yb³⁺ ions measured recently are presented.     

Double heterostructure lasers prepared by LPE on medium-quality GaAs substrates under controlled as vapour conditions

The fact that defect formation in (Ga, Al)As LPE layers can be suppressed by the presence of arsenic vapour is employed to improve properties of double heterostructure (DH) lasers. The controlled vapour pressure (CVP) method is implemented using a modified LPE horizontal carbon boat. Growth kinetics study under near-equilibrium conditions shows that the presence of arsenic vapour diminishes the growth rate of aluminium-containing layers; no such influence has been observed with the GaAs layers. The CVP method, compared with the customary LPE, has a beneficial effect on the DH surface and cross sections perfection, as well as on the lasing characteristics (differential quantum efficiency, threshold current). Observed values of the parameter T₀, characterizing temperature dependence of the laser threshold current, fall into the vicinty of 200 K. The results have been obtained on medium-quality GaAs substrates.     

Cholesteric liquid Crystals: Physical Properties and Molecular-Statistical Theories

It seems certain now that cholesteric liquid crystals (cholesterics, or CLC, for short) have been the first examples of mesomorphic state of matter known to humankind. Apart from the well-known discovery of the two-stage melting of cholesteryl benzoate made by Reinitzer in 1888¹ and cited in almost all the textbooks, one may recall some other papers where the authors, quite unaware of the real significance of their observations, presented undeniable evidence of the liquid crystalline state of matter. E.g., Professor Planer from the Lvov University as early as in 1861 did observe, putting it in the presentday terms, selective reflection from the planar texture of cholesteryl chloride forming a monotropic cholesteric mesophase on cooling.² One should also note that the works of Fergason^3,4 outlining the prospects of practical applications for CLC, did, in fact, precede similar endeavors by Heilmeier in the field of nematics. From the viewpoint of organic chemistry, a number of mesogenic cholesterol derivatives had been already synthesized and thoroughly studied⁵ by the time the name of MBBA (p-methoxybenzylidene-p′-n-butylaniline), to say nothing about cyanobiphenyls, was not yet known to anybody. The mere listing of CLC prospective applications^6,7. seems so impressive that, keeping in mind the above-described historical background, one should expect to find cholesterics in a key position in the field of liquid crystal science.     

Second-order nonlinear susceptibilities of the crystals of some metal tartrates

The relative (with respect to a powder potassium dihydrophosphate crystal) effective second-order nonlinear susceptibilities have been measured for gel-grown single crystals of some tartrates with the general formula MeC₄H₄O₆ · nH₂O on powder samples upon the excitation of the second harmonic of laser radiation with a wavelength of 1.064 μm. The unit-cell parameters and the symmetry space groups have been determined for the single crystals under study.     

The crystal and solution structure of 2,4-dinitrophenyl phenylsulfide determined by X-ray crystallography and13C-NMR spin-lattice relaxation measurements

The crystal structure of 2,4-dinitrophenyl phenylsulfide, C₁₂H₈N₂O₄S, has been determined by single crystal X-ray diffraction. The crystals are monoclinic, space groupP2 _{1/n, Z} = 8, witha = 10.670(2),b = 8.381(4),c = 27.998(18) Å, and = 100.44(4) °. There are two crystallographically independent molecules per asymmetric unit. All intensity measurements were made at room temperature on an Enraf-Nonius CAD-4 automatic diffractometer using monochromatized MoK radiation. The structure was solved by direct methods and refined to a finalR value of 0.032. The two independent molecules have nearly identical geometries, with the most notable difference being the torsion angles involving the sulfur atoms. Thepara-nitro groups are more nearly in the planes of the central phenyl rings than are theortho-nitro groups although there may be nonbonded attractions between the latter. The two phenyls of each molecule are nearly at right angles to each other, as necessitated by steric forces involvingortho hydrogens. The characteristic distortions of the central phenyl rings caused by differing substituent electronegativities and degrees of conjugation are noted. The¹³C-NMR spectrum of the compound was assigned by comparison of calculated with observed chemical shift data. Calculated chemical shifts were obtained by incrementation of the chemical shifts of diphenylsulfide with standard nitro group additivities which were adjusted for the sulfur-nitro group interaction. Where necessary, assignments were confirmed with selective excitation spin-coupled subspectra. Solution behavior of the molecule was determined from spin-lattice (T ₁) relaxation measurements which showed the unsubstituted ring to reorient anisotropically about the C(21)-C(24) bond axis while the substituted ring appears to be governed by the overall isotropic reorientation of the molecular framework, indicating that the sulfur-nitro group interaction restricts anisotropic reorientation about the C(11)-C(14) bond axis.     

Characterization of BGO glass–ceramic materials

Glass–ceramics of the compositions Bi₄Ge₃O₁₂ (BGO) and Bi₂O₃–GeO₂ have been obtained by pouring molten materials onto graphite plates heated below and above the glass transition temperatures. Glass transitions temperatures have been measured by DSC. T_g is lower in the case of BGO than with the Bi₂O₃–GeO₂ glass ceramics material. Crystallization is evaluated by X-ray diffraction and microscopic observations. The XRD measurements confirm the amorphous structure of the samples cast below T_g. The Hruby factor, which measures the stability of the glass–ceramics, is about 0.21 in the case of BGO glass ceramics suggesting a rather stable glass. The fluorescence under U.V. excitation is larger by one order of magnitude in the case of BGO glass–ceramics by comparison to the Bi₂O₃–GeO₂ glasses, suggesting it is a good candidate for scintillation materials. The images obtained under U.V. excitation shows structured luminescent centers in the case of BGO glass–ceramics.