OPTICAL PROPERTIES OF δ-DOPED GaAs AND GaAs/Al0.1Ga0.9As SUPERLATTICES

Radiative transition in δ-doped GaAs superlattices with and without Al_0.1Ga_0.9As barriers is investigated by using photoluminescence at low temperatures. The experimental results show that the transition mechanism of δ-doped superlattices is very different from that of ordinary superlattices. Emission intensity of the transition from the electron first excited state to hole states is obviously stronger than that from the electron ground state to hole states due to larger overlap integral between wavefunctions of electrons in the first excited state and hole states. Based on the effective mass theory we have calculated the self-consistent potentials, optical transition matrix elements and photoluminescence spectra for two different samples. By using this model we can explain the main optical characteristics measured. Moreover, after taking into account the bandgap renormalization energy, good agreement between experiment and theory is obtained.     

75As NMR studies of superconducting LaFeAsO0.9F0.1

We have performed 75As nuclear magnetic resonance measurements on aligned powders of the new LaFeAsO0.9F0.1 superconductor. In the normal state, we find a strong temperature dependence of the spin shift and Korringa behavior of the spin lattice relaxation rate. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin-singlet pairing. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggests that superconducting vortices contribute to the relaxation rate when the field is parallel to the c axis but not for the perpendicular direction.     

Energy sublevels in an Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As quantum wire

We report the successful fabrication of a V-grooveAl_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5As quantum wire system and the temperature-dependent photoluminescence (PL) measurement. The PL spectra are dominated by four features at 681, 642, 635 and 621 nm attributed to the luminescences from quantum wire, top, vertical and side-wall well regions by micro-PL measurements. By the calculations of the energy structure, discrete states (localized sublevels) in the quantum wire region and continuum states (extended along the side-wall and vertical quantum wells) in side-wall and vertical quantum wells have been obtained in both the conduction and valence bands. The calculated excitation energies explain very well the peak positions and their temperature dependence in the photoluminescence measurements.     

Femtosecond laser Fourier transform absorption spectroscopy

A femtosecond mode-locked laser is used for what is believed to be the first time as a broadband infrared source for high-resolution Fourier transform absorption spectroscopy. A demonstration is made with a Cr(4+):YAG laser. The entire nu(1)+nu(3) vibration-rotation band region of acetylene, observed after passing through a single-pass 80-cm-long cell, is simultaneously recorded between 1480 and 1600 nm, in 7.9 s with a signal-to-noise ratio equal to 1000. Two hot bands of the most abundant acetylene isotopologue and the nu(1)+nu(3) band of the (13)C(12)CH(2) are also present. Replacement of the usual conventional tungsten lamp by the bright laser source reduces by about a factor of 150 the recording time needed to get similar results. The noise equivalent absorption coefficient at 1 s averaging is equal to 7x10(-7) cm(-1)Hz(-1/2) per spectral element.     

X-ray absorption spectroscopy study and photocatalyst application of CuO and Cu0.9Ti0.1O nanoparticles

We report detailed investigations on the electronic structure and photocatalyst application of CuO and Cu_0.9Ti_0.1O nanoparticles (NPs). The NPs were prepared by co-precipitation method and subsequent annealing. Crystal structure and morphology of the NPs were investigated by synchrotron X-ray diffraction and high resolution transmission electron microscope, respectively. The local atomic structure around the Cu atoms was investigated by the extended X-ray absorption fine structure (EXAFS) at the Cu K-edge. Electronic structure determination was done using near edge X-ray absorption fine structure (NEXAFS) at the O K-edge, Cu L-edge, Cu K-edge and Ti L-edge. From the structural and electronic structure investigations, it is inferred that the Ti substitutes the Cu in CuO lattice without forming any secondary phases and the valence state of Cu is not affected by the Ti substitution; however the Cu – O bond length is found to be shorten in the Ti doped sample. As prepared NPs exhibit excellent photocatalyst application toward the degradation of methyl orange (MO) and potassium dichromate (PD) pollutant dyes under the visible light irradiation. The mechanism of the photodegradation of MO and PD pollutants, by the smaller sized CuO and larger sized Cu_0.9Ti_0.1O NPs, is briefly discussed.     

Two-photon absorption processes in GaAs/Al x Ga1−x As quantum wells

Summary The two-photon absorption spectra of GaAs/AlGaAs multiple quantum well and superlattice structures have been experimentally investigated by means of the nonlinear luminescence technique in different polarization configurations. A strong excitonic effect overlapping the interband two-photon spectrum has been found and the selection rules for the excitonic transitions have shown to greatly change for different polarizations. The comparison of linear and nonlinear absorption measurements provides important information on the excited states of excitons in multiple quantum wells. Work partially supported by M.P.I.     

Effect of Ga and As vacancies on electronic properties of Ga0.9375Al0.0625As

Electronic properties of Ga_0.9375Al_0.0625As with Ga and As monovacancies are investigated with the density functional theory. A perfect GaAlAs, which specifically produces a radiation wavelength of ～825.6 nm, has been determined in terms of size and number of atoms of substances. We have introduced eight types of vacancies depending on the distance from the Al atom to the vacancy to obtain the changes in charges, band structures, density of states, and optical conductivity. We found that the Fermi level enters into the valence band due to the formations of the Ga or As vacancies so that the vacant materials may show the characteristics of the p-type semiconductor. Interestingly, Ga-vacancy systems make direct band gaps, which are still good for the use in a semiconductor laser. But, As-vacancy systems, where the indirect gaps appear, are not feasible for the same application. It has been found that the latter phenomenon is induced by a newly formed density of state, which comes from the overlapping of hybridised 4s and 4p orbitals of Ga atoms around As vacancy.     

Vibrational spectroscopy of indium-doped Pb0.9Mn0.1Te alloy

Raman scattering and far-infrared reflection spectra of 0.5 at.% In doped Pb_0.9Mn_0.1Te single crystal at temperatures between 10 and 300K are presented. The infrared spectra have been fitted using a modified plasmon-phonon interaction model with an additional oscillator (at about 122 cm^?1) representing a local In-impurity mode. Phonons in this mixed crystal exhibit intermediate mode behavior. The plasma frequency decreases on cooling from 300 to 25K and increases sharply between 20 and 10 K. The results of galvanomagnetic measurements are also presented.     

Local structure of Mn in (Ga,Mn)As probed by X-ray absorption spectroscopy

Local structure of Mn atoms in Ga_1−xMn_xAs epilayers was studied using the X-ray absorption fine structure (XAFS) at Mn K-edge. X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques were used. XAFS spectra for different Mn sites has been calculated and compared with the experimental data. Multi-parameter fitting of the EXAFS data indicates that 15-25% of Mn atoms are in interstitial sites in the as grown films. The Mn-As bond length has been found longer than Ga-As bond length in GaAs for both substitutional (Mn_Ga) and interstitial (Mn_I) sites.     

Weak localization in Al0.25Ga0.75As/In0.2Ga0.8As/GaAs wires

The phase coherence length L_Ø, its temperature dependence and the spin-orbit scattering length L_SO in Al_0.25Ga_0.75As/In_0.2Ga_0.8As/GaAs wires fabricated by electron-beam lithography and CH₄/H₂ Reactive Ion Etching (RIE), were extracted by fitting a 1-dimensional weak localization theory to two-terminal measurements in the temperature range between 2 K and 50 K. The scattering mechanism was found to be Nyquist (electron-electron collisions with small energy transfer).     

Temperature changes in the photoluminescence and the intrinsic absorption edge of the (Ga0.1In0.9)2Se3 crystal

The photoluminescence and the intrinsic absorption edge of the (Ga_0.1In_0.9)₂Se₃ uniaxial crystal are investigated in the temperature range 77–300 K. Exciton and impurity luminescence bands are revealed at low temperatures and the Urbach tail of the absorption edge is found in the temperature range under study. The temperature dependences of the spectral position and the half-width of the luminescence bands, as well as the optical pseudogap and the energy width of the absorption edge, are studied. The mechanisms of radiative recombination and light absorption and the processes of lattice disordering in the (Ga_0.1In_0.9)₂Se₃ crystal are discussed.     

Negative and persistent photoconductivity in p-type Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As heterostructures under uniaxial stress

Abstract

Illumination of a double p-Al_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5As heterostructure by a red light emitting diode results in a negative photoconductivity that, after the diode is switched off, slowly relaxes to a positive persistent photoconductivity, characterised by an approximately 1.5-fold increase of the two-dimensional hole concentration. This metastable state may be explained with a model in which deep electron traps are supposed to be located above the Fermi level on the inverted heterointerface. Under uni-axial compression the hole concentration in the persistent photoconductivity state, as well as in the dark state, demonstrates the same linear decrease.     

Dependence of propagation loss on etching depth in Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As strip-loaded type waveguides

Al_0.3Ga_0.7As/GaAs/Al_0.3Ga_0.7As strip-loaded waveguides were fabricated using a broad-beam electron cyclotron resonance (ECR) ion source. It was found that a very smooth etching profile can be obtained by ECR ion etching and the etching rate of Al_0.3Ga_0.7As is 70 nm min^-1. The propagation losses of strip-loaded type III–V compound semiconductor waveguides with various etching depths were studied by the Fabry-Perot cavity method. It was observed that the reflectance at the cleavage increases slightly with etching depth for TE polarization. The propagation loss is measured as 1.5 dB cm^-1 for etching depth of 0.7 m, less than 1 dB cm^-1 for 0.8 m, and 3.5 dB cm^-1 for 1.1 m.     

Self-mixing interference effects in tunable diode laser absorption spectroscopy

We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (e.g. cell windows) and diffuse reflections (e.g. Spectralon^™ and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy and discuss the implications for real gas detectors.     

Femtosecond multicolor transient absorption spectroscopy of colloidal silver nanoparticles

Time-resolved femtosecond multicolor absorption spectroscopy of silver nanoparticle (NP) colloids with particle diameter in range of 10–30 nm is presented. The amplified femtosecond excitation of the surface plasmon resonance band resulted in transient absorption spectra reflecting the electron-phonon relaxation dynamics, which takes place on the early picosecond time scale. The monitored band with enhanced absorption in the 490–540 nm spectral range exhibited red-shift with increasing pump fluency from 0.4 mJ/cm² to the 1.5 mJ/cm² level. The growth of the relaxation time with increasing pump fluency reveals the temperature dependent relaxation dynamics caused by the nanometer sized electron confinement in the case of silver. This effect was confirmed also by identification of the relaxation time dependence on the particle diameter at constant pump fluency. The complex experimental results revealed nonlinearities both in the laser excitation and electron relaxation processes.     

Femtosecond laser-induced nonlinear spectroscopy for remote sensing of methane

Femtosecond laser-induced nonlinear molecular spectroscopy is applied for sensing atmospheric greenhouse gas methane (CH₄). The high intensity inside the Ti-sapphire femtosecond laser filaments can dissociate the CH₄ molecules into small fragments which emit characteristic fluorescence. Backward CH radical fluorescence is used to quantitatively analyze the pollutant concentration and its remote detection limit. PACS 42.68.Wt; 42.65.Jx; 95.75.Fg