Nonlinear optical spectroscopy of electronic transitions in hexagonal manganites

The hexagonal rare-earth manganites RMnO₃ (R = Sc, Y, Ho, Er, Tm, Yb, Lu) are a group of materials with an unusual combination of magnetic, electric and optical properties. The electronic structure of these materials was studied by second harmonic (SH) spectroscopy in the range from 1.2 to 3.0 eV. Faraday rotation and absorption spectra were measured in the range from 1.0 to 1.6 eV. Broad bands at ∼1.7 eV and ∼2.7 eV are assigned to electronic transitions between Mn³⁺(3d⁴) levels. The SH spectra are discussed on the basis of a recently developed microscopic theory. Received: 26 April 2001 / Published online: 18 July 2001     

Synthesis and field-emission properties of the tungsten oxide nanowire arrays

High-density, uniformly distributed and quasi-aligned tungsten oxide nanowire arrays have been synthesized by a conventional thermal evaporation approach on indium tin oxide (ITO) coated glass substrates without any catalysts. The temperature of the substrate was . The tungsten oxide nanowires are single crystalline with growth direction of [0 1 0]. For commercial applications, field emission properties of tungsten oxide nanowires were studied under a poor vacuum at room temperature. The electron field-emission turn-on field (E_to), defined as the macroscopic field required to produce a current density of , is about . The performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for commercial application in field-emission displays.     

Characterisation of laser-produced tungsten plasma using optical spectroscopy method

This paper describes results of spectroscopic investigation of laser-produced tungsten plasma. The laser intensity on the target surface reached up to 30 GW/cm² depending on the focusing conditions. Optical spectra emitted from plasma plumes which were formed under vacuum conditions in front of the tungsten target due to the interaction of Nd-YAG laser pulses (1.06 μm, 0.5 J), were characterised by means of an optical spectrometer (λ/Δλ= 900) in the wavelength range from 300 to 1100 nm. The spectra were recorded automatically with the use of a CCD detector with exposition time varied from 100 ns to 50 ms. On the basis of WI and WII lines it was possible to estimate electron temperature and electron density which corresponded to the expansion phase of the plasma. T_e and N_e were measured as 1.1 eV and 8×10¹⁶ cm^-3, respectively. The spectra collected by the ion energy analyser showed that the plasma included tungsten ions up to 6+ ion charge. Signals from the ion collector allowed to estimate the average value of ion energy of tungsten as 4.6 keV. Basing on this value the electron temperature corresponding to the initial stage of the plasma formation was estimated to be about 320 eV. Optical microscope investigation showed that laser irradiation caused structural changes on the surface of the target.     

Auger transitions in narrow-gap semiconductors with lax band structure

Auger recombination and ionisation rates are calculated in narrow-gap semiconductor with Lax band structure⁸ such as Pb_1?xSn_xTe, Pb_1?xSn_xSe, Bi_1?xSb_x. Both low-energy (T_e? ε_g) and high-energy (ε_F ? ε_g or Te ? ε) regions are taken into consideration. In the former case we have corrected the results obtained by number of authors^3,4. The latter situation may take place in pinch channel or at the high level of laser exitation. Here we have derived the results except for constant numerical coefficient. We consider the two-particle Auger-process proposed by Emtage³ as well as three-particle ones including additional collision with acoustical phonon, charged impurity or neutral defect. The dependence of Auger transition rates upon the carrier concentration is mainly of the power-type in all variants.     

Vibrational spectroscopy of alkaline tungsten oxyfluoride crystals: structure,lattice dynamics,ordering processes,and phase transitions

Raman scattering (RS) and infrared absorption (IR) of ammonium oxyfluoride crystals (NH₄)₃WO₃F₃, (NH₄)₂KWO₃F₃, and Cs₂(NH₄)WO₃F₃are compared. Conformation of the WO₃F₃ octahedral groups has been established; anomalies have been found close to the transition temperatures in the internal vibrational regions of ammonium and WO₃F₃ groups. The phase transition in (NH₄)₃WO₃F₃ is associated mostly with the ordering of octahedral groups and formation of W O···H N hydrogen bonds. In (NH₄)₂KWO₃F₃ crystal, the transition is not related to the ordering processes; Cs₂(NH₄)WO₃F₃ retains its disordered structure down to 10 K. Copyright © 2010 John Wiley & Sons, Ltd.     

Energy band structure and thermo-mechanical properties of tungsten and tungsten carbides as studied by the LMTO-ASA method

The calculations of the energy band structures of W (b.c.c. lattice), WC (f.c.c. and h.c.p. lattices) and W₂C have been performed by the LMTO-ASA method. The results of the band structure calculations have been used for the evaluations of the unit cell parameters, bulk moduli, Debye temperatures. The values obtained are in good agreement with the experimental data. It is shown that h.c.p. WC has the highest bulk modulus, while W₂C possess the highest density-of-states at the Fermi level. The calculations reveal a noticeable charge transfer from tungsten to carbon in WC, while the effective charges in W₂C are rather small.     

Modification of field-emission currents from tungsten by external magnetic fields

Experimental work is described on the changes produced by an external magnetic field on field emission currents from tungsten needles held at helium temperatures. A steady decrease with field above a critical field is reported together with oscillatory variations of the order of 10 – 15%.     

Contactless electroreflectance spectroscopy of optical transitions in low dimensional semiconductor structures

The authors present the application of contactless electroreflectance (CER) spectroscopy to study optical transitions in low dimensional semiconductor structures including quantum wells (QWs), step-like QWs, quantum dots (QDs), quantum dashes (QDashes), QDs and QDashes embedded in a QW, and QDashes coupled with a QW. For QWs optical transitions between the ground and excited states as well as optical transitions in QW barriers and step-like barriers have been clearly observed in CER spectra. Energies of these transitions have been compared with theoretical calculations and in this way the band structure has been determined for the investigated QWs. For QD and QDash structures optical transitions in QDs and QDashes as well as optical transitions in the wetting layer have been identified. For QDs and QDashes surrounded by a QW, in addition to energies of QD and QDash transitions, energies of optical transitions in the surrounded QW have been measured and the band structure has been determined for the surrounded QW. Finally some differences, which can be observed in CER and photo-reflectance spectra, have been presented and discussed for selected QW and QD structures.     

A new bound-exciton emission band in ZnSe crystals and multiplasmon optical transitions

We have investigated cathodoluminescence both in unannealed ZnSe crystals and in crystals annealed in a Bi melt at a temperature of 1200K for 120 h with subsequent quenching. In the wavelength range 450–480 nm we have detected a new line series I _i ^s -nLO-mPl consisting of the bound-exciton emission line I _i ^s with wavelength λ=455.9 nm and its plasmon and LO-phonon echoes I _i ^s -LO (λ ₁=461.3 nm), I _i ^s -2LO (λ ₂=466.8 nm), I _i ^s -3LO (λ ₃=472.4 nm), and I _i ^s -4LO (λ ₄=478.3 nm). We have determined the mean number of emitted LO phonons N _LO=2.2±0.1 per photon. It is shown that the observed finer structure of the band may be due to multiphonon optical transitions. At low plasma densities (ω _p ≪ω _LO ) the Coulomb interaction causes broadening of the I _i ^s -nLO series. In samples with denser plasma, in which the condition ω _p ⩽ω _LO is met, multiplasmon satellites of the series I _i ^s -nLO-mPl are observed. Theoretical calculations of the shape of the emission band agree with experiment. Fiz. Tverd. Tela (St. Petersburg) 41, 1176–1180 (July 1999)     

Photoelectron spectroscopy studies of the band structure of silver halides

X-ray photoelectron spectroscopy has been used to probe the valence bands of the silver halides. Previous ultraviolet photoemission and optical absorption experiments together with the theoretical band structure calculations form the basis for interpretation of the spectra. Density of states maxima from the halogen p levels are clearly resolved from those of the silver 4d states. Additional splittings due to k space symmetry are observable and in the case of AgCl and AgBr, give excellent conformation of existing band structure calculations. The role of spin-orbit coupling is shown to be unimportant in determining the primary shape of the photoelectron spectrum. The spectrum of AgF shows the inversion of silver 4d and halogen 2p levels suggested by the optical absorption spectra and predicted by several recent band calculations. A new interpretation is proposed for the AgF optical spectrum in which the observed excitons are due to the forbidden Γ₁₂→ Γ₁ and Γ_25′ → Γ₁ tr the high temperature body centered cubic structure. These spectra clearly show the broadening of the I 5p levels which causes the band gap to decrease above the transition temperature.     

Two-electron band to band transitions in silicon

Radiative two-electron band to band transitions are investigated experimentally in silicon. The question of phonon-participation is investigated. It turns out that at low temperature the phononless transitions dominate, whereas above room-temperature the phonon-assisted transitions become more important. A theoretical discussion leads to the conclusion that at least one participating phonon is an acoustical one.     

Photo field emission spectroscopy of Γ-P AND Γ-〈013〉 bands of tungsten

Photo field emission currents from clean (013) and clean and barium covered (111) regions of a tungsten tip have been measured in a field emission microscope using two wavelengths of laser radiation and phase-sensitive detection. The photo field emission current-voltage characteristics show various slopes and shoulders. These shoulders are considered as correlated with optical transitions between energy bands of tungsten. The final and initial energies are evaluated from the excitation energy and the known lowering of the barrier maximum. The obtained results are in fairly good agreement with the band structure of tungsten calculated by Christensen and Feuerbacher [1]. Except direct transitions which are prevailing also some indirect transitions were found. The slopes of the photocurrent characteristics are discussed.     

Quasiparticle band structure and optical spectrum of LiBr

The electronic structures of the Fe-doped perovskite ruthenates BaRu_1?x Fe _x O₃ with x = 0, 0.25, 0.5, 0.625, 0.75, and 1 are investigated through density-functional calculations. Large exchange splitting and small crystal field splitting are found in BaFeO₃, and a contrary scenario can take place on BaRuO₃ as expected since the Ru atom has a highly extended 4d orbital. The small exchange splitting and extended 4d states are the reasons why the obtained spin magnetic moment (0.628μ _B ) is significantly lower than the spin only value (2μ _B ) for the t _2g 3↑ t _2g 1↓ electronic configuration for Ru⁴⁺ ion. Further investigations suggest that Fe substitution at the Ru sites can suppress the bandwidths of Ru 4d orbital, leading to the half-metallic behaviour in BaRu_1?x Fe _x O₃ with x = 0.625 and 0.75. The different orbital feature of the Ru⁴⁺ ions in BaRu_0.375Fe_0.625O₃ is presented, which reflects the influence of Fe dopant on Ru 4d orbitals.     

GaSe valence band structure from angle-resolved photoemission spectroscopy

Energy-wavevector dispersion curves for the GaSe valence bands obtained from angle-resolved photoemission spectra are compared with pseudopotential band calculations. It is found that the third density-of-states feature below the top of the valence bands (peak C) is related to the Ga-Se bond rather than the Ga-Ga bond. A peak not appearing in the angle-integrated spectra appears as the most intense feature at normal emission, and helps to identify completely the nature of the valence bands down to 7–8 eV below the Fermi energy.     

Compensation of field-induced frequency shifts in Ramsey spectroscopy of optical clock transitions

We have extended Ramsey spectroscopy by stepping the probe frequency during the two Ramsey excitation pulses to compensate frequency shifts induced by the excitation itself. This makes precision Ramsey spectroscopy applicable even for transitions that have Stark and Zeeman shifts comparable to the spectroscopic resolution. The method enables a new way to evaluate and compensate key frequency shifts, which benefits in particular, optical clocks based on magnetic field-induced, spectroscopy, two-photon transitions, or heavily forbidden transitions.     

Magnetic field-induced spectroscopy of forbidden optical transitions with application to lattice-based optical atomic clocks

We develop a method of spectroscopy that uses a weak static magnetic field to enable direct optical excitation of forbidden electric-dipole transitions that are otherwise prohibitively weak. The power of this scheme is demonstrated using the important application of optical atomic clocks based on neutral atoms confined to an optical lattice. The simple experimental implementation of this method--a single clock laser combined with a dc magnetic field--relaxes stringent requirements in current lattice-based clocks (e.g., magnetic field shielding and light polarization), and could therefore expedite the realization of the extraordinary performance level predicted for these clocks. We estimate that a clock using alkaline-earth-like atoms such as Yb could achieve a fractional frequency uncertainty of well below 10(-17) for the metrologically preferred even isotopes.     

Energy band structure of spin-1 condensates in optical lattices

The energy band structure of spin-1 condensates with repulsive spin-independent and either ferromagnetic or antiferromagnetic spin-dependent interactions in one-dimensional (1D) periodic optical lattices is discussed. Within the two-mode approximation, Bloch bands of spin-1 condensates are presented. The results show that the Bloch bands exhibit a complex structure as the atom density of m F=0 hyperfine state increases: bands splitting, reversion, intersection and loop structure are excited subsequently. The complex band structure should be related to the tunneling and spin-mixing dynamics.