Measurements of insulator band parameters using combination of single-electron and two-electron spectroscopy

We describe the application of low energy time-of-flight coincidence (e,2e) spectroscopy for measurements of the energy band parameters of a dielectric. The (e,2e) spectrometer can operate also in a single-electron mode by switching off coincidence conditions, and can be used for recording electron energy loss spectra (EELS). Thus, the combination of (e,2e) and EELS allows the measurement of energy gap E_g, valence bandwidth ΔE_val, electron affinity χ and excitonic levels position E_ex of a dielectric. The energy band parameters of LiF film deposited on Si(001) surface are measured: ΔE_val=      

Effect of varying effective index and random deviations of domain boundary on sum-frequency-generation in quasi-phase-matching waveguides

The effect of varying effective index and random deviations of domain boundary on sum frequency generation (SFG) in quasi-phase-matching waveguides are analyzed and analytical expressions of the conversion efficiency are provided for the first time to our knowledge. By simplifying the coupled equations of SFG in waveguide, we provide analytical expressions of the conversion efficiency for the first time. Four models for variations of effective index and random errors of domains are investigated. The results demonstrate that, when effective index varying regularly along the waveguide, the normalized conversion efficiency of SFG is reduced and the curves of are no longer sinc² function but ripple profiles, which is different from the situation in bulk crystals; with the same magnitude, random period error is of much more significance than random duty cycle error, which is similar with the case in bulk crystals.     

Controlled Stark shifts in Er-doped crystalline and amorphous waveguides for quantum state storage

We present measurements of the linear Stark effect on the ⁴I_15/2 → ⁴I_13/2 transition in an Er³⁺-doped proton-exchanged LiNbO₃ crystalline waveguide and an Er³⁺-doped silicate fiber. The measurements were made using spectral hole burning techniques at temperatures below 4 K. We measured an effective Stark coefficient (Δμ_eχ)/(h) = 25 ± 1 kHz/V cm⁻¹ in the crystalline waveguide and  kHz/V cm⁻¹ in the silicate fiber. These results confirm the potential of erbium-doped waveguides for quantum state storage based on controlled reversible inhomogeneous broadening.     

Mode competition in acousto-optically Q-switched planar waveguide lasers

Multi-mode rate equations have been developed to investigate mode competition in high-power acousto-optically Q-switched planar waveguide lasers. The mode competition arises from coupling effects and temporal losses in the transform between guided modes and free-space propagation. Pulse-to-pulse instability and temporal beam distortions are enlarged by mode competition when the laser works in the multi-mode regime. The influence of parasitic oscillation is also discussed. A Nd:YAG planar waveguide laser has been established with a folded hybrid/unstable resonator. A maximum average power of 83 W with a beam propagation factor is obtained. The theoretical simulation agrees well with the experimental observation.     

Theoretical study of local lattice structure of Mn in perovskite fluorides A2MF4 (A=K, Rb; M=Mg, Zn, Cd) series

A theoretical method for investigating the inter-relation between the molecular structure and electronic structure has been established on the basis of the 252×252 complete energy matrices for a 3d⁵ configuration ion in a tetragonal ligand field. By means of this method, which is independent of the X-ray diffraction, the local structure of the paramagnetic Mn²⁺ ion in perovskite fluorides A₂MF₄ (A=K, Rb; M=Zn, Mg, Cd) are determined directly by analyzing the EPR spectrum of octahedral Mn²⁺ center in A₂MF₄ crystals and the optical absorption spectrum of the (MnF₆)⁴⁻ cluster. It is shown that, comparing with the octahedral cubic structure, the local micro-structure in the vicinity of Mn²⁺ displays an elongated distortion when and a compressed distortion when , and ΔR vs. as well as ΔR vs. in the distortion region is, respectively, approximately linear. Simultaneously, the theoretical zero-field-splitting parameters , and are in good agreement with the experimental values.     

Low-temperature measurements of magnetic susceptibility and specific heat of Eu2Ti2O7—An XY pyrochlore

Magnetic susceptibility obtained from magnetization measurement (for fields H=0.1 and 1.0 T) of polycrystalline Eu₂Ti₂O₇ shows two distinct features. Firstly, increases on cooling below 300 K and attains a temperature-independent constant value at 68 K (T_max). Secondly, shows an antiferromagnetic increase below 4.9±0.1 K. The former behavior is explained by crystal field (CF) theory. CF levels and wave functions of ground and excited states are determined accurately from analyses of and earlier reported Mössbauer and optical spectra. Analysis of vs. 1/T curve at low temperatures gives the classical nearest-neighbor exchange interaction J^cl=−0.76 K and a weak dipolar interaction D_nn=0.0056 K. C_P of polycrystalline sample of Eu₂Ti₂O₇ and Y₂Ti₂O₇ are measured between 1.8-35 and 1.8-120 K respectively and θ_D vs. T (K) curves are calculated. At 4 K, θ_D of Eu₂Ti₂O₇ shows a kink and dC_P/dT curve show a maximum. Optical results show energy exchange between Eu³⁺ ions at intrinsic and extrinsic (defect) sites via super-exchange interaction at low temperature which may account for the observed anomalous behavior of and C_P.     

Ground state electronic and magnetic properties of RCrSb3 (R=La, Ce, Nd, Gd and Dy): A first principles study

The electronic density of states (DOS) and magnetic moments of rare-earth antimonides (RCrSb₃) have been studied by the first principles full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). For the exchange-correlation potential, the LSDA+U method is used. The effective moments of LaCrSb₃, CeCrSb₃, NdCrSb₃, GdCrSb₃, and DyCrSb₃ were found to be , , , and respectively. The exchange-splittings of Cr-3d state electrons and 4f-states of rare earth elements were analyzed to explain the magnetic nature of these systems. The Cr atom plays a significant role on the magnetic properties due to the hybridization between Cr-3d and Sb-5p state orbitals. The results obtained are compared and found to be in close agreement with the available data.     

Giant intersubband polariton splitting in InAs/AlSb microcavities

The optical response of the intersubband excitation of multiple InAs/AlSb quantum wells embedded in a planar semiconductor microcavity has been studied through angle-dependent reflectance measurements. Using a resonator based on total internal reflection, a strong coupling is demonstrated between the intersubband optical transition and the cavity photon, with the attendant formation of intersubband polaritons. A giant vacuum-Rabi splitting 2Ω_R was observed both at liquid helium temperatures () as well as at 300 K (), for a transition energy . The observed ratio is a record high value (14%) for any strongly-coupled systems, and demonstrates the huge potential of this material for the achievement of the ultra-strong coupling regime predicted theoretically.     

Integral plug-in RF module in a CO2 hybrid-waveguide laser: Its performance and overall evaluation

This work describes the performance of a compact “in-house built” radio–frequency (RF)-excited CO₂ slab waveguide laser which has the innovation of having a plugged-in RF generator–amplifier module directly connected into the positive electrode of the laser head. The design circuit parameters include a matching circuit and a feed-through element as a whole. The overall laser performance takes into account the waveguide dimension (y-axis) as approximately one-tenth of the free space transverse dimension (x-axis). The optical resonator is calculated to be in the regime of the negative branch for unstable confocal resonators, having focal lengths of and with geometrical amplification of 1.108. Optical output coupling mirror was set to 9.7%. The calculated waveguide length is 37.73 cm whilst the total resonator length was adjusted to 42 cm to allow coupling losses less than 1%. The laser operational efficiency was about 12% and the output beam quality of 1.13 which is close to the ideal Gaussian beam. The optical output power was accomplished by playing with different gas compositions to have a final optimized gas proportion of 1:1:2.7:0.3 correspondingly to CO₂, N₂, He and Xe as admixture.     

Prospects of reactor neutrino experiments

Since the energy of a reactor neutrino is a few MeV, all , and oscillations are accessible by reactor neutrino experiments. KamLAND observed the oscillation and currently Double Chooz, RENO and Dayabay experiments are under construction aiming to detect oscillation. There are still good prospects for future reactor neutrino experiments after them. For example, there is room to further improve sin²2θ₁₃ accuracy at a baseline of ∼1.5 km, a very precise sin²2θ₁₂ measurement and the determination of mass hierarchy may be possible at a baseline ∼50 km, and if KamLAND is enlarged to the SuperKamiokande size, better measurement of and sin²2θ₁₂ will be anticipated. It is important to take into account such possibilities when planning future neutrino program after θ₁₃ is measured by current experiments.     

Equation of state of cubic hafnium(IV) nitride having Th3P4 -type structure

Pressure dependence of the specific volume, V(P), of the recently discovered high-pressure compound Hf₃N₄ having cubic Th₃P₄-type structure (c- Hf₃N₄) has been measured at room temperature up to 43.9 GPa in a diamond anvil cell using energy-dispersive X-ray powder diffraction combined with synchrotron radiation. A least-square fit of the Birch-Murnaghan equation of state to the experimental V(P)-data yielded for c- Hf₃N₄ the bulk modulus of and its first pressure derivative of . For fixed at 4 the bulk modulus of c- Hf₃N₄ was determined to be . The obtained B₀-value is only insignificantly below that estimated in preliminary measurements. Existing theoretical predictions for B₀ scatter around the present experimental data. The observation of a high bulk modulus of c- Hf₃N₄ supports the suggestion that this compound could have high hardness.     

Crystal field effect and geometric frustration in Er2Ti2O7 —an XY antiferromagnetic pyrochlore

Magnetic susceptibility of powder Er₂Ti₂O₇ (ErT) is measured between 300 K and 80 K. shows a Curie-Weiss (CW) type behaviour with   ErTiO_3.5 and . A crystal field (CF) analysis of our experimental data, g-values (g_∥=0.27 and g_⊥=7.8) and the positions of two CF levels (reported earlier from an inelastic neutron scattering study) provide CF parameters and CF levels of the ground ⁴I_15/2 and excited multiplets of ErT. The theoretical follows a CW-type behaviour, with . Single-ion magnetic anisotropy (χ_⊥−χ_∥) is 9500×10⁻⁶ emu/mol ErTiO_3.5 at 300 K, which increases by ∼54 times at 10 K and ErT resembles an XY planar system. It can be inferred from CF analysis that the earlier observed change of from −13 K to −22 K below 50 K is not due to the CF effect. Nuclear hyperfine (HF) levels of ¹⁶⁷ErT and ¹⁶⁶ErT are calculated and the theoretical curve of vs. T (K) for T<T_N matches the observed results. Mössbauer lines expected for ¹⁶⁶ErT are also predicted.     

Hyperfine interaction in Co2SiO4 investigated by high resolution neutron spectroscopy

We have investigated the hyperfine interaction in Co₂SiO₄ by inelastic neutron scattering with a high resolution back-scattering neutron spectrometer. The energy spectrum measured from a Co₂SiO₄ powder sample revealed inelastic peaks at at T=3.5 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature . The inelastic peaks have been interpreted to be due to the transition between hyperfine-split nuclear level of the ⁵⁹Co isotopes with spin . The temperature dependence of the energy of the inelastic peak in Co₂SiO₄ showed that this energy can be considered to be the order parameter of the antiferromagnetic phase transition. The determined hyperfine splitting in Co₂SiO₄ deviates from the linear relationship between the ordered electronic magnetic moment and the hyperfine splitting in Co, Co-P amorphous alloys and CoO presumably due to the presence of unquenched orbital moment. These results are very similar to those of CoF₂ recently reported by Chatterji and Schneider [7].     

Theoretical study of polymeric metal clad optical waveguide polarizer

Planar optical waveguides consisting of thin dielectric films and buffer layers with metal cladding have been investigated theoretically. A computer program was written to calculate the exact zeroes of complex eigenvalue equation for TE and TM modes in multilayer metal clad waveguide polarizer. Numerical results and illustrations are given for Polycarbonate waveguide with other polymers as buffer and Al, Ag and Au as cladding metals at . It is also shown that, using thin (finite) films of metal produce more efficient polarizers as compared to semi-infinite metal films. Effect of low index buffer layer on attenuation of TM/TE modes is also investigated.     

Optical phonon modes and structure of ZnGa2Se4 and ZnGa2S4

We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa₂Se₄ and ZnGa₂S₄. Most of the compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry (defect stannite), has been reported. For ZnGa₂Se₄ we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group for the investigated crystals of ZnGa₂Se₄.Regarding ZnGa₂S₄ we have found three modes exclusively showing Raman activity (2A⊕1B₁), and only eight modes showing infrared as well as Raman activity (3B₂⊕5E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa₂S₄ most likely crystallizes in space group .