Cascaded Raman shifting of high-peak-power nanosecond pulses in As₂S₃ and As₂Se₃ optical fibers

We report efficient cascaded Raman scattering of near-IR nanosecond pulses in large-core (65 μm diameter) As?S? and As?Se? optical fibers. Raman scattering dominates other spectral broadening mechanisms, such as four-wave mixing, modulation instability, and soliton dynamics, because the fibers have large normal group-velocity dispersion in the spectral range of interest. With ~2 ns pump pulses at a wavelength of 1.9 μm, four Stokes peaks, all with peak powers greater than 1 kW, have been measured.     

High Q light-emitting Si-rich Si₃N₄ microdisks

We report on the optical properties of active silicon (Si)-rich Si?N? microdisk cavities in the visible range. We have studied the correlation between the quality (Q) factor of the cavities and the active material deposition parameters. Microphotoluminescence measurements revealed subangstrom whispering galley modes resonances and a maximum Q of 10? around 760 nm. These values improve significantly the best results reported so far for Si-based light-emitting circular resonators in the visible range. In contrast to what is reported for Si-rich SiO?-based microcavities, we demonstrate the absence of a spectral widening at high pump fluxes associated to carrier absorption mechanisms, which allows high emitted power without degrading the Q of the cavity. These results open the route toward the monolithic integration of those structures into more complex circuits including Si photodetectors.     

High-efficiency single-crystal third-harmonic generation in BiB₃O₆

Third-harmonic generation of high-intensity, sub-100-fs idler pulses from a Ti:sapphire-laser-pumped optical parametric amplifier is demonstrated by using a single nonlinear crystal of BiB?O? (BIBO). Maximum internal energy conversion as high as 11% from the fundamental to the third harmonic is achieved by phase- and group-velocity matching for the direct cubic nonlinear process together with the velocity-mismatched cascading quadratic processes.     

In-plane transport and enhanced thermoelectric performance in thin films of the topological insulators Bi₂Te₃ and Bi₂Se₃

Several small-band-gap semiconductors are now known to protect metallic surface states as a consequence of the topology of the bulk electron wave functions. The known "topological insulators" with this behavior include the important thermoelectric materials Bi?Te? and Bi?Se?, whose surfaces are observed in photoemission experiments to have an unusual electronic structure with a single Dirac cone. We study in-plane (i.e., horizontal) transport in thin films made of these materials. The surface states from top and bottom surfaces hybridize, and conventional diffusive transport predicts that the tunable hybridization-induced band gap leads to increased thermoelectric performance at low temperatures. Beyond simple diffusive transport, the conductivity shows a crossover from the spin-orbit-induced antilocalization at a single surface to ordinary localization.     

Symmetry-breaking lattice distortion in Sr₃Ru₂O₇

The electronic nematic phase of Sr?Ru?O? is investigated by high-resolution in-plane thermal expansion measurements in magnetic fields close to 8 T applied at various angles Θ off the c axis. At Θ < 10° we observe a very small (10??) lattice distortion which breaks the fourfold in-plane symmetry, resulting in nematic domains with interchanged a and b axis. At Θ ? 10° the domains are almost fully aligned and thermal expansion indicates an area-preserving lattice distortion of order 2 × 10?? which is likely related to orbital ordering. Since the system is located in the immediate vicinity of a metamagnetic quantum critical end point, the results represent the first observation of a structural relaxation driven by quantum criticality.     

Magnetic field induced dehybridization of the electromagnons in multiferroic TbMnO₃

We have studied the impact of the magnetic field on the electromagnon excitations in TbMnO? crystal. Applying a magnetic field along the c axis, we show that the electromagnons transform into pure antiferromagnetic modes, losing their polar character. Entering in the paraelectric phase, we are able to track the spectral weight transfer from the electromagnons to the magnon excitations and we discuss the magnetic excitations underlying the electromagnons. We also point out the phonons involved in the phase transition process. This reveals that the Mn-O distance plays a key role in understanding the ferroelectricity and the polar character of the electromagnons. Magnetic field measurements along the b axis allow us to detect a new electromagnon resonance in agreement with a Heisenberg model.     

Hybrid Si-LiNbO₃ microring electro-optically tunable resonators for active photonic devices

Hybrid Si-LiNbO? electro-optic tunable ring resonators have been proposed and demonstrated as a path to achieving ultracompact and high-speed electro-optic devices. Free standing single crystal LiNbO? microplatelets (~mm long and ~1 μm thick) were obtained from a z-cut LiNbO? substrate by ion implantation and thermal treatment. The platelets were transferred and thermally bonded on top of Si resonators that were fabricated in a Si-on-insulator platform by a 0.18 μm standard complementary metal-oxide-semiconductor process. For the hybrid microring resonator, a free spectral range of 16.5 nm, a finesse F of ~1.67 × 102, a Q-factor of ~1.68 × 10?, and an effective r coefficient of ~1.7 pm/V were achieved for the TE mode. These values are in good agreement with the calculated results.     

Kittel law in BiFeO₃ ultrathin films: a first-principles-based study

A first-principles-based effective Hamiltonian is used to investigate the thickness dependency of the size of straight-walled domains in ultrathin films made of the multiferroic BiFeO? (BFO) material. It is found that the Kittel law is followed, as in ferroelectric or ferromagnetic films. However, an original real-space decomposition of the different energetic terms of this effective Hamiltonian allows the discovery that the microscopic origins of such a law in BFO films dramatically differ from those in ferroelectric or ferromagnetic films. In particular, interactions between tilting of oxygen octahedra around the domain walls and magnetoelectric couplings near the surface (and away from the domain walls) play an important role in the observance of the Kittel law in the studied BFO films.     

Gate-voltage control of chemical potential and weak antilocalization in Bi₂Se₃

We report that Bi?Se? thin films can be epitaxially grown on SrTiO? substrates, which allow for very large tunablity in carrier density with a back gate. The observed low field magnetoconductivity due to weak antilocalization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest a much suppressed bulk conductivity at large negative gate voltages and a possible role of surface states in the WAL phenomena.     

Control of light polarization by acousto-optic diffraction from leaky acoustic wave in LiNbO₃

The acousto-optic control of light polarization due to diffraction by leaky acoustic waves in ZX-LiNbO? has been demonstrated. The randomly polarized light of 633 nm wavelength is converted by the anisotropic diffraction into two beams with mutually orthogonal polarizations, the relative intensities of which depend on the light incidence angle and acoustic frequency. Variation in acoustic frequency from 108 to 112 MHz rotates the polarization of the output optical beam by 90°. The acousto-optic control is accomplished entirely by electronic means and can be applied for implementation of fast polarization converters.     

Combination transitions due to stretching and librations of OH⁻ ions in LiNbO₃

A new absorption band has been detected at 4009 cm?1 in stoichiometric LiNbO? single crystals. Based on present and earlier experiments and recent calculations, this band has been assigned as a combination transition involving the OH stretching and an OH librational mode. The librational mode participating in this combination is of lower fundamental frequency than that contributing to the combination band at 4415 cm1 observed by Gr?ne and Kapphan [J. Phys. Condens. Matter 7, 3051 (1995)].     

Polarization switching without domain formation at the intrinsic coercive field in ultrathin ferroelectric PbTiO₃

Polarization switching in ferroelectrics has been thought to occur only through the nucleation and growth of new domains. Here we use in situ synchrotron x-ray scattering to monitor switching controlled by applied chemical potential. In sufficiently thin PbTiO? films, nucleation is suppressed and switching occurs by a continuous mechanism, i.e., by uniform decrease and inversion of the polarization without domain formation. The observed lattice parameter shows that the electric field in the film during switching reaches the theoretical intrinsic coercive field.     

Cycloidal order of 4f moments as a probe of chiral domains in DyMnO₃

Using soft x-ray diffraction at the Dy-M? resonance, pronounced circular dichroism in the ferroelectric phase of DyMnO? is observed in connection with sizable b and c components of the Dy-4f magnetic moments. This provides strong evidence for cycloidal order of the 4f moments, corroborating that inversion-symmetry breaking in this material is not accomplished by the Mn spins alone. The 4f circular dichroism allows us to image multiferroic domains that are imprinted on the surface of DyMnO? using the local charging by the x-ray beam via the photoelectric effect.     

Octave spanning supercontinuum in an As₂S₃ taper using ultralow pump pulse energy

An octave spanning spectrum is generated in an As?S? taper via 77 pJ pulses from an ultrafast fiber laser. Using a previously developed tapering method, we construct a 1.3 μm taper that has a zero-dispersion wavelength around 1.4 μm. The low two-photon absorption of sulfide-based chalcogenide fiber allows for higher input powers than previous efforts in selenium-based chalcogenide tapered fibers. This higher power handling capability combined with input pulse chirp compensation allows an octave spanning spectrum to be generated directly from the taper using the unamplified laser output.     

Broadband, rapidly tunable Ti:sapphire-pumped BiB₃O₆ femtosecond optical parametric oscillator

We report a femtosecond optical parametric oscillator (OPO) based on the nonlinear material BiB?O? (BIBO) pumped directly by a Kerr lens mode-locked Ti:sapphire laser. Using a 1.5 mm long BIBO crystal cut at θ=11.4° for collinear type I (e→o+o) phase matching in the xz optical plane, femtosecond signal pulses across 1.4-1.6 μm, and idler pulses across 1.6-1.87 μm spectral range are generated, limited by the reflectivity bandwidth of the OPO mirrors. The high nonlinear gain and large spectral acceptance for type I interaction in the xz plane of BIBO permit rapid and continuous tuning across the entire range by simple fine adjustment of OPO cavity delay or through small changes in the pump wavelength, without varying any other parameters. Additionally, owing to the near-zero group velocity mismatch and dispersion, the OPO supports broad spectrum as wide as 33 nm, which results in self-compressed signal pulses. For 150 fs pump pulses, signal pulses with durations down to 106 fs with a time-bandwidth product of 0.48 are obtained without the need for intracavity dispersion compensation.     

1073 nm continuous-wave Nd:YVO₄ laser with Type I phase-matching LiB₃O₅

Continuous output from a Nd:YVO? crystal around 1073 nm was studied. The output of 1064 nm was suppressed with a Type I phase-matching LiB?O? (LBO) crystal rotated by 45°. With a pump power of 4.5 W, a 300 mW continuous output at 1073 nm was achieved for what we believe is the first time, and the optical-to-optical efficiency was 7%. With an adjustable LBO crystal, the wavelength is tunable from 1073.2 to 1073.5 nm. Even simultaneous output of 1066 and 1073 nm was realized by adjusting the LBO crystal. The experiments proved related theoretical analysis.     

Cerenkov third-harmonic generation via cascaded χ(2) processes in a periodic-poled LiTaO₃ waveguide

We report a type of quasi-phase-matched (QPM) Cerenkov third-harmonic generation (CTHG) in a periodic-poled LiTaO? waveguide. The CTHG results from a guided-to-guided second-harmonic generation cascaded with a guided-to-radiated sum-frequency generation (SFG) in the waveguide. In the guided-to-radiated SFG process, nonlinear interactions with participating and nonparticipating reciprocal vectors would lead to different CTHG radiations. In addition, the power and temperature detuning characters of QPM CTHG were studied. Theoretical predictions were in good agreement with experimental results.     

Detection of internal cracks and ultrasound characterization of nanostructured Bi₂Te₃-based thermoelectrics via acoustic microscopy

The search for thermoelectric (TE) materials for highly efficient devices aims at improving the TE efficiency and broadening their areas of applications. We created nanostructured thermoelectric Bi-Sb-Te-family materials by high energy (ball milling) pre-treatment of the parent materials followed by high-pressure/high-temperature treatment. Bi_0.5Sb_1.5Te₃ compositions with the superfluous maintenance of tellurium was used for the synthesis of the samples with p-type electrical conductivity. Acoustic microscopy was used to study elastic properties and bulk irregularities and to detection of internal cracks both in the parent materials and in the created nanostructured samples. The data has been used for optimization of parameters of synthesis of nanostructured thermoelectrics.     

Dynamic modelling and simulation of ¹⁵N separation by chemical exchange in NO, NO₂-HNO₃ system

The dynamic behaviour of a 1?N separation process by chemical exchange in a NO, NO?-HNO? system has been analysed based on an accurate mathematical model. A nonlinear system of first-order partial differential equations was determined by considering the multiple exchange reactions between the components of the gaseous mixture and the liquid phase constituents. The mathematical model of the process describes the space-time variation of the 1?N mole fraction in gas and liquid phases and provides a better understanding of operating limits and decision support in process design and optimisation.     

Oscillatory spin polarization and magneto-optical Kerr effect in Fe₃O₄ thin films on GaAs(001)

The spin dependent properties of epitaxial Fe?O? thin films on GaAs(001) are studied by the ferromagnetic proximity polarization (FPP) effect and magneto-optical Kerr effect (MOKE). Both FPP and MOKE show oscillations with respect to Fe?O? film thickness, and the oscillations are large enough to induce repeated sign reversals. We attribute the oscillatory behavior to spin-polarized quantum well states forming in the Fe?O? film. Quantum confinement of the t(2g) states near the Fermi level provides an explanation for the similar thickness dependences of the FPP and MOKE oscillations.