Optical cavity mode dynamics and coherent phonon generation in high-Q micropillar resonators

International audienceWe study the temporal dynamics of photoexcited carriers in distributed Bragg reflector based semiconductor micropillars at room temperature. Their influence on the process of coherent phonon generation and detection is analyzed by means of pump-probe microscopy. The dependence of the measured mechanical signatures on laser-cavity detuning is explained through a model that accounts for the varying light-cavity coupling existent during the ultrashort times that pump and probe pulses dwell within the structure. To do so, we first explain the optical mode dynamics with an electron-hole diffusion model that accounts for the escape of carriers from the probed area, as well as their recombination in the bulk and on the free surfaces. We thus show that the latter is the most influential factor for pillars below ∼10μm, where 3D confinement of the optical and mechanical fields becomes relevant

Phonon Bloch oscillations in acoustic-cavity structures

We describe a semiconductor multilayer structure based in acoustic phonon cavities and achievable with MBE technology, designed to display acoustic phonon Bloch oscillations. We show that forward and backscattering Raman spectra give a direct measure of the created phononic Wannier-Stark ladder. We also discuss the use of femtosecond laser impulsions for the generation and direct probe of the induced phonon Bloch oscillations. We propose a gedanken experiment based in an integrated phonon source-structure-detector device, and we present calculations of pump and probe time dependent optical reflectivity that evidence temporal beatings in agreement with the Wannier-Stark ladder energy splitting.Comment: PDF file including 4 figure

Uncoupled excitons in semiconductor microcavities detected in resonant Raman scattering

We present an outgoing resonant Raman-scattering study of a GaAs/AlGaAs based microcavity embedded in a p-i-n junction. The p-i-n junction allows the vertical electric field to be varied, permitting control of exciton-photon detuning and quenching of photoluminescence which otherwise obscures the inelastic light scattering signals. Peaks corresponding to the upper and lower polariton branches are observed in the resonant Raman cross sections, along with a third peak at the energy of uncoupled excitons. This third peak, attributed to disorder activated Raman scattering, provides clear evidence for the existence of uncoupled exciton reservoir states in microcavities in the strong-coupling regime

Scaling rules in optomechanical semiconductor micropillars

International audienceSemiconductor pillar microcavities have recently emerged as a promising optomechanical platform in the unprecedented 20-GHz frequency range. Currently established models for the mechanical behavior of micropillars, however, rely on complete numerical simulations or semianalytical approaches, which makes their application to experiments notoriously difficult. Here we overcome this challenge with an effective model by reducing the full, hybridized mechanical mode picture of a micropillar to an approach that captures the observed global trends. We show experimentally the validity of this approach by studying the lateral size dependence of the frequency, amplitude, and lifetime of the mechanical modes of square-section pillar microcavities, using room-temperature pump-probe microscopy. General scaling rules for these quantities are found and explained through simple phenomenological models of the physical phenomena involved. We show that the energy shift ω m of the modes due to confinement is dependent on the inverse of their frequency ω 0 and lateral size L (ω m ∝ 1/ω 0 L 2) and that the mode lifetime τ is linear with pillar size and inversely proportional to their frequency (τ ∝ L/ω 0). The mode amplitude is in turn inversely proportional to the lateral size of the considered resonators. This is related to the dependence of the optomechanical coupling rate (g 0 ∝ 1/L) with the spatial extent of the confined electromagnetic and mechanical fields. Using a numerical model based on the finite-element method, we determine the magnitude and size dependence of g 0 and, by combining the results with the experimental data, we discuss the attainable single-photon cooperativity in these systems. The effective models proposed and the scaling rules found constitute an important tool in micropillar optomechanics and in the future development of more complex micropillar based devices

Magneto-transport study of intra- and intergrain transitions in the magnetic superconductors RuSr2GdCu2O8 and RuSr2(Gd1.5Ce0.5)Cu2O10

A characterization of the magnetic superconductors RuSr2GdCu2O8 [Ru-(1212)] and RuSr2(Gd1.5Ce0.5)Cu2O10 [Ru-(1222)] through resistance measurements as a function of temperature and magnetic field is presented. Two peaks in the derivative of the resistive curves are identified as intra- and intergrain superconducting transitions. Strong intragrain granularity effects are observed, and explained by considering the antiphase boundaries between structural domains of coherently rotated RuO6 octahedra as intragrain Josephson-junctions. A different field dependence of the intragrain transition temperature in these compounds was found. For Ru-(1212) it remains unchanged up to 0.1 T, decreasing for higher fields. In Ru-(1222) it smoothly diminishes with the increase in field even for a value as low as 100 Oe. These results are interpreted as a consequence of a spin-flop transition of the Ru moments. The large separation between the RuO2 layers in Ru-(1222) promotes a weak interlayer coupling, leading the magnetic transition to occur at lower fields. The suppression rate of the intragrain transition temperature is about five times higher for Ru-(1222), a result we relate to an enhancement of the 2D character of the vortex structure. A distinctive difference with conventional cuprates is the sharp increase in amplitude of the intergrain peak in both systems, as the field is raised, which is ascribed to percolation through a fraction of high quality intergrain junctions.Comment: Submitted for Physical Review

Structural analysis, magnetic and transport properties of the (Ru1-xCox)Sr2GdCu2O8 system

The effects of Co substitution on structural and superconducting properties of RuSr2GdCu2O8 compound have been studied. Rietveld refinements of the X-ray diffraction patterns indicate that the cobalt ion progressively replaces ruthenium sites. This replacement induces significant changes on the crystal structure and on the magnetic and superconducting properties. The effects Co substitution on the superconducting behaviour, and more particulary on the changes induce by the hole doping mechanism, were investigated in (Ru1-xCox)Sr2GdCu2O8 by a "bond valence sum" analysis with Co content from x= 0.0 to x = 0.2. The weak ferromagnetic transition at Tm= 138.2 K is shifted to lower temperature, and suppressed at higher Co content. From the crystallographic point of view the Ru-O(1)-Cu bond angle, associated to the rotation of the RuO6 octahedra, around the c-axis remain essetially constant when Ru is substituted by Co. Furthermore, increasing Co content has the effect to increase the weak ferromagnetic moment, which may be interpreted as the main responsible for breaking the delicate balance between magnetic and superconducting ordering.Comment: 21 pages, 8 figure

Hole-doping dependence of percolative phase separation in Pr_(0.5-delta)Ca_(0.2+delta)Sr_(0.3)MnO_(3) around half doping

We address the problem of the percolative phase separation in polycrystalline samples of Pr$_{0.5-\delta}$Ca$_{0.2+\delta}$Sr$_{0.3}$MnO$_3$ for $-0.04\leq \delta \leq 0.04$ (hole doping $n$ between 0.46 and 0.54). We perform measurements of X-ray diffraction, dc magnetization, ESR, and electrical resistivity. These samples show at $T_C$ a paramagnetic (PM) to ferromagnetic (FM) transition, however, we found that for $n>0.50$ there is a coexistence of both of these phases below $T_C$. On lowering $T$ below the charge-ordering (CO) temperature $T_{CO}$ all the samples exhibit a coexistence between the FM metallic and CO (antiferromagnetic) phases. In the whole $T$ range the FM phase fraction ($X$) decreases with increasing $n$. Furthermore, we show that only for $n\leq 0.50$ the metallic fraction is above the critical percolation threshold $X_C\simeq 15.5$. As a consequence, these samples show very different magnetoresistance properties. In addition, for $n\leq 0.50$ we observe a percolative metal-insulator transition at $T_{MI}$, and for $T_{MI}<T<T_{CO}$ the insulating-like behavior generated by the enlargement of $X$ with increasing $T$ is well described by the percolation law $\rho ^{-1}=\sigma \sim (X-X_C)^t$, where $t$ is a critical exponent. On the basis of the values obtained for this exponent we discuss different possible percolation mechanisms, and suggest that a more deep understanding of geometric and dimensionality effects is needed in phase separated manganites. We present a complete $T$ vs $n$ phase diagram showing the magnetic and electric properties of the studied compound around half doping.Comment: 9 text pages + 12 figures, submitted to Phys. Rev.

Spin Glass Behavior in RuSr2Gd1.5Ce0.5Cu2O10

The dynamics of the magnetic properties of polycrystalline RuSr2Gd1.5Ce0.5Cu2O10 (Ru-1222) have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magneto-superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O2 planes below Tc ~ 40 K. The exact nature of Ru spins magnetic ordering is still debated and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac susceptibility vs. T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin glass with magnetically interacting clusters. Such interpretation is supported by themoremanaent magnetization (TRM) measurements at T = 60 K. TRM relaxations are well described by a stretched exponential relation, and present significant ageing effects.Comment: 4 pages, 6 figures, submitted to Phys. Rev.

Ruthenocuprates RuSr2(Eu,Ce)2Cu2O10: Intrinsic magnetic multilayers

We report ac susceptibility data on RuSr_2(Eu,Ce)_2Cu_2O_(10-y) (Ru-1222, Ce content x=0.5 and 1.0), RuSr_2GdCu_2O_8 (Ru-1212) and SrRuO_3. Both Ru-1222 (x=0.5, 1.0) sample types exhibit unexpected magnetic dynamics in low magnetic fields: logarithmic time relaxation, switching behavior, and `inverted' hysteresis loops. Neither Ru-1212 nor SrRuO_3 exhibit such magnetic dynamics. The results are interpreted as evidence of the complex magnetic order in Ru-1222. We propose a specific multilayer model to explain the data, and note that superconductivity in the ruthenocuprate is compatible with both the presence and absence of the magnetic dynamics.Comment: 9 pages, 11 figures, Revtex; submitted to Phys.Rev.

Antiferromagnetic Order of the Ru and Gd in Superconducting RuSr2GdCu2O8

Neutron diffraction has been used to study the magnetic order in RuSr{2}GdCu2O8. The Ru moments order antiferromagnetically at T{N}=136(2)K, coincident with the previously reported onset of ferromagnetism. Neighboring spins are antiparallel in all three directions, with a low T moment of 1.18(6) mu {B} along the c-axis. Our measurements put an upper limit of ~0.1 mu{B} to any net zero-field moment, with fields exceeding ~0.4T needed to induce a measurable magnetization. The Gd ions order independently at T{N}=2.50(2)K with the same spin configuration. PACS numbers: 74.72.Jt, 75.25.+z, 74.25.Ha, 75.30.KzComment: Four pages, Latex, 5 eps figure