We propose an under-ground experiment to detect the general relativistic effects due to the curva... more We propose an under-ground experiment to detect the general relativistic effects due to the curvature of space-time around the Earth (de Sitter effect) and to rotation of the planet (dragging of the inertial frames or Lense-Thirring effect). It is based on the comparison between the IERS value of the Earth rotation vector and corresponding measurements obtained by a tri-axial laser detector of rotation. The proposed detector consists of six large ring-lasers arranged along three orthogonal axes. In about two years of data taking, the 1% sensitivity required for the measurement of the Lense-Thirring drag can be reached with square rings of 6 m side, assuming a shot noise limited sensitivity (20prad/s/ √ Hz). The multi-gyros system, composed of rings whose planes are perpendicular to one or the other of three orthogonal axes, can be built in several ways. Here, we consider cubic and octahedron structures. The symmetries of the proposed configurations provide mathematical relations that can be used to study the stability of the scale factors, the relative orientations or the ring-laser planes, very important to get rid of systematics in long-term measurements, which are required in order to determine the relativistic effects.
... The decay of the SiH stretching vibrations was observed to occur via the internal, bending m... more ... The decay of the SiH stretching vibrations was observed to occur via the internal, bending modes of the defect. Unlike hydrogen, oxygen forms a briging bond between two silicon atoms, with three vibrational modes of the C2v symmetry site. ...
We report ultrafast optical tuning of the reflection properties of MQW AlGaAs/InAlGaAs 1D photoni... more We report ultrafast optical tuning of the reflection properties of MQW AlGaAs/InAlGaAs 1D photonic crystal waveguides using a reflection geometry, pump-probe technique. Very fast pump-induced shifts of photonic resonances between 800 -900 nm by up to 18 nm nanometers are demonstrated. This is followed by a fast decay which varies from 5 to 30 ps depending on the sample dimensions. Together with the maximum reflectivity change of ∆R/R > 30%, this demonstrates excellent potential for application of photonic crystals as ultrafast nonlinear switches with decay times depending on the structure period.
We present far-infrared (FIR) modulated photoluminescence measurements of self-assembled InAs/GaA... more We present far-infrared (FIR) modulated photoluminescence measurements of self-assembled InAs/GaAs quantum dots. The sample was annealed to remove any indium profile roughness from within the quantum dots and thus from the inhomogeneous broadening. The annealed sample shows resonant FIR excitations just as the unannealed sample, but exhibits spectral hole burning unlike the unannealed sample. The results indicate that indium roughness is one of the primary inhomogeneous broadening mechanisms in self-assembled quantum dots.
Accurate determination of trap density in the active region of mid-infrared narrow-bandgap detect... more Accurate determination of trap density in the active region of mid-infrared narrow-bandgap detectors is crucial in the development towards background-limited performance at higher operating temperatures. We have used both optical and electrical measurements to determine the trap density in InSb/InAlSb nonequilibrium detector structures. Both of these techniques result in very good agreement with trap densities of 5×1014 cm-3.
We report on two pulse, degenerate four-wave mixing (DFWM) measurements on shallow InxGa1-xN/GaN ... more We report on two pulse, degenerate four-wave mixing (DFWM) measurements on shallow InxGa1-xN/GaN multiquantum wells (MQWs) grown on sapphire substrates. These reveal pulse length limited signal decays. We have found a 10:1 resonant enhancement of the DFWM signal at the excitonic transition frequencies which thereby give a sharp discrimination of the discrete excitonic contributions within the featureless distribution seen in absorption spectra. The exciton resonances have peak positions, which yield good overall agreement with a full k•p model calculation for the quantum well (QW) energy levels and optical transition matrix elements. InxGa1-xN/GaN MQWs generally exhibit strongly inhomogeneously broadened excitation spectra due to indium fluctuation effects; this approach therefore affords a practical method to extract information on the excited excitonic states not available previously.
We report the observation of giant quantum coherence effects in the localized modes of ionized hy... more We report the observation of giant quantum coherence effects in the localized modes of ionized hydrogen in synthetic fluorite. Infrared free induction decay experiments on the substitutional H- center show dramatic modulations at negative delay times due to interference between multiple vibrational levels. Spectrally resolving the degenerate four wave mixing signal allows unambiguous assignments of the participating vibrational states. The dependence of the signal intensity upon the delay path between the exciting free electron laser pulses can be accounted for in terms of the resonant third order polarization with a common dephasing time for the excited states.
We propose an under-ground experiment to detect the general relativistic effects due to the curva... more We propose an under-ground experiment to detect the general relativistic effects due to the curvature of space-time around the Earth (de Sitter effect) and to rotation of the planet (dragging of the inertial frames or Lense-Thirring effect). It is based on the comparison between the IERS value of the Earth rotation vector and corresponding measurements obtained by a tri-axial laser detector of rotation. The proposed detector consists of six large ring-lasers arranged along three orthogonal axes. In about two years of data taking, the 1% sensitivity required for the measurement of the Lense-Thirring drag can be reached with square rings of 6 m side, assuming a shot noise limited sensitivity (20prad/s/ √ Hz). The multi-gyros system, composed of rings whose planes are perpendicular to one or the other of three orthogonal axes, can be built in several ways. Here, we consider cubic and octahedron structures. The symmetries of the proposed configurations provide mathematical relations that can be used to study the stability of the scale factors, the relative orientations or the ring-laser planes, very important to get rid of systematics in long-term measurements, which are required in order to determine the relativistic effects.
... The decay of the SiH stretching vibrations was observed to occur via the internal, bending m... more ... The decay of the SiH stretching vibrations was observed to occur via the internal, bending modes of the defect. Unlike hydrogen, oxygen forms a briging bond between two silicon atoms, with three vibrational modes of the C2v symmetry site. ...
We report ultrafast optical tuning of the reflection properties of MQW AlGaAs/InAlGaAs 1D photoni... more We report ultrafast optical tuning of the reflection properties of MQW AlGaAs/InAlGaAs 1D photonic crystal waveguides using a reflection geometry, pump-probe technique. Very fast pump-induced shifts of photonic resonances between 800 -900 nm by up to 18 nm nanometers are demonstrated. This is followed by a fast decay which varies from 5 to 30 ps depending on the sample dimensions. Together with the maximum reflectivity change of ∆R/R > 30%, this demonstrates excellent potential for application of photonic crystals as ultrafast nonlinear switches with decay times depending on the structure period.
We present far-infrared (FIR) modulated photoluminescence measurements of self-assembled InAs/GaA... more We present far-infrared (FIR) modulated photoluminescence measurements of self-assembled InAs/GaAs quantum dots. The sample was annealed to remove any indium profile roughness from within the quantum dots and thus from the inhomogeneous broadening. The annealed sample shows resonant FIR excitations just as the unannealed sample, but exhibits spectral hole burning unlike the unannealed sample. The results indicate that indium roughness is one of the primary inhomogeneous broadening mechanisms in self-assembled quantum dots.
Accurate determination of trap density in the active region of mid-infrared narrow-bandgap detect... more Accurate determination of trap density in the active region of mid-infrared narrow-bandgap detectors is crucial in the development towards background-limited performance at higher operating temperatures. We have used both optical and electrical measurements to determine the trap density in InSb/InAlSb nonequilibrium detector structures. Both of these techniques result in very good agreement with trap densities of 5×1014 cm-3.
We report on two pulse, degenerate four-wave mixing (DFWM) measurements on shallow InxGa1-xN/GaN ... more We report on two pulse, degenerate four-wave mixing (DFWM) measurements on shallow InxGa1-xN/GaN multiquantum wells (MQWs) grown on sapphire substrates. These reveal pulse length limited signal decays. We have found a 10:1 resonant enhancement of the DFWM signal at the excitonic transition frequencies which thereby give a sharp discrimination of the discrete excitonic contributions within the featureless distribution seen in absorption spectra. The exciton resonances have peak positions, which yield good overall agreement with a full k•p model calculation for the quantum well (QW) energy levels and optical transition matrix elements. InxGa1-xN/GaN MQWs generally exhibit strongly inhomogeneously broadened excitation spectra due to indium fluctuation effects; this approach therefore affords a practical method to extract information on the excited excitonic states not available previously.
We report the observation of giant quantum coherence effects in the localized modes of ionized hy... more We report the observation of giant quantum coherence effects in the localized modes of ionized hydrogen in synthetic fluorite. Infrared free induction decay experiments on the substitutional H- center show dramatic modulations at negative delay times due to interference between multiple vibrational levels. Spectrally resolving the degenerate four wave mixing signal allows unambiguous assignments of the participating vibrational states. The dependence of the signal intensity upon the delay path between the exciting free electron laser pulses can be accounted for in terms of the resonant third order polarization with a common dephasing time for the excited states.
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Papers by J-p. R. Wells