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Indirect calorimetry is the recommended, most accurate way to measure resting energy expenditure (REE) in critically ill, mechanically ventilated patients. We tested the agreement of two systems: the Mindray metabolic system (the system to be validated) and the GE S/5 metabolic system (the reference system). We also compared the measurements obtained to commonly used predictive equations.This was a prospective single-center study, in a general 16-bed intensive care unit, with critically ill, mechanically ventilated patients eligible to undergo indirect calorimetry. REE was measured successively during the same session with two 30-min measurements. The agreement and bias between oxygen consumption, carbon dioxide production, REE, and respiratory quotient obtained by Mindray and GE systems were compared using Bland-Altman plots. A priori we defined an acceptable within-method error to be 20% or less and an acceptable between-methods error to be 30% or less, according to Critchley and Critchley.Forty measurements were performed with 16 participants. All measurements were included in the final analysis. The mean REE was 2478 ± 650 kcal/d for the GE system and 2166 ± 415 kcal/d for the Mindray system (P ˂ 0.0001), for a difference of 12.6%. This difference in REE is related to the variations between the two devices in both oxygen consumption and carbon dioxide production.The Mindray metabolic system, compared to the GE S/5 metabolic system (the reference method used), measured REE with a mean difference of 12.6%. The Mindray-measured REE was within an error limit we defined a priori.

We have fabricated two-dimensional photonic crystals for the mid-infrared spectral range. These photonic crystals are engineered in order to show optical modes in resonance with quantum dot intersublevel transitions. A novel fabrication technique was developed to obtain suspended membranes in a planar geometry. The membranes are 3 μm thick and the lattice periodicities vary between 3.5 μm to 6.5 μm. We show that the photonic crystals modes can be probed by the room temperature emissivity using a photomodulation technique. Bloch-modes of the photonic crystals are observed around 11 μm wavelength. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-dimensional photonic crystals are promising structures for photonic applications. Here, we show that the optical properties of two-dimensional photonic crystal membranes fabricated from silicon-on-insulator substrates can be probed at room temperature by the internal continuous wave photoluminescence of Ge/Si self-assembled islands. Quality factors up to 15 000, limited by the spectrometer resolution, are measured. We also show that two-dimensional photonic crystals can be fabricated as well with germanium-on-insulator substrates by using the same processing techniques. The optical properties of these two-dimensional photonic crystals can be probed by the room-temperature photoluminescence associated with the direct band gap of the bulk Ge layer. Enhanced resonant extraction efficiency can be obtained for wavelengths longer than 1.55 μm. The quality factors of H1 hexagonal cavities are found in good agreement with those obtained by finite-difference time domain calculations.

We have investigated pure germanium two-dimensional photonic crystals. The photonic crystals which exhibit resonances in the near infrared spectral range were fabricated on germanium-on-insulator substrates using standard silicon-based processing. The germanium-on-insulator substrate consists of a thin layer of pure germanium-on-oxide deposited on a silicon substrate. The optical properties are probed by the direct band gap optical recombination of pure germanium at room temperature. Resonant optical modes are evidenced between 1.68 and 1.53 μm in different type of hexagonal cavities (H1–H5). The spectral position of the modes is controlled by the lattice periodicity and air filling factor of the photonic crystals. Close to the Ge band edge, the quality factors are limited by the bulk material absorption.

We report on polaron dynamic measurements in InAs/GaAs semiconductor quantum dots. The polaron dynamics is monitored by transient nonlinear spectroscopy by recording the transmission variation as a function of the pulse area of an infrared picosecond excitation. A non-monotonous transmission is observed for pulse areas larger than a π pulse. We attribute this transmission dependence with pulse area to a signature of damped optical Rabi oscillation of polarons. The damping of the Rabi oscillation is modeled by two level optical Bloch equations. A short dephasing time T2 around 5 ps is deduced for the resonantly excited polaron, one order of magnitude smaller than the relaxation time. We show that the decoherence of the polaron state is not limited by its slow relaxation but rather by the fast dephasing of its phonon component.

We report on polaron decay in InAs/GaAs self-assembled quantum dots. The polarons are probed by pump–probe spectroscopy through their optical intersublevel absorption around 62 meV (20 μm wavelength). A T1 polaron lifetime of the order of tens of picosecond is deduced from the low-temperature pump–probe measurements. We show that a long-lived component can be additionally observed on the pump–probe measurements. The spectral dependence of this long-lived component is, however, not correlated to the polaron absorption. It is thus not a signature of polaron relaxation quenching. The origin of this long-lived component is attributed to the two-phonon absorption of the bulk GaAs substrate.

A strain Bir–Pikus Hamiltonian Hst, associated with a 20-band sp3s* k⋅p Hamiltonian Hkp, is used to describe the valence band and the first two conduction bands all over the Brillouin zone. Because the local (in k space) deformation potentials Ⅺu and Ⅺd used in pseudopotential method are unusable in k⋅p theory, we show that taking into account the Bir–Pikus parameters (a, b) of the Brillouin zone center in the Hst Hamiltonian allows one to describe the dispersion relation in the whole Brillouin zone. The method is applied to strained Si on a relaxed Si1−xGex alloy. The values of the energy band gap, and of the Δ2−4 conduction band splitting between the four equivalent in-plane valleys Δ4 and the two valleys along the growth direction Δ2 are in very good agreement with those reported in other publications. The small value of the spin-orbit splitting in silicon is taken explicitly into account. We show that the valence band splitting is consequently not proportional to the stress.

Polaron dynamics in n-doped InAs/GaAs self-assembled quantum dots is investigated using far infrared pump-probe spectroscopy around 22 μm wavelength of the s-p intersublevel transition. Despite the strong coupling between the confined electron and the phonon continuum, forming a priori an everlasting polaron, we show experimentally that the polaron actually relaxes over a tens-of-picoseconds time range even at detuning far away from the phonon resonance. Measured decay times scale from 45 ps to 70 ps at low temperature depending on the transition energy. The energy dependence of this relaxation time is consistent with the polaron nature of the elementary excitation in InAs/GaAs self-assembled quantum dots. As the temperature increases, the polaron decay time slightly decreases, with still a value of 37 ps at 300 K. The experiment shows that the polaron relaxation is driven by the instability of its one-phonon component. It is consistent with a phonon part disintegrating into two acoustic phonons as proposed theoretically by Li, Nakayama and Arakawa in 1999 [4].

A direct matrix solution of the Boltzmann transport equation is developed to investigate impact ionization and other hot carrier phenomena in compound semiconductor devices. The model is applied to GaAs. A full-band calculation of the semiconductor structure is performed using k · p theory. An original determination of the distribution function in k-space as a function of electric field and temperature is achieved. The dependence of the ionization rate on energy, of the drift velocity and of the ionization coefficient on electric field are in good agreement with Monte Carlo data at 300 K and validate the modeling approach. The variation of the impact ionization coefficient on temperature and electric field in GaAs is discussed.

We have investigated the optical properties of quantum wire arrays grown on vicinal surfaces with lateral period of 180 A. Due to the specificity of the CdTe, for which the preferential orientations of step edges during the growth are along the (1 0 0) direction, vicinal substrates misoriented around the (1 0 0) direction are used. The 1D periodic potential modulation in CdTe/CdMnTe tilted superlattice is evidenced experimentally by the energy redshift of the photoluminescence observed when the superlattice is vertical. Such a behavior of the ground state in the 1D structure is accounted quantitatively by a theoretical approach which uses a sinusoidal modulation potential to describe the lateral confinement.

We observe in strained GaN self-assembled quantum dots grown on an AlN layer, a dramatic modification of the optical emission spectra as the dot size varies. In “large” quantum dots with an average height of 4.1 nm, the photoluminescence (PL) peak is centered at 2.95 eV, nearly 0.5 eV below the bulk GaN bandgap. We attribute this enormous redshift to a giant 5.5 MV cm−1 piezoelectric field present in our dots. Temperature-dependent PL studies reveal the strongly zero-dimensional character of this QD system and are consistent with an intrinsic PL mechanism.

The eight-band k · p method and the envelope function approximation is used to describe the infrared inter-subband absorption in Si–Si1−xGex–Si quantum wells. The valence and conduction bands are explicitly taken into account leading to an accurate description of the mixing of the conduction and valence band functions. Both as-grown and interdiffused (annealed) quantum wells are discussed.

Spin transport and dynamics is one of the crucial issues of spintronics, regardless of whether the system of interest is magnetic or nonmagnetic: semiconductors are of special interest because in such systems the charge carrier dynamics can be accurately treated through single-particle approximation, making use of the precise knowledge of wavefunctions. Here the single-electron, possibly spin-dependent wavefunction is well suited to provide reliable fully quantum mechanical models, or to derive semiclassical theoretical models, which eventually can be extended to other kinds of systems. In this context transport operators play a key role and, in particular, the probability current is a fundamental concept in quantum mechanics, which connects the wave-like description of a quasi-particle to the notion of transport current.

Based on a proper definition of the current operators for non-quadratic Hamiltonians, we derive the expression for the transport current which involves the derivative of the imaginary part of the free-electron current, highlighting peculiarities of the extra terms. The expression of the probability current, when Spin-Orbit Interaction (SOI) is taken into account, requires a reformulation of the boudary conditions. This is especially important for tunnel heterojunctions made of non-centrosymmetric semiconductors. Therefore, we consider a model case: tunneling of conduction electrons through a [110]-oriented GaAs barrier. The new boundary conditions are reduced to two set of equations: the first one expresses the discontinuity of the envelope function at the interface while the other one expresses the discontinuity of the derivative of the envelope function.

We propose a novel set of boundary conditions, based on the continuity of a generalized velocity and on the continuity of the probability current at the interface of heterojunctions, which is well suited to construct the solution of the tunneling problem when spin-orbit interaction is taken into account. We illustrate this procedure in a model case: tunneling of conduction electrons through a [110]-oriented GaAs barrier. In that case, the new boundary conditions reduce to two set of equations: the first one expresses the discontinuity of the envelope function at the interface while the other one is close to the standard condition on the derivative of the envelope function.

We give a general formulation for both the Luttinger and the Bir-Pikus Hamiltonians with a spin quantization parallel to the [hhk] direction. This allows one to obtain the hole subbands for any growth direction parallel to [hhk]. The results are given explicitly in the [001], [111], [110], and [112] directions. We show that the axial approximation is exact if the wave vector parallel to the plane (hhk) is equal to zero for (hhk)=(001) (a well-known result) or (111) but that this is not strictly true for (110) or (112). We clarify the link between the axial mass and the cyclotron mass. We then present a method of numerical calculation that is suited to any asymmetric quantum well. We discuss the efficiency and the limits of the Broido-Sham transformation and we show that this transformation is not applicable for the [112] direction even as a rough approximation. Finally, we give the hole subband dispersion in strained ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te-CdTe-${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te quantum wells where, according to the growth direction, the quantum well may or may not be piezoelectric.

We report experimental and theoretical results on interband and intersubband transitions in GaN quantum wells with strained AlN barriers. All of the samples are grown by molecular-beam epitaxy on sapphire (0001) substrates. The results show that even at room temperature, strong electron localization occurs in the plane of the quantum wells due to the combined effect of monolayer thickness fluctuations and the high internal field in the GaN layers. We also demonstrate that the intersubband absorption is systematically blueshifted in n-doped quantum wells with respect to nominally undoped samples as a result of strong many-body effects, namely the exchange interaction. The results for both undoped and doped quantum wells are in good agreement with simulations.

We have investigated various optical non-linearities in resonance with intersublevel transitions in InAs/GaAs self-assembled quantum dots. The infrared pulses were delivered by CLIO at Orsay (France) and enabled (i) the evidence of second-harmonic generation with giant susceptibilities, (ii) the first measurement of a 70 ps long polaron lifetime through noise improved pump-probed experiments and (iii) a photon echo experiment with a 15 ps long extracted dephasing time.

New boundary conditions are derived for tunnel-heterojunctions, where the effective Hamiltonian is a generic power of the momentum-operator. A novel expression of probability-current operator, which can be also applied in presence of the D'yakonov-Perel (DP) Hamiltonian, has to be used. We test our technique on the interface between two semi-infinite media, with on one side a free-electron-like material and on the other side the [110]- oriented GaAs barrier.

International audience; We present a systematic construction of the probability-and spin-current operators, based on a momentum power expansion of effective Hamiltonians. These operators play an essential role in transport problems related to semiconductor heterostructures, in particular when spin-orbit interaction is taken into account. The result, valid whatever the momentum power and including the linear (Bychkov-Rashba) term as well as the cubic (Dresselhaus or D'yakonov-Perel) term, is of special importance for spintronics.

A k⋅p model is used to theoretically investigate the energy and lattice temperature dependence of both transverse and longitudinal “curvature” electron effective masses in silicon. The temperature dependence of the carrier concentration conduction effective masses in the range of 10–550 K is also examined. Our results highlight the energy dependence of the longitudinal effective mass, usually considered to be equal to the band-edge effective mass, which varies from 0.917 to 1.6m0 when the carrier energy ranges from the bottom of the conduction band up to 1.5 eV. This energy dependence should have a significant impact on electronic transport simulations using drift–diffusion, hydrodynamic, or Monte Carlo methods, particularly for hot-carrier phenomena in microelectronic devices.

We have investigated the optical properties of tensile-strained germanium photonic wires. The photonic wires patterned by electron beam lithography (50 μm long, 1 μm wide and 500 nm thick) are obtained by growing a n-doped germanium film on a GaAs substrate. Tensile strain is transferred in the germanium layer using a Si₃N₄ stressor. Tensile strain around 0.4% achieved by the technique corresponds to an optical recombination of tensile-strained germanium involving light hole band around 1690 nm at room temperature. We show that the waveguided emission associated with a single tensile-strained germanium wire increases superlinearly as a function of the illuminated length. A 20% decrease of the spectral broadening is observed as the pump intensity is increased. All these features are signatures of optical gain. A 80 cm⁻¹ modal optical gain is derived from the variable strip length method. This value is accounted for by the calculated gain material value using a 30 band k · p formalism. These germanium wires represent potential building blocks for integration of nanoscale optical sources on silicon.

We propose a procedure, based on momentum power series expansion Hamiltonian of nth general order, to obtain a coherent expression of the probability current operator that is valid also when Spin-Orbit Interaction (SOI) terms are included. We prove that we recover the standard definition when the free electron-like term is included in the Hamiltonian, but when taking into account higher order Spin-Orbit Interaction (SOI) terms, a more general definition of the probability current operator is mandatory, due to its different symmetrization, compared to the Hermitian velocity operator expression.

We have investigated the room temperature optical properties of 50 µm length, 11 µm large and 500 nm thick tensile-strained germanium photonic wires. Tensile strain is transferred in the germanium layer using a Si 3 N 4 film stressor. An optical recombination of tensile-strained germanium involving light hole band is observed around 1690 nm at room temperature corresponding to a strain magnitude around 0.4%. We show that the waveguided emission associated with a single tensile-strained germanium wire increases superlinearly as a function of the illuminated length. A decrease of the spectral broadening is observed as the pump intensity is increased. A 80 cm−1 modal optical gain is derived from the variable strip length method.

We show that effective Hamiltonians in solids lead to specific problems when dealing with spin-orbit interaction. We prove that the usual construction, where the velocity operator is simply deduced from the velocity calculated using the Hamilton relation, has a restricted validity and that there is an absolute necessity to modify the standard current-tensor definition. We derive proper symmetrized expressions in the case of a Hamiltonian which has linear and cubic dependence versus momentum components.

We show that the recombination energy of the direct band gap photoluminescence of germanium can be controlled by an external mechanical stress. The deformation is provided by an apparatus used for blister test. This strain leads to a significant change of the room temperature direct band gap recombination of germanium. An energy red-shift up to 60 meV is demonstrated for the room temperature photoluminescence of a thin germanium membrane (125 nm wavelength shift from 1535 to 1660 nm). This photoluminescence shift is correlated to the inplane tensile strain present in the film. A biaxial tensile strain larger than 0.6 % is achieved by this method. The wavelength shift is correlated to the predicted band gap shift as obtained from a 30 band k. p formalism. An excellent agreement is obtained between the experimental band gap shift and the theoretical one. This mechanical deformation allows to approach the direct band gap condition for germanium. We will discuss the possibility to achieve lasing with n-doped layers and the application of an external mechanical stress.

Self-assembled InGaN islands were grown by molecular-beam epitaxy on GaN, following a Stranski–Krastanow growth mode. Atomic force microscopy revealed that their dimensions were small enough to expect zero-dimensional quantum effects: the islands were typically 27 nm wide and 2.9 nm high. Strong blue-violet photoluminescence of the dots is observed, persisting up to room temperature. The temperature dependence of the photoluminescence is analyzed and compared to that of InGaN quantum well and bulk samples.

Using a 30 band k.p formalism, we calculate the band structure and the optical gain of tensilely-strained germanium. We show that the room temperature emission of germanium-on-insulator can be significantly enhanced by n-doping of germanium.

Due to the absence of inversion symmetry, spin-orbit interaction leads to a very particular topology of the evanescent states in zinc-blende semiconductors, which may consist of loops connecting different spin sub-bands at the zone center. The spin-vector motion along such loops is analytically or numerically studied. A surprising picture emerges from this detailed analysis. Namely, the two spin sub-bands do not correspond to opposite spin states near the Brillouin-zone center and merge with identical spins at larger wave vector. This determines the spin-filtering capabilities of the semiconductor barrier.

In non-centrosymmetric semiconductors with zinc-blende structure grown along the [110] crystallographic direction, electrons with up and down spins undergo different quantum phase shifts upon tunneling, which can be wieved as resulting from spin precession around a complex magnetic field. There is no spin filtering but a pure spin dephasing. The phase shift of the transmitted wave is proportional to the overall barrier-material thickness. We show that a device incorporating a number of resonant tunnel barriers constitutes an efficient quantum-phase shifter.

We discuss possible tunneling phenomena associated with complex wave vectors along directions where the spin degeneracy is lifted in noncentrosymetric semiconductors. We show that the result drastically depends on the direction. In the [110] direction, no solution can be calculated in the usual way assuming that the wave function and its derivative are continuous. A method for obtaining physical solutions is given and consequences are drawn. As a result, there is no spin filtering in such a direction but the spin undergoes a precession through the barrier with the rotation angle being proportional to the barrier thickness. In a direction close to [001] we find a spin-filter effect in close agreement with the model discussed by Perel' et al. [Phys. Rev. B 67, 201304(R) (2003)].

Etude theorique de la variation de la conductivite (σ) avec la forme et la longueur de correlation ζ de la fonction d'autocorrelation associee a la rugosite. La conductivite depend fortement du choix de la fonction d'autocorrelation pour ζ>>k F −1 , ou k F est le vecteur d'onde de Fermi. Pour les films metalliques, la variation moyenne de σ avec l'epaisseur d ne peut etre approximee par la loi en puissance, σαd s . Obtention d'effets similaires pour les puits quantiques semiconducteurs

We have investigated the thermal emission of midinfrared dielectric photonic crystals. We show that the optical properties of the photonic crystals can be probed by the photomodulation of the room-temperature thermal emission. The technique is illustrated in the case of membrane-type two-dimensional photonic crystals. The GaAs-based photonic crystal membranes were fabricated by wafer bonding followed by standard e-beam lithography and etching processes. Thermal radiation collected from the surface of the sample allows one to investigate experimentally a broad spectral range from 500 to $1500\text{ }{\text{cm}}^{\ensuremath{-}1}$. Zone-center modes at the $\ensuremath{\Gamma}$ point coupled to the radiative continuum are evidenced around $900\text{ }{\text{cm}}^{\ensuremath{-}1}$ ($11\text{ }\ensuremath{\mu}\text{m}$ wavelength). The lattice periodicity of the photonic crystals allows one to spectrally tailor the emissivity. Three-dimensional finite-difference time-domain modeling is used to explain the thermal radiation spectra of the photonic crystal characterized by optical sources embedded in the whole structure.

In contrast to oxidized silicon layers, photoluminescence in fresh porous silicon layers is quenched when the temperature decreases. Moreover, the spectra become structured below 100 K and display two large holes centered at 630 and 750 nm, respectively. We explain these experimental results by a model of electric dipole-dipole coupling between the photocreated carriers in the silicon crystallites and Si-H surface vibrations. We demonstrate here that the Morse potential, which is often employed to represent diatomic molecules, is not valid in our case. We compare the energy levels and the oscillator strengths of a Morse well and of a topologically similar well with a square base and adjustable slope: although the energy levels can be matched quite closely, the oscillator strengths are drastically different. We show that only the second well accounts for the experimental results for both energies and oscillator strengths.

We report on the energy band gap and band lineup of $\mathrm{SiGe}∕\mathrm{Si}$ heterostructures either in the case of coherently strained quantum wells or in the case of $\mathrm{SiGe}∕\mathrm{Si}$ self-assembled islands. We take into account the strain field and the quantum confinement effects through an accurate description of the conduction band including the $\ensuremath{\Delta}$ and $L$ bands. The strain field is calculated using a microscopic valence force field theory. The conduction-band diagram and energies are obtained from a 30-band $\mathbf{k}∙\mathbf{p}$ Hamiltonian accounting for the strain through the Bir-Pikus Hamiltonian. The band-edge description is first given for biaxially strained pseudomorphic $\mathrm{SiGe}$ layers. In $\mathrm{SiGe}$ quantum wells grown on relaxed silicon, the band line-up switches from type I to type II depending on the value of the average valence band offset. Applying the 30-band formalism to the case of heterostructures grown on relaxed silicon germanium buffer layers indicates that a better agreement with experimental data is obtained for a valence-band offset value $\mathrm{\ensuremath{\Delta}}{E}_{v}=0.54x$ where $x$ is the Ge composition. For this parameter, a type-II band lineup is thus expected for all compositions of pseudomorphic $\mathrm{SiGe}$/relaxed Si heterostructures. For $\mathrm{GeSi}∕\mathrm{Si}$ islands, we take into account the strain relaxation in the surrounding Si matrix. A type-II band lineup is predicted for all Ge compositions. The near-infrared interband recombination energy of the islands is calculated as a function of their $\mathrm{SiGe}$ composition.

Using third and forth order perturbation, we have derived main GaAs band parameters (such as EP ) from both experimental results (m *, g *, ...) and theoretical results (overall band structure from Cohen-Chelikowski pseudo-potential calculations). The analysis of the set of data leads to a drastic change to the "admitted" value of some parameters (E'P ) from the only experimental results and show inconsistency if theoretical results are furthermore taken into account.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on a method to calculate the band diagram of quantum wells using a 30-band $\mathbf{k}\mathbf{∙}\mathbf{p}$ Hamiltonian. The subbands are calculated over the entire Brillouin zone. Such an approach is useful not only for a valence band description but also for a conduction band description when the energy minimum is not located at the center of the Brillouin zone as is the case for Si, SiGe, and Ge. The method also provides information about confinement in $L$ or $X$ valleys for III-V heterostructures. Such a formalism is useful for both optical calculation and for transport modeling. As an illustration, the scattering rates for holes confined in a quantum well are calculated. The accurate density of states provided by the 30-band $\mathbf{k}\mathbf{∙}\mathbf{p}$ method gives access to very different scattering rates as compared with those obtained following an effective mass approach.

We calculate evanescent waves in GaAs‐like III‐V semiconductors throughout the forbidden band gap taking into account both the absence of inversion center and the spin‐orbit coupling. We find that the evanescent energy bands are spin‐split and that the evanescent wave functions only exist in limited energy and wave‐vector domains. Such tunnel barriers can be used as solid‐state spin injectors.