Asymmetric Relaxation Research Articles

X-ray studies and time-resolved photoluminescence on optically pumped antimonide-based midinfrared type-II lasers

We report on high-resolution X-ray diffraction and time-resolved photoluminescence (TR-PL) studies of antimonide-based midinfrared (MIR) type-II laser samples. A structural characterization taking into account asymmetrical strain, layer tilting, and relaxation enables an accurate determination of the average lattice constant of the active region and the composition of the cladding layers. By designing the antimonide-to-arsenide interfaces, we achieve exact lattice matching of the active region to the substrate. Non-radiative recombination processes are investigated with time-resolved photoluminescence. The samples are also characterized under optically pumped laser operation. By an examination of the time-integrated and time-resolved amplified spontaneous emission (TR-ASE), we investigate the modal gain and gain dynamics. The variable stripe length method is combined with the TR-PL approach. Compared to the time-integrated gain spectra the spectral dependence of the maximum and minimum time-resolved gain shows a broad plateau. The full width half maximum (FWHM) of the TR-ASE pulse is 5.5±0.5 ps. Thus, short pulses in this range should be achievable upon laser operation. The active regions of the laser structures investigated here are promising subunits of type-II quantum cascade lasers.

E' CENTERS IN SILICON DIOXIDE: FIRST-PRINCIPLES MOLECULAR DYNAMICS STUDIES

The point-like defects known as E' centers are the most abundant natural defects in silicon dioxide ( SiO2) and have been identified as unpaired sp3dangling bonds. Their importance stems from the fact that they deeply affect the quality of electronic and optical devices. For this reason, particular attention has been paid in their characterization since the early 1960s. In this work, we review theoretical and experimental results concerning these kinds of defects, focusing on the related charge and spin states. In particular, the defect known as E' in crystalline quartz and its analogous E'γin amorphous SiO2, detected by electron spin resonance, are shown to be due to the Si dangling bonds that arise either upon removal or displacement of an oxygen atom in a SiO2network, accompanied by an asymmetric relaxation of the network.

Effect of the Chain Length on the Inter- and Intramolecular Dynamics of Liquid Oligo(ethylene glycol)s

Precise complex permittivity spectra over the frequency range 0.15 GHz </= nu </= 89 GHz are reported for monodisperse (EGn, 1 </= n </= 6) and polydisperse (PEG174, PEG300, PEG400) oligo(ethylene glycol)s at 25 degrees C. Up to about 20 GHz, the relaxation behavior of all samples can be reasonably described with empirical functions that reflect a broad and asymmetric relaxation time distribution, like the Havriliak-Negami function. However, these functions deviate systematically at higher frequencies and do not allow one to rationalize the concentration dependence of the spectra on dilution in dichloromethane or when going from the diol to the corresponding dimethyl ether. It is shown that a coherent description can be achieved by using a superposition of Debye-type relaxation processes. This approach allows the separation of end-group effects connected with the relaxation of the hydrogen bond network from intramolecular dipole relaxation processes caused by the reorientation of chain segments.

Effect of asymmetric stress relaxation on the polarization-dependent transmission characteristics of a CO2 laser-written long-period fiber grating.

The effect of stress-induced birefringence on polarization-dependent transmission characteristics was thoroughly investigated for a long-period fiber grating (LPFG) that was fabricated by use of a stress relaxation method with a CO2 laser. A series of two-dimensional axial stress profiles for one complete period of a LPFG was measured with an optical tomographic measurement technique. We found that the asymmetry in the stress distribution of the cladding was much larger than in the core of the LPFG. The splitting of polarization-dependent loss peaks in the optical transmission spectrum was calculated based on the measured asymmetric stress profiles and was compared with an experimental result.

Polarizability anisotropy relaxation in liquid ethanol: A molecular dynamics study

Molecular dynamics simulation is used to study the relaxation of the polarizability anisotropy in liquid ethanol at temperatures of 298 and 348 K. Ethanol molecules are represented by a four-site semi-flexible model in which the internal degree of freedom, corresponding to the torsional motion around the C–O bond, is taken into consideration. The molecular polarizability is calculated using an interaction-site model, based on the modified dipole-induced dipole model developed by Thole. The collective polarizability induced by intermolecular interactions is included using first-order perturbation theory and calculated considering both center–center and site–site models. Results are analyzed in terms of projected variables that allow the decomposition of the total relaxation into orientational and collision-induced components, both of which are influenced by molecular flexibility. We compare our data with the results of low-frequency depolarized Rayleigh light scattering experiments, examining the possibility of separating different relaxation processes from spectroscopic signals in the time and frequency domains. We find that even though the largest contributor to polarizability anisotropy dynamics is orientational relaxation, collision-induced contributions are important, especially at shorter times. Moreover, we show that torsional motion also plays a significant role in the fast decay of the polarizability anisotropy of the system. Although liquid ethanol is strongly associated and its dipolar relaxation is highly collective, we find orientational pair correlation effects on polarizability relaxation to be negligible. A comparison with simulation data previously obtained for methanol suggests that increasing the alcohol chain length decreases the relative importance of OH dynamics in polarizability relaxation. We further show that, as expected for such a structured liquid composed of strongly asymmetric molecules, polarizability and dipole relaxation processes exhibit marked differences.

Experimental evidence for fractional time evolution in glass forming materials

The infinitesimal generator of time evolution in the standard equation for exponential (Debye) relaxation is replaced with the infinitesimal generator of composite fractional translations. Composite fractional translations are defined as a combination of translation and the fractional time evolution introduced in [Physica A, 221 (1995) 89]. The fractional differential equation for composite fractional relaxation is solved. The resulting dynamical susceptibility is used to fit broad band dielectric spectroscopy data of glycerol. The composite fractional susceptibility function can exhibit an asymmetric relaxation peak and an excess wing at high frequencies in the imaginary part. Nevertheless it contains only a single stretching exponent. Qualitative and quantitative agreement with dielectric data for glycerol is found that extends into the excess wing. The fits require fewer parameters than traditional fit functions and can extend over up to 13 decades in frequency.

Surface structural analysis of LiF( [formula omitted]) thin films grown on Pt( [formula omitted

The surface structure of a multilayer LiF(1 0 0) thin film grown on Pt(1 1 1) from the vapor has been determined by the automated tensor low energy electron diffraction method. The final structure, which refined to a Pendry R-factor ( R P) of 0.24, had a surface corrugation ( Δ 1) of 0.24±0.04 Å due to the Li + being displaced towards the bulk, leaving the initially coplanar F − unshifted. A similar intralayer corrugation due to the movement of the Li + was also observed in the layer immediately under the surface layer, although to a lesser degree: Δ 2=0.07±0.04 Å. This asymmetric relaxation resulted in the reduction of the first interlayer spacing, d(F 2–Li 1), to 1.77±0.06 Å from the ideal value of 2.01 Å. The second interlayer spacing, d(Li 3–F 2), was within error bars of the bulk value, 2.01 Å.

Physical mechanisms responsible for the relaxation time distribution in disordered dielectrics

The relaxation time distribution function F(τ) is calculated in the framework of the random-field theory. The function F(τ) is expressed through the distribution function f(E) of a random electric field E with due regard for the derived dependence of the relaxation time τ on the electric field. The distribution function F(τ) is calculated in terms of the statistical theory within the random-field approximation. The nonlinear random-field contributions and spatial effects of correlations between randomly distributed electric dipoles are taken into account. The calculations are performed for a mixed ferroelectric glassy phase in which the short-range and long-range polar orders coexist. It is demonstrated that the inclusion of nonlinear contributions of the random field leads to an asymmetric relaxation time distribution function F(τ), whereas allowance made only for the linear random-field contributions results in a symmetric function F(τ). A comparison of the calculated functions F(τ) with empirical functions derived from the Cole-Cole (CC), Davidson-Cole (DC), Kohlrausch-Williams-Watts (KWW), and Havriliak-Negami (HN) laws for the dielectric response shows that these laws correspond to disordered systems in which the long-range and short-range orders coexist. Different forms of the function F(τ) are determined by either linear (the CC law) or nonlinear (the DC, KWW, and HN laws) contributions of the random field.

Asymmetric stretch vibrational energy relaxation of OClO in liquid water

Motivated by a femtosecond experiment and a study of the vibrational energy relaxation (VER) of asymmetric stretch excited OClO(aq), molecular dynamics (MD) simulations of OClO(aq) are used to determine force correlation functions (CF), important in the VER of OClO(aq). Together with a golden rule formula, these simulations are utilized to estimate the lifetime of the OClO asymmetric stretch mode, excited with one quantum. The calculated lifetime of 9 ps agrees well with the model of Poulsen and coworkers, thereby supporting the findings of their paper, that the geminate recombination of OClO primarily induces excitations in the OClO asymmetric stretch mode.

Quantum Size Effects in Low-Temperature Growth of Pb Islands on Si(111)7×7 Surfaces

Flat-top Pb islands with critical and magic thickness have been observed in the Pb/Si(111)7×7 system at 200 K by scanning tunneling microscopy. The growth behavior, different from that in the Stranski-Krastanov mode, arises from a quantum size effect. Quantized states are detected in the current–voltage (I–V) spectra on the Pb islands of varying thickness. Our observation of asymmetrical and oscillatory relaxation in the island thickness reveals that the charge distribution of confined electrons can influence the interlayer spacing. A simple model based on the infinite potential well can explain well all of our results.

Theoretical description of hole localization in a quartz Al center: The importance of exact electron exchange

The ``classical'' model of the ${[\mathrm{AlO}}_{4}{]}^{0}$ defect center in irradiated quartz, an Al impurity having replaced a four-coordinated Si atom, is that a hole forms in a nonbonding orbital of an oxygen atom, with consequent asymmetric relaxation along that particular Al-O direction. This model has been proposed years ago, based on the analysis of the electron-paramagnetic-resonance spectra of Al-containing crystalline ${\mathrm{SiO}}_{2}$ and analysis of Hartree-Fock cluster model calculations. Three recent theoretical studies based on first-principle density-functional theory (DFT) and band-structure plane-wave calculations proposed an alternative model where the hole is completely delocalized over four oxygen neighbors to the Al impurity, at 0 K. Using cluster models containing as many as 104 Si and O atoms and various theoretical approaches, we show that the delocalized picture is an artifact of the DFT approach and that a fully localized hole is obtained when an exact treatment of the exchange term is used. The validity of this conclusion is based on the direct comparison of computed and measured quantities such as the ${}^{17}\mathrm{O}$ hyperfine and ${}^{27}\mathrm{Al},$ ${}^{29}\mathrm{Si}$ superhyperfine coupling parameters, the ${}^{27}\mathrm{Al}$ nuclear quadrupole effect, and the derivable local distortion around the defect. This work shows that great care is needed when DFT is used to describe localized holes in insulators.

Picosecond polarized fluorescence studies of anisotropic fluid media. II. Experimental studies of molecular order and motion in jet aligned rhodamine 6G and resorufin solutions

New polarized time resolved fluorescence techniques are implemented to determine the full angular motion of a probe molecule in an anisotropic environment. Studies of rhodamine 6G and resorufin molecules aligned in a free ethylene glycol jet show that the presence of net molecular order is accompanied by a distinct anisotropy in alignment relaxation following photoselection. Diffusion coefficients for φ and θ motion (D∥ and D⊥) in a jet fixed axis system are determined from the cylindrically symmetric and asymmetric alignment relaxation rates for the isotropic and anisotropic regions of the jet. The presence of net negative molecular alignment as the free jet is formed is seen to correspond to restricted φ motion (D∥&amp;lt;D⊥), with a net positive steady state alignment the anisotropy in the diffusion dynamics is reversed. The differences in D∥ and D⊥ are attributed to anisotropy in the solvent viscosity as a consequence of flow. The combination of linear and circular polarization techniques is seen to provide useful information on cylindrical asymmetry and relaxation dynamics hitherto unobserved by conventional fluorescence polarization techniques.

Comparison of Dielectric Relaxations of Water Mixtures of Poly(vinylpyrrolidone) and 1-Vinyl-2-pyrrolidinone

Microwave dielectric studies on the dynamics of water in poly(vinylpyrrolidone) (PVP)−water mixtures and 1-vinyl-2-pyrrolidinone (VP)−water mixtures were performed to compare the relaxation processes of water in polymer−water and monomeric compound−water mixtures. For the PVP−water mixtures, a relaxation peak for water was observed and the shape of the absorption curve was symmetrical about the logalism of frequency, which the Cole−Cole equation describes well. In this case, the polymer behaves as a geometric constraint for rotational motion of water molecules, and a variation of the local structure of water yields the symmetrical relaxation curve. On the other hand, one relaxation peak due to the rotational motion of the dipoles of water and VP molecules was also observed for VP−water mixtures. However, the shape of the absorption curve is asymmetric and is described well by the KWW function. Cooperative motion of water and VP molecules resulted in a variation of local structure which is observed as an asymmetric relaxation curve.

Shape of dielectric relaxation curves of ethylene glycol oligomer–water mixtures

Dielectric measurements of water mixtures of ethylene glycol oligomer (EGO) with 1–6 repeat units were carried out in the frequency range of 100 MHz–30 GHz at 25 °C. One relaxation process due to water and EGO was observed for each mixture. If the number of repeat units of EGO is larger than three, the water mixtures show a broad and symmetric relaxation curve. On the other hand, if the number of repeat units of EGO is two or less, the mixtures show a broad and asymmetric relaxation curve. The two types of relaxation curves observed in the EGO–water mixtures reflect the size of the EGO molecule. The asymmetric relaxation curve is due to the cooperative motion of water and EGO molecules in the EGO–water cluster for smaller EGO–water mixtures. In contrast, the symmetric dielectric relaxation curve is a result of the variation of local structure in larger EGO–water mixtures. The larger EGO molecules cannot move cooperatively and behave as a geometrical constraint to the motion of water clusters.

Structural characterization of SiGe step graded buffer layers grown on prestructured Si[001] substrates by molecular beam epitaxy

A SiGe graded buffer with a Si 1− x Ge x /Si superlattice on top was grown on top of long, line shaped mesa structures being 2–10 μm in width. Although extended over a length of 2 mm the drastic reduction of the dislocation density, as recently reported for small mesas, was maintained if the mesas were aligned along certain crystallographic directions. Depending on the crystallographic orientation, an asymmetric strain relaxation was observed. Regarding band engineering this adds geometry as a new free parameter.

Ballistic effects and intersubband excitations in multiple quantum well structures

We have studied the transport properties of electrons in asymmetric quantum well structures upon far-infrared optical excitation of carriers from the lowest subband into the continuum. Here the photocurrent consists of a coherent component originating from ballistic transport upon excitation, and of an incoherent part associated with asymmetric diffusion and relaxation processes, which occur after the coherence has been lost. The signature of the coherent contribution is provided by a sign reversal of the photocurrent upon changing the excitation energy. This sign reversal arises from the energy-dependent interference between continuum states, which have a twofold degeneracy characterized by positive and negative momenta. The interference effect also allows us to estimate the coherent mean free path ( >20 nm at 77K). In specifically designed device structures, we use both the coherent and incoherent components in order to achieve a pronounced photovoltaic infrared response for detector applications.

Two-dimensional high-field electron spin echo spectroscopy of nitroxide spin probes

Relaxation of a highly asymmetric nitroxide spin probe in the frozen toluene solution was studied below `rigid limit' temperatures by means of two-dimensional high-field electron spin echo spectroscopy. It was shown that observed relaxation cannot be explained by reorientation motion of the nitroxide. A model is proposed in which an asymmetric relaxation is due to the modulation of the g-values caused by the fluctuations of the environment of the nitroxide. This model qualitatively agrees with the relaxation observed. The estimation of the correlation times of such modulation is done.

Proton Magnetic Relaxation Studies of Various Guest Molecules in Clathrate Hydrates

The rotational dynamics of guest molecules in clathrate hydrates is studied by varying both the cage geometries of the host lattice and the properties of the cyclic guest species, as dipole moments, diameters, and moments of inertia. Proton spin−lattice relaxation rates have been obtained over a temperature range 20−263 K. The relaxation is caused by intramolecular dipole−dipole interactions only. The asymmetric relaxation rate versus inverse temperature curves have been fitted by using the Davidson−Cole spectral density J(ωτ, β). Shape parameters β between 0.1 and 0.3 are found for the reorientation of guests in their cages. Correlation times lying between 2.57 × 10-12 and 4.97 × 10-10 s and low activation energies indicate high guest mobility due to weak van der Waals interactions between guest and host. The reorientation of the guest molecules depends on the different cage types and guest geometries as well. The motions are specific for each cage type. For different guest molecules in the same cage type the correlation times increase with increasing diameters and moments of inertia of the molecules.

Crystallographic tilting in lattice-mismatched heteroepitaxy: A Dodson–Tsao relaxation approach

Asymmetric strain relaxation and the concomitant misorientation (tilt) of the lattice planes of the epitaxial layer in lattice-mismatched heterostructures is analyzed theoretically. The kinetic relaxation model of Dodson and Tsao [Appl. Phys. Lett. 51, 1325 (1987); 52, 852(E) (1988)] is extended to the growth on vicinal surfaces. We calculate the equilibrium tilt values as well as the evolution of tilt during strain relaxation as a function of material parameters, initial defect densities, and substrate miscut angle. Literature data are interpreted within the framework of the model.

Nuclear spin relaxation in electrolyte glasses

Nuclear spin relaxation (NSR) experiments in the laboratory and rotating frame were performed between about 2 K and room temperature in the following electrolyte glasses: LiCl · 7H 2O, LiBr · 7H 2O, ZnCl 2 · 7H 2O, and NaOH · 7H 2O. The data exhibit common features for all the glasses independent of the probe nuclei. In the low temperature region below T ∗ , the temperature and frequency dependence of the NSR data obey approximately a power law of the form 1/ T 1 ∝ T 1+ α ω − β , which can be interpreted consistently in the framework of low-frequency excitations of disorder modes. Above T ∗ , the NSR rates become dominated by diffusive motion of alkali ions resulting in an asymmetric relaxation rate peak near T g. The findings were analyzed by means of a non-Debye relation for the motion-induced NSR.