Inhomogeneous Width Research Articles

NMR Evidences of the Coupling between Conduction Electrons and Molecular Degrees of Freedom in the Exotic Member of the Bechgaard Salt (TMTSF)2FSO3

We performed Se and F-NMR measurements on single crystals of (TMTSF)2FSO3 to characterize the electronic structures of different phases in the Temperature-Pressure phase diagram, determined by precise transport measurements [Jo et al., Phys. Rev. B67, 014516 (2003)]. We claim that such varieties of electronic states in the refined phase diagram are caused by strong couplings of the conduction electrons with FSO3 anions, especially with the permanent electric dipoles on the anions. We suggest that as temperature decreases, the FSO3 anions form orientational ordering through two steps; first only the tetrahedrons form an orientational order leaving the orientations of the electronic dipoles in random (transition I); then the dipoles form a perfect orientational order at a lower temperature (transition II). In the intermediate temperature range between transitions I and II, we found an appreciable enhancement of homogeneous and inhomogeneous widths of 77Se-NMR spectrum. From the analysis of the angular dependence of the linewidth, we attributed these anomalies to the intramolecular charge disproportionation or imbalance and its slow dynamics caused by the coupling with the permanent electric dipole of anion. Results of 19F-NMR relaxation and lineshape measurements support this picture very well. Electronic structures at higher pressures up to 1.25 GPa are discussed on the basis of the results of the 77Se and 19F-NMR measurements.

Regimes of Generation in Low-Q Distributed-Feedback Lasers with Strong Inhomogeneous Broadening of the Active Medium

We study the influence of the ratio between the relaxation rates of the field in a cavity and the polarization of active centers on the dynamic properties of the distributed-feedback lasers by means of 1D numerical simulation. The model of a two-level active medium with strong inhomogeneous broadening of the spectral line under CW wide-band pumping that provides two- or several-mode lasing in the vicinity of the Bragg photonic band gap is used. Evolution of the dynamic spectra and oscillograms of the laser emission with decreasing Q-factor of the Bragg resonator is analyzed. It is shown, in particular, that under conditions of the dominant role of the superradiant effects, there are unique opportunities for control of both quantitative and qualitative characteristics of lasing, including the spectral width, duration, and coherence length of various pulse components of the output radiation.

Bayesian Analysis of an Excitonic Absorption Spectrum in a Cu2O Thin Film Sandwiched by Paired MgO Plates

We analyzed the absorption spectrum of a Cu2O thin film sandwiched by paired MgO plates with the Metropolis algorithm of Bayesian estimation to estimate the energy shift of a yellow excitonic band gap. In the absorption spectrum, discrete excitonic resonant transitions with homogeneous and inhomogeneous widths are superimposed upon the tail parts of excitonic continuum and band-to-band absorption bands. On the basis of the Metropolis samplings with 2.5 million steps after sufficient burn-in steps, the spectral parameters of the excitonic resonant transitions can be estimated satisfactorily with a high degree of accuracy. In particular, we demonstrated that the yellow excitonic band gap in the Cu2O thin film shifts to the lower energy side than that of Cu2O bulk crystals. This result is consistent with the stress relaxation model for such Cu2O thin films, which was reported in Eur. Phys. J. B 86, 194 (2013).

Phase Memory Control in an Inhomogeneously Broadened Ensemble of Three-Level Systems and Stimulated Photon Echo Formation

Phase memory in a three-level system that is associated with the correspondence of isochromates of inhomogeneously broadened lines excited by lasers at various resonant frequencies with a common energy level in different time intervals is studied. It is shown that external spatially inhomogeneous electric fields can control such phase memory and could be used to determine the optimum conditions for forming a stimulated photon echo in a threelevel system.

The influence of coordinated defects on inhomogeneous broadening in cubic lattices

The joint probability distribution function (JPDF) of electric field gradient (EFG) tensor components in cubic materials is dominated by coordinated pairings of defects in shells near probe nuclei. The contributions from these inner shell combinations and their surrounding structures contain the essential physics that determine the PAC-relevant quantities derived from them. The JPDF can be used to predict the nature of inhomogeneous broadening (IHB) in perturbed angular correlation (PAC) experiments by modeling the G2 spectrum and finding expectation values for Vzz and η. The ease with which this can be done depends upon the representation of the JPDF. Expanding on an earlier work by Czjzek et al. (Hyperfine Interact. 14, 189–194, 1983), Evenson et al. (Hyperfine Interact. 237, 119, 2016) provide a set of coordinates constructed from the EFG tensor invariants they named W1 and W2. Using this parameterization, the JPDF in cubic structures was constructed using a point charge model in which a single trapped defect (TD) is the nearest neighbor to a probe nucleus. Individual defects on nearby lattice sites pair with the TD to provide a locus of points in the W1−W2 plane around which an amorphous-like distribution of probability density grows. Interestingly, however, marginal, separable PDFs appear adequate to model IHB relevant cases. We present cases from simulations in cubic materials illustrating the importance of these near-shell coordinations.

Coherent Population Trapping in An Inhomogeneously Broadened \(\Lambda\)-System with Multiple Excited States

We present a theoretical model using density matrix approach to study the phenomenon of coherent population trapping (CPT) in the \(\Lambda\)-type Doppler broadened system with multiple excited states. A multi-level \(\Lambda\)-system is formed by considering the closely spaced hyperfine levels in the D2 line of \(^{87}\)Rb. The presence of closely spaced hyperfine levels affects the transparency window and cause asymmetry in absorption profiles. We observe the sharp CPT dip when the frequency difference of applied fields is equal to the frequency separation between two ground levels. This dip demonstrates that the system is trapped in the dark state. By performing a thermal averaging in the Doppler-broadened \(\Lambda\)-system, we have shown that the transparency window becomes narrower and its linewidth decreases at higher temperature. This study has potential applications in precision measurements due to the small linewidth of the transparency window.

Topologically appropriate coordinates for (V z z , η) joint probability distributions

Inhomogeneous broadening (IHB) of hyperfine interactions in materials arises from a distribution of electric field gradients (EFGs) due to randomly distributed defects contributing non-uniformly to the EFG at probe sites. Hyperfine experiments reflect the inhomogeneous distribution of defects through the joint probability distribution function (PDF) of Vzz and η determined by the defect concentration, crystal structure, and defect sites in the crystal. Czjzek showed how to choose coordinates in the (Vzz, η) plane that are consistent with the physical constraints and ordering convention for these EFG parameters. Here we show how to transform to a new set of coordinates that decreases the distortion inherent in Czjzek’s representation. These new coordinates allow one to express the joint PDF for random distributions of defects in a form reasonably approximated by the product of two independent marginal distributions. This paper focuses on these topologically appropriate coordinates, with simple examples drawn from Czjzek’s work and from our simulations of point defects in cubic lattices as well as random amorphous distributions of defects. Detailed simulations have been carried out for IHB in cubic structures and point charge models relevant to perturbed angular correlation (PAC) experiments.

The Effect of Homogenous and Inhomogeneous Broadening and Gain Compression Factor on Dynamical Characteristics and Modulation of Tunneling Injection InGaAs/GaAs Quantum Dot Lasers

The Effect of Homogenous and Inhomogeneous Broadening and Gain Compression Factor on Dynamical Characteristics and Modulation of Tunneling Injection InGaAs/GaAs Quantum Dot Lasers

Evaluating the effects of nonlinear optical gain and thermal carrier escape on the performance of InGaAs/GaAs self-assembled quantum dot lasers

The present paper reports the effects of nonlinear optical gain and thermal carrier escape on light-current characteristics in self-assembled quantum dot (QD) lasers based on a rate equation model. Comparison of the obtained simulated results with empirical findings showed that it is necessary to consider the effects of thermal carrier escape and nonlinear optical gain in order to evaluate dynamic and static characteristics of self-assembled QD lasers. Moreover, it was revealed that the describing terms of the thermal carrier escape from QDs is not enough in order to describe the effects of temperature on dynamics of carrier escape. Also, it was found that rate of (thermal) carriers stimulation depends directly on the inhomogeneous broadening (IHB). In other words, changes of IHB effects on the dynamics of thermal carrier changes. In addition, the results showed that optimized amount of homogenous broadening (HB) in which the maximum emission occurs in the central mode depended on both IHB and temperature. Comparing two conditions of linear and total optical gain illustrated that output power decreases if linear gain is replaced by total gain; also, lasing emissions broadens and number of lasing modes increases by increase in current. Investigating the effect of carrier’s relaxation lifetime on output power showed that increase of relaxation lifetime increases the threshold current and decreases the output power and slope efficiency. Furthermore, increase in relaxation lifetime caused decline in maximum output power and broadening of lasing spectrum.

Method of separation of homogeneous and inhomogeneous broadenings of absorption and luminescence spectra of colloidal quantum dots

On the basis of the universal Kennard–Stepanov–van Roosbroeck–Shockley relation, a general approach is proposed for separation of the contributions of homogeneous and inhomogeneous broadenings in the absorption and luminescence spectra of colloidal quantum dots. The applicability of the developed method is demonstrated on published experimental data.

Inhomogeneous and Homogeneous Line Broadening of Optical Spectra of PTCDA Molecules Adsorbed at Step Edges of Alkali Halide Surfaces

We report a detailed analysis of the line shapes of the 0–0 transitions in the fluorescence (FL) and fluorescence excitation (FLE) spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules adsorbed at terrace and step edge sites of (100)-oriented alkali halide films (KCl and NaCl). At low temperatures (6–20 K), we find very narrow FLE lines with a fwhm of 4.5 cm–1 (3.0 cm–1) on KCl (NaCl). These line shapes are dominated by inhomogeneous broadening related to the structural variation of the environment of the PTCDA molecules. We explain this site broadening on the basis of structural models for the adsorption sites at the step edges from earlier scanning microscopy data and density functional theory calculations. With increasing temperatures, the 0–0 lines in the FL and FLE spectra broaden; e.g., in the FL, the fwhm increases to 26 cm–1 (18 cm–1) at 100 K on the KCl (NaCl) surface. This temperature induced broadening is of Lorentzian shape and can be described by the theory of Hsu an...

Coronal loop seismology using damping of standing kink oscillations by mode coupling

Context. Kink oscillations of solar coronal loops are frequently observed to be strongly damped. The damping can be explained by mode coupling on the condition that loops have a finite inhomogeneous layer between the higher density core and lower density background. The damping rate depends on the loop density contrast ratio and inhomogeneous layer width. Aims. The theoretical description for mode coupling of kink waves has been extended to include the initial Gaussian damping regime in addition to the exponential asymptotic state. Observation of these damping regimes would provide information about the structuring of the coronal loop and so provide a seismological tool. Methods. We consider three examples of standing kink oscillations observed by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO) for which the general damping profile (Gaussian and exponential regimes) can be fitted. Determining the Gaussian and exponential damping times allows us to perform seismological inversions for the loop density contrast ratio and the inhomogeneous layer width normalised to the loop radius. The layer width and loop minor radius are found separately by comparing the observed loop intensity profile with forward modelling based on our seismological results. Results. The seismological method which allows the density contrast ratio and inhomogeneous layer width to be simultaneously determined from the kink mode damping profile has been applied to observational data for the first time. This allows the internal and external Alfven speeds to be calculated, and estimates for the magnetic field strength can be dramatically improved using the given plasma density. Conclusions. The kink mode damping rate can be used as a powerful diagnostic tool to determine the coronal loop density profile. This information can be used for further calculations such as the magnetic field strength or phase mixing rate.

Electronic band structure and optical gain of GaNxBiyAs1−x−y/GaAs pyramidal quantum dots

The electronic band structure and optical gain of GaNxBiyAs1−x−y/GaAs pyramidal quantum dots (QDs) are investigated using the 16-band k ⋅ p model with constant strain. The optical gain is calculated taking both homogeneous and inhomogeneous broadenings into consideration. The effective band gap falls as we increase the composition of nitrogen (N) and bismuth (Bi) and with an appropriate choice of composition we can tune the emission wavelength to span within 1.3 μm–1.55 μm, for device application in fiber technology. The extent of this red shift is more profound in QDs compared with bulk material due to quantum confinement. Other factors affecting the emission characteristics include virtual crystal, strain profile, band anticrossing (BAC), and valence band anticrossing (VBAC). The strain profile has a profound impact on the electronic structure, specially the valence band of QDs, which can be determined using the composition distribution of wave functions. All these factors eventually affect the optical gain spectrum. With an increase in QD size, we observe a red shift in the emission energy and emergence of secondary peaks owing to transitions or greater energy compared with the fundamental transition.

Modeling of carrier dynamics in InGaAs/GaAs self-assembled quantum dot lasers.

In this study, a theoretical model is used to simulate the lasing characteristics of InGaAs/GaAs self-assembled quantum dot lasers. The rate equation for InGaAs/GaAs is numerically solved using the fourth-order Runge-Kutta method. We consider the homogeneous and inhomogeneous broadening of the optical gain, both with and without considering the nonlinear gain. The results indicate that for a certain injected current, there is a specific homogeneous broadening (HB) with the best lasing performance when HB is near, comparable, or equal to inhomogeneous broadening for every laser-injected current. We show that if the linear optical gain is replaced by the total gain, the output power will reduce a little and the peaks will be more broadened and the maximum of the gain spectrum decreases. The time evolution of the photon number for different currents and different HBs is shown. The results indicate that by increasing the HB, the number of photons in the steady state and the delay in the starting oscillation will increase. In addition, we show that there is threshold coverage for quantum dots to begin lasing and an optimum quantum dot coverage in which the self-assembled quantum dot lasers operate with the lowest possible threshold current and maximum output power.

Homogeneous and inhomogeneous broadenings and the Voigt line shapes in the phase-resolved and intensity sum-frequency generation vibrational spectroscopy.

In this report, we show that the ability to measure the sub-1 cm(-1) resolution phase-resolved and intensity high-resolution broadband sum frequency generation vibrational spectra of the -CN stretch vibration of the Langmuir-Blodgett (LB) monolayer of the 4-n-octyl-4'-cyanobiphenyl (8CB) on the z-cut α-quartz surface allows the direct comparison and understanding of the homogeneous and inhomogeneous broadenings in the imaginary and intensity SFG vibrational spectral line shapes in detail. The difference of the full width at half maximum (FWHM) of the imaginary and intensity sum-frequency generation vibrational spectroscopy spectra of the same vibrational mode is the signature of the Voigt line shape and it measures the relative contribution to the overall line shape from the homogeneous and inhomogeneous broadenings in SFG vibrational spectra. From the phase-resolved and intensity spectra, we found that the FWHM of the 2238.00 ± 0.02 cm(-1) peak in the phase-resolved imaginary and intensity spectra is 19.2 ± 0.2 cm(-1) and 21.6 ± 0.4 cm(-1), respectively, for the -CN group of the 8CB LB monolayer on the z-cut α-quartz crystal surface. The FWHM width difference of 2.4 cm(-1) agrees quantitatively with a Voigt line shape with a homogeneous broadening half width of Γ = 5.29 ± 0.08 cm(-1) and an inhomogeneous standard derivation width Δω = 5.42 ± 0.07 cm(-1). These results shed new lights on the understanding and interpretation of the line shapes of both the phase-resolved and the intensity SFG vibrational spectra, as well as other incoherent and coherent spectroscopic techniques in general.

Time-delayed behaviors of transient four-wave mixing signal intensity in inverted semiconductor with carrier-injection pumping

An analytical expression of transient four-wave mixing (TFWM) in inverted semiconductor with carrier-injection pumping was derived from both the density matrix equation and the complex stochastic stationary statistical method of incoherent light. Numerical analysis showed that the TFWM decayed decay is towards the limit of extreme homogeneous and inhomogeneous broadenings in atoms and the decaying time is inversely proportional to half the power of the net carrier densities for a low carrier-density injection and other high carrier-density injection, while it obeys an usual exponential decay with other decaying time that is inversely proportional to half the power of the net carrier density or it obeys an unusual exponential decay with the decaying time that is inversely proportional to a third power of the net carrier density for a moderate carrier-density injection. The results can be applied to studying ultrafast carrier dephasing in the inverted semiconductors such as semiconductor laser amplifier and semiconductor optical amplifier.

Operating Spin Echo in the Quantum Regime for an Atomic-Ensemble Quantum Memory.

Spin echo is a powerful technique to extend atomic or nuclear coherence times by overcoming the dephasing due to inhomogeneous broadenings. However, there are disputes about the feasibility of applying this technique to an ensemble-based quantum memory at the single-quanta level. In this experimental study, we find that noise due to imperfections of the rephasing pulses has both intense superradiant and weak isotropic parts. By properly arranging the beam directions and optimizing the pulse fidelities, we successfully manage to operate the spin echo technique in the quantum regime by observing nonclassical photon-photon correlations as well as the quantum behavior of retrieved photons. Our work for the first time demonstrates the feasibility of harnessing the spin echo method to extend the lifetime of ensemble-based quantum memories at the single-quanta level.

Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides.

The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.

Analytical Expression for the Electric Field of the Single Mode Laser Homogeneous Broadening in the Pulse Regime

The simplest model of the laser is that of a single mode system homogenously broadened. The dynamical behavior of this laser is described by three differential equations, called Haken-Lorenz equations[1], similar to the Lorenz model [1] already known to predict deterministic chaos. In previous recent work [5-7] we have proposed a simple harmonic expansion method to obtain a series of harmonics terms that yield analytical solutions to the laser equations. ¶This method allows us to derive an analytical expression of the laser field amplitude when this last undergoes a periodic oscillations around zero mean value. We also obtain an analytical expression of the pulsing frequency. Keywords : Laser, instabilities and chaos

Comparative study of the rare earth environment in rare earth metaborate glass (REB3O6, RE = La, Nd) and in sodium borate glasses

The RE environment in the RE-metaborate glass (RE=La3+ or Nd3+) has been characterized by Nd LIII-edge EXAFS and Nd3+ and Eu3+ optical spectroscopy. The spectroscopic data have been compared with that of crystalline RE-metaborate and of two sodium borate glasses containing RE, a Na-poor borate glass (20mol% Na2O) bearing almost no NBOs (non-bridging oxygens), and a Na-rich borate glass (35mol% Na2O) bearing a significant number of NBOs. Spectroscopic data indicate that the RE–O ionicity and mean distance increase in the order Na-rich borate glass<RE-metaborate glass<Na-poor borate glass<RE-metaborate crystal. The distribution of RE3+ sites in the RE-metaborate glass is wider than in the two sodium borate glasses, as revealed by a larger RE–O distance distribution and larger inhomogeneous widths of optical transitions. Data are consistent with a model in which both NBOs and BOs take part to the coordination sphere of RE3+, like in the crystalline RE-metaborate phase, but they point out stronger RE3+–NBO bonds. The structural and bonding differences involving the RE3+ ions, between the glassy and crystalline forms of RE-metaborate, may be the basis of the high nucleation barrier observed in this congruent composition.