Quasiresonant Energy Transfer Research Articles

The investigations of Dy:YAG and Dy,Tb:YAG as potentially efficient GaN blue LD pumped solid state yellow laser crystals

With the fast development of GaN blue laser diode, Dy3+ doped laser crystals are receiving renewed interests due to their potential yellow laser emission under GaN laser diode pumping. In this study, Dy:YAG and Dy,Tb:YAG with high quality have been grown successfully with Czochralski method. Their spectroscopic properties as well as the energy transfer mechanism between Dy3+ and Tb3+ were discussed in details. The 583 nm emission cross-section of Dy:YAG crystal is improved by co-doping of Tb3+, which indicates the efficient quasi-resonant energy transfer between the Dy3+-6H13/2 state and Tb3+-7F4 state is realized. The thermal properties of Dy:YAG, including specific heat and thermal conductivity, were investigated. The high thermal conductivity suggests the Dy:YAG crystal should be very suitable for high-power laser operation. In addition, considering the high phonon energy of Dy:YAG and Dy,Tb:YAG crystals, the depletion of population on the lower laser level (6H13/2) of Dy3+ could be accelerated. The results suggesting that both Dy:YAG and Dy,Tb:YAG could be highly appraised as excellent competitors for all solid state yellow lasers.

Laplacian spectra of a class of small-world networks and their applications.

One of the most crucial domains of interdisciplinary research is the relationship between the dynamics and structural characteristics. In this paper, we introduce a family of small-world networks, parameterized through a variable d controlling the scale of graph completeness or of network clustering. We study the Laplacian eigenvalues of these networks, which are determined through analytic recursive equations. This allows us to analyze the spectra in depth and to determine the corresponding spectral dimension. Based on these results, we consider the networks in the framework of generalized Gaussian structures, whose physical behavior is exemplified on the relaxation dynamics and on the fluorescence depolarization under quasiresonant energy transfer. Although the networks have the same number of nodes (beads) and edges (springs) as the dual Sierpinski gaskets, they display rather different dynamic behavior.

Quasi-resonant vibrational energy transfer in N2 + N2 collisions: Effect of the long-range interaction

State-to-state vibrational rate coefficients for the collision of two vibrationally excited nitrogen molecules have been calculated using quasiclassical trajectories and networked computing. For this purpose two potential energy surfaces formulated using completely different interaction models have been used. The differences in the calculated rates have been rationalized in terms of the dependence of quasi-resonant transition probabilities on the collision energy and the impact parameter. The effect of the different long-range interaction on the calculated scattering properties is discussed.

Studies on pulsed optogalvanic effect in Eu/Ne hollow cathode discharge.

The optogalvanic (OG) effect has been observed in a Eu/Ne hollow cathode discharge lamp using pulsed laser irradiation. An OG spectrum is recorded in dye laser wavelength region 574–602 nm using a boxcar-averager. In total 41 atomic lines are observed. Of these, 38 lines are assigned to neon transitions. Two lines observed corresponding to wavelengths 576.519 and 601.815 nm are assigned to europium transitions; (4f 7 6s 2 , S 8 7/2 →4f 7 6s6p , zP 6 7/2 ) and (4f 7 6s 2 , S 8 7/2 →4f 7 6s6p , zP 8 9/2 ), respectively, and the remaining line at 582.475 nm could not be assigned. The effect of the discharge current on europium as well as neon OG signals is also studied. At moderate discharge current values, an extra positive peak is observed in neon OG signal for the transition (1s 5 →2p 2 ) at 588.189 nm, which is explained by Penning-ionization process using the quasi-resonant energy transfer interactions between excited neon and europium atoms lying in 2p 2 and D 10 9/2 states, respectively.

Laser-induced optogalvanic signal oscillations in miniature neon glow discharge plasma

Laser-induced optogalvanic (OG) signal oscillations detected in miniature neon glow discharge plasma are investigated using a discharge equivalent-circuit model. The damped oscillations in OG signal are generated when a pulsed dye laser is tuned to a specific neon transition (1s5→2p2) at 588.2 nm under the discharge conditions where dynamic resistance changes its sign. Penning ionization via quasi-resonant energy transfer collisions between neon gas atoms in metastable state and sputtered electrode atoms in ground state is discussed to explain the negative differential resistance properties of discharge plasma that are attributed to oscillations in the OG signal. The experimentally observed results are simulated by analyzing the behavior of an equivalent discharge-OG circuit. Good agreement between theoretically calculated and experimental results is observed. It is found that discharge plasma is more sensitive and less stable in close vicinity to dynamic resistance sign inversion, which can be useful for weak-optical-transition OG detection.

Laplacian spectra of recursive treelike small-world polymer networks: Analytical solutions and applications

A central issue in the study of polymer physics is to understand the relation between the geometrical properties of macromolecules and various dynamics, most of which are encoded in the Laplacian spectra of a related graph describing the macrostructural structure. In this paper, we introduce a family of treelike polymer networks with a parameter, which has the same size as the Vicsek fractals modeling regular hyperbranched polymers. We study some relevant properties of the networks and show that they have an exponentially decaying degree distribution and exhibit the small-world behavior. We then study the Laplacian eigenvalues and their corresponding eigenvectors of the networks under consideration, with both quantities being determined through the recursive relations deduced from the network structure. Using the obtained recursive relations we can find all the eigenvalues and eigenvectors for the networks with any size. Finally, as some applications, we use the eigenvalues to study analytically or semi-analytically three dynamical processes occurring in the networks, including random walks, relaxation dynamics in the framework of generalized Gaussian structure, as well as the fluorescence depolarization under quasiresonant energy transfer. Moreover, we compare the results with those corresponding to Vicsek fractals, and show that the dynamics differ greatly for the two network families, which thus enables us to distinguish between them.

Energy transfer and energy level decay processes in Tm3+-doped tellurite glass

The primary excited state decay and energy transfer processes in singly Tm3+-doped TeO2:ZnO:Bi2O3:GeO2 (TZBG) glass relating to the 3F4 → 3H6 ∼1.85 μm laser transition have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the 3H4 manifold at 794 nm, the 3H5 manifold at 1220 nm, and 3F4 manifold at 1760 nm has established that the 3H5 manifold is entirely quenched by multiphonon relaxation in tellurite glass. The luminescence from the 3H4 manifold with an emission peak at 1465 nm suffers strong suppression due to cross relaxation that populates the 3F4 level with a near quadratic dependence on the Tm3+ concentration. The 3F4 lifetime becomes longer as the Tm3+ concentration increases due to energy migration and decreases to 2.92 ms when [Tm3+] = 4 mol. % as a result of quasi-resonant energy transfer to free OH− radicals present in the glass at concentrations between 1 × 1018 cm−3 and 2 × 1018 cm−3. Judd-Ofelt theory in conjunction with absorption measurements were used to obtain the radiative lifetimes and branching ratios of the energy levels located below 25 000 cm−1. The spectroscopic parameters, the cross relaxation and Tm3+(3F4) → OH− energy transfer rates were used in a numerical model for laser transitions emitting at 2335 nm and 1865 nm.

Energy transfer among rare earth ions induced by annealing process of Tm [sbnd]Er codoped aluminum oxide thin films

Er Tm codoped amorphous aluminum oxide (a-Al 2O 3) thin films have been prepared by pulsed laser deposition. Efficient photoluminescence (PL) in the region of 1400–1700 nm with two peaks centered at 1533 nm and 1620 nm were observed with pumping at the wavelength of 791 nm. The PL performance has been investigated as a function of annealing temperature, which was varied from 650 to 850 °C in air. Infrared emission was improved by annealing, and energy transfer processes occurred obviously for annealing temperatures between 800 and 850 °C. All possible energy transfer channels were investigated and our results suggest that the quasi-resonant energy transfer and cross relaxation between Tm 3+ and Er 3+ play an important role in the evolution of the luminescent response.

Ozone-stimulated emission due to atomic oxygen population inversions in an argon microwave plasma torch

It is shown that, in a microwave torch discharge in an argon jet injected into an oxygen atmosphere at normal pressure, quasi-resonant energy transfer from metastable argon atoms to molecules of oxygen and ozone generated in the torch shell and, then, to oxygen atoms produced via the dissociation of molecular oxygen and ozone leads to the inverse population of metastable levels of atomic oxygen. As a result, the excited atomic oxygen with population inversions becomes a gain medium for lasing at wavelengths of 844.6 and 777.3 nm (the 33P–33S and 35P–35S transitions). It is shown that an increase in the ozone density is accompanied by an increase in both the lasing efficiency at these wavelength and the emission intensity of the plasma-forming argon at a wavelength of 811.15 nm (the 2P04s–2P04p transition). When the torch operates unstably, the production of singlet oxygen suppresses ozone generation; as a result, the lasing effect at these wavelengths disappears.

Spectra of Husimi cacti: Exact results and applications

Starting from exact relations for finite Husimi cacti we determine their complete spectra to very high accuracy. The Husimi cacti are dual structures to the dendrimers but, distinct from these, contain loops. Our solution makes use of a judicious analysis of the normal modes. Although close to those of dendrimers, the spectra of Husimi cacti differ. From the wealth of applications for measurable quantities which depend only on the spectra, we display for Husimi cacti the behavior of the fluorescence depolarization under quasiresonant Forster energy transfer.

ColdHe+H2collisions near dissociation

Cross sections for He+H{sub 2} collisions are reported for rovibrational states near dissociation at translational energies less than 1000 cm{sup -1}. In contrast to our previously reported cross sections for lower-lying initial rotational states, the excited rotational states near dissociation give rise to shape resonances for energies between 0.001 and 1 cm{sup -1}. The emergence of these resonances with increasing rotational level is opposite to the trend found for He+CO where the shape resonance strength decreases with rotational level for low-lying states. Quasiresonant energy transfer and low energy excitation thresholds are also discussed and it is found that there are 11 rovibrationally excited states that are stable against collision at ultracold temperatures.

Energy transfer and trapping in regular hyperbranched macromolecules

Fluorescence depolarization under dipolar quasiresonant energy transfer allows to monitor the presence of the excitation on the originally excited chromophore. However, parallel to the radiative energy decay, there is also the possibility of loss due to energy trapping. Here we focus on the situation when the transfer occurs over a fractal hyperbranched macromolecule, whose center is a trap. Remarkably, the eigenvalues needed for the determination of the decay can be obtained through an iterative procedure, a fact which allows to study very large systems.

Optical Control of Excitons in a Pair of Quantum Dots Coupled by the Dipole-Dipole Interaction

We demonstrate coherent nonlinear-optical control of excitons in a pair of quantum dots (QDs) coupled via dipolar interaction. The single-exciton population in the first QD is controlled by resonant picosecond excitation, giving rise to Rabi oscillations. As a result, the exciton transition in the second QD is spectrally shifted and concomitant Rabi oscillations are observed. We identify coupling between permanent excitonic dipole moments as the dominant interaction mechanism, whereas quasiresonant (Förster) energy transfer is weak. Such control schemes based on dipolar interaction are a prerequisite for realizing scalable quantum logic gates.

Monitoring energy transfer in hyperbranched macromolecules through fluorescence depolarization

Fluorescence depolarization under dipolar quasiresonant energy transfer allows to monitor the presence of the excitation on the originally excited chromophore. Moreover, a simple model relates the arrangement of the chromophores to the depolarization decay via the eigenvalues of the corresponding connectivity matrix. Now, for important classes of polymers (such as dendrimers and fractal hyperbranched macromolecules) these eigenvalues can be obtained without the need of diagonalizing the corresponding connectivity matrices, a fact which allows to study very large systems also. We determine the fluorescence depolarization behavior of hyperbranched systems and emphasize the possibilities offered by such measurements in differentiating between the underlying geometries.

Intermolecular Vibrational Relaxation in Mixtures of Excited Benzophenone and Anthraquinone Molecules with Water Vapor

The intermolecular vibrational energy transfer from triplet molecules of benzophenone and anthraquinone to H2O molecules has been investigated. To determine the rates of establishment of vibrational (V-V) and thermal (V-T) equilibrium in a vibrational quasi-continuum of mixed singlet-triplet levels, the dependences of the decay rates and intensities of the fast and slow components of delayed fluorescence on the H2O vapor pressure have been investigated. For V-V relaxation, the efficiencies β1 and the mean energies 〈ΔE〉 transferred per collision in mixtures with H2O and other polyatomic foreign gases have been compared. It has been established that the efficiencies β1 for quasi-resonant vibrational energy transfer (V-V) from benzophenone and anthraquinone to H2O are an order of magnitude lower than the gas-kinetic ones and lower than those obtained under the same experimental conditions for such foreign gases as C5H12, SF6, and CCl4, and decrease with increasing temperature in the 433–513 K range. It has been concluded that the mechanism of V-V relaxation in mixtures with H2O are determined by long-range attractive forces. In mixtures with H2O, no acceleration of V-T relaxation due to the formation of hydrogen bonds has been revealed. The low-efficiency thermalization process (V-T relaxation) is controlled by short-range repulsive forces, and the differences between the intensities β2 for mixtures of benzophenone and anthraquinone with H2O and other polyatomic gases are determined by the change in the reduced mass of interacting molecules.

Cold collisions between argon atoms and hydrogen molecules

by considering a complex square-well potential. We demonstrate that many of the important features of a numerically exact coupled-channel calculation may be qualitatively understood in terms of this simple model. W ith the aid of the complex scattering length, a kinetic model may be constructed for describing the relaxation of vibrationally or rotationally excited trapped molecules 2 . The relaxation may be due to direct collisional quenching or to the formation and decay of van der Waals complexes. When the end-over-end angular momentum of the complex is zero and the vibrational stretching quantum number is the lar gest possible integer that allows the complex to be bound, it is possible to use effective range theory to compute the lifetimes for predissociation 2,21 . The numerical tests that confirmed the reliability of the effective range theory were performed on He H 2 that supported only a single bound state. In the present work, we perform similar tests for Ar H 2 where several bound states exist. It was previously demonstrated 21 that predissociation of the most weakly bound state of a van der Waals complex is influenced by the same quasiresonant vibration-rotation energy transfer that is found in atom-diatom collisions 22 . It was also shown that the proximity of closed quasiresonant channel thresholds provides a strong influence on the lifetimes 21 . In the present work, we study the analytic structure of the threshold behavior obtained using both parametrizations of the Ar-H2 potential-ener gy surface and investigate whether quasiresonant dynamics has any influence on the more deeply bound states of the complex. The paper is organized as follows: in Sec. II, we describe a few qualitative features of the potential-energy surface that are important for our investigations. Section III provides a review of the standard close-coupling formalism that we used to obtain an extensive database of rovibrational rate coefficients and predissociation lifetimes for both of the parametrized surfaces. In Sec. IV, we demonstrate how a complex square-well potential may be used to provide a qualitative description of several of the key results that are found from the numerically exact coupled-channel calculations. These results are presented in Sec. V. Conclusions to our investigation are given in Sec. VI.

Discreteness and quasiresonances in weak turbulence of capillary waves.

A numerical study is presented which deals with the kinematics of quasiresonant energy transfer in a system of capillary waves with a discrete wave number space in a periodic box. For a given set of initially excited modes and a given level of resonance broadening, the modes of the system are partitioned into two classes, one active, the other forbidden. For very weak nonlinearity the active modes are very sparse. It is possible that this sparsity explains discrepancies between the values of the Kolmogorov constant measured in numerical simulations of weakly turbulent cascades and the theoretical values obtained from the continuum theory. There is a critical level of nonlinearity below which the set of active modes has finite radius in wave number space. In this regime, an energy cascade to dissipative scales may not be possible and the usual Kolmogorov spectrum predicted by the continuum theory not realized.

On the role of interaction anisotropy in vibrational relaxation of HF and HCl by Ar

A coupled states quantum scattering study of vibrational relaxation in Ar + HF and Ar + HCl collisions is presented. The energy transfer in these systems exhibits a quasiresonant character resulting in a strongly inverted rotational population of the diatomic molecules after collision. The high anisotropy of interaction is shown to be determining the quasiresonant vibrational energy transfer by favoring the population of high rotational energy levels. The role of interaction anisotropy in atom-diatomic molecule collisions is discussed.

Broadening and shifting coefficients of Raman isotropic Q( j)( j=0,1,2) lines for pure H 2: coupled states and semiclassical calculations. : Comparison with experiments

Quantum coupled states (CS) and semiclassical Robert–Bonamy (RB) calculations of pure H 2 broadening and shifting coefficients of Q( j) rovibrational lines have been performed using a simple atom–atom potential energy surface (PES). The main interest is to test the validity of the semiclassical method versus the CS calculation. This is the first comparison of the two methods for a diatom–diatom system for which rotation–rotation (R–R) energy transfers contribute to the broadening and shifting coefficients. Agreement between the two methods is found acceptable on the total result, but is excellent when resonant or quasi-resonant rotational energy transfer do not occur. Comparison with experiment confirms the empirical laws used to model the temperature dependence of the broadening and shifting coefficients. It also permits us to analyze through the semiclassical model, the weakness and advantages of the atom–atom PES used.

Quasiresonant Energy Transfer in Ultracold Atom-Diatom Collisions

Quantum and classical quasiresonant vibration-rotation energy transfer is investigated for ultracold He-H{sub 2} collisions. Classical trajectory computations show that extremely strong correlations between {Delta}j and {Delta}v persist at low energies, though the changes themselves are less than one quantum. Quantum computations show that quasiresonant transitions occur in the limit of zero collision energy but that threshold effects become important and that some quasiresonant channels close. The qualitative similarity between classical and quantum results suggests that they share a common mechanism. {copyright} {ital 1999} {ital The American Physical Society}