Electronic Transition Rates Research Articles

Theory for Electron Excitation in Dielectrics under an Intense Linear and Circularly Polarized Laser Fields

We report a Keldysh-like model for the electron transition rate in dielectrics under an intense circularly polarized laser. We assume a parabolic two-band system and the Houston function as the tim...

Rapid Evaluation of All-Optical Modulation in Quantum Cascade Laser Using Photoluminescence Spectrum

All-optical modulation in a quantum cascade laser (QCL) can be rapidly evaluated using a photoluminescence (PL) spectrum. The PL spectrum reflects the electron transition rate of the sub-bands of the active region of a QCL. The peak position, amplitude, and displacement of the PL spectrum can be used to determine the optimal wavelength, spot position, the laser incident angle, optical power, and displacement in the electric field of the near-infrared laser. This PL spectrum approach is rapid, accurate, and simple. Furthermore, it can be used to optimize high-speed all-optical modulation of QCLs for use in free-space optical communication and molecular-detection applications.

A comparative study of different methods for calculating electronic transition rates.

We present a comprehensive comparison of the following mixed quantum-classical methods for calculating electronic transition rates: (1) nonequilibrium Fermi's golden rule, (2) mixed quantum-classical Liouville method, (3) mean-field (Ehrenfest) mixed quantum-classical method, and (4) fewest switches surface-hopping method (in diabatic and adiabatic representations). The comparison is performed on the Garg-Onuchic-Ambegaokar benchmark charge-transfer model, over a broad range of temperatures and electronic coupling strengths, with different nonequilibrium initial states, in the normal and inverted regimes. Under weak to moderate electronic coupling, the nonequilibrium Fermi's golden rule rates are found to be in good agreement with the rates obtained via the mixed quantum-classical Liouville method that coincides with the fully quantum-mechanically exact results for the model system under study. Our results suggest that the nonequilibrium Fermi's golden rule can serve as an inexpensive yet accurate alternative to Ehrenfest and the fewest switches surface-hopping methods.

AlGaAs/GaAs Triple Quantum Well Photodetector at $5~\mu \text{m}$ Wavelength—A Simulation Study

A novel AlGaAs/GaAs triple quantum well (QW) photodetector at 5- $\mu \text{m}$ wavelength has been proposed. The photodetector is designed with a double-resonance condition to achieve short tunneling time and large electron escape probability. The interface optical phonon modes and the phonon-assisted electron transition rates have been calculated. The electron escape probability is as high as 0.8 in the proposed photodetector, which is over two times higher than traditional phonon-assisted tunneling devices. Compared with QW infrared photodetectors, the noise current of the photodetector can be reduced dramatically ( $> 10^{13}$ times at 77 K and >4000 times at 300 K). In addition, the effects of delta doping on the absorption coefficients and scattering times of the triple QW photodetector are also investigated in detail. Delta doping at the middle of the well is much more desirable than uniform doping, in order to achieve large escape probability and high absorption coefficient.

Ultra-sensitive graphene photodetector with plasmonic structure

We report a graphene-based photodetector with ultra-high photoresponsivity and wavelength selectivity, targeting at the mid-infrared (MIR) regime. To enhance the spectral selectivity, a gold-grating structure is designed and implemented under the graphene layer to excite surface plasmon polaritons (SPPs). The electromagnetic field with specific wavelength can be guided to and confined within the designed subwavelength structures. The graphene layer contacted by metal is slightly p-type doped due to gold grating, improving the interband transition rate of electrons. The built-in potential established in the contact region facilitates the separation of non-equilibrium carriers generated on graphene layer, leading to a photovoltage. With optimized structural design the photodetector exhibits excellent photoresponsivity of 1 V/μW at the wavelength of 9 μm.

Non-Condon equilibrium Fermi's golden rule electronic transition rate constants via the linearized semiclassical method.

In this paper, we test the accuracy of the linearized semiclassical (LSC) expression for the equilibrium Fermi's golden rule rate constant for electronic transitions in the presence of non-Condon effects. We do so by performing a comparison with the exact quantum-mechanical result for a model where the donor and acceptor potential energy surfaces are parabolic and identical except for shifts in the equilibrium energy and geometry, and the coupling between them is linear in the nuclear coordinates. Since non-Condon effects may or may not give rise to conical intersections, both possibilities are examined by considering: (1) A modified Garg-Onuchic-Ambegaokar model for charge transfer in the condensed phase, where the donor-acceptor coupling is linear in the primary mode coordinate, and for which non-Condon effects do not give rise to a conical intersection; (2) the linear vibronic coupling model for electronic transitions in gas phase molecules, where non-Condon effects give rise to conical intersections. We also present a comprehensive comparison between the linearized semiclassical expression and a progression of more approximate expressions. The comparison is performed over a wide range of frictions and temperatures for model (1) and over a wide range of temperatures for model (2). The linearized semiclassical method is found to reproduce the exact quantum-mechanical result remarkably well for both models over the entire range of parameters under consideration. In contrast, more approximate expressions are observed to deviate considerably from the exact result in some regions of parameter space.

Nanowire Optoelectronics

Abstract Semiconductor nanowires have been used in a variety of passive and active optoelectronic devices including waveguides, photodetectors, solar cells, light-emitting diodes (LEDs), lasers, sensors, and optical antennas. We review the optical properties of these nanowires in terms of absorption, guiding, and radiation of light, which may be termed light management. Analysis of the interaction of light with long cylindrical/hexagonal structures with subwavelength diameters identifies radial resonant modes, such as Leaky Mode Resonances, or Whispering Gallery modes. The two-dimensional treatment should incorporate axial variations in “volumetric modes,”which have so far been presented in terms of Fabry–Perot (FP), and helical resonance modes. We report on finite-difference timedomain (FDTD) simulations with the aim of identifying the dependence of these modes on geometry (length, width), tapering, shape (cylindrical, hexagonal), core–shell versus core-only, and dielectric cores with semiconductor shells. This demonstrates how nanowires (NWs) form excellent optical cavities without the need for top and bottommirrors. However, optically equivalent structures such as hexagonal and cylindrical wires can have very different optoelectronic properties meaning that light management alone does not sufficiently describe the observed enhancement in upward (absorption) and downward transitions (emission) of light inNWs; rather, the electronic transition rates should be considered. We discuss this “rate management” scheme showing its strong dimensional dependence, making a case for photonic integrated circuits (PICs) that can take advantage of the confluence of the desirable optical and electronic properties of these nanostructures.

Pseudotunnel phototransitions in heterostructures with quantum wells III Two-photon charge transfer between wells

This paper discusses phototransitions in a heterostructure that contains deep quantum wells of various depths separated by a barrier that is impenetrable for tunnelling. The tunnelling-Hamiltonian formalism is used in fourth-order perturbation theory to obtain expressions for the two-photon electron-transition rates between states in an upper size-quantization subband of the valence band in a wide well and a lower subband of the conduction band in a narrow well.

Dynamics of electronic transitions and frequency dependence of negative capacitance in semiconductor diodes under high forward bias

We observed qualitatively dissimilar frequency dependence of negative capacitance under high charge injection in two sets of functionally different junction diodes: III-V based light emitting and Si-based non-light emitting diodes. Using an advanced approach based on bias activated differential capacitance, we developed a generalized understanding of negative capacitance phenomenon which can be extended to any diode based device structure. We explained the observations as the mutual competition of fast and slow electronic transition rates which are different in different devices. This study can be useful in understanding the interfacial effects in semiconductor heterostructures and may lead to superior device functionality.

Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion-atomsp3d5s*tight-binding simulations

GaN quantum dots QDs grown on AlN substrates are strong candidates for UV and near-infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: i crystal atomicity;ii strained active region; iii piezoelectricity; and iv spontaneous polarization pyroelectricity. Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long-range fields and the size quantization on an equal footing. In this work, single-particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c-axis are studied using a coupled molecular mechanics-atomistic 20-band sp3d5s* tight-binding VFF-TB framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized containing ~3 million atoms and of hexagonal disk shape having height and base length of 4.1 and 17nm, respectively. It is found that, in contrast to the well-studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center known as quantum confined Stark effect, pronounced non-degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.

Time-Dependent Density Functional Theory Study on Electronic Excited-State Hydrogen Bonding of Benzonitrile in Methanol Solution

In this work, the time dependent density functional theory (TDDFT) method was used to investigate the hydrogen bonding dynamics of benzonitrile (PhCN) as hydrogen acceptor in hydrogen donating solvent methanol (MeOH). The ground-state geometry optimizations and the electronic transition energies of the isolated PhCN and MeOH monomers and the two hydrogen-bonded PhCN–MeOH dimers are calculated by the DFT and TDDFT method respectively. According to the results, the hydrogen bond takes the responsibility of the geometric structure change and electronic transfer of the molecules involved. As well, the intermolecular hydrogen-bond C≡N···H–O is strengthened in electronically excited states of the hydrogen-bonded PhCN–MeOHa (planar structure) and PhCN–MeOHb (perpendicular structure) as a result of the lower excitation energy and the electronic spectral redshifts. Despite the different structure, the effects of hydrogen bond on PhCN–MeOHa and PhCN–MeOHb are considered the same, which serves as a proof that geometric structure has little contribution to the structural and energy change in hydrogen-bonded complexes. However, in high-lying singlet states, the structure can cause the divergence of electronic transition rate between the two hydrogen-bonded complexes, even if within the same transition path. What’s more, the extent of hydrogen bond effect on PhCN and MeOH is different between the low-lying excited states and the high-lying excited states.

Exciton extraction in nanocrystal solar cells

The efficiency of exciton extraction from nanocrystals is defined as the ratio of the number of extracted excitons to the number of absorbed photons. This efficiency is studied here based on a phenomenological model parameterized by recent experimental results in CdSe nanocrystals. Several possibilities of interplay among all possible electronic transition rates (Auger recombination, impact ionization, radiative recombination and electron–hole escape) are explored showing the possibility of obtaining efficiencies above 100% when multiple exciton generation processes are taken into account.

Analytical description of spin-Rabi oscillation controlled electronic transitions rates between weakly coupled pairs of paramagnetic states withS=12

We report on an analytical description of spin-dependent electronic transition rates which are controlled by a radiation induced spin-Rabi oscillation of weakly spin-exchange and spin-dipolar coupled paramagnetic states (S=1/2). The oscillation components (the Fourier content) of the net transition rates within spin-pair ensembles are derived for randomly distributed spin resonances with account of a possible correlation between the two distributions that correspond to the two individual pair partners. The results presented here show that when electrically or optically detected Rabi spectroscopy is conducted under an increasing driving field B_ 1, the Rabi spectrum evolves from a single resonance peak at s=\Omega_R, where \Omega_R=\gamma B_1 is the Rabi frequency (\gamma is the gyromagnetic ratio), to three peaks at s= \Omega_R, s=2\Omega_R, and at low s<< \Omega_R. The crossover between the two regimes takes place when \Omega_R exceeds the expectation value \delta_0 of the difference of the Zeeman energies within the pairs, which corresponds to the broadening of the magnetic resonance lines in the presence of disorder caused by hyperfine field or distributions of Lande g-factors. We capture this crossover by analytically calculating the shapes of all three peaks at arbitrary relation between \Omega_R and \delta_0. When the peaks are well-developed their widths are \Delta s ~ \delta_0^2/\Omega_R.

On transition rates in surface hopping

Trajectory surface hopping (TSH) is one of the most widely used quantum-classical algorithms for nonadiabatic molecular dynamics. Despite its empirical effectiveness and popularity, a rigorous derivation of TSH as the classical limit of a combined quantum electron-nuclear dynamics is still missing. In this work, we aim to elucidate the theoretical basis for the widely used hopping rules. Naturally, we concentrate thereby on the formal aspects of the TSH. Using a Gaussian wave packet limit, we derive the transition rates governing the hopping process at a simple avoided level crossing. In this derivation, which gives insight into the physics underlying the hopping process, some essential features of the standard TSH algorithm are retrieved, namely (i) non-zero electronic transition rate ("hopping probability") at avoided crossings; (ii) rescaling of the nuclear velocities to conserve total energy; (iii) electronic transition rates linear in the nonadiabatic coupling vectors. The well-known Landau-Zener model is then used for illustration.

Preparation and Optimization of Long Persistent Luminescent Sr&lt;SUB&gt;4&lt;/SUB&gt;Al&lt;SUB&gt;14&lt;/SUB&gt;O&lt;SUB&gt;25&lt;/SUB&gt;:(Eu,Dy) Phosphor Materials

Sr4Al14O25:(Eu,Dy) phosphor materials were prepared by solid state reaction. The dependence of long per- sistent luminescence from Sr4Al14O25:(Eu 2+ , Dy 3+ ) on the amounts of H3BO3, the solid state reaction temperature, and the contents of Eu were studied. It was found that the addition of H3BO3 as the flux agent was critical for the formation of the blue-green emitting Sr4Al14O25 phase. Sr4Al14O25 phosphors with persistent luminescence of 490 nm and after- glow time more than 24 h was obtained for the sample prepared at 1400℃ with the doping amounts of H3BO3 of 10at% and Eu/Al atomic ratio of 0.03, respectively. Furthermore, the dependence of persistent luminescence intensity on Eu content indicates that the persistent luminescence process is mainly controlled by the electron transition rate from deep traps to Eu 2+ levels.

Auger spectrum of a water molecule after single and double core ionization

The high intensity of free electron lasers opens up the possibility to perform single-shot molecule scattering experiments. However, even for small molecules, radiation damage induced by absorption of high intense x-ray radiation is not yet fully understood. One of the striking effects which occurs under intense x-ray illumination is the creation of double core ionized molecules in considerable quantity. To provide insight into this process, we have studied the dynamics of water molecules in single and double core ionized states by means of electronic transition rate calculations and ab initio molecular dynamics (MD) simulations. From the MD trajectories, photoionization and Auger transition rates were computed based on electronic continuum wavefunctions obtained by explicit integration of the coupled radial Schrödinger equations. These rates served to solve the master equations for the populations of the relevant electronic states. To account for the nuclear dynamics during the core hole lifetime, the calculated electron emission spectra for different molecular geometries were incoherently accumulated according to the obtained time-dependent populations, thus neglecting possible interference effects between different decay pathways. We find that, in contrast to the single core ionized water molecule, the nuclear dynamics for the double core ionized water molecule during the core hole lifetime leaves a clear fingerprint in the resulting electron emission spectra. The lifetime of the double core ionized water was found to be significantly shorter than half of the single core hole lifetime.

Auger Spectrum of a Water Molecule after Single and Double Core-Ionization by Intense X-Ray Radiation

The high intensity of new x-ray sources such as Free Electron Lasers (FEL) offers the possibility to do single-shot molecule diffraction experiments. Even for small molecules, the dynamics induced by the radiation damage in such experiments are not yet fully understood. In particular, double core-ionized molecules are expected to be created in considerable quantity. We have therefore studied the electronic and nuclear dynamics of water molecules in single and double core ionized states by means of electronic transition rates and ab initio molecular dynamics (MD) simulations. From MD trajectories Auger transition rates were computed based on continuum electronic wavefunctions obtained by explicit integration of the coupled radial Schrödinger equations. The calculated spectra for different molecular geometries were accumulated to account for the effects of nuclear dynamics during the core-hole lifetime. In contrast to the single core ionized water molecule, we found that dissociation dynamics of double core-ionized water have strong effect on the resulting electron emission spectra. In addition, we found that the single core hole lifetime is slightly smaller than the value obtained in earlier theoretical works. Finally, we predict that the lifetime of double core ionized states is significantly lower than half of the lifetime of a single core hole.

Radiative one- and two-electron transitions into the emptyKshell of He-like ions

The branching ratios between the single and double electron radiative transitions to empty K shell in He-like ions with 2s2p configuration are evaluated for 15 ions with 4{<=}Z{<=}26 using fully relativistic multiconfiguration Dirac-Fock wavefunctions in the active space approximation. The effects of configuration interaction and Breit contributions on the transition parameters have been analyzed in detail. Though the influence of Breit interaction on the electric dipole allowed one-electron radiative transitions is negligible, it substantially changes the spin-forbidden rates and the two-electron one-photon transition probabilities. Also, while the single electron transition rates are gauge independent, the correlated double-electron probabilities are found to be gauge sensitive. The probable uncertainties in the computed transition rates have been evaluated by considering the line strengths and the differences between the calculated and experimental transition energies as accuracy indicators. The present results are compared with other available experimental and theoretical data.

Challenges of laser-cooling molecular ions

The direct laser cooling of neutral diatomic molecules in molecular beams suggests that trapped molecular ions can also be laser cooled. The long storage time and spatial localization of trapped molecular ions provides an opportunity for multi-step cooling strategies, but also requires careful consideration of rare molecular transitions. We briefly summarize the requirements that a diatomic molecule must meet for laser cooling, and we identify a few potential molecular ion candidates. We then carry out a detailed computational study of the candidates BH+ and AlH+, including improvedab initio calculations of the electronic state potential energy surfaces and transition rates for rare dissociation events. On the basis of an analysis of the population dynamics, we determine which transitions must be addressed for laser cooling, and compare experimental schemes using continuous-wave and pulsed lasers.

Nuclear physics studies using high energy lasers

Abstract This paper presents several mechanisms expected to be responsible for nuclear excitation in laser-heated plasma. Attempts to detect such processes in nuclei initially in their ground state are discussed in 181Ta and 235U cases. A calculation of the nuclear excitation by electronic transition (NEET) rate is presented in a 201Hg plasma at local thermodynamic equilibrium. We also report on a new way of de-exciting isomeric states in plasma. This indirect decay mode is illustrated in the case of the 84Rb isomer nucleus.