Red Stark Research Articles

Charge transport through the multiple end zigzag edge states of armchair graphene nanoribbons and heterojunctions.

This comprehensive study investigates charge transport through the multiple end zigzag edge states of finite-size armchair graphene nanoribbons/boron nitride nanoribbons (n-AGNR/w-BNNR) junctions under a longitudinal electric field, where n and w denote the widths of the AGNRs and the BNNRs, respectively. In 13-atom wide AGNR segments, the edge states exhibit a blue Stark shift in response to the electric field, with only the long decay length zigzag edge states showing significant interaction with the red Stark shift subband states. Charge tunneling through such edge states assisted by the subband states is elucidated in the spectra of the transmission coefficient. In the 13-AGNR/6-BNNR heterojunction, notable influences on the energy levels of the end zigzag edge states of 13-AGNRs induced by BNNR segments are observed. We demonstrate the modulation of these energy levels in resonant tunneling situations, as depicted by bias-dependent transmission coefficient spectra. Intriguing nonthermal broadening of tunneling current shows a significant peak-to-valley ratio. Our findings highlight the promising potential of n-AGNR/w-BNNR heterojunctions with long decay length edge states in the realm of GNR-based single electron transistors at room temperature.

Dynamic Stark effect in β and γ carotenes induced by photoexcitation of bacteriochlorophyll c in chlorosomes from Chloroflexus aurantiacus.

Chlorosomes of green bacteria can be considered as a prototype of future artificial light-harvesting devices due to their unique property of self-assembly of a large number of bacteriochlorophyll (BChl) c/d/e molecules into compact aggregates. The presence of carotenoids (Cars) in chlorosomes is very important for photoprotection, light harvesting and structure stabilization. In this work, we studied for the first time the electrochromic band shift (Stark effect) in Cars of the phototrophic filamentous green bacterium Chloroflexus (Cfx.) aurantiacus induced by fs light excitation of the main pigment, BChl c. The high accuracy of the spectral measurements permitted us to extract a small wavy spectral feature, which, obviously, can be associated with the dynamic shift of the Car absorption band. A global analysis of spectroscopy data and theoretical modeling of absorption spectra showed that near 60% of Cars exhibited a red Stark shift of ~ 25cm-1 and the remaining 40% exhibited a blue shift. We interpreted this finding as evidence of various orientations of Car in chlorosomes. We estimated the average value of the light-induced electric field strength in the place of Car molecules as ~ 106V/cm and the average distance between Car and the neighboring BChl c as ~ 10Å. We concluded that the dynamics of the Car electrochromic band shift mainly reflected the dynamics of exciton migration through the chlorosome toward the baseplate within ~ 1ps. Our work has unambiguously shown that Cars are sensitive indicators of light-induced internal electric fields in chlorosomes.

Effect of periodic potential on exciton states in semiconductor carbon nanotubes

We develop a theoretical background to treat exciton states in semiconductor single-walled carbon nanotubes (SWCNTs) in the presence of a periodic potential induced by a surface acoustic wave (SAW) propagating along SWCNT. The formalism accounts for the electronic band splitting into the Floquet sub-bands induced by the Bragg scattering on the SAW potential. Optical transitions between the Floquet states and correlated electron–hole pairs (excitons) are numerically examined. Formation of new van Hove singularities within the edges of Floquet sub-bands and associated transfer of the exciton oscillator strengths resulting in the photoluminescence quenching are predicted. The simulations demonstrate the exciton energy red Stark shift and reduction in the exciton binding energy. Comparison of our results with reported theoretical and experimental studies is provided.

Visualizing the coupling between red and blue stark states using photoionization microscopy.

In nonhydrogenic atoms in a dc electric field, the finite size of the ionic core introduces a coupling between quasibound Stark states that leads to avoided crossings between states that would otherwise cross. Near an avoided crossing, the interacting states may have decay amplitudes that cancel each other, decoupling one of the states from the ionization continuum. This well-known interference narrowing effect, observed as a strongly electric field-dependent decrease in the ionization rate, was previously observed in several atoms. Here we use photoionization microscopy to visualize interference narrowing in helium atoms, thereby explicitly revealing the mechanism by which Stark states decay. The interference narrowing allows measurements of the nodal patterns of red Stark states, which are otherwise not observable due to their intrinsic short lifetime.

Blue and red Stark shifts in single Si/SiO2quantum wells

The quantum-confined Stark shift (QCSS) of the ground and the first excited electron energy states in single Si/SiO2quantum wells is calculated. The QCSS for the ground state is always negative (blue), while the QCSS for the first excited state is positive, in general, but it can turn out to be negative when the electric field intensity becomes high enough. A remarkable reduction (∼50%) of the Stark shifts is obtained when the existence of nonabrupt interfaces as thin as 10 Å is taken into account. The nonabrupt interfaces can also change the sign of the Stark shift in the case of the excited states.

Very large blue and red Stark shifts of the excitonic transition in InGaAs-InP-InAsP antisymmetric coupled quantum wells

A new strained InGaAs-InP-InAsP antisymmetric coupled quantum well (CQW) structure with both very large blue and red quantum-confined Stark shifts for the first heavy-hole-to-electron excitonic transition, Ehh1fEe1, is studied theoretically in this paper. In the antisymmetric coupled quantum well, an antisymmetric-like pair of potential profiles between the shallow-deep conduction band profile and the deepshallow valence band profile are formed. The sub-band eigen-energies, E, and the associated envelope wave functions in the CQW structures with or without an applied electric field are calculated by the transfer-matrix method. The effect of strain on the pseudomorphic layers has been taken into account. Results indicate that the strained InGaAs-InP-InAsP antisymmetric CQW structure exhibits significant enhancement of the blue and red Stark effects in the Ehh1fEe1 transition. The influences of various antisymmetric CQW structural parameters, such as the total well width, the individual well width, the central barrier thickness and the composition of the strained layer on the quantum-confined Stark shift, as well as the envelope wave function overlap, are studied systematically. These strong Stark effects in the antisymmetric CQW structure may have potential applications in sophisticated new electronic devices, such as optical switching devices.

Quantum-confined Stark effects in semiconductor quantum disks

Quantum-confined Stark effects (QCSE's) on excitons in semiconductor quantum disks with finite-potential barriers have been calculated as a function of disk size parameters by a variational calculation in an effort to examine possible application to optical devices. The calculations agree with experimental data reported so far. Although the exciton binding energy, E/sub b/, for smaller diameters is large at zero bias, it decreases more with increasing electric field, which is contrary to the E/sub b/ behavior in a spherical quantum dot and quantum well. This larger decrease results in a smaller red Stark shift. Both the red Stark shift and the oscillator strength can be controlled by changing disk diameter and height. The analysis shows that favorable QCSE characteristics, i.e., a large red Stark shift at a small electric field with large oscillator strength, can be obtained.

Strong blueshift of the excitonic transition in the InGaAs/InP/InAsP antisymmetric coupled quantum wells

A new strained InGaAs/InP/InAsP antisymmetric coupled-quantum-well (CQW) structure with significant enhancement of the blue and red Stark effects in the first heavy-hole-to-electron excitonic transition is proposed in this letter. The calculated amount of blueshift is about 48 meV as the applied electric field varied from 0 to 90 kV/cm and the red Stark shift of about 56 meV can be achieved with an applied electric field in the 0 to −90 kV/cm range. The results of the strong Stark effect in the antisymmetric CQW structure may have potential applications in sophisticated new electronic devices, such as optical switching devices and tunable lasers.

Effect of ordering, interface imperfections and clusters, and external electric fields on optical spectra of Si–SiGe heterostructures

A microscopic theory of optical transitions in Si–SiGe heterostructures which offers a direct comparison of the strength of the optical transitions in ordered and disordered structures is presented herein, and suggests a new enhancement mechanism via localization at interface imperfections and Ge islands, and shows that even ultrashort period Si–Ge superlattices exhibit a strong red Stark shift in place of the expected blueshift familiar from analogous GaAs–AlAs systems.

Electro-absorption in a polydiacetylene

The electro-absorption in a DCH-polydiacetylene has been investigated both experimentally and theoretically. In addition to the red Stark shift of the exciton, a significant feature in the difference spectrum is observed at a higher energy. Within an interacting electron model (extended Peierls-Hubbard model), the roles played by the dipole-allowed one-photon states and the dipole-forbidden two-photon states are studied in detail. It is found that the high-energy signals in the difference spectrum are due to the strong field-induced mixing of the one-photon state at the edge of a continuum structure, or ‘conduction band’ in the language of band theory, with its neighboring two-photon states. Contributions to the electro-absorption from the one- and two-photon states cancel deep within the band. Three-photon resonance due to the band edge is expected in third-harmonic generation (THG) and has been seen in both a polydiacetylene and in polysilanes.

Asymmetric Fabry-Perot p-i-n multiple quantum well optical modulators grown on silicon and GaAs substrates

In the present work, GaAs/AlGaAs multiple quantum well p-i-n optical modulators were fabricated on Si (MQW/Si) and GaAs (MQW/GaAs) substrates. Although an appreciable red Stark shift was observed with an applied reverse bias, a rather low reflection modulation (limited to changes of only a few percent) was obtained. Nevertheless, successful operation of the MQW/Si device as a ligth modulator is demonstrated using a −5 V square wave drive at 1.5 kHz. A negative differential photoconductivity region exhibited by the MQW/Si structure may lead to optical bistability in the future, provided the p-i-n diode characteristics are improved.

Skin Permeability to Oxygen in Apples Stored in Controlled Atmosphere

Abstract The skin permeability to O2-diffusion and the mass transfer equilibrium constant for ‘Super Golden Delicious’ spur and ‘Red Stark’ (Malus domestica Borkh.) have been studied using a kinetic approach. A direct proportionality between the O2-exchange initial rate and the environmental PO2 is shown by a linear regression line passing near the origin of the axes. This confirms the suitability of the adopted mass-transfer equations and allows the calculation of the two involved constants: the equilibrium mass-transfer constant and the kinetic constant, which represents a suitable measure of permeability of the apple skin. As apple-skin permeability to O2 increases with fruit ripening, to reach a maximum near the commercial harvesting time, this parameter could be used as a ripening index.

Local velocity evaluation in a laser-produced plasma

We show that the flight velocities in a laser-produced plasma at various distances from the target can be deduced from measured Doppler shifts of spectral self-reversal for the Be IV Ly-α line with the help of a simple model. The red Stark- shifts are taken into account.

Vibronic Scheme of the Relaxed Excited State of the F Center

The vibronic scheme of the relaxed excited state (RES) of the F center which includes the \(2 \varGamma_{1}^{+}\), \(\varGamma_{3}^{+}\), and \(\varGamma_{4}^{-}\)-mode interactions has been developed. Formulae of the morphic effects have been derived. The vibronic parameter values for NaF, KF, KCl, KBr, RbF, RbCl, and RbBr are determined from the consistent analysis of the temperature dependence of the radiative lifetime and of the Stark polarization. Available data of the temperature dependence of the stress-induced polarization are consistently explained. Using parameter values, the dipole moment and stress coupling coefficient (\(\varGamma_{3}^{+}\)-mode) are determined; the red Stark shift is interpreted. By including the spin-orbit interaction, the forms of the magnetic effects are derived. The spin-orbit interaction constant, the orbital g -factor, and the spin polarization in the RES are determined from the analysis of experimental data.

New theory of superconductivity

Based on three earlier papers which treat electromagnetic, elastogravitational, and radiant-nonradiant thermal phenomena in terms of six types of electric or nonelectric charges, the authors classify states of matter as hyperefficient, efficient, semiefficient, and hypoefficient in transmitting a particular type of charge, by means of a generalization of Ohm's law to two or three dimensions. Conventional states of matter (solid, liquid, gas, vacuum) are associated with torsional (gravitational) charges. Applications are made to electric superconductivity of crystals at elevated temperatures, and to frequency shift (gravitational red shift, Lamb shift, and Zeeman and Stark effects).

Vibronic Problem for the Relaxed Excited State of the F-center in Alkali Halides. II. Analysis of the Experiments

Various experiments on the relaxed excited state of the F -center are analyzed in the framework of the vibronic model which consists of the 2 s and 2 p electronic states interacting with the s - and p -like local mode phonons. By comparing the theoretical calculations based on the exact solutions of the vibronic Hamiltonian with observed values of various quantities, the values of three parameters: 2 s -2 p energy difference Δ with phonon energy as the unit, the s - and p -mode coupling constants S 0 and S 1 , are determined consistently. The best fit values for KCl are Δ ≃1.25, S 0 ≃1.0, S 1 ≃1.5. The red Stark shift of the emission band can be explained by taking into account the interference of the adiabatic potential energy surfaces of the 2 s and 2 p states caused by the s -mode interaction.