Stark Field Research Articles

Optoelectronic Switching of a Carbon Nanotube Chiral Junction Imaged with Nanometer Spatial Resolution

Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting-metallic junction. We image topography, laser absorption at 532 nm, and measure I-V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting-metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.

Reply to “Comment on ‘Test of the Stark-effect theory using photoionization microscopy’ ”

The frame-transformation (FT) theory which describes the dynamics of nonhydrogenic atoms in an external uniform electric field was tested for the process of photoionization in a Stark field by Zhao et al. [Phys. Rev. A 86, 053413 (2012)]. Differential cross sections from the FT theory were found to be inconsistent with those from the fully quantum-mechanical coupled-channel theory. The discrepancy was attributed to the frame transformation of irregular wave functions. In a recent investigation, Giannakeas et al. [Phys. Rev. A 91, 067401 (2015)] draw a different conclusion. They show that the FT theory generates irregular wave functions in good agreement with exact solutions for low angular momenta, although an obvious disagreement is seen for high angular momenta. We performed test calculations for numerous Stark states and found that our original conclusion remains valid, namely, the Fano-Harmin frame transformation for the irregular wave function is inaccurate.

Determining the relaxation constants of a molecular gas in a continuous laser field

The processes of collisional relaxation in 13CH3F polar gas are considered, and the nonmonotonous character of the dependence of the signal of delayed optical nutations on the duration of the Stark field is explained. A damping constant is introduced for the matrix of optical coherence.

Probing the plasma near high power wave launchers in fusion devices for static and dynamic electric fields (invited).

An exploratory study was carried out in the long-pulse tokamak Tore Supra, to determine if electric fields in the plasma around high-power, RF wave launchers could be measured with non-intrusive, passive, optical emission spectroscopy. The focus was in particular on the use of the external electric field Stark effect. The feasibility was found to be strongly dependent on the spatial extent of the electric fields and overlap between regions of strong (>∼1 kV/cm) electric fields and regions of plasma particle recycling and plasma-induced, spectral line emission. Most amenable to the measurement was the RF electric field in edge plasma, in front of a lower hybrid heating and current drive launcher. Electric field strengths and direction, derived from fitting the acquired spectra to a model including time-dependent Stark effect and the tokamak-range magnetic field Zeeman-effect, were found to be in good agreement with full-wave modeling of the observed launcher.

Tunable nonlinear double-core PT-symmetric waveguides.

We propose a scheme for creating a tunable, highly nonlinear defect in a one-dimensional photonic crystal with an embedded atomic cell filled by a mixture of two isotopes of three-level atoms. The defect creates an effective double-core waveguide. The induced refractive index for a probe field can be made parity-time (PT) symmetric by means of a proper combination of a control field and a Stark field; hence the complex defect potential obtained contains double-well real and linear imaginary parts. We also show that it is possible to form various stable nonlinear defect modes supported by the focusing nonlinearity of the system.

Surface ionisation of molecular H2 and atomic H Rydberg states at doped silicon surfaces

The detection of ions or electrons from the surface ionisation of molecular H2 and atomic H Rydberg states incident at doped Si surfaces is investigated experimentally to analyse the effect of the dopant charge distribution on the surface-ionisation processes. In both experimental studies, the molecular H2 and atomic H Rydberg states are generated via two-colour vacuum ultraviolet--ultraviolet (VUV–UV) resonant excitation. For H2, various Stark states of the N+ = 2, n = 17 manifold are populated in the presence of an electric field. The variation of the observed surface-ionisation signal with surface dopant concentration and type, shows similar characteristics for all the Stark states. A comparison is made between these ion-detected surface-ionisation profiles and those obtained via electron detection. Different trends as a function of dopant concentration and type are observed for the two cases, explained by the greater effect of surface charges on the post-ionisation ion trajectory compared to the electron trajectory. For the atomic-H Rydberg states with principal quantum number populated in the absence of a Stark field, the observed behaviour is similar to the interaction of molecular H2 Rydberg states at the same surfaces, and these measurements confirm that the observed effects are attributable to the nature of the target surface rather than the specific atomic or molecular Rydberg species.

Built-in voltage of organic bulk heterojuction p-i-n solar cells measured by electroabsorption spectroscopy

We investigate the influence of the built-in voltage on the performance of organic bulk heterojuction solar cells that are based on a p-i-n structure. Electrical doping in the hole and the electron transport layer allows to tune their work function and hence to adjust the built-in voltage: Changing the doping concentration from 0.5 to 32 wt% induces a shift of the work function towards the transport levels and increases the built-in voltage. To determine the built-in voltage, we use electroabsorption spectroscopy which is based on an evaluation of the spectra caused by a change in absorption due to an electric field (Stark effect). For a model system with a bulk heterojunction of BF-DPB and C60, we show that higher doping concentrations in both the electron and the hole transport layer increase the built-in voltage, leading to an enhanced short circuit current and solar cell performance.

Ultrafast Stark-Induced Polaritonic Switches

A laser pulse, several meV red detuned from the excitonic line of a quantum well, has been shown to induce an almost instantaneous and rigid shift of the lower and upper polariton branches. Here we demonstrate that through this shift ultrafast all-optical control of the polariton population in a semiconductor microcavity should be achievable. In the proposed setup, a Stark field is used to bring the lower polariton branch in or out of resonance with a quasiresonant continuous-wave laser, thereby favoring or inhibiting the injection of polaritons into the cavity. Moreover, we show that this technique allows for the implementation of optical switches with extremely high repetition rates.

Separability and dynamical symmetry of Quantum Dots

The separability and Runge–Lenz-type dynamical symmetry of the internal dynamics of certain two-electron Quantum Dots, found by Simonović et al. (2003), are traced back to that of the perturbed Kepler problem. A large class of axially symmetric perturbing potentials which allow for separation in parabolic coordinates can easily be found. Apart from the 2:1 anisotropic harmonic trapping potential considered in Simonović and Nazmitdinov (2013), they include a constant electric field parallel to the magnetic field (Stark effect), the ring-shaped Hartmann potential, etc. The harmonic case is studied in detail.

Full Color Modulation of Firefly Luciferase through Engineering with Unified Stark Effect

The firefly luciferase has been a unique marking tool used in various bioimaging techniques. Extensive color modulation is strongly required to meet special marking demands; however, intentional and accurate wavelength tuning has yet to be achieved. Here, we demonstrate that the color shift of the firefly chromophore (OxyLH2-1) by internal and external fields can be described as a unified Stark shift. Electrostatic microenvironmental effects on fluorescent spectroscopy are modeled in vacuo through effective electric fields by using time-dependent density functional theory. A complete visible fluorescence spectrum of firefly chromophore is depicted, which enables one to control the emission in a specific color. As an application, the widely observed pH-correlated color shift is proved to be associated with the local Stark field generated by the trace water-ions (vicinal hydronium and hydroxide ions) at active sites close to the OxyLH2-1.

Habitat type and distance to edge affect movement behavior of juvenile pond‐breeding salamanders

AbstractBehavioral strategies of natal dispersers in response to human‐altered habitat have far‐reaching implications for functional connectivity and local population dynamics. Spotted salamanders Ambystoma maculatum are forest‐dependent, pond‐breeding amphibians which metamorphose and disperse out of aquatic habitat with limited knowledge of the terrestrial habitat. We quantified the fine‐scale movement behavior and search strategies of recently metamorphosed spotted salamanders in three different habitat types (field, early successional forest and forest) and at varying distances from both hard (field and forest) and soft (early successional forest and forest) edges using fluorescent powder tracking. We found that salamanders moved straighter and with fewer turns through field habitat compared with both forest and early successional habitat. Salamanders significantly oriented movement toward forest when released in the field and when released on the edge between the forest and field. We found that salamander movement in the forest and early successional forest was approximated by a correlated random walk. Based on these results, dispersing spotted salamanders exhibit strong edge‐mediated behavior when differences between habitats are stark (forest and field) and can perceive forest habitat from distances of at least 10 m. These results indicate that dispersing juvenile salamanders exhibit reasonable behavioral rules when moving through habitat types of differing quality. Knowledge of these behavioral rules will improve predictions of the effects of habitat type and configuration on amphibian survival and dispersion in altered landscapes.

Fraunhofer-type absorption line splitting and polarization in confocal double-pulse laser induced plasma

Strong line splitting and polarization are observed in Fraunhofer-type absorption lines in Pb, Sn, Si, Cd, In, and Zn in confocal double-pulse laser induced plasma (DP-LIP) experiments. This effect is detectable using medium laser power densities: (~1–2)×1013W/m2 for the first laser pulse and 1×1014W/m2 for the second laser pulse. Polarization and splitting effects exist only during the second laser pulse (~7ns). Absorption line polarization and splitting phenomena may be explained by a high overall magnetic field and motional Stark effect caused by the second laser pulse inside the laser plasma created by the first pulse.

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile

The relaxation dynamics of the dye D35 has been characterized by transient absorption spectroscopy in acetonitrile and on TiO(2) and ZrO(2) thin films. In acetonitrile, upon photoexcitation of the dye via the S(0) → S(1) transition, we observed ultrafast solvation dynamics with subpicosecond time constants. Subsequent decay of the S(1) excited state absorption (ESA) band with a 7.1 ps time constant is tentatively assigned to structural relaxation in the excited state, and a spectral decay with 203 ps time constant results from internal conversion (IC) back to S(0). On TiO(2), we observed fast (<90 fs) electron injection from the S(1) state of D35 into the TiO(2) conduction band, followed by a biphasic dynamics arising from changes in a transient Stark field at the interface, with time constants of 0.8 and 12 ps, resulting in a characteristic blue-shift of the S(0) → S(1) absorption band. Several processes can contribute to this spectral shift: (i) photoexcitation induces immediate formation of D35˙(+) radical cations, which initially form electron-cation complexes; (ii) dissociation of these complexes generates mobile electrons, and when they start diffusing in the mesoporous TiO(2), the local electrostatic field may change; (iii) this may trigger the reorientation of D35 molecules in the changing electric field. A slower spectral decay on a nanosecond timescale is interpreted as a reduction of the local Stark field, as mobile electrons move deeper into TiO(2) and are progressively screened. Multiexponential electron-cation recombination occurs on much longer timescales, with time constants of 30 μs, 170 μs and 1.4 ms. For D35 adsorbed on ZrO(2), there is no clear evidence for a transient Stark shift, which suggests that initially formed cation-electron (trap state) complexes do not dissociate to form mobile conduction band electrons. Multiexponential decay with time constants of 4, 35, and 550 ps is assigned to recombination between cations and trapped electrons, and also to a fraction of D35 molecules in S(1) which decay by IC to S(0). Differential steady-state absorption spectra of D35˙(+) in acetonitrile and dichloromethane provide access to the complete D(0) → D(1) band. The absorption spectra of D35 and D35˙(+) are well described by TDDFT calculations employing the MPW1K functional.

Visualization of cation diffusion at the TiO2 interface in dye sensitized photoelectrosynthesis cells (DSPEC)

Time-resolved, UV-vis spectroscopic measurements of Li+ diffusion in mesoscopic TiO2 photoanodes were conducted in dye sensitized photoelectrosynthesis cells (DSPECs) under operating conditions. In these experiments the spectral response of TiO2 derivatized with [Ru(bpy)2(4,4′-((HO)2PO)2bpy)]2+ (RuP, where bpy is 2,2′-bipyridine, (4,4′-((HO)2PO)2bpy) is [2,2′-bipyridine]-4,4′-diphosphonic acid) arises from electric field (Stark) effects on the metal-to-ligand-charge transfer (MLCT) absorption spectrum of RuP, which is screened by cation intercalation. These results verify that Li+ diffusion is coupled to electron injection and to electron recombination/extraction at the TiO2 interface. Li+ doping levels depend on the competition between dynamics of its intercalation and electron recombination/transport. For a DSPEC operating in aqueous solution at pH 4.5, the observed rate constants for Li+ intercalation and release were 0.22 s−1 and 0.014 s−1, respectively. Both processes were considerably slower in the more viscous solvent propylene carbonate with Li+ release rate constants <2 × 10−4 s−1. Accumulation of Li+ under these conditions shifts conduction band/trap states to less negative potentials, increasing electron lifetime in TiO2.

Dynamic Stark shift of the3R1Rydberg state of CH3I

Stark shift of the 3R1 Rydberg state of CH3I is measured for different Stark field intensities. Photodissociation products, generated after predissociation of the state, were detected when the molecules were excited with photons resonant with the energy difference between the ground and the shifted 3R1 state, allowing the shift to be quantified. Qualitative agreement has been found with a 1D model.

Fidelity for kicked atoms with gravity near a quantum resonance

Kicked atoms under a constant Stark or gravity field are investigated for experimental setups with cold and ultracold atoms. The parametric stability of the quantum dynamics is studied using the fidelity. In the case of a quantum resonance, it is shown that the behavior of the fidelity depends on arithmetic properties of the gravity parameter. Close to a quantum resonance, the long-time asymptotics of the fidelity is studied by means of a pseudoclassical approximation introduced by Fishman et al. [J. Stat. Phys. 110, 911 (2003)]. The long-time decay of fidelity arises from the tunneling out of pseudoclassical stable islands, and a simple ansatz is proposed which satisfactorily reproduces the main features observed in numerical simulations.

Spectroscopic evidence for the effect of the ortho H2O spin on the electron transfer in photosynthesis

A spin mechanism for electron transfer control in the reaction center of purple bacteria in photosynthesis is proposed. Rotation and conversion of the ortho/para spin isomers of two H2O molecules located near the special pair of the reaction center are treated as the sources of the coherent modulations of transient kinetics. Modulation of the collective wave function of the reaction center electrons by the total proton spin of ortho H2O is a key feature allowing the molecule to play the role of a gate controlling the electron transfer. The iron atom in the reaction center with the gradient magnetic field is treated as a catalyst removing the strict forbiddenness of H2O ortho/para conversion. It is shown that the modulation of the reaction center stimulated emission kinetics observed in the field of femtosecond pulses coincides with the rotational transitions of ortho/para H2O. Influence of the effect of the electric field (Stark effect) on the level displacement and ortho/para conversion rate is discussed.

Semiclassical Szegö limit of resonance clusters for the hydrogen atom Stark Hamiltonian

We study the weighted averages of resonance clusters for the hydrogen atom with a Stark electric field in the weak field limit. We prove a semiclassical Szego-type theorem for resonance clusters showing that the limiting distribution of the resonance shifts concentrates on the classical energy surface corresponding to a rescaled eigenvalue of the hydrogen atom Hamiltonian. This result extends Szego-type results on eigenvalue clusters to resonance clusters. There are two new features in this work: first, the study of resonance clusters requires the use of non-self-adjoint operators, and second, the Stark pertur- bation is unbounded so control of the perturbation is achieved using localization properties of coherent states corresponding to hydrogen atom eigenvalues.

Spontaneous emission of the non-Wiener type

The spontaneous emission of a quantum particle and superradiation of an ensemble of identical quantum particles in a vacuum electromagnetic field with zero photon density are examined under the conditions of significant Stark particle and field interaction. New fundamental effects are established: suppression of spontaneous emission by the Stark interaction, an additional “decay” shift in energy of the decaying level as a consequence of Stark interaction unrelated to the Lamb and Stark level shifts, excitation conservation phenomena in a sufficiently dense ensemble of identical particles and suppression of superradiaton in the decay of an ensemble of excited quantum particles of a certain density. The main equations describing the emission processes under conditions of significant Stark interaction are obtained in the effective Hamiltonian representation of quantum stochastic differential equations. It is proved that the Stark interaction between a single quantum particle and a broadband electromagnetic field is represented as a quantum Poisson process and the stochastic differential equations are of the non-Wiener (generalized Langevin) type. From the examined case of spontaneous emission of a quantum particle, the main rules are formulated for studying open systems in the effective Hamiltonian representation.

Terahertz lasers based on optically pumped four-level asymmetrical double quantum wells

A prototype of terahertz laser based on four-level asymmetric double GaAs/Al x Ga 1− x As quantum well (QW) structures is presented. The prominent feature of this prototype is its close proximity of subband 0 and 1. An ensemble Monte Carlo method is adopted to analyze the electron population on each subband in this 4-level double QW. The close proximity of two bottom levels is found to have greatly improved the optically pumping efficiency. Also, an enhanced population inversion between subband 3 and 2 is obtained even at room temperature. Finally, the electric field Stark effect in QW is shown to be helpful for the close proximity of subband 0 and 1 in double QW.