Stark Effect Research Articles

Nelineynoe pogloshchenie i fotolyuminestsentsiya nanotetrapodov CdTe s nakonechnikami CdSe pri nerezonansnom vozbuzhdenii eksitonov

The nonlinear absorption and photoluminescence of CdTe/CdSe nanotetrapod colloids have been studied by the pump–probe method for the case of single-photon nonresonance excitation of excitons. A competition between the short-wave and long-wave shifts of the peak of photoluminescence associated with an indirect electron–hole transition, which is observed with increasing excitation radiation intensity, has been found and explained. The former shift is associated with an increase in the one-dimensional exciton radius after the exciton state occupation, and the latter shift may be attributed to the charge-induced Stark effect and local heating of nanotetrapods as a result of electron–phonon interaction at nonresonance excitation of the system.

Multistate interferometric measurement of the nonlinear ac Stark shift

We demonstrate measurement of quadratic AC Stark shifts between Zeeman sublevels in an $^{87}$Rb Bose--Einstein condensate using a multi-state atomic interferometer. The interferometer can detect a quadratic shift without being affected by relatively large state-independent shifts, thereby improving the measurement precision. We measure quadratic shifts in the total spin $F = 2$ state due to the light being near-resonant to the D$_1$ line. The agreement between the measured and theoretical detuning dependences of the quadratic shifts confirms the validity of the measurement. We also present results on the suppression of nonlinear spin evolution using near-resonant dual-color light pulses with opposite quadratic shifts.

Direct calibration of laser intensity via Ramsey interferometry for cold atom imaging.

A majority of ultracold atom experiments utilize resonant absorption imaging techniques to obtain the atomic density. To make well-controlled quantitative measurements, the optical intensity of the probe beam must be precisely calibrated in units of the atomic saturation intensity Isat. In quantum gas experiments, the atomic sample is enclosed in an ultra-high vacuum system that introduces loss and limits optical access; this precludes a direct determination of the intensity. Here, we use quantum coherence to create a robust technique for measuring the probe beam intensity in units of Isat via Ramsey interferometry. Our technique characterizes the ac Stark shift of the atomic levels due to an off-resonant probe beam. Furthermore, this technique gives access to the spatial variation of the probe intensity at the location of the atomic cloud. By directly measuring the probe intensity just before the imaging sensor our method in addition yields a direct calibration of imaging system losses as well as the quantum efficiency of the sensor.

Stark effect in nonhydrogenic low-dimensional excitons

Excitons in two dimensions and other low-dimensional semiconductors are known to deviate from simple Wannier model descriptions due to nonlocal dielectric screening. As a consequence, energy levels do not follow a simple hydrogenic Rydberg series. Recently, a Kratzer model including a repulsive core potential has been suggested as an analytically solvable model of nonhydrogenic excitons. We adopt this model to describe both static and dynamic Stark effects in low-dimensional semiconductors. An exact formula for the exciton polarizability valid for arbitrary dimension and core potential is obtained. Moreover, analytical oscillator strengths allow for analyses of the dynamic Stark effect in various dimensions and materials.

Highly Sensitive Hydrogen Sulfide Sensor Based on GaN/GaInN Heterostructure

Accurate detection of gases such as hydrogen sulfide in the exhaled human breath is of great interest for medical professionals as it can possibly help in the early detection of organ malfunction and other diseases. GaInN heterostructure sensors are sensitive to the changes in the surface potential caused by the adsorption of gas molecules. A quantum well (QW) placed close to the surface experiences a change in the quantum‐confined Stark effect and as a result shifts its photoluminescence signal. Several parameters of the GaInN sensors grown by metal organic vapor phase epitaxy are optimized such as the GaN cap layer, QW thickness, and doping concentration. Moreover, how various metal functionalization layers can improve its sensitivity and selectivity is investigated. Gold (Au) and Silver (Ag) shows sensitivity to hydrogen sulfide in the 10–100 parts per billion (ppb) range. Ammonia gas is also detected in the 5–10 range (ppm) with a sensor structure covered with a thin gold layer.

Long-Range Hot Charge Transfer Exciton Dissociation in an Organic/2D Semiconductor Hybrid Excitonic Heterostructure.

Whether and how an electron-hole pair at the donor-acceptor interface separates from their mutual Coulombic interaction has been a long-standing question for both fundamental interests and optoelectronic applications. This question is particularly interesting but yet to be unraveled in the emerging mixed-dimensional organic/2D semiconductor excitonic heterostructures where the Coulomb interaction is poorly screened. Here, by tracking the characteristic electroabsorption (Stark effect) signal from separated charges using transient absorption spectroscopy, we directly follow the electron-hole pair separation process in a model organic/2D heterostructure, vanadium oxide phthalocyanine/monolayer MoS2. After sub-100 fs photoinduced interfacial electron transfer, we observe a barrier-less long-range electron-hole pair separation to free carriers within 1 ps by hot charge transfer exciton dissociation. Further experiment reveals the key role of the charge delocalization in organic layers sustained by the local crystallinity, while the inherent in-plane delocalization of the 2D semiconductor has a negligible contribution to charge pair separation. This study reconciles the seemingly contradicting charge transfer exciton emission and dissociation process and is important to the future development of efficient organic/2D semiconductor optoelectronic devices.

State Manipulation via the All‐Microwave Protocol in the Superconducting Qutrits

The quantum gate is the foundation of quantum computation and simulation in superconducting circuits. In recent years, quantum systems with three energy levels have attracted much attention. Therefore, it is essential to study the protocol of the quantum‐state transfer in qutrits. Herein, the all‐microwave schemes are studied to realize the state transfer and quantum gates. First, the ∣fg>–∣ge> transition is designed in a superconducting circuit with a relatively large coupling. We measured the Stark shift in cases where the experimental parameters did not satisfy the dispersive condition. Then, by carefully calibrating the pulse parameters, the regular Rabi oscillation and the state transfer are realized between the second excited state of two qubits via stimulated Raman adiabatic passage.

Tunable electronic and optical properties of MoTe2/InSe heterostructure via external electric field and strain engineering

Based on first-principles calculation under density functional theory, the geometry, electronic and optical properties of the MoTe2/InSe heterojunction have been investigated. The results reveal that the MoTe2/InSe heterojunction has a typical type-Ⅱ band alignment and exhibits an indirect bandgap of 0.99 eV. In addition, the Z-scheme electron transport mechanism is capable of efficiently separating photogenerated carriers. The bandgap of the heterostructure changes regularly under applied electric field and exhibits a significant Giant Stark effect. Under an applied electric field of 0.5 V Å−1, the band alignment of the heterojunction shifts from type-Ⅱ to type-I. The application of strain produced comparable changes in the heterojunction. More importantly, the transition from semiconductor to metal is completed in the heterostructure under the applied electric field and strain. Furthermore, the MoTe2/InSe heterojunction retains the optical properties of two monolayers and produces greater light absorption on this basis, especially for UV light. The above results offer a theoretical basis for the application of MoTe2/InSe heterostructure in the next generation of photodetectors.

Spectral properties of size-invariant shape transformation.

Size-invariant shape transformation is a technique of changing the shape of a domain while preserving its sizes under the Lebesgue measure. In quantum-confined systems, this transformation leads to so-called quantum shape effects in the physical properties of confined particles associated with the Dirichlet spectrum of the confining medium. Here we show that the geometric couplings between levels generated by the size-invariant shape transformations cause nonuniform scaling in the eigenspectra. In particular, the nonuniform level scaling, in the direction of increasing quantum shape effect, is characterized by two distinct spectral features: lowering of the first eigenvalue (ground-state reduction) and changing of the spectral gaps (energy level splitting or degeneracy formation depending on the symmetries). We explain the ground-state reduction by the increase in local breadth (i.e., parts of the domain becoming less confined) that is associated with the sphericity of these local portions of the domain. We accurately quantify the sphericity using two different measures: the radius of the inscribed n-sphere and the Hausdorff distance. Due to Rayleigh-Faber-Krahn inequality, the greater the sphericity, the lower the first eigenvalue. Then level splitting or degeneracy, depending on the symmetries of the initial configuration, becomes a direct consequence of size invariance dictating the eigenvalues to have the same asymptotic behavior due to Weyl law. Such level splittings may be interpreted as geometric analogs of Stark and Zeeman effects. Furthermore, we find that the ground-state reduction causes a quantum thermal avalanche which is the underlying reason for the peculiar effect of spontaneous transitions to lower entropy states in systems exhibiting the quantum shape effect. Unusual spectral characteristics of size-preserving transformations can assist in designing confinement geometries that could lead to classically inconceivable quantum thermal machines.

Effect of collisions on motional Stark broadening of spectral lines

The shape of spectral lines in strongly magnetized plasmas is investigated theoretically in the framework of astrophysical research. Under certain conditions, the broadening of lines observed on white dwarf spectra is dominated by the so-called motional Stark effect, which occurs due to the Lorentz electric field v→×B→. If the plasma density is high enough, collisions between the emitter and perturbers change the atomic velocity v→ and the resulting motional Stark broadening is modified. We illustrate this modification through dedicated line shape calculations based on a stochastic model for the atomic velocity. A universal line shape formula, which is applicable to any isolated atomic line, is proposed for implementation in line shape codes.

Supersymmetry as an Algebraic Approach to the Jaynes-Cumming Model with Stark Shift and Kerr-Like Medium

Supersymmetry as an Algebraic Approach to the Jaynes-Cumming Model with Stark Shift and Kerr-Like Medium

Role of dynamic Stark shifts in strong-field excitation and subsequent ionization

Role of dynamic Stark shifts in strong-field excitation and subsequent ionization

Recent Advances on GaN-Based Micro-LEDs.

GaN-based micro-size light-emitting diodes (µLEDs) have a variety of attractive and distinctive advantages for display, visible-light communication (VLC), and other novel applications. The smaller size of LEDs affords them the benefits of enhanced current expansion, fewer self-heating effects, and higher current density bearing capacity. Low external quantum efficiency (EQE) resulting from non-radiative recombination and quantum confined stark effect (QCSE) is a serious barrier for application of µLEDs. In this work, the reasons for the poor EQE of µLEDs are reviewed, as are the optimization techniques for improving the EQE of µLEDs.

Stark Effect Modeling in Raman Spectra Analysis of Histological Preparations of Colorectal Adenocarcinomas

Stark Effect Modeling in Raman Spectra Analysis of Histological Preparations of Colorectal Adenocarcinomas

Multiple strategies for evaluating the uncertainty of blackbody radiation frequency shift in an optical clock

The Stark shift due to the blackbody radiation (BBR) is a crucial obstacle limiting the performance of optical clocks. Here five different methods are reported to evaluate an ytterbium (Yb) clock’s BBR shift in the same science chamber for the first time. The intercomparison shows that incorporating in-chamber temperature information can reduce the uncertainty by one order of magnitude. Notably, a simulation model is developed and optimized through calibration with in-chamber sensors, which can suppress the BBR uncertainty close to 1 × 10−18 at room temperature and provide more credible results. Moreover, the 10−19 level uncertainty is further identified by surrounding the atoms in the cryogenic environment. It is worth noting that the above results were obtained without BBR shield and external temperature control. These comprehensive evaluation results provide a valuable reference for different uncertainty requirements or building advanced optical clocks towards total uncertainty in the 10−19 region.

Semiconductor Membranes for Electrostatic Exciton Trapping in Optically Addressable Quantum Transport Devices

Combining the capabilities of gate-defined quantum transport devices in $\mathrm{Ga}\mathrm{As}$-based heterostructures and of optically addressed self-assembled quantum dots could open up broad perspectives in quantum technologies. For example, interfacing stationary solid-state qubits with photonic quantum states would open up a pathway towards the realization of a quantum network with extended quantum processing capacity in each node. While gated devices allow very flexible confinement of electrons or holes, the confinement of excitons without some element of self-assembly is much harder. To address this limitation, we introduce a technique to realize exciton traps in quantum wells via local electric fields by thinning a heterostructure down to a 220-nm-thick membrane. We show that mobilities over $1\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ can be retained and that quantum point contacts and Coulomb oscillations can be observed on this structure, which implies that the thinning does not compromise the heterostructure quality. Furthermore, the local lowering of the exciton energy via the quantum-confined Stark effect is confirmed, thus forming exciton traps. These results lay the technological foundations for devices like single-photon sources, spin-photon interfaces and eventually quantum network nodes in $\mathrm{Ga}\mathrm{As}$ quantum wells, realized entirely with a top-down fabrication process.

Electron Accumulation Tuning by Surface-to-Volume Scaling of Nanostructured InN Grown on GaN(001) for Narrow-Bandgap Optoelectronics

The existence of an uncontrolled electron accumulation layer near the surface of InN thin films is an obstacle for the development of reliable InN-based devices for use in narrow-bandgap optoelectronics. In this article, we show that this can be regulated by modulating the surface of the InN grown on GaN(001). By increasing the surface-to-volume ratio, we can demonstrate a reduction in the surface carrier concentration from ∼1018 to ∼1017 cm–3. These controlled changes are despite the idea that donor-type surface states, which contribute to conduction band electrons are reported to be the main origin of the surface charge density. Additionally, by evaluating the surface carrier concentration through modeling of photoluminescence (PL) spectroscopy, we have found a failure of the Burstein–Moss theory. Conversely, modeling of the longitudinal optical phonon–plasmon coupled modes measured using Raman spectroscopy, simulations of InN structures using the k·p method, and Hall effect measurements, where possible, showed an excellent correlation of the surface electron concentrations. The large inhomogeneous broadening in the PL, which overwhelms any broadening due to the Burstein–Moss effect, is understood to be the result of varying Stark shifts due to varying strain throughout high surface-to-volume nanostructures, which dramatically affects the spatially indirect nature of the electron–hole recombination. Finally, our findings demonstrate how the electron population of 2D and 3D InN nanostructures can be tuned by structural features, such as porosity and/or the surface-to-volume ratio.

Red InGaN nanowire LED with bulk active region directly grown on p-Si (111).

A red nanowire LED with an InGaN bulk active region, directly grown on a p-Si (111) substrate, is demonstrated. The LED exhibits relatively good wavelength stability upon increasing injection current and narrowing of the linewidth without quantum confined Stark effect. Efficiency droop sets in at relatively high injection current. The output power and external quantum efficiency are 0.55 mW and 1.4% at 20 mA (20 A/cm2) with peak wavelength of 640 nm, reaching 2.3% at 70 mA with peak wavelength of 625 nm. The operation on the p-Si substrate results in large carrier injection currents due to a naturally formed tunnel junction at the n-GaN/p-Si interface and is ideal for device integration.

Spectra and excitation properties of (AlP)8: Effect of external electric fields

AbstractOn the basis of density functional theory (DFT), the geometry and infrared spectrum of the (AlP)8 cluster have been calculated under external electric fields (EEFs). In addition, on the basis of time‐dependent DFT, the ultraviolet–visible absorption spectra, oscillator strengths, wavelengths, and hole–electron orbits of the first 20 excited states have been calculated. Under EEFs, the energy of (AlP)8 gradually decreases, the dipole moment increases, and the molecular configuration significantly changes. In the infrared spectrum, the vibration frequency corresponding to the stretching vibration of the Al–P bond decreases, and a red shift occurs. With increasing EEF, the infrared spectrum splits and shows an obvious Stark effect; the ultraviolet absorption intensity is enhanced, and the molecular excitation energy decreases. Additionally, the excitation wavelength increases with increasing EEF. It is conclusively shown that the (AlP)8 cluster is easily excited under an EEF. Separation of the holes and electrons of the (AlP)8 cluster is obvious. Theoretical investigation of the spectra and excitation properties of (AlP)8 is an important step toward a comprehensive understanding of the effects of EEFs on the molecular structure, stability, and dynamics.

Time-correlated luminescence blinking in InGaN single quantum wells

The blinking phenomenon in InGaN single quantum wells is a phenomenon where localized photoluminescence changes over time. Understanding its physics is important for the manufacture of more efficient light emission diodes. We present a study using two InGaN single quantum well samples, emitting at 460 and 510 nm wavelength, respectively. We confirmed that the luminescence intensity fluctuates in localized blinking regions, and we found that these optical variations are not random but are instead correlated in pairs, with either positive or negative coefficient, to a distant reference blinking point. Measurements were performed to obtain standard deviation and cross-correlation maps. Invoking the quantum confined Stark effect, we realized a simple phenomenological model that shows how charge carriers are exchanged among pairs of adjacent opposite correlation regions. As a result, it is suggested that the phenomenon is caused by fluctuations in the number of these exchanged carriers. Our model gives an explanation for the blinking phenomenon in InGaN single quantum wells, and it is important for a deeper understanding to InGaN-based materials.