Stark Energy Shift Research Articles

Electromagnetically induced transparency spectra of cesium Rydberg atoms decorated by radio-frequency fields

<sec>The large electric dipole moment of the Rydberg atom allows for strong coupling with weak electric fields, and is widely used in electric field measurements because of its reproducibility, precision and stability. The combination of Rydberg atoms and electromagnetically induced transparency (EIT) technology has been used for detecting and characterizing radio-frequency (RF) electric fields. </sec><sec>In this work, by selecting probe light (852 nm), dressed light (1470 nm), and coupled light (780 nm), the Rydberg state (49<i>P</i><sub>3/2</sub>) of Cs atom is prepared by using a three-photon excitation scheme through using all-infrared light excitation of Rydberg atoms. We experimentally observe the EIT spectra of the Rydberg states decorated by radio-frequency electric fields, which optically detects Rydberg atoms. The effect of the amplitude and frequency of the RF electric field on the spectrum is explored in light of changes in the EIT spectrum. The results show that in the region of weak electric field, only the ac Stark energy shift and spectral broadening occur. As the electric field is further enhanced, the sideband phenomenon occurs in both the primary peak and secondary peak of the EIT. In the region of strong field, the Rydberg energy level produces a series of Floquet states with higher-order terms, as well as state shifting and mixing, resulting in asymmetry in the spectra of the EIT sideband peaks. The effect of frequency on the shielding effect of the Cs vapor cell is further discussed based on the shift of the main peak of the EIT.</sec><sec>The demodulation of the electric field in a range of 50 Hz–1 kHz with a fidelity of 95% is achieved by modulating the low-frequency electric field to the RF electric field. The results can provide valuable references for spectral detection and traceable measurements of low-frequency electric fields.</sec>

Roles of the transition amplitude phases in photoelectron asymmetry of single strong attosecond pulse

The angular distributions of the photoelectrons in ionization of hydrogen atom by both circularly and linearly polarized intense extreme ultraviolet (XUV) attosecond pulse are investigated by numerically solving the time-dependent Schrödinger equation. We clearly identify nonperturbative features in studying the asymmetrical photoelectron angular distributions in the polarization plane for the XUV photon energy (16.3 eV) close to the ionization threshold, while such nonperturbative features are absent for higher photon energy (36 eV) in the same pulse intensity region. In addition to the carrier-envelope phase (CEP) dependence, the ejection asymmetry of the photoelectron is also sensitive to the relative phases of transition amplitudes in absorbing one photon and two photons. As a consequence, the CEPs corresponding to the maximal (or zero) asymmetry obviously vary as the pulse intensity increases in a moderately large region from 1 × 1015 W cm−2 to 30 × 1015 W cm−2. We attribute the intensity dependence of the transition amplitude phases to a consequence of the depletion of population as well as the Stark energy shift of the initial state. We show that the relative phases of transition amplitudes can be precisely decoded from the pulse intensity dependence of the ejection asymmetry and those phases are insensitive to the ellipticity of the laser pulse.

Theoretical study of constant current scanning tunneling spectroscopy in Pb overlayers

We present a theoretical study of the constant current scanning tunneling spectroscopy of quantum well states localized in Pb(111) overlayers on Cu(111) surfaces. The distance-voltage characteristic of the tunneling junction is obtained with a mixed approach. The wave packet propagation technique is applied to describe electron tunneling from the tip into the sample, and the density functional calculations provide the necessary inputs for the one-dimensional model potential representation of the system. The excited-state population decay mechanisms via inelastic electron-electron and electron-phonon interactions are taken into account with a bias-dependent absorbing potential introduced in the metal. Our results are in good agreement with recent experimental studies [Phys. Rev. Lett. 102, 196102 (2009), Phys. Rev. B 81, 205438 (2010)] over the energy range where the free-electron description of the Pb overlayer used here applies. We find that at high bias the quantum well states experience a Stark energy shift and partially acquire a character of field emission resonances. The present model study also sheds light at the experimentally observed departure of the energies of the quantum well states from the particle-in-a-box prediction for the bias above 4 eV. The measured trend can be consistently explained as due to the departure of the realistic Pb band structure in the $\ensuremath{\Gamma}\ensuremath{-}\text{L}$ direction from free-electron parabola when the electron momentum approaches the $\ensuremath{\Gamma}$ point.

Stark effects on bound polarons in polar rectangular quantum wires

The impurity binding energy in a polar rectangular quantum wire under an applied electric field is studied by a variational approach. The electron- and ion-longitudinal-optical (LO) phonon interactions are both taken into account in the calculations. The numerical results for the GaAs rectangular quantum wires show that the binding energies and Stark energy-shift are both sensitive to the size, shape of the wire section, and the position of the impurity. The binding energy increases with decreasing the section area. The LO phonon effect lowers the binding energy of the on-center impurity but raises that of the impurity near the wire surface, and gives a qualitatively similar contribution to the Stark effect.

Pressure influence on bound polarons in a strained wurtzite GaN/AlxGa1−xN heterojunction under an electric field

The binding energies of bound polarons near the interface of a strained wurtzite GaN/AlxGa1−xN heterojunction are studied by using a modified LLP variational method and a simplified coherent potential approximation under hydrostatic pressure and an external electric field. Considering the biaxial strain due to lattice mismatch or epitaxial growth, the uniaxial strain effects and the influences of the electron–phonon interaction as well as impurity–phonon interaction including the effects of interface-optical phonon modes and half-space phonon modes, the binding energies as functions of pressure, the impurity position, areal electron density and the phonon effect on the Stark energy shift are investigated. The numerical result shows that the contributions from the interface optical phonon mode with higher frequency and the LO-like half space mode to the binding energy and the Stark energy shift are important and obviously increase with increasing hydrostatic pressure, whereas the interface optical phonon mode with lower frequency and the TO-like half space mode are extremely small and are insensitive to the impurity position and hydrostatic pressure. It is also shown that the conductive band bending should not be neglected.

Screening influence on the Stark effect of impurity states in strained wurtzite GaN/AlxGa1−xN heterojunctions under pressure

The screening effect of the random-phase-approximation on the states of shallow donor impurities in free strained wurtzite GaN/AlxGa1–xN heterojunctions under hydrostatic pressure and an external electric field is investigated by using a variational method and a simplified coherent potential approximation. The variations of Stark energy shift with electric field, impurity position, Al component and areal electron density are discussed. Our results show that the screening dramatically reduces both the blue and red shifts as well as the binding energies of impurity states. For a given impurity position, the change in binding energy is more sensitive to the increase in hydrostatic pressure in the presence of the screening effect than that in the absence of the screening effect. The weakening of the blue and red shifts, induced by the screening effect, strengthens gradually with the increase of electric field. Furthermore, the screening effect weakens the mixture crystal effect, thereby influencing the Stark effect. The screening effect strengthens the influence of energy band bending on binding energy due to the areal electron density.

Pressure influence on the Stark effect of impurity states in a strained wurtzite GaN/AlxGa1-xN heterojunction

A variational method is adopted to investigate the properties of shallow impurity states near the interface in a free strained wurtzite GaN/AlxGa1-xN heterojunction under hydrostatic pressure and external electric field by using a simplified coherent potential approximation. Considering the biaxial strain due to lattice mismatch or epitaxial growth and the uniaxial strains effects, we investigated the Stark energy shift led by an external electric field for impurity states as functions of pressure as well as the impurity position, Al component and areal electron density. The numerical result shows that the binding energy near linearly increases with pressure from 0 to 10 GPa. It is also found that the binding energy as a function of the electric field perpendicular to the interface shows an un-linear red shift or a blue shift for different impurity positions. The effect of increasing x on blue shift is more significant than that on the red shift for the impurity in the channel near the interface. The pressure influence on the Stark shift is more obvious with increase of electric field and the distance between an impurity and the interface. The increase of pressure decreases the blue shift but increases the red shift.

Bound polaron in a spherical quantum dot under an electric field

A variational approach is used to study the ground state of a bound polaron in a spherical quantum dot under an external electric field. The binding energy of the hydrogenic impurity state is calculated by taking the interaction of an electron with both the confined longitudinal optical phonons and the surface optical phonons into account. The interaction between impurity and longitudinal optical phonons has also been considered to obtain the binding energy of a bound polaron. It shows that the polaron effects give significant corrections to the binding energy and its Stark energy shift. The external electric field increases the phonon contributions to the binding energy.

A New Desirable Molecular Species for Stark Deceleration

We demonstrate a new molecular sample (CH radical) for Stark deceleration by Monte Carlo simulation, study the dependences of the decelerating effect on the different phase angles, stage number and decelerating voltages, and obtain some new optimized conditions and parameters. We also calculate the Stark energy shift of different quantum states of CH radical in the Stark decelerator electrostatic field and their populations in a supersonic CH molecular beam. Our study shows that the mean velocity of the supersonic CH molecular beam in the 2∏1/2 state can be decelerated from 380m/s to 24m/s, and the corresponding translation temperature is lowered from 2.8K to 27mK. It is thus clear that the CH radical is a new and desirable candidate for Stark deceleration.

Velocity distribution of the pulsed ND 3 molecular beam selected by a quadrupole Stark velocity filter

We selected the slow velocity components from the ND 3 molecules with Maxwell–Boltzmann distribution at room temperature by using a pulsed nozzle and a quadrupole Stark guide. The velocity distribution after the filtering system shows good agreement with theory based on the Stark energy shift in molecular rotational states. We can detect the component of 60 m/s as the slowest column, which corresponds to 4 K of the thermal energy. This result promises that molecules in the mK region would be obtainable by combining this technique with a pre-cooling process like He buffer gas cooling.

Polarisation independent δ-strained InGaAs/InGaAsPquantum well waveguide modulator

A δ-strained InGaAs/InGaAsP MQW modulator has been fabricated and tested. The device shows fully polarisation independent transmission characteristics. Less than 0.8 dB polarisation sensitivity has been achieved for a wide range of wavelength. The extinction ratio is > 20 dB at a reverse bias voltage of 3 V. Almost the same Stark energy shift for heavy holes and light holes has been demonstrated in this structure.

Polaron Effects on the Stark Shift of a Bound Polaron in Finite Quantum Wells

With both LO and SO modes being taken into account, the influence of electron-phonon interaction on QCSE of a bound polaron in a finite quantum well (QW) is investigated by means of the perturbation-variation method. It is found that the presence of an ionized impurity decreases the energy shift and weakens the effect of a finite barrier height. LO and SO mode effects will give significant corrections to the Stark energy shift individually and their influences on the shift just are contrary.

Polaron effects on the Stark shift of a bound polaron in infinite quantum wells

With both longitudinal optical (LO) and surface optical (SO) modes included, the influence of electron-phonon interactions on the quantum confined Stark effect in quantum wells is investigated by means of the variational technique. For an infinite GaAs-GaAlAs quantum well with a shallow donor located at its centre and subjected to an external electric field, the energy shifts of a conduction electron are calculated approximately. It is found that the presence of an ionized impurity decreases the energy shift. The LO mode and SO mode effect will give significant corrections to the Stark energy shift individually and their influences on the shift are just opposed.