Linear Stark Research Articles

Band‐like Charge Photogeneration at a Crystalline Organic Donor/Acceptor Interface

AbstractOrganic photovoltaic cells possess desirable practical characteristics, such as the potential for low‐cost fabrication on flexible substrates, but they lag behind their inorganic counterparts in performance due in part to fundamental energy loss mechanisms, such as overcoming the charge transfer (CT) state binding energy when photogenerated charge is transferred across the donor/acceptor interface. However, recent work has suggested that crystalline interfaces can reduce this binding energy due to enhanced CT state delocalization. Solar cells based on rubrene and C60 are investigated as an archetypal system because it allows the degree of crystallinity to be moldulated from a highly disordered to highly ordered system. Using a postdeposition annealing method to transform as‐deposited amorphous rubrene thin films into ones that are highly crystalline, it is shown that the CT state of a highly crystalline rubrene/C60 heterojunction undergoes extreme delocalization parallel to the interface leading to a band‐like state that exhibits a linear Stark effect. This state parallels the direct charge formation of inorganic solar cells and reduces energetic losses by 220 meV compared with 12 other archetypal heterojunctions reported in the literature.

Stark effect of doped two-dimensional transition metal dichalcogenides

The bandgap of two-dimensional (2D) semiconductors can be efficiently tuned by a gate electric field, which is the so-called the Stark effect. We report that doping, which is essential in realistic devices, will substantially change the Stark effect of few-layer transition metal dichalcogenides in unexpected ways. Particularly, in bilayer structures, because of the competition between strong quantum confinement and the intrinsic screening length, electron and hole dopings exhibit surprisingly different Stark effects: doped electrons actively screen the external field and result in a nonlinear Stark effect; however, doped holes do not effectively screen the external field, causing a linear Stark effect that is the same as that of undoped materials. Our further analysis shows that this unusual doping effect is not limited within transition metal dichalcogenides but general for 2D structures. Therefore, doping plays a much more crucial role in functional 2D devices, and this unusual Stark effect also provides a new degree of freedom to tune bandgaps and optical properties of 2D materials.

Solvent-Independent Anharmonicity for Carbonyl Oscillators.

The physical origins of vibrational frequency shifts have been extensively studied in order to understand noncovalent intermolecular interactions in the condensed phase. In the case of carbonyls, vibrational solvatochromism, MD simulations, and vibrational Stark spectroscopy suggest that the frequency shifts observed in simple solvents arise predominately from the environment's electric field due to the vibrational Stark effect. This is contrary to many previously invoked descriptions of vibrational frequency shifts, such as bond polarization, whereby the bond's force constant and/or partial nuclear charges are altered due to the environment, often illustrated in terms of favored resonance structures. Here we test these hypotheses using vibrational solvatochromism as measured using 2D IR to assess the solvent dependence of the bond anharmonicity. These results indicate that the carbonyl bond's anharmonicity is independent of solvent as tested using hexanes, DMSO, and D2O and is supported by simulated 2D spectra. In support of the linear vibrational Stark effect, these 2D IR measurements are consistent with the assertion that the Stark tuning rate is unperturbed by the electric field generated by both hydrogen and non-hydrogen bonding environments and further extends the general applicability of carbonyl probes for studying intermolecular interactions.

Using electric fields for pulse compression and group-velocity control

In this article, we experimentally demonstrate a way of controlling the group velocity of an optical pulse by using a combination of spectral hole burning, the slow-light effect, and the linear Stark effect in a rare-earth-ion–doped crystal. The group velocity can be changed continuously by a factor of 20 without significant pulse distortion or absorption of the pulse energy. With a similar technique, an optical pulse can also be compressed in time. Theoretical simulations were developed to simulate the group-velocity control and the pulse compression processes. The group velocity as well as the pulse reshaping are solely controlled by external voltages which makes it promising in quantum information and quantum communication processes. It is also proposed that the group velocity can be changed even more in an Er-doped crystal while at the same time having a transmission band matching the telecommunication wavelength.Received 21 October 2016DOI:https://doi.org/10.1103/PhysRevA.95.032104©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLight propagation, transmission & absorptionQuantum state engineeringStark effectPhysical SystemsRare-earth doped crystalsTechniquesCoherent controlOptical pumpingRabi modelAtomic, Molecular & OpticalGeneral Physics

Ultimate capabilities for few-cycle pulse formation via resonant interaction of XUV radiation with IR-field-dressed atoms

We perform an ab initio study of the ultimate capabilities and limits of applicability of the method for few-cycle pulse formation via the resonant interaction of extreme ultraviolet (XUV) radiation with atoms dressed by a moderately strong infrared (IR) laser field proposed in two earlier works [Y. V. Radeonychev et al., Phys. Rev. Lett. 105, 183902 (2010) and V. A. Polovinkin et al., Opt. Lett. 36, 2296 (2011)]. Taking into account all the multiphoton processes in the systems under consideration on the basis of numerical solution of the three-dimensional time-dependent Schr\odinger equation (TDSE) in the single-active-electron approximation, we show the possibilities to produce 1.1-fs pulses from 124.6-nm XUV radiation via the linear Stark effect in atomic hydrogen, as well as 500-as pulses from 58.4-nm XUV radiation via excited-state ionization in helium. We derive a generalized analytical solution, which takes into account the interplay between sub-laser-cycle Stark effect and excited-state ionization and allows us to analyze the results of TDSE calculations. We found that the ultimate intensity of the IR field suitable for few-cycle pulse formation via the linear Stark effect or excited-state ionization is limited by the threshold for atomic ionization from the resonant excited state or the ground state, respectively. We show that the pulses with shorter duration can be produced in the medium of ions with higher values of the ionization potential.

DC Stark addressing for quantum memory in Tm:YAG

We observed a linear DC Stark effect for 3 H 6 – 3 H 4 optical transition of Tm 3+ ions in Y 3 Al 5 O 12 . We observed that application of electric field pulse suppresses the two-pulse photon echo signal. If we then apply a second electric pulse of opposite polarity the echo signal is restored again, which indicates the linear nature of the observed effect. The effect is present despite the D 2 symmetry of the Tm 3+ sites that prohibits a linear Stark effect. Experimental data analysis shows that the observed electric field influence can be attributed to defects that break the local crystal field symmetry near Tm 3+ ions. Using this effect we demonstrate selective retrieval of light pulses in two-pulse photon echo.

Quasi-classical description of the stark effect for an electron in an image-potential state

The quasi-classical approximation is used for establishing the positions of the classical turning points of an electron in an image-potential state exposed to an electric field of arbitrary strength perpendicular to the metal surface. It is demonstrated that the behavior of the system in electric fields of different directions is fundamentally different, which makes the dynamics of the low-dimensional system qualitatively different from that of its three-dimensional analog. The electric field strength leading to transition from the tunnel ionization regime to the regime of above-barrier decomposition is determined. The wavefunction of the bound electron state, which explicitly takes into account the influence of the electric field, is expressed in terms of elliptic integrals. The quantization condition is formulated, and the linear and quadratic in the field corrections to the electron energy are found. It is demonstrated that the difference between the linear Stark effect calculated by means of the perturbation theory and the quasi-classical energy shift in a weak field rapidly decreases with increasing quantum level number.

Probing different regimes of strong field light–matter interaction with semiconductor quantum dots and few cavity photons

In this work we present an extensive experimental and theoretical investigation of different regimes of strong field light–matter interaction for cavity-driven quantum dot (QD) cavity systems. The electric field enhancement inside a high-Q micropillar cavity facilitates exceptionally strong interaction with few cavity photons, enabling the simultaneous investigation for a wide range of QD-laser detuning. In case of a resonant drive, the formation of dressed states and a Mollow triplet sideband splitting of up to 45 μeV is measured for a mean cavity photon number . In the asymptotic limit of the linear AC Stark effect we systematically investigate the power and detuning dependence of more than 400 QDs. Some QD-cavity systems exhibit an unexpected anomalous Stark shift, which can be explained by an extended dressed 4-level QD model. We provide a detailed analysis of the QD-cavity systems properties enabling this novel effect. The experimental results are successfully reproduced using a polaron master equation approach for the QD-cavity system, which includes the driving laser field, exciton-cavity and exciton-phonon interactions.

Stark echo modulation for quantum memories

Quantum memories for optical and microwave photons provide key functionalities in quantum processing and communications. Here we propose a protocol well adapted to solid state ensemble based memories coupled to cavities. It is called Stark Echo Modulation Memory (SEMM), and allows large storage bandwidths and low noise. This is achieved in a echo like sequence combined with phase shifts induced by small electric fields through the linear Stark effect. We investigated the protocol for rare earth nuclear spins and found a high suppression of unwanted collective emissions that is compatible with single photon level operation. Broadband storage together with high fidelity for the Stark retrieval process is also demonstrated. SEMM could be used to store optical or microwave photons in ions and/or spins. This includes NV centers in diamond and rare earth doped crystals, which are among the most promising solid-state quantum memories.

Slow-light-based optical frequency shifter

We demonstrate experimentally and theoretically a controllable way of shifting the frequency of an optical pulse by using a combination of spectral hole burning, slow light effect, and linear Stark effect in a rare-earth-ion doped crystal. We claim that the solid angle of acceptance of a frequency shift structure can be close to $2\pi$, which means that the frequency shifter could work not only for optical pulses propagating in a specific spatial mode but also for randomly scattered light. As the frequency shift is controlled solely by an external electric field, it works also for weak coherent light fields, and can e.g. be used as a frequency shifter for quantum memory devices in quantum communication.

The influence of a permanent dipole moment on the tunnelling ionization of a CO molecule

We show that the linear Stark effect due to a molecule permanent dipole moment does not influence the rate of molecule ionization by laser radiation in the multiphoton limit. However, it may influence the ionization rate in the tunnelling limit. A CO molecule is considered as an example. Its valence orbitals have σ symmetry. In this case, the tunnel effect rate is maximal if the permanent dipole moment (the molecule axis) is oriented along the direction of the laser-wave electric field, and the role of the linear Stark effect is the most noticeable. This situation differs from that when the valence orbitals have π symmetry which we have considered previously (Kornev and Zon 2014 Laser Phys. 24 115302). In that case, the tunnel effect rate is maximal if the molecule axis is oriented perpendicularly to the electric field direction and the role of the linear Stark effect is less significant. We consider the tunnel effect in both dc and ac fields, accounting for perturbation of vibrational motion by an external field. We show that the influence of the permanent dipole moment does not vanish even after averaging the tunnelling rate over the molecule orientations.

Azimuthal asymmetry of the extracted electron in field ionization of a hydrogen atom with orbital angular momentum

The tunneling ionization of a hydrogen atom excited in the presence of a static electric field is investigated for the case where, before being extracted, the electron has an orbital angular momentum L perpendicular to the field E. The escaping electron has a nonzero mean transverse velocity <vT> in the direction of E cross <L>. This asymmetry is similar to the Collins effect in the fragmentation into hadrons of a transversely polarized quark. In addition, the linear Stark effect make <L> and <vT> oscillate in time. The degree of asymmetry is calculated at leading order in E for an initial state of maximum transverse <L>. The conditions for the observation of this asymmetry are discussed.

Interplay of Bias-Driven Charging and the Vibrational Stark Effect in Molecular Junctions.

We observe large, reversible, bias driven changes in the vibrational energies of PCBM based on simultaneous transport and surface-enhanced Raman spectroscopy (SERS) measurements on PCBM-gold junctions. A combination of linear and quadratic shifts in vibrational energies with voltage is analyzed and compared with similar measurements involving C60-gold junctions. A theoretical model based on density functional theory (DFT) calculations suggests that both a vibrational Stark effect and bias-induced charging of the junction contribute to the shifts in vibrational energies. In the PCBM case, a linear vibrational Stark effect is observed due to the permanent electric dipole moment of PCBM. The vibrational Stark shifts shown here for PCBM junctions are comparable to or larger than the charging effects that dominate in C60 junctions.

Vibrational Stark Effects for Diverse Carbonyl Probes Applied to the Re-Interpretation of IR and Raman Data in Terms of Electric Fields at Enzyme Active Sites

Carbonyl groups are ubiquitious in biology and found in many different forms including ketones, oxoesters (e.g. lipids, acyl-intermediates), thioesters (e.g. acetyl-CoA, fatty acid biosynthesis), and amides (e.g. DNA, RNA, proteins). Previous work in our lab has utilized the vibrational Stark effect (VSE) of ketones on non-natural amino acids [1] and substrate analogs [2] to quantify the electric field within several proteins, based on the vibration's linear Stark tuning rate. In order to further probe diverse biophysical systems and environments, we have measured the sensitivity of a functionally-relevant collection of carbonyl-containing compounds by vibrational Stark spectroscopy (VSS) to obtain the Stark tuning rate, and further calibrated them to an absolute electric field utilizing vibrational solvatochromism and MD simulations. The VSE provides a quantitative framework for interpreting observed frequency shifts in biophysical systems, providing physical insights into binding and catalysis. As such, this calibration to an electric field provides a unifying model with which to re-examine a substantial body of literature on highly unusual IR and Raman data of inhibitors at the enzyme active site, which have been previously attributed to substrate polarization and bond distortion [e.g. 3]. The generalizability of this interpretation to many enzymatic systems further suggests the importance of enzyme electrostatics in modulating function and may have significant applications in protein design and engineering.

Electric field gradient in microplasmas

&#61472;Linear Stark splitting of the H&#946; Balmer line components and spatially resolved optical emission spectroscopy (OES) measurements were used to estimate the electric &#64257;eld gradient in the cathode sheath region (&#8764;70 &#956;m long) of an atmospheric pressure direct current argon &#64258;ow-stabilized microplasma jet. Also, plasma parameters in the negative glow region were investigated by both OES and electrical diagnostics. OES was used to diagnose near-cathode region (negative glow and cathode sheath) of the microplasma jet in order to measure both, the basic discharge parameters, in the plasma bulk, and electric &#64257;eld distribution along the cathode sheath. In addition, the main mechanisms responsible for H&#946; spectral line broadenings were investigated.

Vibrational and Electronic Stark Effects in Green Fluorescent Protein

The use of unnatural amino acids to probe non-covalent interactions, dynamics, and folding in proteins is growing rapidly. Specifically, para-cyanophenylalanine (pCNF) is an attractive probe of electrostatics because of its sensitivity to electric fields and its minimal structural perturbation when replacing phenylalanine or tyrosine. However, due to the potential for hydrogen bonding to the nitrile moiety there is potential for the introduction of nonnative interactions, which can cloud our understanding of native proteins. Usual attempts to quantify the extent of nitrile perturbation include functional assays or structural measurements. While structure and function are undoubtedly linked to electrostatic environment, they are not direct reporters of it. An intrinsic reporter of electrostatic environment would allow for the direct comparison of environments with and without an extrinsic nitrile probe. Boxer and coworkers have shown that the intrinsic fluorophore of green fluorescent protein (GFP) is sensitive to electric fields in a manner consistent with a linear Stark effect. Here we incorporate multiple pCNF probes into buried sites near the GFP fluorophore and measure changes in the electric field environment from both the nitriles and the fluorophore in response to a series of amino acid mutations. Our results show that pCNF probes can be incorporated near the GFP fluorophore without affecting the electric field changes experienced by the fluorophore. Additionally, changes in electric field measured from the nitrile are in agreement with those measured from the fluorophore. These results suggest that, for this model system, nitrile probes do not significantly perturb the native electrostatic environment. The agreement of changes in electric field from vibrational and electronic Stark effect probes inside the beta barrel of GFP suggests nitrile frequencies can be interpreted in terms of the Stark effect even when they participate in hydrogen bonding.

X-ray absorption spectroscopy in electrical fields: An element-selective probe of atomic polarization

The authors report x-ray absorption spectroscopy of samples in large electrical fields. The observed effect is linear in electric field and only observable in polar materials. It can be thought of as the x-ray analogue of the linear Stark effect. It may open up a new area of research by studying electrical polarization with element specificity for multiconstituent functional materials

State purified deceleration of SD radicals by a Stark decelerator

ABSTRACTDeceleration of supersonic beams of SD radicals by a linear Stark decelerator was demonstrated for the first time. SD radicals produced by an electric discharge of supersonic beams mainly occupied the lowest two rotational states of and in the 2Π3/2 state. Laser-induced fluorescence of the decelerated beam showed that, due to the proximity of the Stark shift between these two rotational states, the final radical packet still contained both the and rotational states when the decelerator was operated at a constant phase below 50°. We show that by controlling the timing sequence of the Stark decelerator, it is possible to create a decelerated molecular packet containing only a single rotational state of either or of the SD radicals with a tunable velocity. The creation of a cold radical beam containing a single rotation–vibration state will be a useful tool for the study of cold collisions and reactions in the quantum regime.

Analytical studies on the coherent AC Stark effect of an open λ-type drive-probe system

The Doppler-free absorptive lineshape and its physical parameters are analytically formulated for an open λ-type coherently prepared drive-probe system. The individual contribution of drive and probe fields on AC Stark splitting is demonstrated through power broadened lineshape. We exhibit the sharpness and asymmetric nature of Stark splitting for probe field induced quantum coherence and atomic coherence. The coherent linear Stark shift due to probe and drive fields is classified by means of various physical controlling parameters. It is shown that the direction of the AC Stark shift towards red or blue region can be controlled via probe field induced quantum coherence.

Probing interactions between Rydberg atoms with large electric dipole moments in amplitude-modulated electric fields

Dipole-dipole interactions between helium atoms in Rydberg-Stark states with principal quantum number $n=53$ and approximately linear Stark energy shifts, resulting from induced electric dipole moments of approximately 7900 D, have been investigated experimentally. The experiments were performed in pulsed supersonic metastable helium beams, with particle number densities of up to $\sim10^9$ cm$^{-3}$. In the presence of amplitude-modulated, radio-frequency electric fields, changes in the spectral intensity distributions associated with the transitions to these states that are attributed to dipole-dipole interactions within the ensembles of excited atoms have been observed. The experimental results are in excellent agreement with calculations of the Rydberg energy level structure carried out using Floquet methods, and excitations shared by up to 4 atoms. The use of these Rydberg-Stark states as sensors for non-resonant broadband radio-frequency electrical noise is also discussed.