Ultrashort and metastable doping of the ZnO surface by photoexcited defects

Shallow donors in semiconductors are known to form impurity bands that induce metallic conduction at sufficient doping densities. The perhaps most direct analogy to such doping in optically excited semiconductors is the photoexcitation of deep electronic defect or dopant levels creating defect excitons (DX) which may act like shallow donors. In this work, we use time- and angle-resolved photoelectron spectroscopy to observe and characterize DX at the surface of ZnO. The DX are created on a femtosecond timescale upon photoexcitation and have a spatial extend of few nanometers that is confined to the ZnO surface. The localized electronic levels lie at 150 meV below the Fermi energy, very similar to the shallow donor states induced by hydrogen doping [Deinert et al, Phys. Rev. B, 91, 235313 (2015)]. The transient dopants exhibit a multi-step decay ranging from hundred’s of picoseconds to 77 μs and even longer. By enhancing the DX density, a Mott transition occurs, enabling the ultrafast metallization of the ZnO surface, which we described previously [Gierster et al. Nat. Commun. 12, 978 (2021)]. Depending on the defect density, the duration of the photoinduced metallization ranges from picoseconds to μs and longer, corresponding to the decay dynamics of the DX. The metastable lifetime of the DX is consistent with the observation of persistent photoconductivity (PPC) in ZnO reported in literature [Madel et al., J. Appl. Phys. 121, 124301 (2017)]. In agreement with theory on PPC [Lany and Zunger, Phys. Rev. B 72, 035215 (2005)], the deep defects are attributed to oxygen vacancies due to their energetic position in the band gap and their formation by surface photolysis upon UV illumination. We show that the photoexcitation of these defects is analogous to chemical doping and enables the transient control of material properties such as the electrical conductivity from ultrafast to metastable timescales. The same mechanism should be at play in other semiconductor compounds with deep defects

Theory of dark resonances for alkali vapors in a buffer-gas cell

We develop an analytical theory of dark resonances that accounts for the full atomic-level structure, as well as all field-induced effects such as coherence preparation, optical pumping, ac Stark shifts, and power broadening. The analysis uses a model based on relaxation constants that assumes the total collisional depolarization of the excited state. A good qualitative agreement with experiments for Cs in Ne is obtained.Comment: 16 pages; 7 figures; revtex4. Accepted for publication in PR

Triplicated P-wave measurements for waveform tomography of the mantle transition zone

Triplicated body waves sample the mantle transition zone more extensively than any other wave type, and interact strongly with the discontinuities at 410 km and 660 km. Since the seismograms bear a strong imprint of these geodynamically interesting features, it is highly desirable to invert them for structure of the transition zone. This has rarely been attempted, due to a mismatch between the complex and band-limited data and the (ray-theoretical) modelling methods. Here we present a data processing and modelling strategy to harness such broadband seismograms for finite-frequency tomography. We include triplicated P-waves (epicentral distance range between 14 and 30°) across their entire broadband frequency range, for both deep and shallow sources. We show that is it possible to predict the complex sequence of arrivals in these seismograms, but only after a careful effort to estimate source time functions and other source parameters from data, variables that strongly influence the waveforms. Modelled and observed waveforms then yield decent cross-correlation fits, from which we measure finite-frequency traveltime anomalies. We discuss two such data sets, for North America and Europe, and conclude that their signal quality and azimuthal coverage should be adequate for tomographic inversion. In order to compute sensitivity kernels at the pertinent high body wave frequencies, we use fully numerical forward modelling of the seismic wavefield through a spherically symmetric Earth

Pseudoheterodyne near-field imaging at kHz repetition rates via quadrature-assisted discrete demodulation

Scattering-type scanning near-field optical microscopy enables the measurement of optical constants of a surface beyond the diffraction limit. Its compatibility with pulsed sources is hampered by the requirement of a high-repetition rate imposed by lock-in detection. We describe a sampling method, called quadrature-assisted discrete (quad) demodulation, which circumvents this constraint. Quad demodulation operates by measuring the optical signal and the modulation phases for each individual light pulse. This method retrieves the near-field signal in the pseudoheterodyne mode, as proven by retraction curves and near-field images. Measurement of the near-field using a pulsed femtosecond amplifier and quad demodulation is in agreement with results obtained using a CW laser and the standard lock-in detection method

Ultrafast evolution of the complex dielectric function of monolayer WS₂ after photoexcitation

Transition metal dichalcogenides emerged as ideal materials for the investigation of exciton physics. Retrieving the excitonic signature in optical spectra, and tracking their time evolution upon photoexcitation requires appropriate analysis procedures, particularly when comparing different measurements, experimental techniques, samples, and substrates. In this work, we investigate the ultrafast time evolution of the exciton resonance of a monolayer of WS2 deposited on fused silica and Si/SiO2, and using two different measurement techniques: time-resolved reflectance and transmittance contrast. By modelling the dielectric function of the exciton with a Lorentz oscillator, using a Fresnell equations formalism, we derive analytical expressions of the exciton lineshape in both cases. The 2D linearized model introduced by Li et al. [Y. Li and T. F. Heinz, 2D Mater., 2018, 5, 025021] is used for the transmittance of the transparent substrate and a Fresnel transfer matrix method [O. Stenzel, The Physics of Thin Film Optical Spectra, Springer Series in Surface Science, 2016] is used to derive the reflectance in the case of the layered Si/SiO2 substrate. By fitting two models to the time-dependent optical spectra, we extract and quantify the time evolution of the parameter describing the excitonic resonance. We find a remarkable agreement between the extracted dynamics from both experiments despite the different side conditions, showing the equivalence and reliability of the two analysis methods in use. With this work, we pave the way to the resilient comparison of the exciton dynamics from different samples, measurements technique and substrates

Uncovering the (un-)occupied electronic structure of a buried hybrid interface

The energy level alignment at organic/inorganic (o/i) semiconductor interfaces is crucial for any light-emitting or -harvesting functionality. Essential is the access to both occupied and unoccupied electronic states directly at the interface, which is often deeply buried underneath thick organic films and challenging to characterize. We use several complementary experimental techniques to determine the electronic structure of p-quinquephenyl pyridine (5P-Py) adsorbed on ZnO(10-10). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9 eV and causes an occupied in-gap state (IGS) directly below the Fermi level $E_\text{F}$. Adsorption of upright-standing 5P-Py also leads to a strong work function reduction of up to 2.1 eV and to a similar IGS. The latter is then used as an initial state for the transient population of three normally unoccupied molecular levels through optical excitation and, due to its localization right at the o/i interface, provides interfacial sensitivity, even for thick 5P-Py films. We observe two final states above the vacuum level and one bound state at around 2 eV above $E_\text{F}$, which we attribute to the 5P-Py LUMO. By the separate study of anchoring group and organic dye combined with the exploitation of the occupied IGS for selective interfacial photoexcitation this work provides a new pathway for characterizing the electronic structure at buried o/i interfaces

O₂ reduction at a DMSO/Cu(111) model battery interface

In order to develop a better understanding of electrochemical $\mathrm{O_2}$reduction in non-aqueous solvents, we apply two-photon photoelectronspectroscopy to probe the dynamics of $\mathrm{O_2}$ reduction at aDMSO/Cu(111) model battery interface. By analyzing the temporal evolution ofthe photoemission signal, we observe the formation of $\mathrm{O_2^-}$ from atrapped electron state at the DMSO/vacuum interface. We find the verticalbinding energy of $\mathrm{O_2^-}$ to be 3.80 $\pm$ 0.05 eV, in good agreementwith previous results from electrochemical measurements, but with improvedaccuracy, potentially serving as a basis for future calculations on thekinetics of electron transfer at electrode interfaces. Modelling the$\mathrm{O_2}$ diffusion through the DMSO layer enables us to quantify theactivation energy of diffusion (31 $\pm$ 6 meV), the diffusion constant (1$\pm$ 1$\cdot 10^{-8}$ cm$^2$/s), and the reaction quenching distance forelectron transfer to $\mathrm{O_2}$ in DMSO (12.4 $\pm$ 0.4 \unicode{x212B}),a critical value for evaluating possible mechanisms for electrochemical sidereactions. These results ultimately will inform the development andoptimization of metal-air batteries in non-aqueous solvents.<br

Light-driven Rotary Molecular Motors for Out-of-Equilibrium Systems

This chapter discusses whether rotary molecular motors are able to operate away from thermal equilibrium and whether this work on the molecular scale can be amplified across length scales and light-driven rotary motors. The chapter analyzes examples from literature demonstrating the ability of molecular motors to perform work, starting with examples in which a net output is generated by single molecules, followed by larger (supramolecular) systems. The molecular motors are overcrowded alkene switches, and the initial establishment of a photostationary state can be recognized as the photochemical equilibrium described for photoswitches. Surface immobilization is the possibilities to overcome Brownian motion and give opportunity to turn the relative rotation of one motor half with respect to the other into an absolute rotation of the motor with respect to the surface. Self-assembly opens up the possibility to work with very complex systems and enhance functions through cooperativity without the need of complicated synthesis of elaborate single molecules.</p

Atom-molecule dark states in a Bose-Einstein condensate

We have created a dark quantum superposition state of a Rb Bose-Einstein condensate (BEC) and a degenerate gas of Rb$_2$ ground state molecules in a specific ro-vibrational state using two-color photoassociation. As a signature for the decoupling of this coherent atom-molecule gas from the light field we observe a striking suppression of photoassociation loss. In our experiment the maximal molecule population in the dark state is limited to about 100 Rb$_2$ molecules due to laser induced decay. The experimental findings can be well described by a simple three mode model.Comment: 4 pages, 6 figure

Instaseis: instant global seismograms based on a broadband waveform database

We present a new method and implementation (Instaseis) to store global Green's functions in a database which allows for near-instantaneous (on the order of milliseconds) extraction of arbitrary seismograms. Using the axisymmetric spectral element method (AxiSEM), the generation of these databases, based on reciprocity of the Green's functions, is very efficient and is approximately half as expensive as a single AxiSEM forward run. Thus, this enables the computation of full databases at half the cost of the computation of seismograms for a single source in the previous scheme and allows to compute databases at the highest frequencies globally observed. By storing the basis coefficients of the numerical scheme (Lagrange polynomials), the Green's functions are 4th order accurate in space and the spatial discretization respects discontinuities in the velocity model exactly. High-order temporal interpolation using Lanczos resampling allows to retrieve seismograms at any sampling rate. AxiSEM is easily adaptable to arbitrary spherically symmetric models of Earth as well as other planets. In this paper, we present the basic rationale and details of the method as well as benchmarks and illustrate a variety of applications. The code is open source and available with extensive documentation at www.instaseis.net