Spin-polarized samples and spin mixtures of quantum degenerate fermionic atoms are prepared in selected excited Bloch bands of an optical checkerboard square lattice. For the spin-polarized case, extreme band lifetimes above 10 s are observed, reflecting the suppression of collisions by Pauli's exclusion principle. For spin mixtures, lifetimes are reduced by an order of magnitude by two-body collisions between different spin components, but still remarkably large values of about 1 s are found. By analyzing momentum spectra, we can directly observe the orbital character of the optical lattice. The observations demonstrated here form the basis for exploring the physics of Fermi gases with two paired spin components in orbital optical lattices, including the regime of unitarity.

We explore Josephson-like dynamics of a Bose-Einstein condensate of rubidium atoms in the second Bloch band of an optical square lattice providing a double well structure with two inequivalent, degenerate energy minima. This oscillation is a direct signature of the orbital changing collisions predicted to arise in this system in addition to the conventional on-site collisions. The observed oscillation frequency scales with the relative strength of these collisional interactions, which can be readily tuned via a distortion of the unit cell. The observations are compared to a quantum model of two single-particle modes and to a semiclassical multiband tight-binding simulation of 12×12 tubular sites of the lattice. Both models reproduce the observed oscillatory dynamics and show the correct dependence of the oscillation frequency on the ratio between the strengths of the on-site and orbital changing collision processes.

The phenomenon of metastability can shape dynamical processes on all temporal and spatial scales. Here, we induce metastable dynamics by pumping ultracold bosonic atoms from the lowest band of an optical lattice to an excitation band, via a sudden quench of the unit cell. The subsequent relaxation process to the lowest band displays a sequence of stages, which include a metastable stage, during which the atom loss from the excitation band is strongly suppressed. Using classical-field simulations and analytical arguments, we provide an explanation for this experimental observation, in which we show that the transient condensed state of the atoms in the excitation band is a dark state with regard to collisional decay and tunneling to a low-energy orbital. Therefore the metastable state is stabilized by destructive interference due the chiral phase pattern of the condensed state. Our experimental and theoretical study provides a detailed understanding of the different stages of a paradigmatic example of many-body relaxation dynamics.

In this paper, a direct chemical vapor deposition (CVD) approach is applied for the first time to synthesize high quality copper oxide (CuO), copper tungstate (CuWO4) and tungsten oxide (WO3) on F:SnO2 (FTO) substrates for photocatalytic water splitting.

We show that charge carrier mobilities can be measured by reflection time resolved THz spectroscopy (R-TRTS) even for thin films on metal contacts, such as polycrystalline Cu2SnZnSe4 grown on molybdenum. In the measurement a reduced THz reflection upon photo-excitation is observed in contrast to increased THz reflection commonly observed on insulating substrates, and which excludes standard analytic R-TRTS analyses. Instead, a numerical transfer matrix method is used to model the THz reflection from which we derive carrier mobilities of 100 cm2/Vs consistent with literature. We show that R-TRTS on metal substrates is ~100x less sensitive compared to measurements on insulating substrates. These sensitivity of these R-TRTS measurements can be increased by using lower substrate refractive indices, lower substrate conductivities, thicker sample layers or higher THz probe frequencies.

We track charge carriers dynamics from femtoseconds to nanoseconds in Cu2ZnSn(S/Se)(4) thin films by time resolved terahertz spectroscopy (TRTS) and time resolved micro wave conductivity measurements (TRMC). This includes trapping of photo carriers into band tail states followed by recombination with a time constant of 9.1 ns. The relatively long life time is likely caused by the localization into long living trap states rather than by a low concentration of recombination centers.

Excitation correlation photoluminescence (ECPL) measurements are often analyzed in the approximation of a cross correlation of charge carrier populations generated by the two delayed pulses. In semiconductors, this approach is valid for a linear non-radiative recombination path, but not for a non-linear recombination rate as in the general Shockley-Read-Hall recombination scenario. Here, the evolution of the ECPL signal was studied for deep trap recombination following Shockley-Read-Hall statistics. Analytic solutions can be obtained for a fast minority trapping regime and steady state recombination. For the steady state case, our results show that the quadratic radiative term plays only a minor role, and that the shape of the measured signal is mostly determined by the non-linearity of the recombination itself. We find that measurements with unbalanced intense pump and probe pulses can directly provide information about the dominant non-radiative recombination mechanism. The signal traces follow the charge carrier concentrations, despite the complex origins of the signal, thus showing that ECPL can be applied to study charge carrier dynamics in semiconductors without requiring elaborate calculations. The model is compared with measurements on a reference sample with alternating layers of InGaAs/InAlAs that were additionally cross-checked with time resolved optical pump terahertz probe measurements and found to be in excellent agreement.

The relaxation and transport dynamics of photoexcited carriers in p-type stoichiometric polycrystalline CuInSe2 is investigated by optical-pump terahertz-probe spectroscopy. For all time delays and temperatures studied, the optically measured photoconductivity exhibits a characteristic free carrier Drude response, which allows analyzing the carrier scattering and relaxation phenomena in detail. The hot carrier distribution initially present after photoexcitation is found to relax within the first 200 ps by electron-phonon interaction with polar longitudinal optical (LO) phonons. The relaxed carrier distribution found after 200 ps indicates room temperature minority carrier mobilities close to 1000 cm(2)/Vs, in excellent agreement with Hall effect carrier mobilities previously determined for n-type single crystals. Analysis of the temperature dependence shows that the mobility at low temperatures is limited by ionized impurity scattering, while at room temperature the scattering of electrons with polar LO phonons dominates.

Short-Abstract—Heterointerfaces are often the crucial part of high-performance optoelectronic devices. Examples are critical interfaces of multi junction solar cells, III-V on Si(100), or III-V on Ge(100). Until recently, III-V-compound based triple-junction solar cells demonstrated the highest conversion efficiencies worldwide reaching almost their practical limit. Thus, meanwhile research is also addressing four to five junction solar cells and lately a four-junction configuration with optimized band gaps including GaInP/GaAs and InGaAsP/InGaAs tandem cells has been realized yielding a new record efficiency of 44.7%. The paper discusses issues of critical epitaxial, MOVPE-based preparation and analysis routes including atomic scale control and minority carrier lifetime analysis.

We use optical pump Thz probe spectroscopy to access the microscopic mobilities and fast charge carrier dynamics processes in polycrystalline chalcopyrite and kesterite thin films grown by coevaporation. In order to avoid complicating effects from the presence of Ga-gradients, ternary CuInSe2 samples were used as a model system. Significantly different DC mobilities were found for stoichiometric and Cu-poor samples, respectively. While the stoichiometric samples exhibit Drude-like free carrier mobilities with DC mobilities up to 1200cm2/Vs at room temperature, the Cu-poor samples show non-Drude behavior, with much lower DC mobilities, indicative of carrier localization. Kesterite materials are found to show even stronger signatures of carrier localization than the chalcopyrites.

Multiple quantum well (InAlAs/InGaAs) based photovoltaic cells and subsystems were grown by molecular beam epitaxy on (001) InP substrates to study the dynamics of the intersubband transitions and the photocurrent behavior with ultrafast and continuous wave (cw) laser spectroscopy, respectively. The surface recombination velocity was determined with photoluminescence and time domain THz spectroscopy by varying the well thickness. Two-photon absorption was observed in a photovoltaic device as an increase in photocurrent from room temperature down to 5 K by illumination of the structures at two different wavelengths.

The electron injection dynamics of two perylene dyes bound to ZnO and TiO(2) nanoparticles was investigated with femtosecond transient absorption simultaneously monitoring the rise of the cationic and the decay of the excited state. Electron transfer from the chromophores was slower when attached to ZnO compared to TiO(2). The excited state decay and the cationic state rise showed very good agreement at early times, indicating direct electron injection into the conduction band for both semiconductors in absence of intermediate states.

This chapter presents a report on transient absorption experiments in ultrahigh vacuum that probe heterogeneous electron transfer from the molecular chromophore perylene via different molecular anchor-bridge groups into nm-structured anatase TiO 2 layers. Transport of electrons through molecular units is an important topic in the field of molecular electronics and future device modeling. The strength of electronic coupling between a molecular chromophore and a semiconductor can be varied with different molecular bridge units inserted between the chromophore and anchor group. The measured electron transfer times show conducting or insulating behavior of the respective bridge unit, -C=C- and -C-C-. Heterogeneous electron transfer times were measured with different bridge-anchor groups in ultrahigh vacuum where the wide band limit is realized for the perylene/TiO 2 system. The experimental data should allow for a more direct comparison with theoretical calculations because they are neither complicated by the presence of a solvent environment nor by specific Franck–Condon factors.

We demonstrate the operation of a 100 kHz noncollinear optical parametric amplifier that is pumped by just a few microjoules of 800 nm pulses with 50 fs duration. The device delivers sub-20 fs pulses tunable from 460 nm to beyond 1 microm and pulse energies up to 750 nJ when it is pumped with 7 microJ of energy. The design of the single-stage amplifier has been carefully optimized, and the design considerations are discussed.

Time-dependent two-photon photoemission spectra are used to resolve the femtosecond dynamics of hot electrons at the energetically lowest surface resonance of reconstructed InP(100). Two different cases are studied, where electrons either are lifted into the surface resonance via a direct optical transition or are captured from bulk states. These data are the first of this kind recorded with a time resolution below 70 fs. The microscopic analysis shows that electron-phonon scattering is a major mechanism for electron transfer between surface and bulk states.

Electron transfer from the excited aromatic donor perylene to TiO2 occurred with 10 fs time constant via the conjugated -CH = CH- bridge unit compared to 57 fs in the presence of the saturated -CH2-CH2- bridge unit.

An improved setup for femtosecond two-photon photoemission spectroscopy (TR-2PPE) is presented. Two noncollinear optical parametric amplifiers (NOPA) were operated simultaneously at a repetition rate of 150 kHz. The frequencies of the NOPA outputs were tuned such that subsequent second harmonic generation (SHG) provided the two different ultraviolet wavelengths required for the pump and probe pulse. The width of the crosscorrelation function (CC) for pump and probe pulse was sub-30 fs (FWHM).The performance of this assembly was tested by measuring the lifetime of image potential states (IS) on Cu(111) and Ag(111) surfaces. We present here for the first time lifetimes of the IS that were determined directly from the measured decay of the 2PPE signal.

Hot electron dynamics was investigated, with a focus on scattering between bulk states and the C1 surface state that is formed on the (2×4)-reconstructed In-rich surface of InP(100). The latter surface was prepared via metal organic chemical vapor deposition (MOCVD) and monitored by reflectance anisotropy spectroscopy (RAS/RDS). Two-color twophoton photoemission (2PPE) was employed with laser pulses of about 50 fs duration. Hot electrons were generated in bulk states about 0.5 eV above the C1 surface state, thereby avoiding any significant direct optical population of the surface state. A time constant of 35 fs was determined from the experimental data for electron scattering from isoenergetic bulk states to the C1 surface state by analyzing the rise of a C1-specific peak in the 2PPE spectrum. The decay of this C1 peak was ascribed to energy relaxation of the photo-generated electrons to bulk states below the surface state. Analogous measurements were carried out with a (2×1/2×2)-reconstructed P-rich surface of InP(100) that was also grown via MOCVD. No sign of a surface statewas detected in the 2PPE spectra for the latter surface in the corresponding energy range of the conduction band.