Ernst Knoesel

Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling

We demonstrate a simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot. The method relies on electro-optic sampling in a noncollinear geometry for the terahertz radiation and the visible probe beam, coupled with multichannel detection. The approach provides time resolution that is comparable to that of conventional electro-optic sampling measurements.

Temperature dependence of surface state lifetimes, dephasing rates and binding energies on Cu(111) studied with time-resolved photoemission

Abstract The ultrafast electron dynamics of surface states on Cu(111) is investigated as a function of temperature between 25 K and 460 K employing time-resolved two-photon photoemission (2PPE) spectroscopy. Analysis of the thermally-induced energy shift of the unoccupied n = 1 image potential state based on a multiple reflection model allows a precise determination of the position of the upper edge ( L 6+ ) of the sp-gap in the (111)-directin( E L 6+ (T) − E F = 4.15eV− (0.26meV/K) T ). We find that the lifetime of the n = 1 image state decreases from22 ± 3 fs at 25 K to14 ± 3 fs at 350 K. This is attributed to the increasing penetration depth of the image state wave function into the bulk at higher temperatures, where the image state crosses the band edge. The phonon contribution to the electronic dephasing of the n = 0 surface state and the n = 1 image state on Cu(111) is determined from their temperature-dependent linewidths using three-level optical Bloch equations and is found to correlate with their wave function overlap with bulk states.

Paradox in Wave-Particle Duality

We report on the simultaneous determination of complementary wave and particle aspects of light in a double-slit type “welcher-weg” experiment beyond the limitations set by Bohr’s Principle of Complementarity. Applying classical logic, we verify the presence of sharp interference in the single photon regime, while reliably maintaining the information about the particular pinhole through which each individual photon had passed. This experiment poses interesting questions on the validity of Complementarity in cases where measurements techniques that avoid Heisenberg’s uncertainty principle and quantum entanglement are employed. We further argue that the application of classical concepts of waves and particles as embodied in Complementarity leads to a logical inconsistency in the interpretation of this experiment.

Variable Electron-Phonon Coupling in Isolated Metallic Carbon Nanotubes Observed by Raman Scattering

We report the existence of broad and weakly asymmetric features in the high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of defined chiral index. A significant variation in peak width (from 12 cm(-1) to 110 cm(-1)) is observed as a function of the nanotubes chiral structure. When the nanotubes are electrostatically gated, the peak widths decrease. The broadness of the Raman features is understood as the consequence of coupling of the phonon to electron-hole pairs, the strength of which varies with the nanotube chiral index and the position of the Fermi energy.

Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.

CAN WE CONTROL LIFETIMES OF ELECTRONIC STATES AT SURFACES BY ADSORBATE RESONANCES

Abstract Femtosecond time-resolved two-photon photoemission is used to study the excited electronic states of a Cu(111) surface covered with a monolayer of physisorbed O 2 on top of one to five Xe spacer layers. The spectra are dominated by a state 4.0 eV above E F whose lifetime depends strongly on the number of Xe spacer layers. The lifetime decreases with increasing number of spacer layers from (650±30) fs for one Xe layer to (90±10) fs for five layers. To explain this trend we propose a model where the image potential states couple to a negative-ion resonance of the O 2 molecule.

Why Kastner analysis does not apply to a modified Afshar experiment

In an analysis of the Afshar experiment R.E. Kastner points out that the selection system used in this experiment randomly separates the photons that go to the detectors, and therefore no which-way information is obtained. In this paper we present a modified but equivalent version of the Afshar experiment that does not contain a selection device. The double-slit is replaced by two separate coherent laser beams that overlap under a small angle. At the intersection of the beams an interference pattern can be inferred in a non-perturbative manner, which confirms the existence of a superposition state. In the far field the beams separate without the use of a lens system. Momentum conservation warranties that which-way information is preserved. We also propose an alternative sequence of Stern-Gerlach devices that represents a close analogue to the Afshar experimental set up.

Electronic Charge Transport in Sapphire Studied by Optical-Pump/THz-Probe Spectroscopy

THz time-domain spectroscopy (THz TDS) with ultrafast photo-excitation is applied to probe the complex conductivity of the charge carriers in sapphire over the temperature range of 40 - 350 K. A comparison of the measured complex conductivity to the Drude model yields the carrier scattering rate and density. The dependence of the carrier scattering rate on temperature and sample purity is used to identify the scattering mechanisms in sapphire. In the higher temperature range, scattering is determined by intrinsic phonon processes, but impurity scattering becomes dominant at low temperatures in typical optical-grade samples. In high-purity samples, however, impurity scattering remains negligible down to 40 K, and carrier mobilities exceeding 10,000 cm2/Vs can be achieved.

Controlled modification of surface state lifetimes by physisorbed adsorbates

Femtosecond time-resolved two-photon photoemission is used to study the influence of physisorbed xenon and oxygen adlayers on the lifetime of image potential states and interfacial quantum well states on Cu(111). Adsorption of 0 to 3 layers of Xe leads to a pronounced increase of the n equals 1 image state lifetime from 22 fs to 300 fs, respectively. However, for adsorbate heterostructures consisting of one monolayer (ML) O2 on top of Xe spacer layers with variable thickness it is found that the lifetime of an oxygen induced quantum well state (0.35 eV below Evac) decreases from 650 fs to 90 fs when the number of spacer layers is raised from 1 ML to 5 ML. The results can be semiquantitatively reproduced by a model calculation which accounts for the modified image potential due to the Xe and O2 adlayers. The changes of the lifetimes are explained by the differences in the penetration of the excited state wave function into the Cu substrate.

Growth and structural properties of Bi(Fe x Sc1− x )O3 thin films

Epitaxial Bi(Fe x Sc1− x )O3 thin films with a range of compositions were fabricated by pulsed laser deposition on SrTiO3 (001) substrates with a BiFeO3 buffer layer. X-ray diffraction and transmission electron microscopy reveal that this composition series forms a solid solution in the thin film form. Second harmonic generation measurements showed a maximum at x = 0.7, which may be associated with a phase transition. The present BiScO3 films did not exhibit ferroelectric or antiferroelectric behaviour at the field levels which could be probed.