B. Redlich

Time-Resolved Holography with Photoelectrons

The intefererence pattern produced by photoelectrons provides holographic snapshots of the photoionization process. Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.

Coherent control of Rydberg states in silicon

Laser cooling and electromagnetic traps have led to a revolution in atomic physics, yielding dramatic discoveries ranging from Bose–Einstein condensation to the quantum control of single atoms. Of particular interest, because they can be used in the quantum control of one atom by another, are excited Rydberg states, where wavefunctions are expanded from their ground-state extents of less than 0.1 nm to several nanometres and even beyond; this allows atoms far enough apart to be non-interacting in their ground states to strongly interact in their excited states. For eventual application of such states, a solid-state implementation is very desirable. Here we demonstrate the coherent control of impurity wavefunctions in the most ubiquitous donor in a semiconductor, namely phosphorus-doped silicon. In our experiments, we use a free-electron laser to stimulate and observe photon echoes, the orbital analogue of the Hahn spin echo, and Rabi oscillations familiar from magnetic resonance spectroscopy. As well as extending atomic physicists’ explorations of quantum phenomena to the solid state, our work adds coherent terahertz radiation, as a particularly precise regulator of orbitals in solids, to the list of controls, such as pressure and chemical composition, already familiar to materials scientists.

Silicon as a model ion trap: Time domain measurements of donor Rydberg states

One of the great successes of quantum physics is the description of the long-lived Rydberg states of atoms and ions. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr–Sommerfeld formula, shifted to the far-infrared because of the small effective mass and high dielectric constant. Manipulation of Rydberg states in free atoms and ions by single and multiphoton processes has been tremendously productive since the development of pulsed visible laser spectroscopy. The analogous manipulations have not been conducted for donor impurities in silicon. Here, we use the FELIX pulsed free electron laser to perform time-domain measurements of the Rydberg state dynamics in phosphorus- and arsenic-doped silicon and we have obtained lifetimes consistent with frequency domain linewidths for isotopically purified silicon. This implies that the dominant decoherence mechanism for excited Rydberg states is lifetime broadening, just as for atoms in ion traps. The experiments are important because they represent a step toward coherent control and manipulation of atomic-like quantum levels in the most common semiconductor and complement magnetic resonance experiments in the literature, which show extraordinarily long spin lattice relaxation times—key to many well known schemes for quantum computing qubits—for the same impurities. Our results, taken together with the magnetic resonance data and progress in precise placement of single impurities, suggest that doped silicon, the basis for modern microelectronics, is also a model ion trap.

Terahertz polarization conversion with quartz waveplate sets.

We present the results of calculation and experimental testing of an achromatic polarization converter and a composite terahertz waveplate (WP), which are represented by sets of plane-parallel birefringent plates with in-plane birefringence axis. The calculations took into account the effect of interference, which was especially prominent when plates were separated by an air gap. The possibility of development of a spectrum analyzer design based on a set of WPs is also discussed.

Terahertz Raman laser based on silicon doped with phosphorus

Raman-type stimulated emission at frequencies between 5.0 and 5.2?THz as well as between 6.1 and 6.4?THz has been realized in silicon crystals doped by phosphorus donors. The Raman laser operates at around 5?K under optical excitation by a pulsed, frequency-tunable infrared free electron laser. The frequencies of the observed laser emission are close to the frequencies of the intracenter laser lines which originate from the 2p0 and 2p± phosphorus states. The Stokes shift of 3.16?THz is equal to the difference between the energies of the phosphorus ground state, 1s(A1), and the 1s(E) excited state.

Terahertz lasing from silicon by infrared Raman scattering on bismuth centers

Stimulated emission at terahertz frequencies (4.5–5.8 THz) has been realized by electronic Raman scattering of infrared radiation on bismuth donor centers in silicon at low temperatures. The Stokes shift of the observed laser emission is 40.53 meV which corresponds to the bismuth intracenter transition between the 1s(A1) ground state and the excited 1s(E) state. The laser has a low optical threshold and the largest frequency coverage in comparison with other Raman silicon lasers based on shallow donor centers. Time-resolved pump spectra enable the separation of donor and Raman lasing.

Electronic structure and characterization of a uranyl di-15-crown-5 complex with an unprecedented sandwich structure.

Understanding of the nature and extent of chemical bonding in uranyl coordination complexes is crucial for the design of new ligands for nuclear waste separation, uranium extraction from seawater, and other applications. We report here the synthesis, infrared spectroscopic characterization, and quantum chemical studies of a molecular uranyl-di-15-crown-5 complex. The structure and bonding of this unique complex featuring a distinctive 6-fold coplanar coordination staggered sandwich structure and an unusual non-perpendicular orientation of the uranyl moiety are evaluated using density functional theory and chemical bonding analyses. The results provide fundamental understanding of the coordination interaction of uranyl with oxygen-donor ligands.

Multifrequency terahertz lasing from codoped silicon crystals

Stimulated terahertz emission in the range from 4.5 to 6.4 THz has been realized from a single silicon crystal doped by two hydrogen-like donor centers, phosphorus and antimony, when pumped by midinfrared radiation from a free electron laser. Intracenter as well as Raman lasing has been observed. Simultaneous laser emission from both donors occurs when the pump photon energy is sufficient for photoionization of the antimony donors. The laser processes of both donors are not influenced by each other. Therefore the codoping approach can be extended to other group-V donors including more than two dopants in a single crystal.

Breakdown Products of Gaseous Polycyclic Aromatic Hydrocarbons Investigated with Infrared Ion Spectroscopy

We report on a common fragment ion formed during the electron-ionization-induced fragmentation of three different three-ring polycyclic aromatic hydrocarbons (PAHs), fluorene (C13H10), 9,10-dihydrophenanthrene (C14H12), and 9,10-dihydroanthracene (C14H12). The infrared spectra of the mass-isolated product ions with m/z = 165 were obtained in a Fourier transform ion cyclotron resonance mass spectrometer whose cell was placed inside the optical cavity of an infrared free-electron laser, thus providing the high photon fluence required for efficient infrared multiple-photon dissociation. The infrared spectra of the m/z = 165 species generated from the three different precursors were found to be similar, suggesting the formation of a single isomer. Theoretical calculations using density functional theory (DFT) revealed the fragments identity as the closed-shell fluorenyl cation. Decomposition pathways from each parent precursor to the fluorenyl ion are proposed on the basis of DFT calculations. The identification of a single fragmentation product from three different PAHs supports the notion of the existence of common decomposition pathways of PAHs in general and can aid in understanding the fragmentation chemistry of astronomical PAH species.

A THz spectrometer combining the free electron laser FLARE with 33 T magnetic fields

The free electron laser Free electron Laser for Advanced spectroscopy and high Resolution Experiments (FLARE) at the FELIX Laboratory generates powerful radiation in the frequency range of 0.3–3 THz. This light, in combination with 33 T Bitter magnets at the High Field Magnet Laboratory, provides the unique opportunity to perform THz magneto spectroscopy with light intensities many orders of magnitude higher than provided by conventional sources. The performance of the THz spectrometer is measured via high-field electron spin resonance (ESR) in the paramagnetic benchmark system 2,2-diphenyl-1-picrylhydrazyl (DPPH). The narrow ESR linewidth of DPPH allows us to resolve a fine structure with 3 GHz spacing, demonstrating a considerable coherence of the individual THz micropulses of FLARE. The spectral resolution Δ ν / ν is better than 0.1%, which is an order of magnitude higher than typical values for a rf-linac based free electron laser. The observed coherence of the high power THz micropulses is a prerequi...