Stefan Kuck

Dynamics of molecular self-ordering in tetraphenyl porphyrin monolayers on metallic substrates

A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.

Spectroscopy and excited-state absorption of Ni2+-doped MgAl2O4

Abstract Luminescence, ground-state and excited-state absorption and lifetime measurements on Ni2+-doped MgAl2O4 were performed. Excited-state absorption bands at 1700 nm in the near infrared and at 750 and 580 nm in the visible region due to the transitions from the 3T2 (3F) metastable state to the 3T1(3F), 1T2(1D), and 3T1(3P) states were observed and ESA cross sections were determined. The 3T2-3T1(3F) ESA band overlaps with the emission band and prevents lasing of Ni2+ : MgAl2O4 at room temperature.

Preparation and spectroscopy of Yb2+-doped Y3Al5O12, YAlO3, and LiBaF3

We report on the preparation and spectroscopic properties of Yb 2 + in Y 3 Al 5 O 12 , YAlO 3 and LiBaF 3 with respect to the realization of a tunable solid-state laser based on a 5d-4f transition of Yb 2 + . We observe broad emission bands in the region of 470 nm (LiBaF 3 ) and 580 nm (YAlO 3 ) due to the 4f-5d transition correlated with the divalent Yb ion. Exciton-like states, formed by a delocalized electron in the conduction band and an Yb 3 + ion, are also involved in the excitation and emission mechanism.

The up-conversion of near-infrared excitation radiation in Ho3+-doped LiYF4

The up-conversion processes due to the continuous wave excitation of the holmium 5I4 multiplet in LiYF4 crystal are investigated. Despite a very low absorption cross-section for the transition from the holmium 5I8 ground state to the 5I4 multiplet, a bright green up-converted emission originating from the 5S2 multiplet is observed. The excitation and excited state absorption spectra indicate that the up-conversion process is based mainly on excited state absorption from the metastable 5I7 multiplet. Some features of the up-converted green emission are characteristic for the photon avalanche process.

Tunable room-temperature laser action of Cr4+-doped Y3ScxAl5−xO12

A new class of tunable room-temperature Cr4+ lasers is presented: Cr4+-doped Y3ScxAl5−xO12 garnets. With increasing scandium content the emission shifts to longer wavelengths due to the weakening of the crystal field. Free running laser wavelengths are 1440, 1498, 1548 and 1584 nm for x = 0, 0.5, 1.0 and 1.5, respectively. Continuous tunability is obtained from 1309 to 1628 nm.

Electronic and vibronic transitions of the Cr4+-doped garnets Lu3Al5O12, Y3Al5O12, Y3Ga5O12 and Gd3Ga5O12

Abstract The Cr4+-doped stoichiometric garnets Lu3Al5O12 (LAG), Y3Al5O12 (YAG), Y3Ga5O12 (YGG), and Gd3Ga5O12 (GGG) are spectroscopically investigated. All of these crystals exhibit broadband absorption around 1 μm and broadband emission in the spectral range between 1.1 and 1.8 μm. At low temperatures zero-phonon transitions as well as pronounced phonon sidebands are observed. The energies of the coupling phonons and the spin-orbit splittings of the energy levels are analyzed in detail.

1.9-µm and 2.0-µm laser diode pumping of Cr 2+ :ZnSe and Cr 2+ :CdMnTe

Efficient diode pumping at wavelengths of 1.9 and 2.0microm of a Cr(2+): ZnSe laser with an output power of 105 mW and a slope efficiency of 35% with respect to the absorbed pump power is presented. In addition, Cr(2+): CdMnTe has been laser diode pumped as well as operated in the continuous-wave regime, to the best of our knowledge for the first time.

PASSIVE Q SWITCHING OF A DIODE-PUMPED 946-NM ND:YAG LASER WITH 1.6-W AVERAGE OUTPUT POWER

As much as 1.6-W average output power was emitted in a simple setup from a diffusion-bonded Nd:YAG rod with 70-100-ns Q-switched pulses at 946 nm and repetition frequencies between 15 and 45 kHz at 22-W incident diode-pump power. A Cr(4+):YAG crystal with a bleachable loss of approximately 2.5% and a length of 0.5 mm was used as a saturable absorber. The extraction efficiency was 47% in comparison with the continuous-wave laser output power of 3.37 W in the free-running regime.

Comparative study of the spectroscopic properties of Cr4+-doped LiAlO2 and LiGaO2

Abstract A detailed spectroscopic study of the optical characteristics of the tetrahedrally coordinated Cr4+ ion in LiAlO2 and LiGaO2 is given. From absorption and excitation measurements the crystal field parameter Dq and the Racah parameter B were determined to be Dq=1065 cm−1, B=450 cm−1, and Dq/B=2.4 for LiAlO2 and Dq=1055 cm−1, B=428 cm−1, and Dq/B=2.5 for LiGaO2. For the Racah parameter C only a lower limit can be given, i.e. 2417 cm−1 for LiAlO2 and 2667 cm−1 for LiGaO2. Due to the strong crystal field splitting — caused by the low site symmetry — the 3 B ( 3 T 2 ) crystal field component is the metastable and thus the emitting level. In the low-temperature absorption and emission spectra the expected three spin–orbit components of the 3 B level are found at 8273, 8296, and 8300 cm−1 for Cr4+:LiAlO2 and 8610, 8623, and 8632 cm−1 for Cr4+:LiGaO2. The emission lifetime of Cr4+ in LiAlO2 is 95 μs at 10 K and single exponential. In Mg-codoped LiAlO2 and in LiGaO2 the Cr4+ decay is double exponential. In Cr,Mg:LiAlO2 two centers can be clearly distinguished, while in Cr:LiGaO2 a variety of centers are observed, probably due to different charge compensation processes between Li, Ga, and Cr. The quantum efficiencies at room temperature are 42% for Cr:LiAlO2 and 23% for Cr:LiGaO2. Already at low temperature nonradiative decay processes occur. The temperature dependence of the lifetimes were analyzed with the model of Struck and Fonger. Excited state absorption measurements indicate that in the spectral region of the emission the excited state absorption cross-section is larger than the stimulated emission cross-section. Therefore laser oscillation is unlikely in these systems.

Observation of photon cascade emission in the Pr3+-doped perovskite KMgF3

Abstract Pr 3+ -doped KMgF 3 and YF 3 were investigated by means of high energetic excitation with synchrotron radiation. In the emission spectra bands due to the intraconfigurational 4f 2 →4f 2 transitions originating from the 1 S 0 and 3 P 0 multiplets of the Pr 3+ ion have been identified for both materials. In the excitation spectra in the ultraviolet/vacuum-ultraviolet spectral range weak lines due to the 1 S 0 as well as broad bands due to the interconfigurational 4f 2 –4f5d transitions are observed. The quantum efficiency of the Pr 3+ :KMgF 3 emission in the 400–750 nm spectral range is found to be higher than unity by comparison with that of Pr 3+ :YF 3 .