Fritz Luty

Bi-directional holographic information storage based on the optical reorientation of FA centers in KCl:Na☆

Abstract The dichroism of F A centers in KCl is used to store erasable, high-resolution and high-contrast Bragg-angle holograms. The photochromic system has a bi-directional photo-response, high-writing sensitivity, practically non-destructive low-temperature read-out, and permits good recyclability. The holograms have nearly unlimited lifetime at freezer temperatures.

Low temperature electro-optical effects from off-axis Li+ in FA centers☆

Abstract Measurements of the electric field modulated absorption of KC1:Li F A centers (field applied perpendicular to center axis), and its spectral-, field- and temperature dependence are explained by an ‘off-axis’ center model in which the Li + ion can occupy four equivalent pocket states, displaced in 〈110〉 direction from the F A center axis.

Formation, optical properties, and laser operation of F- 2 centers in LiF

Formation conditions, optical properties, and lasing behavior of F−2 color centers in LiF crystals are investigated. The optical pumping cycle of the three‐electron defect is found to involve intermediate transitions of small efficiency into some metastable energy levels (having a lifetime of several seconds). This process could either be intrinsic to the F−2 defects (e.g., intersystem crossing into quartet states) or it could be caused by (low efficiency) optical ionization of F−2 centers and electron capture in unstable traps. While cw laser oscillation is prevented by a resulting bottleneck, efficient tunable pulsed laser operation in the 1.1–1.2‐μm range is possible at room temperature.

Study of the paraelectric behavior of OH− ions in alkali halides with optical and caloric methods-I. Statics of dipole alignment

Abstract The electric field induced alignment of substitutional OH− ions in a variety of alkali halide crystals was studied, using electro-optical and electro-caloric techniques. As basis for the electro-optical studies, the u.v. absorption of OH− in various crystals was thoroughly investigated at different temperatures, and in several cases oscillator strength values were determined. The energy position of the u.v. absorption of OH− in 13 alkali halides follows closely an Ivery-relation, which is discussed in terms of a charge-transfer model. The observed electric field induced zero-moment changes of the u.v. absorption, which depend strongly on light polarization, field direction and temperature, can be quantitatively accounted for by a paraelectric alignment model of permanent dipoles with moment p. The anisotropy of the OH− electro-dichroism reveals dipole orientation in NaCl, KCl, KBr, RBCl, RbBr and RbI, orientation in KI, and orientation in CsBr. The determined dipole moment values p show little variation with the host material, which excludes sizeable contributions to p from off-center shifts of the OH− ion. The observed saturation of the electro-dichroism indicates a peculiar ‘mixed’ polarization behavior of the optical transition, which will be discussed. Reversible electro-caloric measurements, performed on several OH− systems, reveal a field dependence and anisotropy in the high field range, which yield dipole orientation and dipole moment values in agreement with the electrooptical results.

Tunable cw laser operation in the 1.45–2.16 μm range based on F+2‐like centers in O−2 doped NaCl, KCl, and KBr crystals

In additively colored NaCl, KCl, and KBr crystals doped with O−2 anionic molecular impurities, a laser active defect is formed by optically induced association of F centers with oxygen‐vacancy defect pairs. The formed defect has F+2 ‐like properties in terms of optical transition energies, Stokes shift, and optical gain. It can be regarded as an F+2 center attached in different possible configurations to a neighboring double negatively charged oxygen anion impurity which replaces a halide ion. Stable cw laser operation could be realized, so far, in the hosts NaCl, KCl, and KBr. Pumped with 10 W at 1.064 μm the NaCl center laser was tunable from 1.45 to 1.74 μm with 1.3 W peak output power at 1.60 μm. The KCl and KBr systems, both pumped at 1.32/1.34 μm, were tunable from 1.66 to 1.97 μm with 170 mW peak output, and from 1.86 to 2.16 μm with 30‐mW peak output, respectively. Depending on the host material the laser crystals do or do not require auxiliary light exposure exciting the higher‐energy transitions...

Tuneable laser operation of F+2 and (F+2)A centers in OH- and SH- doped alkali halides

Abstract Stabilization and tuneable laser operation of F + 2 and (F + 2 ) A centers using OH - and SH - doped alkali halide crystals are reported. The new stabilization technique improves previously described laser systems and produced a new F + 2 (KBr) laser (1.72–2 μm), covering an important gap around 2 μm. All crystals can easily be reactivated for laser operation after extended periods of storage at room temperature.

Electromodulation spectroscopy of localized excitations in crystals

Abstract Electromodulation spectroscopy has become a powerful tool for the study of electronic transitions of point defects with strong electron-phonon coupling, i.e., broad absorption and emission bands. The potential and physical significance of the method will be illustrated using a few selected examples of well characterized defect structures in ionic crystals. Starting with a simple symmetric F-center it is shown that electromodulation in absorption and emission allows the investigation of excited states with parity-forbidden optical transitions, and is important for the study of the electron-phonon interaction. For more complex defects with lower symmetry, more detailed information (like the presence and magnitude of internal electric fields and relative displacements of the wave-functions of different electronic states) can be obtained using both first and second harmonic modulation techniques. The potential of field-modulation spectroscopy in the study of point-defects with a central instability (“off-center defects”) and of tunneling molecular defects in crystals will be briefly discussed.

Stark effect of Z1 and Z2 centers in KC1

Abstract Electric field modulation of Z1 and Z2 center absorption was measured in KC1: Sr. The obtained Stark spectrum for quadratic field modulation and preliminary measurements on linear field modulation of electrically aligned Z1 centers are interpreted by a specific inversion asymmetric center model, which accounts for several other experimental observations too.

Study of the reorientation kinetics of OH− dipoles in alkali halides by optical methods

Abstract The reorientation kinetics of OH − dipoles in various alkali halides were studied by measurements of the time dependence of the electric field induced optical dichroism of the u.v. absorption. For temperatures below 5°K reorientation by 90° transitions and one-phonon relaxation ( τ r ∝ T −1 ) were found predominant, while for T > 5°K a ( τ r ∝ T −4 )-relation indicates multiphonon processes.

Laser operation near 5 μm of vibrationally excited F-center/CN− molecule defect pairs in CsCl crystals, pumped in the visible

Abstract The association of F-centers to CN - defects in alkali halide crystals forms “F H (CN - )” complexes which under excitation of their electronic absorption transitions in the visible produce optical emission from the vibrationally excited CN - molecular states around 5 μm. In the host CsCl, with the most efficient F-center/CN - molecule coupling, laser oscillation has been obtained at temperatures up to 77 K on the three strongest vibrational emission transitions at 2000, 2025 and 2050 cm -1 . Threshold pump powers are in the mW range.