Optical properties of Se2- and Se2 color centers in the red selenium ultramarine with the sodalite structure.

The unique optical properties of Se2- radicals located in the cages of the sodalite structure are reported. By means of luminescence, photoluminescence excitation, and absorption spectroscopy, three different centers are identified. Two of them are Se2- anions in sites with presumably a tetrahedral Na4(4+) coordination and a Na3(3+) environment with cation deficiency, respectively, giving rise to a red luminescence band with two different progressions. The third center is the intermediate Se2 molecule, created photochemically by UV laser excitation. It induces an additional blue luminescence. The electronic properties of the Se2- centers, particularly in the excited states, are significantly influenced by steric constraints imposed by the limited space in the sodalite host polyhedra. Thus, the sodalite structure can be viewed as a model system for studying effects of this kind on chromophores imbedded in the cages of the zeolite-type lattice.

Long-term stable mode locking of a visible diode laser with phase-conjugate feedback.

We present a new method to obtain long-term stable phase-conjugate feedback (pcf) with a photorefractive crystal for a mode-locked semiconductor laser. The phase-conjugate mirror is a barium titanate crystal in a self-pumped internal reflection configuration, and the laser is a visible, antireflection-coated AlGaInP diode. We describe a new dynamic writing procedure for growth of an index grating in the photorefractive crystal that leads to stable pcf and discuss the reasons for this stability, which is the major advantage of this method over previously reported ones. Using this superior method, we achieve stable mode-locked operation of the laser with pulse widths of ~30 ps and a timing jitter of less than 2 ps.

Nonlinear optical interaction in semiconductor lasers due to population pulsations.

Population pulsations are demonstrated as a basic mechanism for nonlinear optical interaction in a semiconductor laser by detecting the optical beat frequency in the noise spectrum of the injection current.

Bandwidth-limited picosecond pulse generation in an actively mode-locked GaAs laser with intracavity chirp compensation.

A Gires-Tournois interferometer has been applied for intracavity chirp compensation of an actively mode-locked GaAs laser. Fourier-transform-limited pulses as short as 4.6 psec have been obtained at 790 nm.

Mott-transition-induced period doubling in transient-grating experiments in CdS.

The occurrence of higher-order Fourier components in an originally sinusoidal free-carrier index grating produced in CdS by interference of two picosecond light pulses is demonstrated by simultaneously monitoring the decay of the first- and second-order diffraction intensity of a transparent probe pulse. A pronounced contribution with doubled period can be attributed to an exciton phase grating formed by binding of electrons and holes of the free-carrier grating.