A detailed description of the He-Ne/ 127 I 2 laser wavelength standards developed at the National Metrology Institute of Turkey (UME), using high-frequency modulation and featuring a long-term relative instability of a few parts in 10 14 , is presented. The frequency dependence of the lasers on the iodine gas pressure, modulation width and saturating field intensity have been studied with a very low uncertainty level, and the common effect of these parameters on the output frequency has been demonstrated. A new method of accurately measuring modulation width has been realized, making an important contribution to the improvement of reproducibility of this type of wavelength standard. An international comparison between the laser standards of the Physikalisch-Technische Bundesanstalt (PTB) and the UME has been performed. Several iodine cells studied in a laser with an accurate servo-system, reveal a correlation between the spacing of the molecular resonances, the gas contamination in the iodine cell, and the output frequencies of the laser when locked to different iodine components.

No abstract

Photoinduced absorption (PA) of hydrogenated amorphous silicon (a-Si:H) was studied by excitation spectra analysis. The nonlinear PA response taken at different excitation photon energies was found to follow the corresponding absorption coefficient for intrinsic and p-doped samples in the 80–300 K temperature range. Significant contribution of the electronic PA in the observed response was demonstrated. Thermal effects are discussed. A simple model of trapping during the pump with consequent recombination from the localized state was found to be in good agreement with experimental observations.

No abstract

No abstract