We demonstrate efficient generation of picosecond narrow-bandwidth pulses by frequency mixing of broadband opposite chirped pulses in a type I doubling crystal. This procedure allows us to produce picosecond pulses that are perfectly synchronized with femtosecond pulses. The experiment shows a decrease of the initial bandwidth by a factor of more than 30, while a high conversion efficiency is maintained.
High-mobility low-temperature ( 600 • C) unhydrogenated in situ doped polysilicon thin film transistors (TFTs) are made. Polysilicon layers are grown by a low pressure chemical vapour deposition (LPCVD) technique and crystallized in a vacuum by thermal annealing. The source and drain regions are in situ doped. The gate insulator is made of an atmospheric pressure chemical vapour deposition (APCVD) silicon dioxide. Hydrogen passivation is not performed on the transistors. One type of transistor is made of two polysilicon layers, the other one is fabricated from a single polysilicon layer. The electrical properties are better for transistors made of a single polysilicon layer: a low threshold voltage (1.2 V), a subthreshold slope S = 0.7 V/dec, a high field effect mobility (≈100 cm 2 V −1 s −1 ) and an on/off-state current ratio higher than 10 7 for a drain voltage V ds = 1 V. At low drain voltage, for both transistors, the off-state current results from a pure thermal emission of trapped carriers. However, at high drain voltage, the electrical behaviour is different: in the case of single polysilicon TFTs, the current obeys the field-assisted (Poole-Frenkel) thermal emission model of trapped carriers while for TFTs made of two polysilicon layers, the higher off-state current results from a field-enhanced thermal emission.
We demonstrate efficient frequency tripling of 1057-chirped pulses, using adapted chirping and thick KDP crystals. These millijoules broadband pulses at 352 nm have been compressed to 220-fs duration by use of a UV grating-pair compressor. The technique is scalable to kilojoule petatwatt lasers.
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