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内容記述 |
In l977, Yang and De Luca proposed the feasibility of generating tunable laser sources spanning the wavelength range from 165 to 260 nm by using rare-earth doped wide band gap dielectric hosts. Their proposal that was purely based on spectroscopic data resulted to the first rare-earth doped fluoride laser in the ultraviolet region (250-400nm) using Ce<sup>3+</sup>:YLiF<small>4</small> in 1979. Succeeding research have resulted to tunable ultravioret rare-earth doped fluoride lasers such as Ce<sup>3＋</sup>:YLiSAF and Ce<sup>3+</sup>:LiCAF. The feasibility of having a tunable vacuum ultraviolet (100-190nm) laser from rare-earth doped fluorides have been confirmed by the first demonstration of 172-nm emission from Nd<sup>3+</sup>:LaF<small>3</small> in 1985 and a second report in 1992 after improvement of the pumping scheme. Over twenty years after the first report, research is still geared towards finding a suitable host for the development of the shortest wavelength solid-state laser. This work aims to identify Nd<sup>3+</sup>-doped fluoride hosts that could serve as the shortest wavelength solid-state laser material. Among the fluoride hosts that this work considers are YLiF<small>4</small>, LaF<small>3</small>, and (La<small>1-x</small>,Ba<small>x</small>)F<small>3-</small>x. Their optical properties in the vacuum ultraviolet region are characterized in terms of transparency, fluorescence spectra, and lifetime. Although, Nd<sup>3+</sup>:YLiF<small>4</small> is found to be sufficiently transparent in the vacuum. ultraviolet region with a long lifetime (21.7ns) it has a longer wavelength absorption edge and its fluorescence emission at around 18 1 nm is longer than the reported emission from Nd<sup>3+</sup>:LaF<small>3</small>. Based on this preliminary study, a new material is developed, which is Nd<sup>3+</sup>:(La<small>1-</small>x,Bax)F<small>3-</small>x (x=0.1) or Nd<sup>3+</sup>:(La<small>0.9</small>,Ba<small>0.1</small>)F<small>2.9</small>. LaF<small>3</small> is mixed with BaF2 in order to shift the absorption edge to a shorter wavelength. For the growth of this new material, the micro-pulling down method was utilized because it is capable of fast and economical crystal growth. Characterization of the vacuum ultraviolet optical properties of Nd<sup>3+</sup>:(La<small>0.9</small>,Ba<small>0.1</small>)F<small>2.9</small> reveals that it has a shorter wavelength absorption edge at 180 nm with better transparency in the vacuum ultraviolet region, a more intense and broader vacuum ultraviolet fluorescence with a bandwidth of 12 nm, thereby enabling better tenability and amplification of as short as 4.3 fs pulses. Its emission wavelength at l78 nm is, however, longer and its fluorescence decay time at 6.1 ns is faster. In this regard, Nd<sup>3+</sup>:(La<small>0.9</small>,Ba<small>0.1</small>)F<small>2.9</small> would be more suitable as a vacuum ultraviolet scintillator because of its high light yield and fast decay time. To investigate the feasibility of a solid-state pump source, a CZ method-grown Nd<sup>3+</sup>:LaF<small>3</small> cut into a cuboid was excited by the third harmonics of a (290 nm) Ti:sapphire regenerative amplifier. Fluorescence characteristics are similar for the femtosecond and nanosecond pumping schemes. However, for the femtosecond case, fluorescence could have been through frequency up-conversion by energy transfer. The effect of doping concentration was also investigated both in the infrared and vacuum ultraviolet regions. The effect of Nd<sup>3+</sup> concentration needs further study in order to identify whether concentration quenching would pray an important role in improving light yield in the vacuum ultraviolet region. By carrying out further work in this direction, the optimum concentration level would be determined. With the improvement of the quality a1d size of Nd<sup>3+</sup> -doped fluorides, an efficient and reliable vacuum ultraviolet laser will be realizable. |
要旨・審査要旨 (272.6 kB)
