Page 59 - Tunable Lasers Handbook

40 R. C. Sze and D. G. Harris

band remain virtually unchanged, whereas the 351-nm band shows marked
changes in both.
The energy stored in XeF resides in a multitude of rotational states, which
must be collisionally coupled on time scales that are short compared to the stim-
ulated emission rate in order to achieve narrowband lasing. The appearance of
clusters of rotational lines lasing relatively independently suggests that the rota-
tional relaxation rates in the B and/or X states may be too slow to allow narrow-
band lasing. Indeed, it is difficult to achieve efficient injection locking when the
small signal gain is much greater than the threshold gain [37.38].
2.7.4 XeF (C-+A)
The XeF molecule also emits a broad continuum between 470 and 500 nm
from the C+A transition (l rL2n). The A state is repulsive, without a potential
well, so the emission is a true continuum, allowing narrowband lasing as well as
continuous tuning across the emission spectrum. The excitation sources have
been both short-pulse and long-pulse electron beams. Under short-pulse excita-
tion (10 MW/cm; for 10 ns) the media has optical absorption during the electron
beam deposition time and then gain (3Wcm) in the plasma afterglow. Narrow-
band tuning as well as injection seeding has been used to tune across the gain
profile [39-43]. The media show gain throughout the energy deposition pulse
under low-power long-pulse electron beam excitation (250 kW/cm3 for 700 ns).
However strong lasing is reached only after 300 ns [44].

2.1.5 XeCl(308 nrn)
The C state of XeCl molecule lies approximately 230 cm-1 below the B state.
Additionally, the ground state is bound by 255 cm-1, lasing in the B+X bands
occurs predominantly on the 0-1 band but also weakly on the 0-2 and e3 bands
[45]. Although XeCl lasers have been made to operate narrow band, attempts to
injection seed amplifiers have shown a strong wavelength dependence [46], which
has been attributed to saturation of the lower vibrational levels [47]. Owing to the
long gas lifetime and ability to use inexpensive nonquartz optics, XeCl has been
the preferred excimer to test line-narrowing techniques and novel resonators.

2.7.6 Other Rare Gas Halide Excirners
Lasing has been observed in several other rare gas halides, and although
these systems have not been developed to the extent of those already discussed
they do offer potentially tunable radiation. Excimer emission has been observed
at 175.0 nm in ArCl [27], 222 nm in KrCl [48,49], and 281.8 nm in XeBr [50],
which are believed to be excimers with repulsive ground states. A short operat-
ing lifetime for XeBr has not yet been thoroughly addressed [51]. There has
been renewed interest in KrCl because it offers potentially higher efficiency than
XeCl [52.53]. The pulse lengths have been extended to 185 ns, but nothing has
been pursued in the area of spectral control [54,55].