Kinetics of optically pumped Ar metastables

Abstract

Optically pumped lasers that use metastable excited states of Ar have been demonstrated using both pulsed and CW excitation. In terms of Paschen labeling of the states of Ar, the laser system uses excitation of the 2 p 9 - 1 s 5 transition, and lases on the 2 p 10 - 1 s 5 line. Collisional transfer of population from 2 p 9 to 2 p 10 is achieved using He as the buffer gas. For the purpose of modeling and developing this laser, rate constants for state-to-state transfer in Ar ( 2 p i ) + Ar / He mixtures are needed. As the 2 p 10 level can radiate down to 1 s 4 , this lower level also plays a significant role in the laser kinetics. Consequently, rate constants for the relaxation of 1 s 4 by Ar and He are also required. In the present study we have used pulsed laser excitation techniques to measure rate constants of relevance to the optically pumped metastable Ar laser.

Full text

Kinetics of optically pumped Ar metastables
Jiande Han and Michael C. Heaven*
Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA
*Corresponding author: mheaven@emory.edu
Received September 26, 2014; accepted October 13, 2014;
posted October 16, 2014 (Doc. ID 223910); published November 14, 2014
Optically pumped lasers that use metastable excited states of Ar have been demonstrated using both pulsed and CW
excitation. In terms of Paschen labeling of the states of Ar, the laser system uses excitation of the 2p9-1s5transition,
and lases on the 2p10-1s5line. Collisional transfer of population from 2p9to 2p10 is achieved using He as the buffer
gas. For the purpose of modeling and developing this laser, rate constants for state-to-state transfer in Ar2pi
Ar∕He mixtures are needed. As the 2p10 level can radiate down to 1s4, this lower level also plays a significant role
in the laser kinetics. Consequently, rate constants for the relaxation of 1s4by Ar and He are also required. In the
present study we have used pulsed laser excitation techniques to measure rate constants of relevance to the optically
pumped metastable Ar laser. © 2014 Optical Society of America
OCIS codes: (140.1340) Atomic gas lasers; (140.3380) Laser materials; (140.3460) Lasers; (140.3480) Lasers,
diode-pumped.
http://dx.doi.org/10.1364/OL.39.006541
Diode-pumped alkali vapor lasers (DPALs) have been ac-
tively developed over the past decade [1,2]. These devi-
ces have yielded powers up to 1 kW [3] and have the
potential to provide far higher powers with excellent
beam quality. There are, however, some technical chal-
lenges for DPALs that are associated with the chemically
aggressive nature of alkali metal vapors [4,5].
It has been recognized that rare gas atoms, promoted
to metastable electronically excited states (Rg), have
spectroscopic properties that closely resemble those of
the alkali metals [6]. The metastables are generated by
promoting one of the np6valence electrons to produce
the np5n1sconfiguration. The spin triplet from this
configuration has well-known metastable components
[7]. Excitation of the n1selectron to the n1p
manifold can then be used to drive an optically pumped
laser. The energy transfer step needed to create a popu-
lation inversion can be accomplished using He as the col-
lision partner. Pulsed lasers of this kind have been
demonstrated using Ne,Ar
,Kr
, and Xe[8,9], and
CW operation has been achieved for Ar[10]. The advan-
tage of this approach is that the lasing medium is chemi-
cally inert.
Figure 1shows the energy levels that are used for the
optically pumped Rglasers. The levels are labeled using
both Racah and Paschen notation. For convenience, we
use the latter in the following text. The system is excited
via the 2p9-1s5transition and lases on the 2p10-1s5line.
Note, however, that there is radiative branching to the
1s4level (≈20% of the spontaneous decay). In order to
adequately understand these lasers, the rate constants
for collisional energy transfer of population between
the 2pilevels and the rate constants for 1s4-1s5transfer
need to be determined.
To date, the ArHe laser system has been examined
in the most detail [8–11], and the focus of the present
study is on the ArAr and ArHe energy transfer
rate constants. The Ar2piAr kinetics have been ex-
amined at low pressures (<8Torr) in two previous stud-
ies [12,13], and the Ar1s4Ar relaxation rate constant
has also been determined [14]. Surprisingly, we were un-
able to find published reports for Ar2piHe and
Ar1s4He kinetics, but a value of 2.10.2×
10−15 cm3s−1was reported for Ar1s5He →Ar1s4
He endothermic transfer [15].
Rawlins et al. recently reported a CW optically pumped
Arlaser [10]. In this device, a microplasma array was
used to generate Armetastables in the presence of
He at a pressure of 1 atm. An optical-to-optical power
conversion efficiency of 55% was achieved. Modeling
of this laser, using the room temperature rate constants
reported here, indicated that the ArHe 1s4-1s5rate
constant was too small to be consistent with the laser
performance. For the laser system, lineshape data
yielded a local temperature of 600 K within the discharge.
Consequently, Rawlins et al. [10] suggested that the tem-
perature dependence of the 1s4relaxation rate constant
may account for the discrepancy between the observa-
tions and the model.
Here, we report room temperature measurements of
the Ar2piHe∕Ar and Ar1s4He∕Ar rate constants,
and preliminary data for the temperature dependence of
the Ar1s4He rate constant.
Metastable Ar1s5atoms were generated by a pulsed
electrical discharge, and the 2p8,2p9,or2p10 states were
populated by pulsed laser excitation of the 2pi←1s5tran-
sitions. Parallel plate electrodes were used to generate a
glow discharge in pure Ar and Ar/He mixtures. The elec-
trodes were square 2.5×2.5cm stainless steel plates sep-
arated by 0.5 cm. One plate was held at ground while a
Fig. 1. Partial energy level diagram for Ar.
November 15, 2014 / Vol. 39, No. 22 / OPTICS LETTERS 6541
0146-9592/14/226541-04$15.00/0 © 2014 Optical Society of America