IEEE JOURNAL OF QUANTUM ELECTRONICS, NOVEMBER
1973
1119
Correspondence
Improved Temporal Stability of Polymethine Laser Dyes in
Aqueous Solutions
C. E. HACKETT andC. F. DEWEY, JR.
Abstract-Polymethine laser dyes which fluoresce in the near infrared in
organic solvents generally exhibit poor temporal stability at room
temperature. Two typical laser-pumped polymethine dyes in aqueous
solutions containing nonionic surfactants are investigated. Temporal
stability was improved 20-fold, as compared to organic solutions, with no
significant changes in laser-pumpingthreshold or tuning range.
700
600
800
WAVELENGTH (NANOMETERS)
WAVELENGTH (NANOMETERS)
Polymethine dye solutions were the first organic materials to
operate successfully as lasing media [l], [2]. Since 1966 over 30
polymethine chromophoric dyes have exhibited lasing action in
organic solvents;a bibliography of these dyes may be found
[3].in
In using these dyes,we have shared with others the frustration of
observingthelasing
efficiency of preparedorganicsolutidns
deteriorate erratically in times ranging from a few hours to a few
days.
In an attempt to increase the
shelf life of solubilized polymethinelaserdyes,andsimultaneouslydecreasethetemperature
dependence of the solvent refractive index, we have investigated
two typical dyes
in aqueous solutions containing the nonionic surfactant Triton” X-100 [4].’ The dyes were DTDC iodide (3,3‘diethyl-2,2’-thiadicarbocyanineiodide) [2] and DMOTC iodide
(3,3’-dimethyl-2,2’-oxatricarbocyanine
iodide) [5], which
we
shall refer to as D-715 and D-735, respectively [3].
I
I
- _,
‘., - ,
’
0
0
The absorption ( A ) and fluorescent (0
emission spectra for
500
600 700 700
600
800
800
the two dyes in ethanolic and in aqueous solution are given in
WAVELENGTH(NANOMETERS)
WAVELENGTH (NANOMETERS)
Fig. 1. Thesespectrawereobtained
using conventional spec(b)
(d )
trofluorometric techniques. The flporescent emission data were
Fig. 1. Normalizedlinearabsorption
( A ) and fluorescence ( f l spectra for two
obtainedwithexcitationat
632.8 nm;fluorescencewasalso
chromophoric polymethine dyes, D-715 and D-735, solubilized in ethanol and
observedusinganexcitationwavelength
of675 nm to more
water. Fluorescent excitation at 632.8 nm using a He-Ne laser.
closely simulate the emission spectra produced by ruby-laser excitation, and the data exhibited no substantial differences from
those shown in Fig. 1.
TABLE I
Curveslabeled ( F - A ) representfluorescencespectrawhich
ABSORPTION
CROSS
SECTIONS
PER
MOLECULE
AT DYEFOR DYES D-715
have been noticeablyaltered by self-absorption, and this selfAND D-735
absorption is a general feature of polymethine dyes we have used
to produce laser action in the near infrared (at least at the conABSORPTIONCROSS-SECTION
centrationsrequiredfor
efficient laserpumping).Such
a. selfIiYE
PER MOLECULE
AT
6 9 4 . 3 nm
SOLVENT
absorption is thought to be responsible for the increase in laseroutput wavelength with increasing dye concentration which was
observed by Miyazoe and Maeda [5]. InFig.l(c),thecurves
715
C2H50H
5 . 9 x 10-l~
(cm2)
labeled (F) and (F-A) have been normalized to illustratethe
general distortion of the fluorescence spectrum caused by selfabsorption.
715
H20 + 1.5%
2.8 x
(cm2)
The absorption cross section per dye molecule was calculated
TRITON X-100
from these experiments. Table I gives values of the absorption
cross section at the ruby-laser emission wavelength (694.3 nm)
for both dyes in ethanolic and aqueous solutions.
I
Manuscript received March 21, 1973: revised July 5, 1973. This research was supported by the Advanced Research Projects Agency and monitored by the Army
Research Office, Durham, N.C.
The authors arewith the Department of Mechanical Engineering, Massachusetts
Institute of Technology, Cambridge, Mass. 02139.
E! Registered trademark of Rohm & Haas Co.
’ Eastman Kodak Co., Rochester, N.Y. 14650, Cat. No. 13075.
735
C2H50H
735
H20
+
1.5%
9.6
X
(an2)
2.4
X
(cm’)
TRITON X-100
+
Noze: Dyes in ethanolic (C,H,OH) and aqueous (H,O
1.5-percent
Triton X-I00 surfactant) solutions, measured at the ruby-laser .emission
wavelength (694.3 nm).
I120
IEEE J O U R N A L O F QUANTUM ELECTRONICS, NOVEMBER
OUTPUT WAVELENGTH (NANOMETERS)
Fig, 2. Relative output intensity for wavelength-tunable lasing action in the near
infrared for two dyes, D-715 and D-735, in ethanol [(A) and (O)] andin aqueous
solution [(O)and (1 I)].
I-
3
protected from ultraviolet light and temperatures above
2S°C
subsequenttosolubilization,andthedecompositionwasnot
related to the number of laser excitation pulses. The solutions
were exposed to the atmosphere.
It is well known that alcohols absorb water when exposed to
theatmosphere.Thepresence
of freewatermoleculeswith
strong electrical polarization (i.e., in the absence of the micellar
structurecreated by asurfactant)couldberesponsiblefor
degradation of the lasing properties of the dye solution.
Evidenceobtainedbyothers
[7]-[10l2 usingsolutions
of
Rhodamine 6G in organic, aqueous, and solid solutions suggests
that laser dyes may exhibit degradation
solely from the effects of
pumping radiation. The rate of photobleaching is influenced by
the physical nature of the solvent and the p H of the solubilized
mixture.While we havenotobservedadegradation
of any
polymethine laser dyes attributable to several hundred ruby-lase
pumping cycles, a result in agreement with Sorokin et al. [6] and
Hollier and Macomber [ 111, this effect could be sensitive to solvent properties such as viscosity and deserves further study.
REFERENCES
.
T I M EA F T E R
Fig. 3.
1973
SOLUBILIZATION (HOURS)
Relative laser output intensity for aqueous and ethanolic solutions of dye
D-735 as a function of time after solubilization.
P.P. Sorokinand J.R.Lankard.“Stimulated
emission observed from an
organic dye, chloroaluminum phthalocyanine,” IBM J . Res. Dewlop., vol. IO,
pp. 162-163, Mar. 1966.
F. P. Schaefer, W. Schmidt, and J. Volze, “Organic dye solution laser,” Appl.
Phys. Lelt., vol. 9, pp. 306-309, Oct. 16, 1966.
C. F. Dewey, Jr., “Excitation of gases using wavelength-tunable lasers,” in
Modern Oplical Mezhods in GasdynamicResearch. D.Dosanjh,Ed.
New
York: Plenum Press, 1971, pp. 221-270.
0. G. Peterson, S. A . Tuccio, and B. B. Snavely, “CW operation of an organic
dye solution laser,” Appl. Phys. Leu.. vol. 17. pp. 245-247, Sept. 1970.
Y.MiyaroeandM.Maeda,“Stimulated
emission from 19 polymethine
dyes-Laser action over thecontinuousrange
710-1060 mp,” Appl. Phys.
L e t f . , vol. 12, pp. 206-208, March 1, 1968.
P. P. Sorokin, J. R. Lankard, E. C. Hammond, and V. L. Moruzzi, “Laserpumped stimulated emission fromorganic dyes: Experimental studies and
analyticalcomparisons,” I B M J . Res.Deaelop., vol. 11. pp, 130-148, Mar.
1967.
E. P. Ippen, C. V. Shank, and A. Dienes, “Rapid photobleaching of organic
laser dyes in continuouslyoperated devics,” IEEE J . QunnfumElecfron.
(Corresp.), vol. QE-7, pp. 178-179, Apr. 1971.
1. P.
Kaminow,
J.
W.
Stulz,
E. A. Chandross,and
C . A . Pryde,
“Photobleaching of organic laser dyes in solid matrices,” Appl. Opr., vol. 1 I ,
pp. 1563-1567, July 1972.
D. Beer and J. Weber. “Photobleaching of organic laser dyes,” Opt. Commun.,
vol. 5 , pp. 307-309, July 1972.
A. D. Britt and W. B. Moniz, “The effect of pH on photobleachingof organic
laser dyes.” IEEE J . Quantum Electron. (Corresp.), vol. QE-8, pp. 913-914.
Dec. 1972.
R. A. Hollier and J. D.Macomber.“Lightsource
responsible forthe
A p p l . Opt., vol. 1 1 , pp.
deterioration of cryptocyanineQ-switches.”
1360-1364, June 1972.
Lasing characteristics of the four dye-solvent combinations
wereinvestigated using a Q-switched ruby-laserpumpanda
multiple-pass dye-laser pumpingconfiguration ([3], andFig.
I(c)). The peak excitation power was about
20 MW, and the temporal history of the dye-laser output followed closely the 40-11s
ruby pulse. A diffraction grating (blazed at 1.6-pm first order,
600 lines/mm, and used in second order) was the dispersive ele40ment and the rear cavity reflector, and the output mirror was
percent reflective.
Efficient wavelength-tunable lasing action in the near infrared
was observed in both organic and aqueous solutions, as shown in
Fig. 2. While no attempt was made to optimize either the dye
concentration or the optical cavity parameters, the output energy
of the dye laser E, was typically 5 percent of the pump energy
E I Nnear the peaksof the tuning curves. These results, when compared with previous experiments using organic solvents [5], [6],
suggest thatwater-basedpolymethinesolutions
will provide
wavelength-tunable laser performance covering the near infrared
region of the spectrum with power outputs and tuning ranges
High Repetition-Rate Effects in TEA Lasers
comparable to organic solutions. Broadly speaking, we find the
dye laser conversion efficiency (!?,/EIN) with aqueous solutions
ROBERT T. BROWN
containing 0.5-2 percent by volume Triton X-100 (or similar surfactants) to be20-30 percent smaller than theefficiency obtained
with ethanol (C,H,0H).2
Abslract-Experimental studies have been conducted into the dominant
Thetemporalstability
of aqueousdyesolutionsatroom
temperature is strikingly superior to the performance of dyesin effects and possible limitations of TEA CO, laser operation at high repetiorganic solvents. This is illustrated in Fig. 3, where it is seen that tion rates. These studies were carried out in a small channel which provided
for a uniform gas flow at atmospheric pressure transverse to a finnedthe dye D-735 in water exhibited satisfactory laser performance
cathode double-discharge structure. Measurements taken over a range of
overtimesmorethan
20-fold greaterthanacomparable
circuit and gas conditions showed that the major limitation on the repetition
In these
experiments,
both
dyes
were
ethanolic
solution.
rate in TEA C O z lasers is the onset of arcing in the discharge rather than
‘Thedatacomparingthe
relative lasing efficiencies of ethanoland
water
solutions were obtained at dye concentrations of
molar with equalpump
energies. I n our large cross-section dye-laser cell (1.5 X 1.5 cm) with multiple-pass
pumping, lasing efficiency has been found to be substantially independent of concentration in the concentration range employed here.
Manuscript received April 11, 1973. This work was supported in part by the U.S.
Naval Ordnance Laboratory.
The author is with the United Aircraft Research Laboratories,
East Hartford,
Conn. 06108.