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Evidence for diffusion independent triplet quenching in the rhodamine 6G ethylene glycol cw dye laser system
Volume
7, number
3
EVIDENCE
OPTICS COMMUNICATIONS
FOR DIFFUSION
IN THE RHODAMINE
INDEPENDENT
6G ETHYLENE
C.V. SHANK, J. EDIGHOFFER, Bell Telephone
GLYCOL
March
TRIPLET
QUENCHING
CW DYE LASER
SYSTEM
A. DIENES and E.P. IPPEN
Laboratories, Holmdel, New Jersey 07733, Received
1973
3 January
USA
1973
Experiments have been performed that indicate that the triplet quenching mechanism in the highly viscous rhodamine 6G ethylene glycol laser system is diffusion independent. Despite the fact that known triplet quenchers are ineffective in this system very efficient laser action, up to 34.5% efficiency was observed.
Recently Runge and Rosenberg [l] described an experiment where dye laser action was obtained in a free flowing stream of cresyl violet dissolved in ethylene glycol. The high viscosity ethylene glycol solvent was used in order to obtain a stable stream of high optical quality from the nozzle. In their experiments the laser was pumped with intracavity mode-locked pulses. In this communication we report the observation of cw laser action in a rhodamine 6G ethylene glycol mixture pumped by an argon laser. Previously published work on rhodamine 6G dissolved in methanol [2, 31 indicated that either oxygen or some other quencher was required to quench the triplet state to achieve continuous laser action. It was observed that laser action could be quenched by bubbling pure nitrogen through the methanol solution. The results of these previous experiments cast some doubt on whether cw laser action in rhodamine 6G could be obtained in a high viscosity solvent such as ethylene glycol.(Viscosity 19.9 CP compared to methanol with viscosity 0.58 cP.) Since the diffusion rate is inversely proportional to the viscosity, a triplet quenching mechanism relying on the diffusion of a quencher molecule such as oxygen or COT (cycloctatetraene) would be expected to be ineffective in ethylene glycol. In fact experimentally we have determined that dissolved oxygen or COT do not effect rhodamine 6G laser action in ethylene glycol. Experiments were carried out in which nitrogen was bubbled through the ethylene glycol mixture for 24 hours with no change in laser output power 176
or tunability. Similarly oxygen was bubbled through the solvent with no apparent change in laser power. Finally COT was dissolved in the dye solution up to 5 percent concentration with again no apparent influence on laser operation. The results of these observations indicate to us that the rhodamine 6G triplet state is quenched in the solvent environment itself rather than relying on a diffusive quencher. Possibly the vibrational levels of the ethylene glycol act to depopulate the triplet. One other possibility is that the intersystem crossing rate k,, is reduced in the ethylene glycol solvent. The dye laser cavity and pumping configuration is similar to that previously described [4, 51. The flow cell nozzle used differs slightly from that used by Runge et al. and allowed flow velocities to reach 10 meters a second. The nozzle was made with two razor blades forming a 0.01” by 0.5” slit. On the ends of the slit two guides of approximately 0. I” helped contain the flow at high velocities. The operating characteristics of the laser were determined by inserting a calibrated loss consisting of two glass plates which introduce a variable loss depending on the angular deviation from Brewsters angle. In fig.1 we have plotted power coupled out of the cavity versus output coupling loss at 5900 ,A. Note that the single pass gain is nearly 10% and that over 400 mW of power are coupled out with 1.2 W of pumping power. At the power output peak the threshold is 320 mW indicating nearly a 47% slope efficiency and a 34.5%
Volume
7, number
OPTICS COMMUNICATIONS
3
450
..O. 400
0 0
WAVELENGTH 5900% 1.2 WATTS PUMP POWER
0
350
References 0
300
[
P.K. Runge tron. 8 [ B.B. Snavely 728. [ 31 O.G.
0
l2 8 250
0
S! 0
g 5 200
0
z 0
150 l 100
0 0 0
M 0
, 2
I
I
I1
4
6
6
I
IO 12 % LOSS
I 14
II-1 16
I8
20
1973
overall efficiency. The laser was tunable from 5620 to 6270 A. In summary we have observed efficient laser action in a rhodamine 6G ethylene glycol solution in which triplet quenching appears to be independent of diffusion.
0 0
C
March
R. Rosenberg, IEEE J. Quantum 910. F.P. Schaefer, Letters 28A
7, number
3
EVIDENCE
OPTICS COMMUNICATIONS
FOR DIFFUSION
IN THE RHODAMINE
INDEPENDENT
6G ETHYLENE
C.V. SHANK, J. EDIGHOFFER, Bell Telephone
GLYCOL
March
TRIPLET
QUENCHING
CW DYE LASER
SYSTEM
A. DIENES and E.P. IPPEN
Laboratories, Holmdel, New Jersey 07733, Received
1973
3 January
USA
1973
Experiments have been performed that indicate that the triplet quenching mechanism in the highly viscous rhodamine 6G ethylene glycol laser system is diffusion independent. Despite the fact that known triplet quenchers are ineffective in this system very efficient laser action, up to 34.5% efficiency was observed.
Recently Runge and Rosenberg [l] described an experiment where dye laser action was obtained in a free flowing stream of cresyl violet dissolved in ethylene glycol. The high viscosity ethylene glycol solvent was used in order to obtain a stable stream of high optical quality from the nozzle. In their experiments the laser was pumped with intracavity mode-locked pulses. In this communication we report the observation of cw laser action in a rhodamine 6G ethylene glycol mixture pumped by an argon laser. Previously published work on rhodamine 6G dissolved in methanol [2, 31 indicated that either oxygen or some other quencher was required to quench the triplet state to achieve continuous laser action. It was observed that laser action could be quenched by bubbling pure nitrogen through the methanol solution. The results of these previous experiments cast some doubt on whether cw laser action in rhodamine 6G could be obtained in a high viscosity solvent such as ethylene glycol.(Viscosity 19.9 CP compared to methanol with viscosity 0.58 cP.) Since the diffusion rate is inversely proportional to the viscosity, a triplet quenching mechanism relying on the diffusion of a quencher molecule such as oxygen or COT (cycloctatetraene) would be expected to be ineffective in ethylene glycol. In fact experimentally we have determined that dissolved oxygen or COT do not effect rhodamine 6G laser action in ethylene glycol. Experiments were carried out in which nitrogen was bubbled through the ethylene glycol mixture for 24 hours with no change in laser output power 176
or tunability. Similarly oxygen was bubbled through the solvent with no apparent change in laser power. Finally COT was dissolved in the dye solution up to 5 percent concentration with again no apparent influence on laser operation. The results of these observations indicate to us that the rhodamine 6G triplet state is quenched in the solvent environment itself rather than relying on a diffusive quencher. Possibly the vibrational levels of the ethylene glycol act to depopulate the triplet. One other possibility is that the intersystem crossing rate k,, is reduced in the ethylene glycol solvent. The dye laser cavity and pumping configuration is similar to that previously described [4, 51. The flow cell nozzle used differs slightly from that used by Runge et al. and allowed flow velocities to reach 10 meters a second. The nozzle was made with two razor blades forming a 0.01” by 0.5” slit. On the ends of the slit two guides of approximately 0. I” helped contain the flow at high velocities. The operating characteristics of the laser were determined by inserting a calibrated loss consisting of two glass plates which introduce a variable loss depending on the angular deviation from Brewsters angle. In fig.1 we have plotted power coupled out of the cavity versus output coupling loss at 5900 ,A. Note that the single pass gain is nearly 10% and that over 400 mW of power are coupled out with 1.2 W of pumping power. At the power output peak the threshold is 320 mW indicating nearly a 47% slope efficiency and a 34.5%
Volume
7, number
OPTICS COMMUNICATIONS
3
450
..O. 400
0 0
WAVELENGTH 5900% 1.2 WATTS PUMP POWER
0
350
References 0
300
[
P.K. Runge tron. 8 [ B.B. Snavely 728. [ 31 O.G.
0
l2 8 250
0
S! 0
g 5 200
0
z 0
150 l 100
0 0 0
M 0
, 2
I
I
I1
4
6
6
I
IO 12 % LOSS
I 14
II-1 16
I8
20
1973
overall efficiency. The laser was tunable from 5620 to 6270 A. In summary we have observed efficient laser action in a rhodamine 6G ethylene glycol solution in which triplet quenching appears to be independent of diffusion.
0 0
C
March
R. Rosenberg, IEEE J. Quantum 910. F.P. Schaefer, Letters 28A