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Time delayed picosecond optical pulse transmission in liquid DODCI solutions
Volume 32, number 1
OPTICS COMMUNICATIONS
January 1980
TIME DELAYED PICOSECOND OPTICAL PULSE TRANSMISSION IN LIQUID DODCI SOLUTIONS "~" I.C. KHOO *, Y.G. FUH and R.F. CODE Department of Physics and Erindale College, University of Toronto, Toronto, Canada M5S 1A 7
Received 27 August 1979
A broad anomalousincrease in the optical transmission of DODCIdissolved in glycerine has been observed during the first 150 ps delay followingintense optical excitation at 604 nm. The anomaly preceded a normal absorption recovery, and is ascribed to time-dependent interactions between the solvent and the excited states of the DODCImolecule.
During the past few years, there has been considerable interest in the study [1-6] of various optical and physical properties of a common mode-locking dye DODCI (3,3'-diethyloxadicarbocyanine iodide). For example, the determination of the excited state lifetimes has given rise to several reports of interesting and somewhat controversial behaviour. Recent studies [5,6], however, indicate that the differences in the various lifetimes that have been measured may be explained by the influence of factors such as the due concentration, the specific solvent, and on the presence or absence of a short-lived excited state photoisomer. Mialocq et al. [5] have pointed out that photoisomers will not be present in DODCI solutions if single pulse excitation is used. Excitation by the entire picosecond pulse train from a flash-lamp pumped dye laser was demonstrated to favour photoisomer buildup. The absorption recovery time obtained in their single pulse pump-probe experiment agrees very well with the accepted results of Magde and Windsor [3], and Shank and Ippen [4]. Mialocq's report also stated that they did not observe the nonlinear optical processes mentioned by Busch et al. such as stimulated emission or amplification. One must remark here that there is a basic difResearch supported in part by grants from the Natural Sciences and Engineering Council of Canada. * Present address: Department of Physics and Astronomy, Wayne State University, Detroit, Michigan48202, USA.
ference between Mialocq's experiment and that of Busch. In the former, the probe is at the same frequency as the pump, whereas in the latter, the probe is a continuum generated by part of the pump beam. The continuum contains lower frequency components than the pump which naturally experience gain by the usual dye laser action. Gain for a probe pulse of the same frequency as the pump is not possible if the dye molecules act only as saturable absorbers. However, under some favorable conditions, the time dependent solute-solvent interaction [7-11 ], sometimes known as the "solvent cage effect" may well provide a suitable mechanism for amplification or gain The solvent cage effect has been discussed theoretically in detail by Chin et al. [10,11], who also made some qualitative measurements. Essentially the theory accounts for the effects of significantly long solutesolvent reorientation times in viscous solutions by representing the absorption emission process of a dye molecule by a four-band model rather than by the usual two-band model. (In both cases singlet-triplet crossing transitions, and transitions to higher singlet levels are neglected.) The transmitted probe intensity will be enhanced by the saturation of the dye absorption established by the pump pulse. In addition, the probe may be amplified by stimulated emissions between the excited and ground state bands. Since the emission cross sections are modified by interactions with the solvent, the probe transmission in the presence of amplification will no longer be a monotonical191
Volume 82, number 1
OPTICS COMMUNICATIONS
ly decaying function beginning at zero time delay. The transmission should exhibit a broad peak at delay times on the order of the solute-solvent relaxation time following photoexcitation. In this paper we report tile study of time delayed optical transmission of DODCI dissolved in ethanol, ethylene glycol and glycerine to determine the effect of increased solvent viscosity. The experimental setup is schematically shown in fig. 1, typical of a pumpprobe experiment. The flash-lamp pumped dye laser (Electro-Photonics Ltd. model 33) emits a whole pulse train (lasting 1/as) of picosecond pulses (about 5 ps pulse length). One of the more intense pulses in the middle of the pulse train is selected by an ElectroPhotonics switch out unit (model 134). This single pulse is divided by the first beam-splitter into an intense pump beam and a weak probe which is optically delayed. The pmnp and probe are focused into a 1 mm cell filled with solutions of DODCI. After traversing the cell, the probe beam is further delayed before it is recorded by the photo-diode and oscilloscope.
rid --
/ ~ . - . - @ ~--
TO
SCOPE
PD
January 1980
By adjusting the trigger on the scope, four traces of tile probe out versus probe in are recorded (two with and two without the saturating pump). This gives four data points for each delay and the results are averaged. In a separate experiment, we have studied the transmission of the pump beam through some of the solutions as a function of intensity. Beyond about 40 MW/cm 2, the transmission showed evidence of saturation. In the present experiment, the pump beam had an estimated intensity of 80 MW/cm 2 to ensure adequate saturation of the dye molecules. To overcome the influence of fluctuations in the pump intensity from shot to shot, the pump intensity was monitored by the related incident probe intensity. Only data points corresponding to a pump intensity sufficient to ensure saturation were accepted. The transmitted probe intensities were then normalized with respect to their corresponding pump intensites. The samples used were 2.5 × 10 -5 M/L solutions of DODC1 in ethanol, ethylene glycol and glycerine. The laser was set at 6040 A, within the overlapping region of the absorption and emission spectra. For all three solutions this wavelength corresponds to the peak of the emission spectrum and to an emission absorption cross-section ratio of 2.5 : 1. (There is a noticeable shift in the emission and absorption spectra as a function of the solvent used. This solvent shift accounts for the differences in the relative probe intensity transmitted for different solutions, but otherwise it will not change our conclusions about the observed transmission curves at the laser wavelength used.) Fig. 2 shows the plot of optical density difference of the probe beam transmission (with and without the saturating pump beam) as a function of the pumpprobe delay, i.e., we have plotted log e T(with pump) - log e T(without pump),
\
1
Fig. 1. Experimental setup: mode locked dye laser MLDL; single pulse selector SPS; beam splitter BS; variable optical delay VOD; mirror M; shutter S; photo-diode PD; lens L; dye cell DC. 192
where T is the linear probe beam transmission coefficient. For ethanol, where the solvent cage effect of expected to be insignificant (since measurements of solute-solvent relaxation times on similar dyes give time constants less than 10 ps), the curve is a monotonic decaying function starting at zero delay time. On the basis that this curve may be fitted by an exponential, we get an absorption recovery time of 0.9 -+ 0.2 ns, in good agreement with the measurements by Mialocq and others.
Volume 32, number 1
OPTICS COMMUNICATIONS
• ethanol • ethylene
glycol
o glycerine °,,°,
Z
•
NM
2
0
I 50
I 100
I 150
I 200
I 250
/ 3vO
~ 3 0
/ 4~0
,
DELAY[ps] Fig. 2. Relative transmission of probe pulse as a function of pump-probe delay for ethanol ethylene glycol and glycerine. For DODCI dissolved in ethylene glycol, the plot shows an extended flat region. Within experimental errors, no noticeable peak exists. Data points at large delay (not shown in plot) indicate a monotonic decay which, when approximated by an exponential, gives a decay constant of 1.4 + 0.2 ns. This is also in agreement with the measurement of Derkacheva et al., who made a rather extensive measurement of the lifetimes of several dyes dissolved in different solvents. The most interesting results were obtained for DODCI dissolved in glycerine. We observed a distinct maximum centered at about 150 ps delay; this broad peak was reproducible in separate runs. The large delay time transmission (up to 1.1 ns) was also measured and was found to be a decaying function with a constant of about 1.3 -+ 0.2 ns. (To the best of the authors' knowledge, this lifetime in glycerine has not been reported.) The rather slow decay observed at large delay indicates that the absorption recovery time is not significantly shortened by stimulated emissions. Nevertheless, the delayed peak is a signature that the probe experiences some gain besides saturation absorption in accordance with the four band model discussed
January 1980
earlier. That the peak is centered at about 150 ps delay indicates a solute-solvent relaxation time constant of that order. To check whether the probe is amplified in glycerine solution, the delay was set at the center of the broad peak region and the absolute probe transmission was measured. We found that there was still an overall loss in the probe intensity. This showed that while the gain mechanism due to the cage effect was present, it was not sufficient to compensate for all losses. The absence of an overall gain, or amplification, for the probe beam may be attributed to many factors. Among the various parameters affecting the probe gain are: the ratio of the emission to the absorption cross section at the laser wavelength, the intensity of the pump and probe, the variation of solute-solvent relaxation times with respect to the fluorescence and stimulated emission lifetimes, the dye concentration, etc. An optimum condition for the probe gain may be found by a detailed analytical study of the population build up in the levels involved in the gain process and the interplay of these parameters. But such a study is clearly outside the scope of the present communication. Our results, despite their preliminary nature, do point to the possibility of pulse amplification by time delayed superposition of laser pulses in dye molecules *. Detailed studies and analysis on DODCI and other dyes are in progress and will be published elsewhere. We acknowledge helpful discussions with D. Faubert and S.L. Chin, and the Ontario-Quebec Permanent Commission for travel support to one of us
(RFC). :~ Recently Herman and Serinko predicted the possibility of amplification by laser superposition in liquid crystals, see ref. [ 12]. The effect, however, has a magneto-optical origin.
References [ 1 ] E.G. Arthurs, D.J. Bradley and A.G. Roddie, Applied Phys. Lett. 20 (1972) 125. [2] G.E. Busch, K.S. Greve, G.L. Olson, R.P. Jones and P.M. Rentzepis, Chem. Phys. Lett. 33 (1975) 412,417. [3] D. Magde and M.W. Windsor, Chem. Phys. Lett. 27 (1974) 31. 193
Volume 82, number 1
OPTICS COMMUNICATIONS
[4] C.V. Shank and E.P. Ippen, Appl. Phys. Lett. 26 (1975) 62. [5] J. Jaraudias, P. Goujon and J.C. Mialocq, Chem. Phys. Lett. 45 (1977) 107. [6] L.D. Derkacheva, V.A. Petukhov and E.G. Treneva, Opt. Spectrosc. 41 (1976) 574. [7] G. Mouron, B. Drouin, M. Bergeron and M.M. DenariezRoberge, IEEE J. Quant. Electronics 9 (1973) 745.
194
[8] [9] [I0] [11]
January 198(3
N.G. Bakkshiev, Opt. Spectrosc. 10 (1961) 379. E. Lippert, Acc. Chem. Res. 3 (1970) 74, S.L. Chin, A. Zardecki, Phys. Rev. AI3 (1976) 1528. D. Faubert, S.L. Chin, M. Cormier and M. Boloten, Can. J. of Phys., Vol. 57 (1979) 160. [12] R.M. Herman and R.J. Serinko, Phys. Rev. A19 (1979) 1757.
OPTICS COMMUNICATIONS
January 1980
TIME DELAYED PICOSECOND OPTICAL PULSE TRANSMISSION IN LIQUID DODCI SOLUTIONS "~" I.C. KHOO *, Y.G. FUH and R.F. CODE Department of Physics and Erindale College, University of Toronto, Toronto, Canada M5S 1A 7
Received 27 August 1979
A broad anomalousincrease in the optical transmission of DODCIdissolved in glycerine has been observed during the first 150 ps delay followingintense optical excitation at 604 nm. The anomaly preceded a normal absorption recovery, and is ascribed to time-dependent interactions between the solvent and the excited states of the DODCImolecule.
During the past few years, there has been considerable interest in the study [1-6] of various optical and physical properties of a common mode-locking dye DODCI (3,3'-diethyloxadicarbocyanine iodide). For example, the determination of the excited state lifetimes has given rise to several reports of interesting and somewhat controversial behaviour. Recent studies [5,6], however, indicate that the differences in the various lifetimes that have been measured may be explained by the influence of factors such as the due concentration, the specific solvent, and on the presence or absence of a short-lived excited state photoisomer. Mialocq et al. [5] have pointed out that photoisomers will not be present in DODCI solutions if single pulse excitation is used. Excitation by the entire picosecond pulse train from a flash-lamp pumped dye laser was demonstrated to favour photoisomer buildup. The absorption recovery time obtained in their single pulse pump-probe experiment agrees very well with the accepted results of Magde and Windsor [3], and Shank and Ippen [4]. Mialocq's report also stated that they did not observe the nonlinear optical processes mentioned by Busch et al. such as stimulated emission or amplification. One must remark here that there is a basic difResearch supported in part by grants from the Natural Sciences and Engineering Council of Canada. * Present address: Department of Physics and Astronomy, Wayne State University, Detroit, Michigan48202, USA.
ference between Mialocq's experiment and that of Busch. In the former, the probe is at the same frequency as the pump, whereas in the latter, the probe is a continuum generated by part of the pump beam. The continuum contains lower frequency components than the pump which naturally experience gain by the usual dye laser action. Gain for a probe pulse of the same frequency as the pump is not possible if the dye molecules act only as saturable absorbers. However, under some favorable conditions, the time dependent solute-solvent interaction [7-11 ], sometimes known as the "solvent cage effect" may well provide a suitable mechanism for amplification or gain The solvent cage effect has been discussed theoretically in detail by Chin et al. [10,11], who also made some qualitative measurements. Essentially the theory accounts for the effects of significantly long solutesolvent reorientation times in viscous solutions by representing the absorption emission process of a dye molecule by a four-band model rather than by the usual two-band model. (In both cases singlet-triplet crossing transitions, and transitions to higher singlet levels are neglected.) The transmitted probe intensity will be enhanced by the saturation of the dye absorption established by the pump pulse. In addition, the probe may be amplified by stimulated emissions between the excited and ground state bands. Since the emission cross sections are modified by interactions with the solvent, the probe transmission in the presence of amplification will no longer be a monotonical191
Volume 82, number 1
OPTICS COMMUNICATIONS
ly decaying function beginning at zero time delay. The transmission should exhibit a broad peak at delay times on the order of the solute-solvent relaxation time following photoexcitation. In this paper we report tile study of time delayed optical transmission of DODCI dissolved in ethanol, ethylene glycol and glycerine to determine the effect of increased solvent viscosity. The experimental setup is schematically shown in fig. 1, typical of a pumpprobe experiment. The flash-lamp pumped dye laser (Electro-Photonics Ltd. model 33) emits a whole pulse train (lasting 1/as) of picosecond pulses (about 5 ps pulse length). One of the more intense pulses in the middle of the pulse train is selected by an ElectroPhotonics switch out unit (model 134). This single pulse is divided by the first beam-splitter into an intense pump beam and a weak probe which is optically delayed. The pmnp and probe are focused into a 1 mm cell filled with solutions of DODCI. After traversing the cell, the probe beam is further delayed before it is recorded by the photo-diode and oscilloscope.
rid --
/ ~ . - . - @ ~--
TO
SCOPE
PD
January 1980
By adjusting the trigger on the scope, four traces of tile probe out versus probe in are recorded (two with and two without the saturating pump). This gives four data points for each delay and the results are averaged. In a separate experiment, we have studied the transmission of the pump beam through some of the solutions as a function of intensity. Beyond about 40 MW/cm 2, the transmission showed evidence of saturation. In the present experiment, the pump beam had an estimated intensity of 80 MW/cm 2 to ensure adequate saturation of the dye molecules. To overcome the influence of fluctuations in the pump intensity from shot to shot, the pump intensity was monitored by the related incident probe intensity. Only data points corresponding to a pump intensity sufficient to ensure saturation were accepted. The transmitted probe intensities were then normalized with respect to their corresponding pump intensites. The samples used were 2.5 × 10 -5 M/L solutions of DODC1 in ethanol, ethylene glycol and glycerine. The laser was set at 6040 A, within the overlapping region of the absorption and emission spectra. For all three solutions this wavelength corresponds to the peak of the emission spectrum and to an emission absorption cross-section ratio of 2.5 : 1. (There is a noticeable shift in the emission and absorption spectra as a function of the solvent used. This solvent shift accounts for the differences in the relative probe intensity transmitted for different solutions, but otherwise it will not change our conclusions about the observed transmission curves at the laser wavelength used.) Fig. 2 shows the plot of optical density difference of the probe beam transmission (with and without the saturating pump beam) as a function of the pumpprobe delay, i.e., we have plotted log e T(with pump) - log e T(without pump),
\
1
Fig. 1. Experimental setup: mode locked dye laser MLDL; single pulse selector SPS; beam splitter BS; variable optical delay VOD; mirror M; shutter S; photo-diode PD; lens L; dye cell DC. 192
where T is the linear probe beam transmission coefficient. For ethanol, where the solvent cage effect of expected to be insignificant (since measurements of solute-solvent relaxation times on similar dyes give time constants less than 10 ps), the curve is a monotonic decaying function starting at zero delay time. On the basis that this curve may be fitted by an exponential, we get an absorption recovery time of 0.9 -+ 0.2 ns, in good agreement with the measurements by Mialocq and others.
Volume 32, number 1
OPTICS COMMUNICATIONS
• ethanol • ethylene
glycol
o glycerine °,,°,
Z
•
NM
2
0
I 50
I 100
I 150
I 200
I 250
/ 3vO
~ 3 0
/ 4~0
,
DELAY[ps] Fig. 2. Relative transmission of probe pulse as a function of pump-probe delay for ethanol ethylene glycol and glycerine. For DODCI dissolved in ethylene glycol, the plot shows an extended flat region. Within experimental errors, no noticeable peak exists. Data points at large delay (not shown in plot) indicate a monotonic decay which, when approximated by an exponential, gives a decay constant of 1.4 + 0.2 ns. This is also in agreement with the measurement of Derkacheva et al., who made a rather extensive measurement of the lifetimes of several dyes dissolved in different solvents. The most interesting results were obtained for DODCI dissolved in glycerine. We observed a distinct maximum centered at about 150 ps delay; this broad peak was reproducible in separate runs. The large delay time transmission (up to 1.1 ns) was also measured and was found to be a decaying function with a constant of about 1.3 -+ 0.2 ns. (To the best of the authors' knowledge, this lifetime in glycerine has not been reported.) The rather slow decay observed at large delay indicates that the absorption recovery time is not significantly shortened by stimulated emissions. Nevertheless, the delayed peak is a signature that the probe experiences some gain besides saturation absorption in accordance with the four band model discussed
January 1980
earlier. That the peak is centered at about 150 ps delay indicates a solute-solvent relaxation time constant of that order. To check whether the probe is amplified in glycerine solution, the delay was set at the center of the broad peak region and the absolute probe transmission was measured. We found that there was still an overall loss in the probe intensity. This showed that while the gain mechanism due to the cage effect was present, it was not sufficient to compensate for all losses. The absence of an overall gain, or amplification, for the probe beam may be attributed to many factors. Among the various parameters affecting the probe gain are: the ratio of the emission to the absorption cross section at the laser wavelength, the intensity of the pump and probe, the variation of solute-solvent relaxation times with respect to the fluorescence and stimulated emission lifetimes, the dye concentration, etc. An optimum condition for the probe gain may be found by a detailed analytical study of the population build up in the levels involved in the gain process and the interplay of these parameters. But such a study is clearly outside the scope of the present communication. Our results, despite their preliminary nature, do point to the possibility of pulse amplification by time delayed superposition of laser pulses in dye molecules *. Detailed studies and analysis on DODCI and other dyes are in progress and will be published elsewhere. We acknowledge helpful discussions with D. Faubert and S.L. Chin, and the Ontario-Quebec Permanent Commission for travel support to one of us
(RFC). :~ Recently Herman and Serinko predicted the possibility of amplification by laser superposition in liquid crystals, see ref. [ 12]. The effect, however, has a magneto-optical origin.
References [ 1 ] E.G. Arthurs, D.J. Bradley and A.G. Roddie, Applied Phys. Lett. 20 (1972) 125. [2] G.E. Busch, K.S. Greve, G.L. Olson, R.P. Jones and P.M. Rentzepis, Chem. Phys. Lett. 33 (1975) 412,417. [3] D. Magde and M.W. Windsor, Chem. Phys. Lett. 27 (1974) 31. 193
Volume 82, number 1
OPTICS COMMUNICATIONS
[4] C.V. Shank and E.P. Ippen, Appl. Phys. Lett. 26 (1975) 62. [5] J. Jaraudias, P. Goujon and J.C. Mialocq, Chem. Phys. Lett. 45 (1977) 107. [6] L.D. Derkacheva, V.A. Petukhov and E.G. Treneva, Opt. Spectrosc. 41 (1976) 574. [7] G. Mouron, B. Drouin, M. Bergeron and M.M. DenariezRoberge, IEEE J. Quant. Electronics 9 (1973) 745.
194
[8] [9] [I0] [11]
January 198(3
N.G. Bakkshiev, Opt. Spectrosc. 10 (1961) 379. E. Lippert, Acc. Chem. Res. 3 (1970) 74, S.L. Chin, A. Zardecki, Phys. Rev. AI3 (1976) 1528. D. Faubert, S.L. Chin, M. Cormier and M. Boloten, Can. J. of Phys., Vol. 57 (1979) 160. [12] R.M. Herman and R.J. Serinko, Phys. Rev. A19 (1979) 1757.