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Intensity dependent quenching of two - photon fluorescence displays of a mode - locked ruby laser
Volume 2, number 1
OPTICS COMMUNICATIONS
INTENSITY TWO-PHOTON
FLUORESCENCE
DEPENDENT DISPLAYS
D. J. BRADLEY, The Queen’s
Uniz~ersity
May/June 1970
QUENCHING OF
OF
A MODE-LOCKED
RUBY
LASER
T. MORROW and M. S. PETTY of Belfast,
Received
1
Belfast,
Northem
Ireland
June 1970
The effects of quenching of two-photon fluorescence in Rhodamine 6G and 9, lo-diphenylanthracene in measurements of Dicosecond oulse durations are discussed and the separate experimental demonstration of quenching is described. The convenient two-photon fluorescence display technique [l] is widely used for the measurement of ultra-short pulses from neodymium [2-41, ruby [5,6] and organic dye [7 -91 mode-locked lasers. Similar autocorrelation fluorescence patterns are obtained from the picosecond pulses of an ideally mode-locked laser and from freerunning lasers of the same oscillating bandwidth, and the proper interpretation of the data depends on the contrast ratio recorded. For completely mode-locked pulses a peak contrast of 3 is predicted by theory [lo] and this has been experimentally confirmed with picosecond pulses from neodymium lasers employing photoelectric [3] and photographic 14,111 recording. We report anomalous effects in the two-photon fluorescence patterns produced by a mode-locked ruby laser. The central peaks are flattened or even reversed. The decrease in fluorescence intensity at the centre of the pulse-correlation pattern is most marked when Rhodamine 6G is employed but the reversed profile also appears in 9, lo-diphenylanthracene (DPA) with high power laser pulses. This effect, which does not appear to occur in the fluorescence patterns of mode-locked neodymium lasers, has not been reported by other authors [5,6] describing the measurement of picosecond ruby pulses. Because of its importance in the measurement of ruby, and possibly dye, laser pulse durations we have further investigated this effect and have confirmed an intensity dependent quenching of two-photon fluorescence. The mode-locked laser consisted of a 6 mm diameter 7.5 cm long, high optical quality Brewster angled, Czochralski, ruby rod immersed in a coaxial quartz tube circulating water jacket and pumped with a helical flash tube. Q-switching and mode-locking was achieved by immersing a
100% reflectivity dielectric concave mirror (focal length 30 cm) in a 0.3 mm layer of l,l’-diethyl-2,2’-dicarbocyanine iodide (DDI) in methanol. A bi-concave lens (focal length 15 cm) inside the cavity provided compensation for thermal lensing in the ruby rod [5]. The output beam through the second ‘70% reflectivity wedged mirror of the 150 cm cavity was employed, with beam splitters, for simultaneous recording of (i) the two-photon fluorescence pulse profile in a triangular configuration [3], (ii) the laser output spectrum with a plane Fabry-Perot interferometer and (iii) the intensity of the pulse train with a bi-planar photodiode and Tektronix 519 oscilloscope of combined rise-time = 0.2 nsec. Employing a photo-electric triggered, Krytron switched, Pockels cell a single pulse could be selected from the train for separate examination. The laser oscillated in a low order pure transverse mode with a beam divergence < 1 mrad and output energies in the pulse trains of up to 200 m. Fig. 1 shows a microdensitometer trace of a Rhodamine 6G two-photon fluorescence track of a mode-locked pulse train (photographed on H.P.4 film with 75 mm lens at f/5.6) with an X2 density step on the right-hand side to measure the contrast ratio [4]. The 20% reversal, at the centre of the pattern, has a half-width of N 5 psec which corresponds to the laser spectral width of = 5 cm-l. This reversal was observed with a wide range of dye concentrations (low2 - 10-5 M) and with laser energies 50-200 mJ from the oscillator, and up to 1 J employing an amplifier stage. The effect was seen with different ruby with samples of Rhodamine 1,.ods in the oscillator, :JG from different manufacturers, and also when single-pulses were selected from the train and amplified. 1
\‘olume
2, numlx~r
OPTICS
1
Fig. 1. Microdensitometer
trace
of tno-photon fluorescence auto-correlation pattern step of X 2 is shomm on right-hand side.)
The two-photon fluorescence tracks produced by the oscillator pulse train in DPA (10m3 M solution in cyclohexane) were similar to those obtained in Rhodamine 6G from mode-locked neodymium lasers, with a central spike, whose width corresponded to the laser spectral bandwidth [3,4]. The measured contrast ratios were also always higher (- 2.0 - 2.3) than those obtained with Rhodamine 6G (- 2.0). When the pulse train was amplified to 0.5 - 1.0 J the central spike of the fluorescence track in DPA also disappeared, with a resulting reduced contrast ratio. As an additional diagnostic, the two-photon fluorescence patterns were examined with a T.R.W. image-tube streak camera capable of 50 psec time resolution [9]. The Rhodamine 6G patterns were reversed for all of the pulses in a train while in the DPA patterns the central reversal occurred only with the higher power pulses in the middle of the train. Fig. 2 shows how the fluorescence track centres fill in and the high-contrast spike (width N 5 psec) reappears as the pulse peak intensity falls along the train. The change from the reversed pattern to the normal spike occurs for a = 20% drop in laser pulse power and the reversed structure has approximately the same width as the central spike in later pulses. The Fabry-Perot interferograms were simultaneously streaked with a second image 2
May/June
COMMUNICATIONS
in Rhodamine
6G.
1970
(A densitS
tube camera to confirm that the laser spectral width (N 5 cm-l) did not change from pulse to pulse. These results suggested that an intensity de pendent quenching of the two-photon fluorescence The electronic energy levels of was occurring. Rhodamine 6G (derived approximately from corrected absorption and fluorescence spectra) are shown in fig. 3. Two photon absorption of ruby laser light excites a dye molecule to the second excited singlet state (S2), from which it decays rapidly by non-radiative interconversion to the bottom vibrational level of the first excited singlet state (Sl) with subsequent fluorescence emission. At high light intensities excitation by absorption of a third photon to higher excited states (S,) could compete with the S2 to S1 relaxation process. Fluorescence quenching would then result provided the S, to S1 relaxation process were less efficient than S2 to Sf relaxation. This situation would occur if S, were dissociative in character. Stimulated emission [12] from S1 to SO, induced by 6943 A ruby radiation, could also lead to an intensity dependent fluorescence quench ing since this will reduce the the two-photon fluorescence intensity observed normal to the laser beam. To determine the relative importance of these
Volume
2, number
1
OPTICS COMMUNICATIONS
RIay/June 1970
6
5
4
6943 i;
I z -3 5 : w2
---__
1
69C3i
STIMULATED EMISSION
‘I -_
0 Fig.3.Energy
lops Fig. 2. Streak photograph of two-photon fluorescence autocorrelation patterns from a pulse train in 9, lodiphenylanthracene.
two quenching processes a separate experiment was carried out with the arrangement shown in fig. 4. The ruby laser pulse train was divided by a 90% transmission beam splitter. After filtering, the second harmonic pulses, generated by 10% of the fundamental beam in the phase matched KDP crystal, collided with the remainder of the ruby fundamental pulse train in the centre of a 50 cm cell containing a 10-5 M solution of Rhodamine 6G in ethanol. While the total energy content of the 3472 A pulses was = 100 nJ compared with = 70 mJ in the 6943 A pulses the
so
FLUORESCENCE
--
I
level diagram
for Rhodamine
6G molecule
fluorescence produced was at least an order of magnitude more intense. In the absence of the fundamental ruby beam the fluorescence intensity fell off exponentially inside the cell as expected with strong single photon absorption to the 52 level. Fig. 5 shows the result obtained when the second harmonic and fundamental frequencies collided in the cell. At the point of collision the fluorescence intensity is markedly reduced and the hole in the fluorescence curve lasts for at least 100 psec. The duration of the ruby pulses was determined to be = 15 psec by simultaneous two-photon fluorescence pattern measurements in DPA*. This result confirms that fluorescence quenching by 6943 A light is significant and the shape of the curve in fig. 5 would indicate that stimulated emission was more important in this experiment, since single photon absorption to higher excited states could occur for only N 10 psec after collision. The reversed structure in the two-photon
* The fluorescence lifetime is N 5 nsec and pulses in the train are separated by N 10 nsec, so both twophoton and single photon fluorescence will have decayed before the arrival of the succeeding pulse. The non-radiative vibrational relaxation processes are three orders of magnitude faster than the pulse separation.
3
Volume
2, number
1
OPTICS COMMUNICATIONS
TO
PHOTO
May/June
1970
DIODE
I
TO TWO PHOTON FLUORESCENCE APPARATUS
Fig. 4. Experimental
arrangement
for observation
of fluorescence
fluorescence patterns can be explained by considering single photon absorption to higher excited states and stimulated emission, both leading to a departure from the normal quadratic in-
quenching
in Rhodamine
6G by 6943 k radiation.
tensity dependence law. Substituting the modified fluorescence intensity-laser pulse intensity relationship, before carrying out spatial and time averaging, produced [13] modified two-photon
-A-
_ii 6943
i
Fig. 5. Microdensitometer
4
3472 ii
traceOof single photon fluorescence track produced by oppositely 3472 A pulses. (Ordinates are arbitrary linear density scales.)
travelling
6943 A and
Volume
2, number
1
OPTICS COMMUNICATIONS
fluorescence profiles in good agreement with the above experimental results. This fluorescence quenching, which is strongly intensity dependent, can thus lead to anomalous results in two-photon fluorescence measurements of ultra-short ruby laser pulse durations. Considerable care must be taken in interpreting the fluorescence patterns and, in particular, the pulse intensity employed should be kept as low as possible consistent with obtaining a usable track. Since an integrated fluorescence profile is obtained with pulse trains, it would be safer to switch-out a single pulse for measurement. The effect is likely to be less important in twophoton fluorescence measurements of neodymium lasers which excite to the Sl level of Rhodamine 6G, but it must be guarded against because of the results obtained with DPA, which is also excited to the S1 level by two-photon absorption of ruby laser light. We plan to investigate more qunatitatively twophoton fluorescence quenching in Rhodamine 6G, DPA and other organic dyes employed for the measurement of picosecond pulses from dye lasers [8,9].
We would like to thank Dr. M. H. Key, Dr. G. H. C. New and Mr. F. O’Neill for stimulating discussions and Mr. R. J. Frame for making the microdensitometer traces. This research has
May/June
1970
been sponsored in part by Air Force Cambridge Research Laboratories through the European Office of Aerospace Research, 0. A. R., U. S. A. F under Contract F61052-70-C-0003.
REFERENCES [l] J. A. Giordmaine, [2] [3] [4] [5] [G] [7] [B] [9] [lOI [ll]
[12] [13]
P. M. Rentzepis, S. 1,. Shapiro and K. W. Wecht, Appl. Phys. Letters 11 (1967) 216. D. J. BradleT, G. H. C. New. B. Sutherland and S. J. Caughey, Phys. Letters 28A (19G9) 532. S. L. Shapiro and M. A. Duguny, PhJ-s. Letters 28A (19G9) 698. D. J. Bradley, G. H. C. Nen and S. J. CnugheJ-, Phys. Letters 30A (19G9) 78. M. E. Xack, IEEE J. Quantum Electron. QE-4 (1968) 1015. R.Cubeddu, R. Polloui, C. A. Sacchi and 0. Svelte, IEEE J. Quantum Electron. QE-5 (19ti9) 470. D. J. Bradley and A. J. F. Durrant, Whys. Letters 2GA (1968) 73. B. H. Soffer and J. W. Linn, J. Appl. Phys. 39 (1968) 558. D. J. Bradley, A. J. F’. Durrant, F. O’Neill and B. Sutherland, Phys. Letters 30A (1969) 535. H. P. Weber and R. Dandliker, IEEE J. Quantum Electron. QE-4 (1968) 1009. D. 3. Bradley, G. H. C. New and S. J. Caughcy, I.P.P.S. Conf. Non-Linear Optics, Belfast, to be published. 11. D. Galanin, B. P. Kirsanov and Z. A. Chizhikova, JETP Letters 9 (1969) 304. D. J. Bradley, T. Morrow, G.H.C. New and RI. S. Petty, to be published.
OPTICS COMMUNICATIONS
INTENSITY TWO-PHOTON
FLUORESCENCE
DEPENDENT DISPLAYS
D. J. BRADLEY, The Queen’s
Uniz~ersity
May/June 1970
QUENCHING OF
OF
A MODE-LOCKED
RUBY
LASER
T. MORROW and M. S. PETTY of Belfast,
Received
1
Belfast,
Northem
Ireland
June 1970
The effects of quenching of two-photon fluorescence in Rhodamine 6G and 9, lo-diphenylanthracene in measurements of Dicosecond oulse durations are discussed and the separate experimental demonstration of quenching is described. The convenient two-photon fluorescence display technique [l] is widely used for the measurement of ultra-short pulses from neodymium [2-41, ruby [5,6] and organic dye [7 -91 mode-locked lasers. Similar autocorrelation fluorescence patterns are obtained from the picosecond pulses of an ideally mode-locked laser and from freerunning lasers of the same oscillating bandwidth, and the proper interpretation of the data depends on the contrast ratio recorded. For completely mode-locked pulses a peak contrast of 3 is predicted by theory [lo] and this has been experimentally confirmed with picosecond pulses from neodymium lasers employing photoelectric [3] and photographic 14,111 recording. We report anomalous effects in the two-photon fluorescence patterns produced by a mode-locked ruby laser. The central peaks are flattened or even reversed. The decrease in fluorescence intensity at the centre of the pulse-correlation pattern is most marked when Rhodamine 6G is employed but the reversed profile also appears in 9, lo-diphenylanthracene (DPA) with high power laser pulses. This effect, which does not appear to occur in the fluorescence patterns of mode-locked neodymium lasers, has not been reported by other authors [5,6] describing the measurement of picosecond ruby pulses. Because of its importance in the measurement of ruby, and possibly dye, laser pulse durations we have further investigated this effect and have confirmed an intensity dependent quenching of two-photon fluorescence. The mode-locked laser consisted of a 6 mm diameter 7.5 cm long, high optical quality Brewster angled, Czochralski, ruby rod immersed in a coaxial quartz tube circulating water jacket and pumped with a helical flash tube. Q-switching and mode-locking was achieved by immersing a
100% reflectivity dielectric concave mirror (focal length 30 cm) in a 0.3 mm layer of l,l’-diethyl-2,2’-dicarbocyanine iodide (DDI) in methanol. A bi-concave lens (focal length 15 cm) inside the cavity provided compensation for thermal lensing in the ruby rod [5]. The output beam through the second ‘70% reflectivity wedged mirror of the 150 cm cavity was employed, with beam splitters, for simultaneous recording of (i) the two-photon fluorescence pulse profile in a triangular configuration [3], (ii) the laser output spectrum with a plane Fabry-Perot interferometer and (iii) the intensity of the pulse train with a bi-planar photodiode and Tektronix 519 oscilloscope of combined rise-time = 0.2 nsec. Employing a photo-electric triggered, Krytron switched, Pockels cell a single pulse could be selected from the train for separate examination. The laser oscillated in a low order pure transverse mode with a beam divergence < 1 mrad and output energies in the pulse trains of up to 200 m. Fig. 1 shows a microdensitometer trace of a Rhodamine 6G two-photon fluorescence track of a mode-locked pulse train (photographed on H.P.4 film with 75 mm lens at f/5.6) with an X2 density step on the right-hand side to measure the contrast ratio [4]. The 20% reversal, at the centre of the pattern, has a half-width of N 5 psec which corresponds to the laser spectral width of = 5 cm-l. This reversal was observed with a wide range of dye concentrations (low2 - 10-5 M) and with laser energies 50-200 mJ from the oscillator, and up to 1 J employing an amplifier stage. The effect was seen with different ruby with samples of Rhodamine 1,.ods in the oscillator, :JG from different manufacturers, and also when single-pulses were selected from the train and amplified. 1
\‘olume
2, numlx~r
OPTICS
1
Fig. 1. Microdensitometer
trace
of tno-photon fluorescence auto-correlation pattern step of X 2 is shomm on right-hand side.)
The two-photon fluorescence tracks produced by the oscillator pulse train in DPA (10m3 M solution in cyclohexane) were similar to those obtained in Rhodamine 6G from mode-locked neodymium lasers, with a central spike, whose width corresponded to the laser spectral bandwidth [3,4]. The measured contrast ratios were also always higher (- 2.0 - 2.3) than those obtained with Rhodamine 6G (- 2.0). When the pulse train was amplified to 0.5 - 1.0 J the central spike of the fluorescence track in DPA also disappeared, with a resulting reduced contrast ratio. As an additional diagnostic, the two-photon fluorescence patterns were examined with a T.R.W. image-tube streak camera capable of 50 psec time resolution [9]. The Rhodamine 6G patterns were reversed for all of the pulses in a train while in the DPA patterns the central reversal occurred only with the higher power pulses in the middle of the train. Fig. 2 shows how the fluorescence track centres fill in and the high-contrast spike (width N 5 psec) reappears as the pulse peak intensity falls along the train. The change from the reversed pattern to the normal spike occurs for a = 20% drop in laser pulse power and the reversed structure has approximately the same width as the central spike in later pulses. The Fabry-Perot interferograms were simultaneously streaked with a second image 2
May/June
COMMUNICATIONS
in Rhodamine
6G.
1970
(A densitS
tube camera to confirm that the laser spectral width (N 5 cm-l) did not change from pulse to pulse. These results suggested that an intensity de pendent quenching of the two-photon fluorescence The electronic energy levels of was occurring. Rhodamine 6G (derived approximately from corrected absorption and fluorescence spectra) are shown in fig. 3. Two photon absorption of ruby laser light excites a dye molecule to the second excited singlet state (S2), from which it decays rapidly by non-radiative interconversion to the bottom vibrational level of the first excited singlet state (Sl) with subsequent fluorescence emission. At high light intensities excitation by absorption of a third photon to higher excited states (S,) could compete with the S2 to S1 relaxation process. Fluorescence quenching would then result provided the S, to S1 relaxation process were less efficient than S2 to Sf relaxation. This situation would occur if S, were dissociative in character. Stimulated emission [12] from S1 to SO, induced by 6943 A ruby radiation, could also lead to an intensity dependent fluorescence quench ing since this will reduce the the two-photon fluorescence intensity observed normal to the laser beam. To determine the relative importance of these
Volume
2, number
1
OPTICS COMMUNICATIONS
RIay/June 1970
6
5
4
6943 i;
I z -3 5 : w2
---__
1
69C3i
STIMULATED EMISSION
‘I -_
0 Fig.3.Energy
lops Fig. 2. Streak photograph of two-photon fluorescence autocorrelation patterns from a pulse train in 9, lodiphenylanthracene.
two quenching processes a separate experiment was carried out with the arrangement shown in fig. 4. The ruby laser pulse train was divided by a 90% transmission beam splitter. After filtering, the second harmonic pulses, generated by 10% of the fundamental beam in the phase matched KDP crystal, collided with the remainder of the ruby fundamental pulse train in the centre of a 50 cm cell containing a 10-5 M solution of Rhodamine 6G in ethanol. While the total energy content of the 3472 A pulses was = 100 nJ compared with = 70 mJ in the 6943 A pulses the
so
FLUORESCENCE
--
I
level diagram
for Rhodamine
6G molecule
fluorescence produced was at least an order of magnitude more intense. In the absence of the fundamental ruby beam the fluorescence intensity fell off exponentially inside the cell as expected with strong single photon absorption to the 52 level. Fig. 5 shows the result obtained when the second harmonic and fundamental frequencies collided in the cell. At the point of collision the fluorescence intensity is markedly reduced and the hole in the fluorescence curve lasts for at least 100 psec. The duration of the ruby pulses was determined to be = 15 psec by simultaneous two-photon fluorescence pattern measurements in DPA*. This result confirms that fluorescence quenching by 6943 A light is significant and the shape of the curve in fig. 5 would indicate that stimulated emission was more important in this experiment, since single photon absorption to higher excited states could occur for only N 10 psec after collision. The reversed structure in the two-photon
* The fluorescence lifetime is N 5 nsec and pulses in the train are separated by N 10 nsec, so both twophoton and single photon fluorescence will have decayed before the arrival of the succeeding pulse. The non-radiative vibrational relaxation processes are three orders of magnitude faster than the pulse separation.
3
Volume
2, number
1
OPTICS COMMUNICATIONS
TO
PHOTO
May/June
1970
DIODE
I
TO TWO PHOTON FLUORESCENCE APPARATUS
Fig. 4. Experimental
arrangement
for observation
of fluorescence
fluorescence patterns can be explained by considering single photon absorption to higher excited states and stimulated emission, both leading to a departure from the normal quadratic in-
quenching
in Rhodamine
6G by 6943 k radiation.
tensity dependence law. Substituting the modified fluorescence intensity-laser pulse intensity relationship, before carrying out spatial and time averaging, produced [13] modified two-photon
-A-
_ii 6943
i
Fig. 5. Microdensitometer
4
3472 ii
traceOof single photon fluorescence track produced by oppositely 3472 A pulses. (Ordinates are arbitrary linear density scales.)
travelling
6943 A and
Volume
2, number
1
OPTICS COMMUNICATIONS
fluorescence profiles in good agreement with the above experimental results. This fluorescence quenching, which is strongly intensity dependent, can thus lead to anomalous results in two-photon fluorescence measurements of ultra-short ruby laser pulse durations. Considerable care must be taken in interpreting the fluorescence patterns and, in particular, the pulse intensity employed should be kept as low as possible consistent with obtaining a usable track. Since an integrated fluorescence profile is obtained with pulse trains, it would be safer to switch-out a single pulse for measurement. The effect is likely to be less important in twophoton fluorescence measurements of neodymium lasers which excite to the Sl level of Rhodamine 6G, but it must be guarded against because of the results obtained with DPA, which is also excited to the S1 level by two-photon absorption of ruby laser light. We plan to investigate more qunatitatively twophoton fluorescence quenching in Rhodamine 6G, DPA and other organic dyes employed for the measurement of picosecond pulses from dye lasers [8,9].
We would like to thank Dr. M. H. Key, Dr. G. H. C. New and Mr. F. O’Neill for stimulating discussions and Mr. R. J. Frame for making the microdensitometer traces. This research has
May/June
1970
been sponsored in part by Air Force Cambridge Research Laboratories through the European Office of Aerospace Research, 0. A. R., U. S. A. F under Contract F61052-70-C-0003.
REFERENCES [l] J. A. Giordmaine, [2] [3] [4] [5] [G] [7] [B] [9] [lOI [ll]
[12] [13]
P. M. Rentzepis, S. 1,. Shapiro and K. W. Wecht, Appl. Phys. Letters 11 (1967) 216. D. J. BradleT, G. H. C. New. B. Sutherland and S. J. Caughey, Phys. Letters 28A (19G9) 532. S. L. Shapiro and M. A. Duguny, PhJ-s. Letters 28A (19G9) 698. D. J. Bradley, G. H. C. Nen and S. J. CnugheJ-, Phys. Letters 30A (19G9) 78. M. E. Xack, IEEE J. Quantum Electron. QE-4 (1968) 1015. R.Cubeddu, R. Polloui, C. A. Sacchi and 0. Svelte, IEEE J. Quantum Electron. QE-5 (19ti9) 470. D. J. Bradley and A. J. F. Durrant, Whys. Letters 2GA (1968) 73. B. H. Soffer and J. W. Linn, J. Appl. Phys. 39 (1968) 558. D. J. Bradley, A. J. F’. Durrant, F. O’Neill and B. Sutherland, Phys. Letters 30A (1969) 535. H. P. Weber and R. Dandliker, IEEE J. Quantum Electron. QE-4 (1968) 1009. D. 3. Bradley, G. H. C. New and S. J. Caughcy, I.P.P.S. Conf. Non-Linear Optics, Belfast, to be published. 11. D. Galanin, B. P. Kirsanov and Z. A. Chizhikova, JETP Letters 9 (1969) 304. D. J. Bradley, T. Morrow, G.H.C. New and RI. S. Petty, to be published.