Light Source Responsible for the Deterioration ofCryptocyanine Q-Switches

Roberta A. Hollier and James D. Macomber

Ultraviolet radiation, in a band of wavelengths near 300 nm, produced by the flashlamps used to pumpthe ruby rod, is responsible for nearly all the photochemical decomposition (irreversible bleaching) ofmethanolic solutions of cryptocyanine used as passive shutters (Q-switches) in high-peak-power rubylasers. The laser beam itself has little or no irreversible effect upon cryptocyanine.

Introduction

Cryptocyanine (sometimes spelled kryptocyanine:I.U.C. name, I-ethyl4-{3- [1-ethyl-4(1H)-quinolyl-idene]propenyl} quinolinium iodide) is one of severaldyes commonly used as a passive shutter (Q-switch)for high-peak-power ruby lasers. Typically, a dilute(-10-1 M)solution of the dye in methanol or acetoni-trile, in a 1-cm silica or quartz spectrophotometer cell,is placed between the ruby rod and one of the endmirrors of the laser cavity. The shuttering actionresults from reversible bleaching of the dye (due tooptical saturation of an intense narrow absorptionband in the wavelength region of 706 nm) by the laserlight.

A small amount of irreversible bleaching of crypto-cyanine (due to photochemical decomposition) ac-companies the essentially total optical saturation thatoccurs each time the laser is fired. Unless the dye isreplenished after each firing, the peak power and dura-tion of the output laser pulse change from shot to shotin a gradual but eventually drastic manner. Saturableabsorber Q-switches would be much more convenientto use if this decomposition could be prevented. Be-fore one can devise a method to prevent decomposi-tion, however, he must first know whether the damageis done by the laser beam itself or by other light thatalso illuminates the sample when the laser is fired.This other light consists of incoherent spontaneousemission from the ruby rod and from the xenon flash-lamps used to pump the rod. Light from both of thesesources escapes from one end of the chamber housingthe rod and lamps and strikes the Q-switching cell.

The authors are with the Chemistry Department, LouisianaState University, Baton Rouge, Louisiana 70803.

Received 22 December 1971.

Photodecomposition of cryptocyanine has beenstudied previously by several authors. 1-5 Jeffreys'illuminated a methanolic solution of the dye with whitelight from a tungsten lamp. He assumed that thedecomposition that resulted was due to photons in the700-nm wavelength region but gave no evidence tosupport his assumption. Soffer2 was the first to em-ploy a solution (solvent unspecified) of cryptocyanineas a Q-switch; he reported 2 no detectable degradationof the dye after using it for some 100 laser shots.He has since admitted 3 that photodecomposition wasinitially a problem, although he has not reported thecause of the decomposition or the precautions he took toprevent it. Brown and Stone4 exposed acetonitrilicsolutions of cryptocyanine to fluorescent light, to sun-light, and to the light produced by a ruby laser during200 firings. They found measurable decomposition inall three cases, but by far the most destructive sourcewas solar radiation. Since sunlight is richer in uv radi-ation than a desk lamp (their source of fluorescent light),their results suggest that uv is the cause of irreversiblebleaching. On the other hand, the color temperatures ofthe flashlamps used to pump ruby lasers are about thesame as that of the sun's photosphere. This means thatuv light should have also illuminated the dye if it were inthe laser cavity in a quartz cell, so that one is surprisedby their finding of minimal decomposition of Q-switchsolutions after many shots. Although the flux of thesolar radiation incident upon their samples was givenin their paper, the flux of the fluorescent light and theflashlamp light was not. Therefore, no firm conclusionsabout the relative importance of red and uv light in thephotodecomposition of cryptocyanine. can be drawnfrom their results.

Finally, Booth5 reported substantial deterioration ofmethanolic solutions of cryptocyanine in a (presumablyquartz) spectrophotometer cell used to Q-switch a rubylaser and noted that the amount of decompositionproduced was a linear function of the number of times

1360 APPLIED OPTICS / Vol. 11, No. 6 / June 1972

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NUMBER OF EXPOSURESFig. 1. Dependence of the absorbance of a methanolic solutionof cryptocyanine (-10- 6 M) upon the number of prior exposuresof that solution to the light from laser flashlamps. The electricalenergy discharged through the lamps during each exposure was2020 J; the laser itself was prevented from firing by coveringone of the cavity end mirrors with a black cloth. Trianglesrepresent a run of exposures made with a glass plate betweenthe flashlamps and the quartz cell containing the dye solution.Circles represent a run of exposures made without the glassplate. The lines are best fit to the points in the least-squaressense. Absorbances were measured after each exposure at a

wavelength of 706 nm.

the laser was fired. Although he noted the possibilitythat the uv radiation from the pumping flashlampmight have been involved in the decomposition process,he assumed that the laser pulse itself was primarilyresponsible. Booth determined that the rate of de-composition (change in absorbance6 per laser shot)increased when the voltage across the capacitors dis-charged through the flashlamp during the firing processwas increased. Since, in these experiments, the numberof photons incident upon the dye from both the flash-lamp and the laser beam increased when this voltagewas increased, the relative importance of these twolight sources in the breakdown of the dye cannot beassessed from his data.

Experiments

We have performed three experiments that we believehave finally answered the question: does the decom-position of cryptocyanine in passive shutters producedwhen the laser is fired result from the laser light itselfor from light coming out of the xenon flashlamps usedto pump the ruby?

In the first experiment, we prevented the laser fromfiring by placing a black felt cloth between the ruby andthe TIR prism that is used as the rear reflector. Weexposed quartz cells containing methanolic solutions ofcryptocyanine in the laser cavity to only the lightfrom the xenon flashlamps plus spontaneous lumines-cence from the ruby rod. We did this in eight runsof about eight exposures each. A fresh dye solution

was prepared at the beginning of each run, and theabsorbance of the sample was measured after each ex-posure. Prior to each exposure, the voltage across thecapacitors discharged through the flashlamps was setto provide a predetermined pump energy. All theexposures in a given run were made at the same energysetting, but the energy was changed from run to run.After each run, the absorbance of the dye solution wasplotted against the number of exposures and the least-squares best-fit straight line through the points wasdetermined. The results of a typical run appear inFig. 1. Afterall the runs had been completed, theslopes of the various lines were plotted against pumpenergy, and the least-squares best-fit straight linethrough those points was determined. Those points(represented by circles) and the line are shown in Fig. 2.

Next, three runs were made with the black felt clothremoved from the laser cavity so that the laser firedthrough the dye during each exposure. Runs at lowpump energies with the laser firing were not possiblebecause the large initial absorbance of the Q-switchsolutions (required to enable accurate measurements tobe made of the decrease in absorbance from shot toshot) produced a pumping threshold for lasing above1225 J. The slopes.of the lines best fitting the resultsof these runs are also plotted (as triangles) in Fig. 2.

The fact that both triangles and circles fall close tothe same line indicates that decomposition occurswhether or not the laser fires, and that the amount of

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FLASH LAMP ENERGY (Kilojoules)Fig. 2. Dependence of the rate of decrease in absorbance perexposure upon the amount of energy discharged through theflashlamps during the exposure. The mean decrease in absorb-ance (of 706 nm light by methanolic solutions of cryptocyanine)per exposure (to fashlamp light) during a run of several exposureswas measured first with the laser pevented from firing [both with(square) and without (circles) shielding the sample by means of aglass plate during the run], and then with the laser firing [bothwith (diamond) and without (triangles) shielding]. The line isleast-squares-best-fit to the circles. Error bars, representing68% confidence limits, are based only on the statistical uncer-tainty in the rate of decrease in absorbance during a run and donot reflect variations in experimental conditions from one run

to the next.

June 1972 / Vol. 11, No. 6 / APPLIED OPTICS 1361

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Fig. 3. Dependence of the decrease in absorbance upon theamount of energy in the laser output. Each point represents asingle exposure of a methanolic solution of cryptocyanine to boththe light from the flashlamps and the spontaneous and stimulatedemission from the ruby rod. The energy discharged through theflashlamps during the exposures was 1600 J (circles), 2020 J(triangles), or 2500 J (squares). Vertical error bars representthe estimated accuracy of a single measurement of absorbanceat a wavelength of 706 n; horizontal error bars represent thecombined effects of the inaccuracy of the optical calorimeter(given by the manufacturer) and the uncertainty in extrapolatingthe energy readings from the cooling curves produced by the

calorimeter.

photochemical destruction produced by one shot at agiven pump energy is the same in either event. Byway of contrast, a plot of the decrease in absorbanceproduced by a given shot (with the laser firing) againstthe total output laser power for the shot, reveals littlecorrelation (Fig. 3). Parallel measurements made withcommercially available cryptocyanine Q-switch solu-tions produced substantially the same results as thosemade with the solutions prepared fresh in our lab-oratory, although the scatter of the correspondingdata points was greater.

The vertical error bars in Fig. 2 are derived from astatistical analysis of the expected uncertainty in slopefor each run; they represent 68% confidence limits.Deviations in excess of the expected error limits of thepoints (both circles and tri angles) from the line mayarise from systematic error in the calibration of the(volt) meter that indicates the electrical energy storedin the capacitors to be discharged through the flash-lamps, from random errors in the meter, and from non-reproducibility in the behavior of the components of thelaser system (notably the capacitors, the fashlamps,and possibly the temperature of the dye solution) fromrun to run.

To confirm the results of these experiments and toeliminate the possibility that spontaneous emission inthe 700-nm region (from either the fashlamp or theruby or both) might be producing the breakdown of thedye, two further experiments were performed. In thefirst of these, methanolic solutions of cryptocyanine

were prepared in spectrophotometer cells as before andwere illuminated outside the laser cavity by a xenon-mercury lamp. A grating monochromator was in-serted between the lamp and the sample, and a bandof radiation centered at a different wavelength in theinterval from 200 nm to 800 nm was selected by meansof the monochromator for sample illumination duringeach run. A run in this case consisted of two (400-800-nm) or four (200-400-nm) exposures of 30 mmn each,with a fresh dye solution prepared for each exposure.The total intensity of the light in each wavelength bandwas estimated from a spectral distribution curve de-scribed as typical by Oriel Optics Corporation, a com-mercial manufacturer of lamp systems using compo-nents equivalent to ours. The average decrease inabsorbance per exposure in each run, divided by thearea under the lamp curve in the corresponding band, isplotted in Fig. 4; the absorption spectrum of crypto-cyanine in the 200-800-nm interval is displayed in thissame figure.

Light of wavelength shorter than 200 nm is absorbedby the air in the path between the light source and thesample, and hence such light cannot contribute sig-nificantly to the destruction of the dye. Light of wave-length longer than 800 nm (ir) contains photons hav-ing energy insufficient to product electronic excitationof molecules in the sample. Therefore, since electronicexcitation of a molecule by absorption of a photon isusually a necessary preliminary to photochemical de-composition of that molecule (or any other molecule towhich that energy could subsequently be transferred byintermolecular processes), ir radiation cannot con-

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Fig. 4. Dependence of the decrease in absorbance at 706 nmupon the wavelength of the arc lamp light to which the samplehad been exposed previously. Circles represent measurementsmade on 3-4 August 1971 and triangles, on 31 August-7 Sep-tember 1971. The measured decreases in the absorbance havebeen normalized to account for the variation in the intensity ofthe arc lamp with wavelength. The solid line represents theabsorption spectrum of a typical sample for purposes of com-parison with the decomposition data. Experimental details and

a discussion of errors are given in the text.

1362 APPLIED OPTICS / Vol. 11, No. 6 / June 1972

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tribute directly to the decomposition either. In-frared radiation can sometimes produce chemical de-composition indirectly; vibrational and rotational ex-citation of the sample molecules induced directly bythe incident photons can be converted to translationalenergy (heat) by means of relaxation processes. Thisheat, in turn, can lead to thermal decomposition ofmolecules in the sample. In fact, we observed anoticeable increase in the temperature of the dye cellduring each run of flashlamp exposures.' We havemeasured the thermal stability of cryptocyanine indilute methanolic solution however and determinedthat the total temperature rise produced during a run(r.1°C) is not sufficient to produce chemical de-composition. It is therefore unlikely that light withwavelength longer than 800 nm contributes signifi-cantly to the deterioration of Q-switch solutions in ourexperiments.

The vertical error bars in Fig. 4 represent the un-certainty in determining the area under the spectraldistribution curve of the lamp combined with the lackof reproducibility in the amount of photodecomposi-tion per exposure for the various runs; the confidencelimits are 68%. The horizontal error bars representthe bandpass of the monochromator. The accuracy ofthe data in the photochemical experiments is furtherinfluenced by long-term variations in the intensity andspectral distribution of the lamp output and by theassumption that the transmittance of the monochro-mator was effectively constant in the 2 00-800-nmregion.

In the third, and last, experiment, glass sheets (or-dinary plate glass 2.38 mm thick) were inserted into theoptical train (inside the laser cavity) on both sides ofthe dye cell, and two runs like those performed duringthe first experiment were made. During one run, thelaser fired each time the lamps flashed; during theother run, the laser was prevented from firing. Nosignificant decomposition was observed after eitherrun. The data obtained from the latter series of ex-posures are displayed in Fig. 1; the decomposition ratesfor both runs appear (square and diamond) in Fig. 2.The absorbance of the glass (air reference) was measuredfrom 200 nm to 800 nm. It was found that the sheetswere nearly transparent from 350 nm to 800 nm andessentially opaque (absorbance greater than 1.0) atwavelengths shorter than 320 nm.

The cryptocyanine used in these experiments waspurchased from the Aldrich Chemical Company;the methanol was supplied by Mallinckrodt ChemicalWorks (Anhydrous Analytical Reagent, lot VEE).These chemicals were used without further purification.Absorbance was measured by means of a double-beam,recording spectrophotometer (Cary 14, Applied PhysicsCorporation). The intensity of the light source inthis instrument is too low to produce a measurableamount of decomposition of cryptocyanine during themeasurement of absorbance at any wavelength. Puremethanol in a cell having optical properties matchedwith those of the cell containing the sample was used asthe reference, except where noted otherwise. The

laser was an Optics Technology High Peak PowerRuby model 130, the lamps of which were never causedto flash more frequently than once every 5 min duringthese experiments. The laser output was monitored bymeans of a Korad KJ-2 Optical Calorimeter (UnionCarbide Corporation) followed by a microvoltmeter(model 150A, Keithley Instruments) and an inklessstrip-chart dc recorder (Esterline-Angus Minigraph,Esterline Corporation). The arc lamp and power sup-ply used in the photochemical experiments were man-ufactured by Englehard-Hanovia (1000-W compactarc lamp 537B-9 and 1000-W ac power supply). Thelamp housing was manufactured by Schoeffel (Uni-versal Housing LH 151N). The monochromator (250mm, Bausch & Lomb 33-86-40-01) had a dispersion of6.6 nm/mm and was used with both entrance andexits slits set at 3.0 nm.

Conclusions

The results of the three experiments described inthe previous section indicate clearly that the ruby laserbeam has little or no effect upon the deterioration ofQ-switch solutions consisting of cryptocyanine dis-solved in methanol. Instead, the principal culpritseems to be uv light in the 300-nm wavelength regionfrom the pumping flashlamps. We plan further ex-periments to investigate the photodecomposition ofcrytocyanine in other solvents and eventually of othersaturable dyes as well. We plan also to determine themechanism of photochemical decomposition in thesesystems.

We need not wait for the results of future experi-mentation, however, to suggest a method for inhibitingthe irreversible bleaching of saturable dyes used aspassive Q-switches in high-peak-power lasers. Sincea glass sheet placed between the flashlamp/ruby rodhousing and the Q-switch cell greatly reduces the rateof photochemical decomposition of the dye containedtherein, simply replacing quartz cells by glass ones maybe sufficient.

In the LSU Chemistry Department, we thank JamesG. Traynham for assistance with organic nomencla-ture; James W. Robinson for the loan of an arc lamp,its housing, and power supply; Sean P. McGlynn forthe loan of a monochromator; and Joseph Wander forhelpful criticisms of the manuscript. We thankBernard H. Soffer of Korad/Union Carbide (now atHughes Research Labs) for discussions of his unpub-lished work on this subject. One of us (R. A. H.) isindebted to the Sisters of Mt. Carmel (New Orleans)for releasing her from her ordinary duties for thesestudies.

This work was supported in part by the NationalInstitute of Neurological Diseases and Stroke.

References1. R. A. Jeffreys, Ind. Chim. BeIg, Suppl. 2, 495 (1959).2. B. H. Soffer, J. Appl. Phys. 35, 2551 (1964).3. Private communication to one of us (J. D. M.) during the Fifth

International Congress on Quantum Electronics (1969).

June 1972 / Vol. 11, No. 6 / APPLIED OPTICS 1363

4. R. M. Brown and R. J. Stone, Appl. Opt. 8, 2356 (1969).

5. B. L. Booth, Appl. Opt. 8, 2559 (1969).

6. Absorbance is the logarithm to the base 10 of the reciprocalof the transmittance. It is to be distinguished from absorb-

tance, the fraction of the incident beam absorbed by the sample.7. It is likely that most of the temperature rise was actually

produced by the conduction of heat from the air forced throughthe laser head to cool the ruby rod and fiashlamps, the tem-perature of which is elevated above that of the room by theblower motor. This suggests even more strongly that irradiation plays no more than a trivial part in the photochem-ical decomposition of the dye.

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1364 APPLIED OPTICS / Vol. 11, No. 6 / June 1972