Photochemistry and Photobiology Vol. 42, No. 3, pp. 331 - 334, 1985 Printed in Great Britain. All rights reserved

003 1 -8655/85 $03 .OO + 0.00 Copyright 0 1985 Pergamon Press Ltd

TECHNICAL NOTE

DYES AND DYE MIXTURES USEFUL FOR GENERATION OF UV IN A FLASHLAMP DRIVEN TUNABLE DYE LASER

JOHN WHEELER and JOHN CALKINS*

Albert B. Chandler Medical Center, Department of Radiation Medicine, University of Kentucky, Lexington, KY 40536, USA

(Received 21 February 1985; accepted 10 May 1985)

Abstract-We have used a flashlamp driven tunable dye laser combined with angle tuned frequency doubling crystals for producing UV-B radiation for action spectra studies of various organisms. Optimum UV-B power generation is needed to provide biologically effective doses at wavelengths greater than 300 nm. Optimizing power will also serve to lengthen the lifetime of dyes and other laser components at shorter wavelengths where UV-B output is more than adequate. While much information is available on dyes and dye performance from manufacturers, little information is available on the use of dyes and dye mixtures for providing the continuous high power spectrum of wavelengths necessary for biological UV action spectroscopy. We have examined a number of dyes and dye mixtures for optimal laser performance at wavelengths from 260 to 330 nm. The dyes and dye mixtures discussed here provide adequate power output in the UV-B wavelength range and have allowed us to perform numerous UV-B action spectra studies using the tunable dye laser.

INTRODUCTION

Biological responses to non-ionizing radiations such as killing and mutation often require delivering amounts of energy or power to the target organism which are near the upper limits of output of the currently available sources, arc-monochrometers and tunable dye lasers. The problems of generating adequate power are particularly troublesome for wavelengths longer than 300 nm where the biological effectiveness for damage begins to fall. The biologic- aleffectiveness may be 103-104 times less effective for production of a given biological response compared to the energy required for response at wavelengths shorter than 300 nm [Webb and Brown, 1976; Mackay et al., 1976; Setlow, 1974).

The use of the tunable dye laser combined with frequency doubling crystals has been in progress in this laboratory since 1980. Using the dyes and dye mixtures noted here, the laser system generates an adequate level of power for efficient UV-B action spectra studies of the organisms we have tested. It is, however, desirable to operate the laser system at its optimal efficiency even at wavelengths where the system generates power far above the biological requirements. The flash tube, dyes and frequency doubling crystals last longer and are less subject to damage when the system operates efficiently; also a lower optical pumping energy is needed. By lowering the energy input to the flash tube for each flash or “shot”, the damaging effects of the optical pumping flash on the lasing dye and on the flash tube itself are reduced.

*To whom correspondence should be addressed.

Proper dye selection is critical in obtaining an efficiently operating system. Manufacturers of dyes and lasers provide information about dyes and dye performance. However, the lasing range of commer- cially available dyes does not provide all the wavelengths needed for UV action spectra studies. Mixtures of dyes, for which much less information is available, were used to generate UV in wavelength regions in which available pure dyes were not usable. We have investigated dyes and dye mixtures in a largely empirical manner. There have been reports that mixtures of dyes can improve the power output or tuning range of laser dyes (Lin and Dienes, 1973; Pavlopoulos, 1978; Burlamacchi and Cutler, 1977; Sebastian and Sathianandan, 1980). While a theore- tical approach to formulating dye mixtures would seem the most desirable approach, we have not been particularly successful in predicting the behavior of dye combinations. We have found, as noted here, some dye combinations which seem to work well within the constraints of our system and its require- ments.

MATERIALS AND METHODS

We used and discarded considerable volumes of dye solutions. If the dye is dissolved in methanol, the disposal of spent dye solutions is relatively simple. The methanol is distilled for re-use and the small residue is disposed of in accordance with standard chemical waste procedures. AH dyes and dye combinations reported here were dissolved in methanol, as we have not found any obvious advantage in using alternative solvents. Redistilled methanol was used in most cases, being supplemented with ACS certified grade methanol which was added to compensate for various losses. Dyes were purchased from Exciton Chemical Co. and were not further purified or treated; the Exciton designation of dyes is used in this paper.

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332 JOHN WHEELER and JOHN CALKINS

The dyes were operated in a Phase-R DL 1400 dye laser (for more details see Calkins et a/., 1983). The system is equipped with a “triax” flash tube, a system where water is circulated between the flashlamp and the circulating dye. As previously noted, caffeine (1 glt) added to the cooling water greatly improved the dye lifetime (Calkins et al., 1982). and was used in all cases.

Wavelengths were set using a Beckman DU spec- trophotometer which had been calibrated with a mercury discharge calibration source and a HeNe laser. Power output was measured by a Scientech Power Meter Model 361. The primary detector for this system is a sensitive thermopile with an internal resistive heater system for verification of the absolute calibration. The thermopile absorption surface exceeds the size of the laser output beam. Thus, the total radiant power (not power density) was measured.

After frequency doubling, the main beam is separated from the second harmonic by a 60” quartz dispersing prism as well as a UV transmissive Schott glass filter. Both main beam and UV pass through the quartz dispersing prism, a quartz turning prism, and a first surface mirror prior to power measurements; the UV beam path includes the same components but is also filtered through the Schott glass filter. The power levels noted in the results represent the usable power output of the system and have not been corrected for losses through the various optical compo- nents. We removed the doubling crystals from the beam to measure main beam power.

All frequency doubling operations utilized an angle tuned KDP crystal (3 cm in length) purchased from Quantum Technology Inc. Three crystals were used, which were cut to 530,610 and 630 nm. The crystals were mounted using AR wicdows and Fluorinert Electronic Liquid type FC 104 as the optical coupling fluid (kindly supplied by the 3M company). The main beam was focused at the center of the crystal using a20 inch (nominal) focal length quartz focusing lens.

RESULTS AND DISCUSSION

The main beam and frequency doubled power output of the system measured at 5 nm intervals from 260-330 nm is illustrated in Fig. 1. The operating efficiency of frequency doubling was determined by computing the ratio of UV power divided by the main beam power measured without the crystal. The operating efficiency (in percent) is plotted in Fig. 2.

There are a number of parameters which control the laser operation and power output. The voltage applied to the flash tube and the repetition rate give the most direct output power control. Laser and crystal tuning were relatively stable but required slight adjustments from time to time. The most variable parameter was dye temperature and its relationship to cooling water temperature. The dye and water were regulated to maintain a constant differential between dye temperature and water temperature but small variations occurred during the control cycles and influenced the UV power output. The main beam power output was generally more stable than the frequency doubled power. Some operating parameters which increase main beam power reduce the UV output. For instance, shorten- ing the laser cavity can increase main beam power but reduce the UV output (we operated with a 1.42 m cavity). The main beam power can be increased by raising the voltage on the condenser discharging through the flash tube (thus increasing the optical pumping) causing the visible output to be consider-

Figure 1. Measurements of main beam power (filled symbols) at double the wavelength of the UV measurements and frequency doubled power output from 260 nm to 330 nm at 15 kV (O), 16 kV (A) and

17 kV (0).

Technical Note

I I I I I I I I I I I I 1

333

4 CRYSTAL CUT FOR 4 I- CRYSTAL CUT FOR /+ CRYSTAL CUT Fc4 63odil t 530 nM 610 nM

Figure 2. Doubling efficiency of KDP crystals calculated as UV powerhain beam power.

ably increased (Fig. 1). At low power levels the UV output of a crystal will increase in proportion to the square of the main beam power. However, at higher powers the UV output becomes only a linear function of main beam power and in practice little gain in UV output may be achieved by continuing to raise the flash tube voltage above an optimum operating level (Fig. 1).

The dyes and dye mixtures which we have found useful are indicated in Figure 1. Dyes deteriorate with use; however, the dye solutions noted in Fig. 1 all have lifetimes which we consider generally satis- factory, lasting 10000 to 30000 shots per liter of dye. If it is desirable to maintain a constant dose rate, as the dye deteriorates, the flash tube power can be increased to maintain a given output power level. It is a relatively minor operation requiring 5 to 10 min to remove a spent dye solution and replace it with a fresh dye solution of the same type.

We have found the results of mixing dyes to be highly unpredictable. For instance, Coumarin 523 (CS23)t lases well at 523 nm and can be used in combination with several other dyes. If, however, C523 is mixed with a small amount of Rhodamine 560 or Rhodamine 57.5 it will lase (poorly) at 500 nm. When the Rhodamine 560 concentration is increased sufficiently the mixture will lase at 540 nm but not at shorter wavelengths, and can be used up to 550 nm. In general, among the laser dyes we have tested, mixtures of the better laser dyes function better than

tilbbrevicrtions: nm, nanometer; C523, coumarin 523; KDP, potassium dihydrogen phosphate; AR, anti- reflection.

mixtures containing poorer dyes. It has been prop- osed that mixtures containing “booster dyes” which may not necessarily themselves be lasing dyes might form an improved mixture (Pavlopoulos, 1981); we have not tested any such combinations.

Dye lasers are often pumped by a monochromatic source, such as a nitrogen laser. In such systems if the laser dye is a poor absorber of energy at the pumping wavelength then a mixture of dye may increase lasing energy by transferring energy from a good pump energy absorber to the lasing dye. The advantages of such mixtures are obvious (Lin and Dienes, 1973; Speiser and Katraro, 1978; Cox and Matise, 1980). The rationale for using dye mixtures in flashlamp pumped lasers is more obscure. The flashlamp pro- duces a broad spectral distribution of pumping radiation, some of which will be absorbed by the lasing dye. However, dye mixtures have been shown to increase lasing energy in flashlamp driven systems. It is possible, but unlikely, that both dyes in a mixture might contribute to lasing. The more probable condi- tion is that one dye absorbs energy and transfers it to the second dye, the donor dye absorbing energy at a shorter wavelength than the lasing dye.

Energy transfer from one excited molecule to other molecules in solution can involve a variety of mechanisms, both by radiative and non-radiative processes (Forster, 1965; Lin and Dienes, 1973). The precise quantitative determination of the rate of energy transfer in two dye solutions is most difficult, either experimentally or theoretically. Temperature, viscosity, dye concentrations, and optical properties of the solvent as well as chemical interaciions of the dyes all effect the rate of energy transfer. Efficient

334 JOHN WHEELER and JOHN CALKINS

energy transfer rtquires the fluorescent emission spectrum of the donor dye to strongly overlap the absorption spectrum of the acceptor dye. Factors other than the optical spectra of the donor and acceptor dyes are also critical since we observe concentration dependent shifts of the optical lasing wavelength due to the relative concentrations of the donor and acceptor dye. However, the dependence of power output upon absolute dye concentration as we have tested it seems to be relatively noncritical near the optimum operating concentrations. Changes of 10 to 20% in the concentration of the dye or dye mixture often produce no detectable change in power output.

We have noted here some of our experiences regarding techniques for the production of UV from 260-330 nm. It would seem to us to be very useful if others working in the dye laser area might publish similar information. There is no doubt that new dyes and dye combinations will become known which function better than those we have found. An active exchange of such observations among the increasing numbers of dye laser users would benefit all research in this new area.

Acknowledgemenis-This research was supported in part by a grant from the National Institutes of Health (RR01620).

REFERENCES Burlamacchi, P. and D. Cutter (1977) Oprics Cnmm.

22, 28F287. Calkins, J . , E. Colley and J . Hazle (1982) Optics

Comm. 42,275-277. Calkins , J., E. Col ley , J . Hazle and M. A .

Cox, A. J. and B. K. Matise (1980) Chem. Phys. Lett.

Forster, Th. (1965) In Modern Quantum Chemistry (Edited by 0. Sinanoglu), Part 111, pp. 93-137. Academic Press, New York.

Lin, C. and A . Dienes (1973) J . Appl. Phys. 44, 5050- 5052.

Mackay, D . , A . Eisenstark, R. B. Webb and M. S. Brown (1976) Phorochern. Photobiol. 24, 337-343.

Pavlopoulos, T. G. (1978) Optics Comm. 24, 170-174. Pavlopoulos, T. G . (1981) Optics Comm. 38, 299-302. Sebastian, P. J. and K. Sathianandan (1980) Optics

Setlow, R. B . (1974) Proc. Natl. Acad. Sci. USA 71,

Speiser, S . and R. Katraro (1978) Optics Comm. 27,

Webb, R. B. and M. S. Brown (1976) Phorochem.

Hannan (1983) Phorochem. Phoiobiol. 37, 669.

76, 125-128.

Comm. 35, 113-114.

3363-3366.

287-291.

Photobiol. 24,425-432.