Tunable organic dye lasers - Analytical Chemistry (ACS Publications)

DEVELOPMEST of the continuously tunable organic dye laser makes a whole new world accessible to spectroscopists, photochemists, and indeed to all scientists interested in the interaction of light and matter. The key to this new world is continuous tunability. As light sources, the fixed-frequency lasers developed in the last decade have proffered high intensity. extreme spectral purity, coherence, and collimation to a n essentially revolutionary degree. These characteristics have already been vigorously exploited in research in Rayleigh scattering, Brillouin scattering, Raman scattering. and nonlinear optical interactions on time scales ranging from picosecond t o steady state. For spectroscopy or photochemical applications, however, these fixed-frequency lasers suffer one serious limitation-namely, that laser emission occurs only a t certain narrow, discrete wavelengths that are characteristic of the lasing material and hence are not a t the disposal of the chemist. However, if the chemist chooses a different laser-active medium and hence a different discrete laser wavelength, the wavelength of laser emission is not a variable over which he has any “fine-grained” control. The tunable dye laser shares the intensity, monochromaticity, coherence,

J . PIERCE WEBB Research Laboratories Eastman Kodak Co. Rochester, NY 14650

and collimation characteristics of the fixed-frequency lasers and in addition completes the arsenal by providing continuous wavelength tunability. What differentiates an organic dye molecule from other laser-active materials and affords the dye laser its remarkable capacity to be continuously tuned over hundreds of angstroms? The answer, qualitatively, is t h a t a dye has a broad, continuous fluorescence spectrum rather than one or a series of narrow, discrete fluorescent emission lines characteristic of other lasing materials. Lasing, of course, can occur only a t wavelengths where natural fluorescence is amplified. i.e., by a “chain reaction” process where incident photons interact with excited-state molecules, atoms. or ions, stimulating them downward to a lower energy state with (stimulated fluorescent) emission of a second photon of energy (n-avelength) equal to that of the triggering photon. Thus, continuous tunability requires a continuous emission spectrum from the laser-active materia 1. Before examining the absorption and emission characteristics of the dyes more closely, and then developing a detailed theory of dye laser operation, it is worthwhile t o review qualitatively the physical form of a typical laser. As indicated in

Figure 1, a laser-active material with gain G is placed between two mirrors. The laser-active material could be a suitable organic dye dissolved in a solvent, Nd-++-doped glass, Cr+ + +-doped A1203,He-Ke gaseous mixture, etc. The mirrors could be planar and accurately parallel or curved in such a way that radiation trapped bet ween them will reflect back upon itself and retraverse the active medium many times without “walking out.” If a population inr-ersion has been induced in the lasing material so that there are more molecules in a higher energy level than a lower one, then trapped radiation with a frequency v equal t o this energylevel difference/h will be amplified by stimulated emission during each pass through the active region. The population inversion is essential for amplification, since the cross sections for stimulated emission and absorption are equal. The quantum mechanical process of stimulated emission requires t h a t each secondary photon emitted in a stimulated transition have the same wavelength, direction of propagation, and phase as the triggering photon. Thus, stimulated emission continues to funnel energy preferentially into the “trapped radiation” mode. The cource of this energy is the excitation that “pumps” the laser-active material to its ex-

Tunable Organic Dye Lasers Potential uses of dye lasers are enhanced by their relative simplicity when compared to tunable parametric oscillators and even to many fixed-frequency lasers

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ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972

Report for Analytical Chemists

cited state, leading to its population inversion. Laser threshold occurs in a mode when the round-trip photon gain (from stimulated emission) exceeds the round-trip losses (from mirror transmission, scattering within the medium, absorption at the laser wavelength, etc.) in the mode. Organic Dyes as Laser-Active Media

After this qualitative review of a generalized laser, let us turn to organic dyes as laser-active molecules and then develop a theory of dye-laser operation. An organic dye molecule is a large, complicated quantum mechanical system. Rhodamine 6G, a particularly efficient laser dye, is illustrated in Figure 2. TThen one considers the conjugated chain t h a t comprises the chromophore of a dye, it is useful to approximate this complex system as a core plus a 7-electron cloud. The core is comprised of the constituent atomic nuclei plus the tightly bound, inner-shell electrons, all bound together by o-bonding electrons. These r-bonding electrons are outer-shell, valence electrons, but they are well localized (between atoms) in the u bonds and do not participate in optical transitions. I n addition to valence electrons localized in bonding u orbitals, there

are other valence electrons in X orbitals. The ?T orbitals of adjacent atoms overlap, forming, in essence, large (molecule-sized) wave functions that are delocalized over most of the molecule. The electrons in these delocalized x wave functions are not so tightly bound as the a-bonding electrons, and they are the electrons that are involved in the near uv, visible, and near ir optical transitions. T o a fair degree of approximation, the x electrons can be thought of as a one-dimensional electron gas of more-or-less free electrons. This picture has been extensively and quantitatively developed by Hans Kuhn in his “Electron Gas Theory of the Color of Dyes” articles ( I ) . Thus, a (spatially ) longer chromophore, i.e., a longer conjugated chain with its associated x electrons, can be considered a longer “box” for the 7 electron wave functions, and the energy-level spacing is correspondingly reduced. Everything else being equal, then absorption and fluorescence will occur a t longer wavelengths for longer chromophores. This is borne out rather well quantitatively in cyanine dyes of the same family but with various conjugated chain lengths. Because the molecule is so large and has so many constituents, i.e., so many bonds and internal degrees of freedom, there is a great deal of

opportunity for vibration and rotation along different bonds, and the simple “electron-in-a-box” energy levels are split into many, virtually continuous levels. Figure 3 is a schematic representation of the r-orbital energy levels of a dye molecule. Each submanifold, denoted by S,and T,, is a single “electron-in-a-box” level, and it is split into a continuum of vibrational levels (heavy lines) and rotational levels. Excited electronic states (1, 2, etc.) can be either singlet or triplet, with triplet submanifolds usually slightly lower in energy than the corresponding singlet levels. Excitation of the first excited singlet state (by absorption of pump light) and emission (spontaneous or stimulated) are indicated by heavy, yertical arrows. It is important to note that allowed (electric dipole) radiative transitions occur only between states of the same multiplicity, i.e.,

ocooE -

Figure 2.

Rhodamine 6G molecule

Figure 1. Typical laser cavity Mi, output mirror, partially transmitting: Mz, totally reflecting mirror; G, laser active material; €, excitation

ri ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972 ’

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SINGLET STATES

SPECTRAL IMAGING, INC.

Figure 3. Sche. matic representation of r - o r b i t a l energy levels of dye molecule

TRIPLET STATES

I

Z

Q c

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Heavy horizontal lines represent vibrational states; lighter lines represent rotational fine structure. Excita. tion and laser ernis. sion are represented by transitions A b and B -+ a, respectively. Other transitions represent losses in laser process

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k S T . Thermal equilibration is the most rapid relaxation process, occurring on a picosecond time scale, so the bulk of the excited dye molecules ends up in B , i.e., in the lowest vibrational and rotational states of the first electronic excited manifold. Laser action occurs through amplification (by stimulated emission) of naturally occurring, spontaneous emission from level B back to the ground electronic manifold. Once more, the most probable transition is to an excited vibrational state, a, in So rather than to the ground vibrational state A . Thermal equilibration occurs rapidly from these laser-terminus a states, and the dye molecule population in the So manifold maintains itself in a Boltzmann distribution. i.e., with only the lowest levels around A significantly populated. This fast thermal equilibration in the ground electronic manifold means that the molecular terminal states of the laser, the a states. will always be emptied. This rapid emptying of the terminal states means that population inversion between a-B levels is achieved relatively easily; laser threshold for

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rhodamine 6G can be achieved with fractional excited-state populations n*/n 0.1%. H

Dye Laser Threshold

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We observed t h a t laser threshold occurs when a high-enough population inversion is reached for photon gain from stimulated emission t o exceed photon losses a t the potential laser wavelength. I n addition t o extrinsic losses (imperfect mirror reflectivity, etc.) , loss mechanisms intrinsic to the dye are crucial for laser operation and ultimately determine the suitability of a dye for laser use. These intrinsic loss mechanisms include low quantum yield (nonradiative deexcitation from the excited singlet state, either by internal conversion or intersystem crossing) and self-absorption by the dye itself of fluorescent emission a t the laser wavelength. Intersystem crossing to the triplet manifold is particularly deleterious because of the long lifetime TT of this metastable state. Dye molecules continue to accumulate in T I where they are not only lost to the laser emission process, butf a r worse-where they can absorb photons from the potential laser mode by being promoted to excited triplet states in T 2 . Triplet-triplet absorption is a severe enough problem in general to limit dye laser operation to the much more restricted class of “low triplet yield dyes” (small k S T j ,and/or to fast systems where excitation peaks in times of the order of IZRT-l, or to systems with “triplet quencher” additives that reduce TT to the order of kS,-l. F a s t excitation generally means pumping by a Q-spoiled laser, or exotic fast flashlamp. Clearly, continuous wave (CUT) or long-pulse operation is feasible only when the triplet state is quenched preferentially compared t o the excited singlet. Fortunately, triplet quenchers have been discovered for a considerable variety of dyes ( 2 j , and intensive research continues t o expand the list of dyes for which triplet quenchers are available. The growth of intensity I of a laser beam as it propagates through the excited dye can be described by a Beer’s law gain equation of the form

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_ % --G I The gain coefficient G is given by (8)

G = n"uQ(A) - no~,y(h)nTaT(A)

-

(2)

where no,nit,and nT are the (number) densities of singlet groundstate, singlet excited-state, and triplet-state molecules, respectively; as(h) and (TT(A) are the (molecular) singlet-singlet and triplet-triplet absorption cross sections; and r is the average extrinsic loss per unit length of dye, incorporating all intracavity losses except the dye absorbances. The average intracavity loss owing to mirror reflectivities R1 and RP, for example, is given by r = --In(& X R 2 ) / 2 1, where 1 is the length of the dye cell. The stimulated emission cross section ug(A) can be shown ( % 4 ) to be uQ( A )

= A4F( A ) /8rrr,y~?7~ (3)

where rNis the spontaneous singlet decay time, A and c are the wavelength and velocity of light in vacuum, 7 is the index of refraction of the solution, and F ( h ) is the fluorescence line shape normalized so

1'

F (A) d~ equals the quantum

yield 4. The total dye concentration n is equal t o the sum of no,n", and nT. Equation 2 can thus be rewritten as

+

G = nH(ug us) nT(aT- us) - nus - r

(4)

Since the cross sections can be measured directly or calculated from measurable quantities, G is a function of four independent variables: h (through the wavelength-dependent cross sections), n", n T , and r and n* and nT can vary both spatially and temporally following the excitation intensity ( 5 ) . If the intracavity laser intensity I is high, n" can also depend on I through stimulated emission and absorption terms, and Equations 1 and 4 must be solved simultaneously. Great simplification is effected, and the essential physics is displayed if n" is assumed spatially uniform. This approximation corresponds experimentally to a cylin-

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drical dye cell uniformly pumped by a parallel flashlamp imaged in the cell and operated near threshold where I is low enough so stimulated processes do not change nn and I significantly over the length of the cell. With this spatially uniform gain coefficient, i t is not necessary to integrate laser-beam intensity over an intracavity round trip. Laser threshold, in particular, is defined by G = 0, and the threshold inversion is trivially derived from Equation 4. (n*/n)G=O=

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Equation 5 is to be evaluated a t the wavelength where laser emission occurs. This will occur a t some wavelength greater than that of the singlet-singlet absorption maximum, where generally u s ( h ) > T ~ as , a function of lasing wavelength X (nm) and normalized extrinsic loss r/n (cm*/dye molecule) Iso-loss contours are intersections of constant-loss planes with t h e zero-gain surface: t h e dotted line representing locus of minima of these intersections i s critical threshold inversion f o r self. tuning. Figure taken from Peterson et al. (3)

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ANALYTICAL CHEMISTRY, VOL. 44, NO. 6. h l A Y 1972

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Output wavelength (nm) Figure 5. Tuning range of flashlamp-pumped rhodamine 6G laser with four intracavity prisms Flashlamp input, 100 J; laser output energy, > / a J/pulse at 590 nm.

straining possible laser action to this specified (low-loss, retrorefleeted) wavelength is equivalent to choosing a specific, constant-wavelength plane in Figure 4. The value of the extrinsic loss a t the specified wavelength is still arbitrary, depending upon losses introduced by the dispersive element, mirror reflectivity, etc. Laser threshold in this externally tuned configuration occurs a t the intersection of the constant, “laser wavelength” plane with that iso-loss contour in the zero-gain surface whose value is the value of r/n a t the laser wavelength. This configuration with a dispersive loss in the cavity is denoted “externally tuned,” since threshold is no longer constrained to appear on the locus of iso-loss minima. Tuning is accomplished simply by rotating the grating or the cavity mirror adjacent to the prism, so a different wavelength will become the retroreflected, low-loss wavelength for the dispersive cavity. Figure 5 shows the tuning range of a prism-tuned, flashlamp-pumped rhodamine 6G dye laser ( 7 ) . Introduction of a dispersive element in the cavity not only pro-


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Figure taken from Strome

and Webb (7)


vides for tuning but also narrows the spectral width of the laser. The spectral width ( F W H M ) of a n untuned (self-tuned) laser, operated xell above threshold, is typically 30-40 A. With four prisms in the cavity, the spectral width of the same laser operated under the same conditions fell to less than 2 b ( 7 ) . Further spectral narrowing to 0.01 A is possible if a titled Fabry-Perot etalon or Lyot filter is introduced into a cavity that is already coarsetuned (8, 9 ) . This spectral narrowing is remarkably efficient; output-power reductions of less than a factor of 2 are reported with coarse tuning and of only a factor of 4 with a Fabry-Perot filter in the cavity. Dye Lasers: State of the Art and Intrinsic Limitations

K e have noted that laser emission from a given dye can be tuned over seveml hundreds of angstroms. Solvent and pH changes may induce shifts (of several hundred angstroms) and/or extend the tuning range for a given dye substantially. Extreme examples of this pH-dependent tuning-range extension are

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umbelliferone and 4-methylumbelliferone (10, 1 1 ) , where each dye can be tuned over 1500 A in critically adjusted, slightly acidic solutions excited by a fast (2-3-nsec rise time) , pulsed Ir;, laser. These broad tuning ranges represent laser emission from excited neutral and protonated exciplex states of the dye that are simultaneously present in the critically adjusted solution. The spectral range of laser emission can be extended further by substituting a different fluorescent dye (and possibly different mirrors and excitation sources peaked a t different wavelengths to match the new dye, as well). Pulsed laser emission from 3410 to 1168 A has been reported from a variety of organic dyes ( 1 2 - 1 5 ) . Generally speaking, the near-infrared dyes have been excited with Q-spoiled lasers, either ruby or neodymium. Niyazoe and 1Iaeda ( 1 5 ) report overlapping tuning ranges from 19 polymethine dyes that cover the range continuously from 7100 to 10,600 A. Little is reported about the stability of these infrared dyes, but rapid photodegradation of the dye molecules in solution is to be anticipated. Lack of dye stability can 1)robably be expected to limit any great extension of the tuning range toward longer wavelengths. The stability problem with infrared dyes is intrinsic, in that longer wavelength emission implies closer energy-level spacing and hence a longer conjugated chain to give a larger “free-electron box.” These larger molecules with correspondingly more bonds are inherently less stable. Tunable infrared radiation can he obtained further into the infrared, however, by mixing the output of a tunable dye laser with another intense radiation source in a nonlinear crystal (oriented for phase matching a t the difference frequency) and utilizing the difference frequency. Dewey and Hocker (16) have demonstrated the feasibility of this scheme, in the 3-4-p region. They split the output of a Q-spoiled ruby laser, used one part to pump a tunable D T T C iodide dye laser, and then combined the dye-laser output with the second part of the ruby-laser

output in a LiNbOs crystal t o get approximately 6 kW of tunable infrared power. This scheme should extend tunable sources as far into the infrared as transmission characteristics of the nonlinear crystals permit. This is about 4.5 p for LiNb03, but Dewey and Hocker believe that proustite (Ag3AsS3) and pyrargyrite (Ag3SbS3) may extend the range to 13 p. They also speculate t h a t the second harmonic of ruby (0.347 p ) and continuously tunable dye lasers operating from 0.4 to 0.7 p should cover the 0.7-2-p region. Visible and near-ultraviolet dye lasers are pumped by ultravioletrich fast flashlamps and shortpulse lasers (S, or Q-spoiled, frequency-doubled neodymium and r u b y ) . A commercially available, Si laser-pumped dye laser provides continuous coverage from 3600 to 6500 A with nine interchangeable dye solutions. Peak power in (repetitive) laser pulses is in the kilowatt range with nanosecond-range pulse durations. Fast flashlamp-pumped dye lasers cover essentially the same spectral range with peak powers over 10 k W and submicrosecond t o microsecond pulse duration. Output pulse energies exceeding a joule with peak powers over a megawatt have been achieved with xanthene dyes. Stability is generally not a problem with these dyes. Tunable power can be obtained further into the ultraviolet by frequency doubling the output of a dye laser. For example, Johnson and Swagel have frequency doubled a rhodamine B dye laser by use of a KDP crystal oriented for index matching ( 1 7 ) . They achieved a 3-K wide output a t 2950 A from a 160-A wide input a t 5900 A, which could be fine tuned by changing the index-matching orientation angle. A significant recent development is the cw dye laser. Following Snavely and Schafer’s work on oxygen-quenching of the rhodamine 6G triplet state ( 1 8 ) ,Peterson et al. developed a truly continuous rhodamine 6G laser t h a t was pumped with an argon ion laser ( 1 9 ) . Since then, tunable cw dye lasers using fluorescein, rhodamine 6G, and rhodamine B with triplet quench-

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ANALYTICAL CHEMISTRY, VOL. 44,

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is the new look inunbreakable Nalgene"Burets 'hese burets do everything glass does . . except break. Now you an have an individuallycalirated transparent buret iat will not break in normal se. Only a slight meniscus lakes readings easier, more ccurate. The crystal clear crylic body, tip, and leakroof stopcock are unaf?cted by all t h e usual tiants. The Teflon TFE plug ever needs lubrication, is a leasure to turn. Stopcock ssembly easily removed for traight-through cleaning. learly the precision burets )r industrial labs, schools, Id in the field. Sizes: 10, 5, 50, 100 ml. (Cat. No. 550). Order from your Lab upply Dealer. Ask him for ur Catalog, or write Dept. 317,Nalgene Labware Divion, Rochester, N. Y. 14602.

ers have been reported ( d 0 , Z I ) t h a t span the spectrum from 5200 t o 6600 A. Tuned output powers exceed 100 mW, and spectral bandwidths can be less than 0.01A. Thus far, the only continuous source bright enough t o pump the dyes to a threshold excited-state population density is an argon ion laser focused in the dye solution. This has limited cw laser dyes to efficient xanthene-type dyes t h a t ahsorb the primary 4880 and 5145-A argon laser lines; the Stokes-shifted spectral output range is correspondingly limited. Probably the only real limitation of the dye laser is t h a t it cannot be Q-spoiled to give tremendously high peak Dowers. This limitation is in&si; to dyes because their high spontaneous fluorescence rates preclude any integrated pumping up of excited-state population for times greater than their spontaneous lifetimes ( a few nanoseconds). Compared with ruby and neodymium with their millisecond lifetimes during which excited-state populations can be pumped up, dyes obviously do not provide the energy-storage capacity for extremely energetic pulses. For pumping times longer than a few nanoseconds, dye laser output will be in equilibrium with pumping intensity. For the scientist in need of tunable, coherent, monochromatic, collimated light a t cw power levels approaching a watt or with pulsed energies measured in joules and powers in megawatts, the organic dye laser should prove invaluable. The potential impact of dye lasers is further emphasized by their inherent simplicity (compared t o tunable parametric oscillators and even to many fixed-frequency lasers) and the low cost of the laser-active material. References

(1) Hans Kuhn, Fortsch. Chem. Org. Naturst., XVI, 170 (1958); XVII. 404 (IRKRI ,. . ..,.

B. MarlinR, D.W. Gregg, and L. Wood, Appl. Phys. Lett., 17,527 (1970). (3) 0. G. Peterson, J. P. Webb, W. C. McColgin, and J. H. Eherly, J . Appl. Phvs.. 42,1917 (1971). (4) B. B. Snavely, Proc. IEEE, 57, 1374 (2) J.

(1969). , ~ ~ ~ ~ , (5) S. A. Tuccio and F. Strome, Appl. Opt., 11,64 (1972). (6)J. P. Webb, W. C. Mecolain, 0. G. CIRCLE 146 ON READER SERVICE CARD

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Peterson, D. L. Stockman, and J. H. Eberly, J . Chem. Phys., 53, 4227 (1970). (7) F. C. Strome, Jr., and J. P. Webb, A w l . Opt., IO, 1348 (1971). (8) D. J. Bradley, A. J. F. Durrant, G.M. Gale, M. Moore, and P. D. Smith, IEEE J . Quant. Electron., 4, 707 f,. l. W >. __,

(9) H. Walther and J. L. Hull, Appl. Phys. Lett.. 17,239 (1970). (10) C. V. Shank, A. Dienes, A. M.

Trozzolo, and J. A. Meyer, ibid., 16,

405 (1970). (11) A. Dienes, C. V. Shank, and A. M. Trozzolo, zbzd., 17, 189 (1970). (12) H. W.Furomoto and H. L. Ceccon, IEEE J . Quant. Electron., 6, 262

i~-".-,. iwm (13) L. D. Derkacheva, A. I. Krymova, A. F. Vompe, and I. I. Levkoev, Opt. Spectrosc., 25,723 (1968). (14) L. D. Derkacheva, A. I. Krymova, V. I. Malyshev, and A. S. Markin, JETP Lett., 12. 468 (1968). (15) Y. Miyasoe and M. Maeda, Appl. Phws. Lett.., 12., 206 (1MR). (l6),C. F. Dewey and L. 0. Hocker, zbtd., 18, 58 (1971). (17) F. M. Johnson and M. W. Swagel, Avvl. Ovt.. 10. 1624 (1971). (l8fB. B. Snavely and F. P. Schifer, Phws. Lett., 28A, 728 (1969). (19) 0.G. Peterson, S. A. Tuccio, and B. B. Snavely, Appl. Phys. Lett., 17, 245 (1970). (20) S. A. Tuccio, 1971 IEEElOSA Conf. ~

~

on Laser Eng. and Appl., IEEE J . Quant. Electron., 7,12 (1971). (21) M. Hercber and H. A. Pike, 1971 IEEE/OSA Conf. on Laser Eng. and Appl., ibid., 13 (1971).

. * i n *

J. Pierce Webb is an experimental physicist in the Solid State and Mo-

lecular Physics Laboratory a t the Research Laboratories of Eastman Kodak Co., Rochester, N Y . H e has been engaged in organic dye laser research since joining Kodak in 1968. Dr. W e b b received his BA, summa cum laude in physics, f r o m Harvard College 1967 and earned his P h D in 1968 from Stanford University for critical opalescent lightscattering measurements in He8. H e is a member of the American Physical Society and Sigma Xi.

Tunable organic dye lasers - Analytical Chemistry (ACS Publications)

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