DEVELOPMENT
of t h e continuously
tunable organic dye laser makes a whole new world accessi ble to spectroscopists, photochemists, and indeed to all scientists in terested in the interaction of light and matter. T h e key to this new world is continuous tunability. As light sources, the fixed-fre quency lasers developed in t h e last decade have proffered high inten sity, extreme spectral purity, co herence, and collimation to a n es sentially revolutionary degree. These characteristics have already been vigorously exploited in r e search in Rayleigh scattering, Brillouin scattering, R a m a n scattering, and nonlinear optical interactions on time scales ranging from picosec ond to steady state. For spectroscopy or photochemi cal applications, howe\ r er, these fixed-frequency lasers suffer one serious limitation—namely, that laser emission occurs only at cer tain narrow, discrete wavelengths t h a t 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 h a s any "fine-grained" control. T h e tunable dye laser shares the inten sity, 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. W h a t differentiates an organic dye molecule from other laser-ac tive materials and affords the dye laser its remarkable capacity to be continuously tuned over hundreds of angstroms? T h e answer, quali tatively, is t h a t a d y e h a s a broad, continuous fluorescence spectrum rather t h a n one or a series of nar row, 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 (stim ulated fluorescent) emission of a second photon of energy (wave length) equal to t h a t of the trigger ing photon. Thus, continuous tun ability requires a continuous emis sion spectrum from the laser-active material. Before examining the absorption and emission characteristics of t h e dyes more closely, and then devel oping a detailed theory of dye laser operation, it is worthwhile to re view 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. T h e laser-active material could be a suitable organic dye dis solved in a solvent, N d + + + -doped glass, C r + + +-doped A1 2 0 3 , H e - N e gaseous mixture, etc. T h e mirrors could be planar and accurately parallel or curved in such a w a y t h a t radiation trapped between them will reflect back upon itself and retraverse t h e active medium many times without "walking out." If a population inversion has been induced in t h e lasing material so t h a t there are more molecules in a higher energy level than a lower one, then trapped radiation with a frequency ν equal to this energylevel difference//! will be amplified by stimulated emission during each pass through the active region. The population inversion is essen tial for amplification, since the cross sections for stimulated emission and absorption are equal. T h e 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 prop agation, and phase as the triggering photon. Thus, stimulated emission continues to funnel energy prefer entially into the "trapped radia tion" mode. T h e source of this en ergy is the excitation t h a t " p u m p s " 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
30 A ·
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Report for Analytical Chemists
cited state, leading to its popula tion 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 or ganic dyes as laser-active mole cules and then develop a theory of dye-laser operation. An organic dye molecule is a large, compli cated quantum mechanical system. Rhodamine 6G, a particularly effi cient laser dye, is illustrated in Fig ure 2. When one considers the conju gated chain that comprises the chromophore of a dye, it is useful to approximate this complex sys tem as a core plus a ir-electron cloud. The core is comprised of the constituent atomic nuclei plus the tightly bound, inner-shell electrons, all bound together by «--bonding electrons. These σ-bonding elec trons are outer-shell, valence elec trons, but they are well localized (between atoms) in the σ bonds and do not participate in optical transi tions. In addition to valence electrons localized in bonding σ orbitals, there Figure 1.
are other valence electrons in -π or bitals. The -π orbitals of adjacent atoms overlap, forming, in essence, large (molecule-sized) wave func tions that are delocalized over most of the molecule. The electrons in these delocalized π wave functions are not so tightly bound as the σ-bonding electrons, and they are the electrons that are involved in the near uv, visible, and near ir op tical transitions. To a fair degree of approxima tion, the τ 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 (1). Thus, a (spatially) longer chromo phore, i.e., a longer conjugated chain with its associated -π elec trons, can be considered a longer "box" for the π electron wave func tions, and the energy-level spacing is correspondingly reduced. Every thing else being equal, then absorp tion and fluorescence will occur at 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 rota tion along different bonds, and the simple "electron-in-a-box" energy levels are split into many, virtually continuous levels. Figure 3 is a schematic repre sentation of the 7r-orbital energy levels of a dye molecule. Each submanifold, denoted by
