edited by
GALEN W. EWlNG Seton Hall University SoMh Orange. New Jesey 07079
XCIV. Dye Laser Instrumentation Robert 6. Gre'en West Virginia Universiry Morgantown, West Virginia 26506
In the ten years since stimulated coherent emission was first observed from an organic dye, the status of the laser has undergone a subtle but distinct change. The use of the laser was initially confined t o the research laboratory as the exclusive province of the physicist. In recent years, dye laser technology and applications have grown rapidly and chemists are becoming the dominant user group. T o accommdate this growth and diversification of interest, dye lasers have hecome more reliable, easier t o operate, and available a t alower cost. The purpose of this paper is t o make dye laser technology more accessible to scientists and teachers ofvarying background and t o describe the special capabilities of dye lasers, along with their limitations. Although other "tunable" lasers are available, their tuning ranges are extremely limited andlor the complexity of the assoeiated equipment to achieve proper operation is prohibitive for other than specialized research. The dye laser possesses broad tunability and high power in a relatively simple laboratory device. Dye solutions are especially convenient as the active medium in a laser because a high optical quality can be obtained and cooling is simply achieved by a flow system. Moreover, the liquid dye is self-repairing whereas damage t o a solid-state medium is generally permanent. The properties attributed to "fixed-wavelength" lasers such as monochromaticity, coherence, and collimation are also integral to the dye 1.~01
Srveral papers have deserihed the theory undrrlying dye laser operation in detml, bur n briridiscussmn w~llsrn.a to eluridare rhc fundamental reasons for the unique nature of laser light (1, 2). Initially, i t should he painted out that the term "dye" has been assimilated into the vocabulary of laser technology and this paper will abide by this well-established convention. I n the proper sense, "dye" means compounds which absorb energy (light) strongly in the visible part of the snectrum. In connection with lasers it will be u&d in a broader sense t o include oreanic wmpuundr which h m r n strong absorprion lmnd a n w here in the r q l o n irum the ultmviolet to t he near infrared. A conspicuous distinction between the spectra of organic dyes, on the one hand, and atoms or ions, on the other, is the width of their absorption bands; the former usually
covering several tens of nanometers. This is readily understandable since a typical dye molecule may possess fifty or more atoms. Obviously, there is no simple interpretation of the quantum mechanical svstem of complex organic molecules, but a diagram such as Figure 1 should illustrate the important process% involved in the achievement of laser action. When a molecule absorbs a quantum of energy of appropriate wavelength, a transition can take place t o an upper electronic state (St). Such a transition will usually originate in the lowest vibrational level of the ground electronic state (A) since thislevel is significnntly populated at n m n temperature. I he must probaldr opt~calinternalon t n m thuuound s u t e will hrwiththevlbrationally
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SINGLET STATES
TRIPLET STATES
..*/ A
Figure 1. Energy level diagram fwa typical organic molecule.
and rotatianally excited states of the first excited singlet manifold (Franck-Condon principle). The excited singlet state has afinite lifetime on the order of 1 0 V sec for organic molecules. During this time interval any absorbed energy in excess of the lowest vibrational energy level (B) is rapidly dissipated through intermolecular collisions and other mechanisms. The l w of this energy t o (Continued on page A366)
Dr. Robert B. Green is Assistant Professor of Chemistry a t West Virginia University, Morgantown. He received a B.S. in Chemistry from Oklahoma State University in 1966 and a Ph.D. in Analytical Chemistry from Ohio University in 1974. Dr. Green served as a National Research Council postdoctoral research associate a t the National Bureauof Standards, Washington, D.C. from September 1974 to August 1976. In the years between 1966 and 1970, he was a chemist with Monsanto Company and Jefferson Chemical Company, Ine. His major research interest is the application of lasers to the solution of problems in analytical chemistry. Dr. Green participated in some of the initial investigations of intracavity absorption of laser emission. More recent work has involved the characterization of analytical atomic fluorescence with eantinuous wave dye laser excitation and the development of techniques for detecting laser-excited absorntion nrocesses bv electri. cnl methods. Other awns of interest are mdevular luminrscrnce, isou,pe analysis, h e r profdmp of analytical emission sources, and laser probing of chemiluminescence. He has authored and co-authored several papers in these fields. He is a member of the American Chemical Soeiety, Phi Lambda Upsilon, Phi Kappa Phi, and Sigma Xi.
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DYE LASER AND ACCESSORIES ADVERTISING IN THIS ISSUE Candela Corp.
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Volume 54, Number 9, September 1977 / A365
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Chemical lnstrun~entation heating the solution or partition into other modes of vibration and rotation is called internal conversion and occurs without the emission of radiation. Once the excited molecule has relaxed to the lowest vibrational level in SI,several routes for energy dissipation are available. There is a possibility that intersvstem crossine can occur to the trinlet ) whencr thrrrcan be return to ,taw ( T ifrom thr pound state with emission of a photon ~phusphorrsccncr~, or murr likely, loss of the energy from the long-lived triplet state through internal conversion. When a transition occurs from the lowest vibrational level in the excited singlet state t o one of the rotational-vibrational levels (a) in the ground state vieldine a ohoton. this swntaneous emission is called fluorescence. Sinelet-singlrt tronsitionsarr generally more probable than single-triplet trat~sitions,but when thr energy difference is small between singlet and triplet levels (as is the ease in certain dye molecules), intersystem crossing can be competitive with fluorescence. The intermediate level (B) can also be depopulated by stimulated emission. In stimulated emission, an excited molecule is perturbed by a spontaneously emitted photon and emits its photon prematurely. This can only occur if the initiating photon has exactly the energy of the one that otherwise would have been emitted spontaneously. The resulting emission falls precisely in phase with the wave that trieeered its release and is .... identical in wavelength. This procesr has a prohnhility rhnt isdrpendmr upcm the dtnslty of excited specie-. If the density of e x cited species and photons is high enough, stimulated emission will predominate. The precedence of stimulated emission over spontaneous emission is the basis for achievine laser action (lieht amolification hv stirnulnred emission of radiation). For this tu wrur, o p8,pulntion inversmn must take place, that is, there must be more molecules in the excited state than the ground state. The Boltzman distribution predicts that this is difficult, if not impossible, for s two-level system. By following an absorption-emission cycle, one can see that population inversion is relatively easy for many organic molecules since their energy levels constitute a multilevel system. The initial state for laser emission (B) is populated in averyshart time once the molecule is excited. The terminal states for laser emission (a) are essentially empty since thermal equilibration of these levels occurs rapidly a t room temperature. Optical sources are used t o excite the organic molecules. This activating process is known as "pumping". The risetime of a pumping source must he less than the lifetime of the excited state (initial laser level) of the organic molecule. This means that the time required t o invert the population must he sufficiently short or the excited singlet state will he depopulated by spontaneous emission. It is necessary t o enclose the organic dye solution in a structure that prevents photons from escaping in unwanted directions. This can be accomplished by enclosing the laser medium in an optical cavity which, in its simplest form, can consist of a cylindrical tube containing the dye, bounded by two
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(Continued on page A370) A366 / Journal of Chemical Education
Chemical Instrumentation exactly parallel mirrors (Fig. 2). A wave that travels along the axis of this system will grow by stimulated emission until it reaches one of the mirrors where i t will he reflected hack into the active medium. Growth of the wave will continue and if the gain on repeated passages through the dye is sufficient t o compensate for losses within the cavity, a steady wave will he built up. Any wave which is inclined a t an angle t o the long axis of the cavity will he lost after only a few reflections, or perhaps without ever striking one of the mirrors. The laser action is triggered by the first photon to he emitted spontaneously after the system has been pumped t o the excited state. In effect, the waves that are
emitted spontaneously are amplified since they continue to accumulate energy as they are reflected back and forth between the mirrors. If one of the mirrors is semitransparent, a portion of the wave can escape through it, constituting the output of the laser. Organic molecules are intrinsically restricted to generation of stimulated emission over a limited spectral region. Dyes have been found whose laser emission covers the nearultraviolet through the near-infrared. The short wavelength limit is due to the photochemical decomposition of the dye that would he induced bythe high-energy pumping necessary for ultraviolet emission. Dye molecules, whose emission would he in the near-infrared, possess low-lying energy states which would he easily accessible by thermal
excitation, creating reactive species which are subject to decomposition. Several dyes are necessary t o cover the entire wavelength range accessible t o dye lasers (Fig. 3). Tunable coherent radition in the ultraviolet can he obtained from organic dyes hy second harmonic generation in a nonlinear crystal (for further discussion see below and reference (3)).
PUMP
WDRTION
Figure 2. Schematicdiagram of adye laser. MYisthe totally reflecting mirror. M. is the partially hansmining output minor, The dye cell has "Brewster angle" windows. Reflectionlasses of approximately 4 percent occur at each of the interfaceswhen the end windows are normal to the axis of the optical cavity. Bv ssnins these windows at the oolarizino angle. thy will have a 100 percent transmission for light whose electric vector is parallel to the plane of incidence.
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The dye laser may he "tuned" over the gain curve of a particular organic dye molecule (Fig. 3). This can be accomplished in essentiallv the same manner reeardless of the t w e u i d w laser. All l a s ~ r sposses^ longitudinal mode ElruCturc airh the sparing between the modes determmed by the 1en:t h ofthe optlcal cavity. For example, a one meter long He-Ne laser will have several longitudinal modes oscillating under the extremely narrow profile of its discrete emission line (Fig. 4). The coarse tuning gain curve of a dye laser accommodates literally hundreds of oscillating cavity modes. The insertion of a frequency selective device allows the oscillation of one or more of these modes depending upon the device's transmission eharaeteristics. All tuning methods, in this way, seleetively reinforce the desired wavelength-the resolution of the selective device determining the bandwidth of the laser emission. Prisms
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is dependent upon the birefringent properties of crystalline quartz (4). The proper alignment of the crystal axis and polarization vector of several stacked quartz plates of varying thickness produces a narrow wavelength transmission window. When the birefringent filter is inserted within a laser cavity a t Brewster's angle, the gain characteristics of the dye give rise to tunable laser emission as the filter is rotated about the perpendicular to its face. The birefringent filter itself determines the wavelength and not the alignment of the tuning element within the resonance cavity. Therefore, the filter can be removed and replaced without realignment of the optical cavity. An etalon is used singly or in multiples when there is a requirement for narrower linewidths. With (Continued o n page A372)
A370 / Journal of Chemical Education
Chemical instrumentation combinations of etalons of varying thickness (i.e., free spectral range), i t is possible to attain single mode operation ( 1 0 - b m ) . An etalon is a small Fabry-Perot interferometer based on the interference of multiple reflections between two optically coated plates. Two types of etalons are used in dye l a s e r s solid and air-spaced. The solid etalons obviously are limited to a fixed spacing between the coated surfaces and therefore are "tuned" bv tiltine. ,.. which ehanees the effective oath Im@h The wwelength range scanned hai am ;angular dependence and ls n
