optical waveguides. photon plumbing for the chemistry lab: fiber optics, waveguides, and evanescent...

llan Chabay' Center for Analytical Chemlstv National Bureau Of Standards Washington. D.C. 20234

Instrumentation - Optical Waveguides

Photon Plumbing for the Chemistry Lab: Fiber Optics, Waveguides, and Evanescent Waves

as Tools for Chemical Analysis

Guided wave optics has had a tre- mendous impact on several areas of technology in the past several years. Communication by fiber optics, integrated optical circuits for filtering and switching, and fiber optic acoustic, magnetic field, temperature, and pressure sensors are applications that are being developed and exploited. Are these new devices and concepts also important for analytical chemistry? The answer clearly is yes. Optical waveguides are being used replace and improve upon conventional optical components for spectroscopic chemical analysis in a number of ways. This article will outline the relevant concepts and discuss several applications of optical waveguides to spectroscopy for chemical analysis. The emphasis will be on new develop- ments, particularly t h m that involve Raman and fluorescence spectroscopy. Related methods, such as attenuated total reflection in the infrared (1) and nonspectroscopic applications of

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waveguide optics, are not within the scope of this discussion.

A conventional spectroscopic measurement involves the use of a light source, lenses to direct and focus the liiht, a sample cell, a spectral filter, and a light detector. The light from the source to the sample can be colli- mated, transmitted to the vicinity of the sample, and focused onto the sample by lenses and mirrors. Some means of containing the sample and gaining optical access to the sample is needed. Often the sample is in a macroscopic transparent cell positioned at the in- tersection of the incident and signal beams. The signal-containing light, in the form of light scattered, reflected, transmitted, or emitted by the sample, can be collected in the same way by lenses and mirrors and directed onto the detector.

Many components of conventional spectroscopic systems are being re- placed by waveguide optics. Wave- guides are being used to cany incident liiht to the sample and signal-containing light from the sample; to contain the sample and maintain the focused

out hv i, Figure 1. The geometrical path of fight passing through a 8ecllon of optical fiber The index of refractlan of the wmundlngs, the fiber claMing. and the fiber Wlf are denoted by n,, &. and m, respectively. II the path of the ii$i shown enters at the @eat angle of acceptsnoe of the fiber. then nf sin 0 = NA is the nunmrical aperture of the fiber. In terms of the modes. ttm w e shown corresponds to a hiWd%r rode (imt entering far off-axis)

incident light power density throughout the sample; to probe particulate and thin-film samples; and to serve as the wavelength dispersive elements of a spectrometer.

In the next section of this article, concepts and terminology used in waveguide devices and relevant to spectroscopic applications are dis- cussed. The third part illustrates the new spectroscopic applications by de- scribing several types of experiments and instruments that utilize waveguide optics. The last section consists of a summary of the advantages of waveguides for spectroscopic chemical analysis compared to conventional op-

ANALYTICAL CHEMISTRY. VOL. 54. NO. 9, AUGUST 1982 1071 A

tics and an assessment of the possibili- ties for significant future develop- ments in applications.

Concepts and Terminology By constraining electromagnetic

waves to the interior of a structure called a waveguide, optical energy can be transported through a material with minimal loss and at nearly con- stant average energy density over dis- tances of up to several kilometers. The dimensions of the waveguide transverse to the direction of propagation of the light must he of the same order of magnitude as the wavelength of the light. Thus, in the optical region of the electromagnetic spectrum, the waveguide cross-sectional diameters are several micrometers.

Within the waveguide, to remain confined, light must travel parallel to or at a small angle to the axis of the waveguide. In a geometric optical sense, this means that the light reaches the walls of the waveguide at a grazing angle (large angle of incidence with respect to the normal to the interface). Transverse gradients or discontinui- ties in the index of refraction at the walls of the structure serve to reflect the waves and trap the energy within the structure. Outside (prior to) the waveguide, the angle between the incident source beam and the waveguide axis is larger than it is within, since the index of refraction of the waveguide material is larger than that of the medium outside the entrance face of the waveguide. An important pa-

rameter to consider when matching optical components is the maximum angle between waveguide axis (normal to the entry face, usually) and the direction of the light incident on the waveguide. Thii parameter is the numerical aperture (NA), given by N A = nl sin B , where nl is the index of refraction of the external medium. Fig- ure 1 illustrates the angular relation- ships between the light and the waveguide. The numerical aperture, or equivalently, the maximum angle that allows propagation through the waveguide, depends on the relative indices of refraction of the waveguide and the material in contact with the walls of the waveguide. In many cases, optical fibers are coated with a plastic layer as the fiber is drawn. Thii plastic coat, known as cladding, protecta the fiber from abrasion and chemical attack. Within the waveguide, the profde of the index of refraction may be uni. form. or it may have a smooth gradi- ent or a discontinuous “step” profde. The profile of the index across the waveguide cross section can be tai- lored to enhance certain aspects of the propagation of light in the waveguide. Most important of the properties affected by the index profile is the mode of propagation.

The term mode denoteg the pattern. of wave amplitude as a function of po- sition in the waveguide. The electromagnetic wave within the waveguide has a well-defined transverse spatial structure or pattern as it propagates. A beam can propagate in a waveguide

RO Radius, R-

Figure 2. The relationship between field intensity and transverse distance from the center of a symmetric waveguide Is illustrated for the first three modes The fie168 haw an expOnenlially decreasing value beyond ma surlace 01 the waveguide. W d e mS waveguide. he hi- ader modas have a @eater intensity at a given d ia lam man do lover ader modes

in a single mode or in many modes. For example, the simplest transverse mode is one in which the electromagnetic field has minima only at the walla of the guide, and a smoothly varying amplitude between the walls. The fundamental and two higher order modes in an optical fiber are illustrated schematically in Figure 2. The origin corresponds to the center of the fiber with the wall at radius Ro. The mode in the fiber is symmetric about the origin. The ordinate value is proportional to the amplitude of the electromagnetic field. The dielectric surrounding the fiber must have an index of refraction lower than that of the fiber, but, for the purpse of illus- tration, the surrounding index was chosen to be large enough that the “tails” of the field clearly extend into that dielectric.

The mode of propagation is deter- mined by the characteristics of the incident beam and the waveguide, and by the angle of entry of the beam with respect to the axis of the waveguide. Light emerging from the waveguide has a pattern that is indicative of the mode or modes of propagation in the waveguide. The mode of propagation affects the transit time through the waveguide. Lower order modes travel through the waveguide faster than higher order modes. A lower order mode corresponds to a beam that travels at a smaller angle to the axis than does the geometrical beam corre- sponding to a higher order mode. The lower order mode therefore has a shorter path to traverse in the waveguide. This effect is known as modal dispersion. Another dispersive phe- nomenon is material dispersion, which is due to the wavelength dependence of the index of refraction of the waveguide material and is equivalent to a wavelength-dependent speed of light in the waveguide. Modal dispersion and material dispersion are important effects to consider in using waveguides with pulsed sources. The effect of the dispersion is to alter the pulse shape and width. Scattering of light out of the waveguide is a problem, particularly in transmitting shorter wave- lengths, whether the light source is pulsed or continuous. Optical fibers are available with thicknesses and index of refraction profiles that pref- erentially support either single mode or multimode propagation. The degree of attenuation and the temporal and spatial characteristics of the beam are affected by the choice of fiber type. In any optical fiber, the intensity of light scattered from the fiber is proportional to the fourth power of the wavelength and is therefore a more serious problem in the blue end of the spectrum.

Within the waveguide, the ampli-

1072A ANALYTICAL CHEMISTRY, VOL. 54. NO. 9, AUGUST 1982

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Prism y = 0-

I

-Sample f i t )

Figure 3. The path of a beam totally reflected within a prism and the associated evanescent field is illuseated me index 01 refraction of the sample is 1885 than hat of the pisrn. In me lower parlot the tipure. me dependence of intensity on distBncB, y. horn the Interface is shorn and dicated

penetration depm. d,.. Is in-

tude of the electromagnetic field decreases very rapidly at the walls. Light that reaches the boundary from within, where the material has a higher index of refraction than the surrounding material, will be totally internally reflected at the interface. However, the amplitude of the field of the light does not drop abruptly to zero at that boundary. The amplitude has a tail that decreases exponentially in the direction of an outward normal to the boundary extending into the second medium as shown in Figure 2. The tail represents an electromagnetic field that oscillates at the optical frequency of the incident light, hut that does not propagate through the second medium. Thus, the field is strong only very near the interface between the two media.

This protruding field, balled the evanescent field, can be used to study absorption or to excite fluorescence emission and Raman scattering. The evanescent field produced at the surface of a waveguide is often referred to as a “leaky mode,” so ih this case the leaks in the waveguide may be inten- tionally promoted and profitably used.

The boundary at which total internal reflection occurs can be the walls of an optical fiber, or the face of a prism. The latter case is illustrated in Figure 3. Light reaching the interface from within the hemicylindrical prism

at an angle of incidence less than the critical angle is partially transmitted as a propagating wave through the interface and beyond. At an angle of incidence with respect to the interface that is equal to or greater than the critical angle, the light is totally internally reflected. The critical angle is defmed by v = sin-’ (nl/nz) for n ~ < n2, where n2 is the index of refraction of the prism and nl the index of the material on the prism (a sample, for instance). As the angle of incidence is increased beyond the critical angle, the dmtance over which the evaws- cent wave decays to l/e of its value at the interface decreases to a minimum value of ahout 0.1 of the wavelength at a few degrees beyond the critical angle. This distance is labelled dp and is often referred to as the penetration depth, though that term cannot be taken too literally since the field is continuously decreasing away from the interface. In Figure 3, the lower portion illustrates the relationship between the intensity of the evanescence and the distance from the interface.

By interposing a layer of Ag about 50 nm thick between the prism and the sample, the intensity of the evanescent field can be increased by one to two orders of magnitude. The incident field can couple to the electrons in the metal layer when the angle of incidence through the prism is such that

the photon momentum along the surface matches that of the electrons in the metal. The optimum coupling angle in thii case is a few degrees larger than the critical angle. At this angle, collective excitation of the relatively free electrons in the metal occurs if the incident field contains a component of the field normal to the surface, that is, if the field is polarized in the plane of the incident and reflected beams. Much of the incident field energy is then temporarily stored in the metal layer, resulting in en- hanced fields in the adjacent region of the evanescent wave. The optimum coupling angle can be found empirical- ly by rotating the prism with respect to the incident beam until the reflected light passes through a minimum beyond the critical angle. The collective excitation of the electrons is known as a surface plasmon polariton. The depth of penetration (now mea- sured from the metal/sample interface) into the sample region is a function of the angle of incidence only and is not affected by the presence or ab- sence of a metal layer between sample and prism.

With these general notions of waveguides and some essential parameters governing their use in mind, the applications of waveguides to chemical analysis can he examined.

Applications to Chemistry Applications of waveguide tech-

niques to chemical analysis cnn be separated into four categories. These are 1) pipelines from incident or signal sources; 2) temporal or spectral dispersion elements; 3) the sample itself as waveguide; and 4) sources of evanescent waves. In this section, the types of chemical information obtained and the varieties of instrumentation developed with optical waveguide tecbnolo- gy will be illustrated with examples from the recent literature.

Conveying the light from one point to another by means of an optical fiber is a straightforward application which, in some circumstances, has several advantages over conventional optical methods. Light sources and signal detection instruments can be placed far from the measurement site. If the sample is inaccessible to the instruments directly, due to heat, vibration, high electric or magnetic fields, radio- activity, or restricted physical access due to size of the sample or its loca- tion, fiber optics often can be used successfully. An optical fiber that is used simply to convey light over some distance can withstand extraneous perturbations much more easily than the instruments themselves can. Thus, measurements of coheient Raman sig- n a l s generated in a flame and in a jet engine (Z) , fluorescence signals from

1074A ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982

radioactive materials in a site up to a kilometer from a sheltered laboratory (3), and fluorescence and reflectance spectroscopic measurements within a living organism (4,5,6) have been studied with the help of light pipes.

used to illuminate several samples by coupling the source to an array of fibers that guides the light to the samples. Similarly, several samples can be coupled to a single apparatus for detection and analysis. Such a multiplex arrangement can be useful in a situation in which several locations must be monitored (3). The multiplex fiber system itself is relatively inexpensive. The cost of the detection and analysis instruments and the light source is minimized. Multiple angular measurements also can be facilitated by an array of optical fibers held at the requisite angles with respect to a sample (7). The single source and detector can then remain fixed.

Optical fibers have been used to alter the profile of a beam. A cable (composed of many small, individual optical fibers) that has a circular cross section at one end and a thin rectangular cross section at the other can be used to convert the profile of a laser beam incident on the sample from circular to rectangular. This is useful in minimizing thermal damage on absorbing materials (8). The flexibility of the fiber bundle and ita small size make it convenient in this application, compared to conventional solutions, such as cylindrical lenses and sample rotation.

The use of fiber optica as temporal and spatial dispersion elements has been demonstrated in two experi- menta. A set of fiber optic delay lines connected to a single photodetector and spectrometer was used to con- struct two time-multiplexed optical spectrometers (9). An array of fibers was placed at the output plane of the spectrometer such that each fiber in- tercepted a portion of the emerging spectrum. A pulsed optical signal ar- rived at the output of the spectrometer and was selectively dispersed in time as the individual portions of the spectrum passed through fibers of different length to arrive sequentially at the single photcdetectar. In a related experiment, a single fiber was used as a time-of-flight dispersion element in place of a standard spectrometer (10). In this case the 1.1-km optical fiber had sufficient material dispersion that the photons of different wavelength separated during passage through the length of the fiber. A multiwavelength photon pulse that entered one end of the fiber was resolved into sequential pulses with the delay increasing monotonically as the wavelength de- creased. The times of arrival of the

A single incident liiht source can be

..- ,.,

Ag Layer

Fluorescence and Raman-Scattered

Radiation Exit Prism

I Substrate

Thin-Film i Sample

lgure 4. A slab-lype thin-film waveguide with prisms for coupllng light in and out f the film

The figure 1s not to wale. The metal layer undet he prisms is not necessary. but 608s significantly enhance lhe coupling betwesn lncldent field and waveguide. The coupling strengm is controlled by con- trolling me distance between pism and waveguide. FlYQBscenca and Raman scanered light can be o b served n m l to me plane of me waveguide. vhile bansmmed light can be monitored at me exn prism

pulses a t the detector end of the fiber thus indicate the wavelength of that portion of the incident signal. Both types of multiplex spectrometers just mentioned are restricted to use with very low light intensity signals since they were designed for use with single- photon counting analysis. Despite this, theadvantage of this approach is the acquisition and digitization of a spectrum with a single photodetector in as little as about 100 ns, depending on source duration and number of channels in the spectrometer.

fibers was as the sample itself (11). Raman spectra of the fused silica of the fiber itself and of dopants were obtained. Raman signals were detect- able even from the dopants present in low concentration. The key to the suc- cess of the experiments is the long path length with high power density throughout the length of the fiber. More recent work has continued in this vein (12,13). The long path length also limits the usefulness of this method to samples that are weak- ly absorbing. Of course, these often are the cases that cannot be handled easily by other sampling schemes.

The ability of optical fibers to sus- tain the requisite high power densities over substantial lengths is also essential for producing nonlinear or multi- photon processes. Raman gain or stimulated Raman scattering has been

An early spectroscopic use of optical

observed in several solid fib& systems (12,14,15,16).

Liquid samples can be studied in a similar fashion. If the bore of a capillary is filled with a liquid of higher refractive index than that of the capillary walls, light entering the liquid will

be guided through the liquid. In this case, waveguiding occurs for light entering the liquid at an angle to the axis of the capillary that is greater than the critical angle defined by the capillary wall-liquid interface. For example, that angle is about 17 degrees for benzene in a quartz capillary (17 ) . Modes of many orders may be simultaneously present in this case, and the light is confined to the liquid by total internal reflection at the walls. As in the solid fiber studies, the long path exposed to high power density makes this approach very useful for Raman scatter. ing. Both spontaneous Raman (12,18) and coherent anti-Stokes Raman scattering (CARS) ( 1 7 ) have been observed in liquid-filled capillaries with enhancement factors of,several hundred per meter of capillary length compared to signal levels in conventional sample cells. In the CARS ex. periment. two beams must cross at a certain angle known as the phase. matching angle, which matches the momenta of the photons in the two beams along a direction and maximi- zes the interaction of the beams with each other over a macroscopic distance in the medium. This generates a third beam at another angle to the incident pair. In a 50-pm i.d. capillary, one beam was directed along the axis of the capillary, and the second inci. dent beam crossed the first at an angle of 2 degrees. Thus, the two beams crossed and generated CARS at many points throughout the length of the capillary. The requirement that the two incident beams be phase matched and the quadratic dependence of the enhancement factor on capillary length implied that the beams main.

ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 1077A

tain that phase-matching angle of interaction throughout the sample length.

ple within the capillary is very low, as for a gaseous sample, the waveguide can support a mode that has nodes at the walls (19). Since the optical field has nodes at the walls of the waveguide, the light can travel through the waveguide with very low losses due to the walls. In this situation the index of refraction is low inside the waveguide. Guiding can be envisioned as occur- ring by grazing angle external reflection at the walls. Most of the energy of the light travels down the axis of the waveguide. This fundamental mode in the waveguide is well matched to the Gaussiarl fundamental mode of a laser. Two-photon Doppler free absorption (20), optical pumping (ZO), and CARS (21) have been reported for gases in hollow dielectric waveguides.

Thin films and monolayer assem- blies on surfaces have been investi- gated by waveguide methods that involve slab-type geometries rather than fibers. Films more than about a mi- crometer thick can be studied by propagating the probe light through the film itself as a waveguide. Cou- pling the incident light into the film is accomplished through a prism, as illustrated in Figure 4. In this scheme, evanescent waves are generated at the prism-air interface and couple to the thin film, which supports wave propagation. The coupling strength is controlled by adjusting the air gap between prism and thin film, usually by applying pressure against dust particles on the surface. The gap should be a few hundred nanometers to optimal- ly couple to the film without allowing the presence of the prism to perturb the mode patterns of the film. A thin metal layer, usually Ag, can be used between the prism and air layer to enhance the intensity of the evanescent field, as was mentioned above. Sam- ples can be prepared in layers on the substrate, and light scattered out of the film can be collected normal to the plane of the film (22-25). Light emerging from a second prism at the end of the waveguide can be used to determine transmission in the plane of the thin layer.

Layers of materials that are too thin to support optical propagation can be probed by evanescent waves. These nonpropagating optical fields can excite elastic and inelastic scattering within a fraction of a wavelength from the interface at which the evanescent wave is generated. The interface can be between the surface of an optical fiber (26) or thin-film waveguide (23) and the material of interest. In these cases, a sample film, which can be considerably thinner than 1 pm, is formed

If the index of refraction of the Sam-

on the surface of a fiber or on a slab of glass that acts as a waveguide. Both the cylindrical and slab waveguide with evanescent coupling to the sample offer the advantage that the effec- tive path length can be made relatively long, since the probe beam, confined within the guiding structure, generates the evanescent field throughout its length. This is particu- lary helpful for Raman measurements.

Evanescent waves have been used to probe materials near surfaces without the use of a waveguide structure to confine the probe beam. Raman and fluorescence studies of liquids (27), polymers (28), small particles (29), and solids (30) have been done on prism surfaces a t which total internal reflection of an excitation source produced an evanescent field. The details of the theory of Raman spectroscopy with evanescent excitation have re- cently been published as well (31). In- terpretation of the results, especially the polarization dependence of the scattered light on the angle of incidence of the beam with respect to the surface, is complex. Due to this com- plexity, data on evanescently excited Raman spectra of liquid benzene near a prism surface have been incorrectly interpreted to imply that long-range order exists in the liquid near the surface (27).

Evanescent waves generated at a pTism surface have been used to measure the thickness and refractive index of thin films on the prism (32,33). This technique uses the coupling of the evanescent wave to a thin film which is at a small, variable distance from the prism and the subsequent coupling of the evanescent wave from the waveguiding thin film back to the prism. The angles a t which the light reemerges from the thin-film waveguide and the modes of that light contain sufficient information to determine both the index of refraction and the thickness of the film with good precision.

A very thin layer has been probed by another method using waveguide techniques. In this approach, a lipid bilayer (about 50 A thick) was studied by transmission and light scattering by illuminating only the bilayer with small-diameter optical fibers (34). The bilayer was suspended between two optical fibers in an aqueous medium. Light passed across the bilayer in the plane of the bilayer. Light was ef- ficiently coupled into the bilayer without adding a large component of scattering or absorption from the surrounding bath. A remarkable aspect of this experiment is that the bilayer is not only able to act as a waveguide, but it is also possible to distinguish contributions to the transmitted signal that come from the surface wave at

the bilayer-bath interfaces from the contribution due to the component of the light transmitted through the ten- ter of the bilayer. Light transmitted and scattered from the bilayer was used to probe thermal and current- induced molecular motion in the bilayer.

Summary and Conclusions Many of the applications of wave-

guide optics to chemistry have produced data that could not be obtained in conventional ways and that have contributed significantly to chemical analysis. Optical waveguides and evanescent waves as used in the instruments and experiments outlined above constitute a new class of tools for chemical and analytical spectroscopy. The major advantages as devices can be summarized as follows: minimum interference from extraneous noise sources, multiple connections or networking of sources andlor detec- tors to samples, exposure of samples to optical fields with uniform power density over long path lengths, con- finement of the probe radiation to a dimension commensurate with small sample size or depth, and means of temporal or spectral dispersion. Waveguide optics can be used to ob- tain chemical information from Sam- ples that are not readily accessible physically with conventional devices, due either to the environment surrounding the sample or the need to re- strict the probe radiation to the dimensions of the region of interest in the sample.

One of the limitations of some optical fiber devices is the loss of polarization in transmitting the light through the fibers. Only certain types of optical fiber are capable of sustaining a given polarization condition over lengths of meters. In most optical fibers, light that is incident in a plane- polarized form is quickly converted into elliptical polarized light. Scatter- ing of light out of the waveguide is also a problem, especially in the blue end of the spectrum. The scattered intensity is proportional to the fourth power of the wavelength of the light, and is due to inhomogeneities in the index of refraction in the waveguide. Therefore, transmission losses are considerably higher for short wavelength light than for longer wave- lengths. The minimum loss occurs for light in the near-infrared. The quality of the waveguide material can be important in designing an experiment with waveguide optics because of the scattering centers and absorption bands in the material. These loss mechanisms may not be very important in cases where the length of the waveguide element is short.

Integration of optical elements such

1078A ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982

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as lenses, prisms, gratings, phase modulators, polarizen, solid-state lasers, and beam switches into very compact, monolithic devices is currently being done to a limited degree. In the near future, integrated optics devices will be feasible and available. These devices will allow the use of waveguide techniques for spectroscopy in in- creasingly more powerful ways. By combining light source, waveguide, modulator, prism, lenses, and even a detector array (in some appropriate combination) in a single, small solid- state object, the resistance of the de- vice to external perturbations. the time response, and the cost can be significantly reduced. Another important application of waveguide optics that is being developed is the incorporation of chemically specific agents into the optical fiber itself or into a probe as- sembly directly coupled to the end of the fiber (3). This allows sensitive re- mote detection of specific materials or conditions, such as pH and trace ions.

In the next several years, waveguide optics using more and more sophisti- cated waveguides, leaky modes, optical switches and modulators, and integrated components will become important tools for chemistry and chemical analysis.

(18) Walrafen, G.; Stone, J. J. Appl. Spec- tiose. 1972.26,585.

(19) Marcatili, E. A. G.; Schmeltzer, R. A. Bell Syst. Tech. J. 1964.43, 1783.

(20) Guerra. M. A,; Sanchez. A.; Javan, A. Phys. Reu. Lett. 1977.38.482-84.

(21) Miles, R. B.; Laufer, G.; Bjorklund, G. C. Appl. Phys. Lett. 1977,30,417-19.

(22) Iwarnoto. R.; Kaji. 0.; Masaru, M.; Seiichi, M. Appl. Spectrosc. 1981.35. 584-87.

Appl. Spectrosc. 1980.34,517-21. (23) Rablt. J. F.; Santo, R.; Swalen, J. D.

(24) Swalen, J. D. J. Phys. Chem. 1979.83, 1438-45.

(25) Levy, Y.; Imbert. C.; Cipriani. J.; Ra- cine, S.; Dupeyrat. R. Opt. Commun. 1974, I f , 6669.

(26) O’Connor, P.; Tauc, J. Opt. Commun. 1978,24,135-38.

(27) Carius, W.; Schroter. 0. Z. Phys. Chem. (Leipzig), 1981,262,711-14.

(28) Menetrier. M.; Dupeyrat, R.; Levy, Y.; Imbert. G. Opt. Commun. 1977.21.162.

(29) Benner, R. E.;Owen, J. F.; Chang. R. K. J. Phys. Chem. 1980.84,1602-6.

(30) Mattei, C.; Fornari. B.; Pagannone. M. Solid State Commun. 1980..?6,309- ,‘, I L.

I31 I D ” w e , L.; Vigoureux. J M .Menu,

(32) Tien. P. RPU Mud I’hgs 1977,J9, C J (’hem Phys 1981,74,3639-59 ‘ X I

(Bi)”-Feldman, A,; Farabaugh, E. N. In “ h e r Induced Damace”: NBS Snecial Publication, in preas. -

(34) Braun. H. F.; Michel-Beyerle, M. E. Z Noturfimch. 1978,33A. 1594-96.

Ilan Chabay receiued a BA in chemistry from Clark Uniuersity and a PhD in chemical physics in 1972 from the Uniuersity of Chicago. In 1974 Cha- bay went to the National Bureau of Standards as a n NRCINBS postdoc- toral fellow. He later became a staff member of the NBS Center for Ana- lytical Chemistry. At NBS his re- search interests were size measurement and chemical characterization of fine particles, spontaneous and coherent Raman spectroscopy of con- densed phases, and characterization of molecules a t interfaces. He is now the associate director of the Explora- torium in San Francisco. At this “hands-on” museum of science, human perception, and art, his interests are in improving the public leuel of familiarity with science and increasing the appreciation of the aesthetics of science and its relationship to the arts by means of parti- cipatory exhibits.

IOOOA ANALYTICAL CHEMISTRY. VOL. 54. NO. 9. A W S T 1982

optical waveguides. photon plumbing for the chemistry lab: fiber optics, waveguides, and evanescent...

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