THIN-FILM OPTICAL FILTER FOR INTENSE LASER LIGHT

(FIGURES AND TABLES ARE UNAVAILABLE ON THIS WEBSITE.)

III. IDENTIFICATION AND SIGNIFICANCE OF THE OPPORTUNITY

High-power lasers are used in a variety of research, military, and industrial applications. Due to the intense power of these light sources, they can easily damage other sensitive optical instruments as well as the human eye. Optical filters are therefore required to remove narrow-band-pass intense laser light.

Current optical filters are either of the interference type or absorption type. Although interference filters reflect light only over a limited spectral band (narrow band), they have disadvantages: 1) they are expensive, and 2) they have narrow acceptance angles. Absorption filters also have disadvantages: 1) all of the absorbed light energy is converted to heat, and 2) combinations of absorption filters must be used in order to isolate a particular transmission band of interest. Moreover, both types of filters are subject to damage when used with high-power lasers. Even though the interference filter in theory reflects rather than absorbs unwanted wavelengths, small areas of absorption around imperfections or dust will cause burn damage in interference filters. A new approach is therefore needed in the design of narrow-band-pass optical filters for use with high-power lasers.

The goal of this program is to demonstrate that an optical filter can be built that can absorb virtually all of a high-intensity laserlight beam impinging on it without being damaged. The energy in the incident beam will be dissipated via excitonic fluorescence (1) as incoherent diffuse light. The excitonic filter will therefore not be damaged by intense laser light, as are ordinary absorptive or interference filters. We anticipate that these filters will be used with high-power lasers so that the incoherent diffuse light from the excitonic fluorescence is not a complication and can easily be removed if necessary. The excitonic filters should be inexpensive to produce.

IV. BACKGROUND, TECHNICAL APPROACH, AND ANTICIPATED BENEFITS

IV.A. BACKGROUND

IV.A.l. Absorption Filters

Absorption filters consist either of colored glass or gelatin. These filters are unsuited for use with high-power lasers because all of the energy removed from the light beam is converted to heat, which quickly leads to irreversible damage by ablation, melting, or photochemistry.

IV.A.2. Interference Filters

An interference filter consists of a number evaporated-metal films sandwiching evaporated layers of transparent material. Glass plates cover the film for mechanical protection. Theoretically, in an interference filter all of the energy removed from an incident light beam is reflected so that no heat is generated. In practice, however, interference filters absorb a small amount of light via surface defects as well as by the glass plates and metal films. The result is that heat damage does occur. Also, because these filters work only when rigorous interference requirements are met by the incident-light wave, they have narrow acceptance angles.

IV.B. TECHNICAL APPROACH

The optical filters we propose to build use three physical principles in their construction:

• excitonic effects,
• coupling of light and excitons in thin organic dye films, (2,3)and
• excitonic fluorescence.

Thus, energy removed from the incident-light beam in our proposed filters is first coupled to the exciton field and then removed by excitonic fluorescence rather than by heat.

A qualitative discussion of the principles involved in these excitonic filters is given in the paragraphs that follow. Experimental support for these ideas is given in Section VII: Related Research.

IV.B.1. Excitons Are Highly Localized Electronic Excitations in Matter

An exciton is a highly localized collective electronic excitation in the condensed phase of matter. Excitons of the type that are of interest here can be thought of as instantaneous dipoles "hopping" from one molecule to the next by electromagnetic energy transfer. When the oscillator strength of the molecule is near unity, as it is in a strongly light-absorbing compound such as polymethinum dye, strong excitonic effects are observed. The exciton hops rapidly from molecule to molecule- - so rapidly that the nuclei of an individual molecule never have time to respond, i.e., there is no conversion to excited vibrational modes and thus no conversion of electronic energy to heat. (4,5)

In general, the frequency of the excitons is not a strong function of wave vector, and to a first approximation their energy is independent of wave vector. Photons, the quanta of light, have an energy that is linearly related to their wave vector. It is possible, therefore, to match the phase velocity of the photon and the exciton.

Strong photon-exciton interactions occur in the frequency range where the phase velocities of the photon and exciton are nearly equal. The strong interaction of excitons in the dye with light traveling through the dye leads to a frequency-dependent dielectric constant in the condensed dye phase. This phenomenon occurs near the resonant absorption frequency of a single dye molecule (corresponding to ~550 nm for the heptamethinium dye used as an example later in Section VII.A.2.) The frequency-dependent dielectric constant is given by an equation of the form

w+^2 - w^2 = Constant w-^2 - w2
E(w)

where w+ and w- define the high- and low-energy edges of the "exciton band." Within the band, w-<w<w+, the dielectric constant is negative (and the index of refraction is imaginary). this means that incident light is attenuated exponentially, i.e., there are no propagating modes for the incident radiation within the condensed phase. thus, crystals of strongly absorbing molecular dyes exhibit high reflectance in the visible region of the spectrum just as do metals, which also have negative dielectric constants below the plasma frequency.(6) although not as well known, these excitonic phenomena in molecular dye crystals are analogous to more familiar phenomena that occur in ionic crystals such as nacl as a result of coupling of phonons with infrared light to create quasi-particles called polaritons.(5,6)

IV.B.2. Thin-Film Optical Coupling of Light Energy Into Exciton Energy

Interference effects in thin dye films can be used to enhance the transfer of light energy into the exciton field of the dye. We call this effect "absorptive trapping," and it is described below.

Nearly dielectric behavior (transmission of light) is exhibited by ultrathin discontinuous metal films even for frequencies below the plasma frequency (where normally light is reflected) . A similar effect occurs within the exciton band in a condensed film of dye molecules. The films are so thin that even though the intensity of light falls off exponentially within the film (which in a bulk crystal, for example, leads to metallic-like reflection from the surface), light still reaches the bottom surface of the film, where it is totally reflected (because, as is discussed later, it is incident at an angle greater than the critical angle). This reflected light returns to the upper interface, where it cancels the light reflected from the upper interface. Thus, the amplitude of the electromagnetic radiation (light) is a minimum at both surfaces of the film, i.e., it is Gaussian-like. Within the film strong exciton-photon coupling occurs. The light energy within the dye film therefore has an exciton character.

Two major advantages of filters based on the thin films with excitonic character over interference-type filters stem from the physical principles discussed above. First, the excitonic filter will have a somewhat broader band width over which it is useful. Second, the excitonic filter will have a larger angle of acceptance for light. One reason for these advantages is that the Gaussian-like electric field that is established in the films does not have a well-defined wavelength, but is instead the sum of a large number of Fourier component sine waves. Each of the sine-wave components couple to a different excitonic mode with the matching wave vector, and these modes are spread out over the frequencies from w- to w+ (in Figure 1 and Equation 1). Another is that, because the electric field in the film is Gaussian, it does not have a well-defined wavelength and there is not a unique Bragg angle of incidence for which the interaction of light with the film occurs.

IV.B.3. Absorbed Light Energy Is Dissipated by Excitonic Fluorescence

Within the dye film the light energy propagates with an excitonic-like character due to the strong photon-exciton coupling. If the light energy in the film decays through non-radiative channels (heat), then intense incident radiation on the film will damage it. In order to dissipate large amounts of energy without damage, the light energy in the form of excitons within the film must decay radiatively, that is, through fluorescence.

To decay radiatively, the excitons must reach the surface of the film, where they can couple to the external radiation field. The very thin organic dye films, therefore, enhance radiative decay of the excitons. In theory,there is only one unique frequency where fluorescence can occur: w- . (5) If excitons are created at higher frequencies they must lose the excess energy as heat,which will cause film damage. For this reason, these filters must be used to filter intense light only at frequencies near w-.

IV.B.4. Excitonic Fluorescence of Absorbed Light Energy Is Isotropic

Excitons have an associated momentum and direction of propagation that can be changed by interaction with other quasiparticles. Incondensed phases of dye molecules, the excitons are sufficient ly scattered by other quasiparticles in the solid (e.g.,latticephotons), causing them to lose their coherence. The observable result is that the light energy radiated due to excitonic fluorescence is diffuse and incoherent. (1)

IV.C.ANTICIPATED BENEFITS

By the end of Phase I we aim to demonstrate that 1)excitonic effects are valuable in filter design, particularly where large amounts of energy must be removed from the light beam and large acceptance angles with narrow band pass are required; and 2) that there are methods of depositing organic dyes (e.g.,vacuum deposition) that can be used to produce practical films of good optical quality. A new type of optical filter could be constructed specifically for use with high-power lasers. Such filters might also form the basis for safety products (e.g.,eye protection) to be used in laser laboratories and industries utilizing high-power lasers, for example, excimer lasers.

ThePhase II program would be directed toward producing a prototype filter for evaluation by a potential Phase III partner. Also during Phase II we would investigate further commercial uses of thin organic dye films for other optical devices.

Typical interference filters cost from $150 to $250. Absorption filters cost about $50. We anticipate that the cost of our proposed filters would be near that of absorption filters, since their construction would be similar. This would make them cost-effective compared with interference filters. Moreover,they should last much longer than interference filters when used with high-power lasers.

V.PHASE I RESEARCH OBJECTIVES

V.A.INTRODUCTION

The objective of the PhaseI program is to experimentally verify that thin films of organic dyes can effectively absorb and dissipate large amounts of incident-light energy through the mechanism of excitonic fluorescence. Thin films of organic dyes, then, will form the basis for the design of narrow-band optical filters in which dye films are used in the absorptive-trapping configuration. The Phase I experimental program will be designed to increase our current understanding of excitonic fluorescence and film damage by intense light. We will also attempt to improve the quality of films, which should lead to increased absorption near w- and increased transmission over the rest of the spectrum. The program is divided into the three major task areas summarized below.

V.A.l. Characterization of Excitonic Fluorescence

Objectives:

• Measure the quantum efficiency for the conversion of incident light into excitonic fluorescence
• Measure the fluorescence decay of excitons
• Measure the film-heating dueto energy notradiatedas excitonic fluorescence

Questions to be Answered:

• Is light removed from the incident beam converted to excitonic fluorescence?
• ;Is the fluorescent emission from thin dye films different from isolated dye molecules?
• ;Does the heat generated in the film agree with the fluorescent quantum efficiency measurements?

;V.A.2. Film Damage Experiments

Objectives:

• Measure the ablation or melting of films as a function of light intensity and wavelength.

Questions:

• How much energy can the film remove from high-intensity laser pulses without suffering damage?
• How far from the fluorescent peak can the wavelength of the incident radiation be without damaging the film?

V.A.3. Film Improvement

Objectives:

• Improve film uniformity for better optical filter characteristics.
• Investigate other dyes for use in filters that absorb at different wavelengths.

Questions:

• Does improved film uniformity give better resistance to damage under intense laser light?
• Does improved film uniformity produce narrower absorption bands and better transmission over the remainder of the band?
• Is the fraction of light absorbed from the incident beam by the film increased by better film uniformity?

V.B. SIGNIFICANCE TO FUTURE RESEARCH AND COMMERCIAL APPLICATIONS

Phase I will establish a proof of concept, i.e., that thin organic dye films can absorb intense coherent laser light over a narrow frequency band and reradiate it as diffuse incoherent light. Once this proof of concept is established, we will develop a narrow-band optical-filter prototype. This work will carry on into Phase II with the potential of an early-entry device developed by the end of Phase II.

VI.PHASE I RESEARCH PLAN

VI.A. TASK AREA A: CHARACTERIZATION OF EXCITONIC FLUORESCENCE

VI.A.l. Task 1: Measurement of Fluorescence Efficiencies

The objective of this task is to determine the efficiency with which the dye films convert incoming light into fluorescence. The experimental arrangement will be essentially that shown in Figure 1, except that a photomultiplier tube will be placed against the back side of the film. Based on geometric considerations- -i.e., the distance of the photocathode within the tube from the film, and the photocathode, size- -we will be able to determine what fraction of the fluorescence is intercepted and calculate the total fluorescence intensity. This experimental arrangement should eliminate the major problem with most fluorescence measurements- -that of detecting the excitation light rather than the fluorescence- -because the excitation light is totally reflected from the film/air interface and only fluorescent light can be emitted from the back side of the film. Nonetheless, we intend to check for scattered excitation light by obtaining a spectrum of the detected light and comparing it to the spectrum of the excitation source. We will also investigate angular dependence of the fluorescence intensity.

It should be noted that Figure 1 illustrates the basic configuration of the filter design. For the film to interact strongly with light, it is essential that coupling into the film occur at angles greater than the prism-air critical angle.(1-3)

To determine fluorescence efficiencies, we will also measure the light absorption of the film. This will be relatively straightforward. The intensity of the incoming light will be determined by splitting off a known percentage with an angled-glass beam splitter. The intensity of the light exiting the prism will be measured directly. Both measurements will be conducted with conventional photomultiplier technology.

The key result to be obtained is the fluorescence efficiency vs. excitation wavelength. Laser excitation will be used to achieve an intense narrow-band and well-columnated light. We anticipate using a Coherent Radiation Model 694 tunable dye laser for this work.

We plan to study films of several different dyes, each with an absorption maximum at a different wavelength. Among these will be the heptamethinium dye described in Section VII and pentamethinuim perchlorate dye that is in the same homologous series but with w- at 520 nm. We also intend to study films of dyes that are known to be highly efficient fluorescers, such as the rhodamines, fluoresceins, and cumerines used in dye lasers. It is our experience that fairly uniform films of such dyes can be placed on glass substrates by mechanically rubbing them on. Most of the studies will be conducted with films prepared in this way.

VI.A.2. Task 2: Fluorescence Decay Measurements

It is our hypothesis that the fluorescent emission from thin dye films is very different in character from that of fluorescent emission that comes from isolated dye molecules in solution. The fact that the spectrum of the condensed phase fluorescence is not only frequency-shifted from, but also much narrower than, single-molecule fluorescence (1) is evidence in support of the excitonic- fluorescence hypothesis. A further piece of evidence we intend to gather will come from comparison of the fluorescence decay time of the films with that of dye molecules in solution. Fluorescence lifetimes of isolated molecules typically are in the nanosecond range. There is a good possibility that excitonic fluorescence is faster, approaching the "resonance" or instantaneous fluorescence limit because the excitons are so strongly coupled to the radiation field. We therefore intend to use very short pulses from a mode-locked laser as the excitation source and fast photon counting equipment to detect the fluorescence. All of this equipment is available at the University of Oregon's Shared Laser Laboratory, which we have arranged to use in this Phase I program. (See Section IX: Facilities and Equipment).

VI.A.3. Task 3: Measurement of Film-Heating

In this task we will conduct experiments similar to those in Task 1 except that, as an independent check of the measured fluorescence efficiencies, we will measure heat generated in the film as a function of excitation wavelength. This will be accomplished by immersing the prism in a small glass cuvette containing a liquid in which the dye is insoluble. Since most of the dyes are ionic, we anticipate using a hydrocarbon liquid. The cuvette will then be used as a bomb calorimeter, and temperature increases will be measured with a thermocouple in the liquid. The cuvette will be insulated (except for holes required for the incident and transmitted light) . Control experiments will be performed in which the dye film is omitted. Temperature changes observed in control experiments will be subtracted from changes observed when the film is present, and the heat evolved will be determined using the heat capacity of the prism and liquid.

VI.B. TASK AREA B: FILM DAMAGE EXPERIMENTS

VI.B.1. Task 4: Damage Threshold Experiments

The objective of this task is to determine the amount of energy that can be absorbed and dissipated from dye films without their incurring damage. The basic experimental configuration will be that absorptive trapping. Intense laser light having power densities in the tens of megawatts/squared cm will be shown on the films. This light source will be one of the several dye lasers available at the Shared Laser Laboratory, University of Oregon. Very high-energy light will be obtained by focusing high-quality laser beams (for example, from an argon laser or nitrogen laser pumped dye laser) into a small spot on the film. The films will be assessed for ablation or melting as a function of light intensity, number of pulses, and excitation wavelength. These results will provide a good understanding of the limits to which such films can be pushed when used as optical filters for intense light. As in Task Area A, we will investigate films of a number of different dyes.

VI.C. TASK AREA C: FILM IMPROVEMENT

The objective of the work in this task area is to produce highly uniform dye films. Although films of surprisingly uniform thickness can be produced by carefully smearing the solid dye on a clean glass substrate, it is reasonable to expect that other methods will produce more uniform and perhaps more durable films. Improving film uniformity will result in better optical-filter characteristics, i.e., narrower-band absorption and higher absorptivity. The orientation of dye molecules with the film will be examined using linear dichroism. This work would be expanded in any Phase II continuation of this program.

VI.C.l. Task 5: Vacuum Deposition

Films will be made on glass substrates by vacuum deposition. The solid dyes will be placed in a heated crucible in a vacuum oven with the glass substrate placed over the crucible. This work will be conducted in a vacuum deposition chamber available at Bend Research in its scanning electron microscope laboratory. Dyes used in this work will be nonionic, since nonionic dyes are expected to sublime at lower temperatures than do ionic dyes; thus, decomposition should not be a problem. The fluorescein class of dyes are the most likely candidates for this work. Dye films will be characterized by absorptive-trapping spectra and by using scanning electron microscopy to photograph the films. Improved uniformity in these coatings compared with that of the smeared coatings described above will be indicated by higher absorptivity and narrower absorption-band pass.

VI.C.3. Task 6: Final Report Preparation

A final report containing all of the important results, conclusions, and recommendations will be prepared and submitted to NSF. Based on the success of the Phase I program, a Phase II proposal will be submitted for work aimed at fabricating a prototype filter suitable for evaluation by a potential Phase III commercialization partner.

VII. RELATED RESEARCH

Early experimental work that supports the ideas and concepts upon which this proposal is based is summarized below. Evidence for dissipation of energy by excitonic fluorescence and resistance of thin (200-A) organic-dye films to damage by intense laser light is presented.

The principal investigator did the original work on excitonic fluorescence and absorptive trapping ten years ago, as described in References 1, 2 and 3, and he has kept current in related areas since that time. There is, to our knowledge, no closely related work currently being conducted.

VII.A.l. Experimentally Measured Spectral Properties of Organic-Dye Films

A sketch of the experimental optical arrangement is shown in Figure 1. The base of a glass or quartz prism is coated with a thin film of a strongly absorbing dye. Light is incident to an angle (, which is equal to or greater than the prism-air critical angle. Within a band of wavelengths of incident light strong attenuation occurs, and virtually all of the incident light is scattered as diffuse, non-coherent fluorescence. The strong attenuation is the result of the film's optical cavity-like features and is called absorptive trapping.

Past work(1) has been conducted with a poly-methinium dye (1-7-bisdimethylamino- 4-acetoxy heptamethinium iodide), although in principle any strongly absorbing dye will work. This waxy dye is easily applied to a glass surface by rubbing with a cotton swab. The resulting dye films are less than 200 A thick and of reasonably uniform thickness. The absorptive-trapping spectrum of a film of the hepta-hectamethinium dye is shown in Figure 2a as a solid line. This is the spectrum that is obtained when the spectrophotometer beam is incident at 55 degrees. The spectrum is exceedingly polarization-dependent, and in fact, for the film of the dye used as an example here, all of the absorption intensity occurs with polarized light (E-field perpendicular to the plane of incidence). The reason for this is explained in Reference 1. In a practical filter that is used to absorb unpolarized light, two prism devices would be used at right angles to each other to absorb all of the light. The p-polarized light not absorbed by the film in the incident beam would be removed by a second prism rotated so that the p-polarized light reflected from the first prism enters the second as s-polarized light.

The most important feature in the spectrum is the intense absorption that occurs around 600 nm. At this wavelength only 1/100 of the incident light is transmitted. Over the rest of the visible spectrum--i.e., from 550 nm down to 400 nm--more than half of the incident light is transmitted. Thus, an optical filter based on films of this dye could be used to absorb red light while passing a high percentage of light in the rest of the spectrum. A striking visual effect exhibited by the film and due to absorptive trapping is that when it is viewed from the base of the prism, it is so pale as to be barely visible, due to its thinness; yet when it is viewed through the prism, as in Figure 1, it is intensely blue due to the optical-cavity enhancement of absorption.

The absorptive-trapping spectrum in Figure 2a was taken with an angle of incidence of 55 degrees. However, Reference 3 describes additional experiments done at different angles up to the critical angle during which no difference in the absorption intensity at 600 nm is observed. The critical angle for these experiments was about 43 degrees. Thus, between 55 degrees (the angle at which the spectrum in Figure 2a was taken) and 43 degrees - - or over a 12 degree range- -a device based on this principle should filter out 99% of the incident light.

The shape of this absorptive-trapping spectrum is quite different from that of the dye in solution (Figure 2b); this difference is due to 1) the excitonic interactions between dye molecules (as described in more detail in Section C below), and 2) the optical-cavity nature of the thin dye film (also described in more detail in Section C) .

The light absorbed by the thin dye film is radiated away via fluorescence. The spectrum of this fluorescence is shown as the dotted line in Figure 2a. This fluorescence spectrum is very different from the single-molecule fluorescen (i.e., solution fluorescence) shown in Figure 2b. The spectral shift(2) is a consequence of the fact that in the film excited molecules interact strongly with adjacent molecules, shifting the excited-state energy levels. In solution no such interaction occurs. The interaction leads to excitons and to the excitonic fluorescence in Figure 2a. Note that in the film, fluorescence occurs at nearly the same wavelength as the absorption maximum. Thus, little of the energy of the absorbed radiation need be converted to heat. This is in contrast to single-molecule fluorescence, in which the light is absorbed at a lower wavelength than is the fluorescence, and the difference in energy is converted to vibrational modes, i.e., heat within the molecule that can lead to photo-decomposition or heating of the surrounding solution.

VII.A.2. Film Integrity After Intense Laser Radiation

In order for thin dye films as configured in Figure 1 to function as optical filters for intense laser light, they must not be damaged by absorption of the light and subsequent heating. The evidence that such damage can be avoided comes from work conducted by the principal investigator a number of years ago.(3) This work was not directed toward the development of an optical filter, but rather toward observing two-photon spectra of the films in which absorption of an intense laser-pump beam was maximized using the configuration shown in Figure 3. The probe beam was used to monitor transient (two-photon) absorptions created by the laser light.

The key results were obtained with films of 1-7-bis-dimethylamine 4-acetoxy heptamethinium iodide having the absorption spectrum shown in Figure 2a. The laser-pump beam was tuned into the absorption band of the film to maximize absorption. The laser used was a flash-pumped dye laser producing several megawatts of power per pulse. The laser pulse was timed to coincide with the peak intensity of a polychromatic pulse from a flash lamp used as a probe beam. This probe beam passed through the film several times and then on to a monochromator. Diminutions in probe-beam intensity that occurred during the laser pulse could be attributed to transient light absorption. Small fractions of the laser light and the probe light were fed to photo-multiplier tubes and their output was displayed on an oscilloscope. A typical trace is shown in Figure 4.

The upper trace is the peak of the flash-lamp pulse (increasing light intensity is down) and the lower trace shows the laser pulse. The intensity scale is not the same for both traces, since the laser pulse was millions of times more intense than the flash-lamp pulse. Each trace is a double exposure, the first exposure being the flash lamp fired alone to provide a baseline, and the second resulting when the laser was fired at the flash lamp's peak intensity. To set the time scale in the figure, the laser pulse is about 0.5 ( sec long. What this picture shows is that during the time of laser pulse there was a decrease in intensity of probe-beam light (an upward deflection in the upper trace) at the wavelength that was being probed. This was due to a transient absorption created by the laser pulse and was the type of absorption under investigation.

What is more relevant to the current discussion is that the optical properties of the films were unchanged after the laser pulse- -that is, the probe-beam intensity returned to exactly its initial baseline value. This indicates that no film damage occurred. This is indeed a striking result, since virtually all of the energy of the laser pulse was absorbed by the film and, as discussed below, if even a small percentage of the energy were converted to heat, the film could not be expected to survive.

Let us assume for the moment that the absorbed light was converted to heat and determine the temperature in the film from a 5-megawatt laser pulse (typical of laser powers used in the experiments) The laser beam had a cross-sectional area of about 0.5 cm squared;-thus, the energy was absorbed in a volume 0.5 cm squared x 200 x 10-8 cm, or 10 -6 cm3 . The amount of energy in the pulse is about 2 joules, or about 0.5 cal. If we assume a specific heat of the film to be 1(C/Cal-cm3 , then the temperature would increase by 500,000(C. If even 1/100 of the incident energy were converted to heat, the temperature would rise by 5,000(C. Clearly, there is an effective energy dissipation mechanism at work. It is our belief that the energy from the laser-pump beam was effectively reradiated as excitonic fluorescence from the film.

The films studied were subjected to many hundreds of laser pulses. It was found that as long as the laser light wavelength was relatively close to the wavelength of the excitonic fluorescence- -i.e., the input photon energy was about the same as the photon energy of the fluorescence- -no damage occurred.

On the other hand, as the spread between the photon energy of the incident laser light and the photon energy of the excitonic fluorescence wavelength became greater- -i.e., the laser light was at a lower wavelength than that of the fluorescence- -melting and ablation of the film was observed. When the laser was tuned to 500 nm (110 nm away from the fluorescence peak), oscilloscope traces such as that shown in Figure 5 were typically observed.

During the initial half of the laser pulse, absorption in the probe beam increased as before. However, absorption then began to decrease, and in fact the film became more transparent to the probe light than it was before the laser pulse and remained more transparent after the pulse. The explanation for this trace is that, during the absorption of the laser light, the film began to heat up. Ablation of some of the dye occurred and therefore decreased the optical density of the film in the probe beam. It was observed that after a few laser pulses at 500 nm, the film in the area of the incident laser beam was completely gone. Presumably, the difference in energy between the incoming photons at 500 nm and the fluorescence photons at 610 nm was converted to heat (probably in the form of lattice photons) It was this heat that caused ablation of the film.

We conclude from these results that for such dye films to be useful as optical filters for intense light, they will have to be used to filter wavelengths near that of the excitonic fluorescence.

VII.B. RELATED WORK AT BEND RESEARCH

At Bend Research the closest work to that proposed herein is a project on artificial photosynthesis (DOE Grant No. DE-FG06-85ER13389) In this program we are attempting to convert solar energy to hydrogen by splitting water with photoelectrons generated from the impingement of sunlight on thin porphyrin films. To follow the kinetics of hole-electron function, laser-pump experiments have been conducted at the University of Oregon.

VIII. KEY PERSONNEL AND BIBLIOGRAPHY

It is anticipated that Dr. Walter C. Babcock will serve as principal investigator for the proposed program, with key contributions being made by Dr. George Rayfield. (Dr. Rayfield, a professor of physics at the University of Oregon in Eugene, Oregon, devotes half of his time to the University and half to Bend Research.) Abbreviated resumes for these individuals follow.

Curriculum Vitae: Walter C. Babcock

Education:

• B.A. in Chemistry, University of California, San Diego, 1969
• M.Sc. in Physical Chemistry, University of Oregon, 1970
• Ph.D. in Physical Chemistry, University of Oregon, 1976

Employment:

• Bend Research, Inc., Bend, Oregon, 1976-present, President. Research and development in the areas of coupled-transport, reverse-osmosis and gas-separation membranes; also in advanced ion-exchange materials and thin films of light-absorbing polymers.

Relevant Publications (of 16 total):

Babcock, W.C., and W.T. Simpson. 1975. "Absorptive Trapping in Thin Dye Layers." J. Chem. Phys. 62:2.

Babcock, W.C. 1 97 7 . "Excitonic Fluorescence and Absorptive Trapping Spectra of a Polymethinium Dye." J. Chem. Phys. 67:4770.

Curriculum Vitae: George W. Hayfield

Education:

• B.S. in Physics, Stanford University, 1958
• M.S. in Engineering Science, University of California, Berkeley, 1 9 64
• Ph.D. in Physics, University of California, Berkeley, 1964

Positions:

• NASA Moffet Field California, 1956, Aero-test technician.
• Sylvania Corp., Mt. View, California, 1958, Microwave-tube engineer.
• University of Pennsylvania, Philadelphia, 1964-1967, Assistant Professor of Physics.
• University of Oregon, Eugene, 1967-1968, Assistant Professor of Physics.
• University of Oregon, Eugene, 1968-1985, Associate Professor of Physics.
• University of Oregon, Eugene, 1985-present, Professor of Physics.
• U.S. Army Night-Vision Laboratory, 1986-present, consultant.
• Bend Research, Inc., 1987-present, consultant.

Selected Relevant Publications:

Rayfield, G.W., and T.R. Herrmann. 1976 ."A Measurement of the Proton Pump Current Generated by Bacteriorhodopsin in Black Lipid Membranes," Bioch. et Bioche. Acta, 443:623.

Rayfield, G.W., and T.R. Herrmann. 1978 . "The Electrical Response to Light of Bacteriorhodopsin in Planar Membranes," Bioph. J., 21:11.

Rayfield, G.W. 1 9 82 . "Kinetics of the Light-Driven Proton Movement in Model Membranes Containing Bacteriorhodopsin," Biophys. J., 38:79.

Rayfield, G.W. 1983. "Events in Proton Pumping by Bacteriorhodopsin," Biophys. J., 41:109.

IX. FACILITIES AND EQUIPMENT

Bend Research maintains a 17,000 square ft laboratory and office complex with a technical staff of 45. In-house equipment relevant to the performance of the program includes the following:

• scanning electron microscope
• UV-visible spectrophotometer
• vacuum deposition apparatus
• fully-equipped synthetic organic chemistry laboratory
• fluorescenceted work (about 20% of the project) will be conducted by a Bend Research employee at the Shared Laser Laboratory at the University of Oregon. The laboratory is directed by Dr. Bruce S. Hudson. A description of the University facilities and equipment follows.

The University of Oregon Shared Laser Laboratory provides a facility for the communal use of a variety of laser and related equipment by faculty and students engaged in research using lasers. This facility is also available for use by workers from other universities and from industry. The location of several laser devices in a common area and with adequate space permits performance of new experiments without interruption of on-going projects. It also permits experiments involving more than one laser and the efficient use of test and auxiliary equipment. Following is a brief description of the apparatus currently associated with this laboratory.

The laser apparatus available in the Shared Laser Laboratory permits state-of- the-art time resolved fluorescence experiments. Two mode-locked lasers are currently in operation. A Spectra Physics argon ion laser (model 171) and a Spectra Physica Nd:YAG laser (model 3000) provide pulsed output in the IR, visible or near UV region with high repetition rates. The current configuration and properties of the model 3000 ND:YAG laser will be described in detail. The harmonic output of this laser at 532 nm with a pulse duration of about 100 ps and an average power to 1.5 watts is used to synchronously pump a dye laser. This dye laser is cavity dumped resulting in output pulses with a pulse repetition rate of up to 4 MHz in the visible region. The pulse duration is about 5 ps. Harmonic generation produces ultraviolet radiation.

This laser system is used with a fully computer controlled apparatus to perform single-photon-counting fluorescence experiments. An automated, temperature-controlled sample compartment permits interchange of sample and reference cells. The fluorescence is detected by a microchannel plate detector with an intrinsic temporal resolution of 90 ps. This system response sets the basic temporal resolution of the instrument. Extensive numerical deconvolution methods have been implemented that permit extraction of lifetimes 3 to 5 times shorter than the system response. The single-photon- counting technique provides extremely high sensitivity due to the detection of single photomultiplier events per 100 excitation pulses. The resulting histogram of arrival times of photons is stored in a multichannel analyzer and subsequently transferred through a microcomputer to a minicomputer for subsequent data analysis, statistical evaluation and plotting. Numerical deconvolution (including global analysis) and methods of moments programs are available for the extraction of multiple lifetime decays for complex cases. Complete polarization capability and monitoring of the intensity of the excitation beam is available for the performance of anisotropy decay experiments.

An optical gating up-conversion apparatus has been constructed to complement the single-photon-counting device for the measurement of fluorescence and anisotropy decays. With this device the sample fluorescence is summed in a nonlinear optical crystal with the red dye laser radiation. Retardation of the red pulse train by a variable length path results in a sweep of the sample fluorescence signal with a temporal resolution of the pulse duration or about 5 ps. The current configuration permits summation of probe radiation near 600 nm with fluorescence near 450 nm (excited by 300 nm radiation) Fully automated data collection has been implemented.

The mode locked argon ion laser is currently configured to synchronously pump another dye laser producing low power pulses with pulse widths of a few ps at a repetition rate of 81 MHz. The argon laser is also used to provide pulses with a duration of 150 ps at the wavelengths of the major argon ion laser lines.

Q-Switched, Flash Lamp Pumped ND:YAG Lasers and Dye Lasers

The laser laboratory currently has three Quanta-Ray DCR Q-switched, flash lamp pumped ND:YAG lasers. Two of these (a DCR 1-A and a DCR 2-A) have amplifier stages. Each is equipped with harmonic generation capability. The following pulse energies are typical for these devices.

wavelength pulse energy (mJ) (nm) DCR-2 DCR-2A
1064 150 800 532 30 360 355 25 150 266 10 60 213 1

The pulse duration is 7-10 ns and the repetition is up to 50 Hz. These lasers are used to pump tunable dye lasers of home built or commercial design.

One of these ND:YAG lasers is equipped with devices for providing the widest range available of wavelengths in the far ultraviolet and vacuum ultraviolet region. In addition to "5th harmonic" generation at 213 nm, , stimulated Raman shifting in hydrogen gas provides collimated radiation throughout the far UV to wavelengths as short as 150 nm.

Excimer Laser

A Lambda Physik model EMG 150 EST provides high energy pulses according to the following table.

medium ArF KrF XeCl XeF wavelength (nm) 193 248 308 351 pulse energy (mJ) 200 550 450 160 linewidth (FWHM nm) 0.01 0.003 0.01 0.01

This laser is tunable over the range of the excimer emission bands (e.g., 0.3 nm for the 193 and 248 nm bands.) The maximum repetition rate is 25 Hz. The beam dimensions are 5-10 mm (horizontal) by 25 mm (vertical) with a divergence of 0.2 to 0.3 mrad.

High-Resolution Dye Laser

A Coherent model 699 tunable high resolution dye laser is available.

Auxiliary Equipment

Auxiliary equipment available for general use in the laser laboratory includes a Tektronix model 7904 500 MHz oscilloscope, an ISA 0.645 M monochormator, a Spex 1.25 M monochromator, a Tracor Northern optical multichannel analyzer, a high velocity vacuum pumping system, Coherent Radiation and Scientech power meters and numerous photomultipliers. Two IBM PC are used for data collection. A DEC LSI11/73 minicomputer with extensive interfacing is present in the laboratory for computations and plotting.

X. PRIOR, CURRENT, OR PENDING SUPPORT

No prior, current or pending support exists that is similar to the work proposed herein.

XI. CURRENT AND PENDING SUPPORT

Active funding for the proposed period of performance for Walter C. Babcock (WCB) and George W. Rayfield (GWR):

Annual BRI Award Direct Person-Months No. Project Title & Contract No. Period Source Costs WCB GRW

167	Development of a Novel Membrane	    9/1/87	NSF	   $55,633	0.3     0	
	for Extraction of O2 From Air	    2/28/90
	(ISI-8700974)

Proposals under negotiation or under funding review for Walter C. Babcock (WCB) and George W. Rayfield (GRW):

Annual Direct Person-Months Project Title Term Source Costs WCB GRW

Novel Process for Solubilizing Inclusion   6 mos.  NIH    $27,224	   0     2.4
Bodies

XII. POTENTIAL COMMERCIAL APPLICATIONS AND FOLLOW-ON FUNDING COMMITMENT

Bend Research is a pioneering company that develops new, high-technology products 
based on thin films and synthetic membranes.  Founded in 1975, this firm as become an 
internationally recognized leader in the field of thin-film technology.  It is 
the intention of Bend Research to commercialize developments in thin-film 
technology in the area of optical-quality polymeric films.  The proposed work 
is the first step toward the development of a new product needed for the protection 
of sensitive equipment and the human eye.  Additional potential 
applications will be identified during Phase I.

XIV. EQUIVALENT PROPOSALS

No equivalent proposals are under consideration or equivalent awards have been received for the proposed work.

REFERENCES

1. Babcock, W.C., "Excitonic Fluorescence and Absorptive Trapping Spectra of a Polymethinium Dye," J. Chem. Phys., 67(1977)4770.
2. Babcock, W.C., and W.T. Simpson, "Absorptive Trapping in Thin Dye Layers," J. Chem. Phys., 62(1975)2.
3. Babcock, W.C., "Absorptive Trapping, Excitonic Fluorescence and Polariton Absorption in Thin Films of Strongly Absorbing Dyes," Ph.D. Thesis, University of Oregon, 1976.
4. Simpson, W.T., and D.L. Peterson, J. Chem. Phys., 26(1957)5888. See also B.G. Aner and W.T. Simpson, Revs. Mod. Phys, 32(1960)466.
5. Fanconi, P.M., the Exciton-Molecular Vibrational Interaction in Molecular Crystals in the Strong Coupling Limit," Ph.D. Thesis, University of Washington (1968) p. 101.
6. Kittel, C., Introduction to Solid State Physics, 6th Edition. John Wiley, New York, New York (1986) Chapters 10 and 11.