Gelatin films glued to an O-ring are proposed as a new procedure to record interference patterns when a CO2laser is used as an infrared light source. With this method the unwanted effects given by a substrate, on whichthe thin recording film is laid, are avoided. To characterize the medium, interferometric studies includingthe recording of diffraction gratings have been done. Diffraction efficiencies of -30% have been obtainedwhen coherent red light (0.6328-,um wavelength) was sent normally to the gratings. An example of an infraredrecorded hologram is shown.
1. Introduction
The recording and reconstruction of holograms isusually performed with light sources emitting visibleradiation. However, lasers emitting ultraviolet' 2 andinfrared (IR) radiation can also be used to form aninterference pattern given by the object and referencebeams. Recording media which respondtto nonvisibleradiation have been studied; research continues to findmaterials with better qualities.
It has been shown that in some instances holograph-ic elements are more adequate than conventional op-tics to manipulate visible beams, so it is reasonable tothink that holographic elements that modulate IR ra-diation (10.6 gim) will be needed in the future. 3 IRholographic elements can be fabricated by two meth-ods: direct recording of IR interference patterns or bycalculating the desired interference pattern which canbe transferred to a substrate by lithographic methods4 -6 or by writing it directly with electron beams.7 8 Thelithographic procedure presupposes several steps.First, the interference pattern is calculated with a com-puter. Then it is displayed in hard copy or on a CRTscreen. A reduced copy of this display is made byphotography. This reduced interferogram is placedover a substrate that has previously been coated withphotoresist and metal thin films. After some manipu-lations a relief pattern of the interferogram is obtainedwhich then is covered with a thin metal layer to avoid
The author is with Centro de Investigaciones en Optica, Apartadopostal 948, C.P. 37000 Leon, Gto, Mexico.
surface damage and increase reflectivity when IR radi-ation strikes the hologram at the reconstruction step.For more details of the process see Refs. 4-6.
The fabrication of computer-generated hologramsby electron beams also comprises several steps. Thesubstrate (quartz or glass) is coated with a thin film ofchrome and then an electron resist film is laid over it.An electron beam controlled by a computer is used towrite the interferogram on the resist. After this thedevelopment and etching processes proceed and a sur-face relief pattern is obtained. A metal thin film canbe coated over this surface. For more informationabout this process consult Refs. 7 and 8.
Direct recording of IR interference patterns is analternative to the processes outlined above. Materialsthat have been used to record IR patterns are choles-teric liquid crystals,910 bismuth films,'1l-13 cuprousmercuric iodide 12,14-17 wax,11 13 "8 oil,19 plastics, 2 0 gela-tin on glass substrates, 1 8 Plexiglas,1 3 paper,1 0 photo-chromic spiropirans, 2 ' metals (vanadium oxides),22and others. Some of the materials mentioned aboveshow changes in the maximum visible wavelengthtransmittance 9 or in their reflectivity1 4' 16 when IR ra-diation interacts with them; so, to perform a recon-struction of the hologram with IR light a secondarystep is necessary. The pattern is photographed andthe negative is used as a mask in a lithographic methodto produce a reflection hologram. Other materialspresent a surface relief pattern after exposure to IRradiation. This seems to be the more direct methodbecause a thin film of aluminum, or other high IRreflecting material, can be evaporated over their sur-face and reconstruction of the hologram can be per-formed with IR radiation. Unfortunately when thesematerials are used the depth surface modulation at-tained is not large enough to obtain good diffractionefficiency in reconstruction. In this paper we describeintroductory experiments when thin gelatin films,
without substrate, were used to record mid-IR inter-ference patterns (10.6-gm wavelength). The interac-tion of IR radiation with thin gelatin films creates asurface modulation. Section II deals with the fabrica-tion and transmittance of the gelatin films. SectionIII deals with the interferometric characterization ofthe films having different thicknesses, and Sec. IVshows the results when gelatin films were used to re-cord a hologram.
II. Gelatin Thin Films: Manufacture and Transmittance
Thin gelatin films were made with the help of twooptical flats, one of which had a hole of -4-cm diame-ter. They were put in close contact and then placed ona leveled substrate. A mixture of 2% (by weight) gela-tin (U.S.P. gelatin, supplied by J. T. Baker) and water(distilled) was prepared. Small quantities werepoured into the hole of the optical flats. For thin films(<10 m thick) 0.5 m liter of the mixture was em-ployed. To obtain thicker films the quantity men-tioned was doubled, tripled, etc. and films with thick-nesses ranging from 15 to 100 gm were obtained. Thedrying period was -6 h for thin films and about a dayand a half for the thicker ones. This period was afunction of room temperature and relative humidity inthe laboratory. Thicknesses of the films were mea-sured with an interference microscope, for the thinones, and with a micrometer for the thicker ones. Themanufacture of gelatin and polymeric films has beendeveloped throughout the years by the photographicindustry23 and methods to fabricate films have beenreported in connection with the manufacture of DCGfilms,2 4 so any of these methods could be used to fabri-cate gelatin films to record IR radiation.
After the gelatin drying period an O-ring was gluedto the gelatin and with little effort the gelatin film wasdetached from the glass. To measure IR transmit-tance of the films a spectrometer was used; results canbe seen in Fig. 1. It is noted that films of -90-gmthickness show low transmittance at 10.6 gim, -10%,and thin films (15-,gm thickness) show high transmit-tance (90%).
Il1. Interferometric Studies
To find the behavior of thin gelatin films when theyrecord IR interference patterns an interferometric ge-ometry with a stabilized CO2 laser as the light sourcewas used, Fig. 2. Emerging radiation from the CO2laser (6-W output power, nominal) was divided by anantireflection (AR) coated germanium beam splitter.Then Cooper mirrors were used to recombine bothbeams in a region where a gelatin film stood. Therecording area was a square of -6 X 6 mm, the interfer-ence pattern spatial frequency was -7 lines/mm andthe power density of each beam was 12.47 and 14.51 W/cm2 (intensity beam ratio 1.17). Light coming from aHe-Ne laser (0.6328 gim), impinging normally on therecording medium, was used to investigate the forma-tion of the gratings during recording time; first trans-mitted order light intensity was measured with a radi-ometer that was interfaced to a microcomputer
9 10 11 12 (/m.)
Fig. 1. Gelatin film transmittance vs wavelength. Layer thicknessis the parameter.
RADOMETER COMPUTER N
E
Fig. 2. Diagram of the recording-reading geometry used to charac-terize gelatin film as an IR recording medium. A germanium beamsplitter was used to divide the beam: Cooper mirrors redirected thebeam. A He-Ne laser (stabilized) investigated grating formation
during recording time.
through an analog-to-digital converter. Films withdifferent thicknesses (5-60 gim) were used to recordthe interference pattern but only good behavior waspresented by films 20-30 gim thick. This will be ap-parent from the following results.
The effect of recording IR interference patterns ongelatin films is to develop a surface modulation byabsorption of heat. When the recording geometryshown in Fig. 2 is used the modulation presented by thefilm will be an interference grating. One parameterthat represents the quality of the recorded grating is itsdiffraction efficiency which is the percentage of thefirst diffracted order light intensity to the total beamintensity that strikes the grating.
Curves showing the behavior of diffraction efficien-cy as a function of time for different exposure times areshown in Fig. 3. We obtained these curves as follows:First the computer began to take the signal from theradiometer through the A-D converter. Then theshutter was opened for a given period of time (time ofexposure, texp), after which the computer was stopped.Unfortunately there was no means to know the begin-ning or end of the exposure time because the shutterdid not have an electrical connection interfacing withthe computer and controlling the starting point of the
Fig. 3. Diffraction efficiency vs time. Exposure time is the param-eter. Layer thickness is <10 Am.
a)
77%20
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Fig. 5. Diffraction efficiency vs time. Time of exposure is theparameter. Gelatin thickness was -10 m.
77%30- texp=200msec
20- /t~-o-texp=240msec.20 _ / xp=310 msec.o- I 27msec.
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Fig. 6. Diffraction efficiency vs time. Time of exposure is theparameter. Gelatin thickness was -20 Am.
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30-
20-
10 _
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77e,
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30
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Fig. 4. (a) Diffracted orders given by an interference grating re-corded with an appropriate exposure time. (b) Diffracted orderswhen the interference grating was recorded with an overexposure
time.
exposure time. Gelatin films -5-10 gim thick wereused in this part of the experiment. As can be seen inFig. 3 modulation of the layer during exposure timeincreases and reaches a maximum. For short exposuretimes (3 s) the diffracted orders looked like brightspots, Fig. 4(a); however for long exposures they ap-peared as a blur with a nonconstant spatial intensitydistribution, Fig. 4(b). Unfortunately when very thin(<10-gim) gelatin films were used poor repeatability ofresults was found. Degradation of the film duringexposure was severe; in some cases when exposure timewas too long the film was torn. Better diffractionefficiencies were found when gelatin films 10 gimthick were used to record interference gratings. Fig-ure 5 shows behavior of the diffraction efficiency as afunction of time for these films. Diffraction efficien-cies of -16% can be obtained. One interesting point tonote here is that, for an exposure time of 800 ms, the
s0msec.
.1 te= 240 msec.
Time.
Fig. 7. Grating diffraction efficiency behavior as a function of time;spatial frequency was 3 lines/mm; (a) texp = 140 ms; (b) teep = 240 ms.
diffraction efficiency rise reaches a maximum and re-mains steady; however for longer exposures diffractionefficiency reaches a maximum and then decreases untilit reaches a steady state. Again it was noted that forlonger exposures times the gelatin layer was torn.
Good repeatability of results was found when gelatinfilms 20-30 gim thick were used to record interfer-ence gratings; results can be seen in Fig. 6. It is possi-ble to see the behavior described above for thinnerfilms, i.e., the curves reach a maximum and then de-crease until they reach a steady state. Note that dif-fraction efficiencies of 30% can be obtained. Thediffraction efficiency decrease can be attributed to theformation of a second relief grating in the back surfaceof the gelatin layer. The following experiment verifiesthis last statement: Two gratings were recorded withexposures times of 140 and 240 ms, respectively, on a25-gm thick gelatin film; spatial frequency of the pat-tern was 3 lines/mm. Behavior of their diffractionefficiencies as a function of time can be seen in Fig. 7.
Fig. 8. Far-field diffraction patterns given by two IR recorded gratings when normally illuminated by a He-Ne beam. The first row, (a) and
(b), shows two patterns for two gratings recorded with exposure times of 140 and 240 ms. The second row, (c) and (d), shows diffraction pat-terns for the same gratings but with a glass plate wedge placed in close contact with the back surface of the gelatin film and a matching-indexliquid poured between them. Note that the left-hand side patterns (a) and (c) look almost the same; however the right-hand side patterns (b)
and (d) show a difference. No high orders are present in (d).
Note that for the first curve the diffraction efficiencyrises and then remains almost steady. However, thecurve representing the longer, exposure time showsfirst an increase in the diffraction efficiency, reaches amaximum, and then decays. The transmission far-field diffraction pattern given by the gratings when aHe-Ne laser beam was sent perpendicularly to theirsurface can be seen in Fig. 8(a) (texp = 140 ms) and Fig.8(b) (texp = 240 ms). Note the higher orders in this lastphotograph. Diffraction efficiencies, measured forthe first order, were -4% for both. After this a glassplate wedge was placed in contact with the back sur-face of the gelatin film where both gratings had beenrecorded; a matching-index liquid was poured betweenthe glass plate and the gelatin film. The angle be-tween the faces of the glass wedge had a small value(1.50) to avoid multiple reflections which will degradethe far-field diffraction pattern. The far-field pat-terns are those shown in Figs. 8(c) and (d). When wecompare photographs in Figs. 8(a) and (c) it is notedthat there is only a difference in the third diffractedorders; however, for the second grating a noticeablechange exists because most of the higher diffractedorders have vanished leaving only some low orders.By measuring the intensity of the first orders for bothgratings after the glass plate was positioned, it wasfound that for the first grating a diffraction efficiencyof -3% was present and for the second grating a dif-fraction efficiency of -12% was found. When we com-pare these last diffraction efficiency values with thosemeasured before the glass wedge was put in place, it isseen that values for the first grating did not differ too
7%
30 -
20-10- t '~~~~~p 28O..57 m.c.
40 .t27m. 6 1 __teep~20mec
Time
Fig. 9. Diffraction efficiency vs time. Time of exposure is theparameter. Layer thickness was -50um.
much; however, an increase of -8% in the diffractionefficiency exists for the second grating. This meansthat a relief structure had developed at the back sur-face of the gelatin film during recording time, so thatlight impinging the film layer at the reading step wasdiffracted twice.
Thicker films (50 im) than those mentioned abovewere also tested as the recording medium; results areshown in Fig. 9. For exposures times of 220 and 280ms, the curves do not present good behavior becausethey show ripples. Curves for longer exposures times,320 ms, do not represent the real diffraction efficiencybecause for these situations light diffracted along thefirst order was not a bright well-defined circular spotbut an irregular distribution of light; degradation ofthe surface was severe. In this case the film was nottorn as in the previous cases, but the gelatin was melt-ed. Figure 10 shows the recording of two such expo-sures; scale is in millimeters. The central part of eachrecording shows bubbles which appeared when intenseheat melted the gelatin. In the periphery of the expo-sures the recorded interference fringes can be seen.The maximum diffraction efficiency obtained for each
m----I'IMIIFig.10. Gelatin layer -50gm thick with two overexposured record-ings. Note that in the central areas the gelatin has melted due to theintense recording heat. Recorded interference fringes are visible at
the periphery of each recording. Scale is in millimeters.
exposure time taking into account the different thick-nesses of the gelatin layers has been plotted in Fig. 11.It is possible to conclude that, as expected, very thinplates need more energy to produce a modulation be-cause their transmittance is higher. However, itseems that thick plates (>50 gim) are not useful be-cause degradation caused by the recording beams ishigh, which means that diffraction efficiency is low.So thin layers -25 gm thick should be useful to recordholograms with a mean spatial frequency of 7 lines/mm.
Stability of the recording is a parameter to considerwhen choosing a photosensitive medium. When pho-tographic and dichromated gelatin (DCG) films areused to record interference patterns, the developmentand fixing steps cause variations in the thickness of thesensitive layer.2 425 Even more if after these stepsrelative humidity rises significantly layers will degradeand the recorded information will disappear. The useof gelatin layers to record mid-IR interference patternsdoes not include postexposure processes because thedeformation caused by heat is enough to diffract visi-ble light and form the holographic image. However,the presence of high humidity after the IR recordingstep can degrade the gelatin layer glued to the O-ringbecause humidity will affect both surfaces of the gela-tin film not just one as is the case when photographic orDCG films are laid over a substrate. One method toovercome this difficulty is to place the IR elements in acontainer to avoid changes in relative humidity. Thiscover would be transparent to the reading light. An-
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other alternative would be the use of materials that donot change their structure when the relative humiditychanges, possibly some polymers. Two methods wereused to investigate the profile of the IR recorded inter-ference gratings: One was direct observation of thegrating with an ordinary microscope. Figure 12(a)shows a recorded grating that had a spatial frequencyof -7 lines/mm; note the unexposed part. An alterna-tive method was to use an interference microscope; atypical result is shown in Fig.12(b). Interference lineson the right-hand side show us that gelatin film re-mains flat after exposure. Because interference linesseen in the field of view of the interference microscopeshow a contour map of the grating surface we note thatthe relief of the grating shows some spatial changes.Take, for example, the line marked with an arrow; it ispossible to see that the height of the first five peaks(from right to left) is greater than the height of theremainding peaks.
It was mentioned above that the effect of recordingIR interference patterns with gelatin films is to devel-op a surface modulation by heat absorption. Theeffect of change in exposure time on the surface modu-lation can be studied with the help of an interferencemicroscope. Using the configuration depicted in Fig.2 an angle between the recording beams was chosen togive an interference pattern with 3 lines/mm spatialfrequency. Then a gelatin film was placed in the re-cording area and two interference gratings were made,one with an exposure time of 140 ms and the other with240 ms. Then the surface relief of each was investigat-ed with the interference microscope; results can beseen in Fig. 12(c) (texp = 140 ms) and Fig. 12(d) (texp =
240 ms). It is possible to see that modulation of thefirst grating is weaker than the second.
IV. Recording Holograms
A test of the gelatin layers was made by recording anIR hologram. The object was a handmade trianglepunched out of aluminum foil with -3-mm diameter.The angle between recording beams was set to give aninterference pattern with -12-lines/mm spatial fre-quency. Reconstruction made with light coming froma He-Ne laser can be seen in Fig. 13; zero order wasblocked with black paper.
:lol.
e Texe)
6 sec. texp Fig. 11. Maximum diffraction efficiency vs time9 of exposure and exposure energy. Layer thicknessExpoeure Energy J/cm
t t 1 Order Position of Zero - Order(image) Order blocked with
a block paper
Fig. 13. Image given by an IR recorded hologram when reconstruc-tion was made with He-Ne laser light.
V. Comments
More studies are being performed to know better thequalities of gelatin layers working as an IR recordingmedium. These studies comprise the recording ofinterference patterns with different visibilities andspatial frequencies. It is expected that sensitivity ofthe gelatin films will be a few J/cm 2 . This value falls inthe medium range if it is compared with those present-ed by other mid-IR recording materials mentioned inSec. I.
Fig. 12. (a) Interference grating studied with anordinary microscope. Pattern spatial frequency is-7 lines/mm. (b) Image given by an interferencemicroscope when a recorded grating was investigat-ed. Note that the grating surface relief presentsspatial changes. This is evident when the interfer-ence line, marked with an arrow, is seen; the firstfive peaks, taken from right to left, present greaterheight than the rest of the peaks. Spatial frequen-cy is -11 lines/mm. (c) and (d) Field of view of aninterference microscope when two recorded grat-ings are studied. Spatial frequency for both -3
lines/mm. (c) t,,p = 140 ms; (d) tcxp = 240 ms.
Future studies will also use the reading step per-formed with infrared light. To avoid the degradationof information previously recorded on gelatin filmswhen the IR reading light strikes the gelatin film, athin metal high IR reflecting layer will be laid over thegelatin surface.
Thanks are given to J. Z. Malacara-Hernandez, F. J.Cuevas, and Francisco Huerta for interfacing an ana-log-to-digital converter between the radiometer and amicrocomputer. Also thanks are given to the refereesfor comments that improve this paper.
Part of this work was presented at the OSA 1987Annual meeting, Rochester, NY.
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A report of a survey of applicants for research support from theNational Science Foundation in 1985 provides insights into thecharacteristics of this group and of the views of individualinvestigators about NSF's competitive proposal review anddecisionmaking process.
The survey, conducted by NSF's Program Evaluation Staff, wassent to more than 14,000 applicants and drew more than 9,500responses.
Nearly half the applicants were satisfied with the reviewprocess, but a substantial proportion (38 percent) weredissatisfied. Declinees (two-thirds of the applicants) were muchmore likely to be dissatisfied.
The reasons most often volunteered by dissatisfied applicantswere that the reviewers selected by NSF were not sufficientlyexpert in the subject matter of the proposal, or that the reviewswere cursory, conflicting, or did not seem to support the NSFdecision. Reasons such as cronyism, politics or biases ofvarious types were cited less frequently.
Among the respondents, 87 percent were men and 13 percent women.Eighty-nine percent were white, 9 percent Asian, and 2 percentmembers of other racial/ethnic groups.
Twenty-three percent submitted their proposals through one of 21academic institutions that had received 60 or more competitivelyreviewed individual investigator research grants from NSF inFiscal Year 1985; 62 percent submitted through another Ph.D.-granting institution; and 11 percent through a primarilyundergraduate institution. The remaining 4 percent indicated"other."
Copies of the report "Proposal Review at NSF: Perceptions ofPrincipal Investigators," (NSF 88-4) may be obtained by writingto the Forms and Publications Office, National ScienceFoundation, 1800 G Street, N.W., Room 232, Washington, D.C.,20550, or calling (202) 357-7861.
