Infrared recording of transient and permanent interferencegratings with commercial plastics
Sergio Calixto and Carmen Menchaca
Two commercial plastics have been used to record infrared interference patterns when a C02 laser is used as alight source. Recording of transient and permanent interference gratings with different spatial frequencieswas carried out. The diffraction efficiency for red light as a function of time for different exposure times isshown; maximum diffraction efficiencies attained were of the order of 30%. The possibility of using one ofthese plastics as an optical switch is shown.
1. Introduction
Polymers seem to be good materials to record mid-infrared interference patterns because they show lowdiffusivity and low thermal conductivity. In thepast",2 polymers have been used to record infrared (IR)interference patterns; however, more experimentsshould be performed to determine better the responseof polymers to IR radiation. Here we present intro-ductory results of a study made with two commercialplastics. One was a red Plexiglas3 and the other wasLexan.4 Red plastic was used before5 to record tran-sient interference gratings and holograms when anargon laser giving green light (0.5145 Mm) was used as alight source; a simultaneous reading of these diffrac-tive elements was made by sending to the recordingarea a He-Ne laser beam (0.632 Mm), which was poorlyabsorbed by the plastic.
Both plastics, in the form of sheets, were obtained ina store; they received no special preparation before theexperiment. The plastic sheets were cut into blocks of-6 X 5 cm2 ; about thirty exposures were performed oneach block. These materials were rigid so a substratewas not needed to sustain them when they were placedin the optical recording configuration. The plasticsurfaces were smooth when seen with the naked eye.The thickness of the red plastic was -3 mm and therefractive index was 1.494 at 0.589 Mm; while Lexan, apolycarbonate, presented a thickness of -5 mm and arefractive index of -1.582 at 0.6328 ,m. To investi-gate whether the plastics presented birefringence theywere placed between crossed polarizers. The red plas-tic did not show birefringence but Lexan did. Good
The authors are with Centro de Investigaciones en Optica, Apar-tado Postal 948, Leon, GTO, C.P. 37000, Mexico.
light transmission in the visible region was presentedby Lexan; however the red plastic absorbed green,blue, and yellow light and transmitted -80% of redlight5 (Fig. 1). No transmittance of IR radiation (at10.6-Mm wavelength) was observed for both plastics.
To characterize these plastics, interferometric stud-ies were performed. These included the recording ofinterference gratings with different spatial frequen-cies. Depending on the density power of the recordingbeams and on the exposure time two phenomena werenoted. One phenomenon was the transient modifica-tion of the physical properties of the plastic, the otherwas permanent damage. Results obtained when thetransient phenomenon was studied are presented inSec. II of this paper. Section III deals with the perma-nent modification made to the plastics, i.e., the record-ing of permanent gratings.
II. Recording of Transient Gratings
Transient gratings were recorded in Lexan using afrequency stabilized CO2 laser which gave a linearlypolarized beam (TEMOO mode); see Fig. 2. CO2 laserbeam was split into two beams by using a germaniumbeam splitter. These two beams were redirected tothe recording area by two copper mirrors; the powerdensity of each beam was -14 W/cm2. Simultaneous-ly with this recording, a He-Ne laser beam was direct-ed perpendicular to the interference place to performdiffraction efficiency measurements. The transmit-ted first-order light intensity was measured with aradiometer that was interfaced to a microcomputer.This connection between the radiometer and the com-puter was used to store in memory the variations of thefirst-order intensity during exposure time. An elec-tronic shutter, controlled with the computer, was usedto give the exposure time.
Two spatial frequencies of the interference pattern(4 and 10 lines/mm) were selected to test the recordingmedium in transient mode. Figure 3 shows diffraction
0.4-A I I I fv~~~A~~uA~~~A~~j~O | I I 1 220 440 660 880 1i00
Time msec.)(a)
77%2.0
1.6-1.2-
0.8-0.4-
tax p. = 40 msec.Exposure Delay - 20 msec.
: I
0 220 440 660 880 1100Time (msec.)
(b)
Fig. 1. Transmittance of the red Plexiglas as a function of visiblelight wavelength. Fig. 4. Diffraction efficiency behavior when a train of recording
pulses was sent to the IR sensitive medium (Lexan): (a) the delaytime between recording pulses was -100 ms, and in (b) it was -20
ms. The time length for each pulse was 40 ins.
�LASERF
RADIOMETER COMPUTER
Fig. 2. Diagram of the recording-reading geometry used to characterize polymer blocks as IR recording medium.
77%1%-
.8-
.6.
.4-
.2-
0
texp.=500 msec.
tJ r-texp~i =2texp40 miec.
50
40
3020
±0
io 200 300 400 500 -ime (msec)
Fig. 3.Diffraction efficiency behavior of transient gratings as a
function of time when Lexan was used as the recording medium forthree different exposure times. Spatial frequency of the interfer-
ence pattern was -4 lines/mm.
teip = 2.i sec.
tep= 2.0 sec.
: <;.teip 2.3 sec.V.......
AA A -~~~~11.56 T 3.12 Time (sec.)
Fig. 5. Diffraction efficiency behavior during the exposure as afunction of time for permanent gratings when Lexan was used asrecording medium for exposure times; of 2.1, 2.0, and 2.3 s. Thepattern spatial frequency was -4 lines/mm. The arrow for each
Fig. 6. Three fields of view of an interference microscope when interference gratings were investigated. Diffraction efficiency behavior foreach grating presented in (a), (b) and (c) can be seen in Fig. 5in each corresponding curve. Note that modulation of the first grating (a) is like asquare profile, the second grating (b) presents a sinusoidal profile, and the third grating (c) shows low modulation because an overexposure was
given.
efficiency behavior when Lexan was used. WithLexan the heat response was not immediate becausemodulation appeared a few milliseconds after the be-ginning of exposure, i.e., when -280 mJ/cm 2 (10 ms X28 W/cm2) of energy had penetrated the plastic. Thediffraction efficiency of the recorded grating attained amaximum of -0.6% and it remained constant with thisvalue throughout the exposure time. When the shut-ter was closed, diffraction efficiency decayed exponen-tially until it reached the noise level. When spatialfrequency of the interference pattern was increased at10 lines/mm, the maximum diffraction efficiency wasonly around 0.1% for exposure times falling in the 10-80-ms range. This low efficiency is presumably due toa fast degradation of the recorded grating when heatwas interchanged between the nearest interferencelines.
Fast recording and recovering of the normal thermalconditions of Lexan allow its usage as an optical switch.To illustrate this, recording 40 ms long light pulseswere obtained with the electronic shutter. Figure 4shows graphs of diffraction efficiency behavior for redlight as a function of time for two train pulses, eachwith a different delay time. The maximum diffraction
efficiencies for each pulse and the form of each one ofthese pulses are close to the ones presented by thesingle exposure case (Fig. 3). It was attempted to usethe plastic as a switch for times longer than 15 s but itwas noted that Lexan degraded in such a way that nomore recovering of the normal thermal conditions waspresent.
During the recording of transient gratings the dif-fraction effects could be due to surface and/or bulkmodulations. Because blocks of Lexan can be ob-tained only in a limited number of thicknesses, it is notpossible to perform a study of the transmittance of themedium as a function of the thickness to determinehow deep infrared radiation can penetrate. Howeverit is believed that infrared radiation is absorbed com-pletely by Lexan within the first 100 ,m. So, if bulkmodulation exists during the recording time, thisshould not be constant throughout the thickness of theabsorbing layer.
Transient gratings were also observed when the redplastic was placed in the optical configuration butdiffraction efficiencies attained a maximum of-0.01%. This efficiency was much lower than the oneobtained with Lexan.
Fig.7. Profile depth as a function of spatial coordinate. Points represent experimental results taken from Fig.6(b) and the line represents acalculated sinusoidal curve.
so
403 0
2 0
1o
t.xp = 1. .c.l aN 1________ __t.xp = .0 sc.i> _ /v i , tsp = i.4 sec.
.. ..... i _ .i j4A .56
1 312 Time (sec.)
Fig. 8. Diffraction efficiency behavior as a function of time when the red plastic was used as the recording medium for three exposure times.The spatial frequency of the interference pattern was -4 lines/mm. Each arrow points to the end of the exposure time.
1ll. Recording of Permanent Gratings
The effect of permanent damage to the recordingmedia was also studied with the optical configurationdepicted in Fig. 2. Recording times were longer thanthose mentioned in Sec. II. Spatial frequency of theinterference pattern had values of 4, 10, and 15 lines/mm.
Figure 5 shows the typical behavior of the diffractionefficiency as a function of time when Lexan was used asthe recording medium; the measured parameter is ex-posure time (pattern spatial frequency was -4 lines/mm). Each curve is marked with an arrow to show thetime when exposure ended. It is observed that forcurves representing exposure times of 2 and 2.1 s,diffraction efficiency rises and then remains steady.However, the curve representing an exposure time of2.3 s shows that diffraction efficiency rises and thendecays. An interference microscope was used to inves-tigate the cause of this behavior to find the modulationinduced in the plastic by IR radiation. Each photo-graph in Fig. 6 shows the microscope field of view wheneach grating was investigated and they correspond tothe curves presented in Fig. 5. It is possible to notethat the highest modulation is present in Fig. 6(a) (texp= 2.0 s), however, this photograph shows that theprofile of the grating is not sinusoidal but nearlysquare. In Fig. 6(b) (texp = 2.1 s), the profile shown issinusoidal. Figure 6(c) (texp = 2.3 s) shows that thegrating modulation was small. This is due to the over-exposure used. To find out how close the modulationwas to a sinusoidal form in Fig. 6(b), experimental datawere taken from this photograph and plotted against acalculated sinusoidal curve in Fig. 7. The agreement isgood.
Another conclusion inferred from the experimentsperformed with Lexan is that it presented a step re-sponse in relation with the recording of IR radiation.To understand this, typical experimental results areexamined. Figure 5 shows the behavior of diffractionefficiency as a function of time for three exposuretimes. During the experiment various sequential ex-posure times used: 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, and 2.4 s.For each exposure time a new area of the recordingblock was used. When recording with the first twoexposure times (1.8 and 1.9 s), diffraction efficienciesshowed a value which was within the noise gap. When2-s exposure was given a diffraction efficiency of -20%was obtained. This means that by changing the valueof exposure time by 100 ms (from 1.9 to 2.0 s) diffrac-tion efficiency changed its value by -16%. It can beinferred that 1.9 s of the total exposure time softens theplastic and the additional exposure serves to modulatethe surface of the plastic.
Regarding the source of the diffraction mechanism,it is possible that during some time of the exposureperiod bulk modulation could contribute to the dif-fraction of light but most of the contribution is done bysurface modulation. This statement can be inferredfrom the following fact. At the end of the exposuretime when a glass plate was placed in close contact withthe surface of the Lexan block and a matched indexliquid was poured between them, the diffracted spotdisappeared, suggesting that surface modulation wasthe cause of the diffraction phenomenon.
Interference gratings having a spatial frequency of-4 lines/mm were also recorded with the red plastic.Behavior of the diffraction efficiency as a function oftime is shown in Fig. 8. It is observed that the behav-
8 | \ -------- Z-* texp=L.4 sec.4 _ s A~~~I..........................v ~ ~ ~~I v
1.56 312 Tiselsec.)
(a)
tep = i. sec.------------ tesp 1 i.0 sec.
,:~ - tesp = 1.4 ec.
A .............__._
3.16 Time (s*c.)
(b)
Fig. 9. Diffraction efficiency behavior as a function of time whenLexan was used as the recording medium. The spatial frequency ofthe interference pattern was (a) 10 lines/mm and (b) 15 lines/mm.
ior of the diffraction efficiency is similar to that shownin Fig. 5, i.e., in some curves the diffraction efficiencyrises and then remains steady; however, in the lastcurve (t = 1.4 s) it rises and then decays. When wecompare the maximum diffraction efficiencies at-tained with both recording mediums, it is observedthat higher values (10% or more) are present whenLexan records the interference pattern; however, it canalso be concluded that red plastic needs less recordingenergy to obtain a similar diffraction efficiency. Forexample, Lexan needs -58 J/cm 2 (2.1 s X 28 W/cm 2 ,Fig. 5) to attain 30% diffraction efficiency and redplastic needs -30 J/cm 2 (1.1 s X 28 W/cm 2 ) to attain-20% diffraction efficiency. Beside this, we can com-pare these energy values with those published in Refs.6 (fig. 7) where thin gelatin films were used as therecording medium. It is shown in these studies that toattain -27% diffraction efficiency (spatial pattern fre-quency -7 lines/mm), an energy of -10 J/cm2 wasrequired. So Lexan requires -48 J/cm 2 more energy torecord a grating. However, the plastic blocks used inthe present study show less fragility and more patternstability than gelatin films.
Recording of interference gratings showing higherspatial frequencies was done by changing the inter-beam recording angle. Diffraction efficiency behaviorfor Lexan recorded gratings having 10 and 15 lines/mmis shown in Fig. 9; the maximum diffraction efficienciesattained have a lower value than those shown by grat-ings having 4 lines/mm. Red plastic response to thesame spatial frequencies is show in Fig. 10.
Fig. 10. Diffraction efficiency behavior as a function of time whenred plastic was used as recording medium. Spatial frequency of the
interference pattern was (a) 10 lines/mm and (b) 15 lines/mm.
IV. Comments
More studies should be done to fully characterizeboth plastics. Their limited response to interferencepattern spatial frequencies is a drawback; however, atpresent, more studies are being performed to possiblycircumvent this. Diffraction elements having low spa-tial frequencies (a few lines per millimeter) could beused in interferometric tests like the Ronchi one.7
Future studies using these plastics could employ theuse of more powerful CO2 lasers; with the result thatexposures times could be shorter than those presentedthroughout this paper and the interchange of heatbetween recording lines should be low allowing thefabrication of bigger diffractive elements presentinghigher spatial frequencies. Another possible studycould involve the IR reconstruction of these elementsby evaporating a thin high IR reflecting layer overthem. From an economic point of view, recording onplastics presents an advantage because plastics areinexpensive and have widespread use.
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langer, "Plastic Recording Media for Holography at 10.6 gim,"
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Taktakishvili, T. Ya. Chelidze, and V. V. Chichinadze, "PossibleUse of 10.6 ,um Holograms for Plasma Diagnostics," Sov. Tech.Phys. Lett. 2, 425-427 (1977).
3. Rohm & Haas, West Hill, Ontario, red Plexiglas 2423.4. Lexan, General Electric product.5. S. Calixto and R. A. Lessard, "Transient Holograms in Dyed
Plastic," Appl. Opt. 23, 211-213 (1984).6. S. Calixto, "Infrared Recording with Gelatin Films," Appl. Opt.
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