In recent years, with the development of holographyand micro-optics,1 the demands for novel recordingmaterials with diverse capacities have increased rap-idly. Traditional dichromated gelatin2,3 ~DCG! is re-garded as the best volume holographic recordingmaterial because of its high diffraction efficiency,high resolution capacity, high signal-to-noise ratio,and so on. But its shortcoming of image degenera-tion in a humid environment, which is due to hy-drophilous gelatin, confines its wider application.Photoresist4 is used in great quantities to producerelief-type rainbow holograms, but its drawbacks oflow photosensitivity, low spatial resolution, and shortlinear modulation range, as well, make it less suit-able to fabricate micro-optical elements with finestructures.
Photoetching cellulose film ~PCF!, developed by theauthors,5–10 uses commercial cellulose triacetate~TAC! film as its substrate. After being prepro-cessed with a mixed aqueous sodium hydroxide andalcohol solution and being sensitized with an aqueousammonium dichromate solution, homogeneous pho-tosensitive layers of several micrometers thicknessare formed on both sides of the TAC film simulta-
neously. The thickness of the photosensitivelayers—from 2 to 20 mm—can be controlled properlyby the changing of the preprocessing parameters,such as concentration, temperature, and time; athickness of 10 mm is usually chosen in our experi-ments. The preparation procedures are very simpleand don’t need procedures such as coating for DCG orspinning for photoresist. Because the photosensi-tive layers and the substrate are made of the samesubstance, the difference between the substrate ~theTAC! and the photosensitive layers ~the dichromatedcellulose! comprises only partial chemical groups;thus there are no questions of stripping and radialnonuniformity.
Moreover, PCF has an excellent antihumidity ca-pacity because it has hydrophobic cellulose as itsphotosensitive layers. It also possesses some de-sireable characteristics, such as up to a 3000-lineymm-high resolution, a greater than 30% strongreal-time diffraction efficiency, an excellent linearrelief-modulation capacity, light weight, good flex-ibility, simple processing procedure, low cost, etc.In this paper its imaging mechanism and charac-teristics are presented first, then the simultaneousfabrication of a multifunctional integrated opticalelement on both sides of the PCF is described.
2. Mechanism of Image Formation
Dichromate-type recording materials, such as DCGand dichromated poly~vinyl alcohol!, all use dichro-mate as their photosensitizer. The general photo-
The authors are with the Information Optics Institute, SichuanUniversity, 610064 Chengdu, China.
Received 4 February 1998; revised manuscript received 3 April1998.
4330 APPLIED OPTICS y Vol. 37, No. 20 y 10 July 1998
chemical reaction of the chromium ion was given byManivannan11:
Cr~VI! O¡hn
Cr~VI!*, (1)
Cr~VI!* 1 eO¡ Cr~V!, (2)
Cr~V! 1 2eO¡ Cr~III!. (3)
Cr~VI! is finally reduced to Cr~III!, but this reducingprocess is completed through an intermediate, Cr~V!.To verify the proposed introduction of the above re-action scheme into PCF, we use a Model ER-SRC200D electron-spin resonance ~ESR! spectrometermade in Germany to record the ESR spectra of PCFsamples exposed to a mercury lamp for 15 min, withthe sample then set aside for three days in darkness.The exposure conditions are listed as follows: a50-W high-pressure mercury lamp as the exposuresource, an exposure time of 15 min, an exposure dis-tance of 100 cm, and an exposure area of 0.5 cm2.
The various valences of the chromium ion Cr~V!,which has one unpaired d electron, include a para-magnetic species and a stable ESR signal at roomtemperature. In our experiments strong ESR spec-tra of Cr~V! ~see Fig. 1; the g factor equals 1.9780 60.0002! are detected, which shows that, during thephotoreduction reaction of ammonium dichromateand the cellulose polymer, Cr~VI! is reduced to Cr~III!through the intermediate Cr~V!. Moreover, the peakarea of the ESR signal is proportional to the concen-tration of unpaired electrons,12 and in our experi-
ments it is proportional also to the concentration ofCr~V! ions. The bigger the peak area, the higher theCr~V! concentration. So from Fig. 1 we can also findthat, even after PCF is kept in the dark, the strongoxidation capacity of Cr~VI! will produce the dark re-action and increase the concentration of Cr~V! ions,thus making the chemical reaction complete. Wecan use this characteristic to improve the diffractionefficiency, obviously by delaying development afterexposure.
We also have studied the photochemical reaction ofthe cellulose polymer by means of IR spectra. An IRspectrometer can detect the characteristic absor-bances of some functional groups of the polymer12 bycomparison of the relative absorbances of the func-tional groups, so we can deduce the structure of thepolymer and changes in its structure. We used aModel NICOLET-5MX Fourier IR spectrometermade in the U.S. to record the IR absorbance spectraof PCF under different conditions. The curves ofFig. 2 correspond to ~curve 1! a sensitized PCF sam-ple and three PCF samples ~curve 2! exposed to themercury lamp for 30 min and exposed for 30 min andthen developed in a 5% NaOH solution ~curve 3! for30 s and ~curve 4! for 60 s. Comparing curves 1 and2, we can see that the absorbance peak of the etherbond ~OOO! of III ~its characteristic wave-numberrange is 1140–950 cm21!, which is located in themain chain of the cellulose polymer ~Fig. 3!, has beenweakened, corresponding to the decreased concentra-tion of the ether bond. We believe that the decreasein the number of ether bonds is due to fracturing ofthe ether bond during the exposure process, whichleads to a photodegradation reaction and a decreasein the polymeric degree of the cellulose polymer. Sowhen it is developed by the NaOH solution, the sol-ubility of the exposed region of the PCF increases,and a positive surface relief is produced. Mean-while, we can see that, during the exposure and thedevelopment processes, the absorbances of other or-ganic functional groups such as hydroxyl ~OOH! I ~itscharacteristic wave-number range is 3750–3000cm21! and carbonyl ~OAC,! II ~its characteristicwave-number range is 1900–1650 cm21! have beenweakened, which demonstrates that, during the ex-posure process, the breakup of the main chain of thecellulose polymer is accompanied by the breakup ofside chains.
Thus we believe that a real-time PCF hologramconsists of mixed absorption and phase modulationscaused by a color change and a photodegradationreaction. Only the mixed modulation effect leads toa high real-time diffraction efficiency. The devel-oped PCF hologram consists of pure positive surface-relief modulations.
3. Studies of the Characteristics of PhotoetchingCellulose Film
In this section some special characteristics, suchas the real-time effects, the spatial resolution, thelinear relief-modulation effect, and the delayed-
Fig. 1. ESR spectra of the PCF: Curve ~1!, when exposed to themercury lamp for 15 min. Curve ~2!, when laid aside for threedays in darkness.
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development effect, are investigated experimentally.Their potential applications are also presented.
A. Real-Time Effects and Spatial Resolution
PCF has strong absorption in the violet to blue re-gion, so we record real-time holographic gratings byusing two unexpanded beams of a He-Cd laser ~442nm!, of which the beam ratio is 1:1. The resultantgrating holograms are monitored at the Bragg anglewith a beam from a He–Ne laser ~633 nm!.
By measuring the real-time diffraction efficiencyversus the exposure of spatial frequencies of 300linesymm and 600 linesymm ~see Fig. 4!, we find anoptimized exposure power of 2.4 Jycm2. The real-time diffraction efficiencies of different spatial fre-quencies ~200–3000 linesymm! versus exposure arethen investigated at the optimized exposure point~Fig. 5!. The diffraction efficiency is defined as thefirst-order light intensity divided by the incident lightintensity; the maximum real-time diffraction effi-ciency obtained is 30.9%, with a spatial frequency of400 linesymm. ~The best real-time diffraction effi-
ciency of a DCG holographic grating recorded undercomplicated conditions is only 7.5%.13! When thespatial frequency is 3000 linesymm, the real-timediffraction efficiency is 10%, which shows the excel-lent real-time effect and higher resolution of PCF.In addition, we should mention that the optimizedexposure of developed holograms is less than 100mJycm2, although the optimized exposure of real-time holograms is 2.4 Jycm2. This strong real-timeeffect can be used in the fields of holographic inter-ferometry, image subtraction, edge enhancement,character recognition, phase conjugation, and so on.
B. Linear Surface-Relief Modulation
If part of the sensitized PCF is exposed uniformly toa high-pressure mercury lamp or a He-Cd laser andthen developed with a 5% NaOH solution, the con-
Fig. 2. IR spectra of the PCF under various conditions. See text for details.
Fig. 3. Structure of the long-chain molecules in the cellulose poly-mer.
Fig. 4. Real-time diffraction efficiency versus the exposure of thePCF gratings.
4332 APPLIED OPTICS y Vol. 37, No. 20 y 10 July 1998
cave groove of the exposed region can clearly be seen.This result demonstrates the positive photoetchingcharacteristic of PCF.8
A simple method of copying a 20-lineymm silverhalide amplitude grating is chosen to investigate thesurface-relief modulation capacity of PCF. Here weuse a 50-W high-pressure mercury lamp as the lightsource and an exposure distance of 20 cm. The ex-posed PCF is developed with a 5% NaOH solution atroom temperature. Figure 6 shows a surface-reliefinterferogram obtained with a developed PCF grat-ing. From the surface-relief depths of the developedPCF versus exposure ~see Fig. 7!, we can see that thesurface-relief depth is linearly dependent on expo-sure within very wide range ~5 mm or so!. The Ship-ley AZ-1350 photoresist has only a 1-mm linearmodulation region.4 This linear dependence of thesurface-relief depth on exposure is rarely seen in ho-lographic and photoresist materials. It can be usedin the fabrication of continuous-phase diffractive op-tical components or micro-optical elements. In ad-dition, because of its high spatial resolution, it mightbe a useful photoetching material in submicrometermicrolithography.
C. Delayed-Development Effect
As discussed in Section 2, dichromated light-sensitivesystems all possess an obvious dark-reaction effect,which may be harmful to the shelf life of these typesof recording material. But we find that, if after ex-posure they are laid aside for a few hours in darkness,the diffraction efficiency will clearly improve. Thedelayed-development effect could be a practicalmeans of improving the performance of PCF. Toinvestigate the delayed-development effect of PCF,we also use a high-pressure mercury lamp as thelight source to copy a 20-lineymm silver halide am-plitude grating ~its diffraction efficiency is 7.2%!.The diffraction efficiencies of PCF under various pro-cessing conditions and exposure times are given inTable 1. After a few hours of being set aside indarkness after exposure, the diffraction efficiencies ofPCF can clearly be improved. However, if the set-aside time is too long, its diffraction efficiency tendsto decrease. After development, its diffraction effi-ciencies will increase because of pure relief phasemodulation. From Table 1 we can see that PCF pos-sesses a strong delayed-development effect, too.
From the proposed reaction scheme described inSection 2, it can be seen that the Cr~VI! concentrationof dichromate is reduced to Cr~III!, through the inter-mediate Cr~V!. Cr~V! ions formed during the expo-sure process cannot be completely reduced; some ofthem still exist in the photosensitive layer. Keeping
Fig. 5. Real-time diffraction efficiency versus the spatial frequen-cies of the PCF gratings.
Fig. 6. Surface-relief interferogram of a developed PCF grating.
Fig. 7. Surface-relief depth versus the exposure time of developedPCF gratings.
Table 1. Diffraction Efficiencies ~DE! of PCF Gratings under VariousProcessing Conditions and Exposure Timesa
aThe conditions for development were a 2% NaOH solution for 1min at 20 °C.
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the exposed samples set aside for a few hours willmake the remaining Cr~V! ions reduce to Cr~III!, whichleads to the enhancement of the absorbance and thephase modulations in PCF holograms and the im-provement of the diffraction efficiencies of PCF holo-grams. But too long a delay time will make thereduction process extend to unexposed regions, pro-duce noise, and decrease the diffraction efficiencies ofPCF holograms. From Table 1 we can see that 15 hcould be suitable for delayed development. So byseizing the best opportunity to develop the PCF ho-logram and fix the diffraction efficiency, we can ob-tain holograms with high diffraction efficiencies andlow noise.
4. Fabrication of Integrated Micro-Optical Elements
PCF has a unique double-faced photosensitivity ifdifferent functional elements are manufactured onboth its sides, thus providing integrated multifunc-tional elements. In this section we describe the fab-rication of a phase-type micro-Fresnel zone plate14
and a two-dimensional Dammann grating,15 one oneach side of a PCF. The finished element has thefunctions of both beam splitting and focusing. Theelement can transform a collimated beam into 3 3 3spot array at the focal plane, with uniformity of thesplit spot intensity of up to 92%. Applications ofsuch a device include a beam splitter, image process-ing, optical fiber communication, etc. The designparameters and fabrication procedures are presentedhere.
The designed micro-Fresnel zone plate consists of100 alternating annuli whose radii are given by rn 5~ f ln!1y2,14 where n takes on consecutive integer val-ues beginning with 1, f is the focal length of theFresnel zone plate, and l is the wavelength of light.In our experiments, f is 16 cm and l is 633 nm from
a He–Ne laser, so the overall diameter of the achievedmicro-Fresnel zone plate is 6.4 mm and the width ofthe outermost zone is 16 mm. It is easy to manufac-ture such an element for current lithographic tech-niques. Figure 8 shows an enlarged mask formaking a micro-Fresnel zone plate.
The design parameters of the Dammann gratingcome from the Dammann calculated result15: Thereis only one transition point in one period for a three-spot array, and it is located in the 0.368 period, wherea phase difference of p occurs. In our experimentsthe period of the Dammann grating is 37.5 mm. Fig-ure 9 shows an enlarged mask for making a Dam-mann grating.
Fig. 8. Enlarged structure of a Fresnel zone plate as a photo-graphic reduction mask.
Fig. 9. Enlarged structure of a Dammann grating as a photo-graphic reduction mask.
Fig. 10. Optical layout for contact copying of an integrated ele-ment: M, mirror; BS, beam splitter; L, collimating lens; SF, spa-tial filter.
4334 APPLIED OPTICS y Vol. 37, No. 20 y 10 July 1998
The fabrication procedures for an integrateddouble-faced element consist of the following steps:The first step is to make enlarged drawings of theDammann grating and the Fresnel zone plate. Theartwork of the Dammann grating and the Fresnelzone plate at 603 the final dimensions are producedby use of a personal computer and a Hewlett-PackardLaserJet printer. The second step consists of a 603photographic reduction of the above artwork. Thereduced patterns of the Dammann grating and theFresnel zone plate are generated on silver halideemulsions. The third step is simultaneous contactcopying of the amplitude masks of the Dammanngrating and the Fresnel zone plate on either side ofthe PCF. Figure 10 shows a schematic of the opticallayout for contact copying. To avoid coherence noise,one must set the difference between optical paths I andII beyond the coherence length of the He-Cd Laser.The final step is to develop the exposed PCF elementto achieve a phase-type integrated element. By con-trol of the exposure of beams I and II, a desired Dam-mann grating and micro-Fresnel zone plate with aphase difference of p can be obtained.
Figure 11~a! shows a photograph of a 3 3 3 spotarray with approximately equal intensity. Figure11~b! shows the intensity contribution of the 3 3 3spot array; its uniformity is up to 92%. The methodis relatively simple, and, meanwhile, it has goodflexibility—the focal length and the distance betweensplit spots of the integrated element can be varied bychanges of the design parameters, such as the periodof the Dammann grating and the zone radii of themicro-Fresnel zone plate.
5. Conclusions
In reporting a novel recording material—photoetching cellulose film ~PCF!—in this paper wehave presented the latest research on its imagingmechanism and characteristics. We have also de-scribed a new kind of integrated micro-optical ele-ment that takes advantage of PCF’s double-facedphotosensitivity; the obtained results were satisfac-
tory. The integrated element is fabricated on bothsides of the PCF simultaneously, so there is no needfor multistep etching, which would cause alignmenterrors and accumulated etch-depth errors. Com-pared with the multistep registered-etch method, ourfabrication technique is convenient and cost effective.
PCF has many favorable characteristics, such as astrong real-time effect, high resolution, a linearsurface-relief modulation capacity, an excellent anti-humidity capacity, low cost, and a simple preparationprocedure. It can be developed into a practical ho-lographic and photoetching material and can be ex-pected to have wide applications in the fields ofholographic interferometry, image processing, sub-micrometer microlithography, and development ofmicro-optical elements.
The authors appreciate the support of the NationalNatural Sciences Foundation of China.
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