2Centre for Industrial and Engineering Optics, Dublin Institute of Technology, Kevin Street, Dublin, 8, Ireland1School of Physics, Dublin Institute of Technology, Kevin Street, Dublin, 8, Ireland
The first report on the multicolor holographic imagesobtained by superposition of three holograms, eachrecorded at one of three primary wavelengths,appeared in 1964 [1]. Until now, both silver halidephotographic emulsions [2] and dichromated gelatin[3] have been the most common materials used forhigh-efficiency full-color reflection hologram record-ing. However, these materials require wet chemicalprocessing for developing the holograms, which islaborious and costly from the point of commercialapplications. Self-developing photopolymers [4–6],which do not require development, are the idealchoice for the real-time recording and reconstructionof holograms.
In the present paper, for the first time we report onthe sensitizing and quantitative and qualitative anal-yses of an acrylamide-based photopolymer (ABP)for recording multicolor holographic gratings. Holo-grams of a real object were also recorded using thismaterial. ABP material was sensitized for recordingreflectionholographic gratings andhologramsat eachof three primary wavelengths. Qualitative analysis ofthe scattering properties of ABP was carried out bymeasuring the bidirectional scattering distributionfunction (BSDF). Quantitative studies of reflectionholographic gratings were carried out by measuringthe diffraction efficiency and performing spectralcharacterization. For the latter purpose, the chro-maticity points of the recorded wavefronts and thewavefronts reconstructed from the hologram werecompared using the International Commission on Il-lumination (CIE) diagram. The results and the poten-tial issues related to shrinkage in the photopolymer
during holographic recording and its effects on thereconstructed wavelengths are discussed. Reflectionholograms were recorded individually at three pri-mary wavelengths on a single photopolymer layer.
2. Experimental
A. Recording Wavelengths and Recording Material
The wavelengths used for holographic recordingwere selected from the CIE chromaticity chart [7],which provides a simple procedure for predicting therange of colors that can be obtained from amixture ofthe three primary wavelengths. Figure 1 shows theCIE chromaticity chart. The range of colors thatcould be reconstructed lies within a triangle whosevertices are the points corresponding to the recordingwavelengths. A wide spectral range of colors meansthat a large area on the CIE diagram could becovered by this triangle. A greater area on the CIEdiagram can be covered by a polygon or any otherirregular shape formed by using a greater numberof spectral wavelengths. However, the use of morespectral wavelengths complicates the holographic re-cording setup and is constrained by the availabilityof continuous wave lasers [8].The recording material used in these experiments
is an ABP developed at the Centre for Industrial andEngineering Optics. The photopolymer consists of ac-rylamide (monomer), N,N′-methylene-bisacrylamide(cross-linking comonomer), polyvinyl alcohol (bin-der), triethanolamine (electron donor or coinitiator),and primary wavelength sensitive photo initiatordyes. The photopolymer is sensitized using Methy-lene Blue (MB), Erythrosine B (EB), and Acriflavine
(ACF) dye to record at red-green-blue (RGB) wave-lengths, respectively. The photopolymer compositionis given in Table 1.
The mechanism of hologram recording was ex-plained previously [9–12].
The photosensitive layers were prepared by a grav-ity settling coating technique. 600 μl of photopolymersolution was deposited on a 2:5 cm × 7 cm clear glassslide using a microliter pipette. Samples were left fordrying for six to seven hours in a dark room at am-bient conditions. The thickness of the layers afterdrying was 60� 5 μm when measured using a whitelight surface profilometer (ADE Phase Shift Micro-XAM 100). The normalized absorption spectrum ofthe dry layer was measured using a PerkinElmerLambda 900 UV-VIS-NIR absorption spectrometer,shown in Fig. 2. From this spectrum it can beobserved that the material has well-defined peaksof absorption at 473nm, 532nm, and also near633nm wavelengths. It also shows that no interac-tion between the different dye molecules takes place,as the absorption spectrum of each species is clearlydistinguishable.
B. Experimental Setup for BSDF
An important characteristic of the holographic re-cording material is its low scattering property, whichcan be measured as BSDF. BSDF is the ratio of theintensity of the scattered light Pscatter (as a functionof angle θ) to the incident intensity Pincident per unitsolid angle Ωdetector as shown in Eq. (1) [13,14] and ismeasured using the scatterometer shown in Fig. 3
BSDFðθÞ ¼ PscatterðθÞPincidentΩdetector
: ð1Þ
The BSDF, as a function of an angle, was obtainedat two different wavelengths—473nm and 633nm—
so as to cover a wide range of the visible region. This
BSDF was obtained by illuminating the sample witha converging beam, which was spatially filtered andmeasuring the scattered light intensity. The inten-sity of the incident light on the sample placed atthe goniometer axis was 0:8 μW and 0:65 μW for blueand red wavelengths, respectively. The spot radius ofthe laser beam on the sample was 1 cm. The detector(Newport 1830C Optical Power Meter) was placed onthe moving arm to measure the angular dependenceof the intensity. The detector signal and goniometerarm were controlled by a LabVIEW program. Thedistance from the sample to the detector was 24:5 cm,and an aperture of 1mm diameter was placed infront of the detector. This complete scatterometersetup was enclosed in a black box to minimize straylight reaching the detector.
C. Experimental Setup for Recording Reflection Gratings
Reflection gratings were recorded in the photopoly-mer layers sensitized to three wavelengths RGBusing a symmetrical geometry as shown in Fig. 4. Itconsists of an He–Ne laser of wavelength 633nm andtwo diode-pumped solid state lasers (532nm and473nm). These three laser beams were combinedusing a mirror and two dichroic mirrors for greenand blue wavelengths. The combined laser beamswere spatially filtered using a single spatial filterand then collimated. The collimated beam was splitinto two beams using a beam splitter. The two beams
were then diverted onto the photopolymer layerusing two adjustable mirrors to record the holo-graphic gratings.
Spatial frequencies were selected to ensure highspatial frequency recordings and were calculatedusing the Bragg diffraction Eq. (2)
2nΛ sin θB ¼ λ; ð2Þ
where λ is the wavelength of the recording light, n ¼1:51 (average refractive index of the recording mate-rial); Λ is the fringe spacing; and θB is the interbeamangle. This interbeam angle inside the photopolymerlayer can be determined using the Snell law. In ourexperiment, the incident angle 40° was chosen to il-luminate the photopolymer layer outside, and thisangle corresponds to half of the recording interbeamangle, which is θB ¼ 64:8° inside the photopolymerlayer. This value corresponds to spatial frequencies4200 lines=mm, 5000 lines=mm, and 5700 lines=mmfor the wavelengths of 633nm, 532nm, and 473nm,respectively.
For recording the reflection gratings, individualphotopolymer layers were exposed for different ex-posure times, ranging from 20 s to 120 s with anincrement of 20 s for each wavelength. Then the dif-fraction efficiency at each wavelength [red (633nm),green (532nm), and blue (473nm)] of each reflectiongrating was measured by exposing the grating to thecorresponding recording beam wavelength. It has tobe mentioned that the incident angle of the readingbeam was same as the recording beam, and the dif-fraction efficiency measurement was not a real-time measurement. The intensity in the diffractedorder was measured using a photodetector. The re-cording intensities for the RGB wavelengths were1:8mW=cm2, 3mW=cm2, and 4mW=cm2, respec-tively. The percentage of diffraction efficiency, η,was calculated using Eq. (3)
η ¼ I1stIIncident
× 100; ð3Þ
where I1st is first-order intensity, and Iincident is inci-dent intensity. The results are shown in Fig. 6. TheFresnel loss was calculated to be ≈15% for the inci-dent angle of 40° at all three primary wavelengths.
D. Experimental Setup for Spectral Characterization ofMulticolor Reflection Grating
We have characterized the multicolor reflectionholographic gratings recorded in our panchromaticphotopolymer by exposing the material for 80 s tothe combined RGB beam with an intensity of1:8mW=cm2, 3mW=cm2, and 4mW=cm2 at 633nm,532nm, and 473nm wavelengths, respectively. Thespectral response curve is shown in Fig. 8. The spec-tral characteristics of the recorded reflection grat-ings were studied by probing them with a halogenwhite light of 5mm diameter (Avantes, AvaLight-HAL [15]) coupled into a 400 μm optical fiber and
collimated with a collimating lens. The reconstructedlight from the grating was coupled into a second op-tical fiber connected to an Avantes AvaSpec 2048spectrum analyzer. The setup is shown in Fig. 5. Theangle of incidence of the reconstructed beam is ap-proximately 40° to the normal of the photopolymerlayer, which is the same as the recording beam.
3. Results
A. Measurement of BSDF
The BSDF of the air, glass, and uniformly polymer-ized layer at 473nm are shown in Fig. 6. From thegraph it can be observed that the scattering of theuniformly polymerized layer is close to that of theglass and air signals, which indicates that the mate-rial is characterized by very low scattering. The insetof Fig. 6 shows BSDF data of the uniformly poly-merized layer measured at 633nm and 473nm. Asexpected, the scatter is greater at the shorter wave-length of 473nm than the higher wavelength at633nm. From these experiments it can be concludedthat the total scattering of the layers with thicknessof 60 μm is very low and negligible in the presentcomposition of ABP. This is because the photopoly-
mer material is grainless and soft—unlike silver ha-lide photographic emulsions. From the above resultswe believe that this photopolymer material is suita-ble for reflection holographic recording.
B. Measurement of Diffraction Efficiency ofReflection Gratings
The total recording intensity of the two interferingbeams was 1:8mW=cm2 at 633nm. From Fig. 7 it canbe observed that as exposure time increases, diffrac-tion efficiency increases. The recording time for max-imum diffraction efficiency for the three wavelengthsis between 80 and 100 s. However, the maximum dif-fraction efficiencies are different for the three wave-lengths. The maximum diffraction efficiency for thegratings recorded at 633nm, 532nm, and 473nmwavelengths are 11.5%, 6%, and 1.6%, respectively.This decrease in efficiency as the wavelength de-creases is, at least partly, due to the fact that the spa-tial frequency increases as the wavelength decreases.Another possible reason could be that the quantumyield of generation of free radicals is different forthe three dyes, an important parameter that drivesthe polymerization process and results in a refractiveindex modulation between the bright and dark
Fig. 5. Experimental setup used for spectral characterization ofthe recorded reflection holograms/gratings.
Fig. 6. (Color online) BSDF data for the air (circles), glass (lightsquares), and uniformly polymerized layer (triangles) measured at473nm. The inset picture shows the BSDF of the uniformly poly-merized photopolymer layer measured at 633nm (dark squares)and 473nm (triangles).
Fig. 7. (Color online) Diffraction efficiency (percent) of reflectiongratings as a function of time of exposure at red (diamonds), green(squares), and blue (triangles) wavelengths.
Fig. 8. Spectral characteristics of the reconstructed wavefrontfrom the reflection holographic grating recorded using 473nm,532nm, and 633nm.
regions. A relevant point tomention at this juncture isthat the diffraction efficiencies were measured usingdifferent samples for each different wavelength, andthus there should be no possibility that the diffractionefficiency is influenced by the depletion of dye due toprevious recordings in the same sample.While these values of diffraction efficiency reported
here are lower than the best values (diffraction effi-ciency greater than30%at spatial frequencies greaterthan 4500 lines =mm) that we have achieved in reflec-tion gratings [16], the priorities here are to obtainpanchromatic response and fidelity to the originalcolor(s) in the object beam.
C. Spectral Characterization of MulticolorReflection Grating
One of the disadvantages of most holographic re-cording materials is that they often undergo dimen-sional changes either during recording or during anypostexposure treatment. These dimensional changeswould sometimes lead to a decrease or increase in dif-fraction efficiency, deviation from the Bragg angle, orshift in the reconstruction wavelength. These cancause serious problems in display holography and ho-lographic data storage. Figure 8 shows the spectralcharacterization of the ABP layer done using the set-up shown in Fig. 5 (Subsection 2.C). The recordingand reconstruction wavelengths were comparedand are shown in Table 2.From the results of the spectral characterization
studies, it is evident that the panchromatic ABP re-cording material is suitable for recording reflectiongratings at all three primary wavelengths. The spec-
tral response curve (Fig. 8) shows that reconstructedwavelengths of the reflection gratings recorded at633nm, 532nm, and 473nm are at 626nm, 529nm,and 478nm, respectively. The color triangles plottedon the CIE chromaticity diagram, by joining the chro-maticity points for these recorded and reconstructedwavelengths, are shown in Fig. 9. It is observed fromthese preliminary results that the reconstructedwavelengths were shifted toward shorter wave-lengths due to shrinkage (approximately 1.11%) forred andgreenwavelengths. Surprisingly,we observeda shift toward a longer wavelength at the blue wave-length (approximately 1.1%). Although it should beemphasized that there is still no proper reasonableexplanation for this effect that has been observed dur-ing holographic reconstruction, it can be anticipatedthat this effect might be due to swelling.
To demonstrate the use of this panchromatic photo-polymer for display holography application, reflectionholograms of a 10 cent euro coin were recorded at633nm, 473nm, and 532nmwavelengths at separatelocations on the same 60 μmthick photopolymer layerusing Denisyuk single-beam geometry. A photographof the reconstructed images is shown in Fig. 10.
4. Conclusion
An ABP holographic recording material was sensi-tized at the RGB primary wavelengths by using threephotosensitive dyes. BSDF of the photo polymer re-cording material was measured, and scattering ofthe material was found to be very low, and so it canbe efficiently used for holographic recording applica-tions. Reflection gratings that were recorded at4200 lines=mm, 5000 lines=mm, and 5700 lines=mmshowed maximum diffraction efficiencies of 11.5%,6%, and 1.6% at 633nm, 532nm, and 473nm, respec-tively. Spectral properties of the multicolor holo-graphic gratings recorded in the same location inthe photopolymer layerwith the combinedRGBbeamwere characterized using a spectrum analyzer. Therecorded and reconstructed wavelengths were com-pared in the CIE chromaticity diagram. Results showsmall spectral shifts in the reconstruction wave-lengths, which can be attributed to the swelling/shrinking of the photopolymer. Individual primarywavelength reflection holograms were successfullyrecorded and reconstructed in a single panchromaticphotopolymer layer. Further investigations will beundertaken in the future to understand the cause
Fig. 9. (Color online) Comparison of the chromaticity points forthe recorded and reconstructed wavelengths of the multicolorholographic grating.
Fig. 10. (Color online) Photograph of white light holographicreconstruction from a hologram recorded at the three primarywavelengths. The object used was a 10 cent euro coin.
Table 2. Recording Wavelengths and the Corre-sponding Reconstruction Wavelengths
for the wavelength shift during holographic recon-struction. Although work to produce a full-color holo-gramcombining all three primary beams in one singlelight source is at the embryonic stage, dedicated re-search is still being carried out to improve the diffrac-tion efficiency of the holograms by optimizing thecomposition of the recording material (dye concentra-tions and monomers) and exposure conditions andalso doping the photopolymer with specific nanopar-ticles to minimize, if not completely suppress, dimen-sional changes. Hence the results presented in thispaper provide substantial evidence that using ABPfor panchromatic holographic recording display appli-cations is feasible.
The authors would like to acknowledge Technolo-gical Sector Research Strand I for funding the projectand the Facility for Optical Characterisation andSpectroscopy and the Dublin Institute of Technologyfor the laboratory facilities. The authors also greatlyacknowledge Denis B. for helping in the LabVIEWprogram.
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