Investigation of multiple holographic recordingin azo-dye-doped nematic liquid-crystal film
Hongyue Gao,* Jianhua Liu, Fuxi Gan, and Bo MaState Key Laboratory for Advanced Photonic Materials and Devices, Department of Optical Science and Engineering,
Dye-doped nematic liquid crystals have attractedconsiderable attention [1–14] because of extraordina-rily high optical nonlinearity from laser-induced di-rector axis reorientation, Δn∼ 10−2, compared withstandard inorganic optical materials for which Δn∼10−4. Dynamic grating and optical wave mixing havebeen studied in these materials [4–12]. Recently, theformation of a permanent grating led by holographicillumination was reported inMethyl Red (MR)-dopedliquid crystals [15,16]. Optical storage with long-term stability [17] is then a potential application ofthese media. To achieve high memory capacity, mul-tiple holographic recording should be investigated byusing multiplexing techniques in these thin films.We present multiple holographic recordings by
using angular, peristrophic, and spatial multiplexingin planarMR-doped nematic liquid-crystal films. Thecapacity of ten gratings is achieved at one location.We discuss the dependence of the diffraction efficien-cies of multiplexed gratings on storage density. Wealso study the diffractive angle as a function of therecording angle and find two ways to solve problems
of optical recognition for angular and peristrophicmultiplexed gratings that involve a recording angleand a peristrophic rotation angle.
2. Experimental Setup
In our experiment, the nematic liquid-crystal4-pentyl-4’-cyanobiphenyl (5CB) (Sigma-Aldrich,St. Louis, Missouri) is doped with a small amountof MR (0.5 wt. %, Sigma-Aldrich). The mixture ismade uniform by ultrasonic mixing. Two glass sub-strates are first coated with polyvinly alcohol(PVA) (Sigma-Aldrich) and then rubbed in one direc-tion with a lens tissue. Furthermore, the two glasssubstrates are attached to each other by rubbingthem in the same direction, and a 36 μm cell isformed with the thickness controlled by Mylarspacers. The 5CB and MR mixture is poured intothe cell by a capillary action. This process createselongated stress and strain on the polymer andfacilitates alignment of the long axes of the liquid-crystal molecules along the rubbed direction [18],after which a planar MR-doped liquid-crystal filmis fabricated.
The optical transmission spectrum of MR-doped5CB shows that its optical absorption depends onthe wavelength. A holographic effect occurs only atthe laser wavelength at which the absorption
coefficient is large under low power and the film isinsensitive to red light. Therefore, an Arþ laser (λ ¼488nm) and a He–Ne laser (λ ¼ 633nm) are chosenfor recording and reading light, respectively. Here,the effect of the He–Ne laser on the index reflectivegrating generated by the Arþ laser is neglected.Figure 1 illustrates the experimental setup for ho-
lographic recording. There is no external electricfield applied to the liquid-crystal film for all the ex-periments. Two recording beams derived from theArþ laser are s polarized by half-wave plates withthe same intensity of 20mW=cm2 and the same dia-meter of 2:5mm. The incident plane defined by thewave vectors of two writing beams is perpendicularto the surface of the sample. A beam from the He–Nelaser, which is also s polarized and perpendicular tothe surface of the liquid-crystal cell, is used to probethe writing region with an intensity of 8mW=cm2
and a diameter of 2mm.
3. Results and Discussion
To increase the capacity of the film, multiplexingtechniques such as spatial [19], angular [20,21],wavelength [22], phase code [23], and peristrophicmultiplexing [24] should be applied to holographicstorage. In this system, the film has a high degreeof freedom of rotational and linear movement. Angu-lar, peristrophic, and spatial multiplexing methodsare particularly suited for the implementation of thisthin film among multiplexing schemes. A most com-monly used scheme is angular multiplexing, in whichthe recording beams for each page are incident at thesame position of the recording mediumwith differentincident angles [23]. Four gratings have been writtenat one location by this multiplexing method under il-lumination for ∼100 s with the average direction ofthe liquid-crystal molecular alignment, called direc-tor n, parallel to the light polarization direction.Figure 2 shows the diffraction pattern for angularmultiplexed gratings. The light spot numbered 0 isthe probe light. The diffraction lights 1–4 and 10–40are the þ1st-order and −1st-order diffractions from
the gratings, respectively, whose diffraction efficien-cies range from 1.4% to 3.6%, and grating spacingsrange from 3.3 to 13:9 μm, corresponding to incidentangles from 5° to 1:2°.
Here, peristrophic multiplexing is achieved by ro-tation of the film, instead of the reference beam,around the surface normal after each hologram is re-corded. Six gratings are recorded at a location by thismultiplexing. In detail, each grating is stored underillumination for ∼100 s at an optimum recording an-gle of 2° after the previous grating has been recordedand then the film is rotated 30° around the normal todiffraction gratings. Figure 3 illustrates holographicreconstruction in which light spots 1–6 and 10–60 arethe first-order diffractions from these gratings withdiffraction efficiencies ranging from 1.2% to 2.3%.In this system, the angular resolution of angularmultiplexed gratings is as small as 8:8 × 10−2 mrad,and, theoretically, in the optimum recording anglerange of 0.5–5°, 90 gratings can be written at a loca-tion by angular multiplexing. The angular resolutionof peristrophic multiplexed gratings is 8:8mrad at
Fig. 1. Experimental setup for holographic storage: M, mirror;BS, beam splitter; λ=2, half-wave plates; PM, powermeter.
Fig. 2. Diffraction pattern for angular multiplexed gratingsprobed by a He–Ne laser.
Fig. 3. Holographic reconstruction from peristrophic multiplexedgratings probed by a He–Ne laser.
the recording angle of 2°, and then 72 gratings canbe stored at a location for a 0–360° tuning range.However, as the number of gratings increases at alocation, the diffraction efficiencies of the gratingsdecrease. Usually, while a grating is being written,the exposure of the writing beams not only forms thisgrating but also erases the former recorded gratingsat the location, as shown in Fig. 4, which illustratesthe formation process of multiple holographicgratings. From Fig. 4, when the Arþ laser is turnedon as indicated by the first arrow indicating therecording of the first grating, it can be seen thatthe diffraction efficiency increases at a rapid rate.The formation of the grating proceeds when thepump laser is turned off at the time indicated inthe figure, and the diffraction efficiency reaches itsmaximum after several seconds. As the exposurefor the second grating continues, the decrease ofthe diffraction efficiency of the first grating andthe increase of the diffraction efficiency of the secondgrating both vary nonlinearly. In the same way, thebehavior for the diffraction efficiency channels of thefirst and the second gratings is a reverse of the dif-fraction efficiency of the third grating completelyduring the exposure for the third grating at this loca-tion. Obviously, the diffraction efficiencies decreasewith the increase in number of multiplexed gratings.For two gratings stored at a specific position, their
diffraction efficiencies are written by
η1 ≅
�πdΔn1
λ
�2¼
�πdðΔn −ΔneraseÞλ
�2; ð1Þ
η2 ≅
�πdΔn2
λ
�2; ð2Þ
where d denotes the film thickness and λ denotes thewavelength of the diffracted light. The light-inducedrefractive-index changes Δn, Δnerase, and Δn2 de-pend not only on light intensity but also on dyeconcentration, incident angle, light polarization di-rection, the vector direction of the film, and recordingtime [4,7].
Assuming M is large enough, which is the numberof gratings recorded at a location, the average diffrac-tion efficiency is given by
�η∼�πdΔ�n
λ
�2; ð3Þ
where the average refractive-index change is ex-pressed as Δ �n ¼ Δnmax=M, and Δnmax is the maxi-mum refractive-index change of the material. Then,the number of gratings at a location is written as
M ∼πdΔnmax
�η12λ
: ð4Þ
Therefore, at each location the upper limit on thenumber of gratings should be determined by the re-quired lower limit of the average diffraction effi-ciency based on applications.
MR-doped liquid crystals possess large birefrin-gence, n∥ − n⊥ ¼ 0:2∼ 0:3, in the visible to infraredspectral regime, and, theoretically, Δnmax ¼ 0:2∼0:3, where n∥ and n⊥ are the refractive indices forlight polarization parallel and perpendicular to thevector direction, respectively. Experimentally, themaximum refractive-index change of 7:45 × 10−3
was reported in this material [10,15]. For d ¼ 36 μmand λ ¼ 633nm, theoretically, the capacity of a singlelocation can be improved to 24 holograms with anaverage diffraction efficiency of 0.3%, which is highenough to reconstruct a hologram. The storage capa-city of C is given by C ¼ NM, where N is the numberof bits in each stored page [20]. Assuming 1 bit/pixel,N is limited to ∼106 by current technology. Then,the information capacity of 24 Mbits is attained atan area of 5mm2, and the storage density is0:5Gbits=cm2.
In our experiment we recorded ten gratings at alocation by a combination of angular and peristrophicmultiplexing. Figure 5 shows the diffractions fromthese multiplexed gratings with relatively high dif-fraction efficiencies ranging from 0.8% to 2.7%. Thesegratings were kept in the dark at room temperaturefor two years without obvious change. To improve thecapacity, spatial multiplexing can be used with angu-lar and peristrophic multiplexing in which multiplemultiplexed holograms are stored at multiple loca-tions. The storage density of 202 gratings/cm2 isachieved by angular, peristrophic, and spatial multi-plexing methods.
A drawback of multiple storage is the lack of Braggselectivity in this medium because of a Raman–Nathtype diffraction. Here, selective properties of volumegratings that are due to angular [25,26] or
Fig. 4. Formation process of multiple holographic gratings.Diffractions I, II, and III represent the diffraction efficiencies fromthe first, second, and third recorded gratings at a single location,respectively. ON and OFF indicate when the writing light is on oroff.
wavelength [27] deviation from the Bragg conditioncannot be used for data retrieval. We find two effi-cient and simple methods to solve the problems of op-tical recognition. Usually, multiple-order diffractionsfrom a grating are in the same incident plane of twowriting beams. For angular multiplexed gratingsthat have the same incident plane, the diffractive an-gles probed by a given light depend on their respec-tive recording angles at a small probe angle, asshown in the following equations. Since the incidentangles differ, the diffractions from these gratings canbe distinguished by their diffractive angles. Here, therecording angle can play a paramount role in opticalrecognition of angular multiplexed gratings.The grating spacing [18] is written as
Λ ¼ λ2n sin θ ; ð5Þ
where θ denotes the incident angle. Parameter n isthe refractive index of the film at the recordingwavelength and λ is the wavelength of the recordinglight. The requirements for constructive interferenceof the recording light wave can be fulfilled only atparticular angles [28], which is expressed as
Λ×sinθþΛ×sinθm ¼mλn; m¼ 0;1;2 � � � ; ð6Þ
where θm is the mth-order diffractive angle read byone of the recording beams.The probe parameters and Eq. (5) are substituted
into Eq. (6). Then, the diffractive angle for probe lightθ0m, which is the angle between the mth-order probediffraction beam and the normal to the diffractiongrating, is determined by
sinθ0m ¼m2nλpnpλ
sinθ− sinθp; m¼ 0;1;2 � � � ; ð7Þ
where np is the refractive index of the film at theprobe wavelength, λp is the wavelength of the probelight, and θp is the probe angle.
Figure 6 illustrates the dependence of the experi-mental value of sin θ0m and the theoretical value ofm 2nλp
npλ sin θ − sin θp form ¼ 1; 2 on the recording anglein the case of the small probe angle. Apparently, thediffractive angle increases with the increase of therecording angle. Thus, angular multiplexed gratingscan be optically recognized based on different record-ing angles. For peristrophic multiplexed gratings,the first-order diffractions have the same diffractiveangle because of the same incident angle. However,they have different incident planes that depend ontheir respective rotation angles. Therefore, peri-strophic multiplexed gratings can be distinguishedby peristrophic rotation angles.
4. Conclusion
In our experiments we achieved a maximum diffrac-tion efficiency of 21.4% from a grating, which is theratio of the first-order diffraction intensity to theprobe intensity and is relatively high among dye-doped liquid crystals and long storage lifetime forholography in planar MR-doped liquid-crystal film.For its potential applications in holographic memory,we investigated multiple holographic recording bysome optical multiplexing techniques without an ap-plied electric field and solved the problems of opticalrecognition from the lack of Bragg selectivity in mul-tiplexed gratings. Wememorized four gratings by an-gular multiplexing and six gratings by peristrophicmultiplexing at two single locations. Moreover, we re-corded ten gratings at a location by a combination ofangular and peristrophic multiplexing and achievedan optical recording density of 202 gratings=cm2 inthis thin film by combined use of angular, peri-strophic, and spatial multiplexing. We also studied,both experimentally and theoretically, the depen-dence of diffraction efficiencies of multiplexed grat-ings on holographic storage density, which providesan approach to estimate holographic storage capa-city, and the dependence of the diffractive angle on
Fig. 5. (Color online) Holographic reconstruction from angularand peristrophic multiplexed gratings recorded at a singlelocation. Fig. 6. Dependence of the experimental value of sin θ0m for m ¼
1ð•Þ and m ¼ 2ð▴Þ and the theoretical value of 2mnλp sin θ=ðnpλÞ − sin θp for m ¼ 1ð□Þ and m ¼ 2ð▿Þ on the recording anglewith sin θp ¼ 0.
the recording angle to recognize angular multiplexedgratings. Finally, we have demonstrated that angu-lar and peristrophic multiplexed gratings can be dis-tinguished by two schemes that involve recordingangles and peristrophic rotation angles, respectively.Our research has revealed that MR-doped nematicliquid crystals are potential holographic storagemedia and provide a basis for optical address recog-nition of multiple storage with Raman–Nath typediffraction.
This research was supported by the National Nat-ural Science Foundation of China (NSFC), 10574031.
References
1. H. Gao, K. Gu, Z. Zhou, Y. Jiang, and D. Gong, “Diffractionbehavior of an azo-dye-doped nematic liquid crystal withoutapplied electric field,” Curr. Appl. Phys. 8, 31–35 (2008).
2. H. Gao, Z. Zhou, and Y. Jiang, “Holographic image storage andmultiple hologram storage in a planar Methyl Red-dopednematic liquid crystal film,” Appl. Opt. 47, 2437–2442 (2008).
3. P. Pagliusi and G. Cipparrone, “Surface-induced photorefrac-tive-like. effect in pure liquid crystals,” Appl. Phys. Lett. 80,168–170 (2002).
4. L. Lucchetti, M. Gentili, and F. Simoni, “Pretransitional en-hancement of the optical nonlinearity of thin dye-doped liquidcrystals in the nematic phase,” Appl. Phys. Lett. 86,151117 (2005).
5. I. C. Khoo, “Orientational photorefractive effects in nematicliquid crystal films,” IEEE J. Quantum Electron. 32,525–534 (1996).
6. A. Y. G. Fuh, C. C. Liao, K. C. Hsu, C. L. Lu, and C. Y. Tsai,“Dynamic studies of holographic gratings in dye-doped liquid-crystal films,” Opt. Lett. 26, 1767–1769 (2001).
7. H. Gao, Y. Jiang, Z. Zhou, K. Gu, and D. Gong, “The depen-dence of orientational optical nonlinearity in dye-dopedliquid-crystal films on the polarization direction of the record-ing beams,” IEEE J. Quantum Electron. 42, 651–656(2006).
8. P. Pagliusi, R. Macdonald, S. Busch, G. Cipparrone, andM. Kreuzer, “Nonlocal dynamic gratings and energy transferby optical two-beam coupling in a nematic liquid crystal owingto highly sensitive photoelectric reorientation,” J. Opt. Soc.Am. B 18, 1632–1638 (2001).
9. P. Etchegoin and R. T. Phillips, “Stimulated orientational scat-tering and third-order nonlinear optical processes in nematicliquid crystals,” Phys. Rev. E 55, 5603–5612 (1997).
10. I. C. Khoo, S. Slussarenko, B. D. Guenther, M. Y. Shih,P. H. Chen, and W. V. Wood, “Optically induced space-chargefields, dc voltage, and extraordinarily large nonlinearity indye-doped nematic liquid crystals,” Opt. Lett. 23, 253–255(1998).
11. F. Sanchez, P. H. Kayoun, and J. P. Huignard, “Two-wavemixing with gain in liquid crystals at 10:6 μm wavelength,”J. Appl. Phys. 64, 26–31 (1988).
12. A. Brignon, I. Bongrand, B. Loiseaux, and J.-P. Huignard,“Signal-beam amplification by two-wave mixing in a liquid-crystal light valve,” Opt. Lett. 22, 1855–1857 (1997).
13. L. Lucchetti, M. Di Fabrizio, M. Gentili, and F. Simoni,“Optical phase conjugation and efficient wave front correctionof weak light beams by dye doped liquid crystals,” Appl. Phys.Lett. 83, 5389–5391 (2003).
14. I. C. Khoo, H. Li, and Y. Liang, “Self-starting optical phaseconjugation in dyed nematic liquid crystals with a stimulatedthermal-scattering effect,” Opt. Lett. 18, 1490–1492 (1993).
15. A. G.-S. Chen and D. J. Brady, “Surface-stabilized holographyin an azo-dye-doped liquid crystal,” Opt. Lett. 17, 1231–1233(1992).
16. M. Kaczmarek, M.-Y. Shih, R. S. Cudney, and I. C. Khoo, “Elec-trically tunable, optically induced dynamic and permanentgratings in dye-doped liquid crystals,” IEEE J. Quantum Elec-tron. 38, 451–457 (2002).
17. X. Wei, X. Z. Yan, D. R. Zhu, D. Mo, Z. X. Liang, and W. Z. Lin,“Stable optical storage and high-order diffraction in a liquid-crystal polymer film by two-wave mixing,” Appl. Phys. Lett.68, 1913–1914 (1996).
18. I. C. Khoo, Liquid Crystals: Physical Properties and NonlinearOptical Phenomena (Wiley-Interscience, 1994).
19. Y. H. Kang, K. H. Kim, and B. Lee, “Volume hologram schemeusing optical fiber for spatial multiplexing,” Opt. Lett. 22,739–741 (1997).
20. G. W. Burr, F. H. Mok, and D. Psaltis, “Angle and space multi-plexed storage using the 90° geometry,” Opt. Commun. 117,49–55 (1995).
21. M. L. Hsieh, “Versatile holographic data storage system forangularmultiplexing with no upper limit of the angular sweepof the reference beam,” Opt. Eng. 44, 090504 (2005).
22. S. Campbell and P. Yeh, “Sparse-wavelength angle-multiplexed volume holographic memory system: analysisand advances,” Appl. Opt. 35, 2380–2388 (1996).
23. C. C. Chang, K. L. Russell, and G. W. Hu, “Optical holographicmemory using angular-rotationally phase-coded multiplexingin a LiNbO3:Fe crystal,” Appl. Phys. B 72, 307–310 (2001).
24. A. Pu and D. Psaltis, “High-density recording in photopoly-mer-based holographic three-dimensional disks,” Appl. Opt.35, 2389–2398 (1996).
25. F. H. Mok, “Angle-multiplexed storage of 5000 holograms inlithium niobate,” Opt. Lett. 18, 915–917 (1993).
26. S. Tao, D. R. Selviah, and J. E.Midwinter, “Spatioangularmul-tiplexed storage of 750 holograms in a Fe:LiNBO3 crystal,”Opt. Lett. 18, 912–914 (1993).
27. J. Rosen, M. Segev, and A. Yariv, “Wavelength-multiplexedcomputer-generated volume holography,” Opt. Lett. 18,744–746 (1993).
28. P. Yeh, Introduction to Photorefractive Nonlinear Optics(Wiley, 1993).
