Polarization-independent phase-only modulation of a polymer-dispersed liquid crystal (PDLC) is demonstrated. In the low voltage region, PDLC is translucent because of light scattering. Once the voltage exceeds a saturation level, PDLC is highly transparent and exhibits phase-only modulation capability. Although the remaining phase is not too large, it is still sufficient for making adaptive microdevices, such as microlens. A tunable-focus microlens for arrays using PDLC is demonstrated. This kind of microlens is scattering free, polarization independent, and has fast response time

Fan, Yun-Hsing

Lin, Yi-Hsin

Ren, Hongwen

Wu, Shin-Tson

English

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2005 Polarization-independent phase modulation using a polymer-dispersed liquid crystal Hongwen Ren University of Central Florida Yi-Hsin Lin University of Central Florida Yun-Hsing Fan University of Central Florida Shin-Tson Wu University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Ren, Hongwen; Lin, Yi-Hsin; Fan, Yun-Hsing; and Wu, Shin-Tson, "Polarization-independent phase modulation using a polymer-dispersed liquid crystal" (2005). Faculty Bibliography 2000s. 5584. https://stars.library.ucf.edu/facultybib2000/5584 Polarization-independent phase modulation using a polymer-dispersedliquid crystalHongwen Ren, Yi-Hsin Lin, Yun-Hsing Fan, and Shin-Tson Wua!College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816sReceived 9 December 2004; accepted 22 February 2005; published online 1 April 2005dPolarization-independent phase-only modulation of a polymer-dispersed liquid crystal sPDLCd isdemonstrated. In the low voltage region, PDLC is translucent because of light scattering. Once thevoltage exceeds a saturation level, PDLC is highly transparent and exhibits phase-only modulationcapability. Although the remaining phase is not too large, it is still sufficient for making adaptivemicrodevices, such as microlens. A tunable-focus microlens for arrays using PDLC is demonstrated.This kind of microlens is scattering free, polarization independent, and has fast response time. ©2005 American Institute of Physics. fDOI: 10.1063/1.1899749gPhase-only modulation can be used for tunable grating,prism, lens, and other photonic devices. Homogeneous align-ment of liquid crystals sLCsd is commonly used for phase-only modulation,1 although twisted nematic LC cells alsoexhibit such capability in the low voltage regime.2 For phasemodulation using a homogeneous cell, the polarization axissz axisd of the input linearly polarized light salong x axisd isparallel to the LC directors. As the voltage exceeds a thresh-old, the LC directors are reoriented in the z-x plane. Thephase change of the outgoing light varies with voltage as:D=2pdfnesVd−nog /l; where d is the cell gap, ne,o is therefractive index of the extraordinary sor ordinaryd ray, and lis the wavelength. The major advantage of such a homoge-neous cell is that a large phase change can be obtained witha relatively low voltage. However, the homogeneous cell ispolarization dependent and its response time, which dependson d2, is typically ,10 ms for a 5 mm cell gap using a highbirefringence LC operated at 70 °C.3To achieve polarization-independent phase-only modula-tion and fast response time, nanosized polymer-dispersed liq-uid crystal snano-PDLCd droplets have been explored.4–9 In anano-PDLC system, the LC droplets dispersed in the poly-mer matrix are randomly oriented. Since the LC droplet sizeis smaller than a visible wavelength, the light scattering isalmost completely suppressed and the nano-PDLC acts as aphase retarder in the voltage-off state. As the voltage in-creases, the LC directors within the droplets are reorientedalong the electric field direction and, therefore, induce aphase shift. Because of the small LC concentrations,35% d and random droplets distribution, the availablephase change is fairly small. Despite this small phasechange, nano-PDLC has potential applications for phasegratings, microprisms, microlens arrays, color filters, andcolor displays. A critical issue of the nano-PDLC is highoperating voltage s,10 V/mmd. For a 20 mm cell gap, therequired voltage is ,200 Vrms.Unlike nano-PDLC, the conventional PDLC10 has alarger droplet size s,1 mmd and higher LC concentrations,70% d. As a result, PDLC exhibits a lower saturation volt-age s,2–3 V/mmd than nano-PDLC. However, in thevoltage-off state, PDLC strongly scatters visible light be-cause its droplet size is comparable to the wavelength and itsaverage refractive index mismatches with that of polymermatrix. The phase modulation property of PDLC has beeninvestigated previously.11 It was found that phase shift coex-ists with light scattering, but depends on the incident lightpolarization and incident angle. For the normally incidentlight, PDLC does not possess any phase change when theapplied voltage is below the saturation voltage sVsatd.In this letter, we find that the phase-only modulationusing PDLC for the normally incident light exists in the highvoltage region sV.Vsatd. Moreover, such a phase shift ispolarization independent and has fast response time. Al-though the remaining phase change is not too large, it is stillsufficient for making microdevices, such as microprisms andmicrolens. For feasibility demonstration, we fabricate a two-dimensional tunable-focus microlens arrays using PDLC. Incomparison to nano-PDLC, PDLC has a lower operatingvoltage.Figure 1 illustrates the phase modulation mechanism of aPDLC. In Fig. 1sad, the LC droplets sor domainsd are ran-domly dispersed in polymer matrix. Because of the refractiveindex mismatch, light scattering is strong. As the appliedvoltage increases, the LC directors are reoriented along theelectric field direction. As a result, PDLC becomes transpar-ent, as shown in Fig. 1sbd. If the voltage is increased further,more LCs in the droplet cavities are reoriented by the electricadElectronic mail: swu@mail.ucf.eduFIG. 1. LC droplet orientations in a PDLC film at sad V=0, sbd V1, and scdV2.V1.APPLIED PHYSICS LETTERS 86, 141110 s2005d0003-6951/2005/86~14!/141110/3/$22.50 © 2005 American Institute of Physics86, 141110-1field, as shown in Fig. 1scd. From Figs. 1sbd and 1scd, phase-only modulation is still available and is independent of po-larization for the normally incident light.To prepare a PDLC cell, we mixed nematic LC E48sno=1.523 and Dn=0.231d with an UV-curable monomerNOA65 srefractive index np=1.524d at 84:16 wt % ratios.Here, we intentionally used a higher LC concentration inorder to obtain a larger phase change at a lower voltage. Theconventional PDLC with ,65:35 LC/monomer ratios wouldwork equally well but the required voltage is higher. TheLC/monomer mixture was injected into an empty cell whoseinner surfaces were coated with a thin indium-tin-oxidesITOd electrode. The cell gap was measured to be d=22 mm. The filled cell was then exposed to UV light sl,365 nm and 1–15 mW/cm2d at room temperature for30 min to induce phase separation.We observed the phase separation morphology of thePDLC sample using a polarized optical microscope. Theformed morphology indicates that the LC dispersed in poly-mer matrix exists as domains separated by polymer networksrather than isolated LC droplets. This kind of morphology iscommon for the PDLC system having a high LCconcentration.12Next, we measured the transmittance of the PDLC cellusing a He–Ne laser sl=633 nmd beam. A large area photo-diode detector was placed at ,30 cm behind the samplewhich corresponds to ,2° collection angle. A computer con-trolled LabVIEW data acquisition system was used for driv-ing the sample and recording the light transmittance.Figure 2 plots the measured voltage-dependent transmit-tance of the PDLC sample. At V=0, the PDLC sample istranslucent so that the transmittance is low. As the voltageincreases, the transmittance increases. From Fig. 2, the mea-sured contrast ratio is ,9:1, which is lower than a typicalPDLC because of the larger domain size which originatesfrom the higher LC concentration. When the applied voltageexceeds 26 Vrms, the transmittance remains basically un-changed. We define this voltage as saturation voltage sVsatd.At V.Vsat, the PDLC cell remains highly transparent.For each cell, the saturation voltage could vary becauseit depends on the cell gap, LC material, and monomerconcentration.To examine whether the PDLC film exhibits a phase-only modulation capability in the high voltage regime, wemeasured its transmittance at l=633 nm between paralleland crossed polarizers. Results are shown in Fig. 3. Thetransmittance increases for the parallel polarizer configura-tion, while it decreases gradually for the crossed polarizers inthe V.Vsat region. These results imply that PDLC can pro-duce phase-only modulation when the applied voltage ishigher than the saturation voltage. Moreover, rotating thesample does not affect the measurement results shown in Fig.3. This indicates that the phase-only modulation is indepen-dent of the incident light polarization. In the high voltagestate, the LC directors are basically perpendicular to the sub-strates, similar to a homeotropic cell. The input linearly po-larized light always sees the ordinary refractive index, re-gardless of its polarization axis. Thus, the phase modulationis independent of the incident light polarization.From Fig. 3, the saturation voltage of the PDLC sampleis ,1.2 V/mm, which is ,103 lower than that of a nano-PDLC. The phase change sdd of the PDLC sample can becalculated using the following equation:13d = 2 tan−1 ˛T’/Ti , s1dwhere T’ and Ti represent the transmittance at the crossed-and parallel-polarizer configurations, respectively. Figure 4plots the measured voltage-dependent phase change of thePDLC cell. From 26 to 60 Vrms, d decreases gradually be-cause the LC directors inside the droplets continue to bereoriented by the electric field, as schematically shown inFig. 1scd. Although the phase change is small, it is still suf-ficient for making microlens arrays. To increase the phasedifference, several approaches can be implemented, for in-stance, to use a high birefringence LC material, increase LCcell gap, or enhance LC concentration.Response time is a very important parameter for almostevery LC device. We measured the PDLC response time us-ing a square voltage burst at f =1 kHz between 26 and55 Vrms. The measured rise time is ,0.8 ms and decay time,1.9 ms at room temperature. Such a fast response timeresults from the high bias voltage effect.14FIG. 2. Voltage-dependent transmittance of a PDLC film. l=633 nm andcell gap d=22 mm. FIG. 3. Voltage-dependent transmittance of a PDLC cell between parallelsright ordinated and crossed sleft ordinated polarizers. f =1 kHz, l=633 nm, and cell gap d=22 mm.FIG. 4. Measured phase shift of the PDLC cell at different voltages. Cellgap d=22 mm.141110-2 Ren et al. Appl. Phys. Lett. 86, 141110 ~2005!To demonstrate the usefulness of the observed phasechange, we fabricated 2D microlens arrays using the PDLCas electro-optic medium. We first used a lamination methodto prepare polymer microlens arrays.15 The polymer materialused is NOA65 snp=1.524d. The thickness and diameter ofthe formed microlens is ,50 and 450 mm, respectively. Af-terwards, we filled the polymer microlens array cavities withthe NOA65/E48 mixture. The cell was then sealed with a topITO-glass substrate. Finally, we cured the sealed cell usingan UV light. The UV curing condition is the same as men-tioned above.Figure 5sad and 5sbd show the patterned polymer micro-lens arrays before and after filling the PDLC material. Thesetwo images were taken under a polarized optical microscope.From Fig. 5sad, each circular ring corresponds to a concavelens of ,450 mm diameter and lens pitch ,20 mm. In Fig.5sbd, the filled circular region is translucent due to lightscattering.To characterize the focusing properties of the microlens,we illuminated the lens arrays with a collimated unpolarizedHe–Ne laser beam. The transmitted light was detected by aCCD camera. Figure 6 shows the images of the focused spotsproduced by the microlens arrays at different voltages. AtV=0, the focusing effect is obscured because of strong lightscattering. At V=70 Vrms, the focal spots appear but not verysharp. At V=140 Vrms, the focus is at image plane. Thus, thelight intensity increases noticeably. This result means that thepatterned microlens arrays have a tunable focal length in thehigh voltage regime. If we increase the voltage from70 to 180 Vrms, the focal length is tuned continuously from,8 to ,12 cm. In principle, if the LC and polymer have asimilar ordinary refractive index, the focal length of thepolymer/PDLC microlens should reach infinity in the highvoltage regime. The focal length of a microlens is dependenton the lens radius, LC birefringence, and lens thickness. Ifwe want to reduce the focal length, we could reduce the lensdiameter, use a higher birefringence LC, or increase the lensthickness.In comparison with other tunable LC phase modulators,PDLC exhibits a relatively small phase change. This is be-cause the phase phenomenon only exists in the high voltageregime where the LC orientation is close to the saturationlevel. However, the phase modulation using PDLC is polar-ization independent, scattering-free, and has fast responsetime. The operating voltage is lower than that of a nano-PDLC system. Although the tunable phase shift is small, it isstill sufficient for some microdevices.In conclusion, we have demonstrated the phase modula-tion capability using a PDLC film which only exists in thehigh voltage regime. Such a phase modulation is scattering-free, polarization independent, and has fast response time.Although the phase change is small, it is still sufficient formaking tunable-focus microlens arrays.1V. Freedericksz and V. Zolina, Trans. Faraday Soc. 29, 919 s1933d.2N. Konforti, E. Marom, and S. T. Wu, Opt. Lett. 13, 251 s1988d.3S. Gauza, H. Wang, C. H. Wen, S. T. Wu, A. J. Seed, and R. Dabrowski,Jpn. J. Appl. Phys., Part 2 42, 3463 s2003d.4S. Matsumoto, M. Houlbert, T. Hayashi, and K. Kubodera, Appl. Phys.Lett. 69, 1044 s1996d.5L. Vicari, J. Appl. Phys. 81, 6612 s1997d.6T. J. Bunning, L. V. Natarajan, V. P. Tondiglia, G. Dougherty, and R. L.Sutherland, J. Polym. Sci., Part B: Polym. Phys. 35, 2825 s1997d.7H. Ren and S. T. Wu, Appl. Phys. Lett. 81, 3537 s2002d.8D. E. Lucchetta, R. Karapinar, A. Manni, and F. Simoni, J. Appl. Phys.91, 6060 s2002d.9R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, T. J. Bunning, and W.W. Adams, Appl. Phys. Lett. 64, 1074 s1994d.10J. W. Doane, N. A. Vaz, B. G. Wu, and S. Zumer, Appl. Phys. Lett. 48,269 s1986d.11F. Basile, F. Bloisi, L. Vicari, and X. Simoni, Phys. Rev. E 48, 432 s1993d.12P. S. Drzaic, Liquid Crystals Dispersions sWorld Scientific, Singapore,1995d.13S. T. Wu, U. Efron, and L. D. Hess, Appl. Opt. 23, 3911 s1984d.14S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays sWiley, NewYork, 2001d.15H. Ren, Y. H. Fan, and S. T. Wu, Opt. Lett. 29, 1608 s2004d.FIG. 5. Microscope photos of sad concave polymer microlens arrays, and sbdpolymer/PDLC microlens arrays.FIG. 6. Measured CCD images of the 2D microlens arrays at differentvoltages: sad V=0 sbd V=70 Vrms, and scd V=140 Vrms.141110-3 Ren et al. Appl. Phys. Lett. 86, 141110 ~2005!

Polarization-independent phase modulation using a polymer-dispersed liquid crystal

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Polarization-independent phase modulation using a polymer-dispersed liquid crystal

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