Surface-sustained permanent gratings in nematicliquid crystals doped with carbon nanotubes

Wei Lee, Hui-Yu Chen and Sheng-Long YehDepartment of Physics, Chung Yuan Christian University, Chung-Li, Taiwan 32023, Republic of China

wlee@phys.cycu.edu.tw

Abstract: Response properties of photoinduced gratings were studied inplanar cells of liquid crystal doped with multiwalled carbon nanotubes. Thegrating formation time deduced from beam-coupling measurements wasobtained to be ~30 ms. Absorption spectroscopy implies that the permanentgratings are associated with periodically distributed carbonaceous materialadsorbed on the inner surfaces of the cell windows under prolongedillumination to the 514.5-nm beams, giving rise to the persistent light-induced modulation of the easy axis.2002 Optical Society of AmericaOCIS codes: (160.3710) Liquid crystals; (160.5320) Photorefractive materials; (050.1950)Diffraction gratings; (190.7070) Two-wave mixing

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79, 4488–4490 (2001).9. W. Lee, S.-L. Yeh, C.-C. Chang and C.-C. Lee, “Beam coupling in nanotube-doped nematic liquid-crystal

films,” Opt. Express 9, 791–795 (2001), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-9-13-791.10. P. M. Ajayan, “Carbon Nanotubes,” in Nanostructured Materials and Nanotechnology, H. S. Nalwa, ed.

(Academic Press, San Diego 2002), pp.329–360.11. Y. Wang and L.-T. Cheng, “Nonlinear optical properties of fullerenes and charge-transfer complexes of

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1. Introduction

Over the last decade we have witnessed tremendous advances in the development andcharacterization of photorefractive (PR) organic materials including organic crystals, polymercomposites, and liquid crystals (LCs) [1,2]. Since Rudenko and Sukhov [3] first demonstrated

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the photorefractivity of LCs, many researchers have also concentrated their efforts on thisnovel class of material. There are several reasons for pursuing the development of PR LCs inparticular. Compared with polymeric systems, LCs are intriguing owing both to the lowcharacteristic voltage required for application to the material for the realization of wavemixing and to the high modulation of the refractive index induced with a rather low powerdensity of light beams. Most advantageously, PR LC cells are very easy to fabricate, thankspartially to the prosperity in the display industry.

Khoo [4] presented a sophisticated theoretical investigation on the mechanisms of the PReffect in nematic LCs. The effect was interpreted to be derived primarily from orientationalordering within an inhomogeneous bulk space-charge field as a consequence of the Carr–Helfrich effect and flow effect. Recently experimental evidences of surface charge mediatedPR effects have been reported in LC cells [5,6]. While Zhang et al. [5] suggested that the PR-like effect in homeotropically aligned LCs is mediated by surface charges rather than bulkcurrents, Pagliusi and Cipparrone [6] observed the surface charge induced PR-like gratings inpure LC planar cells. It is clear that a full understanding of photorefractivity in LCs ischallenging. This requires additional studies on the relevant issues.

Recently we reported our observations of both self-diffraction and optical amplificationby holographic gratings formed in nematic LC films doped with carbon nanotubes [7–9]. Ourinterest in considering carbon nanotubes as a dopant stems from their novel properties inmechanics, electronics, and nonlinear optics, as mentioned in our earlier publications [7–9]and well documented in Ref. 10. Indeed, our previous studies on the nanotube-doped LCsamples have brought about the achievement of a large nonlinear index-change coefficient n2

~ 10−1 cm2/W [7] as well as of high gains ~15 [8,9]. In the present paper, we discuss furtherthe response characteristics of the orientational PR gratings in the same material. The decaytime was obtained to be several tens of seconds for a persistent grating. This unexpectedlylong decay time constant allowed us to discover the surface phenomenon of the LC cells. It isshown that the prolonged two-wave mixing, employed in the recording of a persistent grating,resulted in an inner-surface alteration of the cell due to the formation of periodicallydistributed adsorbates at the interface between the aligning layer and the LC material. Thesecarbonaceous deposits may well provide charge-trapping sites, leading to a drastic change inLC anchoring properties during the grating recording.

2. Experimental

Glass substrates composed of uniform and transparent indium-tin-oxide films were first spin-coated with polyimide and then rubbed unidirectionally to provide initial planar alignment ofLC. Cells were fabricated with pairs of the treated substrates. The nematic LC sandwiched inthe 25-µm-thick (d) cells is known commercially as E7 (from Merck), a eutectic mixture offour different cyanobiphenyl and cyanotriphenyl compounds with a positive dielectricanisotropy and nematic-to-isotropic transition at 61 °C. At the room temperature, therefractive indices parallel and perpendicular to the director axis are 1.75 and 1.52,respectively, giving the average index of refraction n of 1.6. A trace (0.02% by weight) ofpurified, uncapped multiwalled carbon nanotubes (from SES Research) as the dopant wasthoroughly sonificated in the LC at ~75 °C before incorporation into a sample. Theabsorption spectrum of the nanotubes is shown in Fig. 1. Note that it reveals no apparentfeatures in the visible except for a sole, weak continuum attributed to the scattering by thesuspended particles. The addition of the dopant is made for creating charge-transfercomplexes [11] in the solution to enhance the nonlinearity of the nematic, thereby promotingthe performance of wave-mixing effects [7–9].

To investigate the response properties of our samples we performed a two-wave-mixingexperiment, including the asymmetric beam coupling as reported in Ref. 8, in the tiltedgeometry. The purpose to conduct such an asymmetry is to establish the same PR gratingsthat were previously observed. Because no significant beam coupling was observed at anapplied dc voltage Vdc ≤ 3 V [9], the applied bias in the experiment was set above 3.7 V, the

(C) 2002 OSA 3 June 2002 / Vol. 10, No. 11 / OPTICS EXPRESS 483#1101 - $15.00 US Received April 29, 2002; Revised June 3, 2002

Fréedericksz transition threshold of the sample [7]. Two p-polarized Ar+ laser beams of the514.5-nm (λ) line were superposed onto the sample on the same 3-mm-diameter spot. Thewave-mixing angle α = 0.7° and the incident angle of the two beams, β = 26.2°, inside the LC.The experimental conditions yield a fringe spacing of 27 µm as the grating constant Λaccording to the formula

Λ = λ / 2n sin (α/2) ≈ λ / nα. (1)

Because the dimensionless parameter [12] Q = 2πλd / nΛ2 cos β ≈ 0.1 < 1, the resulting phasegrating is in the Raman–Nath grating regime. The total optical loss contributed by reflection,scattering and minute absorption was estimated to be ~16 cm−1 outside the vicinity of theelectrohydrodynamic instability (Vdc ~ 7 V).

Fig. 1. A typical, normalized linear absorption spectrum of a colloidal solution of multiwalledcarbon nanotubes dispersed in n-hexane against an n-hexane reference at room temperature.The spectrum is featureless and shows a weak continuum due to the scattering by thesuspended particles.

3. Results and discussion

Time-evolved asymmetric-beam-coupling scans were obtained. Typical examples for two 45-mW writing beams with various voltages applied have been presented in an earlier work [8].The formation time of the steady-state refractive-index grating was deduced from the time-evolved beam-coupling data and calculated using the simplest single-carrier model that allowsthe rising signal to be fitted with a single exponential function [13]:

g(t) − 1 = (g − 1) [1 − exp(−t/τ)], (2)

where g(t) represents the beam-coupling ratio of the probe beam [9], the intensity of thetransmitted probe beam in the presence of the pump beam divided by that without thepresence of the pump beam, at time t, g denotes the steady-state beam-coupling ratio, and τ isthe formation time. Fig. 2 shows that, for a constant grating spacing, the grating formationtime decreases as the applied dc voltage is increased, presumably owing to the increasedefficiency of charge-carrier redistribution in the field and effective mobility of carriers. For a1-mW signal beam coupled with a pump of 40 mW, the shortest rise time of 34 ms,accompanied by the greatest beam-coupling ratio of 1.99, occurs at 14 V; further increase ofthe applied voltage causes the rise time to become slightly longer (inset in Fig. 2), probably asa result of the tight confinement of the average director axis along the dc-field direction. It isworth mentioning that the grating formation time based on the two-beam coupling can befurther slightly decreased by decreasing the fringe spacing and increasing the pump/probe

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(C) 2002 OSA 3 June 2002 / Vol. 10, No. 11 / OPTICS EXPRESS 484#1101 - $15.00 US Received April 29, 2002; Revised June 3, 2002

ratio. In any case, the decay time obtained from the same experiment was measured to be ofsimilar scale.

Fig. 2. Voltage dependence of the index-grating formation time of a nanotube-doped nematicfilm with a grating constant of 27 µm. The incident beams are 20 and 1 mW,� and 40 and 1mW,�. Inset, expanded scale for small formation time constants.

The PR performance including the dynamic properties depends strongly on the samplehistory. For a fresh sample written with a transient grating, the decay time obtainedexperimentally was found to follow well-known orientational relaxation dynamics [4]:

τd ∼ γ / K [ (π / d)2 + (2π / Λ)2 ], (3)

where γ is the viscosity coefficient and K the elastic constant. With material properties of γ ~10–2 N s m–2 and K ~ 10–11 N and experimental conditions for d ≈ 25 × 10–6 m and Λ ≈ 27 ×10–6 m, one gets τd ∼ 10–1 s, which is close to our observation of τd ∼ 0.5 ± 0.3 s. Beyond thetransient-grating regime, decay time constants from measurements of self-diffractionefficiencies were observed to possess two distinct time components. We believe that thesurface charge, proposed by Zhang and co-workers [5], along with space charge governs thegrating properties in this stage of grating formation. Such suggestion stems from ourspeculation that the weaker but much longer component for the relaxation (~30 s) arises froma charge accumulation and discharge process of the LC capacitor [14].

The persistent-grating stage can be achieved in our experiment for a 20-minuteillumination at a total writing intensity of 1.2 W/cm2. In order to investigate the formation ofthe persistent grating (Fig. 3) under writing of prolonged illumination or, more precisely,sufficient energy density and, in the hope, to realize how the electric charges are possibly keptat the interface, we have obtained the visible spectra of the fresh samples prior to and after 20-minute illumination by a single cw 514.5-nm laser beam at an intensity of 1.2 W/cm2.(During laser irradiation, no external field was applied across the cells.) The reason we electto apply such simple, conventional absorption spectroscopy, instead of adopting moresophisticated approaches such as micro-Raman or micro-photoluminescence measurements, isfor its ease and yet direct.

As shown in Fig. 4, no significant change is observed in the absorption of the pristine E7sample before and after laser exposure, whereas changes in the broad continua are obvious forthe nanotube-doped E7 cell. We obtained similar spectral results for cell substrates of thetested samples, suggesting that the surface alteration would take place during the beamwriting in a two-wave-mixing experiment. By examining under a microscope the cellsubstrates taken apart from a persistently encoded, nanotube-doped LC sample, we have

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(C) 2002 OSA 3 June 2002 / Vol. 10, No. 11 / OPTICS EXPRESS 485#1101 - $15.00 US Received April 29, 2002; Revised June 3, 2002

observed the surface-sustained continuum phenomenon in the LC bulk: The stripes in goodorder that resemble the interference pattern are ruined during the mechanical interaction withair disturbances and recover when the applied airflow seizes. The deposits in the illuminatedzones, which may be attributed to nonuniform accumulation of simply original carbonaggregates, nanotube complexes, or of photochemical products of multiwalled carbonnanotubes depending on the illumination conditions and surface chemophysical properties ofthe polymeric aligning layer, can serve to trap charge carriers during the later stages of thephase-grating formation. The resulting internal electric field then modulates the LC director-axis reorientation and, in turn, the refractive index to create a phase grating that can diffract alight beam. Although the deposited carbonaceous material itself could very well constitute asurface grating, our observation of substantial polarization dependence for these persistentgratings verifies the additional contribution from LC reorientation [15]. Due to a previousstudy of evident C60 photopolymer produced from a C60-containing chlorobenzene solutionwith a 514.5-nm Ar+ laser beam [14], we reason that high-power-density writing beams wouldcreate more stubborn photoproducts at the interface between the alignment layer and LC andresult in a persistent grating with high degree of depolarization as observed by Khoo [4].

Fig. 3. Micrographic image (×100) of a permanent index grating in a film of the nematic E7doped with carbon nanotubes (top) in comparison with that of a commercialized transmissiongrating of 500 grooves / cm (bottom).

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(C) 2002 OSA 3 June 2002 / Vol. 10, No. 11 / OPTICS EXPRESS 486#1101 - $15.00 US Received April 29, 2002; Revised June 3, 2002

4. Conclusions

To conclude, we have studied the time constants of holographic gratings in the transient,intermediate and permanent regimes in films of nematic LC doped with multiwalled carbonnanotubes. The refractive-index grating is generated as a result of reorientation variation ofthe director axis of the mesophase caused by the photoinduced internal electric field. Theincorporation with carbonaceous dopants, such as carbon nanotubes as a photochargegenerator, provides a prominent function in the formation of the phase grating via theperiodical redistribution of the doping material upon illumination to a light pattern at theinterface between the LC and the aligning layer. The carbonaceous adsorbates may act astraps of photoinduced surface charges, cause the light-induced modulation of the easy axis inthe LC bulk, and sustain the permanent grating through the continuum effect of the LC.Further experiments are being carried out on polarization dependence to clarify thephotoinduced mechanisms at various grating formation stages and the origin of the persistentgrating after prolonged illumination. It can be of interest to discover whether nanotubes canbe turned by light in a similar fashion to photochromic molecules.

Acknowledgments

This work is supported primarily by the National Science Council of the Republic of China under grantNSC-89-2112-M-033-015 and NSC-90-2112-M-033-005, and partially by the Ministry of Education ofthe Republic of China.

(C) 2002 OSA 3 June 2002 / Vol. 10, No. 11 / OPTICS EXPRESS 487#1101 - $15.00 US Received April 29, 2002; Revised June 3, 2002