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Fast responses (20 ms rising time) both in symmetrical two-wave and degenerate-four-wave mixing experiments were observed and investigated in C60 doped 4,4’-n-pentylcyanobiphenyl liquid crystal cells sandwiched between bare ZnSe substrates with an electric field applied parallel to the cell surfaces. The ZnSe material seems responsible for the fast response due to its excellent charge carrier transportation capability. Strong fanning effect and transient features were seen and studied, hinting super strong photorefractive effect in the material system. This low voltage operated liquid crystal based photorefractive approach is promising in real time applications over visible to terahertz regime.
©2012 Optical Society of America
1. Introduction
The exploration of new photorefractive (PR) materials and optimization of existing ones have continued for several decades [1,2]. It is of primary importance to search for PR materials with fast response rate, which is one of key parameters in real time holographic display and optical information processing [3,4]. Organic PR materials, including polymers [2–4] and liquid crystals (LC) [5–13], well known for their high figures of merit, easy processing, and flexibility, are envisioned as promising candidates for practical uses [3,4]. In these organics, the key function groups (constituents) are photoinduced charge generators, transporting agents, trapping sites, and molecules that provide optical nonlinearity [2,7,8]. The low voltage operated liquid crystals are more attractive from a practical point of view. In fact, the early reports on PR effect in LCs [6,7] and following intensive research activities globally [8–13], have demonstrated excellence of the LC PR materials. However, most reported PR effect in nematic LCs is quite slow in response rates [5–7,9–11]. These slow response rates hinder their applications in real time imaging and optical signal processing. Very interestingly, by introducing photoconductive interlayers in between ITO and LC layers, dramatic raises in response rates were attained in LC PR materials [12–14]. All these works revealed the crucial roles played by the photoconducting layers in the associated PR effect. Indeed, the inefficient charge transport in most LC based PR systems strongly limits their performances, since both the dark and photo conductivity are usually made poor purposely to meet the special needs in display, for the sake of low power consumption.
Inspired by the interesting works of Bartkiewicz and his associates [12,13], we proposed a LC based material system to study its PR properties: (1) C60 doped 4,4’-n-pentylcyanobiphenyl (5CB) LC; and (2) semiconductive, optical window material ZnSe. The 5CB was chosen due to its well-known optical nonlinearity, C60 for its photoinduced charge generating and charge carrier trapping capability. The well photoconductive ZnSe may efficiently transport charge carriers within the system, without excluding its partial charge carrier generating and trapping roles. Encouragingly, quite fast response in both two wave mixing (TWM) and degenerate four wave mixing (DFWM) (20 ms rising time) were observed in a C60 doped LC cell sandwiched between two bare ZnSe substrates with a non-tilted configuration, upon applying a DC electric field parallel to the cell surfaces. Gain coefficient ranging from 250 to 300 cm−1 was measured in an LC cell with 50 micron thick spacers. Strong accompanying fanning (scattering) effect was observed, suggesting strong energy transferring from the pumping beam to the weak scattering beams. Transient features were also observed, hinting complicate charge carrier generation, transportation, capturing and compensating processes in the LC based PR system. It seems that the use of ZnSe material is responsible for the fast response, and experimental confirmation was presented, along with potential applications of the findings.
2. Theoretical consideration and proposed approach
In [14,15], 5CB LC were affirmed as an ideal constituent which provides optical nonlinearity in polymeric systems. According to theoretical prediction of [16,17], the lower Tg, the freer of the 5CB molecules, and the higher of the orientational enhancement effect. In the work reported in [14], higher gain coefficients were measured in the specimen with 50% 5CB, in comparison with the specimen with 40% 5CB. It is, of course, natural to apply this picture in describing pure LC specimens. In 5CB LC, transition temperature from crystalline to nematic phases is around 24°C, it is expected that the role of the orientational enhancement effect should be maximized due to higher freedom of the LC molecules. By using the treatment of Refs 16 and 17, and considering the symmetrical incidence (shown in Fig. 1 ), the effective electro-optic coefficients for s- and p-polarization are given by:
where n is the refractive index of the sample, θint the internal interbeam angle, the symmetrical incidence guarantees the maximal effective electrooptic coefficient in both s- and p-polarized light cases. The material constant A = CEO/3- CBR/2, and C = CEO + CBR, CEO and CBR are the coefficients from contributions of electro-optic (EO) and the induced birefringence (BR) effects [16]. These are given by,where N is the density of 5CB molecules, μ the ground-state dipole moment, α⊥ and α// the linear optical polarizabilities of the 5CB molecule parallel and perpendicular to the molecular axis, and β333 the hyperpolarizability of the 5CB molecule. According to the Eq. (1) and (2), the effective electrooptic coefficients resulted from the orientational enhancement effect are proportional to the density of the 5CB molecules and therefore maximize for LC only materials. As a result, the gain coefficient, which is proportional to reff, should increase accordingly. However, compared with the 50% 5CB specimen, very surprisingly, there were no obvious gain coefficient increases in the specimens with 60% 5CB. Contrarily, the response rate becomes slower with increase of 5CB concentration.
Fig. 1 Schematic diagram for measuring TWM and DFWM parameters in LC cells with symmetrical incident beams. Di are detectors, BS beam splitter.
Using glass plate sandwiched C60 doped 5CB LC cells, with the configuration depicted in Fig. 1, there was no appreciable TWM and DFWM signal observed even with over 1000 V voltage was applied across 4~5 mm gap between two electrodes. Considering the three principal functional constituents in making a good PR composite [1,2], there is a major constituent missing in the material composite: photoconducting agents. In fact, in 5CB + polyΠN-vinylcarbazole| ΠPVK) PR material system, when the PVK concentration goes lower, the response rate goes down dramatically: from hundred ms to minutes. This trend suggests the inefficient charge transporting due to PVK reduction is behind the response slowing down. In fact, the interesting works of Bartkiewicz and his associates [12,13] in raising the response rate was indeed involved in adding excellent photoconductive layer: PVK doped with trinitrofluorenone molecules. Indeed, this simple theoretical consideration can provide guidance in the experimental works that follows. No doubt, this surprising response rate difference deserves further investigation to understand the mechanism in the PR effect. Actually, this motivated the research work reported in this paper.
3. Experimental and analysis
Based on the consideration above, we proposed a LC based material combination to study PR effect: (1) C60 doped 5CB LC and (2) photoconductive, optical window material ZnSe. The 5CB would offer necessary optical nonlinearity; C60 photoinduced charge generating and charge carrier trapping capability; and ZnSe charge carrier transportation, plus possible charge carrier generating and trapping roles. In our case, the applied electric field was parallel to the cell surfaces and the LC cell was non-tilted. Based on the theoretical treatments of the Refs 16 and 17, the effective coefficients are optimized in this geometry (refer to Eq. (1)).
All materials chosen for making LC cells were commercially available, including ZnSe plates (Jiahehengde Technology, Beijing), 5CB LC (Merck), and fullerene C60 (Aldrich). Aluminum foil was chosen as spacers of the LC cells also as electrodes. The distance between the two electrodes was 5.0 mm. The structure of the LC cell and the experimental configuration are schematically shown in Fig. 1. Upon application of an electric field, a better anisotropy was established as LC molecules were aligned along the electric field. This results in an induced optical axis in the same direction. Consequently, the transparency of the LC cell increased due to a better organization of LC molecules.
3.1 TWM experiments
With the symmetrical configuration (Fig. 1), we have studied PR properties of the LC cell with TWM and DFWM experiments. The thickness of the LC cell was set by two aluminum foils with thickness d = 50 µm, the crossing angle of beams 1 and 2 from a He-Ne laser at 632.8 nm, was θ = 0.28°, with the corresponding grating spacing Λ = 130.5 µm. The two p-polarized writing beams 1 and 2 were with equal intensities, 150 mW·cm−2. The experimental results shown in Fig. 2(a) were obtained at applied field 0.23 V/μm. It was seen that the transmitted intensities for beams 1 and 2 were not the same, with one increasing and the other decreasing. This suggests that energy transferring from one beam to another occurred. To confirm grating formation in the LC cell, a probing beam, which was counterpropagating to one of the two writing beams, with intensity of 15.0 mW·cm−2 was monitored in comparison with its diffraction beam (see the inset of Fig. 2(a)). The gain coefficients were calculated according to for equal intensity light beams and plotted in Fig. 2(a) against the applied electric field, whereγ0 = I1/I2. The highest gain coefficient measured was 350 cm−1 from the LC cell with 50 µm thick spacers. It should be emphasized that grating formation process is quite fast compared with that in most reported works done in LC PR materials. Figure 2(b) shows typical grating formation dynamics by exhibiting directed transmitted and first the diffraction beam intensities. After turning on writing beams, one of the transmitted beams decreased in power while the first diffraction order increased at the same time. The peak power of the diffracted beam reached was coincided with the time when the directly transmitted beam approached its saturate value. It should be mentioned here that the 20 ms rising time of the diffraction beam makes this PR effect promising in the real time holographic display. This high speed hologram recording is also highly desirable in real time interferometry, and many other applications. It worthwhile to mention that his rising time is consistent with the fastest response observed in other nematic LC cells [18–22]. As stated previously, in the same structured LC cells prepared by bare glass plates, no grating can be formed even as high as 2 V/μm electric field was applied on it. Now, by simply using ZnSe plates in the place of glass plates, so striking changes were seen. Apparently, it suggests that the new findings are closely related to the ZnSe substrates.
Fig. 2 (a) The TWM gain coefficient versus applied electric field, the inset is the energy transferring kinetics curves; (b) The detailed dynamics of the fast PR effect.
Shown in Fig. 3(a) was a TWM dynamic curve obtained at 0.145 V/µm dc electric field applied on the ZnSe based LC cell. Solid energy transferring was evidenced from the graphs. It was also seen that the two transmitted intensities decreased gradually, implying the main beams may transfer energy to the accompanying two to three higher orders seen on each sides or to the strong fanning (scattering) beams, which will be described in details in next subsection. From these observations, it is obvious that there are complicated processes involved in the grating writing in the ZnSe/LC system. Therefore, understanding the underlying mechanism is of importance to further raise response rate and gain coefficients. In the past, the role of the surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers and its impact on associated Friedericksz transition were systematically investigated previously [9,23,24]. In our case, the electric field was applied parallel to the cell surfaces. Similar charge carrier accumulation on electrodes and resultant screening of the electric field might occur in the LC cell. In addition, the carrier charge layers could form near the two ZnSe/LC interfaces.
Fig. 3 (a) Measured TWM energy transferring between the two main beams; (b) Diffracted intensity in a 5CB + C60 LC cell sandwiched with ZnSe substrates with a probing beam in three cases.
To understand possible mechanism associated with carriers’ transportation, possible electric field screening, three straightforward TWM experiments were performed as follows:
- (a) Without applied electric field;
- (b) Preilluminating the LC cell before turning on electric field;
- (c) Preapplying electric field before light illumination.
The related results pertinent to the above experiments are shown in Fig. 3(b). In the first case, the response time was the longest and the diffraction efficiency was the lowest. Compared with other two cases with applied electric field, this suggests that an applied electric field speeds up the charge carriers’ transportation and hence enhances the grating formation in the LC cell. This agrees with the model established in [23,24].
In the second case, the cell was preilluminated before turning on applied field. Both the response rate and diffraction efficiency were the best among all the three cases. It seems that illumination produces charge carrier modulation. When the applied electric field was on, the electroinduced anisotropy and the charge carrier modulation result in LC molecule orientational modulation. This molecule orientational modulation is behind the energy transferring.
In the last case, an electric field was applied before light illumination. Since the electric field could cause charge accumulation near the electrodes, and hence the electric field on the LC cell was screened partially, resulting in a reduced effective electric field. After this, the light modulation produces almost the same charge carrier modulation. The combination of the lower anisotropy and the same carrier modulation result in a relatively weak grating. In addition, the relatively slower carriers drifting slow the rising and decaying process. That is why the diffraction efficiency was lower than that in case (b) and why a longer decaying time was seen.
3.2 Strong fanning effect
The transient feature shown in the TWM and DFWM experiments (also reported in the literatures) has puzzled us greatly when we first observed this effect. However, when we turned our attention towards strong fanning effect accompanied all the observations, it seems this transient behavior might result partially from energy transferring from the strong pumping and signal beams to relatively weak scattering beams. This hypothesis was partially confirmed by the fanning dynamics and spot pattern. When single beam incidences onto a LC cell without applied electric field, there was no apparent fanning effect. Obvious fanning effect occurred at an 0.06 V/μm electric field. The fanning pattern was towards the positive electric field side, since holes are the dominant charge carriers in the LC cell. Shown in Fig. 4(a) and 4(b) are the photographs taken before and after applying electric field, respectively. The typical fanning light intensity dynamics were shown in Fig. 4(c). The angle between the fanning light and the transmitted light was 15°. Over 10 times amplification in fanning light was measured, hinting much stronger gain coefficients in the PR material. It should be pointed out that the strong fanning effect makes this PR composite promising in realizing self- and mutually-pumped phase conjugation.
Fig. 4 (a) and (b): photographs of the transmitted beam spots without/with applied field; (c) Dynamic curves of a selected fanning beam at different applied field.
3.3 DFWM in the LC cells
DFWM process is crucial for optical information processing and phase conjugation and it is often employed in monitoring the grating formation. With a relatively weak p-polarized beam which counterpropagated along one of the two writing beams in TWM setup (refer to Fig. 1), DFWM signals were observed in two cases with p- and s-polarized writing beams. There was no self-diffraction observed in the s-polarized case. Since the s-polarized beams cannot be self-diffracted, the main grating written within the LC cell was stronger than that with p-polarized beams. This is why a relatively strong DFWM signal was obtained in s-polarized case. Depicted in Fig. 5(a) were the DFWM dynamics during the grating formation with two s-polarized and p-polarized light beams.
Fig. 5 (a) A dynamic curve of diffraction intensity in the DFWM configuration with a typical sample prepared by directly depositing ITO layer on top of the sample; Measured external diffraction efficiency versus applied electric field at elevated temperature; (b) Diffracted intensity in a 5CB + C60 LC cell for a continuously incident He–Ne probe beam during a program of applying an electric field.
From Fig. 5(b) it is seen that the diffraction efficiency increased with the electric field, until peaked at 0.10 V/μm. Over 0.10 V/μm, the diffraction efficiency decreased with increasing electric field. This also suggests that electric field and light modulation of carriers are responsible for the grating formation in the LC cell. It seems that the formation of fanning gratings stole energy from the main beams. Great number of growing fanning gratings can dramatically spoil the grating written by the two main beams 1 and 2 in Fig. 1. Therefore, the growing fanning effect with increasing electric field might be one possible reason for the decreasing diffraction efficiency at higher electric field.
3.4 Fast PR effect in LC cells with ZnSe coated ITO glass plates
To confirm the crucial role played by the ZnSe plates, we purposely deposited ZnSe layers on two piece ITO glass plates and then make a homeotropical LC cell with 12.5 µm thickness. Using conventional slanted geometry, the specimen was tilted at an angle β = 45° to the bisector of the two writing beams. It is very exciting to see that a fast response updatable grating can be recorded in the LC cell thus made. Shown in Fig. 6(a) are the typical dynamic curves taken directly from a digital oscilloscope connected to a photodetector monitoring the diffracted signal by using a reading beam from the back of the LC cell. It is encouraging to see the rising time also fell in the 25 ms range, almost the same to the rising time seen in the LC cells made of ZnSe plates. Again, the photoconductive ZnSe layers seem responsible for the fast responsiveness. Intriguingly, the diffraction can reach a steady state in this case. The best diffraction efficiency for the first order was 8.0%. The super low operation voltage threshold in ZnSe based cells was quite striking compared with that conventional ITO glass based cells. It is also seen that the rising time was shortened from 200 ms at 0.2 V to 25 ms at 2.0V. To prove its capability in real time applications, a simple imaging experiment was performed. The setup was similar to that shown in Fig. 1. The photographs taken from the input object and its phase conjugate replica are shown in Fig. 6(b) and 6(c).
Fig. 6 (a)A dynamic curve of diffraction intensity in the two wave mixing configuration with a typical sample prepared by directly depositing ZnSe layer on top of the sample; (b) is the photo of an object and (c) its phase conjugate replica.
To further confirm the crucial role played by the ZnSe plates, we made a homeotropical LC cell with two ITO glass plates, with only one ITO layer covered with a 200 nm thick ZnSe film facing the LC layer. Using the same slanted geometry at tilted angle β = 45° to the bisector of the two beams, it is intriguing to see that quite different response rates obtained for different electrode connection. Shown as the black line in Fig. 7 is the typical dynamic curve of the diffracted signal obtained when the ZnSe-covered substrate served as anode. One can see the response time was 40 ms. Surprisingly, when the ZnSe covered substrate was chosen as cathode, the response time was 658 ms (see the blue line). It should be noted that the light illumination and optics arrangement were all the same before and after switching the electrode connection. The response rate difference might originated from the different desorption rate of the charge carriers from ZnSe surface and ITO surface. Although the microscopic origin of the sort of carriers and exact process is yet to be identified and investigated, this large difference in response rate confirmed again the crucial role played by ZnSe material. Intensive research activities along this line are still undergoing and will be reported in the near future. It deserves to emphasize that this PR material system can be expanded to longer wave regime up to terahertz range.
Fig. 7 A dynamic curve of diffraction intensity in the two wave mixing configuration with a typical sample prepared with two ITO glass plates, with one covered with ZnSe layer on top of ITO layer; the faster response depicted in the black line corresponds to the case when ZnSe coated ITO glass plate was positive electrode, and the other ITO only glass plate substrate was the negative electrode; the blue line exhibits the response curve when the electrode connection were switched.
4. Conclusion
In conclusion, a fast response photorefraction-like effect was observed and studied in C60 doped 4,4’-n-pentylcyanobiphenyl LC cells made of bare ZnSe substrates, and also confirmed with a homeotropical LC crystal cell sandwiched between two nanoscale ZnSe layer coated ITO glass plates. As high as 350 cm−1 gain coefficients were obtained from the LC based PR material, even though strong fanning effect steals energy away from the main writing beams. The excellent photoconductivity and desorption of charge carriers in ZnSe contributes to the fast space charge redistribution, and hence is responsible for the 3 to 4 order of magnitude response rate raise in the PR effect seen. The findings could be used in designing low voltage operated holographic display and optical switch in a broad waveband from visible up to terahertz and real-time interferometry.
Acknowledgments
This work has been supported in part by the grant of the Harbin Institute of Technology under project No. AUGU570000710 and the grant of National Natural Science Foundation of China under project No. 11174067.
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