This study demonstrates all optical switches between the four diffractive light levels of a body-centered tetragonal photonic crystal. The sample is based on holographic polymer-dispersed liquid crystals that are fabricated using a two-beam interference with multiple exposures. The switching mechanism bases on the effective index modulation of the PC that contains a liquid crystal/azo-dye mixture could be controlled by two pumping laser beams. The switching time between the blue-laser-pumped and the blue-and-green-laser-pumped levels is fast. The study also discusses the switching time of the various levels.
© 2012 OSA
The reversible and fast light driven dye-doped liquid crystal (LC) system has attracted considerable attention not only in nonlinear optics but also in all optical switching devices, including the Z-scan , photorefractive grating [2–6], photoalignments [7,8], and photonic crystals [9,10]. Photochromic guest azo dyes control the refractive index of the host LC system by affecting the order parameter, or by isothermally inducing the nematic-isotropic phase transition of the LCs. The absorptions of the photochromic molecules, including the quantum efficiency, absorption cross-section, and the rate constant of the thermal cis-trans back-relaxation (lifetime) of the dye during photoisomerization, govern the characteristics of the switching. Some studies reported that photoisomerization occurs in picosecond timescales at quantum efficiencies below 60% [11–14]. The lifetime of the photoexcited state of molecules that contain azobenzene moieties can be varied from nanoseconds to several hours or even days by changing the donor and acceptor strengths at the molecule ends .
In our previous study , we discussed the optical switch of azo-dye-doped holographic polymer-dispersed liquid crystal (HPDLC) films, which was achieved by controlling the index modulation of the LC and of the polymer matrix. The cis isomer can disrupt the order parameter of LC droplets, which results in the reduction of the LC droplet effective index. In turn, the diffraction beams are switched off. In addition, the concentrations of the trans-cis isomers are in a dynamic equilibrium, which affects by the intensity and wavelength of the pumping laser beams. In this study, two laser beams are used to improve the optical switching of the body-centered tetragonal photonic crystal (BCT PC) based on an azo component-doped HPDLC. The switching time is significantly shortened to less than 1 ms, and gray levels are added in the switching.
2. Sample fabrication
Experimentally in this study, the recording medium for the fabrication of optically switchable PCs is a PDLC film, which consists of an LC (26.2 wt% E7, Merck), polymer (69.3 wt% NOA81, Norland), photoinitiator (1.3 wt% Rose Bengal, Aldrich), and an azo component (3.2 wt% M5C, home-synthesized ). The function of the photoinitiator is to produce free radicals, and then to initiate polymerization of NOA81 with exposure of the film to green-blue laser. The azo component is first added into the liquid crystal and is then vibrated using an ultrasonic oscillator prior to the addition of other components to ensure solubility. M5C is an azobenzene derivative (a 4-pentyloxy-phenyl-4-methoxyphenyldiazene photochromic molecule) whose azobenzenes can undergo reversible photoisomerization between two molecular forms (trans-cis isomers) upon irradiation. The trans isomer is thermally stable as a ground state and transforms into a cis isomer after excitation by blue light (λ = 350–400 nm; π–π* molecular transition). The photoisomerization of cis isomers to trans isomers occurs by itself or is accelerated under visible-light exposure. The time of transition from the trans isomer to the cis isomer is on the scale of a few nanoseconds, which meets the requirements of fast and efficient all-optical switching applications. The cured polymer has a refractive index of ~1.56, and the liquid crystal has ordinary and extraordinary indices of no = 1.5216 and ne = 1.7462, respectively, at 589 nm, 20 °C. Drops of homogeneously mixed compounds are sandwiched between two indium-tin-oxide-coated glasses that are separated by ~20 μm glass spacers to produce a sample.
Three-dimensional (3D) PCs have a great diversity of applications in communication and transmission field than that of 2D gratings, since the additional symmetry (say at the z-axis) also provides light managements that contribute to better photon manipulation [16–18]. In the study, the 3D BCT PCs are fabricated by exposing the sample to light interference using a two-beam interference with a point-by-point multi-exposure in an off-axis holographic setup . In a typical fabrication, a transverse electric (TE)-polarized, continuous-wave, diode-pumped, solid-state laser beam (Verdi, λ□ = 532 nm) is expanded and divided into two TE-polarized beams, namely, the reference (~400 mW/cm2) and object beams (~400 mW/cm2), using a beam splitter. These beams simultaneously illuminate the sample. The former is normally incident to the sample, whereas the latter is incident at an angle of ~39° to the normal incident beam. The sample is placed on a rotating stage that revolves around the reference beam. The sample is exposed four times to a two-beam interference and then subjected to rotations of 90° intervals, i.e., exposed at 0°, 90°, 180°, and 270°. The exposure times are 30, 20, 20, and 30 s, respectively. Because the off-axial holographic recording has an oblique incident beam; the grating vector of resulting grating is not parallel to the surface. Hence, the recorded samples present periodic modulations in z-axis that contribute to fabricate 3D PCs in multi-exposures. Notably, if the exposure time reduces to two exposures (with the sample being rotated by only 0° and 90° but ignored 180° and 270°), the top-view intensity profile is similar to that of the four exposures. However, the side-view intensity profile is different (α≠90°, see the definition in Fig. 1(a) ), in which the elliptic bright regions also tilt with respect to the z-axis.
Figure 1(a) gives the 3D simulated intensity profile of the interference region which is constructed using two-beam interference with four exposures. The figure also gives the illustration of a unit BCT cell. As the sample is exposed to the interference pattern, the photoreactive monomers diffuse to the bright regions (red regions in Fig. 1(a)) of the interference pattern, while the liquid crystal molecules consequently congregate in the dark regions (blue regions in Fig. 1(a)) which locate at the lattice point of the PC. By removing the LCs, Fig. 1(b) shows the top-view scanning electron microscope (SEM) image of the sample in which the polymer surface structure contains a matrix (polymer-rich region) as well as voids (LC-rich region). The primitive cell of the BCT structure has the dimension a = b ≠ c. The a and c lattice constants are ~0.813 ± 0.016 μm and ~3.416 ± 0.068 μm, respectively, and the diameter of the voids that contain the LC droplets is ~0.25 μm . Figure 1(c) presents the far field diffraction image from the sample. The maximum diffraction efficiency of the 1st-order diffractive beams is approximately ~15%.
3. Analysis of the concentration of cis isomer upon irradiation of laser beams
The HPDLC-based BCT sample contains the polymer matrix and voids at the lattice points, which contain the LC/dye mixture. Under pumping light exposure, the guest photoisomers affect the effective index in two ways. First, the cis isomers disrupt the host LC configurations in the voids. As the cis isomer fraction increases after laser light exposure, the effective indices of the LCs simultaneously decrease. Second, the cis isomers reduce the nematic-to-isotropic transition temperature of the LCs [19,20]. The phase transition temperature continuously decreases as the cis isomer concentration increases. If the cis isomer concentration is sufficiently high, the LCs exhibit an isotropic phase. The dynamic equilibrium of the as-pumped trans and cis isomers under various exposure conditions are the key to diffraction beam switching. The cis isomer fraction (Ncis) can be expressed as ,
Hence, in the steady state, the fraction of cis isomers can be simplified asEq. (2) with the parameters of , and XT under various pumping conditions. The simulation assumes that XT is a constant which is independent of the concentration of cis fraction upon pumping. The inset of Fig. 2 gives the concentration of cis isomer upon irradiation of a single blue or green laser beam, by applying the = 50 mW/cm2, = 10 mW/cm2, and XT = 0.7. It is seen that Ncis increases much faster by a blue pumping laser beam than that by a green laser beam. Figure 2 shows the simulated concentration of cis isomers under the excitation by the blue (fixed intensity IB = 100 mW/cm2) and the green pumping laser beams (intensity ranges from 0 to 800 mW/cm2), simultaneously. As the intensity of the green laser increases, the Ncis decreases. The reason is that the green laser transforms the cis isomers generated by the blue pumping laser to trans isomers. In other words, the fraction of cis isomer generated from the simultaneous exposures of the blue and green lasers is lower compared with that generated by a single blue laser.
4. Experimental setup in optical switching
Figure 3 shows the experimental setup used to measure the optical switch properties of the BCT PC. A TE-polarized Ar + laser (green, λ = 514.5 nm, intensity = 400 mW/cm2) beam and a diode laser (blue, λ = 403 nm, intensity = 100 mW/cm2) illuminate the sample at 30°, whereas a He-Ne laser is used to probe the BCT sample. In the experiment, shutters are used to control the exposure time of the green and blue laser beams. An aperture and a color filter were placed behind detector 2 to prevent laser light reflection. The pumping conditions of non-exposure and exposure to green, blue and green, and blue laser beams are simplified into OFF, ONG, ONB + G, and ONB, respectively, for convenience.
5. Experimental results and discussions
Figure 4 shows the diffraction beam intensity when the samples are pumped using a blue and/or a green beam. The exposure time is set at 40 s. Curves 1 to 3 in Fig. 4 represent the diffraction intensities of the He-Ne laser beam from the BCT PC, which is subjected to the OFF-ONG-ONB-OFF, OFF-ONB-ONG-OFF, and OFF-ONB-ONB + G-OFF laser pumping process. As mentioned above, the intensity of the diffraction beam depends on the cis isomer concentration shown in Eq. (2). The intensity weakens when the cis isomer concentration increases upon exposure to either green or blue laser beams. However, the wavelength of the pumping blue laser is close to the absorption band that generates a higher cis isomer concentration than that of the green laser. The high cis isomer concentration induces the transition of the LCs in the voids into an isotropic state. The diffraction intensity is relatively low compared with that pumped by the green laser. As the first pumping green laser is replaced by the blue laser, the diffraction intensity rapidly drops (curve 1). Similarly, the intensity increases as the pumping laser beam sequence is inverted (curve 2). The diffraction intensity rapidly decreases and increases when the blue pumping laser beam is replaced by the green one. However, a comparison of the OFF-ONG and ONB-ONG paths shows that the ONG levels are different, with the ONG level of the latter requiring a few seconds to recover to the former. This phenomenon becomes apparent when the exposure time to the blue laser exceeds several minutes. Thus, the thermal effect may be affecting the order parameter of the LCs. The newly created level is constant until the heat dissipates or even when no external disturbance exists. When the green laser beam is added to the pumping blue laser, the diffraction light intensity increases (curve 3). This result is expected because the green laser not only increases the trans isomer concentration but also promotes the organization of the host LCs by the newly transformed trans isomers. In other words, the trans isomer produced under the green laser beam reconverts the LC structure to the initial configuration. This change is temporary and disappears as soon as the green pumping laser is off.
Figure 5 shows the curves of the AC voltage applied to the shutter, the green laser light, and the diffraction light versus time. The switching times of the transition from ONB + G to ONB and back are calculated on the basis of the curves. The used shutter is a ferroelectric shutter (FLC) shutter (Newport, M50075) that is driven by a 50 Hz AC voltage in order to fast switch the green laser beam. The FLC shutter is ON (OFF) when the AC voltage is 5 V (−5 V). The plots of the green laser intensities from detector 3 are shown in Fig. 5 for reference. In the process, the sample pumps by a sequential ON/OFF period of the green laser, with the blue laser always ON. Hence, the cis isomer concentration is sufficiently high initially. The transition times of the trans-to-cis and cis-to-trans isomers are on the nanosecond scale. The trans isomer concentration generated by the pumping green laser beam immediately affects the order parameter of LCs. The ONB-ONB + G switching times are estimated at ~600 μs, whereas those of ONB + G-ONB are estimated as ~800 μs. Thus, ONB-ONB + G is slightly slower than ONB + G-ONB. When the green laser beam is switched off, the time of isomer transformation from trans to cis may be on the same order as that from cis to trans when the green laser beam is opened. However, LC reconstruction in the system is faster than LC disruption because the LC phase transition from isotropic to nematic is generally faster than that from nematic to isotropic.
For convenience, the intensities of the diffraction beams in these curves can be classified into four levels, namely, level 4 (OFF), level 3 (ONG), level 2 (ONB + G), and level 1 (ONB), according to the exposure conditions. Figure 6 shows the level upsteps and downsteps by combinations of exposures to the two lasers. One of the curves represents a level up that corresponds to the raising of level 1 to level 4 under ONB-ONB + G-ONG-OFF exposure. The other curve represents a level down that corresponds to the decrease from level 4 to 1 under OFF-ONG-ONB + G-ONB exposure. The contrast of levels 4, 3, and 2 are 7.5, 5–6, and 2, respectively, as compared to that of level 1. As seen from the curves, the ONG level is sensitive to the thermal effect during laser optical pumping. Hence, the immediate contrast of the ONG level depends on the path used to reach this level.
Table 1 gives the summarized switching time between the four levels. The switching times of the levels are affected by the guest dye, the governing factors (i.e., quantum efficiency, absorption cross-section, and rate constant of the thermal cis-trans back-relaxation), and the thermal effect. The thermal dissipation and phase transition of LCs apparently inhibit the switching. Thus, the LCs in the ONB-OFF process has a delayed relaxation time during phase transition (curve 1 of Fig. 6). The thermal effect slows down the ONB + G-ONG switching (curve 2 of Fig. 6). Therefore, the ONB-OFF and ONB + G-ONG level transitions that are affected are generally avoided in applications because of their relatively slow switching time. Given the long distance of the trans isomer absorption band from the green laser wavelength, trans isomer production is not efficient. Thus, the overall LC distribution is difficult to change, which results in a relatively slow OFF-ONG switching time. Therefore, the switching times for OFF-ONG, ONB- OFF, and ONB + G-ONG are on the order of tens of seconds. The ONB-OFF switching time can be improved by exposure to the green laser because ONB-ONG transition is faster than that of ONB-OFF. Here, a small amount of trans isomers produced under green laser exposure significantly promotes the recovery of the LC order. In addition, the switching from level 1 (ONB) to level 4 (OFF) in applications can be significantly improved by a short-pulse exposure to green laser. The level recovery from ONB + G to ONG is relatively slow because the ONB + G level contains some trans isomers. The newly produced trans isomers also do not exert a strong effect. The back processes to levels ONG and OFF are long because LC relaxation from a disordered state to the normal state is required.
In conclusion, a four-level optical switching is established using two laser beams that are based on an azo dye-doped HPDLC film. The switching time between the ONB and ONB + G levels is short. Since the electrical switching of nano-sized HPDLCs requires a sufficiently high voltage (~10 V/μm), and is difficult to switch off completely, the optical switching provides us a better switching in these respects. Furthermore, the pumping laser wavelength and intensities can be optimized to increase the contrast ratio of ONB to ONB + G. The CR of the ONB + G-ONG level can be increased by increasing the green laser intensity. The optical switching of PCs based on HPDLC has high potential in electro-optical applications.
The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Grant No. NSC 98-2112-M-006-001-MY3 and NSC 101-2112-M-006-011-MY3. Additionally, this work is partially supported by Advanced Optoelectronic Technology Center as well.
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