This study compares optical switching capabilities of liquid crystal (LC) materials based on different classes of azobenzene dyes. LCs based on molecules containing benzene rings with nearly symmetrical π-π conjugation respond more efficiently to a cw beam than to a nanosecond laser pulse and maintain the changes induced by the beam for tens of hours. Using azo dye molecules containing two benzene rings with push-pull π-π conjugation we demonstrate high photosensitivity to both a cw beam as well as nanosecond laser pulse with only 1 s relaxation of light-induced changes in material properties. Even faster, 1 ms restoration time is obtained for azo dye molecules containing hetaryl (benzothiazole) ring with enhanced push-pull π-π conjugation. These materials respond most efficiently to pulsed excitation while discriminating cw radiation.
© 2010 OSA
All-optical devices rely on materials that change their optical properties under the influence of light. A substantial portion of molecules in the material is transformed into an excited state in this process. The large number of photons required for this transformation is supplied by a high-power laser pulse of a nanosecond or shorter duration. Low-power continuous wave (CW) beams can provide a sufficient number of photons as well, but within longer time periods. The optical response time τ of a material therefore is typically inverse proportional to the intensity I of acting radiation, determined by the quantum efficiency q of photoexcitation and the absorption constant α, τ ~1/αqI.
Consider now a material layer subject to a laser pulse. The thickness of the layer L shall be at least of the order of wavelength of the light, L ~λ, to impact the beam propagation. The number of photons acting on this layer at any given time is determined by the ratio of the layer thickness to the pulse length, L/cτp, where c is the speed of light and τp is the pulse duration. This ratio, L/cτp ~λ/cτp ~1/ωτp (with ω being the optical frequency), is very small in all practical situations. A nanosecond pulse, for example, stretches in space for nearly 30 cm, and only a 10−6th fraction of the available photons act on the material at any given time.
Thus the concentration of photoexcited molecules Nc (concentration of cis isomers of azobenzene in this study), hence, the change in the refractive index of the material induced by a light beam, is determined by the lifetime τc of the photoexcited molecules. In a material with slow relaxation of the photoexcited state, the number of photoexcited molecules is growing during the pulse thus accumulating the effect of the photons available in a beam and maximizing the change of optical properties of the material. In contrast, the optical nonlinearity of materials with practically instantaneous response, such as those characterized by the electronic mechanism of optical nonlinearity, are many orders of magnitude smaller .
The photoisomerization of azobenzene molecules is nearly an ideal mechanism for all-optical processes: it is accumulative in nature due to relatively large lifetime of cis-isomers, and it is fast due to high quantum efficiency and absorption constants. Photoisomerization takes place in picosecond time scales with quantum efficiency ~0.5 [2–4]. The lifetime of the photoexcited state of molecules containing azobenzene moieties can be varied from milliseconds to many hours, and even days, by changing the strength of the donor and acceptor at the ends of the molecule [5,6]. Thus the lifetime of the photoexcited state of azobenzene materials can be large enough to ensure an accumulative effect even for CW laser beams. Finally, azobenzene can form the core of mesogenic molecules yielding materials that blend a photosensitivity typical of semiconductors with the unmatched optical modulating capability of liquid crystals (LCs) [7–10].
In the present paper we demonstrate the opportunity provided by different classes of azobenzene based LC material systems (azo LCs) to efficiently respond to single nanosecond laser pulse, CW beams, or both at 532 nm wavelength of radiation. Particularly, we present materials with response that discriminates between a short laser pulse and a much longer CW beam of the same energy. This is achieved when the lifetime of the photoexcited state of the molecules is longer than the pulse duration, but is short compared to the time required for accumulation of a high concentration of photoexcited molecules in the CW beam.
2. Experiment and results
The first class of materials under study is based on molecules containing benzene rings with nearly symmetrical π-π conjugation: 4-butyl-4’-alkoxyazobenzenes (BA series materials) [Fig. 1(a) ] [7,9]. The mixture of these materials (BA 1005, in the current study) possesses LC phases at room temperature. The azo LC BA-1005 is nematic between 12.5°C and 49°C. These LCs exhibit strong π−π* absorption in the ultraviolet (λmax = 345 nm) related to absorption of trans isomers, and a lower-intensity n-π* absorption band in the visible spectrum of wavelengths (λ = 446 nm) [Fig. 1(d)]. The lifetime of cis isomers in these azo LCs is tens of hours which makes them highly sensitive to CW green laser beams of 532 nm wavelength as well [11–14]. The efficiency of their response to nanosecond laser pulses, while stronger than most nonlinear optical materials, is somewhat reduced, however, by the low absorption at that wavelength .
The concentration of cis isomers in these materials will continuously grow in the ambient light due to the presence of UV components. Thin layers of such materials, particularly those with relatively low clearing temperature, are even transformed into an isotropic state when exposed to ambient light for extended time periods. The long restoration time of their LC state is a substantial drawback for some of their applications.
Optimization of azo LCs for nonlinear optical response to short laser pulses of visible, and particularly, of green wavelengths, requires developing materials with enhanced absorption in visible spectrum and shorter lifetimes of cis-isomers. Quite fortunately, these two features prove to be not only compatible, but fundamentally coupled with each other. We synthesized a series of azobenzene materials containing donor-acceptor pendants for shifting the absorption spectrum of π-π conjugation towards visible wavelengths . Earlier, a similar approach was used for the purpose of reducing the lifetime of cis-isomers .
Figure 1 shows the structures and spectra of azo dye molecules containing two benzene rings with donor-acceptor, or push-pull, π-π conjugation [CP series, Fig. 1(b)] and azo dye molecules containing a hetaryl (benzothiazole) ring with enhanced push-pull π-π conjugation [NB series, Fig. 1(c)]. The synthesis of CP series azo dyes was described earlier in Ref. . The dyes of NB series were synthesized by coupling of the diazonium salts of 2-amino-6-nitrobenzothiazole with N-alkyl-N’-phenylpiperazines or 4-alkoxy-1-phenylpiperidines. N-alkyl-n’-piperazine intermediates were obtained by the reaction of iodobenzene with N-alkylpiperazines in accordance with the general method proposed in Ref. .
The dyes were solved in the nematic liquid crystal (NLC) 4-pentyl-4’-cyanobiphenyl (5CB). The clearing temperature of the host NLC 5CB (Merck), 4-pentyl-4’-cyanobiphenyl, is Tcl = 35°C and is increased by 4.5 °C when doped with 10 wt.% CPND8 and by 8°C when doped with 5 wt.% NB6CBZ.
The absorption coefficient at 532 nm is 320 cm−1 for BA1005 while it is as high as 5360 cm−1 for CPND8(10%)/5CB and 6670 cm−1 for NB6CBZ(5%)/5CB. The peak absorption wavelength of compositions based on the CP series of materials is at 471 nm, whereas the absorption peak at 539 nm for the NB series of materials practically matches the wavelength of the pump beam of interest. All absorption constants reported here are measured for unpolarized beam.
The tests described below show that the lifetime of cis isomers of the CP series of azo dyes is on the order of 1 s, and they are highly responsive both to nanosecond laser pulses, as well as to CW laser beams of 532 nm wavelength. The lifetime of cis isomers of the NB series of azo dyes is on the order of 1 ms, and they prove to be unaffected by CW beams while exhibiting strong response to a nanosecond pulse.
Even though the CP and NB series azo dyes do not possess a LC phase at room temperature and they are used dissolved in LCs at relatively small concentration, their response to short laser pulses has proven to be considerably faster and stronger than that of the BA series of materials .
The test setup shown in Fig. 2(a) consists of pump and probe beam subsets. The probe beam subset uses a diode laser of 635 nm wavelength propagating in the (x,z) plane along the normal to the test cells with planar oriented LC between crossed polarizers. All test LCs were oriented with their optical axis along the y-axis of the coordinate system which makes a 45 degree angle with respect to the polarizer/analyzer axes in the probe beam subset. The pump beam could be alternated between a nanosecond laser pulse provided by the second harmonic of a Nd:YAG laser, and a CW beam of the same wavelength (λ = 532 nm) from a diode-pumped solid state laser. The pump beams, propagating along the z-axis, hence incident at an angle to the LC cells, were polarized along the y-axis, parallel to the orientation of the optical axis of the test LCs. The beam diameter on the LC cells was 1.2 mm for the pulsed laser and 1.5 mm for the CW laser beam. The transmission dynamics for the probe beam were studied for pump pulses of 7.5 ns duration and 1.6 mJ energy (corresponding to 140 mJ/cm2 energy density) and for the power density of the CW pump laser beam equal to 110 mW/cm2.
Figure 2(b)-1 demonstrates the effect of a single laser pulse on the transmission of the probe laser beam for azo LC BA-1005. The energy density in the pulse is insufficient for driving this material into the isotropic state, however, it decreases the order parameter of the material causing a decrease in transmission of the probe beam. Low absorption at 532 nm wavelength results in a relatively slow response of 12 ns. The CW beam induces a nematic-isotropic phase transition within 5 s, Fig. 2(c)-1. This corresponds to an energy density of 550 mJ/cm2. The low transmission state, corresponding to the photoinduced isotropic phase of the LC, persists for 30 hours.
The behavior of LCs containing the CP series of azo dyes, CPND8(10 wt.%) in NLC 5CB, is shown in Fig. 2(b)-2 and Fig. 2(c)-2. This material shows high sensitivity to both pulsed and CW beams. The restoration time to the high transmission state is only 2 s making such materials practical for applications. The switching time to the low transmission state is 3.1 ns for the pulsed beam [Fig. 2(d)] and 420 ms for the CW beam corresponding to an energy density of 46 mJ/cm2.
The restoration time is reduced by another three orders of magnitude (~1 ms) when the NLC 5CB is doped with 5 wt.% of the NB series azo dye NB6CBZ [Fig. 2(b)-3 and Fig. 2(c)-3]. This material is most efficient for pulsed excitation with a response time of only 2.8 ns, while it remains unaffected when subjected to the CW green laser beam.
Figure 3(a) demonstrates output vs. input pulse energy density dependence for the azo LC cells under study, all of them planar oriented and 1.6 μm in thickness. The early stage of nonlinear transmission in these curves is well fit with the function Eout = EsatEin/(Eth + Ein), where Ein, Eout and Esat are input and output pulse energy, and energy at saturation level, correspondingly; Eth is the energy characterizing the threshold for nonlinear transmission. This threshold is 3.5 J/cm2 (39 mJ) for azo LC BA-1005, 0.25 J/cm2 (2.8 mJ) for CPND8/5CB and 0.32 J/cm2 (3.6 mJ) for NB6CBZ/5CB. Note that the decrease in the transmission of the system is noticeable starting at nearly one order of magnitude smaller energy values: 350 mJ/cm2 for BA-1005 and approximately 50 mJ/cm2 for materials containing CPND8 and NB6CBZ. Figure 3(b) demonstrates the inverse proportional dependence of the response time of the materials on the energy of the nanosecond pulses. The response time for CPND8/5CB, as an example, decreases from approximately 90 ns to 2.8 ns when the pulse energy increases from 0.3 mJ (26.5 mJ/cm2) to 6 mJ (530 mJ/cm2).
Output vs. input power density dependence for a CW green laser beam is shown in Fig. 3(c) for the two azo materials that are responsive to CW irradiation. The data correspond to the steady-state of the transmission dynamics curves. The input power density threshold for nonlinear transmission is 376 mW/cm2 (6.6 mW) for BA 1005 and 42 mW/cm2 (0.74 mW) for CPND8/5CB. The response time is approximately 300 s for BA 1005 and 5.7 s for CPND8/5CB at 16 mW/cm2. The observed response time is 30 s for BA 1005 and 1.3 s for CPND8/5CB at a high power density level of ~1 W/cm2.
Note that the LC or mesogenic nature of the azo dyes discussed in this paper is essential for obtaining the large and fast optical nonlinearity. On one hand, large chromophore content yields high photosensitivity and the opportunity for large changes in the optical properties of the material up to the full isothermal nematic-isotropic phase transition [7,8]. On the other hand, as dopants, they do not affect the order parameter of the LC material even at large concentrations. Non-mesogenic azo dyes such as Methyl Red have been extensively used and are still used for enhancing the optical nonlinearity of LC material systems, however, apart from the disadvantages discussed above, they may also result in irreversible processes due to their molecular structure that is not congruent to typical LC molecules [18–20].
The class of materials capable of undergoing optical switching processes in short time scales is rather limited, particularly, when high photosensitivity is an issue [21–23]. We have demonstrated that LC materials based on azobenzene molecules offer the best opportunities by combining fast response times with strong changes in their optical properties and high photosensitivity. The key to the high sensitivity of these materials is their high absorption, high quantum efficiency, the accumulative nature of the photoisomerization process, and the strong impact it has on the optical anisotropy of the LC material. Three different classes of materials were synthesized and tested to demonstrate the feasibility of customizing their ability to respond to CW as well as short laser pulses, or to exhibit a discriminative response to CW or short laser pulses due to the variation of the recovery time of their light-induced state from many hours to 1 ms. Fortunately, from a practical standpoint, the materials with fast relaxation also exhibit enhanced sensitivity to visible wavelengths. Of particular importance is using the new classes of materials discussed here for imparting optical switching capabilities onto diffractive components such as holographic polymer dispersed liquid crystals (HPDLCs) , Polymer-Liquid-Crystal-Polymer (POLICRYPs) systems [25,26], and diffractive waveplates . Their big advantage compared to azobenzene LCs used earlier for that purpose was recently demonstrated for POLICRYPs  using azo dyes of CP series.
This document has been approved for public release. US Army Natick Soldier RD&E Center PAO #: U10-698.
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