The nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer were investigated using single beam Z-scan and optical Kerr effect (OKE) techniques. The nonlinear refractive index of electronic origin (3.1×10−19 m2/W) and the nonlinear absorption coefficient (3.63×10−13 m/W) were determined with 800 nm femtosecond laser pulses at a repetition rate of 1 KHz. The corresponding one-photon and two-photon figures of merit are determined to be 6.05 and 0.94, respectively, at irradiance of 50 GW/cm2. The response time of the observed nonlinearities is estimated to be as fast as 300 fs. These experiment results demonstrate that the polymer is a promising candidate for applications in all-optical switching modulators and nonlinear photonic devices.
© 2012 OSA
At present, intensive research has been carried out for the development of versatile materials with large optical nonlinearities and fast response because of their potential applications such as all-optical switching, optical storage and nonlinear optical devices [1–5]. Among the numerous materials investigated, photoresponsive polymers containing azobenzene chromophores, as a promising candidate, have intrigued considerable interest for their relatively large nonlinearity [6–8]. Although many studies have been performed to investigate the resonant third-order nonlinearity of the azo materials [9–11], they are generally slow and associated with large absorption. For practical use in the ultrafast all-optical switching devices, two figures of merit have to be satisfied: and , where n2 is the nonlinear refractive index, α0 is the linear absorption coefficient, β is the two-photon absorption coefficient, λ is the wavelength, and I is the light irradiance . From this view, research attention has been paid on the nonlinear response in off-resonant region because it brings weak linear absorption and signal loss compared to the conventional resonant excitation. Furthermore, ultrafast response speed is also required for the nonlinear processes since the on-off time of a device depends on the switched time of the nonlinear change in the refractive index.
Previous studies have found that the figures of merit are far away from the target values for Sudan I ethanol solution . To improve the figures of merit, various design strategies have successfully been used to synthesize azo materials with large two-photon absorption cross-section. Azo-containing ionic liquid crystal is becoming one of the primary frontier of research, which combines the peculiar properties of azo dyes, liquid crystal and ionic liquid . The resonant nonlinear response of them have been widely studied with CW or long pulse lasers as excitation source [15, 16]. However, there was no report on the nonlinear response under femtosecond off-resonant excitation in azo-containing ionic liquid crystal materials as far as we know.
In this paper, we present our experimental investigation into the nonlinear response of a newly synthesized azobenzene-containing ionic liquid crystalline polymer in the off-resonant region. Our results show that the polymer possesses a response time of 300 fs with and , indicating its potential applications for optical switching. The influence of thermal effect on the sign of the nonlinear refractive index at high pulse repetition rates is also discussed.
2. Experimental details
The molecular structure of azobenzene-containing ionic liquid crystalline polymer used in the work is shown in Fig. 1(a) . The sample is a supramolecular material prepared by the ionic self-assembly. For the preparation of ionic self-assembly complex, 5 mg/mL sodium polyacrylate (PANa) aqueous solution, obtained from the neutralization of polyacrylate with sodium hydroxide, was added dropwise to NDAZO (3-(6-(4-((4-(dimethylamino)phenyl)diazenyl)phenoxy)hexyl)-1-methyl-1H-imidazol-3-ium bromide) aqueous solution with the concentration of 1 mg/mL, i.e. in a 1:1 molar charge ratio. The precipitated complex was filtrated and washed several times with deionized water to remove residual salts and possible noncomplexed precursors and then dried in vacuum at 60 °C for 24 h. Differential scanning calorimetry (DSC) thermogram of the polymer displayed phase transition peak at 93°C, indicating its crystalline nature. The linear absorption spectrum of the polymer solution in chloroform with a concentration of 2 × 10−3 M is shown in Fig. 1(b). The maximum absorbance is located at 395 nm.
The nonlinear optical measurements were performed using the single-beam Z-scan technique, which is a simple and sensitive method to determine the nonlinear refraction index and nonlinear absorption coefficient. The experimental set-up in our experiment was similar to that reported in reference . All the studies were performed with solution concentrations of 4 × 10−3 M. Our femtosecond experiment setup involves a standard Ti:Sapphire system comprising an oscillator (Coherent, model Mira 900F) and an amplifier (Coherent, model Legend). The model 900 F Mira Ti:sapphire (TiS) oscillator is tunable from 730 to 900 nm and its repetition rate is 80 MHz. We have used 800 nm wavelength, where pulse width mostly remains around 100 fs. We optimize our Ti:Sapphire amplified (TSA) to operate at 800 nm with 100 fs pulse width and at 1 KHz repetition rate. For the Z-scan studies, a 10 cm focal length lens was used to focus the laser pulses into the 1 mm long sample cell. The sample was mounted on a translation stage driven by a computer controlled stepper motor to move along the Z axis around the focal point. A 2-mm-diameter aperture was placed on the Z axis in the far field and a photodiode detector was put behind to record the transmission as a function of the sample position (closed aperture Z-scan). Before dealing with the sample, the Z-scan experiment on CS2 with the thickness of 1 mm was first done with the amplified kilohertz laser (TSA) at 800 nm for testing and calibration. Through measurement and calculation, the third-order nonlinear refractive index of CS2 was evaluated to be 2.1 × 10−19 m2/W, which was in good agreement with previous measurements at this range of pulse duration (2.5 × 10−19 m2/W , 3 × 10−19 m2/W ). So it might be sure that the experiment setup was working normally.
To determine the nonlinear optical response time of the polymer, a standard femtosecond optical Kerr-effect (OKE) experiment was performed. The femtosecond laser pulse was generated from a mode-locked Ti:Sapphire oscillator (Mai Tai HP-1020) generating 100 fs pulses at 800 nm. A regenerative amplifier system (Spectra-Physics, Spitfire Pro) was used to amplify the pulses 106 times at a repetition rate of 1 kHz. The beam was split into two parts with intensity ratio of 10:1 and the polarization of the probe beam was adjusted by 45⁰ to the pump. The two beams were focused by a 50 cm focal length lens and overlapped on the same spot of the sample with a spot size of 200 μm. The generated OKE signal was detected by a silicon photodiode connected to a lock-in amplifier.
3. Results and discussion
The first set of Z-scan experiments was carried out using laser TSA. By assuming a spatially and temporally Gaussian profile for input laser pulses, the normalized energy transmittance TOA under open aperture (OA) condition can be expressed as 
The Z-scan experiments were performed at input irradiance range from 50 to 100 GW/cm2, and strong reverse saturable absorption (RSA) was observed for all of the OA data. Figure 2(a) shows our OA Z-scan experimental data of the polymer solution excited with TSA at 50 GW/cm2. By applying the Eq. (1), the Z-scan experimental data can be fitted very well. The value of is evaluated to be m/W, and it remains constant with increasing irradiance as shown in Fig. 2(b), indicating that the mechanism of the nonlinear absorption is effective 2PA process.
The closed-aperture Z-scan experiments are also measured at different irradiances that range from 50 to 100 GW/cm2. In order to determine the nonlinear refraction contribution, the Z-scan experimental data in closed-aperture configuration are divided by those under the open aperture, which will remove away the influence of the nonlinear absorption on the nonlinear refraction. The valley-peak configuration as shown in Fig. 3(a) , indicates the positive nonlinearity of the sample, which results from self-focusing. The experimental data are fitted by the equation Figure 3(b) shows the relations of ΔTp-v and ΔZp-v/z0 as a function of incident intensity. The peak and valley are separated by approximately 1.7z0, and the difference between normalized peak and valley transmittance shows linear relation with no significant deviation from the intensity dependence. All the results indicate that the nonlinear effect is a third-order response. Similar measurements were performed with the solvent (chloroform) only, and no clear Z-scan curves were recorded for the pump power used. Therefore, the relatively weak contribution of the solvent was negligible.
There are some different processes contributed at short laser pulses to the overall nonlinear refractive index. It has been proven that linearly polarized light in Z-scan measurement will induce a refractive-index associated with the photoisomerization of azo materials, and this effect could occur even if the incident wavelength is not in the absorption band of the sample. In our experiments, no birefringence signal was observed when pumping at 800 nm. Moreover, Rosales et al. showed that the nonlinear refraction effect associated with photoisomerization was intensity dependent . As mentioned previously, the third-order optical nonlinearity obtained in this experiment was intensity independent. Thus, the effect of photoisomerization on the observed nonlinear response could be ignored. The nonlinear refraction was also not induced by laser heating effect because the sign of the nonlinear refractive index due to laser heating effect was negative, but the sign of nonlinear refractive index observed in this experiment was positive. Thus, we attribute the observed nonlinear refraction to the nonresonant electronic nonlinearity, which is due to the electronic response of molecules. A molecule underwent a transition from the ground state to the excitation after absorbing two photons. The dipole moment of the azo molecule changed during such a transition. This change would give birth to electronic nonlinearity.
To evaluate the material requirements for all-optical switching devices, we calculated the one-photon and two-photon figures of merit for the azobenzene-containing ionic liquid crystalline polymer. For λ = 800 nm, β = 3.63 × 10−13 m/W, n2 = 3.1 × 10−19 m2/W, I = 50 GW/cm2, and α = 0.32 cm−1, the figures of merit are calculated to be and , indicating that the polymer has great potential for optical switching applications.
For practical use, ultrafast response times are also required for the nonlinear processes involved. The nonlinear optical response time of the polymer solution was determined using OKE technique. The typical optical Kerr signal of the polymer solution at the concentration of 2 × 10−3 M is illustrated in Fig. 4 . The circles are the experimental results under a pump intensity of 60 GW/cm2. We also measured the OKE signal of chloroform to separate the influence of the solvent, and no detectable OKE signal was observed, indicating that the third-order susceptibility of the solvent is negligible. The OKE signal of the solution is nearly symmetric with a valley centered at zero delay point, and the full width at the half maximum of the Kerr signal is as fast as 300 fs, indicating that the optical response of the sample is very fast.
Next, in order to investigate the variations of the refractive index due to the thermal effect, we performed the Z-scan experiments using the high repetition rate TiS laser with the input irradiance range from 0.2 to 1.0 GW/cm2. In this case, the electronic nonlinear mechanism is suppressed and thermal effects are considered to be a major influence on the behavior of nonlinear optical properties. There is no significant peak or valley characteristic shape for the OA Z-scan data, which indicates that the nonlinear absorption is negligible. The CA Z-scan data show that the polymer solution possesses negative nonlinear refractive index at different input irradiances, as shown in Fig. 5 . By using the theory of the third-order optical nonlinearity to fit the experimental data, we obtained the nonlinear refractive index of −9.1 × 10−18 m2/W, which was about one order of magnitude larger than CS2 , and was comparable with the values of bulk CdS material .
Even though the influence of nonlinear absorption at high repetition rate is insignificant, our spectral measurements give the linear absorption coefficient of this polymer solution at 800 nm. Therefore, the observed thermal lensing effect is mainly caused by linear absorption in this medium. The thermal lensing effect generated by a 100 fs pulse can be accumulated across neighboring pulses because their associated pulse-to-pulse separation tp-p is considerably shorter than the millisecond (ms) order thermal diffusivity time constant, which is the time taken for the generated thermal lensing effect to disappear. Other groups have also observed the nonlinearity induced by the thermal lensing effect under femtosecond pulses excitation. Tang et.al. have reported thermal lensing effect in two transparent liquids . They attributed the negative lensing effect to the excitation of librations and/or vibrations, which overwhelm the simultaneously excited nonresonant skeletal motions. Ganeev et al. observed the thermal lensing effect in CS2 solution due to nonlinear absorption . Whatever the applications envisaged, fundamental investigation of laser heating induced nonlinearity will be valuable in better understanding the physics inherent of the nonlinear optical properties of azo materials. Moreover, thermal lens effect has been used as a spectrometric technique for the measurement of weak absorption.
We have investigated the third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer. The measurements have been conducted by using both Z-scan and OKE techniques with 800 nm, 100 fs laser pulses at a repetition rate of 1KHz. The observed electronic nonlinearities have a response time of 300 fs with the figures of merit: and . Such a good property may allow this polymer to be a good candidate of all-optical switching and storage for all optical processing, which require the large third-order refraction as well as short linear and nonlinear absorptions. In contrast to the positive electronic nonlinearities, negative thermal effect was investigated at high pulse repetition rates. This effect may play an important role in applications of spectrometric technique for measuring weak absorption and thermooptical properties.
This work was supported by the National Natural Science Foundation of China (No. 11174203) and the fund of State Key Laboratory of Advanced Optical Communication Systems and Networks.
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