Optical Limiting Studies on Chalcone Doped PMMA Polymer Film

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ScienceDirect Materials Today: Proceedings 3 (2016) 2163–2168

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Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)

Optical Limiting Studies on Chalcone Doped PMMA Polymer Film T. Chandra Shekhara Shetty*, S. Raghavendra, S.M.Dharmaprakash Department of Studies in Physics, Mangalore University, Mangalagangotri- 574199

Abstract An organic nonlinear optical (NLO) material (2E)-1-(3-chlorophenyl)-3-(3,4-dimethoxyphenyl)prop-2-en-1-one (CPDP) with molecular formula C17H15ClO3has been synthesized and crystallised in methanol solution. The crystals of CPDP were characterised by FTIR spectral technique. The various functional groups present in the crystal are identified. The thermal characteristics of the crystal were determined from the thermo gravimetric analysis (TGA) and differential scanning calorimetry(DSC) technique. CPDP crystals are thermally stable up to 115oC.Thin films of poly(methylmethacrylate) (PMMA) doped with CPDP were prepared on a clean corning glass substrate using a spin coater unit. N, N-dimethylformamide (DMF) is used as solvent. The surface characteristics of the film is studied using scanning electron microscope (SEM).Direct and indirect band gap energy of CPDP doped PMMA is determined using UV–Vis spectral response in the wavelength range 200-1100nm. Single beam Z-scan technique is used to study the nonlinear optical properties like nonlinear absorption coefficient and optical power limiting of the film using a nanosecond Nd-YAG laser operating at 532nm. The results of optical limiting studies show that the film possesses reverse saturable absorption (RSA) due to excited state absorption. The nonlinear optical properties of CPDP have been retained in the presence of PMMA making it a good candidate for optical power limiting applications. © 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords: Nonlinear; Z-scan; Spin coating; PMMA (Poly(methylmethacrylate)); Optical limiting

* Corresponding author. Tel.: +91 9448249153. E-mail address:[email protected] 2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].

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1. Introduction Organic molecules exhibiting strong two photon absorption (TPA) are of great demand because of their practical importance in applications such as three dimensional fluorescence imaging, eye and sensor protection, frequency up lasing, multi photon microscopy and optical signal reshaping etc. [1–5]. The strength of electron donor/acceptor groups, molecular planarity, conjugation length and molecular symmetry are the parameters which affect the TPA cross section[6].The nonlinear optical response of organic molecules can be enhanced by adoptingsuitable design strategies, such as donor–p–donor (D–p–D), donor- acceptor–donor(D–A–D), donor–p– acceptor (D–p–A)and acceptor–donor–acceptor (A–D–A) [7,8].Among the organic materials, chalcones are the promising materials because of their third-order nonlinearity and good optical power limitingproperty[9]. Organic molecules may get degraded or bleached, when they are exposed to intense laser, hence they cannot be used directly inphotonics device applications. To overcome this drawback and to enhance their physical stabilities, a suitable alternative is to dope them intoa polymer matrix. Studies by X.T. Tao et.al.showed that these polymer doped materials retain NLO property and linear optical transmittance [10].Since the material has melting point near to the glass transition temperature (125°C) of PMMA, it can be embedded in PMMA. This paper describes the preparation and NLO properties of CPDP doped PMMA doped films. 2. Experimental: 2.1 Synthesis of CPDP (2E)-1-(3-chlorophenyl)-3-(3,4-dimethoxyphenyl)prop-2-en-1-one (CPDP) was synthesized using standard procedure used in literature[11].Commercially available 3-Chloroacetophenone and 3’-4’-dimethoxy benzaldehyde are purchased from Sigma Aldrich are used directly inthe reaction without further purification. Synthesised compound was purified by re-crystallization using methanol as solvent. The preparation scheme is shown in Fig.1. O O

O

CH3

H

OCH 3 CH 3 OH NaOH

+ OCH 3

Cl OCH 3

OCH 3 Cl (2E)-1-(3-chlorophenyl)-3-(3,4-di methoxyphenyl)prop-2-en-1-one

Fig. 1 Synthesis Scheme of CPDP

2.2 Preparation of CPDP doped PMMA film CPDP doped PMMA films of known concentrations are prepared at ambient temperature. A known quantity of PMMA mixed with dimethylformamide (DMF) stirred well using a magnetic stirrer for 15 hours to form a solution of concentration 10-4mol/litre. Required quantity of CPDP wasalso dissolved separately in DMF and stirred for two hours. Later both the solutions were mixed and stirred for 10 hours using a magnetic stirrer to form homogeneous solutions of different dopant concentrations. The solution is coated on a corning glass plate at a rotation speed of 1000rpm. The mixed solutions were used to prepare films using a spin coater unit. The films were annealed for 3 hours at a temperature of 75°C. A single beam Z-scan technique [12]was used to determine the nonlinear absorption and optical limiting characteristics of the film. 3. Characterization 3.1. U.V. Analysis Optical absorption of CPDP and CPDP doped PMMA were measured using Shimadzu 2550 UV-VIS spectrophotometer. There is small shift in the absorption edge of the material and the transparency in the visible

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region of the spectrum is maintained even after doping. To determine direct and indirect transition band gaps Tauc’s plots are drawn[13]. The Tauc’s plots are shown in the fig 2a and 2b for pure and CPDP doped with PMMA. The direct band gap is obtained by plotting (αhγ)1/2 versus energy(eV) and indirect band gap is obtained by plotting (αhγ)2 verses energy (eV). The values of direct band gap for CPDP are found to be 2.60eV and 2.64eV with PMMA and without PMMA respectively. In the similar manner from the plot in fig.2b, indirect band gap energy of CPDP are calculated as 2.78 eVand 2.82 eV with PMMA and without PMMA respectively.

Fig 2a.Tauc plot for direct transition energy gap

Fig 2b. Tauc plot for indirect transition energy gap

3.2 FTIR Analysis

The Fourier transform infrared spectrum of CPDP was recorded using KBr pellet technique [14]. The FTIR spectra are obtained using IR Prestige-21 Shimadzu FTIR spectrophotometer. The FTIR spectrum is shown in fig.3. The characteristic transmission peaks are consistent with the functional groups present in the material. The assigned values of the absorptions peaks are presented in Table 1 100

80 70

3075 2987 2823

30 3000

2500

2000

1500

1023 841 790

1144

40

1265

50

588

1660

60

1574 1518

Transmittance(%)

90

1000

-1 Wave number (cm )

500

Fig. 3. FTIR spectrum of CPDP

3.3 Thermal studies To investigate the thermal stability [15] of CPDP,thermo gravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) was carried out. Powdered sample of thecrystal was selected for this purpose and the analysis

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was carried outunder the nitrogen atmosphere at a heating rate of 5°C,/min using SDTQ Simultaneous TGA/DSC analyser. The TGA/DSC thermogram corresponding to CPDP is shown in Fig. 4. The DTA curve implies that the material undergoes an irreversible endothermic transition at 114 °C, where melting begins. The peak of the endothermic, which represents the temperature 155 °C at which melting terminates, is corresponding to its melting point. It is clear that there is no phase transition before melting. The sharpness of the peak shows good crystallinity and purity of the sample. The TG curve of this sample indicates that the sample is stable up to 300°C. The exothermic peak of the DTA corresponding to the first phase of the weight loss in the TG curve, indicating that the weight loss is due the degradation of the sample itself. 3.4 Surface morphology studies In order to study the surface morphology of the CPDP doped PMMA films, Scanning Electron Microscope (SEM) images were taken using a Hitachi TM3030 table top SEM with a magnification of X5K. The image shows the uniform distribution of the atoms of the material in the film. Table1. Functional group assignment of CPDP Wave number(cm-1) 3075 2987 2823 1660 1574 1518

Mode of assignment C–H aromatic stretch C–H stretch Aliphatic C-H stretch C=O stretch Aromatic C=C Aromatic C=C

Fig.4. DSC/TGA thermogram of CPDP

Wave number(cm-1) 1265 1144 1023 841 790

Mode of assignment C–O stretch C–H wag C–O stretch =C–H bend C–Cl stretch

Fig.5. SEM image of CPDP doped polymer film with a magnification of X5K.

3.5 Non linear optical studies Using research grade DMF the liquid sample was prepared for Z-scan experiment. About 6 wt% of CPDP have been used as dopant in PMMA . The concentration of the liquid sample is 0.9x10-4mol/L. The liquid sample was taken in 1mm thick quartz cell. An Nd-YAG laser with532 nm laser pulses of 9 ns duration is used for the experiment. The output beam intensity has Gaussian nature and was focussed using a convex lens of focal length 21.5cm.The sample in the quartz cell was moved along z axis with the help of computer controlled stepper motor and each 250μm step. Normalized transmittance data was collected using RJP 735 pyroelectric detector. The experimental data was fitted with standard equation given in literature [12].The open aperture Z-scan graph is shown in fig.6. The nonlinear absorption coefficient, β is dependent on the number of absorptive centers in a unit volume

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[16] and which is given by β=σ2N0= σ2NAd×10-3 where N0 is the molecular density of the sample (in units of cm-3), σ2 is the molecular TPA cross-section (or coefficient) of the compound in units of cm4/GW, d is the concentration of the compound in the solution in units of mol/L, and NA is the Avogadro number. The value of σ2 can be calculated from the known value of β and d.The value of molecular TPA cross-section σ´2 in units of cm4 s and incident photon energy (in Joules) is also reported in the literature [17]. The calculated value of β , σ2 and σ´2 for the doped film is presented in the table 2. Table2: Nonlinear absorption parameters of CPDP doped PMMA film.

β (cm/GW)

σ2 (cm4 /GW)

2.3

4.2×10-19.

σ´2 (cm4 S/photon) 1.5×10-46.

The optical limiting study was performed by keeping the sample at the focus in 1 mm thick quartz cell.By increasing the input laser pulse steadily the output energy from the quartz cell was recorded. Fig.7 shows the optical power limiting behaviour of CPDP in PMMA host. It was observed that the output energy increases linearly for an input energy less than 300μJ/pulse and then it stabilizes to give a constant out energy for the input energy greater than 350μJ/pulse. The variation in the limiting threshold energy isattributed to the variation in conjugation length and also due to the presence of acceptor/ donor end groups [18-19]. However the nonlinearity is predominantly electronic in origin in the case of picosecond laser pulses [20]. Since the nanosecond laser are used in the experiment third- order nonlinear response is attributed to the delocalisation of the electron and the power limiting is due to the two photon absorption mechanism [21, 22]. The nonlinear process with strong fluence dependence indicates the participation of excitedstatesin thenonlinearprocess.

Fig.6: Nonlinear absorption of the PMMA doped CPDP

Fig.7 Optical limiting characteristics of CPDP-PMMA film

4. Conclusion A third order nonlinear organic material is synthesized and the functional groups are confirmed using FTIR. TGA/DSC characteristic shows that the material is stable to quite high temperature a 300°C with a melting point of 114 °C. The material is blended with PMMA and spin coated films were studied using SEM. The linear optical properties are determined using UV studies. The third order nonlinearity is determined using Z-scan technique. The material shows optical limiting characteristics with RSA property. Acknowledgements The authors gratefully acknowledge Coordinator, DST FIST and UGC SAP, Department of Physics, Mangalore University for providing facilities for the characterization of thin films and technical support to carry out the work.

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