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Surface Plasmon Assisted Enhancement in the Nonlinear Optical Properties of Phenothiazine by Gold Nanoparticle Shiju Edappadikkunnummal,† Siji Narendran Nherakkayyil,‡ Vasudevan Kuttippurath,† Divyasree Manathanathu Chalil,† Narayana Rao Desai,§ and Chandrasekharan Keloth*,† †

Laser and Nonlinear Optics Laboratory, Department of Physics, National Institute of Technology, Calicut, Kozhikode 673601, Kerala, India ‡ Department of Physics, C K G M Government College, Perambra, Kozhikode 673525, Kerala, India § School of Physics, University of Hyderabad, Gachibowli, Hyderabad 500046, India S Supporting Information *

ABSTRACT: We report the photophysical method for the synthesis of phenothiazine (PTZ)−gold (Au) nanocomposite (NC), by ablating a Au target in PTZ−dimethylformamide (DMF) solution using a Q-switched Nd:YAG laser delivering 7 ns pulses at 532 nm. The ablation of the Au target as well as the photoionization of PTZ was carried out simultaneously in the same medium with the same laser system. PTZ itself acts as a reducing and stabilizing agent during the formation of Au nanoparticles (NPs). The composite formation was confirmed from the Fourier transform infrared spectroscopy (FTIR) analysis and UV−visible absorption spectrum. The presence of NPs in the composite was evident from the absorption studies and transmission electron microscopy (TEM) analysis. A noticeable reduction in photoluminescence intensity was observed in the composite material, indicating the electron/energy transfer between the constituents. Nonlinear optical (NLO) studies have been done by employing the single beam Z-scan technique that uses 532 nm, 7 ns and 10 Hz laser pulses for excitation. A significant enhancement (∼67 times) in nonlinear optical absorption (NLA) was observed in the composite compared to the constituent moieties, and the reason behind the enhancement could be attributed to both the local field effect and electron/energy transfer. It is observed that the NLA mechanism for pure PTZ (two photon absorption (TPA)) differs from that of PTZ−Au NC (TPA assisted excited state absorption (ESA)). The self-defocusing nature of both the composite and pristine compounds was explored from the closed aperture Z-scan studies. The adopted strategy is found to be useful for designing novel materials with potential applications in photonics.

1. INTRODUCTION

candidates for numerous purposes like optical communications, high density optical data storage, photodynamic therapy for cancer treatment, and optical limiters for eye and instrument safety.4−9 The noble metal NPs exhibit absorption in the visible region due to surface plasmon resonance (SPR).7 SPR frequency is determined by the size and shape of the NPs and the dielectric constant of the embedded medium. The incorporation of NPs with an organic moiety or a polymer matrix was found to enhance both linear and nonlinear optical properties when compared to pristine compounds. Such metal NPs or metal NCs are of great interest due to their application in the improvisation of photonic devices, optical limiting devices, photoconductors, etc.10 The enhanced optical nonlinearity

There has been a rapid technological development in nonlinear optics (NLO) which demands the development of novel materials with excellent applications of various phenomena untangled by this inspiring field. For example, NLO effects are of use, as they manipulate laser beams to produce new optical frequencies, not on hand with existing lasers. Synthesis of such materials with large nonlinearity is an ever growing research field, and many methods are being developed on a regular basis. Investigation of NLO phenomena was started with inorganic materials, which guide the development of conventional NLO materials including KDP and LiNbO3. The considerable nonlinearity of inorganic semiconductors GaAs, ZnS, etc., was however counterpoises with their slow response time. Organic molecular and polymeric materials were then recognized as good NLO materials, as they are cheap and have rapid NLO responses, high damage threshold, structural extensibility, and ease of fabrication.1−3 They are designed as potential © 2017 American Chemical Society

Received: July 3, 2017 Revised: November 9, 2017 Published: November 9, 2017 26976

DOI: 10.1021/acs.jpcc.7b06528 J. Phys. Chem. C 2017, 121, 26976−26986

Article

The Journal of Physical Chemistry C

where PTZ*, PTZ•, PTZ•+, H•, and e ̅ are the excited state of PTZ, PTZ radical, radical cation of PTZ, hydrogen radical, and an electron, respectively. Reduction of Au+ initiated by the photo-oxidation of PTZ in the presence of the laser light and the reaction mechanism can be represented as in Figure 2. Rooted in this information and in

arises due to metal-to-ligand or ligand-to-metal charge transfer or energy transfer and SPR of the metal NPs. The major issues regarding the synthesis of NPs are related to the control of size, shape, and surface functionalization. The synthesis of gold NPs with an effective control over these parameters has been achieved by chemical reduction of Au ions in respective solutions.11−13 However, in this method, removing excess reagents like residual surfactants or ions and functionalizing Au NPs with various molecules were tedious tasks.14 Later, a new method of synthesizing Au NPs by ablating bulk metal in water was introduced.15 Compagnini et al. demonstrated the synthesis of Au NPs by pulsed laser ablation (PLA) in organic solvents such as alkanes, aliphatic alcohols, and polymer or sol− gel matrixes.16 PLA is environmentally friendly and does not require any toxic chemicals for the preparation of nanomaterials. This method is devoid of multistep synthesis procedures, longer reaction time, and high temperature and pressure. The size and nature of the NPs are decided by the laser parameters like wavelength, pulse width, frequency, and ablating medium.17 Here we synthesized Au NCs in PTZ, where PTZ is an organic dye with chemical formula S(C6H4)2NH (Figure 1). It

Figure 2. Schematic representation of photophysical reduction of Au ions by phenothiazine. Au3+ ions get reduced to Au by subsequent photoinduced electron transfer from phenothiazine.

continuation of our inquisitiveness in the synthesis and characterization of new metal NCs, we synthesized a new metal NC derived from PTZ and gold by the PLA technique to explore its linear and NLO properties.

Figure 1. Molecular structure of phenothiazine.

2. EXPERIMENTAL SECTION 2.1. Laser Ablation. In order to prepare Au NPs and PTZ−Au NC, we adopted an eco-friendly technique called PLA which involves plucking of materials from the target surface by a focused laser beam. During the ablation process, the metal surface absorbs much energy in a very short time and melts or gasifies instantly due to immense heat. Metal atoms or clusters simultaneously overcome the binding energy of the metal surface, forming a plume of NPs. Interaction of the plume with the solution results in a sharp expansion and momentary explosion in solution and finally solidifies into spherical NPs of regular shapes and uniform size distribution in the solution.29 PLA has many advantages over other synthesizing techniques: (i) In comparison to chemical aspects, it is a clean and simple synthesis due to the reduced derivative formation, absence of catalyst, simpler starting materials, etc. (ii) There is no need of extreme temperature or pressure for preparation of NPs. (iii) Researchers have the freedom to combine interesting target materials and suitable solvent matrix to fabricate complex NCs or nanostructures, which are extremely important from both a fundamental and technological point of view.30 In our studies, for the ablation process, we have used a Q switched Nd:YAG laser delivering 7 ns pulses at 532 nm wavelength with a repetition rate of 10 Hz. A schematic of the experimental setup of PLA and formation of NPs is shown in Figure 3. We prepared 2 wt % solution of PTZ (≥98%, Sigma-Aldrich) in spectroscopic grade DMF. The gold target (99.9%, 1 mm thickness) was taken in a beaker containing 10 mL of the prepared solution. The target was focused with the laser beam

belongs to a heterocyclic thiazine class of compounds and acts as a stabilizer or inhibitor in chemical reactions. It is an electron affluent heterocyclic compound because of the presence of nitrogen and sulfur atoms. Moreover, its ring has a butterfly conformation that would obstruct the molecular aggregation. These structural features have been of wide concern for the production of functional dyes useful for optoelectronics and nonlinear optical applications.18−21 PTZ is also an electron donor molecule with a low ionization potential. Zhu et al. reported the ability of PTZ to form stable radical cations by electron transfer on photo-oxidation.22 Under UV/vis light, it can easily reduce as well as stabilize Au NPs through interaction with nitrogen and sulfur coordinating sites present in it. Information is available on the capability of reducing and stabilizing Au NPs through feeble covalent and electrostatic interaction with nitrogen and sulfur atoms. Besides that, the planar radical cation produced after oxidization of PTZ has more superior aromatic resonance stabilization than that of PTZ and gives the stability to the NPs.23−26 In our studies, we have used the same laser source simultaneously for laser ablation and photoionization. The formation of stable radical cations of PTZ by photo-oxidation has been studied by many research groups.27,28 As a result of photoionization, PTZ dissociates to electrons and radical cations of PTZ or PTZ and hydrogen radicals as per the following equations hv

PTZ → PTZ* → PTZ•+ + e ̅ hv

PTZ → PTZ* → PTZ• + H• hv

PTZ → PTZ* → PTZ•+ + e ̅ → PTZ• + H•

(1) (2) (3) 26977


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Figure 3. Schematic diagram of pulsed laser ablation and nanoparticle formation.

position of the sample. Recorded data can be numerically fitted to the corresponding theoretical model, from which NLA parameters can easily be estimated. Nonlinear optical refraction (NLR) parameters are derived by closed aperture (CA) Z-scan analysis, in which a small aperture is kept in front of the signal detector. Measured transmittance through the aperture is susceptible to phase distortion and is influenced by both NLA and NLR. The pure NLR part can be extracted by dividing CA transmittance data by OA transmittance data (division method). Z-Scan measurements were conducted using a Q switched Nd:YAG laser (Quanta-Ray INDI-40) of 7 ns pulse width and 10 Hz repetition rate operating at 532 nm wavelength, as the excitation source. Using a beam splitter, the laser beam was split into two, one as the reference beam and the other sent to the sample through a convex lens of focal length 150 mm. The sample taken in a quartz cuvette of 1 mm optical path length was fixed on the computer controlled motorized translational stage, which ensures the precise movement of the sample over a distance of 20 mm on either side of the focus. An aperture of 4 mm diameter was kept in front of the signal detector for CA analysis. The beam waist at the focus was measured to be 17.56 μm, and the Rayleigh range of the laser beam was calculated to be 1.82 mm. We have chosen thin sample approximation for the analysis,31 as the sample thickness is very small when compared with the Rayleigh range. Both the reference beam energy and transmitted beam energy were measured using two identical pyroelectric detectors (RjP-735, Laser Probe. Corp., USA), and the ratio was taken using an energy ratio meter (Rj7620, Laser Probe Corp., USA) simultaneously. During each translation step, the laser pulses were fired into the sample using a single shot mode to minimize the cumulative thermal contributions.

through the liquid medium using a convex lens of focal length 50 mm. We ablated the target for 15 min with 20 mJ laser pulses. Subsequently, it was found that the color of the prepared solution gets changed from light yellow to wine red and that remained stable for a long time. We have also prepared Au NPs in pure DMF only for comparison studies under the identical experimental conditions. 2.2. Z-Scan Analysis. NLO characterization of the samples was done by the Z-scan technique, developed by Sheik Bahae et al.,31 by which both the nonlinear absorption as well as the nonlinear refractive parameters can be extracted. NLA and optical limiting (OL) parameters can be figured out from the open aperture (OA) Z-scan analysis, in which optical transmission of the sample is recorded as a function of input intensity. A schematic diagram of the experimental setup is shown in Figure 4. The sample is placed on a computer

Figure 4. Schematic of the Z-scan experimental setup.

controlled translational stage and is scanned along the direction (z axis) of the laser beam in predetermined steps. The laser beam is tightly focused using a convex lens, and the sample experiences a maximum intensity at the focus, which decreases equally on either side of the focus. Transmitted energy through the sample at different positions was collected by the energy detector, and the corresponding data is plotted against the

3. RESULTS AND DISCUSSION 3.1. Absorption and Photoluminescence Studies. The normalized absorption spectra of PTZ, Au NPs, and PTZ−Au NC are shown in Figure 5. Spectra were taken using a UV−vis spectrophotometer (Shimadzu-UV 2450). The absorption 26978


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Figure 5. (a) Absorption spectra of PTZ and prepared Au NPs (inset) and (b) absorption spectrum of PTZ−Au NC.

in the Au NP SPR band, and due to this overlap, electrons or energy will transfer from PTZ to Au through plasmon coupling, which results in suppressed emission intensity.36 A significant decrease in the emission intensity of PTZ−Au NC on exciting it with a wavelength of 318 nm indicates the PTZ is attached or nearby. We did not observe any significant emission when excited at 530 nm. 3.2. FTIR Analysis. In order to inspect and characterize the significant changes occurring to the PTZ bands as a result of interaction between PTZ and Au NPs upon formation of PTZ−Au NC, the Fourier transform infrared (FTIR) spectroscopy analysis was done. Figure 7 shows the FTIR spectrum

peaks for PTZ were found to be around 318 and 266 nm, while that of Au NPs shows a characteristic surface plasmon absorption peak around 530 nm (transverse mode), which according to Mie theory is the unique property of globular Au NPs. Meanwhile, nonglobular NPs have an additional (longitudinal) SPR peak at a higher wavelength.32 PTZ−Au NC displays a broad dual band absorption nature, and there was a slight blue shift and red shift observed in absorption maxima corresponding to PTZ and Au NPs, respectively; the shift could be due to the morphological modifications. Shape modification and the dielectric constant of the holding medium also contribute to the shift in the SPR band.33,34 The emission spectra of PTZ and PTZ−Au NC at excitation wavelengths of 318 nm (absorption maximum of PTZ) and 530 nm (absorption maximum of Au NPs) were recorded using a fluorometer (PerkinElmer LS 55), and they are shown in Figure 6. PTZ shows a broad emission peak around 460 nm which is in the visible band that makes it a material of immense attention, as they can substitute the expensive semiconductors and other crystalline photonic materials for illumination, displays, etc.35 Photoluminescence (PL) spectra of PTZ fall

Figure 7. FTIR spectra of PTZ and PTZ−Au NCs.

of PTZ and PTZ−Au NC in the region 400−2000 cm−1, since major changes are taking place in this region. Spectra were analyzed according to the literature,37 and the peaks appearing in the spectrum of PTZ indicate the following: asymmetric angle bending of δas (C−S−C) at 531 cm−1, angle bending δs (C−N−C) and δs (N−H) at 659 cm−1, angle bending of δs (N−H) and δs (C−C−C) at 869 cm−1, and symmetric stretching of υs (C−N−C) angle bending of (C−H) at 1260 cm−1. However, in the spectrum of PTZ−Au NC, these peaks show a down shift to 522, 653, 861, and 1247 cm−1, respectively. The observed shifts in the peak of PTZ−Au NC to lower wavenumber are a sign of the decrease in the bond

Figure 6. Photoluminescence spectra of PTZ and PTZ−Au NC at an excitation wavelength of 318 nm. The blue dotted line indicates the photoluminescence spectrum of PTZ−Au when excited at 530 nm. Photographic images of PTZ and PTZ−Au NC (inset). 26979


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Figure 8. (a) TEM image of PTZ−Au NC and size distribution of Au NPs in the composite (inset). (b) Selected area electron diffraction pattern of PTZ−Au NC.

strength of C−S and C−N. The interaction between PTZ and Au occurs at C−N−Au and C−S−Au due to the affinity of Au NPs toward sulfur and nitrogen that will result in electron transfer from S and N atoms to Au atoms. The decrease in the stretching frequency could be attributed to the charge transfer from S and N atoms to Au atoms as a result of the electrostatic interaction which gives the stability to Au NPs.24 3.3. TEM Analysis. For the structural investigation, we have done transmission electron microscopy (TEM) analysis. A drop of the prepared PTZ−Au NC was deposited on a carbon layered copper grid and dried out ahead of the analysis. The existence of Au NPs in PTZ−Au NC was confirmed by the TEM observation. Figure 8a shows the TEM image of PTZ− Au NC taken with a JEOL, JEM-2100, instrument. A Gaussian fitted histogram of the TEM image is also given in the inset of Figure 8a. From the image, it is obvious that the formed particles are globular in shape, and the statistical investigation of the particle size distribution indicates that the size of the formed particle varies from 10 to 40 nm with a peak value around 27.8 nm. The crystal structure and nature of Au NPs in the NC was investigated by selected area electron diffraction (SAED) analysis (Figure 8b). The calculated d values were well matched with the (111), (200), (220), and (331) Miller indices of a face centered cubic (fcc) Au crystal, which also ensures the crystalline nature of the NPs in the composite.38 3.4. Nonlinear Optical Studies. 3.4.1. Nonlinear Absorption Studies. The normalized absorption spectra derived from the Z-scan experiments performed for pure PTZ, Au NPs, and PTZ−Au nanocomposite are shown in Figure 9. The observed transmission curves are symmetric about the focus (z = 0), where it has the lowest transmittance. The symbols indicate the experimental data, and they are found to be fitting well with the theoretical model for TPA in the case of pure PTZ.31,39,40 In that case, the spatial rate of change of intensity can be written as dI = −α0I − βI 2 dz

Figure 9. OA Z-scan signatures of PTZ, Au NPs, and PTZ−Au NC in DMF with an on axis beam intensity of 0.82 GW/cm2.

includes the excited state absorption (ESA) from the plasmonic states of Au NP to higher lying conduction band states and direct TPA in PTZ and Au NP. The partial saturation of the plasmonic states of Au NPs is also introduced. Saturation of absorption (SA) plays a role here, even though it is not visible. This is to take into account the low concentration of NPs compared to PTZ. However, as expected, we observed Is to be very high. In such a case, the net absorption coefficient α(I) contains both saturable absorption and TPA and the intensity variation can be expressed as41 α0 + βeff I α (I ) = I 1+ I (5) s

⎛ ⎞ α0 ⎟ dI ⎜ = −⎜ I − βeff I 2 I ⎟ dz ⎝ 1 + Is ⎠

(4)

and the net absorption coefficient can be written as α(I) = α0 + βI. In the case of Au NPs and PTZ−Au NCs, reverse saturable absorption (RSA) is associated with effective TPA, which

(6)

In eq 5, the first term expresses the SA part and the next term expresses the effective TPA part, which consists of direct TPA and excited state absorption. Then, the normalized transmittance is given by42,43 26980


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Figure 10. Excitation and consecutive relaxation scheme in Au nanoparticles.

∞

T (z ) =

∑ m=0

⎡ αI L ⎤ m ⎢ 0z2eff ⎥ ⎣ 1 + z 02 ⎦ [m + 1]

surface by SPR, and the other induces RSA by free carrier absorption in the conduction band. In our measurements, the effective TPA coefficient was not so large (βeff = 0.55 × 10−10 m/W) that it indicates feeble free carrier absorption. The OA Z-scan data fitted well with the SA associated TPA model, confirming the occurrence of absorption saturation in conjunction with reduced free carrier absorption, which explains the intensity dependent NLA behavior.46 We had to excite the pure PTZ sample at an input intensity of 0.82 GW/cm2, since there was no significant NLA at lower intensities and this could be due to the off-resonant absorption response exhibited by the PTZ−DMF solution. This is evident from the absorption spectrum of PTZ (Figure 5a). The OA Zscan data of PTZ was found to be well fitted to the model for TPA, while that of PTZ−Au composite was fitted well with the model for a combination of TPA and ESA. The NLA mechanism of pristine PTZ as well as PTZ−Au NC under nanosecond pulse excitation can be explained as in Figure 11, in

(7)

where α is the nonlinear absorption coefficient and Leff is the effective sample length given by Leff = (1 − e−α0L)/α0, L is the sample length, and α0 is the linear absorption coefficient. I0 indicates the on axis peak intensity, z is the sample position, πω

2

z 0 = λ0 is the Rayleigh range, ω0 is the beam waist radius at the focal point, and λ is the wavelength of the laser source. The OA Z-scan data are numerically fitted to the theoretical model obtained by eq 5 and eq 7. The data fitted using eq 7 are also shown in Figure 9 (Au NPs and PTZ−Au NC), where the symbols indicate experimental data and the solid lines are the corresponding theoretical model. The imaginary part of the third order NLO susceptibility χ(3) from βeff is given by eq 8. Im χ (3) = n0 2ε0cλβeff /2π

(8)

To study the NLA characteristics, we first performed the OA Z-scan analysis of pristine PTZ, Au NPs, and PTZ−Au NC in DMF. The NLA characteristics of Au NPs had been explored well in the past few years.7,43,44 SPR response was found to be the major contributor for NLO in plasmonic Au nanocrystals, where SPR is the collective oscillation of electrons close to the Fermi surface. In our experiment, since the excitation wavelength is very close to the SPR (resonant excitation), it results in an intense excitation leading to the bleaching of the SPR spectrum (saturation of absorption).44 The SPR decay can take place in three distinct ways, namely, radiative, intraband, and interband transition (Figure 10).45 The excitation of Au NPs with a 532 nm light source will lead to interband (d to sp) and intraband (sp to sp) transitions. Relaxation of the excited atom can be through interband/ intraband transition or radiactive emission. The major part of the SPR decay will happen through the intraband and interband transition, which results in the production of free carriers, and the role of radiative emission is poor, which is evident from the PL emission at 532 nm (Figure 5). Thus, there can be two counter effects: one induces absorption saturation due to the exhaustion of electron in the near proximities of the Fermi

Figure 11. Energy level model explaining the nonlinear absorption mechanism in PTZ and PTZ−Au NC.

which S0 is the ground state and S1 is the excited state of PTZ.47,43 For pristine PTZ, the mechanism of NLA was found to be pure TPA, since it has an absorption peak exactly at 266 nm which is nearly in resonance with the TPA for the excitation wavelength (532 nm). Interaction of laser pulses at 532 nm, 7 ns may excite the PTZ molecule from the ground state to the 26981


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The Journal of Physical Chemistry C Table 1. Obtained NLO Parameters of PTZ, Au NPs, and PTZ−Au NC and a Few Comparisons βeff/βTPA

Im χ(3)

−10

−12

Isat sample Au PTZ PTZ−Au (present study) ZnO:Au, R. Udayabhaskar et al.49 pthalocyanine + 0.5% Au NPs, A. N. Gowda et al.50 graphene oxide−Fe3O4, X. L. Zhanh et al.51

12

2

10 (W/m )

10

(m/W)

βeff = 0.55 βTPA = 0.026 βeff = 1.74 βeff = 1.00 β = 2.5 β = 2.6

3 1.78 4.5 0.25

excited state of PTZ by absorbing two photons together, which is known as TPA. However, it is feeble due to the restricted electron delocalization by the presence of sp3 hybridized nitrogen,48 and hence, PTZ shows the poor NLA behavior. When the PTZ forms a composite with Au NPs, the NLA property of PTZ gets enhanced significantly (∼67 times) and it can be explained as follows. Besides the individual NLA mechanism exhibited by PTZ as well as Au NPs, there could be a possibility of charge/energy transfer between the excited states of Au NPs and that of PTZ. This charge/energy transfer further enhances the total NLA of the composite formed. On analyzing the measured NLA parameters, we observed a considerable enhancement in the NLA behavior in comparison with the constituent moieties (pristine PTZ and Au NPs) when all of the samples were studied with the almost uniform linear transmission (LT) around 80% (Table 1). Since we have conducted the experiment at a high linear transmittance, our result is comparable or better than that of many reported values of the NLA coefficient. The enhancement in NLO properties of PTZ−Au nanocomposites can be attributed to intersystem excitation transfer through both electron and energy transfer, as shown in Figure 12. If there is a resonant energy transfer between a coupled pair

10

(esu)

1.73 0.23 5.71

n2 10

−11

(esu)

6.8 2.2 8.0

Re χ(3) 10−12 (esu) 7.1 3.1 8.7

Moreover, the interaction between PTZ and SPR of Au NPs could lead to a strong charge delocalization between them and hence the dipole moment of the system gets enhanced significantly.49,52,53 The effect of field intensification depends on parameters like excitation wavelength, separation from the NPs, neighboring medium, and the diameters of the core and shell.56 The Z-scan experiment of PTZ−Au NC was done for different input beam intensities (I0 = 0.27, 0.82, 1.37, and 1.88 GW/cm2), to explore the mechanism behind the NLA (Figure 13a), and corresponding NLA coefficients were plotted against on axis input intensities, as shown in Figure 13b. It is found that the NLA coefficient is decreasing with increasing on axis intensity. A drop in the value of βeff against I0 is the mark of ESA, where real excited states contribute to the NLA through ESA. Since ESA depends on the intensity of the excitation source, exciting with high intensity radiation, a significant depletion in the ground state population can happen due to higher excited state absorption cross section and results in the variation of βeff values with I0. However, in the case of pure TPA, the absorption cross section is small compared to ESA. Thus, there will not be any considerable depletion in the ground state population with increasing input intensity, which ultimately makes the NLA coefficient intensity independent.57 3.4.2. Optical Limiting Studies. There are materials which exhibit constant transmittance at lower input fluence and reduced transmittance at a higher input fluence, known as optical limiters. These materials can play crucial roles in fabricating laser safety devices, pulse shaping, pulse compression, mode locking, etc. When input beam fluence crosses a certain threshold value in such a material, limiting action takes place simultaneously. This is because the material starts showing NLA and NLR properties at the threshold value which leads to a decrease in the transmittance. TPA, RSA, ESA, nonlinear scattering, optically induced heating, free carrier absorption, etc., are the mechanisms responsible for inducing limiting action in materials.47 In our optical limiting studies, transmittance data are extracted from the open aperture Z-scan result and plotted against input fluence, as shown in Figure 14a, where symbols are experimental data and solid lines are corresponding theoretical fits. From the graph, we can observe a significant enhancement in the optical limiting action of PTZ−Au NC compared to pure Au NPs and PTZ. Enhanced limiting action could be attributed to the increase in NLA of the NC, which has been discussed already in section 3.4.1. The quality of an optical limiter is determined from its onset value (the input fluence at which the output transmittance starts decreasing) and limiting threshold (value of input fluence where the output transmittance becomes half of the initial value). Optical limiting studies of PTZ−Au NC for different LTs at an input intensity of 0.82 GW/cm2 are shown in Figure 14b. For the sample

Figure 12. Schematic representation of electron/energy transfer between PTZ and Au NPs.

system, the linear and NLA characteristics of the entire system will improve significantly. In our studies on analyzing the emission spectra of PTZ and absorption spectra of Au NPs, it is evident that there is an overlapping between the two spectra, that leads to resonant energy transfer between PTZ and Au NPs.52,53 Here NPs can act as an electron or energy accepting material due to the positive charge on it. Energy transfer plays a crucial role in large NPs as a result of the adequate overlap of emission spectra of PTZ and absorption spectra of Au NPs.54,55 26982


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Figure 13. (a) OA Z-scan curves of PTZ−Au NC at different on axis beam intensities and (b) corresponding βeff versus on axis beam intensity plot.

Figure 14. (a) Optical limiting studies of PTZ, Au NPs, and PTZ−Au NC at an input intensity of 0.82 GW/cm2 and (b) optical limiting studies of PTZ−Au NC with different linear transmittances at peak on axis beam intensity of 0.82 GW/cm2.

having LT 67%, we got the onset and the threshold value of limiting action at 0.33 and 2.59 J/cm2, respectively. We claim these values are higher or comparable to many other reported values which were proposed as good optical limiters so far.47,49,53 For example, V. Mamidala et al. reported that the limiting threshold of the GO + porphyrin and Au + porphyrin complexes were ∼1.9 and 4.3 J/cm2, respectively. B. Anand et al. reported the optical limiting threshold in metal hybrids of functionalized hydrogen exfoliated graphene (f-HEG) as 13.7 and 8.8 J/cm2 for Pt/f-HEG and Pd/f-HEG, respectively. However, in our studies, the limiting threshold is calculated as 2.59 J/cm2, which indicates the composite presented here can have potential usefulness in sensor and eye protection from intense laser radiation. Thus, we can infer the composite as a good choice for optical limiting purposes at the measured wavelength. 3.4.3. Nonlinear Refraction Studies. The NLR parameters were ruled out from the closed aperture Z-scan analysis. For a thin medium (zR ≫ L), one can estimate the effect of nonlinear refraction by dividing the data of a CA Z-scan by that of an OA Z-scan, with both Z-scans being performed at the same incident intensity. The resulting curve can be used for the measurement of phase shift due to nonlinear refraction alone. Thus, the normalized transmittance variation is expressed as58,59

() T (z , ΔΦ ) = 1 + ⎛ ⎞⎛ ⎜( ) + 1⎟⎜( ) ⎝ ⎠⎝ 4

0

z z0

2

z z0

z z0

2

⎞ + 9⎟ ⎠

ΔΦ0 (9)

Here, T is the normalized transmittance for the pure refractive nonlinearity, ΔΦ0 = kn2I0Leff is the on axis nonlinear phase shift at the focus, and I0 is the on axis intensity at the focus (z = 0). Figure 15 shows normalized closed by open Z-scan data of PTZ, Au NPs, and PTZ−Au NC at 0.83 GW/cm2, where symbols denote the experimental values and solid lines are theoretical fits for the transmission equation (eq 9). If Δφ0 < π, we can also evaluate Δϕ0 from the expression ΔTp−v ≅ 0.406|Δφ0|, where ΔTp−v is the difference between the peak and value of normalized transmittance.58 The ΔTp−v values obtained from the Z-scan traces (Figure 15) are very close to the calculated value of 0.406|Δφ0|. The obtained NLR parameters are also tabulated in Table 1. The coefficient nonlinear refractive index can be calculated using the expression n2 (esu) =

cn0λΔϕ0 80π 2I0Leff

(10)

In electrostatic units, n2 is related to Re(χ(3)) by the relation

Re χ (3) = 2n0 2ε0cn2 26983

(11) DOI: 10.1021/acs.jpcc.7b06528 J. Phys. Chem. C 2017, 121, 26976−26986

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composite formation due to the influence of SPR from Au NPs. The NLO properties of the NC are enhanced noticeably compared to the pristine compounds, and the reason for enhancement can be explained in two ways, the local field effects and intersystem energy/charge transfer. From the OA Zscan analysis, it is observed that the cause for NLA and OL of PTZ is TPA, while that of the NC is ESA along with TPA. The defocusing (negative nonlinearity) nature of the composite was explored from the CA Z-scan investigations. The enhanced NLA and OL behavior of the composite guarantees the fruitful use of the same strategy in optoelectronics and nanophotonics applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06528. A detailed description on the theoretical formalisms adopted for the curve fitting of both open aperture and closed aperture Z-scan analysis (PDF)

Figure 15. CA Z-scan data of PTZ, Au NPs, and PTZ−Au NC at a beam intensity of 0.82 GW/cm2.

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On analyzing the CA Z-scan trace, it is clear that PTZ, Au NPs, and PTZ−Au NC show a prefocal transmittance peak followed by a valley, which is the signature of a negative NLR coefficient. Negative nonlinearity under a ns regime is common, and it is due to thermal effects that are generated as a result of interaction between the medium and high fluence laser beam.60 Due to the negative nonlinearity, the Gaussian laser beam generates a gradient in the index of refraction with the minimum refractive index at the beam axis and increases toward the periphery so that the sample behaves like a diverging lens on incoming light results in the defocusing of the beam. The variation in refractive index can be written as Δn = (∂n/∂T)ΔT, where ∂n/∂T is the temperature dependent refractive index and ΔT the laser-induced temperature variation. These effects are significant in cases where the pulse width of the laser source is higher than a few tens of picoseconds. The phase of the propagating beam through the medium will be distorted by the lensing effect of the medium.58 Even though PTZ shows a negative nonlinearity, its value is small compared to Au NPs and PTZ−Au NC and it could be due to constrained delocalization of electrons.48 However, Au NPs show a better nonlinearity and it could be attributed to SPR of Au NPs. Significant enhancement of the NLR property of PTZ−Au NC can be ascribed to charge transfer between PTZ and Au NPs upon composite formation, which introduces an enhanced absorption cross section leading to an enhanced thermal effect.43 Moreover, SPR of Au NPs leads to an intense local field effect, redistribution of population above and below the Fermi level (population lens) and metal NP size, host medium, concentration and excitation source intensity has also played a crucial role in the variation of NLR.61 The decoration of metal NPs in PTZ upon composite formation makes new scattering centers, which will also lead to the enhancement in observed NLR properties.62 The values denote the composite bearing higher potential to be used in photonic applications that exploit NLO activities.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chandrasekharan Keloth: 0000-0003-2287-2339 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to NIT Calicut, Kerala, India, for financial support. We express our gratitude towards Ms. Dijo Prasannan and Mrs. Vijisha M V, NIT Calicut, for their fruitful discussions and helpful suggestions.

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REFERENCES

(1) El-Shishtawy, R. M.; Al-Zahrani, F. A. M.; Afzal, S. M.; Razvi, M. A. N.; Al-amshany, Z. M.; Bakry, A. H.; Asiri, A. M. Synthesis, Linear and Nonlinear Optical Properties of a New Dimethine Cyanine Dye Derived from Phenothiazine. RSC Adv. 2016, 6, 91546−91556. (2) Calvete, M.; Yang, G. Y.; Hanack, M. Porphyrins and Phthalocyanines as Materials for Optical Limiting. Synth. Met. 2004, 141, 231−243. (3) Organic Molecules for Nonlinear Optics and Photonics; Messier, J., Kajzar, F., Prasad, P., Eds.; Kluwer Academic Publisher: Amsterdam, The Netherlands, 1991. (4) Adair, R.; Chase, L. L.; Payne, S. A. Nonlinear Refractive Index of Optical Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 39, 3337−3350. (5) Ganeev, R. A. Nonlinear Refraction and Nonlinear Absorption of Various Media. J. Opt. A: Pure Appl. Opt. 2005, 7, 717−733. (6) Guo, S.; Xu, L.; Wang, H.; You, X.; Ming, N. B. Determination of Optical Nonlinearities in Cu (mpo)2 by Z-Scan Technique. Opt. Quantum Electron. 2003, 35, 693−703. (7) Sudheesh, P.; Siji Narendran, N. K.; Chandrasekharan, K. ThirdOrder Nonlinear Optical Responses in Derivatives of Phenylhydrazone by Z-Scan and Optical Limiting Studies-Influence of Noble Metal Nanoparticles. Opt. Mater. (Amsterdam, Neth.) 2013, 36, 304−309. (8) Senthil, K.; Kalainathan, S.; Hamada, F.; Yamada, M.; Aravindan, P. G. Synthesis, Growth, Structural and HOMO and LUMO, MEP Analysis of a New Stilbazolium Derivative Crystal: A Enhanced ThirdOrder NLO Properties with a High Laser-Induced Damage Threshold for NLO Applications. Opt. Mater. (Amsterdam, Neth.) 2015, 46, 565− 577.

4. CONCLUSION In summary, photophysical synthesis of PTZ−Au NC was done by ablating a Au target in PTZ−DMF solution and its linear and NLO characterizations were also done. A considerable shift in FTIR spectra was observed upon the composite formation. The emission intensity of PTZ decreases substantially upon 26984


Article

The Journal of Physical Chemistry C (9) Sergeeva, E. V.; Puntus, L. N.; Kajzar, F.; Rau, I.; Sahraoui, B.; Pekareva, I. S.; Yu, K.; Bushmarinov, I. S.; Lyssenko, K. A. Keto-Enol Tautomerism and Nonlinear Optical Properties in β -Diketones Containing [ 2. 2 ] Paracyclophane. Opt. Mater. (Amsterdam, Neth.) 2013, 36, 47−52. (10) Mbhele, Z. H.; Salemane, M. G.; Van Sittert, C. G. C. E.; Nedeljković, J. M.; Djoković, V.; Luyt, A. S. Fabrication and Characterization of Silver-Polyvinyl Alcohol Nanocomposites. Chem. Mater. 2003, 15, 5019−5024. (11) Jana, N. R.; Gearheart, L.; Murphy, C. J. Wet Chemical Synthesis of High Aspect Ratio Cylindrical Gold Nanorods. J. Phys. Chem. B 2001, 105, 4065−4067. (12) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. Hybrid Nanoparticles with Block Copolymer Shell Structures. J. Am. Chem. Soc. 1999, 121, 462−463. (13) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R.; Thomas, J. M.; Edwards, P. P.; Faraday, M.; Porter, M. D.; Bright, T. B.; et al. Synthesis of Thiol-Derivatised Gold Nanoparticles in a TwoPhase Liquid−Liquid System. J. Chem. Soc., Chem. Commun. 1994, 0 (7), 801−802. (14) Prochazka, M.; Stepanek, J.; Vlckova, B.; Srnova, I.; Maly, P. Laser Ablation: Preparation of “Chemically Pure” Ag Colloids for Surface-Enhanced Raman Scattering Spectroscopy. J. Mol. Struct. 1997, 410−411, 213−216. (15) Mafuné, F.; Kohno, J.; Takeda, Y.; Kondow, T. Formation of Gold Nanoparticles by Laser Ablation in Aqueous Solution of Surfactant. J. Phys. Chem. B 2001, 105, 5114−5120. (16) Compagnini, G.; Scalisi, A. A.; Puglisi, O. Production of Gold Nanoparticles by Laser Ablation in Liquid Alkanes. J. Appl. Phys. 2003, 94, 7874−7877. (17) Divyasree, M. C.; Chandrasekharan, K. Structural and Nonlinear Optical Characterizations of ZnS/PVP Nanocomposites Synthesized by Pulsed Laser Ablation. Opt. Mater. (Amsterdam, Neth.) 2017, 67, 119−124. (18) Kong, L.; Yang, J. X.; Chen, Q. Y.; Zhang, Q.; Ke, W. D.; Xue, Z. M.; Zhou, H. P.; Wu, J. Y.; Jin, B. K.; Tian, Y. P. Blue-Shift of Photoluminescence Induced by Coupling Effect of a Nanohybrid Composed of Fluorophore-Phenothiazine Derivative and Gold Nanoparticles. J. Nanopart. Res. 2014, 16, 2324. (19) Liu, F.; Wang, H.; Yang, Y.; Xu, H.; Yang, D.; Bo, S.; Liu, J.; Zhen, Z.; Liu, X.; Qiu, L. Using Phenoxazine and Phenothiazine as Electron Donors for Second-Order Nonlinear Optical Chromophore: Enhanced Electro-Optic Activity. Dyes Pigm. 2015, 114, 196−203. (20) Wu, W.; Yang, J.; Hua, J.; Tang, J.; Zhang, L.; Long, Y.; Tian, H. Efficient and Stable Dye-Sensitized Solar Cells Based on Phenothiazine Sensitizers with Thiophene Units. J. Mater. Chem. 2010, 20, 1772. (21) Iqbal, Z.; Wu, W. Q.; Huang, Z. S.; Wang, L.; Kuang, D.-B.; Meier, H.; Cao, D. Trilateral π-Conjugation Extensions of Phenothiazine-Based Dyes Enhance the Photovoltaic Performance of the Dye-Sensitized Solar Cells. Dyes Pigm. 2016, 124, 63−71. (22) Zhu, X. Q.; Dai, Z.; Yu, A.; Wu, S.; Cheng, J. P. Driving Forces for the Mutual Conversions between Phenothiazines and Their Various Reaction Intermediates in Acetonitrile. J. Phys. Chem. B 2008, 112, 11694−11707. (23) Gupta, S.; Singh, A. K.; Jain, R. K.; Chandra, R.; Prakash, R. Phenothiazine-Capped Gold Nanoparticles: Photochemically Assisted Synthesis and Application in Electrosensing of Phosphate Ions. ChemElectroChem 2014, 1, 793−798. (24) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Investigation into the Interaction between Surface-Bound Alkylamines and Gold Nanoparticles. Langmuir 2003, 19, 6277−6282. (25) Cui, H.; Wang, W.; Duan, C. F.; Dong, Y. P.; Guo, J. Z. Synthesis, Characterization, and Electrochemiluminescence of Luminol-Reduced Gold Nanoparticles and Their Application in a Hydrogen Peroxide Sensor. Chem. - Eur. J. 2007, 13, 6975−6984. (26) Rawashdeh, A. M. M.; Austin, S.; Am, M.; Hf, H. Computing the Redox Potentials of Phenothiazine and N -Methylphenothiazine. Basic Sci. Eng. 2005, 14, 195−208.

(27) Alkaitis, S. A.; Beck, G.; Graetzel, M. Laser Photoionization of Phenothiazine in Alcoholic and Aqueous Micellar Solution. Electron Transfer from Triplet States to Metal Ion Acceptors. J. Am. Chem. Soc. 1975, 97, 5723−5729. (28) Ghosh, H. N.; Sapre, A. V.; Palit, D. K.; Mittal, J. P. Picosecond Flash Photolysis Studies on Phenothiazine in Organic and Micellar Solution. J. Phys. Chem. B 1997, 101, 2315−2320. (29) Von Der Linde, D.; Sokolowski-Tinten, K. Physical Mechanisms of Short-Pulse Laser Ablation. Appl. Surf. Sci. 2000, 154−155, 1−10. (30) Semaltianos, N. G. Nanoparticles by Laser Ablation. Crit. Rev. Solid State Mater. Sci. 2010, 35, 105−124. (31) Sheik-Bahae, M.; Said, A. A.; Wei, T. H.; Hagan, D. J.; Van Stryland, E. W. Sensitive Measurement of Optical Nonlinearities Using a Single Beam. IEEE J. Quantum Electron. 1990, 26, 760−769. (32) Smitha, S. L.; Gopchandran, K. G.; Smijesh, N.; Philip, R. SizeDependent Optical Properties of Au Nanorods. Prog. Nat. Sci. 2013, 23, 36−43. (33) Shan, G.; Wang, S.; Fei, X.; Liu, Y.; Yang, G. Heterostructured ZnO/Au Nanoparticles-Based Resonant Raman Scattering for Protein Detection. J. Phys. Chem. B 2009, 113, 1468−1472. (34) Zhang, W. Q.; Lu, Y.; Zhang, T. K.; Xu, W.; Zhang, M.; Yu, S. H. Controlled Synthesis and Biocompatibility of Water-Soluble ZnO Nanorods/Au Nanocomposites with Tunable UV and Visible Emission Intensity. J. Phys. Chem. C 2008, 112, 19872−19877. (35) Sreekumar, G.; Louie Frobel, P. G.; Sreeja, S.; Suresh, S. R.; Mayadevi, S.; Muneera, C. I.; Suchand Sandeep, C. S.; Philip, R.; Mukharjee, C. Nonlinear Absorption and Photoluminescence Emission in Nanocomposite Films of Fuchsine Basic Dye-Polymer System. Chem. Phys. Lett. 2011, 506, 61−65. (36) Lee, M. K.; Kim, T. G.; Kim, W.; Sung, Y. M. Surface Plasmon Resonance (SPR) Electron and Energy Transfer in Noble Metal-Zinc Oxide Composite Nanocrystals. J. Phys. Chem. C 2008, 112, 10079− 10082. (37) Alcolea Palafox, M.; Gil, M.; Nunez, J. L.; Tardajos, G. Study of Phenothiazine and N-Methyl Phenothiazine by Infrared, raman,1H-, and 13C-NMR Spectroscopies. Int. J. Quantum Chem. 2002, 89, 147− 171. (38) Ftouni, J.; Penhoat, M.; Addad, A.; Payen, E.; Rolando, C.; Girardon, J. S. Highly Controlled Synthesis of Nanometric Gold Particles by Citrate Reduction Using the Short Mixing, Heating and Quenching Times Achievable in a Microfluidic Device. Nanoscale 2012, 4, 4450. (39) Taheri, B.; Liu, H.; Jassemnejad, B.; Appling, D.; Powell, R. C.; Song, J. J. Intensity Scan and Two Photon Absorption and Nonlinear Refraction of C 60 in Toluene. Appl. Phys. Lett. 1996, 68, 1317−1319. (40) Sutherland, R. L. Handbook of Nonlinear Optics; Marcel Dekker Inc: New York, 1997. (41) Couris, S.; Koudoumas, E.; Ruth, A. A.; Leach, S. Concentration and Wavelength Dependence of the Effective Third-Order Susceptibility and Optical Limiting of C 60 in Toluene Solution. J. Phys. B: At., Mol. Opt. Phys. 1995, 28, 4537. (42) Gao, Y.; Zhang, X.; Li, Y.; Liu, H.; Wang, Y.; Chang, Q.; Jiao, W.; Song, Y. Saturable Absorption and Reverse Saturable Absorption in Platinum Nanoparticles. Opt. Commun. 2005, 251, 429−433. (43) Sreeramulu, V.; Haldar, K. K.; Patra, A.; Rao, D. N. Nonlinear Optical Switching and Enhanced Nonlinear Optical Response of Au− CdSe Heteronanostructures. J. Phys. Chem. C 2014, 118, 30333− 30341. (44) Philip, R.; Kumar, G. R.; Sandhyarani, N.; Pradeep, T. Picosecond Optical Nonlinearity in Monolayer-Protected Gold, Silver, and Gold-Silver Alloy Nanoclusters. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 13160−13166. (45) Sonnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P. Drastic Reduction of Plasmon Damping in Gold Nanorods. Phys. Rev. Lett. 2002, 88, 774021−774024. (46) Philip, R.; Chantharasupawong, P.; Qian, H.; Jin, R.; Thomas, J. Evolution of Nonlinear Optical Properties: From Gold Atomic Clusters to Plasmonic Nanocrystals. Nano Lett. 2012, 12, 4661−4667. 26985


Article

The Journal of Physical Chemistry C (47) Anand, B.; Kaniyoor, A.; Sai, S. S. S.; Philip, R.; Ramaprabhu, S. Enhanced Optical Limiting in Functionalized Hydrogen Exfoliated Graphene and Its Metal Hybrids. J. Mater. Chem. C 2013, 1, 2773− 2780. (48) Daub, J.; Engl, R.; Kurzawa, J.; Miller, S. E.; Schneider, S.; Stockmann, A.; Wasielewski, M. R. Competition between Conformational Relaxation and Intramolecular Electron Transfer within Phenothiazine - Pyrene Dyads. J. Phys. Chem. A 2001, 105, 5655− 5665. (49) Udayabhaskar, R.; Karthikeyan, B.; Sreekanth, P.; Philip, R. Enhanced Multi-Phonon Raman Scattering and Nonlinear Optical Power Limiting in ZnO: Au Nanostructures. RSC Adv. 2015, 5, 13590−13597. (50) Gowda, A. N.; Kumar, M.; Thomas, A. R.; Philip, R.; Kumar, S. Self-Assembly of Silver and Gold Nanoparticles in a Metal-Free Phthalocyanine Liquid Crystalline Matrix : Structural, Thermal, Electrical and Nonlinear Optical Characterization. Chemistry Select. 2016, 1, 1361−1370. (51) Zhang, X. L.; Zhao, X.; Liu, Z. B.; Shi, S.; Zhou, W. Y.; Tian, J. G.; Xu, Y.-F.; Chen, Y. S. Nonlinear Optical and Optical Limiting Properties of Graphene Oxide−Fe3O4 Hybrid Material. J. Opt. 2011, 13, 075202. (52) Fan, C.; Wang, S.; Hong, J. W.; Bazan, G. C.; Plaxco, K. W.; Heeger, A. J. Beyond Superquenching: Hyper-Efficient Energy Transfer from Conjugated Polymers to Gold Nanoparticles. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 6297−6301. (53) Mamidala, V.; Polavarapu, L.; Balapanuru, J.; Loh, K. P.; Xu, Q. H.; Ji, W. Enhanced Nonlinear Optical Responses in Donor-Acceptor Ionic Complexes via Photo Induced Energy Transfer. Opt. Express 2010, 18, 25928−25935. (54) Kumar, S.; Shibu, E. S.; Pradeep, T.; Sood, A. K. Ultrafast Photoinduced Enhancement of Nonlinear Optical Response in 15Atom Gold Clusters on Indium Tin Oxide Conducting Film. Opt. Express 2013, 21, 8483−8492. (55) Dupuis, B.; Michaut, C.; Jouanin, I.; Delaire, J.; Robin, P.; Feneyrou, P.; Dentan, V. Photoinduced Intramolecular ChargeTransfer Systems Based on Porphyrin−viologen Dyads for Optical Limiting. Chem. Phys. Lett. 1999, 300, 169−176. (56) Tanabe, K. Field Enhancement around Metal Nanoparticles and Nanoshells: A Systematic Investigation. J. Phys. Chem. C 2008, 112, 15721−15728. (57) Anand, B.; Addo Ntim, S.; Sai Muthukumar, V.; Siva Sankara Sai, S.; Philip, R.; Mitra, S. Improved Optical Limiting in Dispersible Carbon Nanotubes and Their Metal Oxide Hybrids. Carbon 2011, 49, 4767−4773. (58) Jayakrishnan, K.; Joseph, A.; Bhattathiripad, J.; Ramesan, M. T.; Chandrasekharan, K.; Siji Narendran, N. K. Reverse Saturable Absorption Studies in Polymerized Indole - Effect of Polymerization in the Phenomenal Enhancement of Third Order Optical Nonlinearity. Opt. Mater. (Amsterdam, Neth.) 2016, 54, 252−261. (59) Liu, X.; Guo, S.; Wang, H.; Hou, L. Theoretical Study on the Closed-Aperture Z-Scan Curves in the Materials with Nonlinear Refraction and Strong Nonlinear Absorption. Opt. Commun. 2001, 197, 431−437. (60) Kuladeep, R.; Jyothi, L.; Prakash, P.; Shekhar, S. M.; Prasad, M. D.; Rao, D. N. Investigation of Optical Limiting Properties of Aluminium Nanoparticles Prepared by Pulsed Laser Ablation in Different Carrier Media. J. Appl. Phys. 2013, 114, 243101. (61) He, T.; Wang, C.; Pan, X.; Wang, Y. Nonlinear Optical Response of Au and Ag Nanoparticles Doped Polyvinylpyrrolidone Thin Films. Phys. Lett. A 2009, 373, 592−595. (62) Biswas, S.; Kole, A. K.; Tiwary, C. S.; Kumbhakar, P. Enhanced Nonlinear Optical Properties of Graphene Oxide−Silver Nanocomposites Measured by Z-Scan Technique. RSC Adv. 2016, 6, 10319−10325.

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