Surface Plasmon Assisted Enhancement in the Nonlinear Optical

Subscriber access provided by READING UNIV

Article

Surface Plasmon Assisted Enhancement in the Nonlinear Optical Properties of Phenothiazine by Gold Nanoparticle Shiju Edappadikkunnummal, Siji Narendran Nherakkayyil, Vasudevan Kuttippurath, Divyasree Manathanathu Chali, Desai Narayana Rao, and Chandrasekharan Keloth J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06528 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Surface Plasmon Assisted Enhancement in the Nonlinear Optical Properties of Phenothiazine by Gold Nanoparticle Edappadikkunnummal Shijua, Nherakkayyil Siji Narendranb, Kuttippurath Vasudevana, Manathanathu Chalil Divyasreea, Narayana Rao Desaic, Keloth Chandrasekharan*a a

Laser and Nonlinear Optics Laboratory, Department of Physics, National Institute of Technology Calicut, Kozhikode 673601, Kerala, India. b

c

Department of Physics, C K G M Govt. College, Perambra, Kozhikode 673525, Kerala, India.

School of Physics, University of Hyderabad, Gachibowli, Hyderabad, 500046, India.

*[email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

Abstract We report the photophysical method for the synthesis of phenothiazine (PTZ)–gold (Au) nanocomposite (NC), by ablating Au target in PTZ-dimethyl formamide (DMF) solution using Q- switched Nd: YAG laser delivering 7 ns pulses at 532 nm. The ablation of Au target as well as the photo-ionization of PTZ was carried out simultaneously in the same medium with the same laser system. PTZ itself acts as 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. 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 single beam Z-Scan technique that uses 532 nm, 7 ns and 10Hz 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 enhancement could be attributed to both 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 PTZAu NC (TPA assisted excited state absorption (ESA)). 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 worth useful for designing novel materials with potential applications in Photonics.


Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction 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 the 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 to 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 having rapid NLO responses, high damage threshold, structural extensibility and ease of fabrication1-3. They are designed as potential candidates for numerous purposes like optical communications, high density optical data storage, photo-dynamic therapy for cancer treatment, optical limiters for eye and instrument safety4-9. The noble metal NPs exhibit an 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 etc10. The enhanced optical nonlinearity 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 solutions11-13. But in this method, removing excess reagents like residual surfactants or ions and functionalizing Au NPs with various molecules were tedious tasks14. Later a new method of synthesizing Au NPs by ablating bulk metal in water was introduced15. Compagnini et al. demonstrated the synthesis of Au NPs by pulsed laser ablation (PLA) in organic solvents such as alkanes, aliphatic alcohols and in polymer or sol-gel matrices16. PLA is environmentally friendly and doesn’t require any toxic chemicals for the preparation of nanomaterials. This method is devoid of multistep synthesis procedures, longer reaction time, high temperature and pressure. The size and nature of the NPs are decided by the laser parameters like wavelength, pulse width, frequency and the ablating medium17. Here we synthesized Au NCs in PTZ; where PTZ is an organic dye with chemical formula S (C6H4)2 NH (Figure 1). It 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 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 29

and sulfur atoms. Moreover, its ring has a butterfly conformation that would obstruct the molecular aggregation.

Figure 1. Molecular structure of Phenothiazine

These structural features have been of wide concern for the production of functional dyes useful for optoelectronics and nonlinear optical applications18-21. 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-oxidation22. 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 superior aromatic resonance stabilization than that of PTZ and gives the stability to the NPs23-26. In our studies, we have used the same laser source simultaneously for laser ablation and photo-ionization. The formation of stable radical cations of PTZ by photo-oxidation has been studied by many research groups27-28. As a result of photo-ionization, PTZ dissociates to electrons and radical cations of PTZ or PTZ and hydrogen radicals as per the following equations.

∗ → ∙ ē

∗ → ∙ ∙

∗ → ∙ ē → ∙ ∙

(1) (2) (3)

Where PTZ*, ∙ , ∙, ∙ , ē are excited state of PTZ, PTZ radical, radical cation of PTZ, hydrogen radical and an electron respectively.


Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 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 PLA technique to explore its linear and NLO properties.

Figure 2. Schematic representation of photophysical reduction of Au ions by phenothiazine. Au3+ ions get reduced to Au by subsequent photo- induced electron transfer from 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 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, forms a plume of NPs. Interaction of the plume with the solution results in a sharp expansion and momentary explosion in solution and finally solidified into spherical NPs of regular shapes and uniform size distribution in the solution29. PLA has got 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 and simpler starting materials etc. (ii) 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 fundamental and technological point of view30. In our studies, for the ablation process we have used a Q switched Nd: YAG laser delivering 7ns pulses at 532 nm wavelength with a repetition rate of 10 Hz. Schematic of the experimental setup of PLA and formation of NPs is shown in Figure 3. 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

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

We prepared 2wt % solution of PTZ (98%, Sigma Aldrich) in spectroscopic grade DMF. The gold target (99.9%, 1mm thickness) was taken in a beaker containing 10 ml of the prepared solution. The target was focused with the laser beam through the liquid medium using a convex lens of focal length 50 mm. We ablated the target for 15 minutes 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 Z-Scan technique, developed by Sheik Bahae et al31, 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. Schematic diagram of

the experimental setup is shown in Figure 4. The sample is placed on a computer 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 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


Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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. Pure NLR part can be extracted by dividing CA transmittance data by OA transmittance data (division method).

Figure 4. Schematic of the Z-Scan experimental setup

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 3 mm diameter was kept in front of the signal detector for CA analysis. 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 analysis31, as sample thickness is very small when compared with Rayleigh range. Both the reference beam energy and transmitted beam energy was measured using two identical pyroelectric detectors (RjP-735, Laser Probe. Corp., USA) and the ratio was taken using an energy ratio meter (Rj-7620, Laser Probe Corp., USA) simultaneously. During each translation step, the laser pulses were fired into the sample using single shot mode to minimize the cumulative thermal contributions.

3. Results and Discussions



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 29

3.1. Absorption and photoluminescence studies The normalized absorption spectra of PTZ, Au NPs and PTZ-Au NC is shown in Figure 5 (a) and figure 5 (b). Spectra were taken using UV-Vis spectrophotometer (Shimadzu-UV 2450). The absorption peaks for PTZ were found to be around 318 nm 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, non-globular NPs have an additional (longitudinal) SPR peak at a higher wavelength32. 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 and the shift could be due to the morphological modifications. Shape modification and dielectric constant of the holding medium also contribute to the shift in SPR band33, 34.

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

The emission spectra of PTZ and PTZ-Au NC at excitation wavelength of 318 nm (absorption maximum of PTZ) and 530 nm (absorption maximum of Au NPs) were recorded using a fluorometer (Perkin Elmer LS 55) is 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 etc35. Photoluminescence (PL) spectra of PTZ falls in the Au NPs SPR band and due to this overlap electrons or energy will transfer from PTZ to Au through plasmon coupling, which results in suppressed emission intensity36. A significant decrease in the emission intensity of PTZ-Au NC on exciting it with a wavelength of 318 nm indicates the PTZ being attached or close to. We didn’t observe any significant emission when excited at 530 nm.


Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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

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 of PTZ and PTZ-Au NC in the region of 400-2000 cm-1, since major changes are taking place in this region. Spectra were analyzed according to the literature37 and the peaks appearing in the spectrum of PTZ indicate the following; the asymmetric angle bending of δas (C-S-C) at 531cm-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. Whereas 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 wave number is a sign of the decrease in the bond 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 towards 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 NPs24.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 29

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

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 it out ahead of the analysis. The existence of Au NPs in PTZ-Au NC was confirmed by the TEM observation. Figure 8 (a) shows the TEM image of PTZ- Au NC taken with JEOL, JEM-2100. Gaussian fitted histogram of the TEM image is also given in the inset of Figure 8 (a). 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. Crystal structure and nature of Au NPs in the NC was investigated by selected area electron diffraction (SAED) analysis (Figure 8 (b)). 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 composite38.


Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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

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 is 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 are found to be fitting well with the theoretical model for TPA in the case of pure PTZ31, 39,40. In that case the spatial rate of change of intensity can written as

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dI = −α 0 I − β I 2 dz

Page 12 of 29

(4)

and net absorption coefficient can be written as, α ( I ) = α 0 + β I In the case of Au NPs and PTZ-Au NCs reverse saturable absorption (RSA) is associated with effective TPA, which 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. Also introduced the partial saturation of the plasmonic states of Au NPs. 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

α (I ) =

α0 1+

I Is

  α dI = − 0 dz 1+ I  Is 

+ β eff I

(5)    I − β eff I 2   

(6)

In Eqn. (5), the first term expresses the SA part and next term expresses the effective TPA part, which consist of direct TPA and excited state absorption. Then, the normalized transmittance is given by42, 43.

T ( z ) =

∞

∑

m = 0

  α I 0 L e ff  z 2 1 + z 02  [m + 1 ]

    

m

(7) Where α is nonlinear absorption coefficient and Leff is the effective sample length given by

Leff = (1 − e−α 0 L ) / α 0 , L is the sample length and α0 is the linear absorption coefficient. I0 indicates the on-axis peak intensity, z is the sample position, z0 =

πω0 2 is the Rayleigh range, ω0 is the beam waist λ

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 Eqn. (5) and Eqn. (7). The data fitted using Eqn. (7) are also 12 ACS Paragon Plus Environment

Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shown in Figure 9 (Au NPs and PTZ-Au NC), where symbol indicates experimental data and solid lines are corresponding theoretical model. The imaginary part of the third order NLO susceptibility χ (3) from βeff is given by Eqn. (8).

Imχ (3) = n 02ε 0 cλβeff / 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 years7, 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 transition, intraband and interband transition (Figure 10)45.

Figure 10. Excitation and consecutive relaxation scheme in Au nanoparticles

The excitation of Au NPs with 532 nm light source will lead 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 Fermi 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 = 0.55x10-10 m/W) that indicate feeble free carrier absorption. The OA Z-Scan data fitted well with the SA associated TPA model confirming the occurrence 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

of absorption saturation in conjunction with reduced free carrier absorption that explains the intensity dependent NLA behavior46. We had to excite the pure PTZ sample at an input intensity of 0.82GW/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 5 (a)). The OA Z-Scan 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. NLA mechanism of pristine PTZ as well as PTZ-Au NC under nanosecond pulse excitation, can be explained as in the Figure 11 in which, S0 is the ground state and S1 is excited states of PTZ47, 43. For pristine PTZ, the mechanism of NLA was found to be pure TPA since it is having 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 excited state of PTZ by absorbing two photons together, which is known as TPA. But it is feeble due to the restricted electron delocalization by the presence of sp3 hybridized nitrogen48 and hence PTZ shows the poor NLA behavior. When the PTZ forms composite with Au NPs, the NLA property of PTZ get 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.

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

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 the samples 14 ACS Paragon Plus Environment

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 NLA coefficient.

Table 1: Obtained NLO parameters of PTZ, Au NC, PTZ-Au NC and few comparisons

Sample

Isat

βeff/ βTPA

1012 2

10-10 (m/W)

(W/m )

Im χ 10

-12

(3)

(esu)

n2

10

-11

(esu)

Re χ 10

-12

(3)

(esu)

Au

3

βeff = 0.55

1.73

6.8

7.1

PTZ

--

βTPA= 0.026

0.23

2.2

3.1

1.78

βeff = 1.74

5.71

8.0

8.7

4.5

βeff = 1.00

--

--

--

0.25

β = 2.5

--

--

--

--

β = 2.6

--

--

--

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

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 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

transfer between PTZ and Au NPs52, 53. Here NPs can act as 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 NPs54, 55. 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 significantly49, 52, 53. The effect of field intensification depends on parameters like excitation wavelength, separation from the NPs, neighboring medium, and the diameters of core and shell56.

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

The Z-Scan experiment of PTZ-Au NC was done for different input beam intensities (I0= 0.27GW/cm2, 0.82GW/cm2, 1.37GW/cm2, 1.88GW/cm2), to explore the mechanism behind the NLA (Figure 13(a)) and corresponding NLA coefficients were plotted against on axis input intensities as shown in Figure 13 (b). It is found that NLA coefficient is decreasing with increasing on axis intensity. 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. Whereas in the case of pure TPA, the absorption cross section is small compared to ESA. So there will not be any considerable depletion in the ground state population with an increasing input intensity which ultimately makes NLA coefficient intensity independent57.


Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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

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 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 materials47. 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 14 (a), where symbols are experimental data and solid lines are corresponding theoretical fit. 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 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 14 (b). For the sample having LT 67%, we got the onset and the threshold value of limiting action at 0.33 J/cm2 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

limiters so far

47,49, 53

Page 18 of 29

. For example V Mamidala et al. reported that the limiting threshold of the GO +

Porphyrin and Au + Porphyrin complexes were ~1.9 J/cm2 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 J/cm2 and 8.8 J/cm2 for Pt/f-HEG and Pd/f-HEG respectively. But in our studies, 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.

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 PTZ-Au NC with different linear transmittances at peak on axis beam intensity of 0.82 GW/cm2

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, 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


Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


z ) z0 T ( z , ∆Φ 0 ) = 1 + ∆Φ 0 z z (( ) 2 + 1)(( ) 2 + 9) z0 z0 4(

(9)

Here, T is the normalized transmittance for the pure refractive nonlinearity, ∆Φ 0 = kn2 I 0 Leff is the onaxis 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 fit for the transmission Eqn. (9). If ∆φ0

Surface Plasmon Assisted Enhancement in the Nonlinear Optical

Recommend Documents