Enhanced third order optical nonlinearity in ultrathin amorphous film of tetraphenyl-porphyrin in picosecond regime

Optics and Laser Technology 119 (2019) 105642

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Enhanhanced third order optical nonlinearity in ultrathin amorphous film of tetraphenyl-porphyrin in picosecond regime

T

⁎

L.M. Claviana, P.C. Rajesh Kumara, , K.V. Anil Kumarb,1, D. Narayana Raoc, N.K. Shihabc, Sanjeev Ganeshb a

Department of Physics, St Joseph Engineering College, Vamanjoor, Mangaluru 575028, India Department of Physics, Mangalore University, Mangalagangothri, Mangaluru 574199, India c Laser Lab, School of Physics, University of Hyderabad, Hyderabad 500046, India b

H I GH L IG H T S

thin film of 5,10,15,20-Tetraphenyl-21H,23H-porphine (TPP) is fabricated. • Quality developed thin film exhibits excellent fluorescence emission. • The film depicts saturation absorption and self-defocusing optical nonlinearity. • TPP effective nonlinear absorption coefficient (β ) is of the order of 10 m/W. • The • Thin film portrays enhanced NLR index (n ) of the order of 10 m /W. eff

2

−6

−13

2

A R T I C LE I N FO

A B S T R A C T

Keywords: Porphyrin Thermal evaporation technique Picosecond pulsed Z-scan technique Nonlinear saturation absorption Nonlinear refraction

The third order optical nonlinearity of ultrathin 5, 10, 15, 20-Tetraphenyl-21H, 23H-porphine (TPP) films fabricated on an ultrasonically cleaned glass-substrate, by high vacuum thermal evaporation method is investigated. The powder X-ray diffraction (XRD) pattern demonstrates the amorphous nature of the fabricated thin film. The atomic force microscopy (AFM) and the field emission scanning electron microscope (FESEM) images show that the surface morphology of thin film composes of randomly oriented particles with the mean surface roughness of 17.73 nm. The structure of TPP thin film portrays a characteristic UV–Visible spectrum due to π-π* transitions in the porphyrin molecule. The photoluminescence spectroscopic study reveals that the TPP exhibits excellent fluorescence emission from S1 singlet excited state. The third order optical nonlinearity is studied using single beam Z-scan technique at 532 nm with picosecond pulsed (Δτ = 30 ps) laser. The TPP thin film exhibits saturation absorption property, with the effective two-photon absorption coefficient (βeff) of the order of 10−6 m/W. The observed nonlinear saturation absorption behavior is largely influenced by one-photon absorption due to the filling effect of the surface states and the localized defect states in the thin film. The closed aperture Z-scan study highlights the self-defocusing nature of the TPP thin film with negative nonlinear refractive index (n2) of the order of 10−13 m2/W. The enhanced n2 value is attributed to the highly polarizable structure of free base TPP molecule and modified electronic band structure due to the strong intermolecular interactions observed in the condensed state.

1. Introduction Organic materials with highly delocalized π- conjugation system have emerged as a predominant choice for the design and fabrication of nonlinear optical (NLO) materials. Because of their high functionality and matured synthesis feasibility which is compatible with the existing

technologies for the fabrication of integrated optical and electro-optical devices, profound research has been undertaken on organic and organometallic compounds over the last two decades. They are quintessential to accomplish the ultimate goal of device miniaturization by going to the molecular level [1]. However, the practical employability of such compounds in modern day functional devices also demands

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Corresponding author. E-mail address: [email protected] (P.C. Rajesh Kumar). 1 Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India. https://doi.org/10.1016/j.optlastec.2019.105642 Received 3 January 2019; Received in revised form 26 April 2019; Accepted 10 June 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.

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2. Experimental details

long-range structural order (∼a few mm to cm). Therefore, purely from an application perspective in the development of photonic technologies, the ability to prepare high quality thin films of analogous π- conjugated organic molecules with well defined optical nonlinearities is vital [2–5]. Among the various NLO materials investigated, a class of π- conjugated organic molecules known as porphyrins and metalloporphyrins have emerged as promising candidates in the world of cutting-edge photonic technology. Their optimum optical non-linearity, ultrafast response time, broadband spectral response and ease of processing makes them one of the superior NLO candidates [6,7]. Porphyrins are a ubiquitous class of naturally occurring compounds with major biological representatives such as chlorophyll, hemes and vitamin B12 [1]. They are involved in various processes such as oxygen binding, electron transfer, catalysis, light harvesting and photodynamic therapy (PDT) [8,9]. Porphyrins and their derivatives are highly versatile molecules, with wide possibilities of tailoring their physicochemical properties through conformational design, metal complexation, extension of π-conjugation network through various axial ligands and peripheral substituent’s in the macrocycle [7,10–13]. In order to optimize the NLO response of porphyrin based materials, several strategies have been reported on symmetrical and asymmetrical porphyrins [14], influence of solvents [15], impact of metallation [6,9,16], influence of peripheral and axial substitution [17–20], porphyrin oligomers [21], porphyrin arrays [22] and covalently linked porphyrin-graphene hybrid materials [23–25]. However the optimization of the NLO response with above approaches was limited due the poor solubility of the porphyrin derivatives, difficulty to obtain resultant compound with high yield and device compatibility [26,27]. To overcome this limitation considerable attention has been provided in developing high quality thin films of porphyrin derivatives for advanced photonic applications [5,28–31]. It is found that the ultrathin film with highly ordered molecular assembly exhibits exceptional physical and chemical properties including very high NLO susceptibility [1]. Li Jiang et al. [32] have reported the enhanced NLO response in electrostatic layer-by-layer self assembled ultrathin films of negatively charged 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin (DHP) and oppositely charged polyethylenimine (PEI) using Z-scan studies. The thin films also have the advantage of controlling the light-matter interaction efficiently with their variable thickness, light confinement ability at the defect centers and the ability to modify electronic band structure etc. [33]. The structural configuration and the molecular arrangement of the thin film essentially depends on its mode of fabrication, thickness of the film, nature of substrate and the stress due to buffer layer [34–38]. The restriction on solubility and the advantage of higher thermal and chemical stability of porphyrin derivatives provides a huge opportunity to explore the nonlinear optical features of porphyrin, in the form of thin films developed using thermal evaporation technique. Although several inorganic NLO thin films have been successfully fabricated using thermal vapor deposition [39], the NLO properties of porphyrin thin films fabricated using the aforementioned technique is yet to be explored. Since the structural integrity of the porphyrin molecule is fully preserved in thermal evaporation technique, it provides added advantage over other methods in preparing thin films and offers a huge scope in optimizing its NLO properties [27]. The porphyrin thin films developed using thermal vapor deposition technique can practically give a massive boost towards the development of current generation all-optical devices. In the present work we report the fabrication of metal free tetraphenylporphyrin thin film on glass substrate via high vacuum thermal vapor deposition technique. The nonlinear optical parameters and the underlying NLO phenomena were explored using Z-Scan method with picosecond pulsed laser at wavelength of 532 nm. The enhanced NLO response in the TPP thin film obtained using remarkably simple fabrication technique has great potential in modern optical device fabrication.

2.1. The fabrication of TPP thin films A dark violet powder of 5,10,15,20 Tetraphenyl 21H,23H-porphine (TPP) procured from Sigma Aldrich was used without further refinement for the preparation of thin films. An ultrasonically cleaned glass substrate was employed as a base material for the fabrication of thin films of TPP via Hind Hi Vacuum thermal evaporation coating unit. The TPP powder was sublimated from a molybdenum boat sample holder, also used as a heating element. The rate of material deposition was maintained at 2.5 nm/s, and recorded using a quartz crystal thickness monitor attached to the film coating system. The thin films were coated over a glass substrate of dimension 37 × 12 × 1 mm, at room temperature under a pressure of 10−5 Torr. Uniform high quality thin films of TPP with thickness of the order of 120 nm were prepared using this method. 2.2. Characterization of TPP thin films The structural analysis of the TPP thin films were analyzed by Rigaku Miniflex 600 powder X-Ray diffractometer, equipped with Cu Kα/40 kV/15 mA radiation source (λ = 1.54056 Å), with Ni Kβ filter at scanning speed of 2 0/min. The ground state absorption spectra of TPP thin film was studied using Shimandzu 1800 UV–Vis spectrophotometer in the wavelength region from 300 to 1100 nm. The steady state emission spectrum of the film was obtained using Fluorolog, Horiba JobinYvon spectroflurometer. Atomic force microscopy (AFM, Agilement 5500) was employed to study the topography and measure the surface roughness of the thin film through non-contact mode. CarlZeiss FESEM was used to obtain the high resolution surface images of the developed TPP thin films. The third order nonlinear optical performance of TPP thin film was studied by employing single beam Z-scan technique proposed by shiek-Bahae et al. [40]. This method measures the optical nonlinearities of samples by detecting the transmittance of a tightly focused Gaussian beam as a function of position of the test sample. The Z-scan system is comprised of frequency doubled Nd:YAG laser source with a repetition rate of 10 Hz, pulse width of 30 ps and a wavelength of 532 nm with a maximum average power of 1.55 mW. The focal length of the lens, the beam waste at focal point (ω0) and the Rayleigh range (z0) was 0.11 m, 2.484 × 10−5 m and 3.641 mm, respectively. The linear transmittance of the TPP thin film at the probing wavelength was found to be 37.79%. The nonlinear transmittance observed through the sample at position ‘z’ with respect to the focal plane (z = 0) in the Z-Scan experiment was measured without an aperture (open aperture Z-scan) to analyze the nonlinear absorption behavior. Whereas the transmittance of the tightly focused beam measured through an aperture at the far field (closed aperture Z-scan) contains the information on nonlinear refraction. The Z-scan system was calibrated by using CS2 as the standard test sample before carrying out the present study. The values of nonlinear coefficients presented here are accurate within 10% error limit and the errors arise due to uncertainties in the estimation of the spot size at focus, measurements of peak intensity, fitting procedures, calibration of neutral density filters etc. 3. Experimental results and discussion 3.1. Powder X-ray diffraction The XRD pattern obtained at room temperature, of the fabricated TPP thin film over a glass substrate is displayed in Fig. 1. The diffraction pattern depicts a broad peak around 2θ = 25.90 indicating that the obtained thin film is amorphous in nature due to the random molecular arrangements. The previously reported studies shows that the TPP powder embodies polymorphic crystalline structures such has triclinic, tetragonal, orthorhombic and monoclinic forms [41,42]. Michio Ashida 2

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“meso-phenyl” substituent’s and the porphine π system is due to the steric-hindrance between the hydrogen atoms on the porphine ring and phenyl rings, resulting in a large dihedral angle between the planes of phenyl groups and porphine π system [52]. The minor splitting observed in the soret absorption band at around 416 nm is due to the interaction between two or more molecules in a unit cell of the aggregate resulting in two or more excitonic transitions (Bx and By) also known as davydov splitting [53]. According to the exciton model developed by Kasha et al. [54] the splitting or shift in absorption band is induced by the interaction of the localized transition dipole moments. The extent of davydov splitting depends upon the number of interacting molecules, distance between the interacting molecule, the angle of transition dipole moments with the aggregate and the angle of their transition dipole moments between neighboring molecules [55].The Gouterman four orbital model parameterizes the porphyrin molecular orbital configuration, which states that the Q and B absorption bands are formed due to excitation from the two highest occupied molecular orbital’s (HOMO) 1a1u(π) and 4a2u(π) to the lowest unoccupied molecular orbital (LUMO) level 5eg(π*) [44,56,57]. The Q band absorption transitions have smaller oscillator strength due to opposite directions of the transition dipoles, resulting in cancellation of electric dipoles and therefore reduction in the absorption intensity. While on the contrary, in the B band absorption region the transition dipoles are parallel and shows large oscillator strength resulting in high intensity absorption bands [58]. The type of electron transitions and the value of optical band gap Eg of TPP thin film can be determined using the equation given in reference [41].

Fig. 1. Powder X-ray diffraction pattern of TPP thin film.

et al. [43] have developed TPP thin film over (0 0 1) plane of KCl substrate using vacuum evaporation method and found that the film depicted monoclinic crystalline structure. This indicates that the order of molecular arrangement in TPP thin film depends both on the fabrication technique and the nature of the substrate.

3.2. Linear optical absorption spectroscopy The UV–Visible absorption spectra of the developed tetraphenylporphyrin thin film displayed in Fig. 2 can be categorized into two regions, the intense soret band (B band) region and the less intense Q band region. The soret band depicts maximum absorption at 437 nm and is attributed to S0 → S2 singlet transition. The four Q band absorption peaks denoted by Qx(0,0) at 656 nm, Qx(1,0) at 598 nm, Qy(0,0) at 562 nm and Qy(1,0) at 524 nm are attributed to S0 → S1 singlet vibronic transitions [44,45]. This absorption spectrum is similar to that of TPP dissolved in benzene solution and TPP dissolved in nematic liquid crystals, which depicted soret band in the region of 400–490 nm and Q band absorption from 510 to 690 nm [46,47]. Therefore, this verifies that the observed vibronic transitions are not due to the simple interference effect observed in the thin films. It was originally identified by J.R Platt [48] as vibration on the basis of relative constant energy difference between Q(1,0) and Q(0,0) absorption peaks. Further from the theoretical and Raman analysis these vibrational transitions are been attributed to vibrations of symmetry A1g, A2g, B1g and B2g [49,50]. These observed absorption peaks are as a result of π-π* transitions between the bonding and anti-bonding molecular orbital of porphyrin rings [51]. It is observed that the phenyl substituent in the meso-positions of the porphine macrocycle have a trivial effect on the absorption spectra. This lack of interaction between

(αhυ) = α 0 (hυ − Eg )r

(1)

where α0 is the constant, α is the linear absorbance corresponding to the frequency ν and Eg is the optical band gap. The dependence of (αhν)1/r on the photon energy (hν) for onset and fundamental energy gaps is plotted and discussed using Tauc’s plot (Fig. 3 and Table 1), where r = 1/2 for allowed direct band gap transitions and r = 2 for allowed indirect band gap transitions. The best plot was obtained for r = ½ indicating that the fabricated TPP thin film is an indirect band gap material. The first optical energy gap Eg corresponds to the onset of the optical absorption and formation of bound electron hole pair, or exciton, also known as ‘‘Frenkel exciton” [59]. The fundamental energy gap is the energy gap between HOMO–LUMO bands. The energy levels in between are either traps or impurity energy levels [55]. The obtained results on linear optical study are in good agreement with the previously reported observations [41]. 3.3. Photoluminescence spectroscopy The Photoluminescence (PL) spectra of the TPP thin film recorded at room temperature is depicted in Fig. 4. The fabricated thin film of TPP

Fig. 2. Linear absorption spectra of pristine TPP thin film.

Fig. 3. Band gap calculation using Tauc’s plot for pristine thin film. 3

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[67].

Table 1 Indirect energy band gaps of TPP thin film.

3.5. Field emission scanning electron microscope

Onset energy gap Fundamental energy gap TPP absorption bands

Qx(0,0)

By

Qy(1,0)

Qy(0,0)

Qx(1,0)

As deposited (eV)

1.71

2.61

2.16

2.03

1.83

Field emission scanning electron microscopy (FESEM) images in Fig. 6 provides further insight into the domain structure of the TPP thin film. It can be clearly observed that the FESEM images are in par with the morphological results procured using AFM. The FESEM images of the TPP thin film reveals that the film is continuous, homogeneous and uniform. From Fig. 6(a) it is observed that the surface of the thin film is textured in the form of small grains and elongated fibre-like structures. The FESEM image obtained over 100 × 100 nm2 surface area (Fig. 6(b)) reveals that the film is composed of closely-packed, randomly oriented, irregular shaped granules of the TPP molecules with some local defects and grain boundaries. 3.6. Third-order nonlinear optical properties The third order nonlinear optical (NLO) properties of TPP thin film is probed by means of a standard Z-Scan technique. This method enables simultaneous measurement of nonlinear absorption and refraction coefficients along with their sign of nonlinearity. The sign of nonlinearity is a very important entity for the practical application in optical signal processing devices. According to the Z-scan theory, by numerically fitting the nonlinear transmission curve with the appropriate equation the order and underlying mechanism of optical nonlinearity can be predicted. In our case, the best fit for the open aperture nonlinear transmission curve and the two photon nonlinear absorption coefficient βeff (m/W), was obtained by using equation [68],

Fig. 4. Photoluminescence spectra of TPP thin film.

was excited at an incident wavelength of 437 nm (max. absorption peak wavelength). The confirmation of negligible contribution by the glass substrate towards the PL spectra at the same excitation wavelength was also recorded. The PL spectra of TPP thin film depicts excellent fluorescence emission peaks at 663 nm denoted as Q(0,0) and at 729 nm denoted as Q (0,1) due to S1 → S0 transitions [60]. The obtained emission spectrum is similar to that of TPP dissolved in benzene solution [60]. The emission spectra is mirror symmetric to the absorption spectra in the Qx region [61]. The steady state fluorescence emission wavelength and intensity is found to be independent of the excitation wavelength, due to the rapid non-radiative internal conversion of the higher excited singlet states Sn to S1 [62]. The perturbation on the fluorescence yield due to the peripheral substitution is frail, owing to the very large dihedral angle between porphin π-system and phenyl rings [60]. We also searched for phosphorescence from the thin film at room temperature by scanning out to 1000 nm, but no phosphorescent emission could be unambiguously detected. Paul G.Seybold and Martin Gouterman have reported the fluorescence quantum yield (ϕf) = 0.11 for TPP in benzene solution at room temperature [63]. The lifetime of Q (0,0) emission band in the same solvent measured by single photon counting was found to be 12.4 ns [64].

⎡1 − T (z , S = 1) = ⎣

α 0 LIs IS + I0 / (1 + x 2)

−

(1 − α 0 L)

βeff I0 L 1 + x2

⎤ ⎦ (2)

where x = z/z0, z is the distance of the sample with respect to the focal point (z = 0), z0 = 2πω02/λ is the Rayleigh range of the incident Gaussian laser beam, λ is the wavelength of the incident laser, ω0 is the Gaussian beam spot radius at focus, α is the linear absorption coefficient, L is the sample thickness, I0 is the on-axis laser intensity at the focal point, and Is is the saturation intensity threshold for nonlinear saturation absorption. Fig. 7 depicts the normalized open aperture (OA) transmittance curve of thermally evaporated TPP thin film over a glass substrate. The measured symmetric bell shaped transmission profile with a maximum at focus, is attributed to the nonlinear saturation absorption (SA) nature of TPP thin film. The experimental curve at the far field is nearly flat indicating linear regime in the far field. In order to obtain approximate normalized transmission values, the experimental transmission values were first divided by the far field linear regime transmission values. The observed saturation absorption behavior is interpreted in-terms of lifetimes of the excited states as depicted in Fig. 8. To determine the value of nonlinear refractive index n2 (m2/W), the Z-Scan experimental data were analyzed using procedure stated by M. Shiek-Bahae et al. [40]. The theoretical normalized closed aperture transmittance was obtained using equation

3.4. Atomic force microscopy The two and three-dimensional (2D and 3D) surface morphology images of TPP thin film recorded at room temperature using Atomic force microscopy (AFM) are as shown in Fig. 5. The surface of the thin film was scanned over an area of 6 × 6 µm2. As observed in Fig. 5(a) the surface of TPP thin film presents randomly oriented small grains and long slender casuarina leaves like shaped particles. From Fig. 5(b) it is observed that the topography of the TPP thin film comprises of stacked aggregates (H- aggregates) formed during the adsorption process [41,65]. The larger sized particles observed on the surface of the film is due to the growth of the particles by means of fusion with adjacent particles in the presence of sufficient surface energy [66]. The AFM observations revealed that the film has a mean square root surface roughness of 17.73 nm. This observed surface roughness is attributed to the non-uniform distribution of grains in the pristine thin film due to the nucleating behavior of the molecules at lower substrate temperature

T (z , Δ∅0 ) ≅ 1 −

(4Δ∅0 x ) (x 2 + 9)(x 2 + 1)

(3)

where Δϕ0 is the on-axis peak nonlinear phase shift with the sample at the focus of the lens. Δϕ0 can be obtained by fitting the experimental closed aperture Z-Scan data with Eq. (3). The third order nonlinear refractive index n2 is then related to Δϕ0 by equation [40].

n2 =

|Δ∅0 |λ 2πI0 Leff

(4)

where Leff = [1 − exp(−α 0 L)] is the effective thickness of the sample. 4

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1.2μm (a)

(b)

Fig. 5. Atomic force microscope image of TPP thin film (a) 2D image (b) 3D image.

Fig. 6. FESEM images of TPP thin film scanned over (a) 1 × 1 µm2 (b) 100 × 100 nm2 surface area.

Also, it should be noted that with such low intensity of incident pulses, the effect of both high-order nonlinearity and nonlinear scattering are mitigated. The TPP thin film has remained stable even after the exposure to laser pulses and no holes were generated. This was confirmed by the identical absorption and emission spectra recorded at the same spot before and after the exposure to laser beam. In order to validate the present results, repeated Z-scan experiments were carried out on the same spot of the film. The obtained Z-scan plot depicted similar nonlinear behavior. This confirms that the films were not damaged in the process of the experiments at which the laser power was operated and the observed results are not due to the ablation damage in the film at the focus. The Fig. 7 reveals that the normalized nonlinear OA transmittance of the TPP thin film is about unity when the sample is far-off from the focal point. The transmittance then gradually increases as the sample approaches the focus and reaches maximum at focal point due to the bleaching of the ground states of TPP molecule, with the increase in the intensity. It is observed that the occurrence of saturation absorption (SA) heavily depends on excitation wavelength, on axis peak intensity, doping concentration and molecular structure [69]. As the wavelength of excitation is shifted closer to the wavelength of resonant absorption, ground state absorption cross section (σ0) increases and vice-versa [11,70,71].The linear absorption of TPP thin film at wavelength of 532 nm which lies on the blue side of Qy(1,0) band is petite but it is high enough for resonance absorption. Therefore we can expect the nonlinear saturation absorption from the singlet state S1 due to the near resonance effect [11]. Based on the literature pertaining to the exited state lifetimes and

Fig. 9 depicts the normalized closed aperture (CA) Z-Scan transmittance curve of TPP thin film coated on a glass substrate. The CA transmittance signal profile is characterized by a peak prior to the focal point (z = 0) and then followed by a valley subsequent to the focal point. This CA transmittance signal pattern indicates negative optical nonlinearity and is a distinct signature of self defocusing behavior of the propagating wave in the film. The imaginary and the real part of the third order susceptibility are obtained from the following equations [40]

Im (χ (3) )=

λcn02 ∊0 βeff 2π

Re (χ (3) ) = 2n02 ∊0 cn2

(5) (6)

where n0 is the linear refractive index, ε0 is the permittivity of free space, and c is the speed of light. From the above equations the values of two photon nonlinear absorption coefficient (βeff), third order nonlinear refractive index (n2), saturation intensity (Is), and the resulting real and imaginary part of third order nonlinear susceptibility (Re χ(3) and Im χ(3)) were obtained at various input on axis peak intensity (Table 2 and Table 3). Reasonably good matches between the experimental data and the theoretical fits are observed indicating that the experimentally detected NLO effects have effective third order characteristics. The OA and CA transmittance curve for glass substrate measured using similar parameters is flat at 532 nm, which infers that the contribution of glass substrate towards the observed optical nonlinearity is zero (Supplementary data). 5

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Fig. 7. Open aperture Z-scan transmittance curve of TPP thin film with variable on axis peak intensities (Dotted plots corresponds to experimental graph and continuous line corresponds to theoretical fit).

time scale [72]. The excited state absorption (ESA)/sequential two photon absorption further occurs from the singlet state S1 to a higher singlet state S2. The relaxation dynamics from the excited singlet state S2 to S1 is surprisingly fast due to the ultra-short lifetime (τS2 = 68 ± 15 fs) of the S2 state [73–75]. The molecule then, from the excited state S1 can either undergo inter-system crossing to triplet state T1, or decay directly back to the ground state S0 nanosecond time-scale. However, it is found that with pico-second pulse excitation the NLA from the triplet transitions is negligible because of the slow intersystem crossing (ISC) rate (∼22 ns) when compared to the excitation pulse width [17,76,77]. The measured high fluorescence quantum yield (ϕf = 0.11) and large fluorescence lifetime (τf = 12.4 ns) in the TPP molecule implies that the S1 → S0 is more dominant channel of Qx(0,0) relaxation in the TPP molecules [20,76,78,79]. The third order nonlinear optical properties of TPP explored by Guanghong Ao et.al. [80] in CH2Cl2 solution using Z-scan technique under nano second and pico-second regime at 532 nm, reported that the material exhibited reverse saturation absorption (RSA) behavior. Conventionally, it has also been found that the optical nonlinearity of thin films are influenced by factors such as the grain size, the surface morphology and the defects [5]. In amorphous thin films, besides two photon nonlinear absorption, one photon absorption (OPA) also contributes towards SA due to the localized defect states for all pulse durations [39,81]. The localized defect states effectively blur the energy gap in the material, resulting in the filling effect of the localized states. Saturation occurs only if the trapping duration by the localized defect states are longer than the laser pulse duration [36,39,81,82]. The transformation of nonlinear absorption nature from RSA in TPP solution, to saturable absorption in TPP thin film can also be attributed to

Fig. 8. Energy level diagram of tetraphenyl porphyrin.

the dynamics of the TPP and similar molecules, a self - consistent standard three level model as shown in Fig. 8, is used to elucidate the observed saturation absorption behavior of the TPP thin film in picosecond regime. The laser light excites molecules from the ground state S0 to the various vibrational–rotational states in the first singlet state S1 (S0 → S1). The molecules then rapidly undergoes vibrational relaxation to a thermally equilibrium level of this electronic state in 20 ± 1.6 ps 6

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Fig. 9. Closed aperture Z-scan transmittance curve of TPP thin film with variable on axis peak intensities (Dotted plots corresponds to experimental graph and continuous line corresponds to theoretical fit).

the surface effect observed in the ultra-thin film. When the TPP thin film is subjected to high intensity laser beam, the excited electrons introduced by laser would be trapped by the surface states in the film rather than in the excited states [26]. In addition, the slight broadening and splitting of the soret band in the solid films of porphyrin derivatives compared to solutions reflects the intermolecular interaction resulting in the modification of the energy level configuration in TPP thin film [21,83]. The conjugated porphyrin molecules are well known for readily forming J or H aggregates in a highly condensed system and thereby further altering the energy level configuration and the NLO properties of the porphyrin materials [29,84]. The H-aggregates in the amorphous morphology of the TPP thin film are characterized by excition energy lower than that of an individual molecule in less ordered domain of the film. These H aggregates may act as exciton traps and can influence the magnitude of the molecular second hyperpolarizability (γh) of the TPP thin films [41,85]. To further comprehend the exact mechanism behind the observed NLA behavior in TPP thin film, the open aperture Z-scans were conducted at different input on axis peak intensities(I0) (Fig. 7). It is apparent from the Fig. 7 that there is no swapping of the nonlinear

Table 3 Third order nonlinear optical refraction parameters of the TPP thin film. Input on axis peak intensity I0 (W/m2)

Nonlinear Refractive Index n2 (m2/W)

6.681 × 1013 2.117 × 1013 1.336 × 1013 5.319 × 1012

−(2.384 −(6.156 −(1.463 −(5.444

± ± ± ±

0.143) × 10−14 0.430) × 10−14 0.08) × 10−13 0.435) × 10−13

Real χ3 (esu)

(3.209 (8.288 (1.969 (7.329

± ± ± ±

0.193) × 10−16 0.580) × 10−16 0.118) × 10−15 0.586) × 10−15

absorption nature of the thin film with the change in input peak intensity. It is observed From the Table 2 that the effective TPA coefficient (βeff) decreases with the increase in the input peak axis intensity (I0). This decrease of βeff value with increase in I0 is attributed to the upsurge in the filling effect of the localized defect states, and therefore results in the enhancement of the one photon absorption contribution to the observed saturation absorption effect [39]. It is also observed from Table 2, that despite the change in the on-axis peak intensities, the Isat value remains constant. This is because the thickness of the film is kept constant resulting in the fixed number of localized defect states

Table 2 Third order nonlinear optical absorption parameters of the TPP thin film. Input on axis peak intensity I0 (W/m2)

Saturation Intensity Is (W/m2)

Two photon nonlinear absorption coefficient βeff (m/W)

6.681 × 1013 4.225 × 1013 8.430 × 1012 5.319 × 1012

0.9 × 109 0.9 × 109 0.9 × 109 0.9 × 109

−(0.99 −(1.57 −(3.65 −(1.50

7

± ± ± ±

0.05) × 10−7 0.08) × 10−7 0.22) × 10−6 0.09) × 10−6

Imaginary χ3 (esu) (5.646 (8.954 (2.082 (8.554

± ± ± ±

0.282) × 10−17 0.448) × 10−17 0.125) × 10−15 0.513) × 10−16

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electrostatic layer-by layer self assembled thin films of 5,10,15,20-tetrakis-(3,4,5-trihydroxy-phenyl)-porphyrine (DHP) and oppositely charged poly{(2, 5-bis (3-bromotrimethylammoniopropoxy)-phenylene-1,4-divinylene)-alt-1,4-(2,5-bis(2-(2-hydroxyethoxy) ethoxy)) phenylenevinylene} (BH-PPV) at 532 nm and 8 ns pulse width. Pengxia Liang et al.[13] have reported the β and n2 of the order of 10−10 m/W and 10−18 m2/W for a series of meso-extended porphyrin derivatives with various electron donating and withdrawing substituent’s at 532 nm and 12 ps pulse width in tetrahydrofuran solvent. The imaginary and real third-order nonlinear optical susceptibility measures the speed of response of material to the perturbation induced by the laser pulses. Arun K. Sinha et al. [92] have reported the χ3 value of 3.25 × 10−10 esu for the thin films of SOG polymer containing triethoxysiloxane substituted TPP using DFWM method at 532 nm. Here the high value of χ3 was attributed to the combined contribution of the porphyrin and polymer molecules. Farooq Khurum Shehzad et al. [19] have reported the χ3 value of the order of 10−10 esu for the supramolecular compounds of tris(alkoxo) ligand functionalized Andersontype heteropolyoxometalate anions with [H2TPP]2+ cation at 532 nm with pulse duration of 6 ns. This enhanced third order nonlinear susceptibility χ3 was due to the easier electronic charge transfer from porphyrin to anions upon irradiation from laser pulses. The obtained χ3 values of TPP thin film is purely due to the porphyrin molecule unlike the value reported in the literature for various compounds, as these χ3 values were influenced by strong coupling unit or the solution.

between the energy band gap [81]. The highly versatile and flexible structure of TPP molecule facilitates the large nonlinear refractive index (n2) as observed in Fig. 9. The absence of central metal ion lowers the symmetry of the TPP molecule (D4h → D2h) and also results in the reduction of its stability. Thus the electron cloud in the free base TPP molecules are more susceptible to distortion in contrast to the stable metal TPP configuration [4,75,86]. The higher frequency vibrations due to the N-H vibrational modes in the free base, which are non-existent in the metal TPP derivatives, also enhances the change in polarizability to the applied electromagnetic field [75]. Here the focus is on the physical mechanism of the nonlinear refraction observed in TPP thin film developed over a glass substrate using thermal evaporation method. Closed aperture scans is performed at intensities where the contribution from the fifth order nonlinear effects are negligible (the value of Δϕ0 estimated is ≤π). In the case of self – defocusing behavior, the sample acts like a thin lens converging the incoming beam at the pre-focal positions and then diverging after focus due to its negative nonlinearity [40]. The CA signal profile of TPP thin film shows equivalent separation of peak and valley with respect to the focal point ( ± 3 mm). According to the model proposed by M. ShiekBahae et al. [40], in closed aperture Z-scan, the value of ΔZp-v ≈ 1.7z0 for non-thermal NLO process and is greater than 1.7z0 for thermal process. In the present case the closed aperture Z-scan depicts ΔZpv = 6 mm, which is approximately equal to 1.7z0, indicating the presence of Kerr-type of nonlinearity [30]. The accumulative thermal effect could play a significant role only in the case of high repetition rates (order of MHz) and long pulse duration (order of ns). As pico-second pulsed laser with pulse width of 30 ps and with a short repetition rate of 10 Hz is used in the present experiment, the influence of thermal effect towards the nonlinearity could be neglected [87]. Consequently, the mechanism behind the observed NLR in TPP thin film is predominantly ascribed to the polarization and population redistribution due to the electronic effects. The nonlinear NLR parameters of thermally evaporated TPP thin film as reported in Table 3, are significantly more than the previously reported values [80]. The increased value of n2 emanates from the strong interaction between individual molecules observed in the densely packed thin films and also could be explained in terms of peak intensities used in this studies [88]. This fact points out that the molecular susceptibility as well as optical properties of the TPP thin film is highly sensitive to the order of molecular arrangement. It is also reasonable to believe that the molecular stacking enhances the coupling of π-electron orbital of the macrocycles with neighbors and hence the third order nonlinear response. This also implies that the Z-scan response depends not only on the instantaneous electronic polarization but also on the orientational and vibrational effects of the molecules [1,4,89]. The significant increase in the n2 value may also be due to the fact that the TPP thin film has larger linear absorption coefficient at 532 nm [90]. The present investigation shows that the n2 and βeff values of TPP thin films are not similar to that of the TPP in solution form. Moreover the observed NLA in-case of the TPP thin film is due to SA and as it is reported in the literature the NLA in TPP solution is due to reverse saturation absorption (RSA) [80]. It is worth noting that this optical nonlinearity pattern of TPP thin film is in accordance with the highly ordered nanosphere films of 5,10,15,20-[1,4-benzodioxane- 6-carboxalde]porphyrin (TEOP) under ns and ps laser excitation [91]. Similar behavior of flip in NLO property was also reported by Guanying Zhu et al. [26] in meso-tetrakis(p-carboxyphenyl)porphyrin (TCPP) solution and the layer-by-layer assembly of composite (TCPP/LDH)n/LDH and (TCPP/P5W30/LDH)n/LDH ultrathin films. The third order nonlinear optical parameters of the fabricated TPP thin film summarized in Table 2 and Table 3 are comparable to some of the recently reported studies on the porphyrin derivatives. Li Jiang et al [88]. have reported β = −1.9 × 10−5 m/W and n2 = −5.57 × 10−12 m2/W for the

4. Conclusion In summary, homogeneous and high quality thin films of TPP were successfully fabricated over a glass substrate using high vacuum thermal evaporation technique. The structural analysis reveals that the obtained thin films are amorphous in nature. The TPP thin film demonstrates excellent linear absorption in the visible region. The morphological studies show that the surface of the thin film is made up of randomly oriented particles with fairly rough surface due to the nucleation of TPP grains. The steady state emission spectral study reveals that the TPP thin film exhibits excellent fluorescence emission from first excited singlet state S1. The open and closed aperture z-scans show that the ultrathin TPP film exhibits saturation absorption and self-defocusing optical nonlinearities. The investigation also reveals that the filling effect of surface states and localized defect states observed in the amorphous TPP thin film has a profound impact on the nonlinear saturation behavior. The UV–Visible study gives an insight that the meso substituted phenyl rings have a minute effect on the nonlinear properties due to the large dihedral angle with the porphyrin macro-cycle. The present study justifies the fact that the nonlinear absorption behavior of the material depends on the laser pulse width, absorption cross sections, and the lifetime of excited states. This work has also enabled us to identify that, in addition to molecular structure, the order of molecular arrangement also performs a vital role in optimizing the nonlinear optical properties of the material. These investigations show that the TPP thin films can be one of the promising NLO materials, whose property may be widely exploited for passive mode-locking or Q switching of ultrafast laser systems. As TPP thin films offer enhanced NLO properties and size compactness, they can be most preferred materials for photonic integration. The present work highly encourages us to design and explore porphyrin-based NLO materials possessing excellent optical nonlinear responses.

Acknowledgements This work has been supported by DAE-BRNS, Govt. of India (Project Number: 34(1)/14/30/2014-BRNS).

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Appendix A. Supplementary material

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