Contribution of two-photon and excited state absorption in ‘axial-bonding’ type hybrid porphyrin arrays under resonant electronic excitation
Introduction
Organic materials with delocalized electrons have attracted basic as well as applied research because of their large nonlinear optical susceptibilities, architectural flexibility, and ease of fabrication. The delocalization of an electron in the molecular frame enhances the optical nonlinearity and this has been one of the most successful strategies for improving nonlinearity. The optical and electronic properties of many oligomeric conjugated systems have been investigated to determine structure–property relations [1] focusing on the third-order nonlinear optical susceptibilities (χ(3)) [2], [3]. Conjugated organic materials have been widely investigated in search of suitable chromophores for all-optical applications like switching and limiting [4]. Metalloporphyrin and metallophthalocyanines form an important class of electronic materials exhibiting nonlinear absorption, due to their large π-electron conjugation, high values of χ(3) and small HOMO–LUMO gaps with two-dimensional conjugated molecular structure [5], [6], [7]. The architectural flexibility like axial substitution of heavy atom and electron-donor subunits linked to the central metal atom [8], [9], [10] in porphyrins provides extra ways of optimizing the nonlinear absorption (NLA).
Excited state absorption (ESA) and two-photon absorption (2PA) are the important processes leading to NLA behaviour in metalloporphyrins. Linear absorption in a chromophore is a χ(1) process and ESA is a sequential χ(1):χ(1) process effectively giving rise to third-order nonlinearity. 2PA itself is a χ(3) process involving four fields. An electronic nonlinear optical process is said to be resonant when the excitation wavelength is closer to any absorbing electronic transition. Under resonant and near resonant excitations, ESA is the dominant mechanism in metalloporphyrins [11], whereas, under nonresonant excitation 2PA dominates the NLA behaviour [11], [12], [13]. Under certain excitation conditions, both ESA and 2PA may be operative simultaneously leading to higher nonlinearities [14]. In this Letter, we discuss the contribution of ESA and 2PA to the NLA with increasing excitation intensity in ‘axial-bonding’ type hybrid porphyrin arrays based on tin(IV) tetratolylporphyrin (SnIVTTP) scaffold under resonant excitation. We present observations of open aperture Z-scan technique and degenerate four wave mixing (DFWM) in the picosecond (ps) regime and attribute the observed NLA to third- and fifth-order optical nonlinearities to the excited states of the molecules.
Section snippets
Axial-bonding type hybrid SnIVTTP arrays
Monomer and dimers are synthesized following the procedure reported in the literature [15]. The nomenclature of monomer molecule is meso-5,10,15,20-(tetratolyl) porphyrinato tin(IV) dihydroxide; [(TTP)SnIV(OH)2]; and is represented as SnTTP in this Letter. The architecture of the trimer arrays is such that SnIV complex of meso-5,10,15,20-(tetratolyl)porphyrin forms the basal scaffolding unit, and free-base, NiII porphyrins occupy the two axial sites via an aryloxy bridge [16]. The nomenclature
Experimental details
Second harmonic (532 nm) radiation from a hybrid mode-locked Nd:YAG laser giving 25 ps pulses at 10 Hz repetition rate is used in our studies. Open aperture Z-scan [18] is carried out by focusing the input beam on to the sample with linear transmission of ∼75% at 532 nm using a lens of 500 mm focal length to 60 μm spot-size at focus giving peak intensities in the range 5–50 GW cm−2. The transmitted light is collected using a photodiode. We have employed the standard backward DFWM phase conjugate
Nonlinear absorption and optical nonlinearities
The open aperture Z-scan curve representing NLA of Sn2-Zn4(TTP)6 hexamer is shown in Fig. 3. NLA shows a totally different behaviour with increasing intensity. In the intensity range of 5–10 GW cm−2 these oligomers show only RSA due to ESA. With the increase of input intensity to 10–25 GW cm−2 we observed SA in RSA due to the saturation of ESA in the S1 → Sn transition. At further higher intensities (35–50 GW cm−2) RSA within saturation of ESA due to 2PA is observed. In Sn-H2 oligomers initial RSA is
Acknowledgements
Financial support from the Department of Atomic Energy – Board of Research in Nuclear Sciences, and Defence Research Development Organization, India, is acknowledged. P.P. Kiran thanks the Council for Scientific and Industrial Research, India, for the Senior Research Fellowship. Authors thank Dr. Suneel Singh, University of Hyderabad, for discussions with rate equations.
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