Resonant plasmon - stimulated nonlinear absorption in three - level systems

Mechanisms of resonant nonlinear absorption in the hybrid exciton-plasmon systems based on gold nanoparticles and macroheterocyclical compounds are presented. We used novel substituted subphthalocyanines (SubPc) with 14 π-electronic system as powerful excitonic 3 – level systems, and 50 nm gold nanoparticles (AuNp) as plasmonic structures. We succeeded to define mechanisms of resonant nonlinearities and to show that local field factor (LFF) and Purcell factor (PF) lead to dramatic enhancement in SubPc nonlinearity. We suggest a theoretical model which describes exciton-plasmon interaction of porphyrin/phthalocyanine related compounds with gold nanoparticles in the resonant high power laser field. Unique optical properties of hybrid structures based on metal nanostructures and chromophore molecules are very promising in nonlinear optics. Nowadays different hybrid plasmonic nanosystems are used as a basic part of optical modulators and switchers [1,2], nanolasers [3,4], high-sensitive sensors [5,6], harmonic generation materials [7], optical limiters etc. Tight “mixing” of plasmonic properties of metal structures with excitonic properties of chromophore is a key feature of resonant nonlinearity, it leads to dramatic changings in optical nonlinear properties in the whole excitonplasmon system. There are two major reasons for it. The local field factor i.e. enhancement of the near field intensity would result to increase of the nonlinear responses, on the other hand enhancement of the decay rate of excited states would lead to acceleration of the rate in a radiative relaxation process. In spite of very different time scales of the processes in a uncoupled regime, when chromophore and plasmon structures don’t interact, in the tightly “mixing” exciton-plasmon regime both processes can have similar time scales. Therefore relaxation and excitation rates can turn from independent values into strong correlated ones. In addition to modification of the rates of electronic transitions in plasmon-coupled excitonic systems, the impact of strong near-field on chromophores can lead to appearance of parametric nonlinearities, such as multiphoton absorption and others. Therefore it is very difficult to predict a character of nonlinear properties in general, without full description of the exciton-plasmon system’s dynamics. Among promising nonlinear optical (NLO) organic materials, macroheterocyclical compounds hold a special place [8] due to their large conjugated electronic system. This is the origin of considerable nonlinear response in such compounds. High structural flexibility of the molecule allows one to construct new materials with prescribed NLO properties. According to this, researchers paid great attention to develop novel NLO devices based on different macroheterocyclical chromophores [9,10]. In particular, subphthalocyanine (SubPc) compounds are good candidates to be used in harmonic generation [11,12] and optical modulating/switching devices [13]. We believe that unique optical properties of plasmonic structures together with noticeable NLO properties of macroheterocyclical compounds are very encouraging to enhance the efficiency of SPbOPEN2015 IOP Publishing Journal of Physics: Conference Series 643 (2015) 012049 doi:10.1088/1742-6596/643/1/012049 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 existing NLO devices, as well as to reduce correspondent thresholds, and allow to discover completely new applications. The first step is a study of light absorption properties. Thus, the subject of our research is an experimental and theoretical study of resonant plasmon-stimulated nonlinear absorption in three-level chromophore systems, investigation of the involved mechanisms. 2. SYNTHESIS of nano-hybrids An aqueous solution of gold nanoparticles (AuNp) capped with citric ion was prepared by reduction of HAuCl4 with sodium citrate as described previously elsewhere [14]. The mean diameter evaluated from transmission electron microscopy (TEM) images was equal to 50±5 nm. Hexa-phenoxy substituted subphthalocyanines, bearing a halogen atom (chlorine) in axial position, were synthesized from corresponding phthalonitriles using the method described in literature [15]. Solution of PhO SubPcBCl (0.6 mL, 3∙10 -5 M) in DMSO was mixed with aqueous solution of AuNP (0.6 mL, 50 nm gold particles), then 0.3 mL of DMSO:H2O (1:1, V:V) was added. This mixture was stirred for 24 h for the stacking to take place. The completeness of reaction was monitored by quenching of free subphthalocyanine luminescence. Figure 1.Scheme of possible AuNp PhO SubPcBCl conjugate formation. There are two approaches to formation of interaction between the target subphthalocyanines and the surface of nanoparticles. First, the formation of new B-O bond between -OH fragments of citrate and central halogen atom of subphthalocyanine (Fig.1A). It resulted from high lability of extra ligand in subphthalocyanine complexes. Second, due to the appearance of π-π interactions between subphthalocyanine macrocycle and surface of nanoparticles (Fig.1B). We suppose, that these two directions of AuNp – PhO SubPcBCl conjugate formation are equally possible. 3. LINEAR properties In order to achieve a strong exciton-plasmon interaction in hybrid structures, it’s necessary to provide both excellent spectral overlap between absorption band of chromophores with plasmonic band of AuNp, and their extremely close spatial arrangement [16]. Figure 2A presents linear extinction spectra of synthesized PhO SubPcBCl, AuNp and hybrid nanostructures. From Figure 2A it follows that energy of chromophores’ transitions from the ground state to the first excited state is tuned into plasmonic resonance. Linear spectrum of “hybrids” isn’t a direct superposition of correspondent spectra of structures’ components. Difference extinction spectrum (Fig.2B), formed by the difference between hybrid’s spectrum and the sum of the PhO SubPcBCl and AuNp spectra, demonstrates a “dip”. We suppose that it is Fano “dip” with the energy splitting about 50 meV. Presence of Fano “dip” in extinction spectra is a widespread proof of strong exciton-plasmon coupling in such hybrid structures. Moreover, Yoshida et al. showed that strong couplings are possible between molecules which have only been directly attached to metal surface [17], therefore absence of the “dip” in the total extinction spectrum of “hybrids” (Fig.2A) is caused by little fraction of molecules which are directly attached onto the metal surface. SPbOPEN2015 IOP Publishing Journal of Physics: Conference Series 643 (2015) 012049 doi:10.1088/1742-6596/643/1/012049