Nonlinear absorption in bis ( 1-butyl-3-methylimidazolium ) tetrathiocyanatozincate ( [ BMIM ] 2 [ Zn ( SCN ) 4 ] ) an ionic liquid with a transition metal in the anionic moiety

Nonlinear absorption has been detected and characterized by open aperture Z-scan in ionic liquids obtained by combination of 1-butyl-3-methyl-imidazolium cations with anions containing a transition metal (Zn) and thiocyanato groups. The laser source was a Ti:Sapphire oscillator (80-fs pulses, λ=810nm, repetition rate of 80.75MHz). This ionic liquid presents quite low heat capacity that favours the development of strong thermal effects at 810nm. Thermal effects and nonlinear absorption make the ionic liquid interesting for optical limiting purposes.


Introduction
The synchronized absorption of two or more photons by an active molecule reaching temporarily an excited state via intermediate states is known as multiphoton absorption (MPA), an intensity-dependent and non-parametric nonlinear effect [1].Although it is a limiting factor in nonlinear optical devices, it is very useful in applications like frequency upconversion lasing, data storage, microfabrication, fluorescence microscopy, optical limiting, optical reshaping or optical stabilization (see for example [1,2] and references therein).In the last twenty years, a lot of effort has been directed to the research on multiphoton absorbing materials such as organic liquids, liquid crystals, polymers, fullerenes, organometallic compounds and biomolecules [1,3].
During the last ten years, a particular kind of salts known as ionic liquids (IL) have been the object of interest of numerous research groups [4,5].Typically, ILs are composed by a large cation (usually organic, containing nitrogen, sulphur or phosphorous and one or more alkyl chains) and an anion which can be mono or polyatomic, organic or inorganic.These emergent materials are involved in interesting applications in fields as electrochemistry, nuclear processing or renewable energies.The technologic development around them has required an enormous effort in the synthesis and in the characterization of their chemical and physical properties, needed for designing any application.Among them, optical properties have been addressed only by few researchers.Optical absorption and fluorescence have been studied in a limited set of imidazoliumbased ionic liquids [6,7], and thermooptic effect induced by different laser sources have been addressed in [8][9][10][11][12][13][14].Nonlinear absorption has not been detected in any of the liquids characterized in those papers at the selected wavelengths.Nevertheless, Sesto and co-workers have designed phosphonium based ionic liquids with cubic susceptibilities basing on the cubic susceptibilities of the constituent ions [15,16].They report the existence of nonlinear absorption in some of them but they did not provide, up to our knowledge, quantitative values of neither the nonlinear absorption coefficient nor the nonlinear cross section.In this work, we study the nonlinear absorption effects in an ionic liquid containing a Zn atom in the anionic moiety and 1-butyl-3methyl-imidazolium [BMIM] as cation.

Results and discussion
As there were no reported preparations for the IL of our interest, we proceed to the synthesis of BMIM 2 Zn(SCN) 4 (Figure 1) which was carried out by treating BMIMCl and KSCN in acetone with anhydrous ZnCl 2 under reflux.

Figure 1
Heat Capacity: The sample was subjected to temperature ramps at 0.25 K•min -1 in the temperature interval (280-338) K using a DSCIII calorimeter from Setaram.From these experiments, heat capacity can be determined.Moreover, information about any physical or chemical process which involves heat exchange -in particular, phase transitions-can be obtained.We estimated the uncertainty of heat capacity measurements in about 0.05 J•g -1 •K -1 , much higher than that usually found for other using this procedure, [17,18] fact which comes from the difficult handling of ILs.
We found that the compound present quite low values, (see Table 1), which makes them quite sensitive to heating and, particularly, to that induced by laser pulsed radiation, as it is shown below.The absorbance spectrum from 280nm-1000nm is shown in Figure 2. The pathlength of the cells was 1mm.The liquid present negligible linear absorbance at the excitation length, 810nm.In Figure 3 we plot the open aperture Z-scan curve obtained with this ionic liquid together with the fit to theoretical model for three-photon absorption.The values of the absorption coefficients estimated by this fit is β 3PA = (1.4±0.5) 10 -3 cm 3 /GW 2 .Figure 3 3. Conclusions Nonlinear absorption has been detected in this newly synthesized ionic liquid.The absorption coefficient has been characterized by the open aperture z-scan technique at 810 nm.The behaviour observed is compatible with three photon absorption.The order of magnitude of the value of the nonlinear absorption coefficient is similar to the measured in other Zn compounds at the same wavelength, ZnO or ZnS, or TiO 2 .