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Nonlinear optical characterization of new ionic liquids by a noise reduced thermally managed EZ-Scan technique

Thermal and nonlinear optical characteristics are evaluated for a novel ionic liquid, BMIOMe.NTf2, by means of the well established thermally managed eclipse Z-scan technique (EZ-scan). Nevertheless, this method still requires samples with good optical quality in order to provide a reasonable signal-noise ratio (SNR) when the material’s nonlinearity is small. Two different procedures where then introduced to improve the SNR against sample linear inhomogeneities and scattering. Both procedures are based in a subtraction of noisy background signals from the nonlinear signal of EZ-scan output. The first is applied to thermal effects and slow sample’s response, based on the perform a subtraction of the initial exposure time from the steady state signal, eliminating the linear noise. In this case, we could verify an increment of SNR from 1:2 to 40:1. The second procedure is related to fast response, e.g., arising from the electronic nonlinear polarizability. This consists in performing two sequential measurement, one with a decade of intensity higher than the first. After performing the subtraction between them, linear background suppression can be verified, with improvement of SNR to 2:1, amount about 10 times the signal value. Performing these procedures, we present a study of properties of ionic liquid BMIOMe.NTf2. This presents a steady cumulative effect of multiple pulse absorption leading to thermal lensing as measured by the thermally managed EZ-scan technique. The photoinduced conductivity effect observed for this ionic liquid is discussed based on the observed photothermal conversion. Also a fast response corresponding to a nonlinear refractive index was also registered when the time evolution is extrapolated for short times.