Nonlinear refraction and absorption in metal cluster compound (n-Bu(4)N)(4)[Mo8Cu12O8S24]: The dispersion behavior and optical limiting effect

We report the observation of time resolved self-defocusing effects in metal cluster compound (n-Bu(4)N)(4)[Mo8Cu12O8S24]. We also present the dispersion of the nonlinear refractive index, n(2), of the compound measured over a spectral range below the resonance 2.45 eV. The time resolved self-defocusing effects clearly indicate that the observed nonlinear refraction should be of electronic origin. The dispersion can be fit by a two-level model. Away from the resonance, n(2) = -2 x 10(-11) cm(2) W-1 M(-1), while at 532 nm, just below the resonance, n(2) = -(2.3 +/- 0.5) x 10(-9) cm(2) W-1 M(-1). In addition, the measurements reveal the involvement of excited state absorption. Based on these observed nonlinearities, optical limiting of the compound has been demonstrated with nanosecond laser pulses of 532 nm wavelength.