Fig. 6.

Absorption spectrum measurement (a) Setup: we use a white light laser (WLL) and a tunable filter (TF) to vary the excitation wavelength. For different excitation intensity and wavelength the fluorescence of the gNDs is filtered out the dichroic mirror (DM) and a long pass filter (LP) and recorded by a CCD. (b) In order to derive the value of the absorption cross section from the fluorescence saturation measurement, we represent the gNDs with two energy levels (the excited state S₁ and the ground state S₀). k_f is the fluorescence rate (we measured the average lifetime (τ_f) in Fig. 2, k_f = 1/ τ_f). The WLL is a pulsed source, we assume a constant excitation rate during the pulse, so the excitation rate during the pulse is $k_{ex}={\sigma }_{abs}{I}_{exc}{T}_{rep}/T_p$.