Nonlinear optical transmission of silk/green fluorescent protein (GFP) films

In this initial work, we demonstrate a technique for preparing thin (10-20 μm) films of silk doped with green fluorescent protein (GFP) by casting/annealing at 20 °C and describe the resulting film characteristics. Notably, the GFP molecules maintain their nonlinear optical properties as evidenced by two-photon fluorescence microscopy and two-photon absorption measurements using near-infrared femtosecond pulses. The fractional transmission of focused near-infrared pulses of 775 nm wavelength, 140 fs pulsewidth was observed to decrease as the incident pulse energy is increased and/or the incident spot size is decreased, indicating that nonlinear absorption is taking place. Visible damage from the pulses is observed in a ∼10 μm film at the highest peak incident fluences, which were in the range of 0.1-0.2 J/cm². Variations in thickness, morphology and GFP concentration of the films make precise specification of the two-photon absorption coefficient difficult. Since these films have potential applications in photonics, we suggest techniques for improving these properties in future generations of films. The suggestions present opportunities for future work.