Infrared photothermal modulation of nonlinear optical response from protein-templated gold nanoclusters

Protein templated gold-nanoclusters (Au-NCs) form unique macromolecular systems with exceptional optical properties and biocompatibility, making them suitable as labels for bioimaging and guided-therapy. They are known to emit strong fluorescence from the nanoclusters, while also exhibiting inherent chemical bond-specific absorption from the protein template. Here, we study the nonlinear optical response of protein templated Au-NCs and report on an interesting coupling mechanism between the infrared (IR) absorption signatures and its nonlinear optical response. Three different proteins, namely, pepsin (PS), bovine serum albumin (BSA), and egg-white (EW) templated Au-NCs are studied and their nonlinear optical properties quantified through two-photon excited fluorescence (TPEF), second-harmonic generation (SHG), and third-harmonic generation measurements. The coupling between the IR absorption and nonlinear optical emission is studied through a modulation transfer technique from the mid-infrared excitation to the lock-in detected nonlinear optical signals. A direct correlation is observed between the IR absorption spectra and the modulation transferred nonlinear optical response, with an increase in SHG for PS-Au-NCs, and a decrease in TPEF for BSA- and EW-Au-NCs in the presence of the mid-IR excitation. This coupling mechanism is attributed to a slow photothermal response modifying the local environment, thereby modulating the nonlinear optical response of the templated Au-NCs.