Carbon nanotubes for ultrafast photonics

Single Wall Carbon Nanotubes (SWNTs) are of great interest in photonics and optoelectronics due to their linear and non-linear optical properties. Features in the optical absorption and emission spectra are generated by excitonic transitions [1–3]. SWNTs show strong third order nonlinearities through these excitonic transitions, as they become more transparent when irradiated by high power (saturable absorption) [4–6], with fast recovery time [4, 5, 7–14]. SWNTs are thus suitable for application as saturable absorbers in passively mode-locked lasers and noise suppression filters [15–17]. SWNT-based saturable absorbers must be carefully designed to keep a balance between modulation depth, which is ruled by the SWNT concentration, and unwanted non-saturable losses, such as scattering from large bundles or entanglements and non-saturable absorption from polymer matrix, surfactants, residual catalyst particles and carbon impurities [6]. To ensure fast device operation, the relaxation time of the saturable absorber must be as short as possible [18]. The relaxation time is one order of magnitude longer in isolated SWNTs, which relax within tens of picoseconds [10–12], than in bundles, which relax within 1 ps [7, 8]. Hence, bundles are useful in SWNT-based saturable absorbers. However, their size must be smaller than the device operation wavelength to avoid non-saturable losses due to scattering [19]. It is thus crucial to find a way to detect and quantify the presence of bundles in a SWNT solution. PLE spectroscopy is a powerful tool to detect the presence of individual SWNTs [20] and bundles [21, 22] in solution. Here, we investigate by PLE spectroscopy the presence of bundles in SWNT solutions, which are then used to prepare polymer composites. Saturable absorption properties of the composites are studied by power dependent absorption. The composites are then employed as mode-lockers in a fibre laser.