Ultrafast Nonlinear Photoresponse Of Single-walled Carbon Nanotubes:a Broadband Degenerate Investigation

Single-wall carbon nanotubes (SWNTs), a model one-dimensional quantum confined system with fascinating physics, have been widely explored for optical and optoelectronic applications. In particular, a broad absorption range combined with an ultrafast relaxation time has raised a great deal of interest in using SWNTs as nonlinear optical materials. It is desirable to obtain quantitative information about the optical properties of SWNTs over an extended spectral range, as it provides important insights into light-matter interactions in such low-dimensional systems. To this end, photoexcitation of SWNTs in the linear regime has been extensively studied and well interpreted. On the other hand, the nonlinear photoresponse in SWNTs, which is governed by photocarrier processes following high-intensity ultrashort pulse irradiation, is equally important. Understanding of the broadband signatures of photoresponse in such an excitation regime may directly impact functionalities in various photonic devices.Understanding of the fundamental photoresponse of carbon nanotubes has broad implications for various photonic and optoelectronic devices. Here, Z-scan and pump-probe spectroscopy performed across 600-2400 nm were combined to give a broadband degenerate mapping of the nonlinear absorption properties of single-walled carbon nanotubes (SWNTs). In contrast to the views obtained from nondegenerate techniques, sizable saturable absorption is observed from the visible to the near-infrared range, including the spectral regions between semiconducting excitonic peaks and metallic tube transitions. In addition, the broadband mapping unambiguously reveals a photobleaching to the photoinduced absorption transition feature within the first semiconducting excitonic band-2100 nm, quantitatively marking the long-wavelength cut-off for saturable absorption of the SWNTs investigated. Our findings present a much clearer physical picture of SWNTs’nonlinear absorption characteristics, and help provide updated design guidelines for SWNT based nonlinear optical devices.