Light Emission from High Density 1D Excitons in Nanotubes
Single-walled carbon nanotubes (SWNTs) are one of the leading candidate materials to realize novel nanoscale photonic devices such as 1D lasers. Their diameter-dependent bandgaps can be utilized for unique multi-wavelength devices. There have been a large number of optical studies on SWNTs, but most of these studies were performed in the weak-excitation, quasi-equilibrium regime. In order to assess their performance characteristics as optoelectronic materials under device-operating conditions, it is crucial to examine their optical properties in highly non-equilibrium situations. Such studies also provide new insight into the bosonic properties of 1D excitons at quantum degenerate densities. Previous studies have shown that an upper limit exists on the exciton density due to very efficient exciton-exciton annihilation (EEA). Furthermore, a recent study predicts that varying the temperature could lead to an increase in the exciton density surpassing this upper limit at room temperature. Therefore, we performed temperature-dependent photoluminescence (PL) on HiPco SWNTs embedded in an i-carrageenan matrix under high resonant excitation. We found that, for each temperature within our range (300 K to 15 K), the PL intensity saturates as a function of pump fluence and the saturation intensity increases from 300 K to a moderate temperature around 100-150 K. Below that critical temperature, the PL intensity decreases with decreasing temperature. Within the framework of diffusion-limited EEA, we successfully estimated the upper limit of the density of 1D excitons in SWNTs as a function of temperature. This result is promising for the development of a new class of nanoscale optoelectronic devices.