Exciton Dynamics and Biexciton Formation in Single-Walled Carbon Nanotubes Studied with Femtosecond Transient Absorption Spectroscopy

Abstract

We used femtosecond transient absorption (TA) spectroscopy to examine the excited state dynamics of singlewalled carbon nanotube (SWNT) bundles embedded in polymer matrices. The SWNTs were excited by a femtosecond pump pulse centered at either 1800, 900, or 550 nm and probed using a white-light continuum extending from 400 to 750 nm. We observed a structured TA spectrum consisting of a series of narrow induced transmission (IT) and induced absorption (IA) bands. The TA spectrum, which is independent of excitation wavelength, appeared on a time scale shorter than our instrument response (200 fs) and persisted for up to 100 ps. TA spectra obtained at a series of pump-probe delay times provided a window through which to monitor the exciton dynamics. We observed three distinct spectral signatures in the time-dependent data: (1) the decay of a broad photobleach, (2) the biphasic decay of narrow IT and IA features, and (3) a dynamical spectral shift of IA bands. These processes were attributed to plasmon relaxation, electron-hole recombination, and lattice relaxation associated with exciton self-trapping, respectively. Analysis of the transient spectrum suggested that it arose from a nonlinear optical response of the SWNT, where excitons produced by the pump pulse modified the transition frequencies of subsequent carrier excitations. The result was a series of IT bands (bleaches) at the ground state absorption frequencies, and associated with each was a corresponding red-shifted absorption band. These induced absorptions were attributed to the formation of biexcitons, fourparticle excitations that are produced through the sequential excitation of two closely spaced electron-hole pairs.

Asymptotic exchange coupling of quasi-one-dimensional excitons in carbon nanotubes

“…Here, I extend our studies to the strong (non-linear) excitation regime whereby photogenerated biexcitonic states may be formed in CNs (observed recently in singlewalled CNs by the femtosecond transient absorption spectroscopy technique 13 ). An analytical (universal) expression is obtained for the biexciton binding energy as a function of the inter-exciton distance and the binding energy of constituent excitons.…”

“…In the latter case, the experimental observation of the non-linear absorption line splitting predicted here would help identify the presence and study the properties of biexcitonic states (including biexcitons formed by excitons of different subbands 13 ) in individual single-walled CNs, due to the fact that when tuned close to a plasmon resonance the exciton relaxes into plasmons at a rate much greater than τ …”

Intraband and intersubband many-body effects in the nonlinear optical response of single-wall carbon nanotubes

“…This inconsistency in the literature is probably due to the use of poly-disperse samples in several of these studies that prevented a fine identification of the spectral changes. As a consequence, the physical interpretation also remained quite elusive : these PA bands were tentatively assigned to transitions to the bi-exciton 10 , to a speculative transition to the two-exciton S 11 manyfold 13 , to the Stark shift induced by photo-induced free carriers 31 or to the coupling with non-equilibrium hot phonons 8 . Finally, we observe a linear relationship between all the nonlinear quantities over time, which clearly shows that they share the same dynamics, independently of the pump power, in agreement with a common physical origin.…”

“…In most of the previous studies however, such effects were blurred because of the spectral congestion of optical transitions due to the use of poly-chiral suspensions 10,11 . As a consequence, most of the previous pumpprobe studies have been interpreted in terms of simple photo-bleaching (PB) and photo-absorption (PA) bands respectively assigned to state filling effects (Pauli blocking in a molecular picture) and absorption from longlived excited states to upper states [11][12][13] .…”

Theory of biexcitons and biexciton-exciton cascade in graphene quantum dots

“…The generation and stability of biexcitons in carbon nanotubes as well as conjugated polymers have been studied extensively both theoretically and experimentally [37][38][39][40][41][42][43][44][45][46], while investigation of biexcitons in graphene quantum dots is beginning to attract attention [47,48].…”