Ultrafast Mid-Infrared Intra-Excitonic Response of Individualized Single-Walled Carbon Nanotubes
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1230-MM02-04
Ultrafast Mid-Infrared Intra-Excitonic Response of Individualized Single-Walled Carbon Nanotubes Jigang Wang1,2 Matt W. Graham,3 Yingzhong Ma,3 Graham R. Fleming,3 and Robert A. Kaindl1 1
Materials Sciences Division, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. 2 Department of Physics and Astronomy and Ames Laboratory, Iowa State University, Ames, IA 50011, U.S.A. 3 Department of Chemistry, University of California at Berkeley and Physical Biosciences Division, E. O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A.
ABSTRACT We present femtosecond mid-infrared (mid-IR) studies of the broadband low-energy response of individualized (6,5) and (7,5) single-walled carbon nanotubes. Strong photoinduced absorption is observed in these semiconducting tubes around 200 meV photon energy. The transition energy and broadly sloping spectral shape are characteristic of quasi-1D intra-excitonic transitions between different relative-momentum states. Our result yields a value of the intra-excitonic absorption cross section of σ||MIR ≈ 4 × 10-15 cm2. INTRODUCTION Quasi-1D confinement and reduced screening of photoexcited charges in single-walled carbon nanotubes (SWNTs) gives rise to strongly enhanced Coulomb interactions and large exciton binding energies. Such amplified electron-hole (e-h) correlations have important implications for both fundamental physics and optoelectronic applications of nanotubes [1]. Exploiting the availability of “individualized” SWNT ensembles, supporting evidence for excitonic behavior has been demonstrated in absorption-luminescence maps [2], two-photon excited luminescence [3, 4], and ultrafast carrier dynamics [5]. In particular, ultrafast studies can access the rapid electronic processes which occur after absorption of optical photons (Fig. 1a), associated with relaxation into the lowest, E11 subband followed by formation, interaction, and annihilation of quasi-1D excitons. Due to symmetry restrictions, however, optical interband probes can detect only a limited subset of the available exciton states. In contrast, intra-excitonic transitions between low-energy levels of excitons with the same cell-periodic symmetry (arrows, Fig. 1b) represent a fundamentally different tool, analogous to atomic absorption spectroscopy [6-8]. Unlike interband absorption – which measures the ability to generate e-h pairs – intra-excitonic absorption detects existing excitons via transitions from their 1s ground state into higher relative-momentum states. These transitions can occur at all center-of-mass momenta K, and are therefore sensitive to genuine exciton populations across momentum space. Additionally, intra-excitonic absorption is also unrestricted by the exciton ground state symmetry since the cell-periodic component of the wavefunction remains unchanged. Applied to individualized SWNTs, intra-excitonic resonances can therefore yield a measure of both bright and dark exciton populations and should occur in the mid-IR after
Pair Energy
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E11 continu
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