Double walled carbon nanotubes (DWCNTs) are an intriguing example of a one-dimensional Moire system, because they differ in diameter and chirality of the inner and outer tubes. For many years, DWCNTs have been described as two weakly coupled single walled carbon nanotubes (SWCNTs) without much electronic interaction. Advances in theoretical modeling, however, predicted diverse coupling possibilities when considering the electronic interactions between twisted layers of low dimensional materials. This coupling can induce the formation flat bands and strongly perturb the electronic states. Experimentally, this should manifest in a reduced bandgap in DWCNTs and a red-shift of the optical transition energies. Chasing this coupling effect, we used resonant Raman spectroscopy to measure the transition energies of semiconducting inner tubes in DWCNTs. The transition energies are indeed reduced, with varying red-shifts for the same inner wall chirality (50 and 150 meV). The shift depends on the electronic type of the host tube that we assign from based on the Radial breathing mode frequencies. The coupling increases from weak to strong by tightening intratube spacing in agreement with theoretical predictions. We discuss our observations in light of joint and localized vibrational and optical excitations in the two tubes making up the DWCNT.
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