The magnitude and spectral response of second-harmonic generation ͑SHG͒ from 0.4 nm single-walled carbon nanotubes ͑CNTs͒ in the channels of AlPO 4 -5 zeolite is reported. The second harmonics ͑SH͒ was found to be polarized perpendicular to the tube axis and maximized by an excitation polarization at 45°to the tube axis. A SH resonance peak at 2 eV was observed, which corresponds to the lowest-energy excitonic state in chiral ͑4,2͒ CNTs. The second-order optical susceptibility ͑2͒ of the system was determined to be 10 −6 esu, which agrees with the large ͑2͒ predicted for small diameter CNTs. These experimental results suggest that SHG can be used to characterize the symmetry and chirality of CNTs.

Pressure induced structural and electronic transitions of Se helical chains confined inside nano-channels are studied. Raman scattering and optical absorption experiments show strong evidence of band gap reduction under high pressure. Ab initio calculations reveal that under hydrostatic compression, the Se chains should elongate and the change in morphology leads to a softening of phonons and narrowing of band gaps, and these signatures are observed in experiments. Our investigation demonstrates a negative compressibility in one dimension.Selenium is a group VI non-metallic element with a band gap of about 2 eV at ambient pressure. To realize metallization for bulk selenium with a concomitant structural phase transition, a large pressure up to tens of GPa is necessary 1-3 . Such phase transition phenomena of polycrystalline selenium have been observed at very high pressures by in situ Raman and X-ray diffraction. The metallization 4 and anomalous liner expansion coefficient under 20 GPa pressure 5 have been examined for crystalline selenium using density functional calculations. Here, we investigate the property of a single Se chain under pressure and we demonstrate that a much lower pressure is sufficient to induce detectable changes in the properties of a confined Se chains inside nano-channels. The pressure induced structural and electronic changes of Selenium chains confined in zeolite channels are studied using Raman spectroscopy and optical absorption spectra, and the results hint at a trend towards metallization as we increase the pressure. Such changes are interpreted using density functional theory (DFT) calculations. The calculations focus on understanding the hydrostatic pressures effect applied on the single trigonal helical