Optical absorption spectroscopy (OAS) is an important method of investigation of material [1]. It analyzes the physical properties, and allows concluding about the modifications of the electronic properties [2]. Single-walled carbon nanotubes (SWCNTs) is a unique allotropic modification of carbon with interesting properties. The application of optical absorption spectroscopy to SWCNTs allows studying the optical, electronic properties of carbon nanotubes. In this paper, I overview the achievements in investigations of the SWCNTs with OAS. I discuss trends in modifications of spectra upon filling of carbon nanotubes [3-5]. This effects testify about the doping with charge transfer. The modified electronic properties can find applications in nanoelectronics, solar cells, catalysts, thermoelectric power generation, light emission, biomedicine.

[1] M. V. Kharlamova. Electronic properties of pristine and modified single-walled carbon nanotubes. Physics-Uspekhi 2013, 56, 1047-1073.

[2] M. V. Kharlamova. Advances in tailoring the electronic properties of single-walled carbon nanotubes. Progress in Materials Science 2016, 77, 125-211.

[3] M. V. Kharlamova et al. Comprehensive spectroscopic characterization of high purity metallicity-sorted single-walled carbon nanotubes. Physica Status Solidi b 2015, 252, 2512-2518.

[4] M. V. Kharlamova et al. Single-walled carbon nanotubes filled with nickel halogenides: atomic structure and doping effect. Physica Status Solidi B 2012, 249, 2328-2332.

[5] M. V. Kharlamova et al. Acceptor doping of single-walled carbon nanotube by encapsulation of zinc halogenides. Eur. Phys. J. B. 2012, 85, 34.