Ultrafast terahertz and optical spectroscopy of 1D van der Waals nanomaterials

In this thesis, we report on the experimental investigation of the optical properties of 1D van der Waals materials, such as carbon and boron nitride, transition metal dichalcogenides nanotubes, and their heterostructures. Despite numerous studies of carbon nanotubes their properties in the THz region were under debate. Recent progress in understanding the nature of the THz conductivity of carbon nanotubes motivated us to further understand their photophysical properties. By using a combination of techniques, we investigated the properties of ultrathin, highly conductive carbon nanotubes films with different thicknesses. The unique photoinduced negative THz conductivity allows us to construct a modulator based on photoinduced transparency. We showed that with an increase in the carbon nanotubes film thickness, the modulation performance increases. Moreover, it was found that the photoconductivity lifetime is short and the device can be operated with the modulation speed more than 300 GHz, and a large modulation depth, up to +80 %. Furthermore, we investigated the influence of doping the carbon nanotubes’ properties and consequently on the modulator performance. Our research reveals the increase of electrical conductivity due to the doping which results in suppression of the THz modulation performance with a simultaneous decrease in the modulation time (increase in the modulation speed). The possible physical explanation is given. Overall, it was shown that the decrease of the MD due to doping needs to be taken into account when designing THz modulators. The second part of this thesis showed the first study of the novel radial 1D van der Waals heterostructure properties. An interesting result was shown: ultrafast dynamic switch from negative to positive photoconductivity was observed and explained in terms of the different temporal dynamics for free-carrier absorption in the carbon nanotubes and MoS2 nanotubes. The combination of optical pump-THz probe and optical pump-optical probe spectroscopy shows the co-existence of free charges and excitons in MoS2 nanotubes