Title: ULTRAFAST OPTICAL RESPONSE AND TRANSPORT PROPERTIES OF STRONTIUM TITANATE-BASED COMPLEX OXIDE NANOSTRUCTURES 

Abstract: 

the silicon-based semiconductor integrated circuits led by Moore's Law approaching their physical limits, the search for a new generation of nanoelectronic and nanophotonic devices is becoming a hot topic in this post-Moore era. The strontium titanate-based complex oxide heterostructure appears to be a promising alternative due to its diverse emergent properties. Being able to control the metal-insulator transition at the polar/nonpolar LaAlO₃/SrTiO₃ interface using conductive atomic force microscopy (c-AFM) lithography has made LaAlO₃/SrTiO₃, in particular, an attractive platform.

Expanding the class of heterostructures which can be controlled at nanoscale dimensions is important for alternative oxide-based nanodevices. In this dissertation, the writing and erasing of nanostructures at the nonpolar/nonpolar oxide interface of CaZrO₃/SrTiO₃ using c-AFM lithography is investigated. Conducting nanostructures as narrow as 1.2 nm at room temperature is achieved. Low-temperature transport measurements based on these nanostructures provides insight into the electronic structure of the CaZrO₃/SrTiO₃ interface. Such extreme nanoscale control, with dimensions comparable to most single-walled carbon nanotubes, holds great promise for oxide-based nanoelectronic devices.

Nanophotonic devices operating at terahertz frequencies, on the other hand, offer unique information for many applications. In this dissertation, broadband nanoscale terahertz generators based on c-AFM lithography defined LaAlO₃/SrTiO₃ nanojunctions are proved to be able to detect the plasmonic response of a single gold nanorod. By femtosecond pulse shaping using a home-built pulse shaper, over 100 THz bandwidth selective difference frequency generation at LaAlO₃/SrTiO₃ nanojunctions is also demonstrated, which has great potential in both studying fundamental light-matter interaction and realizing selective control of rotational or vibrational resonances in nanoparticles. With this unprecedented control of THz field, the two-dimensional (2D) material graphene and its coupling with the quasi-2D LaAlO₃/SrTiO₃ interface are also under investigation. The preliminary data shows evidence for graphene response up to 60 THz. These results help to fill the terahertz gap as well as offer new opportunities for oxide-based nanophotonic devices or even hybrid optoelectronic integrated circuits.