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Broadband THz spectroscopy of nanoscale objects

  • By Maintenance
  • 28 October 2016

Broadband THz spectroscopy for a single nanoscale object

Terahertz frequency (0.1−30 THz) is known as the characteristic frequency for intramolecular and intermolecular motions. Spectroscopy with terahertz radiation has played an important role in fields like chemical sensing, characterization of biological material and drug analysis. Recently, broadband terahertz (10 THz bandwidth) generation and detection at 10 nm scales has been demonstrated using LaAlO3/SrTiO3 (LAO/STO) nanostructures [1]. This unprecedented control of terahertz radiation, on a scale of four orders of magnitude smaller than the diffraction limit, provides a useful technique to investigate a variety of nanoscale objects, such as Au nanorods, PbS quantum dots, graphene and single-layer molybdenum disulfide (MoS2) nanoflakes.

Nanoscale conducting region can be created and erased at LAO/STO interface with the method of conductive atomic force microscope (c-AFM) lithography [2]. These created nanostructures can be used as photodetectors [3] as well as THz generators and detectors. After creating a nanostructure composed of 10nm wide nanowires and a junction with comparable width, ultrafast pulses coming from a Ti: Sapphire laser go through a Michelson interferometer and then focused onto the junction. Photo-induced voltage change across the junction is measured as a function of time delay between the two pulses. When time delay is around 0 fs, there is a sharp decrease in the 4-terminal voltage, indicating the generation of THz radiation. The mechanism behind this phenomenon is the  process, where the nonlinear polarization is a product of the electric field across the junction, the third-order susceptibility at the junction, and two optical fields.

This powerful technique has been proved to be able to probe the response of a single plasmonic Au nanorod [4]. Commercial gold nanorods with a well-defined plasmon resonance peak around 810 nm is deposited onto the surface of LAO/STO sample. After an AFM image is taken to locate an ideal single nanorod, a four terminal structure with a junction located right on top of the nanorod will be designed and created. When the light polarization is parallel to the long axis of nanorod, the THz signal will be largely enhanced. This is due to the plasmonic coupling with induced THz radiation at the nanojunction. With this powerful THz spectrometer as a platform, various other nanoscale materials can be studied, such as PbS quantum dots and single-layer MoS2 nanoflakes. Graphene plasmonics is also under investigation.

  1. Ma, Y., Huang, M., Ryu, S., Bark, C. W., Eom, C.-B., Irvin, P., & Levy, J. "Broadband Terahertz Generation and Detection at 10 nm scale," Nano Letters 13, 2884−2888 (2013).
  2. Cen, C., Thiel, S., Hammerl, G., Schneider, C. W., Anderson, K. E., Hellberg, C. S., Mannhart, J. & Levy, J. "Nanoscale Control of an Interfacial Metal-Insulator Transition at Room Temperature," Nature Materials 7, 298 (2008). 
  3. Irvin, P., Ma, Y., Bogorin, D. F., Cen, C., Bark, C. W., Folkman, C. M., Eom, C.-B. & Levy, J. "Rewritable Nanoscale Oxide Photodetector," Nature Photonics 4, 849-852 (2010).
  4. Jnawali, G., Chen, L., Huang, M., Lee, H., Ryu, S., Podkaminer, J. P., Eom, C.-B., Irvin, P. & Levy, J. "Photoconductive Response of a Single Au Nanorod Coupled to LaAlO3/SrTiO3 Nanowires," APL 106, 211101 (2015).