Bridging between plasmonics and spintronics: the spin plasmons in topological insulators

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University of Delaware
Conventionally, the research of plasmonics and spintronics share little in common due to different physics and applications. The connection of these two fields is seemingly promising in creating advanced devices involving both optical and spintronic applications. To date, there are still limited attempts in connecting the two fields. In this dissertation, we propose to excite and observe the spin plasmons in topological insulators, which is, theoretically, a perfect bridge between plasmonics and spintronics. ☐ Topological insulators (TI) are a group of materials that have unique band structures: the surfaces show linearly dispersed bands, while the bulk shows normal conduction bands and valence bands. The carriers located in the surface states are spin-momentum locked, meaning the spin of the surface carriers are dependent on the direction of their motions. Plasmons are the collective oscillations of electrons, affecting the way how electrons in the materials interact with external electro-magnetic waves. Thus, the optical property of a material is largely dependent on the plasmons in it. Due to the spin-momentum locking, the plasmons that are formed by TI surface carriers (known as the Dirac plasmons), are correlated electron density waves as well as spin density waves. These spin density waves are essentially spin plasmons. The direct experimental evidences of this prediction remain unveiled. Therefore, the ultimate goal of this dissertation is to experimentally reveal the charge density oscillations and spin density oscillations at the mean time. ☐ This dissertation proposed a possible method to observe the propagating spin plasmons using a THz-pump IR-probe method with a magneto-optical Kerr effect (MOKE) system. The direct observation of the spin plasmons were not achieved due to the limitations of the time resolution and the low signal of the plasmonic responses. Future works can still be applied in this direction as we refine the quality of the devices, and also the time resolution of MOKE system. ☐ This dissertation first demonstrates the optimization of the growth of TI Bi2Se3 with molecular beam epitaxy (MBE). A buffer layer of (Bi1-xInx)2Se3 between the substrates and the Bi2Se3 films are utilized to enhance the crystal quality of the films. We established an optimized growth procedure for growing high quality (Bi1-xInx)2Se3 buffer layers. The TI films grown with the enhanced buffer layer show a sheet carrier density down to ~1×1013 cm-2, decreased by a factor of 1/2 compared with the Bi2Se3 films grown without the buffer layer. The optical properties of BIS, including the optical bandgaps and the permittivity in the IR range, were also characterized for further analysis of the plasmons from the devices created with BIS. ☐ A structure of BIS(5nm)/Bi2Se3(50nm)/BIS(50nm)/sapphire substrate was grown with the optimized growth procedure for the excitations of TI Dirac plasmons. Linear gold grating couplers with various periodicities (400nm- 700nm) were fabricated to enable the excitation of propagating Dirac plasmons. TM extinction spectra taken by Fourier transform IR spectroscopy (FTIR) showed a series of plasmonic absorption peaks shifting with the grating periodicity. The plasmonic absorptions were reported to be located in the Terahertz (THz) range. Simulation showed a good agreement between the data and the theoretical dispersion relation of the TI Dirac plasmons.
Dirac plasmon polariton, Spin plasmons, Topological insulators, Plasmonics, Spintronic applications, Advanced devices, Optical applications, Oscillations