laboratoire pierre aigrain
électronique et photonique quantiques
laboratoire pierre aigrain

Seminar, 20 juin 2017 (15h30 conf IV)

Masayoshi Tonouchi, Institute of Laser Engineering, Osaka University
New Methods of THz Time Domain Spectroscopy for Graphene Science

This talk introduces two new methods of THz time domain spectroscopy for
graphene characterization. The one is THz parallel plate waveguide TDS
for ultrathin conductive materials. THz TDS is a strong tool for
material characterization but not for graphene. We have a problem on the
SN ratio of electric field signal amplitude, which is typically just
around 1000 and easily goes down to 300 - 500 considering substrates
even at peak amplitude, and at THz frequencies, the standard deviation
of the transmittance in usual TDS system is 0.5% or higher. Intrinsic
graphene has a transmittance of around 97%, which largely changes
depending on contaminations, such as oxygen, and the substrates by
charge transfer. As a result, sometime the transmittance further
increases, which increases uncertainty in the data. To overcome the
problem, we recently proposed to use THz parallel plate waveguide to
increase interaction length with graphene and THz waves, which allow us
to observe strong absorption of the THz waves when the graphene is
placed in the center of the waveguide. [1,2] New extraction theory has
been provided and the estimation agrees with the existing data. The
other method is temperature programed THz emission spectroscopic
imaging. [3,4] This method provides us a new technique to study
molecular dynamics on graphene. We observe temperature dependence on THz
emission amplitude locally excited from graphene covered InP wafers. The
emission is generated by the photocarrier acceleration at the InP
surface due to built-in-field, which is quite sensitive on the surface
potential. From the temperature dependence of the amplitude, we
successfully estimated the adsorption energy of oxygen on graphene for
the first time, which coincides with some predicted values by the first
principle calculation.

[1] M. Razanoelina, /et. al./, ”Parallel-Plate Waveguide Terahertz Time
Domain Spectroscopy for Ultrathin Conductive Films”, Journal of
Infrared, Millimeter, and Terahertz Waves 36, 1182, 2015.

[2] M. Razanoelina, /et. al/., “Probing low-density carriers in a single
atomic layer using terahertz parallel-plate waveguides”, Optics Express
24, 3885, 2016

[3] Y. Sano, /et. al/., “Imaging molecular adsorption and desorption
dynamics on graphene using terahertz emission spectroscopy” /Scientific
Reports/, 4, 6046, 2014.

[4] F. R. Bagsican,/et. al/., “Adsorption energy of oxygen molecules on
graphene and two-dimensional tungsten disulfide”, /Scientific Reports/,
7, 1774, 2017.