Ultrafast terahertz photoconductivity and near-field imaging of nanoscale inhomogeneities in multilayer epitaxial graphene nanoribbons

We study broadband terahertz (THz) conductivity and ultrafast photoconductivity spectra in lithographically fabricated multilayer epitaxial graphene nanoribbons grown on C-face of 6H-SiC substrate. THz near-field spectroscopy reveals local conductivity variations across nanoscale structural inhomogeneities such as wrinkles and grain boundaries within the multilayer graphene. Ultrabroadband THz far-field spectroscopy ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>0.15</a:mn> <a:mtext>–</a:mtext> <a:mn>16</a:mn> <a:mspace width="0.28em"/> <a:mi>THz</a:mi> </a:mrow> </a:math> ) distinguishes doped graphene layers near the substrate from quasineutral layers (QNLs) further from the substrate. Temperature-dependent THz conductivity spectra are dominated by intraband transitions both in the doped and QNLs. Photoexcitation then alters mainly the response of the QNLs: these exhibit very high carrier mobility and large positive THz photoconductivity with picosecond lifetime. The response of QNLs strongly depends on the carrier temperature <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mi>T</c:mi> <c:mi mathvariant="normal">c</c:mi> </c:msub> </c:math> : the scattering time drops by an order of magnitude down to ∼10 fs upon an increase of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msub> <e:mi>T</e:mi> <e:mi mathvariant="normal">c</e:mi> </e:msub> </e:math> from 50 K to <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:msub> <g:mi>T</g:mi> <g:mi mathvariant="normal">c</g:mi> </g:msub> <g:mo>&gt;</g:mo> <g:mn>1000</g:mn> <g:mspace width="4pt"/> <g:mi mathvariant="normal">K</g:mi> </g:mrow> </g:math> , which is attributed to an enhanced electron-electron and electron-phonon scattering and to an interaction of electrons with mid-gap states.