  Surface states generally degrade semiconductor device performance by raising
the charge injection barrier height, introducing localized trap states,
inducing surface leakage current, and altering the electric potential.
Therefore, there has been an endless effort to use various surface passivation
treatments to suppress the undesirable impacts of the surface states. We show
that the giant built-in electric field created by the surface states can be
harnessed to enable passive wavelength conversion without utilizing any
nonlinear optical phenomena. Photo-excited surface plasmons are coupled to the
surface states to generate an electron gas, which is routed to a nanoantenna
array through the giant electric field created by the surface states. The
induced current on the nanoantennas, which contains mixing product of different
optical frequency components, generates radiation at the beat frequencies of
the incident photons. We utilize the unprecedented functionalities of
plasmon-coupled surface states to demonstrate passive wavelength conversion of
nanojoule optical pulses at a 1550 nm center wavelength to terahertz regime
with record-high efficiencies that exceed nonlinear optical methods by 4-orders
of magnitude. The presented scheme can be used for optical wavelength
conversion to different parts of the electromagnetic spectrum ranging from
microwave to infrared regimes by using appropriate optical beat frequencies.
