  Optical second harmonic generation (SHG) has been measured for the first time
in reflection from the nanometer-thick films (6 to 40 nm) of the topological
insulator Bi2Se3 using 1.51 eV (820 nm) Ti:Sapphire laser photons and revealed
a strong dependence of the integral SHG intensity on the film thickness. The
integral SHG intensity was determined by area integration of the SHG rotational
anisotropy patterns measured for different input-output light polarization
geometries. A ~100-fold enhancement of the integral SHG intensity with
decreasing film thickness has been suggested to result from the
DC-electric-field-induced SHG (EFISHG) effects. Two sources of dynamically
created DC electric field were proposed: (i) the capacitor-type DC electric
field that gradually increases with decreasing film thickness from 40 to 6 nm
due to a dynamical imbalance of photoexcited long-lived carriers between the
opposite-surface Dirac surface states and (ii) an DC electric field associated
with a nonlinearly excited Dirac plasmon, which is responsible for the resonant
enhancement of the integral SHG intensity for the 10 nm thick film with a
Lorentz-shaped resonance of ~1.6 nm full width at half maximum. Additionally to
the general SHG enhancement trends with decreasing film thickness, a relative
decrease of the out-of-plane contribution with respect to the in-plane
contribution was observed. Using a theoretical treatment of the measured SHG
rotational anisotropy patterns, this effect has been suggested to result from
the joint contributions of the linear and quadratic DC electric field effects
to the EFISHG response.

Mesoscale and Nanoscale Physics