  Monolayer transition-metal dichalcogenides (TMDCs) present high second-order
optical nonlinearity, which is extremely desirable for, e.g., frequency
conversion in nonlinear photonic devices. On the other hand, the atomic
thickness of 2D materials naturally leads to low frequency converted
intensities, highlighting the importance to design structures that enhance the
nonlinear response for practical applications. A number of methods to increase
the pump electric field at the 2D material has been reported, relying on
complex plasmonic and/or metasurface structures. Here, we take advantage of the
fact that unstructured substrates with a low refractive index naturally
maximize the pump field at a dielectric interface, offering a simple means to
promote enhanced nonlinear optical effects. In particular, we measured second
harmonic generation (SHG) in MoS2 and WS2 on fluorine tin oxide (FTO), which
presents an epsilon-near-zero point near our 1550-nm pump wavelength. Polarized
SHG measurements reveal an SHG intensity that is one order of magnitude higher
on FTO than on a glass substrate.
