Optical parametric amplification (OPA) is a second-order nonlinear optical process whereby a signal field is amplified by a pump while generating an idler field, and allows the generation of broadly tunable coherent radiation for many photonic applications, e.g. spontaneous parametric down-conversion  for entangled photon pair generation. Here we demonstrate single-pass OPA at the ultimate thickness limit in monolayer transition-metal dichalcogenides . The absence of dispersion-induced phase mismatch, thanks to the vanishing thickness of the nonlinear medium, bypasses phase-matching requirements and achieves ultrabroad amplification bandwidths ( Fig. 1a-b ), with a linear dependence of the amplification gain with pump and signal powers ( Fig. 1c ).
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