Twisted photovoltaics at terahertz frequencies from momentum shift current

The bulk photovoltaic effect (BPVE) converts light into a coherent dc current at zero bias, through what is commonly known as the shift current. This has previously been attributed to displacement of electronic wave function center in real space, when sample excited by light. We reveal that materials like twisted bilayer graphene (TBG) with flatband dispersion are uniquely suited maximize BPVE because they lead an enhanced momentum unlike any mechanism. identify properties quantum geometry, which go beyond geometric tensor, and unrelated Berry charges, physical origin large we observe TBG. Our calculations show TBG band gap several meV exhibits giant range 0.2--1 THz, represents strongest reported so far this frequency two-dimensional material partially persists even room temperature. paper provides design principle for generation, applies broad heterostructures potential overcome so-called ``terahertz gap'' THz sensing.