  Two-dimensional materials with tunable in-plane anisotropic infrared response
promise versatile applications in polarized photodetectors and field-effect
transistors. Black phosphorus is a prominent example. However, it suffers from
poor ambient stability. Here, we report the strain-tunable anisotropic infrared
response of a layered material Nb2SiTe4, whose lattice structure is similar to
the 2H-phase transition metal dichalcogenides (TMDCs) with three different
kinds of building units. Strikingly, some of the strain-tunable optical
transitions are crystallographic axis-dependent, even showing opposite shift
when uniaxial strain is applied along two in-plane principal axes. Moreover,
G0W0-BSE calculations show good agreement with the anisotropic extinction
spectra. The optical selection rules are obtained via group theory analysis,
and the strain induced unusual shift trends are well explained by the orbital
coupling analysis. Our comprehensive study suggests that Nb2SiTe4 is a good
candidate for tunable polarization-sensitive optoelectronic devices.
