  The ultrathin nature and dangling bonds free surface of two-dimensional (2D)
semiconductors allow for significant modifications of their band gap through
strain engineering. Here, thin InSe photodetector devices are biaxially
stretched, finding, a strong band gap tunability upon strain. The applied
biaxial strain is controlled through the substrate expansion upon temperature
increase and the effective strain transfer from the substrate to the thin InSe
is confirmed by Raman spectroscopy. The band gap change upon biaxial strain is
determined through photoluminescence measurements, finding a gauge factor of up
to ~200 meV/%. We further characterize the effect of biaxial strain on the
electrical properties of the InSe devices. In the dark state, a large increase
of the current is observed upon applied strain which gives a piezoresistive
gauge factor value of ~450-1000, ~5-12 times larger than that of other 2D
materials and of state-of-the-art silicon strain gauges. Moreover, the biaxial
strain tuning of the InSe band gap also translates in a strain-induced redshift
of the spectral response of our InSe photodetectors with {\Delta}Ecut-off ~173
meV at a rate of ~360 meV/% of strain, indicating a strong strain tunability of
the spectral bandwidth of the photodetectors.
