  Germanium is a strong candidate as a laser source for silicon photonics. It
is widely accepted that the band structure of germanium can be altered by
tensile strain so as to reduce the energy difference between its direct and
indirect band gaps. However, the conventional deformation potential model most
widely adopted to describe this transformation happens to have been
investigated only up to 1 % uniaxially loaded strains. In this work, we use a
micro-bridge geometry to uniaxially stress germanium along [100] up to
$\varepsilon_{100}$=3.3 % longitudinal strain and then perform
electro-absorption spectroscopy. We accurately measure the energy gap between
the conduction band at the $\Gamma$ point and the light- and heavy-hole valence
bands. While the experimental results agree with the conventional linear
deformation potential theory up to 2 % strain, a significantly nonlinear
behavior is observed at higher strains. We measure the deformation potential of
germanium to be a = -9.1 $\pm$ 0.3 eV and b = -2.32 $\pm$ 0.06 eV and introduce
a second order deformation potential. The experimental results are found to be
well described by tight-binding simulations. These new high strain coefficients
will be suitable for the design of future CMOS-compatible lasers and
opto-electronic devices based on highly strained germanium.
