  We report a large g-factor tunability of a single hole spin in an InGaAs
quantum dot via an electric field. The magnetic field lies in the in-plane
direction x, the direction required for a coherent hole spin. The electrical
field lies along the growth direction z and is changed over a large range, 100
kV/cm. Both electron and hole g-factors are determined by high resolution laser
spectroscopy with resonance fluorescence detection. This, along with the low
electrical-noise environment, gives very high quality experimental results. The
hole g-factor g_xh depends linearly on the electric field Fz, dg_xh/dFz = (8.3
+/- 1.2)* 10^-4 cm/kV, whereas the electron g-factor g_xe is independent of
electric field, dg_xe/dFz = (0.1 +/- 0.3)* 10^-4 cm/kV (results averaged over a
number of quantum dots). The dependence of g_xh on Fz is well reproduced by a
4x4 k.p model demonstrating that the electric field sensitivity arises from a
combination of soft hole confining potential, an In concentration gradient and
a strong dependence of material parameters on In concentration. The electric
field sensitivity of the hole spin can be exploited for electrically-driven
hole spin rotations via the g-tensor modulation technique and based on these
results, a hole spin coupling as large as ~ 1 GHz is expected to be envisaged.
