  Self-mode-locking has become an emerging path to the generation of ultrashort
pulses with vertical-external-cavity surface-emitting lasers. In our work, a
strong Kerr nonlinearity that is so far assumed to give rise to mode-locked
operation is evidenced and a strong nonlinearity enhancement by the microcavity
is revealed. We present wavelength-dependent measurements of the nonlinear
absorption and nonlinear-refractive-index change in a gain chip using the
Z-scan technique. We report negative nonlinear refraction up to 1.5E-11 cm2/W
in magnitude in the (InGa)As/Ga(AsP) material system close to the laser design
wavelength, which can lead to Kerr lensing. We show that by changing the angle
of incidence of the probe beam with respect to the gain chip, the Kerr
nonlinearity can be wavelength-tuned, shifting with the microcavity resonance.
Such findings may ultimately lead to novel concepts with regard to tailored
self-mode-locking behavior achievable by peculiar Kerr-lens chip designs for
cost-effective, robust and compact fs-pulsed semiconductor lasers.
