Efficient adaptive feedback control of ultrafast semiconductor nonlinearities

Summary form only given. We present the first experimental demonstration of adaptive feedback control in ultrafast semiconductor spectroscopy. Our work focuses on broadband continuum nonlinearities, which are an ideal arena for adaptive optimization schemes. This is because the optical pulse shape that produces a desired nonlinear response is very difficult to predict a priori due to the complex physics involved. Our results demonstrate that (i) a posteriori knowledge of the optimum pulse shape gives new insight into the interaction of semiconductors and ultrafast optical pulses; and that (ii) ultrafast semiconductor nonlinearities can be substantially enhanced by almost a factor of 4. In a pump-probe experiment, a pixelated liquid-crystal spatial light modulator is used to shape the spectral phase of 20 fs pulses without modification of the spectral amplitude. The pulse shaping is directed by an evolutionary strategy.