Abstract

Excitonic resonances in direct-gap semiconductors have proved to exhibit similar optical non-linearities as in atoms. One example is the optical Stark effect [1] which occurs when the sample is excited by a strong pump field whose frequency is detuned from the optical transition. The optical Stark effect has been described as a coupling between the electromagnetic field and the resonance with no real absorption. It can also be seen as the creation of a virtual population in the excited state. A transient gain, quadratic in pump intensity, is associated to this effect as already observed in atomic systems [2]. This hyper-Raman process involves two pump photons and occurs at frequency 2ω_p – ω₀, where ω_p and ω₀ are the pump and atomic transition frequencies, respectively (see inset of Fig. 1).

© 1996 Optical Society of America