Huge density-dependent blueshift of indirect excitons in biased coupled quantum wells

A nonlinear Stark shift, in which the wavelength of an optical resonance depends on the intensity of an input optical beam, is of great interest because it can be used in various schemes of optical switching; if an input optical signal strongly changes the absorption of a medium then a second optical signal can be switched on and off in the optical analog of a transistor. This behavior, based on the ac Stark effect, has been demonstrated in GaAs single quantum wells and other systems and has been utilized in switching devices. A limitation of many versions of the ac Stark effect is that a strong light signal must be used to generate a relatively small shift. By analogy with the electrical transistor, what one would like is a device in which a weak input signal produces a large change in the output signal. We report here a system in which the optical resonance wavelength depends extremely sensitively on the intensity of an input light beam. This occurs in conjunction with the dc Stark effect, also known as the quantum confined Stark effect, in coupled GaAs quantum wells. The effect is a manifestation of the many-body renormalization of the exciton energy levels due to their interaction, an effect that has attracted much theoretical interest over the years ~e.g., Refs. 8–11.!