Abstract

We investigate the feasibility of a graded multiple quantum well as an infrared phase modulator. The existence of infrared intraband transitions in multiple quantum wells is well known.¹ By applying an external voltage to the well, the wavelength of the transition can be altered. This shifting absorption center induces a corresponding refractive index change of the material, which provides the basis of the electrooptic phase modulator. The introduction of a linear composition gradient in the well causes the well potential to vary across its width. The external voltage also introduces a linear variation in the potential, but now both inside and outside the well. In the absence of a material gradient, the refractive index change is a quadratic function of the voltage. We show that including a built-in composition gradient changes a weak quadratic electrooptic effect into a strong linear one. In particular, for a Ga₁^__x-Al_xAs well with AlAs barriers we find the voltage needed for a 180° phase change to be of the order of 150 V, 2 orders of magnitude less than a conventional electrooptic material. The combination of index change linearity and low power consumption makes this structure an excellent candidate as a mid-infrared index modulator.

© 1989 Optical Society of America