High-speed mid-IR modulator using Stark shift in step quantum wells

We show in calculations that there is a capability for high speeds with a low applied voltage in modulators based on intersubband transitions in step quantum wells (QWs). A waveguide based on surface plasmons is assumed to achieve the necessary tight confinement of the optical field. In a structure with 8 GaInAs-AlGaInAs-AlInAs step QWs, we obtain a device capacitance of 14 fF corresponding to a RC limitation of electrical f/sub 3 dB/=190 GHz. The extinction ratio of 6.6-/spl mu/m light is 10 dB at an applied voltage of 0.9 V and T=300 K. By simple reasoning, we find that the device capacitance is approximately proportional to the absorption linewidth cubed when the linewidth is considered in the device design. Thus, the linewidth is very decisive for the modulation speed. We propose to place the dopants asymmetrically in the barriers in order to reduce broadening caused by doping induced potential fluctuations. In addition, the doping levels in the outermost barriers of the multi-QW structure are proposed to be reduced and asymmetrical, in order to achieve a uniform electric field over the step QWs, which is shown to increase the achievable f/sub 3 dB/ very markedly.

I. Introduction

Devices based on intersubband transitions in quantum wells (QWs) have received considerable attention in the last decade. The work has mainly been devoted to lasers [1] and detectors [2]. Several schemes for intersubband (IS) modulators have also been proposed and demonstrated. However, generally conventional wide multipass waveguides [2] are employed in IS modulators [3]. They are suitable for experimental investigation of IS absorption, because of the ease of coupling a laser beam into the waveguide, but the optical overlap with the QWs, where the absorption or refractive index shift occurs is not very large.