Voltage-Controlled Negative Index in Vertically Coupled Quantum Dot   Systems
  We demonstrate that voltage-controlled negative index can be obtained in self-organized InAs quantum dot systems. As the bias voltage changes, the refractive index can be adjusted and controlled continuously from -7 to 7. Simultaneously, the absorption of light in the system will be very small. The single-negative index materials and the double-negative index materials can be achieved in different bias voltages. 
Huan Wang, Ka-Di Zhu