Abstract:

Slow and fast light are the propagation of optical signals at group velocities below and above the speed of light in a given medium. There has been great interest in the use of nonlinear optics to engineer slow and fast light dispersion for applications in optical communications and radio-frequency or microwave photonics. Early results in this field were primarily confined to dilute atomic systems. While these results were impressive, they had two major barriers to practical application. First, the wavelengths were not compatible with fiber optic telecommunications. More importantly, the bandwidth obtainable in these experiments was inherently low; 100 kHz or less.

Within the last five years slow and fast light effects have been observed and engineered in a much wider variety of systems. In this work, we detail our efforts to realize slow and fast light in semiconductor systems. There are three primary advantages of semiconductor systems: fiber-compatible wavelengths, larger bandwidth, and simplification of integration with other optical components.

In this work we will explore three different types of physical mechanisms for implementing slow and fast light. The first is electromagnetically induced transparency (EIT). In transporting this process to semiconductors, we initially turn our attention to quantum dots or "artificial atoms". We present simulations of a quantum dot EIT-based device within the context of an optical communications link and we derive results which are generally applicable to a broad class of slow light devices. We then present experimental results realizing EIT in quantum wells by using long-lived electron spin coherence.

The second mechanism we will explore is coherent population oscillations (CPO), also known as carrier density pulsations (CDP). We examine for the first time how both slow and fast light may be achieved in a quantum well semiconductor optical amplifier (SOA) while operating in the gain regime. Again, we simulate the device within an optical link and we study the limitations of the fast light effect using practical metrics such as bit error rate and power penalty. Finally we present some preliminary experimental results.

Our third study revolves around ultrafast intraband effects of spectral hole burning and carrier heating in SOAs. These experiments employ sub-picosecond pulses, demonstrating record-breaking bandwidth for slow and fast light of 1 THz. Our initial demonstration of fast light based on intraband processes achieves an advance of 2.5 times the input pulse width, an important milestone and a record for slow and fast light in semiconductors. Finally, we demonstrate a novel technique for improving performance by chirping the pulse at the input and then recompressing the pulse at the output. In addition to improving the fast light advance observed at gain bias, we observe an unexpected tunable delay effect by changing the sign of the chirp at the input. When both the delay and advance are employed we observe a five-fold increase in temporal shift, more than 10 times the input pulse duration.