Slow Light by Electromagnetically Induced Transparency ( EIT ) in Semiconductors

The ability to controllably slow down the propagation of light pulses can enable a variety of important applications. In recent years, two different approaches have been pursued for slow light. One approach explores dispersion engineering with passive optical elements such as resonators, filters, and photonic bandgap structures. The other approach takes advantage of coherent nonlinear optical processes, such as electromagnetically induced transparency (EIT) and coherent population oscillation (CPO). In an EIT process, destructive interference induced by a quantum coherence renders an opaque medium transparent, leading to a spectrally sharp dip in the absorption spectrum. The corresponding steep dispersion within the transparency window results in a large reduction in the group velocity of light. The steepness of the dispersion depends on the intensity of a pump laser beam, thus providing an effective and convenient mechanism for slowing down the propagation of light in a controllable fashion. While the phenomenon of EIT has been investigated extensively in atomic systems for many years, it has been difficult to realize EIT in semiconductors. The main challenge arises from the fact that typical quantum coherences in semiconductors are very fragile. They decay in a timescale of a few tens of picoseconds even at liquid helium temperatures. Another difficulty in terms of applications is that for an optical pulse to be contained completely in a slow light medium, the effective interaction length of the medium needs to exceed the pulse length. Recently, CONSRT researchers have developed an EIT scheme based on the use of electron spin coherence, i.e. coherent superposition of electron spin states, in a semiconductor waveguide. The electron spin coherence can feature lifetime exceeding 100 ns and can remain robust even at room temperature. In addition, semiconductor waveguides can accommodate long optical interaction lengths. In previous experimental studies, electron spin coherences are induced in the presence of an external magnetic field. By taking advantage of special optical selection rules for light-hole transitions, our scheme also makes it possible to induce electron spin coherence without using an external magnetic field. In semiconductors such as GaAs, the band edge is characterized by a slike conduction band and p-like heavy-hole and light-hole valence bands. Dipole transitions between the conduction and the light-hole valence bands form two V-type three-level systems ( see Figure. 1 ). Using a Transverse Magnetic-polarized (TM) probe field and a Transverse Electric –polarized (TE) pump field, we couple the two electron spin states to a common light-hole valence band state, inducing a coherence superposition of the two electron spin states. Destructive interference induced by the electron spin coherence then leads to a sharp resonance in the transmission spectrum as shown in Fig. 2 (a peak in transmission corresponds to a dip in absorption). We have carried out additional experimental studies to further confirm the physical origin of the transmission resonance. Our experimental studies demonstrate the feasibility of using electron spin coherence to realize EIT and slow light in semiconductor waveguides, opening the door to controlling the propagation of light pulses in a semiconductor waveguide (see Fig. 3). There are, however, still considerable challenges towards the realization of a practical device. For room temperature operation, we expect that quantum dot structures with strong quantum confinement are needed in order to suppress carrier thermalization. In addition, strong absorption of the pump beam in the waveguide currently limits the increase in transmission to under 10%. Further experimental efforts on an improved scheme that can avoid the pump absorption are currently under way.