over - 2005 Taming Slow Light on a Chip

In the November 3, 2005 issue of the scientific journal Nature, the IBM team describes the integrated photonic circuit that allows first to dramatically reduce the speed of light signals and then to actively control it by applying an electric current. This achievement is an important milestone toward development of silicon microchips that integrate both photonics and electronics components. Such on-chip optical integration could pave the way to low-cost optical interconnects that transmit data between chips using light signals over waveguides and fibers rather than electrical signals over copper wires.
This work was partially supported by the Defense Advanced Research Projects Agency (DARPA) through the Defense Sciences Office program Slowing, Storing and Processing Light.

The photonic circuit described in the paper comprises photonic crystal waveguides – carefully engineered silicon nanostructures able to manipulate optical signals on a scale of a wavelength. This ultimate light confinement results in many unusual properties unattainable in conventional silicon photonics structures, like demonstrated in the paper 300-fold reduction of the light speed. The ability to structurally engineer the speed at which the light signals are traveling in the circuit has potential implications for a broad range of possible on-chip nanophotonic components as optical delay lines, optical buffers and even optical memory.

Besides experimental demonstration of slowing the light down, the IBM team also showed a way to electrically control the light velocity by passing electric current across the photonic crystal waveguide. Thus the tiny hot spot is generated in the desired portion of the photonic circuit that locally changes the refractive index of silicon. As a result the speed of light can be effectively tuned within a large range with very low applied electric power. Small footprint of the device of only 0.04mm2 achieved by utilization of nanophotonic waveguides is largely responsible for the device response time that is approximately 1000 times faster than in conventional photonic thermo-optic modulators. The device is easily scalable further to even smaller footprints well below 0.01mm2. At this level the areal density of photonic circuits starts to approach the density of electronic circuitry in computer chips.

Description of the animation

Dark blue bar is a silicon photonic wire waveguide. A packet of light pulses, shown as a train of white balls, is propagating along the silicon waveguide with a speed not much different from a speed of light in vacuum

State 1. Click to enlarge. State1. Click to enlarge.
Photonic wire waveguide is butt-coupled to the silicon photonic crystal waveguide defined by a periodic array of holes etched in a silicon membrane.

When the train of light pulses is entering into photonic crystal waveguide the light speed is significantly reduced due to diffraction in a periodic photonic lattice.

In the paper the speed as low as 1/300 of the speed of light in vacuum is measured.
State 2. Click to enlarge. State2. Click to enlarge.
To control the speed of light electrically two lateral electrical contacts are deposited on top of a silicon membrane. This forms an integrated microheater.

The trick is to position the contacts as close as possible to the waveguide core and, at the same time, to avoid excessive parasitic losses due to absorption in metal
State 3. Click to enlarge. State 3. Click to enlarge.
When the voltage is applied to the contacts the current is passing across the photonic crystal waveguide. This current heats the silicon membrane that in turn results in slight changes in the refractive index of silicon due to thermo-optic effect.

Small changes in the material refractive index results in very large changes of the speed of light. Thus light pulses can be accelerated or decelerated on-demand.
State 4. Click to enlarge. State 4. Click to enlarge.