Xiang Zhang, director of LBNL's materials sciences division, led a study in which a mathematical concept called adiabatic elimination was applied to optical nanowaveguides, allowing crosstalk in densely packed nanowaveguides to be eliminated.
"When nanowaveguides in close proximity are coupled, the light in one waveguide impacts the other. This coupling becomes particularly severe when the separation is less than the diffraction limit, placing a restriction on how close together the waveguides can be placed," he said. "We have demonstrated a scheme that effectively cuts off the crosstalk between them, enabling on demand dynamical control of the coupling between two closely packed waveguides."
According to Zhang's team, adiabatic elimination has a proven track record in atomic physics and other research fields. The approach works by decomposing large dynamical systems into smaller ones by using slow versus fast dynamics.
"Picture three buckets side by side, with the first being filled with water from a tap, the middle being fed from the first bucket though a hole while feeding the third bucket through another hole," said researcher Michael Mrejen. "If the flow rate into the middle bucket is equal to the flow rate out of it, the second bucket will not accumulate water. This, in a basic manner, is adiabatic elimination. The middle bucket allows for some indirect control on the dynamics, compared to the case in which water goes directly from the first bucket to the third bucket."
The researchers applied this concept to a coupled system of optical nanowaveguides by inserting a third waveguide in between the two, with each waveguide only 200nm apart. The middle waveguide operates in a 'dark' mode, in that it doesn't appear to participate in the exchange of light between the two outer waveguides.
"Even though the dark waveguide doesn't seem to be involved, it nonetheless influences the dynamics of the coupled system," said researcher Haim Suchowski. "By selecting the relative geometries of the outer and intermediate waveguides, we achieved adiabatic elimination, which in turn enabled us to control the movement of light through densely packed nanowaveguides. Until now, this has been almost impossible to do."
