A team of physicists from TU Graz, Harvard SEAS, and the University of Texas at Austin has demonstrated a nanoscale device that boosts nonlinear frequency conversion — the process of transforming light into different frequencies — by three orders of magnitude compared to a non-patterned heterostructure.
Published in Nature Nanotechnology, the work combines two metamaterials: a semiconductor heterostructure made from nanoscale layers of gallium arsenide and aluminium gallium arsenide containing asymmetrically coupled multiple quantum wells, topped with a metasurface of titanium dioxide nanopillars.
The quantum wells confine electrons in one dimension, forcing their energy into discrete quantum states. Because the wells are asymmetrically coupled, electrons predominantly move in one direction when exposed to light, creating enhanced nonlinear oscillations that allow light waves to interact far more efficiently than in naturally occurring crystals.
The metasurface on top precisely creates the light polarization required to deflect incoming light so it scatters along the 'one-way street,' further amplifying light-matter interactions. During experiments, the team discovered that at exact normal incidence the enhanced nonlinear polarization cancelled itself out completely. Tilting the sample by just 0.3 degrees broke this symmetry and restored the enhancement.
The resulting effective nonlinear conversion is three orders of magnitude higher than a non-patterned heterostructure and exceeds previously reported values for comparable near-infrared devices. Potential applications include more compact telecom components, quantum communication photon-pair sources, and ultrafast optical signal processing.
