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Indium tin oxide-coated glass. Credit: Adafruit Industries

Though it may not be as highly publicized as world records in sports, researchers at a U.S. university have found a record nonlinear optical response in a commonly available semiconductor (Science, doi: 10.1126/science.aae0330). The results could open new avenues for building tiny structures for many nanophotonics applications.
 
The University of Rochester team, led by OSA Fellow Robert W. Boyd, studied indium tin oxide, a so-called degenerate semiconductor—it can act like a metal or a semiconductor, depending on the level of doping. Indium tin oxide can easily be deposited as a thin film on substrates, and its permittivity drops to zero in the near-infrared range. The oxide also works well with existing semiconductor-fabrication technologies.
 
The change in a material’s refractive index grows large as its permittivity grows small, so scientists expect that it becomes optically nonlinear in the “epsilon-near-zero” frequency range. The Rochester researchers probed the oxide material with near-infrared laser pulses and found that its nonlinear response—specifically, the real part of its refractive index—changed by 0.72 ± 0.025 during the pulses. That figure corresponds to 170 percent of the material's linear response.
 
The experiments also revealed that the recovery time for the indium tin oxide to return to its original refractive index after the laser pulse is 360 fs, suggesting that components made of this material could modulate light at rates of 1.5 THz or better.
 
Perhaps the most challenging part of the work, says team member Israel De Leon, was understanding the physical mechanisms that led to the huge optical nonlinearity observed in the experiments. De Leon, who was a research associate at Rochester, is now at the Tecnologico de Monterrey in Mexico.
 
Next, the researchers will study the practical implications for photonics from having a material with such a large optical nonlinear response, according to De Leon. Furthermore, he adds, it is important to continue searching for other materials that have an equally large intensity-dependent refractive index but that may have lower optical losses.
 
The research group included M. Zahirul Alam, Boyd’s graduate student at the University of Ottawa (Canada).