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Since the advent of subwavelength near-field microscopy techniques in the mid-1990s, scientists have been trying without success to use the methods to study widely used semiconductor materials. The issue: the imaging techniques worked only with longer wavelengths of light.
Now a team of researchers at a US university has pushed the microscopy method into the range of blue light, which opens up new possibilities for studying electrons inside wide-bandgap materials such as silicon and gallium nitride (Light Sci. Appl., doi: 10.1038/s41377-023-01137-y).
Homing in with s-SNOM
Using the technique called scattering-type scanning near-field microscopy (s-SNOM), researchers have probed such nanoscale phenomena as grain defects in perovskite solar cells and ghost polaritons in calcite. The nonlinear microscopy method uses light scattered from a sharp tip that sits just above the material being studied. Some of the scattered light carries information about the material.
Unfortunately for anyone studying economically important semiconductors, s-SNOM requires terahertz and infrared radiation to work properly. Higher-energy photons do not couple to the thin tip of the probe, and focusing the system is difficult.
Three researchers at Brown University worked around the problem by using blue light to generate terahertz radiation from the sample material itself. In their experiments, the team beamed 410-nm femtosecond laser pulses at the tip of an atomic force microscope hovering over a semiconductor sample. The blue light impinging on the material induces terahertz emission through a second-order nonlinear process. In turn, the researchers analyzed the terahertz rays via laser terahertz emission microscopy and obtained nanoscale spatial resolution.
The Brown group tested out the technique on indium arsenide, which has a narrow bandgap of only 0.35 eV, before performing experiments with bulk crystalline silicon. Silicon emits terahertz rays when hit with blue light, but not with red or near-infrared radiation. The researchers also developed a mathematical framework to extract information about the charge-carrier properties of the silicon.
Future studies
The researchers say they are excited to see what new information would be revealed using their method—for example, better insights into semiconductors used in blue LEDs—according to the press release accompanying the study. Daniel Mittleman, the senior author of the paper, says he is currently devising plans to use blue light to study materials that he and others haven’t been able to before.