Expanding the light spectrum at the smallest scale

Since the invention of the laser in 1960, nonlinear optics has aimed to broaden light’s spectral range and create new frequency components. Among the various techniques, supercontinuum (SC) generation stands out for its ability to produce light across a wide portion of the visible and infrared spectrum.

However, traditional SC sources rely on weak third-order optical nonlinearity, requiring long interaction lengths for broad spectral output. In contrast, second-order optical nonlinearity offers far greater efficiency and lower power requirements, though phase mismatching in bulk crystals has historically limited its spectral coverage and overall efficiency.

In a study published in Light: Science & Applications, a collaborative research team from Aalto University, Tampere University, and Peking University, led by Professor Zhipei Sun, has demonstrated a revolutionary method for generating octave-spanning coherent light at the deep-subwavelength scale (<100 nm). Their innovative approach employs phase-matching-free second-order nonlinear optical frequency down-conversion in ultrathin gallium selenide (GaSe) and niobium oxide diiodide (NbOI₂) crystals.

The researchers successfully generated coherent light with a -40 dB spectral width spanning from 565 to 1906 nm via difference-frequency generation. This produced a light source that is five orders of magnitude thinner and requires two to three orders of magnitude less excitation power than conventional coherent broadband light sources based on bulk materials. Furthermore, the conversion efficiency per unit length from the nanometer-thick NbOI₂ crystal exceeded 0.66% per micrometer—about three orders of magnitude higher than typical bulk methods.

To assess the coherence of their generated broadband light, the team employed a Michelson interferometer, revealing an impressive fringe visibility above 0.9—demonstrating superior coherence compared to standard superluminescent diodes and long-pulse SC sources.

This exceptional coherence is due to the difference-frequency generation in the thin GaSe and NbOI₂ crystals, showcasing the power of this phase-matching-free technique for nanoscale coherent broadband light generation. Additionally, the researchers enhanced efficiency and total output power, further solidifying the method’s potential for practical applications.

With this development, “nano rainbows” could revolutionize compact, versatile light sources with applications in metrology, spectroscopy, and telecommunications—pushing the boundaries of light manipulation at the smallest scale.