Nano rainbows advance the light spectrum at the nanoscale

Nano rainbows advance the light spectrum at the nanoscale
by Simon Mansfield
Sydney, Australia (SPX) Jan 14, 2025

Since lasers emerged in 1960, nonlinear optics has sought to expand light's spectral range and generate new frequency components. Among the many techniques, supercontinuum (SC) generation is notable for producing light that spans wide sections of the visible and infrared spectrum. Traditionally, SC sources rely on third-order optical nonlinearity, requiring long interaction lengths to achieve broad spectral coverage. Second-order optical nonlinearity, while more efficient and requiring less power, has historically faced limitations due to phase mismatching in bulk crystals, restricting its utility.

A recent study published in *Light: Science and Applications* introduces an innovative approach to address these challenges. Led by Professor Zhipei Sun, a team of researchers from Aalto University, Tampere University, and Peking University successfully demonstrated a method to generate octave-spanning coherent light at a deep-subwavelength scale, less than 100 nanometers thick. Their work utilizes phase-matching-free second-order nonlinear optical frequency down-conversion within ultrathin crystals of gallium selenide (GaSe) and niobium oxide diiodide (NbOI2).

The team achieved coherent light generation with a spectral range extending from 565 to 1906 nm at a -40 dB width using difference-frequency generation. This approach resulted in a light source five orders of magnitude thinner and requiring two to three orders of magnitude less excitation power compared to conventional bulk material-based coherent broadband sources. Moreover, the nanometer-thick NbOI2 crystal demonstrated a conversion efficiency per unit length exceeding 0.66% per micrometer - nearly three orders of magnitude higher than standard bulk techniques.

To evaluate the coherence of the generated light, researchers used a Michelson interferometer. They recorded a fringe visibility above 0.9, signifying superior coherence relative to conventional superluminescent diodes and long-pulse SC sources. The remarkable coherence stems from the phase-matching-free difference-frequency generation in the thin GaSe and NbOI2 crystals. Enhancements in efficiency and total output power further underscore this method's potential for practical applications.

This advancement could pave the way for compact and versatile "nano rainbow" light sources with applications in fields like metrology, spectroscopy, and telecommunications. It represents a significant leap in manipulating light at the nanoscale, unlocking new possibilities for scientific and industrial innovation.

Research Report:Nanoscale thickness Octave-spanning Coherent Supercontinuum Light Generation