Nanorainbow: Expanding the optical spectrum at the smallest scale

Written by Light Publishing Center, Changchun Optical Research Institute, Precision Mechanical Physics, CAS
Coherent broadband supercontinuum (CBS) generation with phase-matched second-order optical nonlinearity. a) Schematic illustration of CBS generation by DFG process in nanomaterials with deep subwavelength thickness (about 100 nm). The thickness is significantly thinner than the coherence length of the incident beam, allowing free movement of phase matching. b) Tunable CBS generation at different l2 using the spectral width of input beam B2 (Dl2) to label the center wavelength (l2) while beam B1 is fixed at a wavelength of 400 nm. Credit: Susobhan Das et al.
Since the invention of the laser in 1960, nonlinear optics has sought to extend the spectral range of light and create new frequency components. Among various technologies, supercontinuum (SC) generation stands out in that it can generate light over a wide range of visible and infrared spectra.
However, conventional SC sources rely on weak third-order optical nonlinearities and require long interaction lengths to obtain broad spectral output. In contrast, second-order optical nonlinearities offer much better efficiency and lower power requirements, but phase mismatches in the bulk crystal have historically limited their spectral range and overall efficiency.
In a study published in Light: Science & Applications, a joint research team from Aalto University, Tampere University, and Peking University, led by Professor Zhipei Sun, has developed an innovative method to generate coherent light that spans an octave at deep subwavelength scales. I have proven it. (<100nm). Their innovative approach employs second-order nonlinear optical frequency downconversion without phase matching in ultrathin gallium selenide (GaSe) and niobium diiodide oxide (NbOI2) crystals.
The researchers successfully generated coherent light with a -40 dB spectral width spanning 565 to 1906 nm through difference frequency generation. This results in a light source that is five orders of magnitude thinner and requires two to three orders of magnitude less excitation power than traditional coherent broadband light sources based on bulk materials. Furthermore, the conversion efficiency per unit length from nanometer-thick NbOI2 crystals was over 0.66% per micrometer, which is about three orders of magnitude higher than the common bulk method.


Demonstration of CBS applications in gas sensing. Credit: Light: Science & Applications (2025). DOI: 10.1038/s41377-024-01660-6
To assess the coherence of the broadband light produced, the team employed a Michelson interferometer, revealing an impressive fringe visibility of over 0.9. It demonstrates superior coherence compared to standard superluminescent diodes and long-pulsed SC light sources.
This extraordinary coherence is due to difference frequency generation in thin GaSe and NbOI2 crystals, demonstrating the power of this phase-matching-free technique for nanoscale coherent broadband light generation. Furthermore, the researchers improved the efficiency and total output, further solidifying the potential of this method for practical application.
With this development, the “nanorainbow” has the potential to revolutionize compact and versatile light sources, pushing the limits of light manipulation at the smallest scales, with applications in metrology, spectroscopy, and communications.
Further information: Susobhan Das et al, Generation of coherent supercontinuum light spanning an octave of nanoscale thickness, Light: Science & Applications (2025). DOI: 10.1038/s41377-024-01660-6
Provided by: Light Publishing Center, Changchun Institute of Optics, Fine Mechanics and Physics, CAS
Citation: Nanorainbows: Expanding the Light Spectrum at the Smallest Scale (January 13, 2025), from https://phys.org/news/2025-01-nano-rainbows-spectrum-smallest-scale.html 2025 Retrieved January 13th
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