The invention of the laser (Light Amplification by Stimulated Emission of Radiation) in 1960 ushered in a new era of light-based technologies. Today, lasers are indispensable in communication, precision manufacturing, scientific research, etc. However, the direct observation of how stimulated emission initiates and evolves inside a laser system remains a missing piece.  

In a recent study published in Nature, a research team led by Prof. LI Ruxin and Prof. TIAN Ye from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences unraveled this mystery of this fundamental process of laser using a novel form of light - electromagnetic waves with massive electrons that float along the surface of a waveguide surface, dubbed surface plasmon polariton (SPP), which resulted in a powerful and ultra-small coherent surface light source that could fit on a microchip. 

As a comparatively new light form, SPP is gaining more attention over the past decades due to its distinctive characteristics of sub-wavelength confinement and near-field enhancement. A broad scope of interesting applications is available utilizing the SPPs, such as integrated photonic circuits and highly sensitive detectors. One major obstacle towards these applications is the relatively low coupling efficiency that restricts the SPPs to a tiny spatial size and low energy. 

To coherently amplify the SPP, just like a laser, an innovative way was employed in this study which takes advantage of free electrons generated by a femtosecond laser pulse. The free electrons coprapagates with the SPP over the waveguide surface in a synchronous manner, and transfer their energy to the SPP via the stimulated emission. Consequently, the spatial-temporal evolution of the SPP-electron interaction is captured by an ultrafast probe light that records the electric and magnetic field variances of the SPP with femtosecond resolution. 

"This way of making laser-like light could surpass existing methods that use electrons," said Dr. Nicholas Rivera from Harvard University in the News & Views of Nature. "The effects reported here could be amplified and enhanced by structures whose electromagnetic properties vary in space (for example, artificial materials called photonic crystals or metamaterials). Such spatially structured materials can offer marked control over the behaviour of light - a capability that has been used extensively in the field of nanophotonics to shape the spontaneous and stimulated emission of visible and infrared light."  

After showing the stimulated emission process, the researchers worked to turn their observed phenomena into a miniature laser by adding nanotip-based electron sources to the waveguide surface that could provide phase-matched electrons with SPP. Simulations suggested the SPP power could become even stronger when a proper electron pulse is used. The predicted power could even be comparable to much larger photonic lasers. Such a high-power coherent SPP light source will allow applications that require power compensation either as stand-alone components or as 'gain blocks' atop a photonic chip.  

The researchers will continue to improve the peak power of the demonstrated coherent SPP light source, and to make it easily integrated into a photonic chip for the diverse applications therein.