A team of researchers from across the country, led by Alexander Spott, University of California, Santa Barbara, USA, have built the first quantum cascade¹ laser on silicon². The advance may have applications that span from chemical bond spectroscopy and gas sensing, to astronomy and free-space communications. Integrating lasers directly on silicon chips is challenging, but it is much more efficient and compact than coupling external laser light to the chips. The indirect bandgap of silicon makes it difficult to build a laser out of silicon, but diode lasers can be built with III-V materials such as InP or GaAs. By directly bonding an III-V layer on top of the silicon wafer and then using the III-V layers to generate gain for the laser, this same group has integrated a multiple quantum well laser on silicon that operates at 2 µm. Limitations in diode lasers prevent going to longer wavelengths³ where there are many more applications, so the group turned their attention to using quantum cascade lasers instead. 

 

Building a quantum cascade laser on silicon was a challenging task made more difficult by the fact that silicon dioxide becomes heavily absorptive at longer wavelengths in the mid-infrared. 

 

"This meant that not only did we have to build a different type of laser on silicon, we had to build a different silicon waveguide too," Spott explained. "We built a type of waveguide called a SONOI waveguide [silicon-on-nitride-on-insulator], which uses a layer of silicon nitride [SiN] underneath⁴ the silicon waveguide, rather than just SiO2."

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