First Quantum Cascade Laser on Silicon

3-D artistic depiction of multiple Quantum Cascade Lasers integrated above silicon waveguides (Source: A. Spott)

3-D artistic depiction of multiple Quantum Cascade Lasers integrated above silicon waveguides (Source: A. Spott)

A team of researchers from across the country, led by Alexander Spott, University of California, Santa Barbara, have built the first quantum cascade laser on silicon. 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 wave­lengths 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) underneath the silicon wave­guide, rather than just SiO2.”

The break­through could lead to several applications, Spott explained. Traditionally, silicon photonic devices operate at near-infrared wave­lengths, with applications in data trans­mission and tele­communications. However, there is emerging research interest in building these silicon photonic devices for longer mid-infrared wavelengths, for a range of sensing and detection applications, such as chemical bond spectros­copy, gas sensing, astronomy, oceano­graphic sensing, thermal imaging, explosive detection, and free-space communi­cations.

The next step for the team is to improve the heat dissipation to improve the performance of these QCLs and to allow them to make conti­nuous-wave QCLs on silicon. “We generally hope to improve the design to get higher powers and efficiency,” Spott said. “This brings us closer to building fully integrated mid-infrared devices on a silicon chip, such as spectro­meters or gas sensors. Silicon is inex­pensive, the fabri­cation can be scaled up to signi­ficantly reduce the cost of individual chips, and many small devices can be built on the same silicon chip for example multiple dif­ferent types of sensors operating at different mid-infrared wave­lengths.” (Source: OSA)

Link: Optoelectronics Group (A. Spott), University of California, Santa Barbara, USA

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