Facet Reflectors for On-Chip Lasers

An etched facet semiconductor laser with an air gap reflector. (Source: A-Star / OSA)

A systematic study of a simple and general structure for on-chip semi­conductor lasers by A*STAR researchers in Singa­pore sets the scene for much broader application of inte­grated semi­conductor lasers beyond conven­tional silicon-based systems. The ability to use, mani­pulate and sense light is appli­cable to many techno­logies, from data inter­connection and fiber optics to optical sensors and optical storage systems. Tiny lasers are routinely inte­grated into micro­chips for these optoelec­tronics applications using a well-under­stood silicon-based laser structure, but alter­native and poten­tially simpler structures in non-silicon systems have yet to be explored in detail.

One such non-silicon-based appli­cation is a new type of data storage system – heat-assisted magnetic recor­ding (HAMR) – which researchers at the Data Storage Insti­tute have been working on as a next-genera­tion data storage tech­nology. HAMR uses inte­grated lasers for fast and precise micro-spot heating of a magnetic medium, but requires the laser to be formed on alu­minum-titanium-carbide (AlTiC) rather than silicon. This presented Chee-Wei Lee and his colleagues with a significant problem, since the silicon substrate plays in integral role in producing the laser light.

“We needed to develop a generic inte­gration scheme that would allow us to fabri­cate laser devices on different substrates, not just silicon,” says Lee. “For this, a facet reflector structure is very useful, but low facet reflec­tivity is a problem, and using different reflec­tors usually means a more compli­cated fabri­cation process and greater chance of device failure.” The lasers used in such applications turn electrical current into a light emission. They do this by taking light produced by a stack of ultrathin layers of a light-emitting semi­conductor – in this case aluminum-gallium-indium-arsenide – and multi­plying light at the target wave­length using a resonant cavity formed between two reflectors.

By designing a faceted laser structure consi­dering process inte­gration, Lee and his team developed a fabrication scheme that can accom­modate different types of reflectors without addi­tional processing steps. The team then used this fabri­cation scheme to test facet reflectors made by depositing a thin gold film, by chemical modi­fication of the surface, or by etching an air gap.

Studies of the different laser structures and suppor­ting simu­lations revealed that a thin gold layer, less than 100 nm thick, afforded the best per­formance in terms of facet reflec­tivity, minimum lasing current, emission effi­ciency and output power. “We expect our results to serve as benchmark for research and develop­ment on etched facet lasers with different reflec­tors,” says Lee. (Source: A*Star)

Reference: C.-W. Lee et al.: Comparison of III-V/Si on-chip lasers with etched facet reflectors, Appl. Opt. 56, 5086 (2017); DOI: 10.1364/AO.56.005086

Link: Data Storage Inst., Agency for Science, Technology and Research (A*STAR), Singapore

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