Thin-film laser takes a practical approach to silicon photonics

Marie Freebody, marie.freebody@photonics.com

Scientists have successfully built a thin-film laser onto silicon that demonstrates the lowest threshold current densities to date for lasers on silicon integrated with waveguides. The Duke University researchers say that their tiny laser is cheap and simple to produce, making it an ideal component for future chip-scale optical integrated circuits.

Chip-scale optical sources are necessary for optical sensing systems for portable medical diagnostics and environmental monitoring as well as high-bandwidth chip-to-chip optical interconnects in future optical computers. But developing small lasers that are suitable for chip-scale systems is no easy task.

To build a low-power, portable, cost-effective integrated system, the laser power consumption must be minimized and the laser-to-waveguide coupling maximized. Not only that, but for a cost-effective alternative to electronic circuits, the fabrication process must also be simple.

Nan Marie Jokerst and colleagues in the university’s department of electrical and computer engineering opted for a practical approach. The team built a thin-film InGaAs/GaAs edge-emitting single-quantum-well laser that has been integrated onto a silicon substrate.

Nan Marie Jokerst, left, and Sabarni Palit review a chip they fabricated in the Shared Materials Instrumentation Facility – a “clean” setting much like those seen in the semiconductor industry. Courtesy of Duke University Photography.

Thanks to its edge-emitting format, optical signals can be directed into a planar optical waveguide and distributed across the chip or board, mimicking the electrical interconnects found on silicon CMOS integrated circuits.

“Our goal is not tiny lasers, nor the lowest-power lasers. Rather, we seek to use conventional (i.e., low-cost) laser structures that can be used now to build useful systems,” Jokerst explained. “This implies using standard fabrication tools and materials whenever possible, integrating for mechanical robustness, and seeking low-cost solutions to achieve results that may not be the world’s record in anything, but meet the needs of the application.”

Despite Jokerst’s modesty, the electrically pumped laser offers a threshold current density as low as 240 A/cm², which is the lowest reported to date for a facet-embedded thin-film III–V edge-emitting laser.

The laser exhibits peak wavelengths from 995 to 1002 nm in pulsed mode and measures 120 to 150 μm wide and 3.3 to 3.5 μm thick, with a laser cavity length from 800 to 1000 μm. The laser is integrated using a metal back contact for effective heat dissipation, which is critical for systems that incorporate a laser source.

As well as building the laser, the group reported developing a thin-film photodetector, polymer waveguide and microresonator sensors. Details of the integrated structures can be found in a paper on the work published on Oct. 15, 2010, in the journal Optics Letters.

“The thin-film photodetectors (~1 μm thick) can be embedded in the waveguides, and the waveguides can overlap one laser facet for efficient launch into the waveguide,” Jokerst said.

Next on the agenda for the team is integrating a laser source and a micro-resonator sensor onto silicon to create a portable, chip-scale medical diagnostic tool. The goal is to use battery power to operate the chip and to detect multiple target analytes, which will require multiple microresonator sensors to be integrated with multiple photodetectors.

“DNA is one particular target, but this system will also address water quality sensing,” Jokerst said. “Finally, we are also integrating this entire optical sensing system with digital microfluidics to integrate sample processing and optical sensing into a single portable diagnostic.”