Marie Freebody, email@example.com
DURHAM, N.C. – 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.”