Silicon Advances as a Laser Material
POVO, Italy -- One-quarter of the Earth is silicon. Microelectronic manufacturing techniques and the expanding capabilities of integrated circuits have made it even more ubiquitous. The demand for bandwidth and higher processing speeds is driving the transition from electronic circuitry to photonic circuitry, but silicon, the base of the manufacturing infrastructure, has been a poor candidate as a laser source. Now, a novel assembly of silicon nanocrystals has rekindled hopes that silicon will be an effective lasing medium.
Reporting in the Nov. 23, 2000, issue of Nature, researchers at the University of Trento and the University of Catania in Catania described how they introduced the 3-nm-diameter nanocrystals into pure quartz wafers and into silicon dioxide layers on a silicon substrate using ion implantation. The difference in the index of the quartz and the nanocrystal-impregnated region created a planar waveguide.
Experiments with silicon nanocrystals suggest that the material may
be used to create a viable laser. Courtesy of Lorenzo Pavesi, University
They illuminated the waveguide configuration with 2-ps pulses of 390-nm light from a Ti:sapphire laser operating at 82 MHz, varying the length of the excited path by using a movable slit to partially block the excitation beam, and measured the emission from the edge of the waveguide at 800 nm. The emitted intensity fit an amplified spontaneous emission curve with a net gain of 100 ±10 cm21, and, as they increased the net pump power by lengthening the excited path or raising the pulse energy, the linewidth of the emitted radiation decreased, another signature of amplified spontaneous emission.
The team also irradiated the thermally grown device with the laser as a weak 800-nm probe beam was transmitted through the layers. At low pump energies, the probe beam was absorbed, but as the pump energy was increased, the structure amplified the probe beam.
Lorenzo Pavesi from Trento said that the gain is due to population inversion among the radiative interface states that form at the interface with the substrate. He believes that although each nanocrystal's gain is low compared with standard semiconductor laser materials, the individual contribution of nanocrystals is summed when they are deposited densely, so the material could yield macroscopic gain values that compete with existing laser materials.
"The next step," said Pavesi, "is conceptually very simple: You need to form the nanocrystals within a planar or vertical optical cavity to get positive optical feedback for a laser. Then you need to contact the system, inject a current and see if you get lasing."
There are still obstacles to silicon laser efficiency; light emission in the silicon nanocrystals is a slow process, taking microseconds rather than nanoseconds. Pavesi acknowledged that this long time allows nonradiative processes to compete for the excited state energy, but he maintained that a clear step has been taken toward the production of silicon lasers.
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