Silicon-Based Laser Emits in Visible
Although much of the excitement about silicon lasers in this year has centered on silicon Raman lasers investigated at Intel Corp. and at the University of California, Los Angeles, other re- searchers have pursued other promising approaches to silicon-based lasers. Recently, scientists at the University of Cincinnati, working with a group at Nitronex Corp. in Raleigh, N.C., obtained visible lasing from a layer of Eu-doped GaN deposited on a silicon substrate.
Europium ions in a 0.5-µm-thick GaN film deposited by molecular beam epitaxy on a silicon substrate lased at 620 nm when pumped with a nitrogen laser.
Silicon is the fundamental building material for modern electronics and would seem the obvious host for combining electronics and photonics in monolithically integrated devices. Unfortunately, it turns out to be a poor photonic material. Its transparency at telecommunications wavelengths makes it useless as a detector in that spectral region. And its indirect bandgap means that the product of electron-hole recombination is far more likely to be a phonon than a photon. To overcome the indirect-bandgap problem, scientists have turned to Raman lasing, or to doping or combining silicon with materials that can lase effectively.
The Ohio researchers adopted the latter approach. They previously had studied lasing in rare-earth-doped films of GaN on sapphire substrates and sought to extend that technique to silicon substrates. They used substrates developed by Nitronex that overcome the lattice and thermal-expansion mismatch between silicon and GaN by depositing multiple AlGaN layers as buffers between the silicon substrate and the 1-percent-atomic Eu-doped GaN (see figure). A top cladding layer of AlGaN created a planar waveguide of the whole structure.
A 337-nm nitrogen laser with 600-ps pulses optically pumped the laser and created a population inversion in the Eu3+ ions, which subsequently lased at 620 nm. Incident on the upper surface of the structure, the 337-nm radiation achieved threshold lasing at a density of ~117 kW/cm2.
Although the current experiment demonstrated lasing in the red spectral region, the scientists believe that other rare-earth elements could be substituted for europium to create lasers from the ultraviolet through the near-infrared.
As evidence of lasing, the researchers cite the polarization of the output and line narrowing from ~2.3 nm to ~1.9 nm at laser threshold. They also note the existence of distinct longitudinal laser modes in the output spectrum of polished submillimeter cavities. For example, in a 145-µm cavity, the mode spacing was 0.56 nm, corresponding to a theoretical mode spacing of approximately the same value.
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