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Silicon-Based
Laser Emits in Visible
Active layer of europium-doped GaN is
deposited on silicon substrate.
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 researchers 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. 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 A1GaN 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 Eu ions, which subsequently lased at 620 nm. Incident on
the upper surface of the structure, the 337-nm radiation achieved thresh
old lasing at a density of ~117 kW/cm
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
re searchers 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
sub-millimeter 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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