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Topic Name: Researchers bend near-infrared light through optical waveguiding in colloidal crystals
Category: Photonics
Research persons: Professor Paul V. Braun
Location: University of Illinois, United States
Details
Researchers at the University
of Illinois are the first to achieve optical
waveguiding of near-infrared light through features embedded in
self-assembled, three-dimensional photonic crystals. Applications for the
optically active crystals include low-loss waveguides, low-threshold lasers and
on-chip optical circuitry.
Key to the fabrication technique – which uses multi-photon polymerization
and a laser scanning confocal microscope – is a self-assembled, colloidal
material that exhibits a photonic band gap, said Professor Paul V. Braun,
a University Scholar and professor of materials science and engineering.
In previous work, reported in 2002, Braun’s research group was the first to
show that through multi-photon polymerization they could embed a polymer feature
inside a silicon dioxide, self-assembled colloidal crystal.
Now, in a paper accepted for publication in Nature Photonics, and posted on
the journal’s Web site, Braun and his team demonstrate actual optical activity
in waveguides and cavities created in their colloidal crystals.
“Taking our earlier work as a starting point, we built upon recent advances
in theory and computation, improvements in materials growth techniques, and
better colloidal crystallization capabilities to produce this new photonic
material,” said Braun, who also is affiliated with the university’s Beckman
Institute, Frederick Seitz Materials Research Laboratory, and Micro and
Nanotechnology Laboratory.
To make their optically active devices, the researchers begin by assembling a
colloidal crystal of uniform silica spheres that are 900 nanometers in diameter.
After removing the solvent, the researchers fill the spaces between the spheres
with a photoactive monomer. Then they shine laser light through a microscope and
into the crystal, polymerizing the monomer at the desired locations.
Next, they remove the unpolymerized liquid, and then fill the structure with
silicon. Finally, they etch away the silica spheres, leaving the desired optical
features embedded in a three-dimensional photonic crystal.
“Using spheres 900 nanometers in diameter creates a band gap at 1.5
microns, which is the wavelength used by the telecommunications industry for
transmissions through fiber-optical cables,” Braun said. “Creating these
waveguides by coupling colloidal assembly and multi-photon polymerization is
simpler and less expensive than conventional fabrication techniques, especially
for large-area photonic crystals.”
Note for Photonic Crystal
Photonic crystals are periodic optical (nano)structures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons. Photonic crystals occur in nature and in various forms have been studied by science for the last 100 years.
Photonic crystals are composed of periodic dielectric or metallo-dielectric (nano)structures that affect the propagation of electromagnetic waves (EM) in the same way as the periodic potential in a semiconductor crystal affects the electron motion by defining allowed and forbidden electronic energy bands. Essentially, photonic crystals contain regularly repeating internal regions of high and low dielectric constant. Photons (behaving as waves) propagate through this structure - or not - depending on their wavelength. Wavelengths of light (stream of photons) that are allowed to travel are known as "modes". Disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena such as inhibition of spontaneous emission, high-reflecting omni-directional mirrors and low-loss-waveguiding, amongst others.
Note for Lasing Threshold
In a laser, the lasing threshold is the lowest excitation level at which the laser's output is dominated by stimulated emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing excitation. Above threshold, the slope of power vs. excitation is orders of magnitude greater. The linewidth of the laser's emission also becomes orders of magnitude smaller above the threshold than it is below. Above the threshold, the laser is said to be lasing.
The lasing threshold is reached when the optical gain of the laser medium is exactly balanced by the sum of all the losses experienced by light in one round trip of the laser's optical cavity.
This simple analysis is predicated on the laser operating in a steady-state at laser threshold. However, this is not an assumption which can ever be fully satisfied. The problem is that the laser output power varies by orders of magnitude depending on which side of the threshold the laser is operating on. When very close to threshold, the smallest perturbation is able to cause huge swings in the output laser power.
With Braun, co-authors of the paper are Stephanie A. Rinne, a postdoctoral
fellow at the Beckman Institute, and Florencio García-Santamaría, a
postdoctoral research associate in the department of materials science and
engineering.
The work was funded by the U.S.
Army Research Office, National
Science Foundation and the U.S.
Department of Energy.
In figure 2, The opal in this bracelet contains a natural periodic microstructure responsible for its iridescent color. It is essentially a natural photonic crystal, although it does not have a complete photonic band gap.
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