under white light, reflects colors from
red to indigo. The filter the researchers
created was about 25 mm long, but the
technique they used is scalable.
“We have developed a process that can
be modified to create spatially separated
structures with designed optical properties. It is a novel and simple concept to
integrate multiple reflection filters in a
very compact manner,” explains Gan.
Many inexpensive mass-produced
gratings are made by embossing plastic
with a master. But making the master
with nanoscale features typically requires
either expensive focus ion beam milling or
electron beam lithography techniques. In
contrast, this technique allows fabrication
of compact multicolored filters in a
single step using only optics.
Lasing from a Self-Assembled
Researchers in India have developed a mirrorless photonic crystal laser
made out of dye-activated, self-assembled
nanoscale spheres. The tiny laser emits
visible wavelengths at room temperature
and takes only a few hours to build (Phys.
Rev. A 85, 023844).
A team led by Rajesh V. Nair of the
Bhabha Atomic Research Centre (India)
created the laser crystal out of rhodamine-B-dye-doped polystyrene spheres, each
with a diameter of 295 nm. The face-centered cubic (fcc) crystal, roughly 2 cm2
in area, grew from a colloidal suspension
of the spheres in about three hours.
When excited by a low level of energy
from a 532-nm pump laser, the dye spontaneously emits light centered at 600 nm,
which overlaps with the expected first-order “photonic stop gap” along the 
direction of the fcc crystal. As predicted
by theory, the reflectivity spectrum
peaked at that wavelength, with a corresponding trough in the transmittance
spectrum, demonstrating the expected
stop gap. The researchers also detected
second- and third-order stop gaps at
shorter wavelengths due to the optical
quality of the material.
The scientists found that increasing the
excitation energy of the pump laser to at
Rajesh V. Nair, Bhabha Atomic Research Centre
A source excites dye molecules in a nano-photonic structure generating the spontaneously emitted light within the structure.
At higher excitation energy, the stimulated
emission overcomes the spontaneous
emission, which leads to lasing.
least the threshold of 0.7 mJ would cause
the stimulated emission inside the crystal
to overwhelm the spontaneous emission,
thus leading to laser action. The team
attributed the lasing to the enhancement
of the local density of states near the long-wavelength edges of the band and the field
energy distribution within the crystal.
Nair and his colleagues suggest that,
because the refractive index of the crystal
material depends on the intensity of the
incident light, these crystal devices could
be used as optically switchable lasers in
optical circuits. Researchers could also
exploit the direction-dependent light
from these easily fabricated crystals to
enhance solid-state lighting.
— Patricia Daukantas
NEED IR OPTICS? 700nm - 2µm
8 - 14µm
3 - 5µm
WE COVER YOUR
HOW CAN WE HELP YOU?