Optics & Photonics News

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
Photonic Crystal

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 [111] 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

532 nm laser

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.