OPTICAL AMPLIFIERS
Nanocrystals in polymers amplify signals Researchers at Cornell University (Ithaca, NY) have created crystalline thin-film waveguides from polymer composites, raising the potential of growing optical materials of unusual size or geometry and broad tunability. To create this optical gain media, Clifford Pollock and colleagues embedded nanocrystals of chromium-doped forsterite (Cr:forsterite) or chromium-doped diopside (Cr:diopside) in a polymer matrix. Of added interest, this proof-of-concept work encapsulates materials that are hygroscopic or otherwise environmentally sensitive.
The amorphous Cr:forsterite particles were synthesized with a dispersion polymerization process and fired at 750°C to form crystals. The temperature permits a higher chromium-doping level (2.7 ¥ 1020 cm-3) than possible with the 1910°C temperature required to grow Cr:forsterite by the Czochralski method (typically 4.0 ¥ 1018 cm-3). After synthesis, the crystals were dispersed in a polymer consisting of 55% tribromostyrene and 45% napthyl methacrylate, with a refractive index of 1.653 for optimal matching to forsterite. The fluid composite was puddle-cast onto glass substrates to form 1 ¥ 2-cm waveguides of 5- to 10-µm-thick films; nanocrystal concentrations were 0.5% to 13% by weight.
Pump-probe experiments
The grou¥end-pumped the Cr:forsterite film with a Nd:YAG laser operating at 1.06 µm. A continuous-wave Cr:forsterite laser operating at 1.23 µm provided the probe signal, and microscope objectives coupled both pum¥and probe beams in and out of the medium. The measured signal increase varied linearly with pum¥power, rising to just over a factor of two for an input of 7.5 W (see figure).
Composite waveguides of Cr:diopside were formed using a similar technique. The researchers end-pumped a waveguide with 50 mW from an argon-ion laser operating between 459 and 514 nm, while a Ti:sapphire laser provided the probe signal. The medium demonstrated amplification of 1.07 times in the 780-nm region but showed no gain in the 980-nm region, the second fluorescence band of Cr:diopside.
Although they have detected the relative gains mentioned above in the composite materials, the researchers did not demonstrate net gain with either material. The primary mechanism for signal loss is scattering from the nanocrystals, but cracks in the thin film contributed to loss. Coating the glass substrates with a layer of silane before applying the polymer composite eliminates the cracking. The silane acts as a bonding agent between the glass and polymer.
The scattering issue presents a more difficult problem. Although the polymer provides a level of index-matching, the birefringent character of both Cr:forsterite and Cr:diopside, with the implied difference in refractive index along the three axes of the crystals, precludes an exact index match. As a result, this issue cannot be addressed directly.
Pollock does not expect scattering losses to be an insurmountable issue, however. "You`re always going to get some net index mismatch," he says, "but, in principle, we can get the scattering quite low." Potential approaches to this problem include the preferential alignment of the nanocrystals along a principal axis to minimize index mismatch for properly polarized light. Another approach is to reduce crystal size to minimize scatter.
The ability to embed nanocrystals in a polymer composite means that instead of growing crystals that are many centimeters long, it may be possible to extrude crystals of much longer length, according to Pollock. These crystals could even take the form of paint or be pushed from a tube. He thinks that other classes of materials actually offer more promise as optical nanocomposites, including crystals with a cubic structure such as chloride and fluoride. If more funding becomes available, Pollock would investigate the technology for display applications.
Conard Holton