Appl. Phys. Express 5, 092601 (2012)


Lanthanide-doped nanocrystals that can efficiently upconvert infrared radiation into visible light are being studied with a view to applying them in the display, DNA-detection and bio-imaging areas. Now Xiaojie Xue and co-workers from the Toyota Technological Institute, Japan have observed ultraviolet emission from Tb3+/Yb3+ co-doped KY3F10 nanocrystals that were excited with light from a 976-nm laser diode. KY3F10 nanocrystals doped with x% Tb3+ and 5% Yb3+ were synthesized by a hydrothermal method and had an average size of about 45 nm. The emission spectrum of the nanocrystals featured both strong ultraviolet emission at 381 nm and weaker green emission at 544 nm. The emission decay time for the ultraviolet emission was 1.9 ms. The intensity ratio between the ultraviolet–blue emission from the 5D3 level and the green emission from the 5D4 level decreased with increasing Tb3+ concentration. The researchers say that this concentration-dependent phenomenon is evidence of cross-relaxation, and indicates possible upconversion mechanisms based on the energy transfer from Yb3+ to Tb3+ and cross-relaxation processes between Tb3+ pairs


Stronger Nanofibres

J. Lightwave Technol. 29, 1018–1025 (2011)

Nanostructured optical fibres offer custom-designed dispersion, high strength and high fabrication efficiency properties that make them attractive for use in a wide range of miniature highperformance photonic devices. Although a nanostructured fibre can be fabricated in the same way as a microstructured fibre — by drawing a structural preform at the glass-softening temperature — slight non-uniformities in the preform can cause the fibre to deform or even collapse.

Meisong Liao and co-workers in Japan have now circumvented some of the difficulties associated with nanostructured fibre fabrication by analysing the non-uniformity of hole evolution during the fibre-drawing process. The researchers found that hole size is a key factor in the stability of the fibre, and that small holes increase the likelihood of fibre collapse. The researchers claim to have fabricated a fibre with the smallest ever fibre core by using an inflation method to increase the hole size. For a 120-nm long nanostructured fibre, they achieved a glass bridge thickness of only a few tens of nanometres. They also showed that high drawing temperatures cause an increase in surface tension but a significant decrease in viscosity, which leads to fibre distortion.


Suspended-core fibers have a small solid core with a large gas-filled region; if the core is small enough, it becomes a suspended nanowire, opening the door for interesting nonlinear optical effects. Scientists at Toyota Technological Institute (Nagoya, Japan) used a gas-pressure inflation technique to fabricate a suspended lead-silicate-glass-core nanofiber that has a diameter ratio of holey region to core of at least 62, a core diameter of 480 nm, and a length of several hundred meters (see Fig. 4). Optical loss at 1557 nm was 8 ±2 dB/m; single-mode third-harmonic generation (THG) was achieved when pumped with a 1557 nm femtosecond laser.

In fiber-optic communications, systems are now being designed to encode not just the light signal's amplitude but its phase as well, to reach higher data transmission rates. But, as a result, nonlinear phase noise—formerly a nonissue—will become the prime limitation to performance. In response, a team of researchers from the University of Southampton, Chalmers Institute of Technology (Gotebörg, Sweden), University College Cork (Cork, Ireland), OFS (Brøndby, Denmark), Eblana Photonics (Dublin, Ireland), and the University of Athens (Athens, Greece) have developed the first practical all-optical regenerator that can remove both phase and amplitude noise from binary phase-encoded optical signals. The device, which takes advantage of the phase-squeezing ability of phase-sensitive amplifiers, operates on 40 Gbit/s signals and has the potential to operate at much higher rates, say the researchers.

FIGURE 4. A suspended-core fiber with a core diameter of 480 nm is imaged via optical microscope (a) and scanning-electron microscope at various magnifications (b-d). (Courtesy of the Toyota Technological Institute)

From UV to mid-IR

Appl. Phys. Lett. 95, 161103 (2009)

Supercontinua—ultrabroad bandwidth light pulses usually created by propagating intense optical pulses through a strongly nonlinear media — are attractive for many applications including spectroscopy, optical coherence tomography and the creation of tunable ultrafast femtosecond light sources. So far, however, supercontinuum generation has been limited between UV and nearinfrared wavelengths for silica photonic crystal fibres and 0.8–4.5 μm for fluoride fibres. Now, Guanshi Qin and colleagues from Toyota Technological Institute in Japan report that broader operation from the UV to wavelengths as long as 6–8 μm in the midinfrared can be achieved by using a short length of ZrF4–BaF2–LaF3–AlF3–NaF fluoride fibre and a high peak pump power. The team investigated a 2-cm-long step-index fluoride fibre with a core diameter of 9 μm, a numerical aperture of 0.2 and a zero-dispersion wavelength of 1.65 μm. Using a 1,450 nm femtosecond laser emitting a peak power of 50 MW as a pump, supercontinuum light with wavelengths ranging from the UV to 6.28 μm was observed. The spectral broadening is said to be a result of self-phase modulation, Raman scattering and four-wave mixing. According to the researchers, the use of a fibre with a larger numerical aperture, such as air-cladding microstructured fibre, could further extend the wavelength to 8 μm.