Although being far from giving an answer, much progress has taken place in recent years on the realization of photonic platforms where the light flow can be controlled with large flexibility through microscale optical devices. The problem of light trapping is inherently related to the possibility of slowing down the velocity of light. Two main approaches have proved to be effective for realizing slow light in a photonic chip, based on the use of ring resonators and photonic crystals waveguides, respectively. The maturity of these platforms has rapidly improved in the last decade, and now architectures with increased complexity and broadened functional capability can be fabricated.
However, today none of them can offer at the same time microscopic scale light trapping and ultra-small losses, this combination being the primary ingredient for ultra-high quality-factor microresonators. Both photonic crystals and ring-resonator-based devices require high-index-contrast material interfaces, in order to achieve light confinement in a small footprint. But the drawback with high index contrast is the strong sensitivity to any fabrication imperfections, waveguide sidewall roughness, holes displacement in photonic crystal waveguides, and resonances spread in coupled ring resonators. On the other side, low-index-contrast technologies do not permit optical resonators with microscale size.
A new way to think and realize optical microresonators with a very high quality factor is presented in the work by M. Sumetsky and co-workers. This new technology is called Surface Nanoscale Axial Photonics (SNAP) and employs whispering gallery modes (WGMs) circulating circumferentially around the surface of a thin optical fiber. With respect to conventional approaches, the main idea in SNAP is to exploit nanoscale deformations of the fiber radius (and/or equivalent variation of the fiber refractive index) to realize microresonators with ultra-low loss and microscopic dimensions. Thanks to extraordinary smoothness given by the draw process to the fiber surface, orders of magnitude lower loss can be achieved with respect to high-index-contrast waveguides obtained with lithography-based technologies. Moreover, since the propagation of WGMs is primarily azimuthal, the axial propagation of the light is naturally slowed down, such that slow light is achieved without the periodic modulation of the refractive index, as required for instance in photonic crystals waveguides.
Everything works well with SNAP provided that a very accurate, controllable, and reproducible modification of the optical fiber effective radius at the nanoscale is obtained. In this work, two different techniques are effectively used to demonstrate the viability of SNAP as a practical technology: an annealing process performed with a CO2 laser beam and a UV excimer laser beam exposure. Both methods guarantee impressive reproducibility within 2 Å in the effective radius variation, and can be used to realize resonators with tens of micrometer dimensions and a Q factor in excess of 106.
The perception reading the results presented by M. Sumetsky and co-workers is that SNAP can really represent a new effective way to trap photons in a microscale footprint device. Now, we are looking forward to seeing how many benefits SNAP can bring to photonic devices, and in which applications its unique combination of miniaturization and ultra-low loss can replace conventional photonic technologies.
Probably it is too early to say that a new photonic platform is born, but we will discover it soon.
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