Researchers in Switzerland have developed a new principle for introducing second-order optical nonlinearity into silicon nitride chips.
The team from the Photonic Systems Laboratory at EPFL, headed by Professor Camille Brès, induced nonlinearity to convert light where it is not normally possible to do so, for example producing different colours. The silicon nitride resonators developed at EPFL and commercialized by Ligentec in a standard CMOS process, exhibit very low losses so that light circulates in resonators for a very long time.
“When using a green laser pointer for example, the laser itself is not green because these are particularly difficult to manufacture. So we change the frequency of an existing laser. It emits at a frequency which is half that of green, then we double it by using nonlinearity in a crystal which gives us green,” said Prof Brès.
“Our study consists of integrating this functionality but on chips that can be manufactured with standard CMOS techniques developed for electronics. Thanks to this, we will be able to efficiently generate different colours of light on a chip.”
Current photonic chips compatible with CMOS processes use standard photonic materials, such as silicon, which do not possess second-order nonlinearity and therefore are not inherently capable of transforming light in this way. “This turns out to be a barrier to the advancement of technology,” said Brès.
“Non-linearity comes from the interaction between light and matter. This exchange must be long if the process is to be functional and efficient. However, the chip is a small object on which we do not benefit from long distances” said researcher Dr Edgars Nitiss. The light introduced into the resonator is captured and travels the time necessary for the nonlinear interaction to be increased.
This technique boosts the efficiency of the optically reconfigurable quasi-phase-matching in the large-radius silicon nitride microresonators to 47.6 percent/W but imposes a new constraint. “When using a resonator, we are limited in terms of the colours available,” said Brès.
The effectiveness of a nonlinear effect also depends on the phase agreement between the different interacting colours, whereas they inevitably have different propagation speeds in the material.
The researchers found a solution to avoid this constraint and to offer access to a range of several colours despite the use of the resonator. In the resonator, light waves propagate, producing a coherent interaction that changes the properties of the material. A self-organization of the structure is achieved in a completely all-optical manner, which automatically compensates for the phase mismatch regardless of the input colour. This circumvents a critical limitations of resonators while still benefiting from the higher efficiency.
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