However, most of such cavities operate in the surface-emitting configuration, and their incouplings and outcouplings suffer from extremely low efficiencies. This is because the surface emissions from the subwavelength nanocavity modes often have quite large divergence angles. The collection efficiencies from these cavities by a microscope objective at the surface normal direction can easily go below 1%. Low-efficiency operation is a critical problem for the implementation of planar photonic crystal nanocavities in many applications.
A group of researchers from the Universita di Pavia in Italy and the University of St. Andrews in the United Kingdom took a smart approach to significantly enhance the coupling efficiency without significantly compromising the quality factor and the cavity volume. By manipulating the holes around a planar photonic crystal nanocavity, an additional grating is inserted into the original photonic crystal grating. This new grating diffracts the cavity mode in such a way that the original wide divergence angle surface emission is now collimated along the surface normal direction. With proper design and optimization, an efficiency as high as 16% by a NA of 0.5 objective and a quality factor of up to 62,000 have been measured by use of resonant scattering experiments in a modified L7 cavity. The experimental results compare well with the theoretical predictions.
The increase in the incoupling efficiency will enhance the optical intensities inside the cavities and consequently the interactions between light and matter, if we use the same external photon sources. Similarly, the increase in the outcoupling efficiency will generate stronger signals for the detection devices. Combining these two aspects, the modified planar photonic crystal nanocavities reported in this paper can be used to significantly improve the performance of many applications of nanophotonic cavities—for example, low pump power and high output power lasers, high-efficiency single-photon sources, and high-sensitivity biosensors. Let’s see whether the coupling efficiency can be further increased while the quality factor (cavity mode volume) is not reduced (increased).
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