Near field imaging of a plasmon photonic crystal patterned on the facet of a quantum cascade laser

Planar photonic crystal (PPC) has recently attracted much attention as a promising platform for the realization of compact nanocavity devices. Our proposed photonic crystal (PC) structure consists of a periodic hole array with a point defect at the center. The device has been integrated on the facet of a quantum cascade laser working in the mid-infrared region of optical spectrum. Finite-difference time domain (FDTD) simulations have been performed to optimize the design structure. Simulations showed that with a periodicity of the holes (Λ) between 1.3um and 1.4um, the near field enhancement at the center of the cavity on the same level as the top metal surface can be as high as 10 times the incident electric field. The radius of the hole and center cavity radius are 0.45 and 0.2 times Λ. The structure was simulated at experimentally measured operating wavelength (λ=5.98um) of our device. During fabrication, we used a buffer SiO2 layer thickness of 100nm followed by metal-dielectric-metal structure with layer thicknesses of Au - SiO2 -Au (100/20/ 100 nm). Next, the MDM photonic crystal design was fabricated on the MDM coated facet of the QCL using focused ion beam (FIB) milling. The integrated device has been tested using an apertureless mid-infrared near field scanning optical microscopy (a-NSOM). The measurement set-up is based on an inverted microscope coupled with a commercially available Atomic Forced Microscopy (AFM). Using this technique, we could simultaneously measure the topography and NSOM image of the photonic crystal integrated QCL. It showed that the combination of high quality factor and extremely low mode volume of the PC design can squeeze the optical mode within a nanometric spot size ~ 450nm. The experimental results is a proof of concept, although we believe, further optimization and improvisation with different PC designs can lead squeezing the optical mode into a much smaller volume. Such integrated device are capable of focusing radiant infrared light down to nanometer length scale and strongly enhance the near field intensity which can be extremely useful in molecular sensing.

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