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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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