Fano resonance is an intriguing physical phenomenon that could be achieved by engineering a destructive interference between a superradiant bright mode and a subradiant dark mode. A variety of hybrid systems of metasurfaces have been widely explored demonstrating sharp Fano resonances with high sensitivity. However, plasmonic metasurfaces have limitations of Ohmic losses that constrain the achievable quality factor. Meanwhile, the dielectric metasurfaces provide sharp Fano resonance but with limited sensitivity and figure-of-merit (FOM) compared to plasmonic metasurfaces. In this paper, we report an ultra-sharp, ultra-sensitive refractormetric gas sensor based on Fano resonance using all dielectric metasurface. Our proposed design is composed of hybrid system of nano-bar/nano-elliptic all dielectric silicon. The proposed design has reported a sensitivity of 1,852 nm/RIU with a significant quality factor of 1225, leading to a figure-of-merit (FOM) of 411 at the operating wavelength of 5.5128 μm, within the spectral window of the nitric oxide. Our design brings a considerable impact as a cheap and easily fabricated sensor for gas chemical recognition.
Fano resonance is a universal phenomenon that has been widely exploited to achieve spectrally sharp resonances for a variety of applications. Recently, all dielectric metasurface design composed of two silicon resonators, nanodisk and nanobar, exhibits an induced transparency window with ultrahigh quality factor in polarization direction along the nanobar long axis relying on the intriguing physics of Fano resonance. However, the design demonstrates a dispersive behavior within a short spectral range, a phenomenon that hinders its practical usage as a Biosensor due to the lack of the spectral selectivity. In this work, we report the tunable design along the range of wavelengths from 1.2 μm to 1.75 μm through changing the radius of the nanodisk from 200nm to 350nm. Although the design demonstrates a spectral red shift of the reflection with increasing the nanodisk radius, the reflectance behavior is inconsistent indicating the formation of transparency windows in most cases. Worthwhile, the structure design of radius (r=300nm) enables attaining a single sharp Fano resonance addressing the aforementioned short range dispersion obstacle. Meanwhile, the optimized design of radius of (r=300nm) provides a significant quality factor of (~808) and ultrahigh sensitivity of (275nm/RIU) with a figure-of-merit of (148) at the operating wavelength of (λ=1.583μm). The proposed design demonstrates a tremendous impact in refractometric biosensing applications due to its high performance and the ease of fabrication.
KEYWORDS: Photonic crystals, Design and modelling, Waveguides, Biological and chemical sensing, Silicon, Gas sensors, Refractive index, Gases, Sensors, Mid-IR
Mid infrared (MID-IR) band of electromagnetic radiation plays the key role for gas and chemical sensing application because it provides molecular fingerprints for most trace gases and molecules. Photonic crystal cavity coupled waveguide (PC-CWG) based sensors is a remarkable platform for sensing applications due to its capability to confine light modes on small volume displaying high sensitivity and high-quality factors. In this work, we report a novel design of (PC-CWG) based on square lattice of silicon pillars in air with radius of 0.2𝜇m and lattice constant of 1 𝜇m. A waveguide is introduced by removing three columns of silicon pillars, and a microcavity is created by removing a number of silicon pillars forming curved shapes on both sides of the waveguide. The proposed design demonstrates multiple resonances covering a broad spectral range in MID-IR ranging from 2.4 𝜇m to 4.2 𝜇m that represents the bandgap region. Remarkable resonances are observed at operating wavelengths 2.67 𝜇m, 2.88 𝜇m, 3.03 𝜇m, 3.2 𝜇m and 3.5 𝜇m. Moreover, the reported design shows ultra-high sensitivity reaching 2680 nm/RIU with a significant quality factor of Q=6475 giving rise to a figure-of-merit of 5.7 × 106 at the operating wavelength of (𝜆=3.03𝜇m). The suggested photonic crystal design offers simple fabrication and broad applicability for refractive index sensing applications.
Imaging with conventional optical systems has the limitation of the diffraction limit of the point spread function (PSF) where the maximum attainable resolution reaches down to half of the incident wavelength in lateral direction and down to the wavelength in axial direction. In fact, the tradeoff between the lateral and axial confinement is inevitable resulting in a deterioration of the 3D optical image. In this work, we report a metalens design attaining high resolution imaging in lateral and axial directions at 𝜆=3𝜇m. The proposed design employs a 2D square photonic crystal lattice of silicon nanorods with an engineered optical cavity and a silver slit on top of the structure. Our design exhibits a simultaneous sub-diffraction limit in lateral and axial directions reaching down to 0.38λ and to 0.8λ, respectively. The ideal PSF requires the equality of the full width at half maximum (FWHM) for both lateral and axial directions. Hence, the figure of merit (FOM) can be expressed as the ratio between the (FWHM) in lateral and axial directions approaching unity for ideal PSF. The proposed design demonstrates enhanced FOM, up to 0.47, compared to the confocal microscopy that shows FOM of 0.36. Our proposed design provides a great potential usage in the field of biomedical imaging and molecular dynamics.
Hyperbolic metamaterial (HMM) has paved the way for sub-diffraction focusing inside the HMM due to the propagation of large momentum wave vectors in the HMM. However, these high momentum K modes exponentially decay outside the HMM which results in decaying of the focusing resolution in the near field of the HMM. In this work, we introduce both a HMM and a hypergrating structures for sub-wavelength focusing in air. Hypergrating is a structure that combines a HMM with a grating surface. The proposed structure consists of upper metallic slit integrated on HMM based multilayer of doped/intrinsic InAs with lower intrinsic InAs grating surface. HMM based multilayer of doped/intrinsic InAs has the advantage of tuning the focusing wavelength in the mid-IR range. The proposed structure has reported sub-wavelength focusing in air with value reaching 0.08 λ. Hypergrating structure shows focusing resolution enhancement of 0.08λ as compared to 0.15λ for a HMM without lower grating, both at wavelength of 7.3μm. The focusing resolution outside the hypergrating structure is much higher than that is observed in the HMM only due to the introduced lower grating. This structure demonstrates a good candidate for sub -wavelength IR imaging application in air.
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