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1. Broadband subwavelength imaging of flexural elastic waves in flat phononic crystal lenses

   https://doi.org/10.1038/s41598-023-34314-5  

   Danawe, Hrishikesh; Tol, Serife (May 2023, Scientific Reports)

   Abstract Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range. 

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2. Sub-wavelength lateral detection of tissue-approximating masses using an ultrasonic metamaterial lens

   https://doi.org/10.1038/s41467-020-19591-2  

   Walker, Ezekiel L.; Jin, Yuqi; Reyes, Delfino; Neogi, Arup (December 2020, Nature Communications)

   null (Ed.)

   Abstract Practically applied techniques for ultrasonic biomedical imaging employ delay-and-sum (DAS) beamforming which can resolve two objects down to 2.1 λ within the acoustic Fresnel zone. Here, we demonstrate a phononic metamaterial lens (ML) for detection of laterally subwavelength object features in tissue-like phantoms beyond the phononic crystal evanescent zone and Fresnel zone of the emitter. The ML produces metamaterial collimation that spreads 8x less than the emitting transducer. Utilizing collimation, 3.6x greater lateral resolution beyond the Fresnel zone limit was achieved. Both hard objects and tissue approximating masses were examined in gelatin tissue phantoms near the Fresnel zone limit. Lateral dimensions and separation were resolved down to 0.50 λ for hard objects, with tissue approximating masses slightly higher at 0.73 λ . The work represents the application of a metamaterial for spatial characterization, and subwavelength resolution in a biosystem beyond the Fresnel zone limit. 

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3. Multimode experimental band dynamics of resonant nanophotonic lattices

   https://doi.org/10.1364/OE.495835  

   Razmjooei, Nasrin; Magnusson, Robert (August 2023, Optics Express)

   Subwavelength resonant lattices provide a host of interesting spectral expressions on broadside illumination. The resonance mechanism is based on generation of lateral Bloch modes phase matched to evanescent diffraction orders. The leaky mode structure and mode count determine the spectra and the number of resonance states. Here, we study band flips and bound-state transitions in guided-mode resonant structures supporting multiple resonant modes. We present theoretical simulations and experimental results for a subwavelength silicon-nitride lattice integrated with a liquid film with adjustable boundary. The relatively thick liquid waveguiding region supports additional modes such that the first four transverse-electric (TE) leaky modes are present and generate observable resonance signatures. By varying the duty cycle of the basic lattice in experiment, the 4 bands undergo band transitions and band closures as quantified herein. The experimental results taken in the 1400-1600 nm spectral region agree reasonably well with numerical analysis. 

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4. Anisotropic leaky-like perturbation with subwavelength gratings enables zero crosstalk

   https://doi.org/10.1038/s41377-023-01184-5  

   Kabir, Md Faiyaz; Mia, Md Borhan; Ahmed, Ishtiaque; Jaidye, Nafiz; Ahmed, Syed Z.; Kim, Sangsik (June 2023, Light: Science & Applications)

   Abstract Electromagnetic coupling via an evanescent field or radiative wave is a primary characteristic of light, allowing optical signal/power transfer in a photonic circuit but limiting integration density. A leaky mode, which combines both evanescent field and radiative wave, causes stronger coupling and is thus considered not ideal for dense integration. Here we show that a leaky oscillation with anisotropic perturbation rather can achieve completely zero crosstalk realized by subwavelength grating (SWG) metamaterials. The oscillating fields in the SWGs enable coupling coefficients in each direction to counteract each other, resulting in completely zero crosstalk. We experimentally demonstrate such an extraordinarily low coupling between closely spaced identical leaky SWG waveguides, suppressing the crosstalk by ≈40 dB compared to conventional strip waveguides, corresponding to ≈100 times longer coupling length. This leaky-SWG suppresses the crosstalk of transverse–magnetic (TM) mode, which is challenging due to its low confinement, and marks a novel approach in electromagnetic coupling applicable to other spectral regimes and generic devices. 

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5. In-plane beam focusing via integrated photonic gradient-index subwavelength grating metalens

   https://doi.org/10.1364/OE.539591  

   Jaidye, Nafiz; Lee, Jaewhan; Pimbi, Daniel; Kim, Myoung-Hwan; Bernussi, Ayrton; Kim, Sangsik (December 2024, Optics Express)

   We present an in-plane beam converter scheme that can focus a large Gaussian slab mode into a tightly focused spot approximately hundreds of micrometers away from the chip facet. Our approach involves designing the modal expander that converts a photonic waveguide mode to a large Gaussian slab mode and engineering the two-dimensional (2D) gradient-index subwavelength grating arrays that modify modal wavefront to be focused as the beam propagates. The device is designed on a monolithic silicon nitride scheme, which is transparent at the visible wavelength regime and readily available for the complementary metal-oxide-semiconductor process. Our device can be utilized in various chip-scale photonic applications, especially involving biochemical species and target samples ranging from one to tens of micrometer scales. 

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