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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. 

We present polarization-free Bragg filters having subwavelength gratings (SWGs) in the lateral cladding region. This Bragg design expands modal fields toward upper cladding, resulting in enhanced light interaction with sensing analytes. Two device configurations are proposed and examined, one with index-matched coupling between transverse electric (TE) and transverse magnetic (TM) modes and the other one with hybrid-mode (HM) coupling. Both configurations introduce a strong coupling between two orthogonal modes (either TE-TM or HM₁-HM₂) and rotate the polarization of the input wave through Bragg reflection. The arrangements of SWGs help to achieve two configurations with different orthogonal modes, while expanding modal profiles toward the upper cladding region. Our proposed SWG-assisted Bragg gratings with polarization independency eliminate the need for a polarization controller and effectively tailor the modal properties, enhancing the potential of integrated photonic sensing applications. 

We present a single-etched polarization-independent Bragg filter using cladding asymmetry. An air cladding strongly couples TE and TM modes, rotating its polarization status and achieving polarization-insensitive Bragg reflection at the average of both Bragg wavelengths 

A photonic Bragg grating is a fundamental building block that reflects the direction of wave propagation through spatial phase modulation and can be implemented using sidewall corrugation. However, due to the asymmetric aspect ratio of a waveguide cross section, typical Bragg gratings exhibit a strong polarization sensitivity. Here, we show that photonic Bragg gratings with cladding asymmetry can enable polarization-independent notch filters by rotating input polarizations. Such Bragg gratings strongly couple transverse electric (TE) and transverse magnetic (TM) modes propagating in opposite directions, filtering the input signal and reflecting the rotated mode. We analyzed this polarization-rotating Bragg grating using the coupled-mode theory and experimentally demonstrated it on a silicon-on-insulator platform. Our device concept is simple to implement and compatible with other platforms, readily available as polarization transparent Bragg components.