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1. Compact nonvolatile polarization switch using an asymmetric Sb ₂ Se ₃ -loaded silicon waveguide

   https://doi.org/10.1364/OE.482817  

   Jin, Huimin; Niu, Lei; Zheng, Jiajiu; Xu, Peipeng; Majumdar, Arka (January 2023, Optics Express)

   We propose and simulate a compact (∼29.5 µm-long) nonvolatile polarization switch based on an asymmetric Sb₂Se₃-clad silicon photonic waveguide. The polarization state is switched between TM₀and TE₀mode by modifying the phase of nonvolatile Sb₂Se₃between amorphous and crystalline. When the Sb₂Se₃is amorphous, two-mode interference happens in the polarization-rotation section resulting in efficient TE₀-TM₀conversion. On the other hand, when the material is in the crystalline state, there is little polarization conversion because the interference between the two hybridized modes is significantly suppressed, and both TE₀and TM₀modes go through the device without any change. The designed polarization switch has a high polarization extinction ratio of > 20 dB and an ultra-low excess loss of < 0.22 dB in the wavelength range of 1520-1585 nm for both TE₀and TM₀modes. 

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2. Ultra-high extinction-ratio silicon photonic polarization beam splitter with anisotropic metamaterials

   https://doi.org/10.1364/IPRSN.2021.ITh2B.4  

   Ahmed, Syed Z.; Ahmed, Ishtiaque; Mia, Borhan; Jaidye, Nafiz; Kim, Sangsik (January 2021, Optica Publishing Group)

   We present an ultra-high extinction-ratio silicon photonic polarization beam splitter with anisotropic metamaterial claddings. Exceptional coupling with anisotropic metamaterial achieved 58- 60 dB (46-48 dB) extinction-ratios for TE (TM) mode and the large birefringence resulted in a short coupling length (14.5 µm). 

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3. InGaAs Membrane Waveguide: A Promising Platform for Monolithic Integrated Mid-Infrared Optical Gas Sensor

   Kyoung Min Yoo, Jason Midkiff (March 2020, ACS sensors)

   Mid-infrared (mid-IR) absorption spectroscopy based on integrated photonic circuits has shown great promise in trace-gas sensing applications in which the mid-IR radiation directly interacts with the targeted analyte. In this paper, considering monolithic integrated circuits with quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), the InGaAs−InP platform is chosen to fabricate passive waveguide gas sensing devices. Fully suspended InGaAs waveguide devices with holey photonic crystal waveguides (HPCWs) and subwavelength grating cladding waveguides (SWWs) are designed and fabricated for mid-infrared sensing at λ = 6.15 μm in the low-index contrast InGaAs−InP platform. We experimentally detect 5 ppm ammonia with a 1 mm long suspended HPCW and separately with a 3 mm long suspended SWW, with propagation losses of 39.1 and 4.1 dB/cm, respectively. Furthermore, based on the Beer−Lambert infrared absorption law and the experimental results of discrete components, we estimated the minimum detectable gas concentration of 84 ppb from a QCL/QCD integrated SWW sensor. To the best of our knowledge, this is the first demonstration of suspended InGaAs membrane waveguides in the InGaAs−InP platform at such a long wavelength with gas sensing results. Also, this result emphasizes the advantage of SWWs to reduce the total transmission loss and the size of the fully integrated device’s footprint by virtue of its low propagation loss and TM mode compatibility in comparison to HPCWs. This study enables the possibility of monolithic integration of quantum cascade devices with TM polarized characteristics and passive waveguide sensing devices for on-chip mid-IR absorption spectroscopy. 

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4. Stochastic FDTD Modeling of Propagation Loss due to Random Surface Roughness in Sidewalls of Optical Interconnects

   https://doi.org/10.23919/USNC-URSINRSM51531.2021.9336433  

   Guiana, Brian; Zadehgol, Ata (February 2021, 2021 United States National Committee of URSI National Radio Science Meeting (USNC-URSI NRSM))

   null (Ed.)

   The dielectric waveguide (WG) is an important building block of high-speed and high-bandwidth optical and opto-electronic interconnect networks that operate in the THz frequency regime. At the interface of Si/SiO 2 dielectric waveguides with width above w = 2.5 μm and anisotropic surface roughness, transverse electric (TE) mode surface wave propagation can experience a loss of approximately a = 2 dB/cm; however, propagation losses increase rapidly to near a = 44 dB/cm as the width decreases to w = 500 nm, due to increased interaction of surface waves and sidewall surface roughness that exhibits random distribution inherent to the manufacturing process. Previous works have developed analytic expressions for computing propagation loss in a single dielectric waveguide exhibiting random roughness. More recent works report a = 0.4 dB/cm noting the non-trivial estimation errors in previous theoretical formulations which relied on planar approximations, and highlight the discrepancy in planar approximations vs. the 3-D Volume Current Method. A challenge that remains in the path of designing nanoscale optical interconnects is the dearth of efficient 3-D stochastic computational electromagnetic (CEM) models for multiple tightly coupled optical dielectric waveguides that characterize propagation loss due to random surface roughness in waveguide sidewalls. Through a series of theoretical and numerical experiments developed in the method of finite-difference time-domain (FDTD), we aim to develop stochastic CEM models to quantify propagation loss and facilitate signal & power integrity modeling & simulation of arbitrary configurations of multiple tightly-coupled waveguides, and to gain further insights into loss mechanisms due to random surface roughness in optical interconnects. 

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5. Broadband integrated polarization splitter and rotator using subwavelength grating claddings

   https://doi.org/10.1364/OE.479195  

   Borhan_Mia, Md; Jaidye, Nafiz; Ahmed, Ishtiaque; Ahmed, Syed_Z; kim, Sangsik (January 2023, Optics Express)

   We present a broadband integrated photonic polarization splitter and rotator (PSR) using adiabatically tapered coupled waveguides with subwavelength grating (SWG) claddings. The PSR adiabatically rotates and splits the fundamental transverse-magnetic (TM₀) input to the fundamental transverse-electric (TE₀) mode in the coupler waveguide, while passing the TE₀input through the same waveguide. The SWGs work as an anisotropic metamaterial and facilitate modal conversions, making the PSR efficient and broadband. We rigorously present our design approaches in each section and show the SWG effect by comparing with and without the SWG claddings. The coupling coefficients in each segment explicitly show a stronger coupling effect when the SWGs are included, confirmed by the coupled-mode theory simulations. The full numerical simulation shows that the SWG-PSR operates at 1500–1750 nm (≈250 nm) wavelengths with an extinction ratio larger than 20 dB, confirmed by the experiment for the 1490–1590 nm range. The insertion losses are below 1.3 dB. Since our PSR is designed based on adiabatical mode evolution, the proposed PSR is expected to be tolerant to fabrication variations and should be broadly applicable to polarization management in photonic integrated circuits. 

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