Recently, optical bound states in continuum (BICs) incorporated with optical gain have been reported to exhibit lasing. Here, it is shown that each of the four BICs supported by C4v$_{4v}$ symmetric photonic crystal slab can be made to lase, allowing the control over the topological charge of the resulting laser beam. The type of each BIC and their topological charges are identified by imaging the far‐field polarization vortices of the lasing signal. Results are compared with experimentally obtained dispersions, finite element method simulations, and multipole decomposition method based on the microscopic polarization currents in the photonic crystal plane. A demonstration of multimode lasing of two non‐degenerate BICs with opposite topological charges is presented. The momentum space overlap of the BICs results in a unique polarization pattern. The study provides a generalizable example for engineering the topological properties of coherent light.

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The prevalent nature of micro and nanoplastics (MP/NPs) on environmental pollution and health-related issues has led to the development of various methods, usually based on Fourier-transform infrared (FTIR) and Raman spectroscopies, for their detection. Unfortunately, most of the developed techniques are laboratory-based with little focus on in situ detection of MPs. In this review, we aim to give an up-to-date report on the different optical measurement methods that have been exploited in the screening of MPs isolated from their natural environments, such as water. The progress and the potential of portable optical sensors for field studies of MPs are described, including remote sensing methods. We also propose other optical methods to be considered for the development of potential in situ integrated optical devices for continuous detection of MPs and NPs. Integrated optical solutions are especially necessary for the development of robust portable and in situ optical sensors for the quantitative detection and classification of water-based MPs.

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Microplastic (MP) pollution is alarming and poses an imminent threat to the environment with a direct impact on our health and that of fauna in natural water bodies. The understanding of light-MP interactions in water as well as the need for low-cost and robust optical sensors for the detection of MPs that appear everywhere is, therefore, necessary. We have demonstrated the use of a prototype optical sensor in the detection of flat and curved [from polyethylene terephthalate (PET) water bottle] pristine and rough MPs from commercial PET and low-density polyethylene plastics in water. The optical sensor utilizes a photodiode and charge-coupled device (CCD) camera to record simultaneously the specular reflection and the speckle pattern modified by the MPs. In this study, we have exploited the specular reflection in the detection of the pristine samples, whereas the speckle contrast, the normalized standard deviation of the speckle pattern intensity, is utilized in the qualitative estimation of the "effective" surface roughness of the MPs. With the sensor, one can, therefore, detect PET MPs with varying average surface roughness, Ra an indication of MP aging, embedded in water. The prototype can detect the effect of size, type, curvature, transparency, and the translucency of sunken and/or floating MPs in water based on the reflection, scattering, and the (edge) diffraction of light. However, the optical sensor is limited in the discrimination of MP concentration in water. Further modifications to the sensor are needed for its practical implementation in complex natural water bodies and wastewaters.