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We study numerically the effect of periodicity on the plasmon-assisted scattering and absorption of visible light by infinite and finite gratings of circular silver nanowires. The infinite grating is a convenient object of analysis because of the possibility to reduce the scattering problem to one period. We use the well-established method of partial separation of variables however make an important improvement by casting the resulting matrix equation to the Fredholm second-kind type, which guarantees convergence. If the silver wires have sub-wavelength radii, then two types of resonances co-exist and may lead to enhanced reflection and absorption: the plasmon-type and the grating-type. Each type is caused by different complex poles of the field function. The low-Q plasmon poles cluster near the wavelength where dielectric function equals -1. The grating-type poles make multiplets located in close proximity of Rayleigh wavelengths, tending to them if the wires get thinner. They have high Q-factors and, if excited, display intensive near-field patterns. A similar interplay between the two types of resonances takes place for finite gratings of silver wires, the sharpness of the grating-type peak getting greater for longer gratings. By tuning carefully the grating period, one can bring together two resonances and enhance the resonant scattering of light per wire by several times.

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Optical modes and associated linear threshold values of material gain bringing them to lasing are investigated for a VCSEL-type cavity with a quantum well, sandwiched between two distributed Bragg reflectors. They are found as solutions to a specific novel eigenvalue problem with the "active" imaginary part of the quantum well refractive index. For the calculation of the Bragg mirror reflection coefficients, well-established method of the transfer matrices is used. The presented results accurately quantify intuitively predictable lowering of the modal thresholds for the modes whose lasing frequencies lay inside the reflectors rejection bands. Besides, they demonstrate that this approach automatically incorporates the account of overlapping between the active region and the modal E-field patterns and its effect on the thresholds.

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