Embodiments of the invention include a semiconductor light emitting device for emitting a first light at a first wavelength and a wavelength conversion medium arranged to convert at least part of the first light into a second light at a second wavelength. The wavelength conversion medium is disposed between a periodic antenna array and the semiconductor light emitting device. The periodic antenna array includes a plurality of antennas. The periodic antenna array supports surface lattice resonances arising from diffractive coupling of localized surface plasmon resonances in at least one of the antennas.