a semiconductor diode structure including first and second doped semiconductor layers and an active layer between a back surface of the first semiconductor layer and a front surface of the second semiconductor layer, the active layer being arranged for emitting diode output light, resulting from electrical current flow through the diode structure, at a nominal emission vacuum wavelength λ₀ to propagate within the diode structure;

a reflector positioned against a back surface of the second semiconductor layer that internally reflects diode output light incident thereon from within the diode structure;

a wavelength-converting layer positioned with a back surface thereof facing and spaced-apart from a front surface of the first semiconductor layer, the wavelength-converting layer absorbing diode output light at the vacuum wavelength λ₀ and in response emitting down-converted light at a vacuum wavelength λ₁ that is greater than λ₀, at least a portion of the down-converted light exiting the wavelength-converting layer through a front surface thereof; and

an intermediate spacer positioned between the front surface of the first semiconductor layer and the back surface of the wavelength-converting layer and comprising one or more dielectric layers, each dielectric layer of the intermediate spacer being at least partly transparent at the vacuum wavelength λ₀, the intermediate spacer having an effective refractive index n_S less than an effective refractive index n_C of the wavelength-converting layer so that the wavelength-converting layer acts as an optical waveguide supporting one or more laterally propagating optical modes at the vacuum wavelengths λ₀ and λ₁.