A passive radiative cooling structure comprising: one or more cooling stacks ; wherein each cooling stack is configured with a first waveguide layer , a first emissive layer and a thermal conductive layer , wherein the first emissive layer is sandwiched between the first waveguide layer and the thermal conductive layer ; wherein the thermally conductive layer has a proximal end and a distal end; wherein the thermally conductive layer is substantially perpendicular to and in thermal contact at its proximal end with a source of heat to be cooled, and wherein the first waveguide layer comprises a first sublayer and a second sublayer; characterized in that : the first emissive layer is configured to absorb and emit infrared radiation at wavelengths where Earth's atmosphere is transparent; wherein the first sublayer of the first waveguide layer is configured with microlenses to collect and focus the infrared radiation from the first emissive layer onto redirecting coupling elements embedded in the second sublayer of the first waveguide layer , the redirecting coupling elements being parallel to one another with each redirecting coupling element having an injection facet on one end, a narrowing on the other end, and an orientation within the waveguide layer such that the injection facet directs the infrared radiation from the first emissive layer upwards towards the sky, and the narrowing is directed in an opposite direction to inhibit the infrared radiation from being reflected backwards in the opposite direction. one or more cooling stacks ; wherein each cooling stack is configured with a first waveguide layer , a first emissive layer and a thermal conductive layer , wherein the first emissive layer is sandwiched between the first waveguide layer and the thermal conductive layer ; wherein the thermally conductive layer has a proximal end and a distal end; wherein the thermally conductive layer is substantially perpendicular to and in thermal contact at its proximal end with a source of heat to be cooled, and wherein the first waveguide layer comprises a first sublayer and a second sublayer; characterized in that : the first emissive layer is configured to absorb and emit infrared radiation at wavelengths where Earth's atmosphere is transparent; wherein the first sublayer of the first waveguide layer is configured with microlenses to collect and focus the infrared radiation from the first emissive layer onto redirecting coupling elements embedded in the second sublayer of the first waveguide layer , the redirecting coupling elements being parallel to one another with each redirecting coupling element having an injection facet on one end, a narrowing on the other end, and an orientation within the waveguide layer such that the injection facet directs the infrared radiation from the first emissive layer upwards towards the sky, and the narrowing is directed in an opposite direction to inhibit the infrared radiation from being reflected backwards in the opposite direction. the first emissive layer is configured to absorb and emit infrared radiation at wavelengths where Earth's atmosphere is transparent; wherein the first sublayer of the first waveguide layer is configured with microlenses to collect and focus the infrared radiation from the first emissive layer onto redirecting coupling elements embedded in the second sublayer of the first waveguide layer , the redirecting coupling elements being parallel to one another with each redirecting coupling element having an injection facet on one end, a narrowing on the other end, and an orientation within the waveguide layer such that the injection facet directs the infrared radiation from the first emissive layer upwards towards the sky, and the narrowing is directed in an opposite direction to inhibit the infrared radiation from being reflected backwards in the opposite direction. <EOS>