The characterisation of exoplanets is one of the most important goals for astronomy in the coming decades.
Besides the basic parameters of an exoplanet, such as its orbit, mass and size, we also want to know the composition of the planet itself and its atmosphere (if present). This characterisation is only possible by spectroscopy of the starlight seeping through the atmosphere and bouncing off the clouds.
Figure 1. Atmospheric spectrum of an Earth-like exoplanet in the habitable zone; exoplanets may have very different fingerprints of their atmosphere.
Exoplanet spectroscopy
Exoplanet atmospheres can be studied at visible and near-infrared wavelengths (e.g. with NASA’s future Habitable Worlds Observatory) and at mid-infrared wavelengths (e.g. with ESA’s future Large Interferometer For Exoplanets). These spectroscopic windows give complementary information. In both cases the star outshines the planet with a factor of ten million or even a billion, respectively in mid-infrared or in visible/near-infrared. The measurement instrument needs to remove most of the starlight from its view before it can collect any meaningful information about the planet. And even then, it needs to efficiently capture most incoming light particles as exoplanets are notoriously weak light sources.
Single photon spectroscopy
SRON develops a spectrograph based on MKIDs optimised for visible, near-infrared and mid-infrared wavelengths. These detectors identify single photons with an intrinsic energy resolving power of over 100, which enables spectroscopy without dispersive optics such as gratings or a prism. This means less optical elements are needed, allowing for a simpler and more efficient design. Contrary to conventional semiconductor detectors, MKIDs can detect and characterise single photons without the penalties of dark current and read-noise.
MKIDs are based on a superconducting metal whose electric resistance disappears when it is cooled to low temperatures close to absolute zero. To make use of this property, we keep our MKIDs around a temperature of 0.1 degrees above zero—0.1 Kelvin. When a light ray hits the detector, the metal heats up and its superconducting state changes. This affects the electric current running through, which we observe in the readout signal. MKIDs are ideal to link in a large array because they are easy to read out all at once. To distinguish between pixels, we give each detector a slightly different length so their readout signal has a different frequency.
Figure 1: Operation principle of MKID detectors. a) Photograph of one of our MKID pixels. The inductor and capacitor form a superconducting microwave resonator, which is coupled to the readout line. A dielectric lens is placed on top to focus the radiation onto the inductor. b) When a photon is absorbed, it will break up thousands of Cooper pairs into quasiparticles. c) The MKID is a microwave resonator, where the resistance and inductance change upon the absorption of a photon. The change in the resonance curve when a photon is absorbed, which is read out at the equilibrium resonant frequency. d) Response of an MKID to single photons of different colour, which shows the energy-sensitivity of the detector. e) A histogram of measured single photon pulse heights, which demonstrates the state-of-art intrinsic resolving power of MKIDs, here separating different shades of red.
