My research focuses on using Microwave Kinetic Inductance Detectors (MKIDs), a relatively new and promising type of superconducting photon detector to build optical/UV instrumentation for observations of compact objects and planet finding.  The primary attraction of MKIDs is that, unlike other low temperature detectors, they are easy to multiplex into large arrays. The largest arrays of low temperature detectors (LTDs) are currently several hundred pixels in the submillimeter and up to 120 pixels in the optical and X-ray. Much larger arrays are needed for important scientific goals like measuring the CMB polarization, detecting terrestrial planets in the IR/optical/UV, and measuring iron lines from supermassive black holes with future X-ray missions.  MKIDs have the potential to scale to megapixels sizes in the near-IR to X-ray in the coming decades.

    MKIDs work on the principle that incident photons change the surface impedance of a superconductor through the kinetic inductance effect. This change can be accurately measured using a thin film superconducting resonant circuit, resulting in a measurement of the energy and arrival time of the incident photon for the case of optical/UV/X-ray photons or the total photon flux for lower energy photons.  The only real difference between arrays designed for different wavelengths is the method used to couple the photon energy into the MKID - the detectors themselves and the readout are nearly identical. Figure 1 gives an overview of this process. In Figure 1 panel (a), a photon with energy hv > 2∆ (∆ is the superconducting gap energy) is absorbed in a superconducting film cooled to T\Tc, breaking Cooper pairs and creating a number of quasiparticle excitations Nqp = eta hv/∆.  The efficiency of creating quasiparticles eta will be less than one since some of the energy of the photon will end up as vibrations in the lattice called phonons. In this diagram, Cooper pairs (C) are shown at the Fermi level, and the density of states for quasiparticles, Ns(E), is plotted as the shaded area as a function of quasiparticle energy E.

    Panel (b) shows that the increase in quasiparticle density changes the surface impedance Zs = Rs + i omega Ls of the film (represented as the variable inductor), which is used as part of a microwave resonant circuit. The resonant circuit is depicted schematically here as a parallel LC circuit which is capacitively coupled to a through line. The effect of the surface inductance Ls is to increase the total inductance L, while the effect of the surface resistance Rs is to make the inductor slightly lossy (adding a series resistance).

    Panel (c) shows that on resonance, the LC circuit loads the through line, producing a dip in its transmission. The quasiparticles produced by the photon increase both Ls and Rs, which moves the resonance to lower frequency (due to Ls) and makes the dip broader and shallower (due to Rs). Both of these effects contribute to changing the amplitude (c) and phase (d) of a microwave probe signal transmitted past the circuit.  The amplitude and phase curves shown in this illustration are actually the data measured for an aluminum test device at 120 mK (solid lines) and 260 mK (dotted lines), which is of a magnitude similar to what would be expected from a photon event. This choice of circuit design, which has high transmission away from resonance, is very well suited for frequency-domain multiplexing, since multiple resonators operating at slightly different frequencies can all be coupled to the same through line (Figures 2 and 3).

    The primary advantage of MKIDs compared to other low temperature detectors such as superconducting tunnel junctions (STJs) or transition edge sensors (TESs) is that by using resonant circuits with high quality factors, passive frequency domain multiplexing allows up to thousands of resonators to be read out through a single coaxial cable and a single high electron mobility transistor (HEMT) amplifier (Firgure 2.). Large arrays of MKIDs are significantly easier to fabricate and read out than any competing technology. They do not require any superconducting electronics, and their readouts can leverage the tremendous advances in room temperature microwave integrated circuits developed for the wireless communications industry. MKIDs currently achieve sensitivities adequate for ground-based submillimeter and optical astronomy.

For more information, see:

A broadband superconducting detector suitable for use in large arrays. Nature, vol. 425, pp. 817–821, 2003.

Microwave Kinetic Inductance Detectors.  Mazin Thesis.

The physics of superconducting microwave resonators.  Gao Thesis.

Figure 1.  The operational principles of a MKID.

Figure 2.  Demonstration of 0.8 MHz frequency jitter in an optical MKID array.

Figure 3.  SEM of a prototype aluminum/tantalum optical MKID array.

Figure 4.  An X-ray MKID strip detector, from Position sensitive X-ray spectrophotometer using microwave kinetic inductance detectors,