Transition-edge Hot-electron Microbolometer

Our research focuses on the development of a sensitive detector, called a Transition-Edge Hot-Electron Microbolometer (THM). The THM detector is optimized to take precision measurements of the Cosmic Microwave Background (CMB). Arrays of 1000s of these THM detectors would meet the sensitivity level required to measure the faint B-mode polarization signal in the CMB, a remnant of gravitational waves from the inflation era, and tell us even more about what happened in the earliest moments of the universe. A bolometer is a detector that absorbs incident photons and converts their energy into heat. It consists of three parts: an absorber, thermometer, and cold bath (see figure below). Incoming photons thermalize in the absorber, and heat leaves the absorber via a weak link to the cold bath. The power of incident radiation is measured by monitoring the temperature of the absorber.

To measure small differences in incident power a sensitive thermometer is necessary. The THM employs an extremely sensitive thermometer called a Transition-Edge Sensor (TES). A TES is a superconductor which exhibits a transition between a normal and superconducting state. At this transition its resistance drops to zero and there is a sensitive dependence of resistance on temperature. An important component to the operation of bolometers is the thermal link between the detector and the cold bath. Common TES bolometers make use of micro-machined isolation structures, to precisely control the thermal conductance of the link. This thermal conductance affects the noise, time response, saturation level and other characteristics of the detector. These membrane structures are fragile and present both fabrication and design complexities. The Transition-Edge Hot-Electron Microbolometer (THM) makes use of a different type of thermal isolation, one that is controlled by the weak coupling between electrons and phonons (quantized vibrational states of the crystal lattice) in the detector at low temperatures and within small volumes.

The basic design of the THM consists of a micron-sized metal Bismuth absorber overlapping a micron-sized superconducting bilayer Gold/Molybdenum TES (see figure above). The absorber terminates a Niobium superconducting microstrip transmission line which can couple to microwave radiation via a planar antenna (see figure below). The detector is operated at milliKelvin temperatures in order to maximize sensitivity.