A Transition-Edge Hot-Electron Microbolometer
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
about what happened in the earliest moments of the universe.
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.
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.
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.