Measurements of the cosmic microwave background are a powerful probe of the early universe. Part-per-million fluctuations in the intensity of background trace the initial conditions of matter and energy shortly after the Big Bang, mapping the large-scale structure of spacetime. Now, new measurements in linear polarization at sensitivities of a few parts per billion can look behind these initial conditions to test physics at energies a trillion times higher than terrestrial accelerators, and perhaps even provide a glimpse of quantum gravity in action.
Measurements at the part-per-billion level are technically challenging. State-of-the art detector technology has progressed to the point where the limiting factor for instrument sensitivity is shot noise from photon arrival statistics. Once the detector noise falls below the background from photon statistical fluctuations, further sensitivity gains can only be realized by collecting additional photons. A common implementation increases the effective detecting area using an array of individual detectors. The resulting kilo-pixel detector arrays drive instrument complexity and cost. An alternate design uses fewer, but physically larger detectors, each capable of sensing multiple modes of the incident electromagnetic field. Since the signal increases linearly with detector size while the photon noise only increases as the square root, increasing the detector size increases sensitivity to sky signals without requiring large, costly detector arrays.
Scientists and engineers at Goddard have developed a large-area, polarization-sensitive bolometer for multimode optical systems at millimeter through sub-millimeter wavelengths (see figure). It consists of a set of 3-μm wide wires of doped crystalline silicon micromachined from an ion-implanted layer and suspended from an un-doped crystalline silicon frame. The parallel wires absorb light from a single linear polarization, and transport the energy to thermistors located in the supporting end banks. The entire device is maintained at temperature 0.1 K, near absolute zero, so that changes in the intensity of the incident radiation cause corresponding changes in the temperature of the thermistors, which are read out using a cryogenic JFET follower.
A key technical challenge is achieving a large absorbing area while keeping the detector time constant short. The absorbing structure is 12.7 mm long, providing 30 times greater absorbing area than previous state-of-the-art bolometers. The time constant is minimized by replacing the silicon nitride absorber used in previous bolometers with single-crystal silicon. Optical power absorbed in the shallow doped layer is readily conducted out of the absorber by phonons in the underlying bulk crystalline silicon. Measurements on prototype detectors show a time constant of 8 ms, meeting requirements for microwave measurements. Detectors of this type will be used by the Primordial Inflation Explorer (PIXIE) mission to map the microwave sky in polarization, opening a new window to the earliest moments of the universe.