In collaboration with Roger Romani, the optical group is building a portable imaging array of TES pixels for use in astronomical observation. Although we call it an "optical" detector, the pixels are sensitive to near-IR/optical/near-UV photons, making it a more wideband detector than implied by the word optical.
Experimental Setup
A 32 pixel array of tungsten TESs is coupled to a 50 mK substrate that is cooled using an adiabatic demagnetization refrigerator (ADR) purchased from Janis Research. To learn about the physics behind ADRs, check out the ADR Primer from NASA Goddard Space Flight Center.
TES Array
The individual detectors are small (20 micrometers on a side) thin film tungsten pixels deposited on a silicon substrate with aluminum wiring. The arrays are fabricated at the Stanford Nanofabrication Facility. Incident photons are directly absorbed into the TES, rather than through a large absorbing material as in some other TES experiments.
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| A schematic of the ADR | The base stage and wiring. View is from the right of the ADR in the schematic (and flipped vertically) |
| There are several unique aspects of the optical project that are not directly related to TESs. For example, a reflecting metalized mask is placed on top of the pixel array. This serves two purposes. First, photons are reflected away from the biasing wires, also called rails, and the substrate. Photons that hit these areas will still be detected by the TESs, but with an incorrect measured energy value. Second, these photons are redirected back into the pixel. This increases the effective fill factor of the array, which is important when we are trying to collect as much light as possible. Also, incident photons are directly absorbed into the TES, rather than through a large absorbing material, as is common in higher energy TES detectors. |  |
| A photograpgh and cartoon of the relfective mask showing photons reflected onto the pixel |
Second, we directly image onto the TES array. The detector array is coupled to the outside of the ADR through a number of filter windows. The windows are designed such that only light in the wavelengths we are interested in are transmitted through to the TESs. A microscope objective located at the 4.2 K stage focuses the image onto the array.
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| The 32 pixel W TES array | Spectrum of a green laser fit with a gaussian curve |
What are we going to do with all this?
Our objective is to use the TES array as an astronomical observing tool. With high energy resolution, microsecond time-stamping and high quantum efficeny, the optical project is well suited to observing faint and rapidly varying astrophysical sources. Pulsars are the prototypical class of objects for the optical project. The Crab pulsar, for example, dims and fades with a period of about 30 Hz. Phase-binning CCD can, if the period is known a priori, obtain phase-binned spectra, but without simultaneously obtaining intrinsic energy information and with no single photon statistics. For an example of TES based, non-imaging observation of the Crab pulsar see Romani et. al. (1999).