Toward a Detector/Readout Architecture for the Background-Limited Far-IR/Submm Spectrograph (BLISS)

We have built and tested 32-element linear arrays of absorber-coupled transition-edge sensors (TESs) read out with a time-division SQUID multiplexer. This detector/readout architecture is designed for the background-limited far-IR/submm spectrograph (BLISS) which is a broadband (35–433 m), grating spectrometer consisting of six wavebands each with a modest resolution of R ∼ 700. Since BLISS requires the effective noise equivalent power (NEP) of the TESs to equal 1 × 10 ^-19 W/Hz ^1/2 , our detectors consist of very long (1–2 mm), narrow (0.4 m), and thin (0.25 m) Si _x N _y support beams that reduce the thermal conductance G between the substrate and the optical absorber. The thermistors of our lowest noise TESs consist of iridium with T_c=130 mK. We have measured the electrical properties of arrays of these Ir TESs with various meander and straight support beams and absorber shapes and found that G is ∼ 30 fW/K (meander) and ∼ 110 fW/K (straight), the electrical NEP is 2–3 × 10 ^-19 W/Hz ^1/2 (meander and straight), and the response time τ is 10–30 ms (meander) and 2–5 ms (straight). To reduce spurious or “dark” power from heating the arrays, we mounted the arrays into light-tight niobium boxes and added custom L/R and L/C low-pass chip filters into these boxes to intercept dark power from the bias and readout circuit. We found the average dark power equals 1.3 and 4.6 fW for the boxes with L/R and L/C chip filters, respectively. We have built arrays with T_c= 70 mK using molybdenum/copper bilayers and are working to lower the dark power by an order of magnitude so we can demonstrate NEP = 1 × 10 ^-19 W/Hz ^1/2 with these arrays. PACS numbers: 85.25.Pb; 95.85.Gn; 95.85.Fm; 63.22. + m