Low noise terahertz MgB2 hot-electron bolometer mixers with an 11 GHz bandwidth

Abstract

Terahertz (THz) hot-electron bolometer mixers reach a unique combination of low noise, wide noise bandwidth, and high operation temperature when 6 nm thick superconducting MgB2 films are used. We obtained a noise bandwidth of 11 GHz with a minimum receiver noise temperature of 930 K with a 1.63 THz Local Oscillator (LO), and a 5 K operation temperature. At 15 K and 20 K, the noise temperature is 1100 K and 1600 K, respectively. From 0.69 THz to 1.63 THz, the receiver noise increases by only 12%. Device current-voltage characteristics are identical when pumped with LOs from 0.69 THz up to 2.56 THz, and match well with IVs at elevated temperatures. Therefore, the effect of the THz waves on the mixer is totally thermal, due to absorption in the π conduction band of MgB2.

Analysis of the Broad IF-Band Performance of MgB2 HEB Mixers

“…[14]). For MgB2 HEBs, the I(V)-curves under the optimal power 700 GHz, 1.6 THz, and 2.6 THz LOs have been confirmed to correspond well to the I(V) without an LO, but at a temperature close to Tc [22]. The same being valid also for the output noise [25].…”

“…Additionally, a batch of MgB2 devices was fabricated with THz spiral antennas using MgB2 films similar to those for the devices described above. Packaged into a mixer block with a Si lens, the noise temperature was measured with Yfactor technique at a 1.63 THz LO at 5 K, 15 K, 20 K, and 25 K operation temperatures (Fig.2) as a reference (see [22]). The measured minimum noise temperature is about 1000 K, i.e.…”

“…Due to the advancement in ultrathin superconducting MgB2 film fabrication technology [17], [18] , in recent years, MgB2 based HEBs have become a center of interest due to their very fast inelastic electron-phonon interaction (eph 1-2 ps at 40 K) and efficient acoustic matching to both sapphire and SiC substrates [19], [20], [21]. A receiver noise temperature of 1000 K and a noise bandwidth (NBW) of 11 GHz have been reported for MgB2 HEBs using the Yfactor technique at both 700 GHz and 1.63 THz LO frequencies, with an optimal Local Oscillator power of about 10 W [22]. With a critical temperature of 30 K, the mixer showed only a minor reduction of sensitivity for operation temperatures up to 20 K.…”

“…Earlier gain bandwidth measurements showed that for MgB2 thin films the gain is almost constant up to 10 GHz (no roll-off), which is a huge improvement compared to any existing state-of-the-art HEBs. Such gain bandwidth (GBW) does not match the 11 GHz NBW [22], because for HEBs an NBW/GBW ratio of 2 is to be expected [23], [24]. The obvious reason for this discrepancy is the imperfection of the utilized receiver layout, involving the mixer block, the IF amplifier and the interaction between those.…”

Engineering Physics of Superconducting Hot-Electron Bolometer Mixers

“…Therefore, to improve the IF bandwidth, it is more beneficial, to search for other materials that can replace NbN. Interesting progress along this line has been made recently with magnesium diboride (MgB 2 ) (Cunnane et al, 2015;Novoselov and Cherednichenko, 2017). A promising noise bandwidth of up to 11 GHz, combined with a higher operating temperature, has been reported.…”