Stella Bevilacqua

A Room Temperature Bolometer for Terahertz Coherent and Incoherent Detection

We present a novel room temperature bolometer with nanosecond response that can be used both for coherent and incoherent detection through the entire terahertz frequency range. A responsivity of up to 15 V/W, and a noise equivalent power (NEP) ~ 450 pW/Hz0.5 were measured at modulation frequencies from 0.5 kHz to 100 kHz. A conversion gain of -28 dB was demonstrated at an intermediate frequency of 20 MHz with a Local Oscillator power of 0.74 mW. Possible improvements of the bolometer characteristics are discussed.

Low noise MgB2 terahertz hot-electron bolometer mixers

We report on low noise terahertz bolometric mixers made of MgB2 superconducting thin films. For a 10-nm-thick MgB2 film, the lowest mixer noise temperature was 600 K at 600 GHz. For 30 to 10-nm-thick films, the mixer gain bandwidth is an inverse function of the film thickness, reaching 3.4 GHz for the 10-nm film. As the critical temperature of the film decreases, the gain bandwidth also decreases, indicating the importance of high quality thin films for large gain bandwidth mixers. The results indicate the prospect of achieving a mixer gain bandwidth as large as 10-8 GHz for 3 to 5-nm-thick MgB2 films.

Study of IF Bandwidth of

A noise bandwidth (NBW) of 6-7 GHz was obtained for hot-electron bolometer (HEB) mixers made of 10 nm MgB₂ films. A systematic investigation of the (IF) gain bandwidth as a function of the MgB₂ film thickness (30, 15, and 10 nm) is also presented. The gain bandwidth (GBW) of 3.4 GHz was measured for a 10 nm film, corresponding to a mixer time constant of 47 ps. For 10 nm films a reduction of the GBW was observed with the reduction of the critical temperature (<i>Tc</i>). Experimental data were analyzed using the two-temperature model. From the theoretical analysis, the electron-phonon time (τ_{e - ph}), the phonon escape time (τ_esc) and the electron and phonon specific heats (c_e, c_ph) were extrapolated giving the first model for HEB mixers of MgB₂ films.

{\hbox{MgB}}_{2}

A high sensitivity broadband terahertz direct detector based on YBa₂Cu₃O₇ high-Tc superconductor microbolometers is presented. At 77 K, the responsivity of the spiral antenna-integrated microbolometers (1.5 μm ×1.5 μm) is 190 V/W, referenced to the input of the silicon substrate lens, across the frequency range of 330 GHz-1.63 THz in a single device. The response time is approximately 300 ps. Using a room temperature readout, we measure an optical noise equivalent power (NEP) of 20 pW/Hz^0.5 (readout noise limited) for modulation frequencies ranging from 500 Hz to 100 kHz.

Phonon-Cooled Hot-Electron Bolometer Mixers

A high sensitivity broadband terahertz direct detector based on YBa₂Cu₃O₇ high-Tc superconductor microbolometers is presented. At 77 K, the responsivity of the spiral antenna-integrated microbolometers (1.5 μm ×1.5 μm) is 190 V/W, referenced to the input of the silicon substrate lens, across the frequency range of 330 GHz-1.63 THz in a single device. The response time is approximately 300 ps. Using a room temperature readout, we measure an optical noise equivalent power (NEP) of 20 pW/Hz^0.5 (readout noise limited) for modulation frequencies ranging from 500 Hz to 100 kHz.

Terahertz Direct Detection in YBa2Cu3O7 Microbolometers

In this paper, we present a study of the noise and the gain of MgB2 hot-electron bolometer mixers with different critical temperatures (Tc) and at various operation temperatures. At a local oscillator (LO) frequency of 1.63 THz the minimum input receiver noise temperature (Tr) was 700 K with a gain of -18 dB for a device with a Tc of 8.5 K. For a device with a Tc of 22.5 K the corresponding values were 1700 K and -19 dB. For the latter device the Tr was 2150 K at a bath temperature of 12 K, which is not achievable with Nb-compound based HEB mixers. We present and compare different methods for measurements of the HEB mixer gain and the output noise.

Terahertz Direct Detection in

We present a detailed investigation of the responsivity and the noise in room temperature THz direct detectors made of YBa₂Cu₃O_7-x (YBCO) thin-film nano-bolometers. The YBCO nano-bolometers are integrated with planar spiral antennas covering a frequency range from 100 GHz to 2 THz. The detectors were characterized at 1.6 THz, 0.7 THz, 400 GHz and 100 GHz. The maximum electrical responsivity of 70 V/W and a minimum noise equivalent power (NEP) of 50 pW/Hz^0.5 were measured, whereas the highest optical responsivity was 45 V/W. The (1/f) noise in nano-bolometers is independent on the device volume and can be found as (V_N/V)²=6×10^-11×1/f Hz^-1 for a given modulation frequency f and a dc voltage V.

{\hbox{YBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7}

A high sensitivity broadband terahertz direct detector based on YBa₂Cu₃O₇ high-Tc superconductor microbolometers is presented. At 77 K, the responsivity of the spiral antenna-integrated microbolometers (1.5 μm ×1.5 μm) is 190 V/W, referenced to the input of the silicon substrate lens, across the frequency range of 330 GHz-1.63 THz in a single device. The response time is approximately 300 ps. Using a room temperature readout, we measure an optical noise equivalent power (NEP) of 20 pW/Hz^0.5 (readout noise limited) for modulation frequencies ranging from 500 Hz to 100 kHz.

Microbolometers

In this paper, we compare the performance of MgB2 Hot-Electron Bolometer Mixers operating at Local Oscillator frequencies of 0.6 and 1.63 THz. The minimum noise temperatures that were obtained are 700 and 1150 K for 0.6 and 1.63 THz, respectively. The receiver noise bandwidth is of the order of 2.2-3 GHz for 10-nm-thick HEB devices with a Tc of 8.5 K. Sub-micrometer size HEBs were also fabricated with no degradation of the initial film quality when a 20-nm MgB2 film with a Tc of 22 K was used. In the direct detection mode, the maximum voltage responsivity is in the range of 1-2 kV/W at 1.63 THz and the optimal bias current is around 1/4-1/3 of the Ic at 4.2 K.

Effect of the Critical and Operational Temperatures on the Sensitivity of

We have investigated the cryogenic stability of two-finger InP HEMTs aimed for Ka-band ultra-low noise amplifiers (LNAs). Unlike two-finger transistors with a large gate-width above 2 χ 50 μm, the transistors with a small gate-width exhibit unstable cryogenic behavior. The instability is suppressed by adding a source air-bridge. The stabilizing effect of the air-bridge is demonstrated both on device and circuit level. A three-stage 2440 GHz monolithic microwave integrated circuit (MMIC) LNA using a stabilized 100-nm HEMT technology is presented. The amplifier achieves a record noise temperature of 7 K at 25.6 GHz with an average noise of 10.6 K across the whole band at an ambient temperature of 5.5 K. The amplifier gain is 29 dB ± 0.6 dB exhibiting very stable and repeatable operation. To our knowledge, this amplifier presents the lowest noise temperature reported so far for InP cryogenic LNAs covering the Ka-band.