Evgenii Novoselov

Effect of the Critical and Operational Temperatures on the Sensitivity of

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.

{\hbox{MgB}}_{2}

In this paper we consider using the terahertz (THz) time domain spectroscopy (TDS) for non destructive testing and determining the chemical composition of the vanes and rotor-blade spars. A versatile terahertz spectrometer for reflection and transmission has been used for experiments. We consider the features of measured terahertz signal in temporal and spectral domains during propagation through and reflecting from various defects in investigated objects, such as voids and foliation. We discuss requirements are applicable to the setup and are necessary to produce an image of these defects, such as signal-to-noise ratio and a method for registration THz radiation. Obtained results indicated the prospects of the THz TDS method for the inspection of defects and determination of the particularities of chemical composition of aircraft parts.

HEB Mixers

In this work we present experimental investigation of the MgB2 hot-electron bolometer (HEB) for low noise mixing at terahertz frequencies. A dedicated MgB2 thin film deposition system was designed and constructed based on Hybrid Physical-Chemical Deposition. Films as thin as 15nm have a superconducting transition at 35K, with a critical current density <107 A/cm2 (at 4.2K) in bridges as narrow as 500nm, indicating on good connectivity in the film. The gain bandwidth (GBW) was measured by mixing of two THz sources. The GBW is proportional to the film thickness and it is at least 6GHz for 15nm thick devices. Performance of MgB2 HEBs was compared to performance of one of the NbN HEB mixers made for the Herschel Space Observatory (one of the flight units), for which both the GBW and the Noise Bandwidth (NBW) was measured. MgB2 HEB mixers show a GBW at least a factor of three broader compared to the NbN HEB measured in the same set-up.

Nondestructive monitoring of aircraft composites using terahertz radiation

Terahertz (THz) hot-electron bolometer mixers reach a unique combination of low noise, wide noise bandwidth, and high operation temperature when 6nm thick superconducting MgB2 films are used. We obtained a noise bandwidth of 11GHz with a minimum receiver noise temperature of 930K with a 1.63THz Local Oscillator (LO) and a 5K operation temperature. At 15K and 20K, the noise temperature is 1100K and 1600K, respectively. From 0.69THz to 1.63THz the receiver noise increases by only 12%. Device current-voltage characteristics are identical when pumped with LOs from 0.69THz up to 2.56THz, 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.

MgB2 hot electron bolometer mixers for THz heterodyne instruments

We discuss a custom built hybrid physical chemical vapor deposition system for MgB₂ ultrathin film deposition: construction, deposition process development, and optimization. Achieved films on SiC substrates have a critical temperature (<italic>T_c</italic>) ranging from 35 K (10-nm-thick films) to 41 K (40-nm-thick films). The 20-nm-thick unpatterned film had a room temperature resistivity of 13 <italic>μ</italic> Ω·cm, whereas it becomes 50 <italic>μ</italic>Ω·cm in submicrometer size bridges with a critical current density <italic>J_c</italic> (4.2 K) up to 1.2 × 10⁸ A/cm². The lower value of resistivity corresponds to the higher of both <italic>T_c</italic> and <italic>J_c</italic>. The surface roughness, measured with an atomic force microscope, is approximately 1.5 nm.

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

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.

Study of MgB2 ultra-thin films in submicron size bridges

In this paper, we study variation of the MgB2 hot-electron bolometer mixer characteristics such as noise temperature, gain, output noise, and local oscillator (LO) power at 5-, 15-, and 20-K bath temperatures, and at 0.69- and 1.63-THz LO frequencies. The main reason for the noise temperature rising at higher temperatures is a reduction of the mixer gain, which occurs proportionally to the LO power reduction. Contrary to this, the output noise remains constant (for the same bias point).

MgB2 Hot-Electron Bolometer Mixers at Terahertz Frequencies

In this paper, we present a method for low-signal S11 parameter vector measurements of a cryogenic device in the microwave frequency range. In particular, the intermediate frequency (IF) impedance of MgB2 hot-electron bolometer (HEB) mixers was investigated over a frequency range of 100 MHz to 10 GHz. A new cryogenic calibration technique, which employs the HEB as a calibration kit and requires two consecutive thermal cycles, was developed for this purpose. The real part of measured IF impedance showed a strong correlation with the differential resistance (dV/dI) obtained from the dc I-V curves, whereas the imaginary part was capacitive for almost the entire frequency range.

Gain and Noise in THz MgB2 Hot-Electron Bolometer Mixers With a 30-K Critical Temperature

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.

Wideband MgB2 Hot-Electron Bolometer Mixers: IF Impedance Characterisation and Modeling

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.