4. Design, fabrication and experimental studies of a novel
Such bolometer consists of a normal metal absorber strip that is connected to planar antenna via SIN tunnel junctions. By proper dc bias of SIN junctions we can obtain electron cooling that significantly improve bolometer performance. As a result the CEB can receive radiation in sub-mm range with sensitivity down to attowatt per square root Hertz.
As it was mentioned in Science (v.302, N5653, p.2038), the most acknowledged breakthrough of the year 2003 was observation in microwaves the portrait of the earliest universe by Wilkinson Microwave Anisotropy Probe. The observation confirms that the Universe is made up largely of mysterious dark energy and dark matter. For understanding a nature of dark energy and dark matter, the future cosmology needs to get more detailed picture of the cosmic microwave background radiation. Such sensitive measurements at terahertz frequencies are possible only with cryogenic receivers. Another confirmation of the importance of superconducting devices studies and development is awarding to Prof. A. Abrikosov and Prof. V. Ginzburg by the Nobel prise in physics in 2003 for the theory of superconductors.
It has become increasingly evident in the last few years that superconducting devices will play a major role in radiation detection and characterisation in a wide range of the electromagnetic spectrum from the millimetre to the X-ray regions. These superconducting devices offer the prospect of fundamental thermodynamic or quantum-limited sensitivity; spectrally sensitive detection, the ability to manufacture large arrays. Such sensors could have a wide range of applications in very diverse fields such as astronomy, quantum computing, materials characterization, and high-resolution medical imaging.
In particular, hot electron bolometers (HEB) have seen a rapid maturation and can now be utilized on the first generation of applications. Future widespread applications of superconducting sensors have also come into reach because of the very recent development of user-friendly, self-contained ultra-low temperature coolers, which no longer need liquid cryogens. The use of such coolers in not much more complicated compared to the kitchen refrigerator.
The Cold-Electron Bolometer (CEB) proposed at Chalmers University is a novel device, which could be a turning point in realization of modern supersensitive detectors. The CEB is essentially a nanorefrigerator that cools the electrons within a thin metal film by extracting the hottest electrons through normal-insulator-superconductor (NIS) junctions. This effect is similar to that used in a thermoelectric cooler. The refrigeration effect of the junctions allows this detector to operate with extremely low noise in a relatively high temperature environment.
The key element of my research is design, fabrication and studies of such terahertz band cold electron bolometer with attowatt sensitivity, to develop a novel type of bolometer in which electrons are cooled below the phonon temperature. By using the direct electron cooling it is possible to achieve in such cold electron bolometer (CEB) the sensitivity corresponding to 100 mK temperature in a small 3He sorption cooler with basic temperature 260 mK. Such a bolometer is intended for detection of a terahertz-band radiation with ultimate sensitivity of attowatt per square root of Hertz (10-18 W/Hz1/2). Present semiconductor bolometers offer one order of magnitude worse characteristics. A goal is to prove the concept of novel type of superconducting photon-counting submillimetre device, the cold electron bolometer (CEB). Research and development includes areas of solid-state physics, nanoelectronics, nanofabrication, cryoelectronics, low temperature physics, terahertz spectroscopy, etc. Research is carried in close collaboration with leading European scientists from Helsinki University, Rome University, Oxford Instruments, CESR (Toulouse, France), VTT (Finland). The proposal to the 6 EU FP Programme on RTD for building a European consortium “Development of Superconducting Sensor Arrays” is intended to support this activity.
This project is expected to make to advance the technique of radiation detection and characterisation in a wide range of the electromagnetic spectrum from the millimetre to the X-ray regions. These superconducting devices offer the prospect of fundamental thermodynamic or quantum limited sensitivity; spectrally sensitive detection. The first time in bolometer concepts, heating by unavoidable current or voltage bias is replaced by electron cooling by the same dc bias. Such sensors could have a wide range of applications in very diverse fields such as astronomy, quantum computing, materials characterization, and high-resolution medical imaging. Concept of quasiparticle cascade amplifier integrated on the same chip with bolometer greatly improves bolometer performance and allows avoiding noisy semiconducting amplifiers or capricious SQUID readout.
The idea of capacitive coupled normal metal hot electron bolometer is new and it is developed from the concept of a normal metal hot electron bolometer with Andreev mirrors. Further development is achieved with the new concept of electron cooling that is completely technologically compatible to the bolometer.
Development of such bolometer is vital for the radioastronomy space projects, which require ultimate sensitivity. The objects of an astronomical study in submillimeter and infrared wavelengths are the "cold" components of the matter in the Universe (3oK radiation relic of Big Bang, extragalactic sources on the stage of star birth burst, dust in the Solar System, in the Galaxy). The primary goals for observations with bolometers are: a submillimeter wave full sky photometric survey, studies of the spectral energy distribution (SED) of astronomical sources and their variability, determination of 105 - 106 brightest objects of young Universe (between beginning of visible structure formation and half of modern age), investigations of clusters of galaxies (Zeldovich-Syunyaev effect), search for spectral details in the CMB due to processes during "dark time" between the epochs of recombination and secondary heating, study of the anisotropy and polarization of the cosmic microwave background (CMB) with efficient separation of foreground sources.
The objective to develop a novel cold electron bolometer will be achieved in several stages: theoretical studies of heat balance and noise in a nanobolometer, design of the layout for the bolometer structure, experimental studies of CEM at cryogenic temperatures.
Theoretical studies and modelling is based on solving the heat balance equation, modelling of sources of noise and mechanisms of their conversion. The new experimental data for thermal conductivity will be used to make adequate modelling and correct estimations corresponding to experimental results. Design of layout is based on experience in computer-aided design of integrated cryogenic circuits comprising planar antennas and non-linear superconducting sensors. The experience gained by applicant in designing of integrated SIS receiver, Josephson detector and mixer, SQUID amplifier will be transferred to novel cold electron bolometer
Nanofabrication is based on direct e-beam lithography and shadow evaporation of different metals at different angles. Besides that a so-called three-layer technology will be developed as analogue to the Nb 3-layer technology that is a cornerstone for SIS mixer fabrication.
Data measurements and acquisition is based on low-noise measuring set-up and planning use of recently proposed quasiparticle cryogenic amplifier. The latter is a novel device that can dramatically improve performance of cryogenic sensors. Spectral measurements in the Terahertz band are based on original network analyser-spectrometer based on a broadband Josephson high critical temperature oscillator. Such oscillator integrated with a broadband log-periodic antenna covers the frequency range from 100 GHz up to 2-3 THz.
Experimental cryogenic technique is usually based on dilution refrigerator and sorption cooler refrigerator in helium cryostats that one should fill with cryogen every day. Expected new cryogenic technique is based on the first commercial cryogen-free millikelvin refrigerator with closed cycle 4He cooler and 3He sorption cooler designed and fabricated for our project by Oxford Instruments. The rest of cooling from 300 mK down to about 100 mK will be achieved by electron cooling. Such system in simplicity of operation is approaching a kitchen refrigerator, which you need just to switch on and set the desired temperature.