PRIORITY CLAIM

This application is a national phase entry of PCT/FI2018/050851, filed on Nov. 22, 2018, which claims priority to Finnish Patent Application No. 20176051, filed on Nov. 23, 2017, the entire disclosures of which are incorporated by reference herein.

FIELD OF THE INVENTION

The invention is related to the field of detecting and measuring of electromagnetic energy at microwave frequencies. In particular the invention is related to an ultrasensitive detector structure and arrangement that can detect microwave signals of extremely small magnitude.

BACKGROUND

Ultrasensitive detection methods of received electromagnetic radiation comprise calorimetry and bolometry. A calorimeter is a device in which the momentarily increased temperature of a detector element decays exponentially towards that of a surrounding thermal bath. A bolometer has a detector element and a thermal bath, but the approach is to measure power (energy over time), i.e. the mean flux of incoming photons, rather than the exact energy of a single photon.

The schematic illustration ofFIG. 1is, as such, applicable to both a calorimeter and a bolometer. Incoming radiation101is absorbed in an absorber102, so that the absorbed energy heats up a detector element103, the heat capacity of which is C. The absorbed thermal energy is subsequently lost to a heat sink or thermal bath104through some thermal conductivity G. A measurement arrangement105is used to detect and measure either the temperature of the detector element103as an absolute value, and/or related quantities like the rate of change of the temperature.

In order for a bolometer to “keep up” with changes in the power of the incoming signal, the changes must be slow in relation to the thermal time constant of the system, which is essentially C/G. In other words, to make a bolometer sensitive to fast changes would require having a small C and a large G. On the other hand, the thermodynamical lower bound for the noise equivalent power of the system is proportional to the square root of 4*kB*T2*G, where kBis Boltzmann's constant and T is the temperature of the thermal bath. Thus in order to keep noise low one should make G small and T low.

FIG. 2is a schematic illustration of a known ultrasensitive bolometer. The detector element comprises a microscopic length of material that is ohmically conductive and does not become intrinsically superconductive even at low temperatures. This so-called normal conductor can be made of e.g. a gold-palladium alloy, and in the detector structure it comprises a so-called long section201and a short section202. The structure comprises also superconductor parts, i.e. pieces made of a material like aluminum or other metal that becomes superconductive at the temperatures to which the detector is cooled for operation. The superconductor parts comprise an input section203coupled to one end of the long section201, and an output section204coupled to one end of the short section202. A bridge section205is placed across the normal conductor so that it divides its length into the long section201and short section202. One end of the bridge section205is grounded through a capacitive coupling. All sections listed above are enclosed in the thermal bath206.

The short section202is short enough, and bound by superconductor sections at its both ends, so that at the low temperature of the thermal bath the proximity effect makes it weakly superconductive, whereas the long section201mostly retains its ohmic conductivity. The short section202also constitutes a part of a tank circuit, so that its impedance (which, due to said weak superconductivity, depends strongly on temperature) in the tank circuit has a significant effect on the resonance frequency of the tank circuit. The incoming microwave signal heats up the long section201, resulting in a corresponding temperature increase also in the short section202. A resonance frequency measurement circuit207measures the resulting change in the resonance frequency of the tank circuit, so that the output of the resonance frequency measurement circuit207reveals the power of the incoming microwave signal.

FIG. 3illustrates a variation of the principle ofFIG. 2. The difference toFIG. 2is that inFIG. 3the detector element comprises a number of short sections301,302, and303. This helps to increase the impedance of the measurement circuit without making it non-linear, but the general operating principle is the same as inFIG. 2.

Although the detectors ofFIGS. 2 and 3are extremely sensitive, they involve problems related to their calibration. Very small variations in the manufacturing process introduce significant uncertainty in the measurement response. Basically it would be possible to calibrate each manufactured bolometer individually using some known source of microwave radiation, but producing the required reference signals on microwave frequencies at the required accuracy is difficult and expensive.

Prior art documents that describe known high-sensitivity bolometers comprise for example patent publications FI122887B and U.S. Pat. No. 9,255,839B2.

SUMMARY

It is an objective of the present invention to present a detector and detector arrangement for detecting microwave radiation with extremely high sensitivity and relative ease of calibration. Another objective of the invention is to present a method and devices for calibrating a detector of microwave radiation without causing significant increase in the heat capacity of the detector element. Yet another objective of the invention is to present a detector of microwave radiation of the kind described above so that inevitable variations in the manufacturing process do not sacrifice the sensitivity and accuracy of the manufactured detectors.

These and further advantageous objectives of the invention are achieved by providing a heating element in the detector that is closely enough coupled to the other parts of the detector so that the hot electron effect can be exploited, while simultaneously using long enough superconducting leads to block stray heat conduction carried by quasiparticle thermal transport.

A detector of microwave radiation according to an embodiment is characterized by the features recited in the independent claim directed to a detector.

A detector arrangement of microwave radiation according to an embodiment is characterized by the features recited in the independent claim directed to a detector arrangement.

