HTSC Photomixers

High Temperature Superconducting Photomixers:

High Tc materials exhibit two strong responses to excitation by optical signals:ht1
Close to Tc or under very high illumination powers a bolometric response dominates in which the extra electron temperatureintroduced by the photoexcitation closes the superconducting gap and results in a return to the normal state and development of resistance. This has been widely used as a method for enabling bolometric detection of optical infrared and even mm-wave signals but is limited in speed by the thermal relaxation time of the electron subsystem via the lattice temperature to the substrate.

Below Tc another mechanism is possible - photomodulation of the kinetic inductance which arises due to the kinetic energy required to create a superconducting current. Although supercurrents travel through superconductors in a lossless fashio it does requrie energy to get the superconducting particles to move. If a small piece of superconductor is carrying a constant current, and if a section of that piece suddenly looses a fraction of its superconducitng particles , then a charge builds up around that section until the remaining particles can accelerate to return current continuity.

We have investigated these photodetectors using heterodyne optical techniques to drice detectors with modulated optical signals ranging in freqeuency from a few GHz to ~1THz. Theoretial models based on electron thermalisation and BCS theory of the condensate have predicted a response for the two mechanisms which are very different as a function of freqeuency.

The devices fabricated to test this structure are shown in the two images below. They consist of a think (100nm) Tl2Ba2CaCu2O10-x film patterned by lithography to form a microbridge between a square patch antenna and a 50Ohm microstrip transmission line. The antenna and the line are covered by a 200nm gold film to prevent unwanted illumination and improve surface resistance (surface resistance for 100nm thick films is not generalyl as good as for the thicker films used for RF filters)