The available experimental data on non-linear microwave effects in superconductors and some theoretical explanations of these, published since 1964, are reviewed. Various mechanisms of non-linearkty observed under a wide variety of external conditions and in a broad spectrum of materials are discussed. Some examples of application of the non-linearity to frequency mixing are considered and analysed in the framework of a simple model of a fast-response resistive mixer.

The voltage-dependent dielectric constant (ε) of SrTiO3 (STO) thin films is the basis for developing cryogenic capacitors for tunable microwave applications. In this study, the effect of microwave signal level on nonlinear response at 1.7–1.9 GHz was examined by measuring the level of the third order intermodulation distortion (IMD) signal relative to the input signal level. Small signal dielectric properties such as capacitance, tuning, and loss (tan δ) were also measured at 1 MHz, 3 GHz, and 10 GHz, at temperatures from 4.2 to 300 K. Planar capacitors were comprised of highly (100)-oriented, 1 μm thick STO films deposited via magnetron sputtering onto CeO2-buffered (11_02)-oriented sapphire substrates, with 10 μm gaps between the electrodes. Deviations from the anticipated cubic dependence of the third order IMD product on incident power, for incident power ranges from −10 to 22 dBm, were attributed to conductivity nonlinearity. At incident power levels of 22 dBm and with no dc bias applied to the capacitor, the level of the third order IMD product was 21 dB below the fundamental signal level. Application of a 107 V/m dc electric field bias across the capacitor suppressed the third order IMD by an additional 10 dB. The nonlinear properties of thin film STO capacitors as a function of microwave voltage were determined by comparing the experimental and theoretical dependencies of the IMD products.

The nonlinear response of ferroelectric BaxSr1−xTiO3 films to microwave electric field intensity up to ∼3×106 V/m was studied. Two techniques were used for this investigation: (i) 10 GHz pulsed power measurements, and (ii) 4 GHz intermodulation distortion (IMD) measurements. The nonlinear distortion of the resonant curve under microwave pulsed power and generation of the third-order IMD products in microwave resonators using ferroelectric film planar capacitors were measured. The use of microwave pulses and continuous signals enabled the separation of the nonlinear dielectric response from the heating response of the ferroelectric films and the microwave nonlinear parameters of the ferroelectric films to be determined. It is shown that up to a specified value of microwave voltage amplitude the nonlinear response of BaxSr1−xTiO3 film capacitors can be predicted from the small signal capacitance–voltage characteristics. Formulas to estimate power handling capability connected with the field dielectric nonlinearity and the film overheating are derived for the tunable microwave devices based on ferroelectric films.

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