THz mixing of high-order harmonics using $YBa_2Cu_3O_{7-δ}$ nanobridges

Abstract

Superconducting materials are a key for technologies enabling a large number of devices including THz wave mixers and single photon detectors, though limited at very low temperatures for conventional superconductors. High temperature operation could in principle be offered using cuprate superconductors. However, the complexity of the material in thin film form, the extremely short coherence length and material stability, have hindered the realization of THz devices working at liquid nitrogen temperatures. $YBa_2Cu_3O_{7-δ}$ (YBCO) nanodevices have demonstrated non-linear properties typical of Josephson-like behavior, which have the potential for the mixing of AC signals in the THz range due to the large superconducting energy gap. Here, we present AC Josephson functionalities for terahertz waves utilizing Abrikosov vortex motion in nanoscale-confined fully planar YBCO thin film bridges. We observe Shapiro step-like features in the current voltage characteristics when irradiating the device with monochromatic sub-THz waves (100 GHz to 215 GHz) at 77 K. We further explore these nonlinear effects by detecting THz high-order harmonic mixing for signals from 200 GHz up to 1.4 THz using a local oscillator at 100 GHz. Our results open a path to an easy-fabricated HTS nonlinear nanodevice based on dimensional confinement suitable for terahertz applications.

Figures (5)

• Figure 1: Design and characterization of nanobridges heterodyne detectors and measurement set-up. (a) SEM false-color image of a 70nm wide YBCO nanowire integrated with a YBCO spiral antenna on a MgO substrate. The inset shows a zoom-in of the nanowire oriented along a-axis. (b) Resistance as a function of the temperature of the device. (c) Schematic of the measurement setup with an inset showing the device inside the sample box.
• Figure 2: Photo-induced response of a bridge with cross-section of $70 \times 50$ nm$^2$. (a) IVCs for a nanobridge for temperatures 77K and 60K where the critical currents are indicated for both curves (b) Measurement at 77K of the current-voltage characteristic without an applied signal and with an applied frequency $f_{LO}$ = 100GHz, 155GHz, 175GHz, 195GHz and 215GHz. (c) Evolution at 77K of the current voltage characteristic for non-illumination case and for increasing illumination power at $f_{LO}$ = 100GHz.
• Figure 3: Second harmonic mixing using $f_{LO}$ = 100GHz with $f_{S}$ = 201GHz for a bridge with cross-section of $70 \times 50$ nm$^2$. (a) IVC (left axis) and output power response detected at $f_{IF}$ = 1 GHz (right axis) at $T$ = 77K. (b) Differential resistance obtained from IVC from panel (a) as a function of voltage. (c) IVCs measurement (left axis) and output power response (right axis) measured at temperature $T$ = 60K. (d) Differential resistance obtained from panel (c) as a function of voltage. The vertical dashed lines indicate the the voltages corresponding to the minima of differential resistance. (e) Voltage position of current steps from panels (a) and (c) as a function of the step number for $T$ = 77K and $T$ = 60K, respectively. The voltage positions are obtained after fitting a Lorentzian to the corresponding minima of the differential resistance. The standard deviation of the fitting is smaller than the marker size.
• Figure 4: Comparison of tenth harmonic mixing using $f_{LO}$ = 100GHz and $f_{S}$ = 1001GHz in bridges with cross-section of $70 \times 50$ nm$^2$ and $200 \times 50$ nm$^2$, biased at equivalent AC currents at temperatures $T$= 59K and $T$= 58K, respectively. (a) IVC measurement (left axis) and output power response detected a $f_{IF}$ = 1GHz (right axis). (b) Differential resistance $dV/dI$ obtained from the IVCs shown in (a) as a function of voltage. (c) Voltage positions of current steps obtained from fitting a Lorentzian to the minima of the differential resistances shown in (a) and (c) as a function of the step number. The standard deviation of the fitting is smaller than the marker size.
• Figure 5: Detection limit of the nanobridges at 59-58 K. (a) For the nanobridge with cross-section of $200 \times 50$ nm$^2$, both 100GHz current steps (left axis) and harmonic mixing with 1.001THz i.e. the tenth harmonic (right axis) extend up to the flux-flow instability voltage (5.5mV). (b) For the nanobridge of cross-section $70 \times 50$ nm$^2$, intermediate frequency power as a function of the harmonic number n (filled circles) for a constant LO frequency (100GHz). Open circles are corrected for a 15 dB (x30) lower THz source power in the 1.1-1.5 THz range (x12, x13, x14 harmonics). The solid line is a $P_{IF} \propto n^{-10}$ fit. The y-axis is limited to -120 dBm which corresponds to our noise threshold.