Gate-Tunable Photoresponse of Graphene Josephson Junctions at Terahertz Frequencies

Abstract

Graphene Josephson junctions (JJ) provide a promising platform for ultra-broadband quantum sensing of light owing to graphene's frequency-independent absorption, vanishing electronic heat capacity, and weak electron-phonon coupling, which enable rapid suppression of the critical current through radiation-induced electron heating. Existing investigations have been confined to the microwave and infrared regimes, where competing detector technologies are already established; by contrast, the terahertz (THz) band - where sensitivity is most urgently lacking and no mature quantum sensor exists - has remained largerly unexplored. Here we demonstrate a strong photoresponse of graphene JJs at THz frequencies, establishing a first experimental step towards graphene-based THz quantum sensors. Under low-intensity illumination, we observe a pronounced suppression of the critical current that generates a strong photovoltage (Vph) under current bias. By tracking this Vph and independently measuring the electron temperature as a function of absorbed power, we extract a responsivity of 88 kV W^-1 and a noise-equivalent power of 45 aW Hz^-1/2 at 1.7 K. Furthermore, gate tunability of our JJ enables access to a regime where hysteretic current-voltage characteristics persist up to 0.9 K, offering a potential route toward single-photon THz detection beyond millikelvin (mK) temperatures. These findings establish graphene JJ as a versatile platform for broadband cryogenic radiation sensing and point towards their use as quantum sensors at THz frequencies.

Figures (4)

• Figure 1: Device architecture and superconducting transport.(a) Three-dimensional schematic of the van der Waals graphene JJ and the four-terminal measurement circuit. The inset shows an AFM image of a representative cracked NbSe$_2$ electrode; the gap width is $\approx$150 nm and the flake thickness $\approx$25 nm. (b) Optical micrograph of the main device, D1 in this study. The heterostructure consists, from top to bottom, of hBN, monolayer graphene (MLG), cracked NbSe$_2$, hBN, and graphite. (c) Temperature dependence of the four-terminal junction resistance of D1 (at $I_{\rm DC}=0$). $T_\mathrm{c1}\approx 7$ K marks the bulk NbSe$_2$ transition; below $T_\mathrm{c2} \approx 4.8$ K the resistance vanishes. The inset shows the hysteretic I--V behavior of the D2 at 0.9 K. (d) Current--voltage ($I$--$V$) characteristics at 0.7 K (D4) and 1.7 K (D1). The hystersis between switching current $I_c$ and retrapping current $I_r$ at 0.7 K is signature of underdamped Josephson dynamics driven by self-Joule heating. At 1.7 K the hysteresis is absent. For clarity, the $I$–$V$ curve at 0.7 K shown here was measured on Device 4 (D4); all other data are from D1 unless otherwise specified. (e) Critical current $I_{\mathrm{c}}$ (Red curve) and normal-state resistance $R_{\mathrm{n}}$ (Blue curve) as a function of gate voltage $V_g$ at $T = 1.7$ K. CNP$^1$ (CNP$^2$) denotes the charge-neutrality point of peripheral MLG and MLG within the junction, respectively. (f) The differential resistance (d$V$/d$I$) as a function of DC bias, $I_{\rm DC}$. The d$V$/d$I$ curve shows robust superconducting pleatue with large crticial current and clear gate-tunable $I_{\rm c}$. The $I_{\rm c}$ can be extracted by setting a threshold resistance.
• Figure 2: THz photoresponse and power-dependent suppression of the critical current.(a) Optical setup. THz source is collimated by convex lens, co-aligned with a visible guide beam, and focused onto the sample by scanning a convex lens in $x$--$y$ to maximize the $V_{\rm ph}$ signal. IR-blocking filters at 300 K and 50 K suppress spurious blackbody loading. (b) Differential resistance ${\rm d}V/{\rm d}I$ versus DC bias with (red) and without (blue) THz illumination at $V_g = 2$ V. THz irradiation produces a well-defined suppression $\Delta I_{\mathrm{c}}$ of the critical current. (c) Colour map of ${\rm d}V/{\rm d}I$ versus $I_{\rm DC}$ and THz power density. The superconducting window bounded by the white dashed lines ($I_{\mathrm{c}}^{\pm}$) narrows symmetrically with increasing power and eventually disappers. (d) Extracted $I_{\mathrm{c}}(P)$ versus THz power density at 1.7 K with the independently measured dark calibration curve $I_{\mathrm{c}}(T)$.
• Figure 3: Photovoltage, responsivity, and heat-transport analysis.(a) I-V characteristics of GJJ with THz (3.5 THz) ON and OFF. The suppression of $I_{\mathrm{c}}$ is demonstrated. The $V_{\rm ph}$ is shown in the inset. (b)$V_{\mathrm{ph}}$ versus $I_{\rm DC}$ measured at 0.14 THz, 2.5 THz, and 3.5 THz. All three frequencies produce qualitatively identical bias-current profiles; amplitude differences reflect the varying source output powers. (c)$V_{\mathrm{ph}}$ at 0.14 THz versus $I_{\rm DC}$ at three gate voltages for fixed incident THz power. The response is maximized near CNP$^1$, where the vanishing carrier density minimizes the electronic heat capacity of the peripheral MLG. (d)$V_{\rm ph}$ at 3.5 THz versus $I_{\rm DC}$ and $T$. The amplitude decreases with increasing temperature, consistent with a Josephson origin. The region without $V_{\rm ph}$, bounded by dashed lines, corresponds to intact superconductivity under THz irradiation. (e)$V_{\mathrm{ph}}$ versus power density and corresponding calibrated absorbed power $P_{\mathrm{abs}}$ (see text). The linear fit gives a voltage responsivity $R_\mathrm{v} = 87.6$ kV W$^{-1}$. (f) Electron-temperature rise $\Delta T_{\mathrm{e}} = T_{\mathrm{e}} - T_{\rm }$ extracted from the $I_{\mathrm{c}}$ suppression, plotted versus $P_{\mathrm{abs}}$ at three gate voltages. The response is weakest in the heavily electron-doped regime ($V_g = +2$ V) where the heat capacity is largest. (g) The calculated theoretical noise-equivalent power (NEP) due to thermal fluctuations. (h) Polarization dependence of $V_{\mathrm{ph}}$: THz polarization parallel and perpendicular to the bow-tie antenna yield nearly equal amplitudes, indicating that direct free-carrier absorption in the graphene antenna wings dominates over resonant antenna coupling.
• Figure 4: The schematic of heat transport in our graphene JJ. Heat generated in graphene leads diffuses to the junction to and to phonons through electron-phonon coupling.