Efficient photo-Nernst terahertz emission in single heavy-metal films

Abstract

State-of-the-art metallic terahertz (THz) emitters rely predominantly on spintronic heterostructures, where heavy metals serve as passive spin-to-charge converters. Here, we demonstrate efficient THz radiation from standalone Pt nanofilms at cryogenic temperatures and under external magnetic fields. The governing mechanism is identified as the ultrafast photo-Nernst effect, wherein a transient thermal gradient drives a transverse charge current. The THz emission polarity is directly dictated by the sign of the Nernst coefficient, as verified by the phase reversal observed between Pt and W or Ta. Remarkably, both thickness scaling and alloying-induced suppression of thermal conductivity independently amplify the single-layer emission to levels comparable with benchmark spintronic bilayers. These findings redefine the established role of heavy metals from passive spin-sinks to active THz emitters, uncovering a universal emission paradigm applicable across diverse spintronic and quantum materials.

Figures (4)

• Figure 1: Magnetic-field-induced THz emission from a single Pt film.a Schematic of the experimental geometry for THz emission spectroscopy. b Representative time-domain THz waveforms emitted from the single Pt film at 10 K under external magnetic fields, alongside a reference signal from a Pt(3 nm)/CoFeB(3 nm) heterostructure scaled by a factor of 0.1 for amplitude comparison. c Corresponding Fourier-transformed amplitude spectra of the THz transients in (b). d THz peak amplitude as a function of external magnetic field at 10 K. The solid line indicates the fit based on the semiclassical Drude-Boltzmann transport model. The dashed line represents a linear fit to the low-field data ($<$ 3 T), extrapolated to highlight the sublinear deviation.
• Figure 2: Symmetry signatures and pump-fluence dependence of the THz emission.a Time-domain THz waveforms measured at 10 K for opposite magnetic-field directions, with the optical pump incident from the film side (+n) and substrate side (-n). The THz polarity reverses upon flipping either the magnetic field or the excitation geometry. b The THz electric field ($E_x$) detected parallel to $\bm{B}$ at 10 K and 7 T, revealing a negligible THz emission. c THz transients generated by linearly polarized (LP), right-circularly polarized (RCP), and left-circularly polarized (LCP) pump pulses. The inset displays the THz amplitude as a function of the linear polarization angle from $0^\circ$ to $90^\circ$. d Pump fluence dependence of THz emission for bare Pt film and the Pt$_{0.8}$Ti$_{0.2}$ alloy. The THz amplitude of the alloy is scaled down by a factor of 3 for clarity. The solid lines indicate fits to the empirical saturation model.
• Figure 3: Material dependence and thermal evolution of the photo-Nernst emission.a THz amplitudes from various 5-nm-thick single heavy-metal films measured at 10 K and 7 T. Red and blue symbols correspond to front-side and back-side optical pump excitation, respectively. The gray bars represent the electrical conductivity of each metal. b Temperature dependence of the THz amplitude for 5-nm-thick Pt, Pt$_{0.8}$Ti$_{0.2}$ alloy and W films. The amplitude for W is scaled by a factor of -1 for direct comparison.
• Figure 4: Thickness optimization of the Pt film.a Time-domain THz waveforms emitted from Pt films of varying thicknesses, recorded at 10 K and 7 T. The transisents are horizontally offset for clarity. b Left axis: THz peak amplitude (circles) as a function of the Pt film thickness. The solid curve represents a fit accounting for the optical pump absorption and the effective THz conductive screening. Right axis: Independently measured THz transmittance (squares), exhibiting a monotonic decay with increasing film thickness.