A distinctive dipolar terahertz pattern led researchers to examine how a probe tuned to vertical electric fields can detect currents flowing sideways through an ultrathin metallic emitter.
Paper: Spatiotemporal terahertz emission nanoscopy of spintronic photocurrent. AI-generated abstract conceptual image created using ChatGPT/OpenAI
A recent study in Light: Science & Applications uses spatiotemporal terahertz emission nanoscopy (TEN) to map the electromagnetic signatures of ultrafast, spin-driven charge currents with nanoscale spatial sensitivity and femtosecond temporal resolution.
The researchers combined a fiber-coupled spintronic terahertz emitter with scanning near-field microscopy to investigate the electromagnetic fields generated by localized charge currents.
Addressing the Challenge of Ultrafast Current Imaging
Electric charge and spin transport play a critical role in advanced electronics, spintronic devices, and light-driven energy conversion technologies. Capturing these processes across femtosecond timescales remains a major experimental challenge. Terahertz emission spectroscopy (TES) offers a way to investigate ultrafast charge currents by measuring the terahertz radiation that a material emits after femtosecond laser excitation.
Researchers can analyze the radiation's amplitude and polarization to characterize the underlying current behavior. But optical diffraction limits conventional far-field measurements to micrometer-scale spatial resolution. Terahertz emission nanoscopy addresses this limitation by combining terahertz detection with a sharp metallic probe that amplifies local electromagnetic fields.
In this study, the researchers show how a near-field probe can detect the electromagnetic signatures of currents flowing parallel to a material's surface, extending nanoscale terahertz imaging to in-plane current measurements.
Combining Spintronic Emitters with Near-Field Microscopy
The researchers fabricated a spintronic terahertz emitter by depositing a thin metallic trilayer onto the end of a single-mode optical fiber.
The structure included a 1.8 nm-thick cobalt–iron–boron (CoFeB) ferromagnetic layer between 2 nm tungsten (W) and platinum (Pt) layers. The opposite spin Hall properties of the two heavy metals allow efficient conversion of spin currents into charge currents.
To follow the time evolution, the team integrated the fiber-coupled emitter into a scattering-type scanning near-field optical microscope (SNOM).
A sharp metallic tip positioned above the emitter amplified the local out-of-plane electric-field component sampled by the detector. A three-axis piezoelectric stage moved the emitter beneath the stationary tip. The fiber-coupled pump kept the excitation profile and position, pump–probe delay, and terahertz detection response consistent during scanning.
The team recorded spatially resolved terahertz waveforms using pump–probe measurements. This method compares the response at controlled time delays after excitation, allowing the researchers to reconstruct the signal's temporal evolution at different positions. Initial measurements covered a 30 × 30 µm² area with 1 µm scan steps. Finer scans covered a 10 × 10 µm² area with 300 nm steps.
The researchers complemented these measurements with finite-element electromagnetic simulations in COMSOL Multiphysics. They modeled localized in-plane currents and varied the excitation-beam diameter and film conductivity to determine how these parameters affected the detected fields.
Comparison of the simulated and experimental results helped identify the electromagnetic mechanism responsible for the observed spatial contrast.
Revealing the Electromagnetic Signature of In-Plane Currents
The measurements revealed a distinct dipolar pattern in the spatially resolved terahertz signal. The signal formed two opposing regions with positive and negative amplitudes, separated by a low-signal line. The dipole axis lay perpendicular to the emitter's in-plane magnetization. This pattern showed how the probe could detect currents flowing parallel to the metallic surface while remaining primarily sensitive to out-of-plane electric fields.
The analysis showed that localized in-plane currents generate out-of-plane electric-field components above the film. These fields produce the dipolar contrast recorded by the near-field probe.
An idealized point-dipole model for an effectively infinite thin film showed that the out-of-plane field follows a dipolar spatial distribution and decreases with the inverse square of the distance from the source. This behavior explains why the signal is suppressed at the excitation center, where current density is expected to peak, and why the polarity reverses across the central line.
The measured field provides a spatially resolved electromagnetic signature of the underlying in-plane charge currents. Local current magnitude cannot be read directly from that field map.
The temporal measurements also captured terahertz oscillations on subpicosecond timescales, consistent with the ultrafast response of spintronic emitters. Signals recorded 4 and 14 µm from the emitter center showed propagation delays. The authors tentatively attributed these delays to surface plasmon polaritons, electromagnetic excitations that couple with charge oscillations at a metal–dielectric interface.
Frequency-domain analysis showed a corresponding signature of the propagation delay. The measured spectrum peaked near 1 THz, and the spectral-amplitude map reproduced the dipolar pattern observed in the time domain. The spectral-phase map revealed an abrupt 180-degree phase reversal across the central line, alongside a radial phase gradient associated with propagation delays.
The electromagnetic simulations reproduced the measured spatial patterns, including the signal peaks and central suppression. The central signal suppression had an approximately 400 nm width at the paper's 1/e² level, indicating sensitivity to fine spatial variations in the field.
The simulations also yielded a phase velocity of 60% of the speed of light at 1 THz, comparable to values reported for similar surface plasmon polariton modes. The velocity comparison supports the researchers' tentative interpretation, while the surface plasmon polariton assignment remains unconfirmed.
Additional simulations helped identify experimental parameters that influence signal generation and detection. Film conductivity primarily influenced signal strength rather than its spatial distribution. The field amplitude began to saturate at conductivities above 104 S/m. These results provide practical guidance for refining TEN measurements across different materials and device structures.
The measured profile was slightly broader than the Gaussian-current simulation, possibly because of saturation near the excitation center. The roughly 21-hour acquisition also required correction for periodic timing drift.
Toward Nanoscale Imaging of Spin-Driven Charge Transport
The study clarifies how TEN can detect ultrafast in-plane charge currents driven by spin-to-charge conversion through their out-of-plane electromagnetic fields. The near-field signal is the out-of-plane electric field associated with those currents, so spin transport is inferred through the conversion process.
Agreement between measured and simulated dipolar patterns supports this interpretation but does not directly reconstruct the microscopic spin current.
The technique could extend nanoscale terahertz imaging beyond established applications to out-of-plane photocurrents. Researchers could use it to investigate magnetic nanodomains in altermagnets, examine two-dimensional magnetic materials, and map spatial variations in photocurrents across heterogeneous structures. Such studies could improve understanding of spin-to-charge conversion and ultrafast transport in emerging electronic materials.
By explaining the image-forming mechanism behind TEN measurements of in-plane currents, the work provides a basis for applying the method to a broader range of nanostructures and magnetic materials. TEN could help researchers examine photocurrent variations and spin-to-charge conversion across different local environments. Further work will be needed to establish its performance across different systems.
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