The starting point for the study is a well-established measurement tool known as terahertz-emission spectroscopy. In this technique, a femtosecond optical pulse strikes a material and triggers a burst of electrical current; that transient current, in turn, radiates a pulse of terahertz electromagnetic waves into the far field. By analyzing the amplitude and polarization of the emitted terahertz radiation, researchers can reconstruct the magnitude and direction of the underlying currents, making the method a genuinely vectorial probe of ultrafast carrier motion. The catch is diffraction. Like any far-field optical approach, terahertz-emission spectroscopy cannot resolve spatial features smaller than roughly the wavelength of light, limiting its spatial resolution to the micrometer range and blinding it to local current distributions within or near the excitation spot.

Near-field scanning probes offer a way around this limit. In terahertz-emission nanoscopy, a sharp metallic tip of a scattering-type near-field optical microscope acts as a terahertz nano-antenna, confining the interaction to a region set by the tip apex — ideally tens of nanometers. The approach has already yielded striking local insights into ultrafast out-of-plane photocurrents in two-dimensional materials, semiconductors, and metallic nanostructures. But a major obstacle remained: many of the most technologically important ultrafast phenomena involve currents flowing in the plane of a thin film, coupled to the flow of electron spin. Because near-field tips are primarily sensitive to electric fields pointing perpendicular to the sample surface, it has been unclear whether — and how — terahertz-emission nanoscopy could capture these in-plane, spin-driven charge dynamics at all.

To attack the problem, the team built a clever model system: a spintronic terahertz emitter sputtered directly onto the tip of a single-mode optical fiber. The emitter is a metallic trilayer, with a 1.8-nanometer-thick ferromagnetic CoFeB layer sandwiched between 2-nanometer films of tungsten and platinum, two heavy metals whose spin-Hall angles are large but opposite in sign. When a femtosecond laser pulse travels down the fiber and hits the stack, it triggers ultrafast demagnetization of the ferromagnet and injects a burst of spin current into the adjacent heavy metals. The inverse spin Hall effect then converts those spin currents into transverse, in-plane charge currents — and because tungsten and platinum have opposite spin-Hall angles, the two charge currents add constructively, radiating a strong terahertz pulse. Mounting this fiber-coupled emitter on a three-axis piezo stage beneath the stationary tip of a near-field microscope, and raster-scanning it laterally, allowed the researchers to map the terahertz dynamics in real space.

The fiber coupling is more than a packaging convenience; it is the key to making the images quantitatively trustworthy. In conventional setups, moving the tip changes the setup response function with respect to the terahertz detector, while moving the sample changes the optical pump-path length, the excitation profile, and the excitation position. In the new design, the single-mode fiber guarantees a scan-position-independent Gaussian excitation profile with sub-wavelength dimensions, and the flexible fiber maintains a constant pump-probe delay and an invariant detection response throughout the scan. Because the optical path length barely changes as the fiber bends, the temporal delay axis is identical for every pixel of the image, so the spatiotemporal evolution of the terahertz signal can be filmed directly without post-hoc corrections of relative timing.

The resulting movies delivered a surprise. The near-field terahertz signal did not peak where the pump intensity — and hence the current density — was largest, at the center of the Gaussian excitation spot on the fiber core. Instead, the signal traced a counterintuitive dipolar pattern: two lobes of opposite sign centered on the fiber core, with the dipole axis oriented perpendicular to the in-plane magnetization of the ferromagnetic layer. The researchers interpret this pattern using the analytical solution for the electric field surrounding a point-like in-plane dipole embedded in a thin metallic film. The out-of-plane field component decays as one over the distance squared, multiplied by the cosine of the angle relative to the dipole axis — a two-lobe structure with a central zero line, exactly what the experiment shows. Because the charge-current dipole generated by inverse spin Hall conversion is expected to lie orthogonally to the magnetization, the observed orientation confirms that the images are indeed the out-of-plane signature of the spin-driven in-plane charge currents.

This mechanism also explains a persistent ambiguity in the field. Previous terahertz-emission nanoscopy studies had managed to detect signals from spintronic emitters despite the probe’s supposed lack of in-plane sensitivity. The new analysis suggests that in those free-space excitation experiments, the laser spot was likely positioned off-center relative to the tip, allowing the tip to pick up the finite out-of-plane fields at the edges of the in-plane current distribution. In the frequency domain, the team measured the spectral amplitude and phase at a center frequency of about one terahertz, consistent with the 70-femtosecond pump pulse duration. The spectral-amplitude map at one terahertz reproduced the dipolar profile, while the spectral phase showed an abrupt, nanoscale pi phase jump across the dipole’s zero line, plus a constant radial gradient — the frequency-domain fingerprint of a propagation delay along the surface, tentatively attributed to a surface plasmon polariton moving along the air-metal interface.

To put the interpretation on rigorous footing, the team turned to finite-element simulations of the full experimental geometry, including the fiber facet, the 6-nanometer metallic film, and the scanning probe, driven by a Gaussian-shaped in-plane current density. The agreement with experiment is remarkably close: simulated and measured spectral-amplitude maps at one terahertz match for scan step sizes of one micrometer and 300 nanometers, and center line scans reproduce the measured peak positions and the dip shape, with a width of roughly 400 nanometers. The simulations also show that the experiment sits near an optimum: the 10.5-micrometer mode-field diameter of the single-mode fiber maximizes the out-of-plane field generation, while the film conductivity of about one million siemens per meter lies in a regime where the signal amplitude has saturated, making the measurements robust against moderate variations in film quality. Linear fits to the phase gradient yield a subluminal phase velocity of about sixty percent of the speed of light, in line with known surface plasmon-polariton modes.

The implications stretch well beyond one device. Spintronic terahertz emitters are themselves candidate components for future terahertz data-processing technology, and understanding their current distributions at the nanoscale is a step toward engineering them more precisely. More broadly, the demonstration shows that terahertz-emission nanoscopy can serve as a fully vectorial probe, combining its established sensitivity to out-of-plane currents with newly explained access to in-plane, spin-driven charge transport. The authors foresee applications across heterogeneous material systems and nanostructures, including magnetic nanodomains in emerging altermagnets and two-dimensional magnets, as well as photocurrent heterogeneities in a wide range of systems on their native spatiotemporal scales. From ultrafast spin transport in atomically thin materials to spin-to-charge conversion at metal interfaces, the fastest and smallest currents in modern physics may soon be routinely filmed — one femtosecond and a few nanometers at a time.

Subject of Research: Nanoscale spatiotemporal imaging of ultrafast spintronic charge currents using terahertz emission nanoscopy

Article Title: Spatiotemporal terahertz emission nanoscopy of spintronic photocurrent

Article References: Spatiotemporal terahertz emission nanoscopy of spintronic photocurrent. (n.d.). https://doi.org/10.1038/s41377-026-02468-2

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02468-2

Keywords: terahertz emission nanoscopy, spintronics, spintronic terahertz emitter, near-field microscopy, inverse spin Hall effect, ultrafast spin currents, femtosecond dynamics, nanoscale imaging, surface plasmon polariton, CoFeB, spin-to-charge conversion, SNOM

Top-emitting OLEDs are the workhorses of high-resolution active-matrix displays because their light exits away from the substrate, decoupling the emitting aperture from the transistor backplane beneath. They also lend themselves naturally to a Fabry–Perot microcavity, formed between a reflective bottom electrode and a semitransparent top cathode. The cavity narrows the electroluminescence spectrum and concentrates emission toward the forward direction, boosting frontal efficiency and color purity. But the same physics that sharpens the spectrum also makes extraction design unforgiving: much of the generated light ends up confined in waveguide modes traveling laterally through the organic stack and in surface plasmon polaritons hugging the metal electrodes. Fixing the air-side interface alone cannot recover those internal losses.

Conventional internal outcoupling structures, such as wrinkles, corrugations, and strongly curved microlens textures, scatter trapped modes back toward useful directions, but they carry a hidden cost. When the underlying texture is steep, the organic layers deposited on top fail to follow it conformally, so the local optical thickness of the cavity varies across the device and the resonance condition drifts from point to point. Stronger scattering then does not translate into a better device, because the cavity-defined forward emission benefit is simultaneously eroded. The key insight of the new work is that the geometry of an internal extraction layer must be judged not just by how much light it scatters but by how gently it does so.

The researchers call their solution a gentle internal corrugation layer, or GICL. Atomic force microscopy over a 50 by 50 micrometer area revealed a peak-to-valley height of about 1.12 micrometers with root-mean-square roughness of 144.5 nanometers, but the defining feature is the slope: representative surface angles of only 2.1 to 6.9 degrees along one axis and 1.8 to 3.5 degrees along the other, spread over lateral spans of nearly 9 micrometers. Cross-sectional electron microscopy showed the entire multilayer stack following this soft undulation without discontinuities. The contrast with reference structures is instructive. A steep imprinted random microlens array reached surface angles of 25.3 degrees with a peak-to-valley height of 7.87 micrometers, while one wrinkle reference achieved low roughness yet still exhibited angles above 53 degrees, demonstrating that roughness amplitude alone says little about geometric severity.

Finite-difference time-domain simulations confirmed that gentler is genuinely better. Among the compared structures, GICL yielded the highest relative total simulated power, outperforming both wrinkle references and the steep microlens array, even though the latter produced far stronger local field perturbation. Ray tracing sharpened the picture: GICL directed 77.3 percent of input power into the forward collection region, versus 43.5 percent for the microlens array, which instead diverted light into wide-angle, internally reflected, and backward trajectories. Under identical conditions, the calculated light extraction efficiency rose from 37.6 percent for the planar microcavity device to 74.1 percent with GICL, an increase of nearly a factor of two. An intermediate-index clear resin with a refractive index of 1.59 was applied at the outer surface to soften the air interface and let the redistributed light actually escape.

The emissive system matters as much as the geometry. In a microcavity, only the spectral portion of the emitter that overlaps the resonance band contributes efficiently to the enhanced external output, so a broad emitter wastes much of its spectrum on off-resonant wavelengths. The team used a phosphor-assisted thermally activated delayed fluorescence sensitization scheme, in which a TADF host and a phosphorescent assistant funnel excitation energy to a terminal multi-resonance TADF emitter called tCzphB-Fl, producing green emission with a full width at half maximum of only about 19 to 22 nanometers. When the microcavity was formed, the narrowband system’s peak shifted just 2 nanometers and its FWHM narrowed from 22 to 19 nanometers, whereas a conventional phosphorescent comparator shifted 13 nanometers and narrowed from 65 to 25 nanometers, evidence that most of the narrowband emission already sat inside the resonance band.

Device results bore out the combined strategy. All GICL devices shared the same 2.25-volt turn-on voltage and essentially unchanged current density, ruling out electrical effects, and the forward spectra showed only minor changes, ruling out spectral reshaping. Under matched resin conditions, adding GICL raised the external quantum efficiency from 34.8 to 69.2 percent, current efficiency from 302.7 to 378.4 candela per ampere, and power efficiency from 199.3 to 396.7 lumens per watt, with maximum luminance reaching nearly 120,000 candela per square meter. Across five samples the efficiencies averaged 68.2 plus or minus 0.7 percent, indicating good reproducibility. The angular color shift at 60 degrees off-axis also fell from 0.0197 to 0.0086 in CIE 1976 coordinates, and the luminance distribution became more Lambertian, both valuable for display uniformity.

Polarization-resolved measurements identified where the recovered light had been hiding. Because waveguide modes are predominantly transverse-electric like and surface plasmon polaritons are transverse-magnetic like, separating the s- and p-polarized emission provides a mode-sensitive diagnostic. Both polarized components, and the non-polarized total, increased across the viewing-angle range in the corrugated device, confirming genuine extraction rather than mere redistribution between channels. Quantitative modal analysis showed the outcoupled fraction rising from 29.7 to 38.6 percent, with the waveguide fraction dropping from 40.0 to 32.0 percent and the plasmon-related fraction changing only modestly. The roughly 30 percent simulated gain in outcoupled power matched the approximately 25 percent uncorrected device-level increase closely, lending quantitative coherence to the extraction interpretation.

Notably, the same corrugation helped the broad phosphorescent system far less, lifting its current efficiency by only about 15.7 percent compared with 25.0 percent for the narrowband system, a difference the authors attribute to spectral overlap with the cavity resonance rather than to bandwidth alone, since the two emissive chemistries differ in other respects as well. The gains did come with a small price: efficiency roll-off at high brightness was moderately larger in the corrugated devices, possibly reflecting local current crowding on the textured surface, and operational lifetime at 1,000 candela per square meter was slightly shorter, though still comparable at about 101 hours to 95 percent of initial luminance. The broader lesson stands: in microcavity OLEDs, internal texture geometry, outer-interface index matching, and emitter-resonance alignment must be engineered together, and gentleness, it turns out, extracts more light than force.

Subject of Research: Cavity-compatible internal light extraction in top-emitting microcavity OLEDs using a gentle corrugation layer and resonance-matched narrowband emission

Article Title: Cavity‐Compatible Light Extraction in Top‐Emitting Microcavity OLEDs via a Gentle Internal Corrugation Layer and Resonance‐Matched Narrowband Emission

Article References: Kim, Y. R., Nimbalkar, A., & Suh, M. C. (2026). Cavity‐Compatible Light Extraction in Top‐Emitting Microcavity OLEDs via a Gentle Internal Corrugation Layer and Resonance‐Matched Narrowband Emission. Advanced Science, Article e78013. https://doi.org/10.1002/advs.78013

Image Credits: AI Generated

DOI: 10.1002/advs.78013

Keywords: OLED, microcavity, light extraction, external quantum efficiency, corrugation, TADF, narrowband emission, waveguide modes, surface plasmon polariton, display technology, FDTD simulation, phosphorescence

The research team, comprising Hamid Nadgaran, Mohammad Amin Izadi, and Rahman Nouroozi, built their design around a hybrid plasmonic waveguide loaded with periodically poled lithium niobate, or PPLN. Lithium niobate is prized in photonics for its strong second-order nonlinearity, which enables a process called difference-frequency generation, or DFG. In DFG, an intense pump pulse transfers energy to a weaker signal pulse, boosting its power while, under the right conditions, squeezing its duration dramatically. The catch has always been efficiency: conventional titanium-diffused PPLN waveguides have cross-sections measured in tens of micrometers, which dilutes the optical fields and demands punishing pump powers.

In their earlier theoretical work, the same group showed that a conventional Ti:PPLN waveguide could regenerate a 1550-nanometer signal pulse that had broadened to 300 picoseconds and lost over 90 percent of its energy after traversing 50 kilometers of standard single-mode fiber. The results were impressive, compressing the pulse to 1.5 picoseconds with 10.9 decibels of amplification, but they came at a steep price: a pump pulse of roughly 15 watts of peak power, a 20-millimeter interaction length, and an exotic fifth-order super-Gaussian pump shape that would require elaborate pulse-shaping optics such as spatial light modulators to synthesize in the laboratory.

To break through these limits, the team turned to surface plasmon polaritons, electromagnetic waves that ride the boundary between a metal and a dielectric and squeeze light into volumes far smaller than any purely dielectric waveguide can achieve. Bare plasmonic structures, however, suffer from brutal ohmic losses: when the researchers modeled a lithium niobate channel sitting directly on silver, the pump mode’s attenuation reached 43.8 per millimeter, meaning the pump energy would vanish within a microscopic distance and render the device useless even with a 20-watt pump. The solution was a hybrid architecture, in which an ultrathin low-index spacer separates the nonlinear core from the metal, pushing the optical field away from the absorptive silver surface while preserving deep-subwavelength confinement.

The choice of spacer material proved decisive. Common photonic dielectrics like silica and magnesium fluoride have refractive indices too high for this geometry, allowing too much field to accumulate near the metal. The team instead selected Teflon AF 2400, an amorphous fluoropolymer whose refractive index approaches that of vacuum and which is compatible with nanoscale thin-film deposition. After a systematic optimization sweep varying spacer thickness, channel width, and channel height, the final design emerged: a PPLN core measuring 450 by 600 nanometers, an 80-nanometer Teflon AF 2400 spacer, 300-nanometer-wide Teflon sidewalls for tight lateral confinement, and a monolithic silica top-cladding for mechanical robustness, all resting on a 100-nanometer silver substrate.

Finite-difference simulations of this optimized geometry yielded a nonlinear coupling coefficient of 841.78 per square-root-watt per meter, a thirteenfold enhancement over the 63 achieved in conventional Ti:PPLN waveguides. The confinement statistics are striking: 96 percent of the 775-nanometer pump power resides inside the lithium niobate core, with a mere 0.0048 percent penetrating the silver, a 99.8 percent reduction compared with the bare plasmonic case. At 1550 nanometers, the signal propagates as a hybrid surface plasmon polariton, with 65 percent of its power confined in the nonlinear core and only 0.023 percent reaching the metal. This delicate balance between confinement and loss is what makes the device viable.

The payoff appears in the pulse-processing simulations. The team fed their waveguide a signal pulse that had degraded over 50 kilometers of ITU-T G.652 fiber, arriving broadened to 300 picoseconds, chirped, and stripped of 90 percent of its energy. Driven by a simple Gaussian pump pulse with a peak power of just 6 watts and a 25-picosecond duration, the 5-millimeter device compressed the signal to 8.7 picoseconds, a 97.1 percent reduction in duration, while regenerating its peak power to 4 milliwatts, four times the power of the original transmitted pulse. The energy conversion efficiency reached a factor of 30, and the process simultaneously generated a compressed idler pulse at 1560.13 nanometers, exploiting the phase-conjugate relationship between signal and idler to achieve compression that is independent of the input chirp.

Crucially, all of this was achieved with an ordinary Gaussian pump, the natural output of standard mode-locked lasers, eliminating the need for complex external pulse shaping. The team also verified that parasitic third-order nonlinearities, a known hazard of plasmonic systems given silver’s large nonlinear susceptibility, remain negligible in their design. Because the low-index spacer confines the fields so effectively, the self-phase modulation on the signal amounts to roughly 23.72 millionths of a radian and the cross-phase modulation from the pump to 0.125 radians, both far below the 1-radian threshold at which pulse distortion becomes problematic. The pump intensity inside the core, about 2.8 gigawatts per square centimeter, also sits safely below lithium niobate’s picosecond damage threshold, which routinely exceeds 10 gigawatts per square centimeter.

The improvements over the group’s previous design are substantial on every axis: peak pump power cut by roughly 60 percent, device length reduced by 75 percent, and pump complexity eliminated altogether. The authors caution that real-world fabrication will bring challenges, including lithographic dimensional drift, scattering from metal surface roughness at the silver-Teflon interface, and the demanding requirement of sub-micron periodic poling in thin-film lithium niobate. The extreme nanoscale mode also mismatches standard single-mode fibers, so integrated adiabatic mode converters such as inverse tapers would be needed for efficient coupling. Even so, the 4-milliwatt output provides a generous power margin over the microwatt sensitivities of standard telecom receivers, leaving ample optical budget to absorb coupling and insertion losses.

If experimentalists can realize the design, the implications extend beyond telecom signal regeneration. A platform that combines the strong second-order nonlinearity of PPLN with plasmonic field enhancement in a five-millimeter footprint could serve as a building block for compact wavelength converters, optical phase conjugation modules, and even quantum frequency conversion on chip. For now, the work stands as a rigorous theoretical demonstration that the decades-old trade-off between nonlinear efficiency and device size can be broken, not by choosing between dielectrics and metals, but by engineering the space between them with nanometer precision.

Subject of Research: Simultaneous pulse compression and amplification via difference-frequency generation in an optimized hybrid PPLN-loaded plasmonic waveguide

Article Title: Theoretical investigation of efficient simultaneous pulse compression and amplification in an optimized hybrid PPLN-loaded plasmonic waveguide

Article References: Nadgaran, H., Izadi, M. A., & Nouroozi, R. (2026). Theoretical investigation of efficient simultaneous pulse compression and amplification in an optimized hybrid PPLN-loaded plasmonic waveguide. Results in Optics, Article 101170. https://doi.org/10.1016/j.rio.2026.101170

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101170

Keywords: plasmonics, lithium niobate, PPLN, difference-frequency generation, pulse compression, optical amplification, hybrid waveguide, surface plasmon polariton, optical communications, nonlinear optics, Teflon AF 2400, signal regeneration