Release Summary
Hyperbolic phonon polaritons (HPhPs) are hybrid light–matter waves that can confine infrared electromagnetic fields to spatial scales far below the corresponding free-space wavelength. Controlling HPhPs on femtosecond timescales could provide a foundation for future ultrafast and ultracompact nanophotonic devices.
However, directly imaging such ultrafast dynamics has remained challenging because HPhPs are strongly dispersive and their wavelength changes markedly with frequency. Broadband femtosecond laser pulses therefore excite HPhPs over a range of wavelengths, causing their interference patterns to overlap and wash out in real-space images.
In this study, researchers developed an ultrafast infrared near-field optical microscopy technique that combines nanoscale spatial resolution and an approximately 150-femtosecond temporal resolution with a spectral resolution of about 10 cm⁻¹. By using a diffraction grating to spectrally resolve the light scattered from the microscope tip before detection, the technique enables frequency-selective imaging without sacrificing ultrafast time resolution.
Applying the new technique to a van der Waals heterostructure composed of tungsten disulfide (WS2) and hexagonal boron nitride (hBN), the researchers revealed how photoexcited charge carriers generated in WS2 transiently modulate HPhPs propagating in the adjacent hBN. The researchers observed changes in the HPhP electric-field amplitude and, in structures containing a relatively thick WS₂ layer, a change in the HPhP wavelength.
The researchers say, “The results establish a new platform for observing and controlling nanoscale light propagation on femtosecond timescales, opening new opportunities for ultrafast nanophotonics”.
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