This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A collaborative research team has successfully visualized in real space the ultrafast optical modulation of hyperbolic phonon polaritons (HPhPs) in a van der Waals heterostructure composed of hBN and WS2. The research is published in the journal Nano Letters, and was led by Kazuki Kamada of the Institute for Molecular Science (IMS) and Osaka Metropolitan University, along with Dr.
Jun Nishida, assistant professor at IMS, and Takashi Kumagai, associate professor at IMS. HPhPs are hybrid light-matter modes formed through strong coupling between infrared electromagnetic fields and optical phonons in a material. Their hybrid character allows them to coherently transport electromagnetic energy while confining it to spatial scales far below the wavelength of light in free space.
These properties make HPhPs promising for nanoscale light manipulation, high-sensitivity spectroscopy and future nanophotonic technologies. Dynamically controlling HPhPs at ultrafast speeds could provide an important foundation for active nanoscale optical devices. However, directly observing ultrafast control of HPhPs presents a fundamental experimental challenge.
HPhPs are strongly dispersive, meaning that their wavelength changes substantially with the frequency of the incident light. A broadband femtosecond infrared pulse therefore excites HPhPs with many different wavelengths simultaneously. Their interference fringes overlap and become averaged in real-space images, obscuring the underlying propagation patterns.
Simply narrowing the spectrum of the incident pulse is not an ideal solution because doing so would lengthen the pulse and degrade the temporal resolution. To overcome this trade-off, the team introduced a diffraction grating into the detection path of an ultrafast infrared near-field optical microscope. The grating spectrally separates the broadband infrared light scattered from the probe tip of an atomic force microscope before detection.
Because frequency selection is performed during detection rather than by narrowing the incident pulse, the method preserves an approximately 150-femtosecond temporal resolution while achieving a spectral resolution of approximately 10 cm-1 together with nanoscale spatial resolution. The team then applied this new ultrafast nano-imaging technique to van der Waals heterostructures composed of WS2 and hBN. Visible-light pump pulses generated photoexcited charge carriers in WS2, while time-delayed infrared near-field pulses probed the resulting response of HPhPs in the adjacent hBN.
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