More Like this

1. Layer‐Dependent Phonon Polaritons in hBN Resolved by Photo‐Induced Force Spectroscopy

   https://doi.org/10.1002/advs.76087  

   Hajarian, Amin; Seo, Jiwoo; Nam, SungWoo (June 2026, Advanced Science)

   ABSTRACT Hexagonal boron nitride (hBN) is a widely studied van der Waals material that supports highly confined phonon polaritons—hybrid light‐matter quasiparticles arising from the coupling of infrared photons with optical phonons. While prior works—primarily using scattering‐type near‐field optical microscopy (s‐SNOM)—have explored the effect of hBN layer number on the wavelength and propagation characteristics of phonon polaritons, the direct spectroscopic investigation of their resonance energy as a function of layer number has remained largely unaddressed. In this study, we present the first systematic spectroscopic analysis of layer‐dependent phonon polariton resonance frequencies in hBN, using photo‐induced force microscopy (PiFM). By probing hBN with layer numbers ranging from approximately 10 to 60, we uncover clear and opposing trends in the resonance behavior of the two principal Reststrahlen bands. Specifically, we observe that the in‐plane phonon polariton resonance exhibits a blue shift with increasing layer number, while the out‐of‐plane resonance shows a red shift. Additionally, the peak intensity in the out‐of‐plane band increases with layer number, whereas the in‐plane resonance becomes weaker and broader. These findings reveal critical insights into the optical response of hBN across different layer regimes and underscore the importance of layer number as a tuning parameter in nanophotonic applications. 

   more » « less
2. Low Dielectric Medium for Hyperbolic Phonon Polariton Waveguide in van der Waals Heterostructures

   https://doi.org/10.3390/nano14161344  

   Noh, Byung-Il; Reza, Salvio; Hardy, Cassie; Li, Jiahan; Taba, Adib; Mahjouri-Samani, Masoud; Edgar, James H; Dai, Siyuan (August 2024, Nanomaterials)

   Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons—quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently, hexagonal boron nitride (hBN), known for having hyperbolicity in the mid-infrared range, has been used to explore multiple modes with high optical confinement. This opens possibilities for practical polaritonic nanodevices with subdiffractional resolution. However, polariton waves still face exposure to the surrounding environment, leading to significant energy losses. In this work, we propose a simple approach to inducing a hyperbolic phonon polariton (HPhP) waveguide in hBN by incorporating a low dielectric medium, ZrS2. The low dielectric medium serves a dual purpose—it acts as a pathway for polariton propagation, while inducing high optical confinement. We establish the criteria for the HPhP waveguide in vdW heterostructures with various thicknesses of ZrS2 through scattering-type scanning near-field optical microscopy (s-SNOM) and by conducting numerical electromagnetic simulations. Our work presents a feasible and straightforward method for developing practical nanophotonic devices with low optical loss and high confinement, with potential applications such as energy transfer, nano-optical integrated circuits, light trapping, etc. 

   more » « less
3. Ultrabroadband Spacetime Nanoscopy of Terahertz Polaritons in a van der Waals Cavity

   https://doi.org/10.1002/smll.202505899  

   Wehmeier, Lukas; Xu, Suheng; Chen, Xinzhong; Yu, Shang‐Jie; Lin, Yuchen; Dai, Zhenbing; Wang, Heng; Yao, Helen; Hu, Jenny; Homes, Christopher C; et al (February 2026, Small)

   ABSTRACT Guiding, storing, and processing light at the nanoscale hinges on understanding how polaritons — hybrid quasiparticles of light and matter — propagate and interfere in both space and time. This work introduces a synchrotron‐based technique, SYnchrotron SpaceTimE Mapping (SYSTEM), which captures real‐time evolution of polariton wave packets with ∼10 nm spatial and sub‐100 fs temporal resolution across an ultrabroadband 5–50 THz range. Here, SYSTEM directly visualizes the creation, interference, and decay of multiple high‐quality Fabry‐Pérot phonon polariton cavity modes in an α‐MoO₃microcavity. These real‐space, real‐time observations reveal wave‐packet dynamics and cavity resonances with record‐high quality factors (Q ≈ 100) in the single‐digit terahertz regime near 9 THz. SYSTEM thus offers a powerful and broadly applicable platform for probing and engineering ultraslow, deeply subwavelength polaritons, opening new avenues for tailoring light–matter interactions and advancing next‐generation THz nanophotonic technologies. 

   more » « less
4. Tunable Phonon Polariton Hybridization in a Van der Waals Hetero‐Bicrystal

   https://doi.org/10.1002/adma.202401349  

   Wehmeier, Lukas; Yu, Shang‐Jie; Chen, Xinzhong; Mayer, Rafael A; Xiong, Langlang; Yao, Helen; Jiang, Yue; Hu, Jenny; Janzen, Eli; Edgar, James H; et al (August 2024, Advanced Materials)

   Abstract Phonon polaritons, the hybrid quasiparticles resulting from the coupling of photons and lattice vibrations, have gained significant attention in the field of layered van der Waals heterostructures. Particular interest has been paid to hetero‐bicrystals composed of molybdenum oxide (MoO₃) and hexagonal boron nitride (hBN), which feature polariton dispersion tailorable via avoided polariton mode crossings. In this work, the polariton eigenmodes in MoO₃‐hBN hetero‐bicrystals self‐assembled on ultrasmooth gold are systematically studied using synchrotron infrared nanospectroscopy. It is experimentally demonstrated that the spectral gap in bicrystal dispersion and corresponding regimes of negative refraction can be tuned by material layer thickness, and these results are quantitatively matched with a simple analytic model. Polaritonic cavity modes and polariton propagation along “forbidden” directions are also investigated in microscale bicrystals, which arise from the finite in‐plane dimension of the synthesized MoO₃micro‐ribbons. The findings shed light on the unique dispersion properties of polaritons in van der Waals heterostructures and pave the way for applications leveraging deeply sub‐wavelength mid‐infrared light‐matter interactions. 

   more » « less
5. Light people: Professor Mengkun Liu

   https://doi.org/10.1038/s41377-026-02211-x  

   Zheng, Wenjun; Guo, Siqiu (December 2026, Light: Science & Applications)

   Editorial At the frontier where light meets quantum matter, Professor Mengkun Liu has built a distinctive research program that integrates advanced optical nanoscopy, quantum materials, and emergent light–matter interactions. As a Professor in the Department of Physics and Astronomy at Stony Brook University, he leads the Ultrafast & Near-field Infrared Laboratory (UNI-Lab), where his group pioneers infrared-to-terahertz near-field spectroscopy under extreme conditions, enabling direct visualization of polaritons, collective excitations, and quantum phenomena at the nanoscale. Over the past decade, Professor Liu and his collaborators have not only made fundamental discoveries in quantum materials and nanophotonics but have also reshaped experimental capabilities through instrument innovation - ranging from magneto-near-field optical microscopy to synchrotron-based infrared nanoscopy. In this interview, he reflects on the scientific ideas that drive his research, the philosophy behind building a creative research group, and his perspective on the future of optical science at the quantum frontier. 

   more » « less