Mid-infrared nanophotonics can be realized using sub-diffractional light localization and field enhancement with surface phonon polaritons in polar dielectric materials. We experimentally demonstrate second harmonic generation due to the optical field enhancement from critically coupled surface phonon polaritons at the 6H-SiC-air interface, employing an infrared free-electron laser for intense, tunable, and narrowband mid-infrared excitation. Critical coupling to the surface polaritons is achieved using a prism in the Otto geometry with adjustable width of the air gap, providing full control over the excitation conditions along the polariton dispersion. The calculated reflectivity and second harmonic spectra reproduce the full experimental data set with high accuracy, allowing for a quantification of the optical field enhancement. We also reveal the mechanism for low out-coupling efficiency of the second harmonic light in the Otto geometry. Perspectives on surface phonon polariton-based nonlinear sensing and nonlinear waveguide coupling are discussed.
Local control and lateral nanofocusing of hyperbolic phonon polaritons
Korea Advanced Institute of Science and Technology (KAIST)（韩国科学技术院）；Department of Physics, University of Oviedo（奥维耶多大学物理系）；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）；Universidad del Pais Vasco/Euskal Herriko Unibertsitatea (UPV/EHU)（巴斯克地区大学）；Department of Biomicrosystem Technology, Korea University（韩国大学生物微系统技术系）；KU-KIST Graduate School of Converging Science and Technology, Korea University（韩基融合科学技术研究生院）；Center of Research on Nanomaterials and Nanotechnology, CINN (CSIC-Universidad de Oviedo)（纳米材料与纳米技术中心（西班牙国家科学研究委员会-奥维耶多大学））；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯科奎，巴斯克科学基金会）
Measuring momentum-resolved dissipation of phonon-polaritons in LiNbO$_3$ with terahertz driving
Brigham Young University（杨百翰大学）；ETH Zurich（苏黎世联邦理工学院）；Sapienza University of Rome（罗马第一大学）；Université de Strasbourg（斯特拉斯堡大学）；Université Paris Cité（巴黎西岱大学）；Paul Scherrer Institut（保罗谢尔研究所）
The exploitation of phonon-polaritons in nanostructured materials offers a pathway to manipulate infrared (IR) light for nanophotonic applications. Notably, hyperbolic phonons polaritons (HP2) in polar bidimensional crystals have been used to demonstrate strong electromagnetic field confinement, ultraslow group velocities, and long lifetimes (~ up to 8 ps). Here we present nanobelts of α-phase molybdenum trioxide (α-MoO3) as a low-dimensional medium supporting HP2 modes in the mid- and far-IR ranges. By real-space nanoimaging, with IR illuminations provided by synchrotron and tunable lasers, w…
Polaritons can provide strong optical field enhancement allowing to boost light-matter interaction. Here, we experimentally observe enhancement of mid-infrared second-harmonic generation (SHG) using grating-coupled surface phonon polaritons of the 6H-SiC surface. In our experiment, we measure the SHG along the polariton dispersion by changing the incidence angle of the excitation beam. We observe hybridization between the propagating surface phonon polaritons and localized plasmon resonances in the gold grating, evidenced by the modification of the polariton dispersion as we change the area ra…
Near-field mapping has been widely used to study hyperbolic phonon-polaritons in van der Waals crystals. However, an accurate measurement of the polaritonic loss remains challenging because of the inherent complexity of the near-field signal and the substrate-mediated loss. Here we demonstrate that large-area monocrystalline gold flakes, an atomically-flat low-loss substrate for image polaritons, provide a platform for precise near-field measurement of the complex propagation constant of polaritons in van der Waals crystals. As a topical example, we measure propagation loss of the image phonon…
Intersubband polariton -- LO phonon interaction in mid-infrared non-dispersive cavities: experimental demonstration of spontaneous scattering and perspectives towards polariton lasing
Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Here, we study the much less explored coupling between lattice vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Remarkably, strong coupling is achieved for hBN layers as thin as 10 nm. Further, the ul…
Fast Programming of In-Plane Hyperbolic Phonon Polariton Optics Through van der Waals Crystals using the Phase-Change Material In3SbTe2
RWTH Aachen University（亚琛工业大学）；Juelich-Aachen Research Alliance (JARA-FIT)（于利希-亚琛研究联盟）；University of Oviedo（奥维耶多大学）；Center of Research on Nanomaterials and Nanotechnology, CINN (CSIC-Universidad de Oviedo)（纳米材料与纳米技术研究中心，CINN（西班牙国家科研委员会-奥维耶多大学））；Istituto Italiano di Tecnologia, Center for Biomolecular Nanotechnologies（意大利理工学院，生物分子纳米技术中心）
Phonon polaritons in natural anisotropic crystals hold great promise for infrared nano-optics. However, the direct electrical control of these polaritons is difficult, preventing the development of active polaritonic devices. Here we propose the heterostructures of graphene on a biaxial crystal (α-phase molybdenum trioxide) slab and theoretically study the hybridized plasmon-phonon polaritons with dependence on the Fermi level of graphene from three aspects: dispersion relationships, iso-frequency contours, and the quantum spin Hall effects. We demonstrate the distinct wavelength tunability of…
Designing and realizing switchable cavities at the infrared and THz frequencies is crucial for achieving novel integrated photonic circuits. Semiconductors like SiC support the propagation of surface phonon polaritons with the ability to tune carrier concentrations via doping. In this manuscript, we investigate the tunability of coupled surface-phonon-plasmon polariton waves inside an elliptical cavity build from a metal-semiconductor heterostructure. We theoretically study the impact of the extra n- and p-carrier doping on the resonant modes of the cavity and we show that for certain cavity m…
We present combined experimental and numerical work on light-matter interactions at nanometer length scales. We report novel numerical simulations of near-field infrared nanospectroscopy that consider, for the first time, detailed tip geometry and have no free parameters. Our results match published spectral shapes of amplitude and phase measurements even for strongly resonant surface phonon-polariton (SPhP) modes. They also verify published absolute scattering amplitudes for the first time. A novel, ultrabroadband light source enables near-field amplitude and phase acquisition into the far-in…
The coherence of polaritons plays a fundamental role in numerous recent experimental observations, from strong coupling to Bose-Einstein condensation, but accessing the coherence of polaritons is a difficult task due to the high energy of plasmons and excitons. However, surface phonon polaritons offer similar promises at much lower energy. Here, we demonstrate the possibility to use visible ultrafast lasers to generate and time resolve coherent surface phonon polaritons in nanowires. We show that these modes are generated through the ultrafast screening of the surface depletion field. By compa…
Coherent phonon-polaritons have attracted a considerable amount of interest owing to their relevance to nonlinear optics and terahertz (THz)-wave emissions. Therefore, it is important to analyze the THz electric-fields of phonon-polaritons. However, in the majority of previous measurements, only a single component of the THz electric field was detected. In this paper, we demonstrate that pump-probe electro-optical imaging measurements using the Stokes parameters of probe polarization enable the phase-resolved selective detection of THz electric-field components that are associated with the pho…
Generalized 4 $\times$ 4 Matrix Formalism for Light Propagation in Anisotropic Stratified Media: Study of Surface Phonon Polaritons in Polar Dielectric Heterostructures
Hyperbolic phonon polaritons (HPhPs) in hexagonal boron nitride (hBN) enable the direct manipulation of mid-infrared light at nanometer scales, many orders of magnitude below the free-space light wavelength. High resolution monochromated electron energy-loss spectroscopy (EELS) facilitates measurement of excitations with energies extending into the mid-infrared while maintaining nanoscale spatial resolution, making it ideal for detecting HPhPs. The electron beam is a precise source and probe of HPhPs, that allows us to perform novel experiments to observe nanoscale confinement in HPhP structur…
Hyperbolic phonon polaritons (HPhPs) in orthorhombic-phase molybdenum trioxide ($α$-MoO3) show in-plane hyperbolicity, great wavelength compression and ultra-long lifetime, therefore holding great potential in nanophotonic applications. However, its polaritonic response in the far-infrared (FIR) range has long remained unexplored due to challenges in experimental characterization. Here, using monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), we probe HPhPs in $α$-MoO3 in both mid-infrared (MIR) and FIR frequencies and correlate their …
In recent years there has been significant fundamental research into phonon polaritons, owing to their ability to compress light to extremely small dimensions, low-losses, and ability to support anisotropic propagation. In this perspective, after briefly reviewing the present state of mid-infrared optoelectronics, we will assess the potential of phonon-polariton based nanophotonics for infrared (3-100$\mathrmμ$m) light sources, detectors and modulators. These will operate in the Reststrahlen region where conventional semiconductor light sources become ineffective. Drawing on the results from t…
Polar van der Waals (vdW) crystals that support phonon polaritons have recently attracted much attention because they can confine infrared and terahertz (THz) light to deeply subwavelength dimensions, allowing for the guiding and manipulation of light at the nanoscale. The practical applications of these crystals in devices rely strongly on deterministic engineering of their spatially localized electromagnetic field distributions, which has remained challenging. This study demonstrates that polariton interference can be enhanced and tailored by patterning the vdW crystal α-MoO3 into microstruc…
We theoretically propose and study in-plane anisotropic acoustic phonon polaritons (APhPs) based on a layered structure consisting of a monolayer (or few layers) α-phase molybdenum trioxide (α-MoO3) sandwiched between two metal layers. We find that the APhPs in the proposed sandwiched structures are a canalization (highly directional) electromagnetic mode propagating along with the layers and at the same time exhibit extreme electromagnetic-field confinement surpassing any other type of phonon-polariton modes. When a double layer of α-MoO3 is sandwiched by two Au layers, twisting the two α-MoO…
Highly confined and low-loss hyperbolic phonon polaritons (HPhPs) sustained in van der Waals crystals exhibit outstanding capabilities of concentrating long-wave electromagnetic fields deep to the subwavelength region. Precise tuning on the HPhP propagation characteristics remains a great challenge for practical applications such as nanophotonic devices and circuits. Here, we show that by taking advantage of the varying air gaps in a van der Waals α-MoO3 crystal suspended gradiently, it is able to tune the wavelengths and dampings of the HPhPs propagating inside the α-MoO3. The results indicat…
Resonant enhancement of second harmonic generation in the mid-infrared using localized surface phonon polaritons in sub-diffractional nanostructures
我们报告的强烈增强的中红外二次谐波产生（SHG）从SiC纳米柱由于共振激发的局部表面声子极化激元内的Reststrahlen带。的SHG峰的幅度在的谐振频率超过描述的场的本地化程度和分散的线性和非线性光学SiC性能的经验强烈的依赖性。比较结果为相同的纳米结构制成的4 H和6 H SiC多型体，我们证明了局域表面声子极化激元与带折叠弱声子模式的各向异性晶体的相互作用。在6 H-SiC中激发带折叠声子的区域内和区域外调谐双折射模式，当两种模式耦合时，我们观察到已经双折射增强的SHG输出的显著增加。设想这种相互作用的中红外非线性纳米声学的展示功能之一，我们讨论了其在红外光谱范围内具有所需性能的非线性光学材料的有效工程的前景。
