Josue J. Lopez

Efficient plasmonic emission by the quantum Cerenkov effect from hot carriers in graphene

Graphene plasmons have been found to be an exciting plasmonic platform, thanks to their high field confinement and low phase velocity, motivating contemporary research to revisit established concepts in light–matter interaction. In a conceptual breakthrough over 80 years old, Čerenkov showed how charged particles emit shockwaves of light when moving faster than the phase velocity of light in a medium. To modern eyes, the Čerenkov effect offers a direct and ultrafast energy conversion scheme from charge particles to photons. The requirement for relativistic particles, however, makes Čerenkov emission inaccessible to most nanoscale electronic and photonic devices. Here we show that graphene plasmons provide the means to overcome this limitation through their low phase velocity and high field confinement. The interaction between the charge carriers flowing inside graphene and the plasmons enables a highly efficient two-dimensional Čerenkov emission, giving a versatile, tunable and ultrafast conversion mechanism from electrical signal to plasmonic excitation.Quantum Čerenkov Effect from Hot Carriers in Graphene: An Efficient Plasmonic Source Ido Kaminer, Yaniv Tenenbaum Katan, Hrvoje Buljan, Yichen Shen, Ognjen Ilic, Josué J. López, Liang Jie Wong, John D. Joannopoulos, and Marin Soljačić Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge 02139, Massachusetts, USA Physics Department and Solid State Institute, Technion, Haifa 32000, Israel Department of Physics, University of Zagreb, Bijenička c. 32, 10000 Zagreb, Croatia Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075

All-angle negative refraction of highly squeezed plasmon and phonon polaritons in graphene–boron nitride heterostructures

Significance Realizing negative refraction of highly squeezed polaritons is an important step toward the active manipulation of light at the extreme nanoscale. To realize negative refraction, an effective means to tailor the coupling of different polaritons is absolutely necessary yet undeveloped. Here, we predict a viable way to flip the sign of group velocities of hybrid plasmon–phonon–polaritons in graphene–boron nitride (BN) heterostructures. The polaritonic strong coupling enables the all-angle negative refraction phenomena between highly squeezed graphene’s plasmons, BN’s phonon polaritons, and their hybrid polaritons. Due to the combined advantages of tunability, low loss, and ultrahigh confinement provided by these polaritons, graphene–BN heterostructures thus provide fundamental tools to explore the manipulation of light at the extreme nanoscale. A fundamental building block for nanophotonics is the ability to achieve negative refraction of polaritons, because this could enable the demonstration of many unique nanoscale applications such as deep-subwavelength imaging, superlens, and novel guiding. However, to achieve negative refraction of highly squeezed polaritons, such as plasmon polaritons in graphene and phonon polaritons in boron nitride (BN) with their wavelengths squeezed by a factor over 100, requires the ability to flip the sign of their group velocity at will, which is challenging. Here we reveal that the strong coupling between plasmon and phonon polaritons in graphene–BN heterostructures can be used to flip the sign of the group velocity of the resulting hybrid (plasmon–phonon–polariton) modes. We predict all-angle negative refraction between plasmon and phonon polaritons and, even more surprisingly, between hybrid graphene plasmons and between hybrid phonon polaritons. Graphene–BN heterostructures thus provide a versatile platform for the design of nanometasurfaces and nanoimaging elements.

Substrate-Independent Light Confinement in Bioinspired All-Dielectric Surface Resonators

Traditionally, photonic crystal slabs can support resonances that are strongly confined to the slab but also couple to external radiation. However, when a photonic crystal slab is placed on a substrate, the resonance modes become less confined, and as the index contrast between slab and substrate decreases, they eventually disappear. Using the scale structure of the Dione juno butterfly wing as an inspiration, we present a low-index zigzag surface structure that supports resonance modes even without index contrast with the substrate. The zigzag structure supports resonances that are contained away from the substrate, which reduces the interaction between the resonance and the substrate. We experimentally verify the existence of substrate-independent resonances in the visible wavelength regime. Potential applications include substrate-independent structural color and light guiding.

Large Photothermal Effect in Sub‐40 nm h‐BN Nanostructures Patterned Via High‐Resolution Ion Beam

The controlled nanoscale patterning of 2D materials is a promising approach for engineering the optoelectronic, thermal, and mechanical properties of these materials to achieve novel functionalities and devices. Herein, high-resolution patterning of hexagonal boron nitride (h-BN) is demonstrated via both helium and neon ion beams and an optimal dosage range for both ions that serve as a baseline for insulating 2D materials is identified. Through this nanofabrication approach, a grating with a 35 nm pitch, individual structure sizes down to 20 nm, and additional nanostructures created by patterning crystal step edges are demonstrated. Raman spectroscopy is used to study the defects induced by the ion beam patterning and is correlated to scanning probe microscopy. Photothermal and scanning near-field optical microscopy measure the resulting near-field absorption and scattering of the nanostructures. These measurements reveal a large photothermal expansion of nanostructured h-BN that is dependent on the height to width aspect ratio of the nanostructures. This effect is attributed to the large anisotropy of the thermal expansion coefficients of h-BN and the nanostructuring implemented. The photothermal expansion should be present in other van der Waals materials with large anisotropy and can lead to applications such as nanomechanical switches driven by light.