Mid-infrared nanophotonics with hyperbolic phonon polaritons

125 p.El nitruro de boro hexagonal (h-BN) es un material especialmente interesante en óptica subdifracional en el infrarrojo medio. h-BN es un cristal polar que exhibe fonón-polaritones en sus dos bandas Reststrahlen en el infrarrojo medio. En estas bandas, debido a su estructura cristalina anisótropa, únicamente una de las componentes del tensor de permitividad (que es uniaxial) posee signo negativo. Como consecuencia, la propagación de luz a en nitruro de boro en forma de fonón-polaritones hiperbólicos. En esta tesis estudiamos dos de las nanoestructuras más fundamentales en fotónica: nanoantenas lineales y cristales polaritónicos. Las nanoantenas lineales pueden ser analizadas como guías de onda polaritónicas truncadas, que son resonantes cuando el modo polaritónico cumple la condición de Fabry-Pèrot. Las nanoantennas de h-BN son analizadas usando esta interpretación. Un cristal polaritónico, de manera análoga a un cristal fotónico, es una nanoestructura periódica que soporta polaritones, en la que el periodo del patrón es similar a la longitud de onda del polaritón. Estudiamos un cristal polaritónico bidimensional fabricado a partir de una capa fina de h-BN, cuyo modo polaritónico más relevante es un fonón-polariton hiperbólico confinado en la capa fina

Mid-infrared nanophotonics with hyperbolic phonon polaritons

125 p.El nitruro de boro hexagonal (h-BN) es un material especialmente interesante en óptica subdifracional en el infrarrojo medio. h-BN es un cristal polar que exhibe fonón-polaritones en sus dos bandas Reststrahlen en el infrarrojo medio. En estas bandas, debido a su estructura cristalina anisótropa, únicamente una de las componentes del tensor de permitividad (que es uniaxial) posee signo negativo. Como consecuencia, la propagación de luz a en nitruro de boro en forma de fonón-polaritones hiperbólicos. En esta tesis estudiamos dos de las nanoestructuras más fundamentales en fotónica: nanoantenas lineales y cristales polaritónicos. Las nanoantenas lineales pueden ser analizadas como guías de onda polaritónicas truncadas, que son resonantes cuando el modo polaritónico cumple la condición de Fabry-Pèrot. Las nanoantennas de h-BN son analizadas usando esta interpretación. Un cristal polaritónico, de manera análoga a un cristal fotónico, es una nanoestructura periódica que soporta polaritones, en la que el periodo del patrón es similar a la longitud de onda del polaritón. Estudiamos un cristal polaritónico bidimensional fabricado a partir de una capa fina de h-BN, cuyo modo polaritónico más relevante es un fonón-polariton hiperbólico confinado en la capa fina

Remote near-field spectroscopy of vibrational strong coupling between organic molecules and phononic nanoresonators

Vibrational strong coupling (VSC) promises ultrasensitive IR spectroscopy and modification of material properties. Here, nanoscale mapping of VSC between organic molecules and individual IR nanoresonators is achieved by remote near-field spectroscopy. Phonon polariton (PhP) nanoresonators can dramatically enhance the coupling of molecular vibrations and infrared light, enabling ultrasensitive spectroscopies and strong coupling with minute amounts of matter. So far, this coupling and the resulting localized hybrid polariton modes have been studied only by far-field spectroscopy, preventing access to modal near-field patterns and dark modes, which could further our fundamental understanding of nanoscale vibrational strong coupling (VSC). Here we use infrared near-field spectroscopy to study the coupling between the localized modes of PhP nanoresonators made of h-BN and molecular vibrations. For a most direct probing of the resonator-molecule coupling, we avoid the direct near-field interaction between tip and molecules by probing the molecule-free part of partially molecule-covered nanoresonators, which we refer to as remote near-field probing. We obtain spatially and spectrally resolved maps of the hybrid polariton modes, as well as the corresponding coupling strengths, demonstrating VSC on a single PhP nanoresonator level. Our work paves the way for near-field spectroscopy of VSC phenomena not accessible by conventional techniques.This work was supported by the MCIN/AEI/10.13039/501100011033 under the María de Maeztu Units of Excellence Program (CEX2020-001038-M) and the Projects RTI2018-094830-B-100, PID2021-123949OB-I00, PID2019-107432GB-I00 and PID2021-122511OB-I00, as well as by the Graphene Flagship (GrapheneCore3, No. 881603). J.L. and J.H.E. are grateful for support from the Office of Naval Research (Award No. N00014-20-1-2474), for the BN crystal growth. S.V. acknowledges financial support by the Comunidad de Madrid through the Atracción de Talento program (grant no. 2020-T1/IND-20041). C.M.-E., R.E., and J.A. received funding from grant no. IT 1526-22 from the Basque Government for consolidated groups of the Basque University

Enhanced light–matter interaction in 10B monoisotopic boron nitride infrared nanoresonators

Phonon-polaritons, mixed excitations of light coupled to lattice vibrations (phonons), are emerging as a powerful platform for nanophotonic applications. This is because of their ability to concentrate light into extreme sub-wavelength scales and because of their longer phonon lifetimes compared to their plasmonic counterparts. In this work, the infrared properties of phonon-polaritonic nanoresonators made of monoisotopic 10B hexagonal boron nitride (h-BN) are explored, a material with increased phonon-polariton lifetimes compared to naturally abundant h-BN due to reduced photon scattering from randomly distributed isotopes. An average relative improvement of 50% of the quality factor of monoisotopic h-BN nanoresonators is obtained with respect to nanoresonators made of naturally abundant h-BN, allowing for the sensing of nanometric-thick films of molecules through both surface-enhanced absorption spectroscopy and refractive index sensing. Further, even strong coupling between molecular vibrations and the phonon-polariton resonance in monoisotopic h-BN ribbons can be achieved.The authors acknowledge funding from the Graphene Flagship (Core2 and Core3), the Spanish Ministry of Science and Innovation (projects MDM-2016-0618 of the Maria de Maeztu Units of Excellence Programme and national projects (RTI2018-094830-B-100, RTI2018-094861-B-100, and PID2019-107432GB-I00)), and the Basque Government (project GIU18/202, Ekartek project KK-2018/00001, IT1164-19, and PhD fellowships PRE_2018_2_0253 and PRE_2017_2_0052). The h-BN crystal growth was supported by the National Science Foundation, grant CMMI 1538127 and the II-VI Foundation.Peer reviewe

Active and Passive Tuning of Ultranarrow Resonances in Polaritonic Nanoantennas

[EN] Optical nanoantennas are of great importance for photonic devices and spectroscopy due to their capability of squeezing light at the nanoscale and enhancing light-matter interactions. Among them, nanoantennas made of polar crystals supporting phonon polaritons (phononic nanoantennas) exhibit the highest quality factors. This is due to the low optical losses inherent in these materials, which, however, hinder the spectral tuning of the nanoantennas due to their dielectric nature. Here, active and passive tuning of ultranarrow resonances in phononic nanoantennas is realized over a wide spectral range (approximate to 35 cm(-1), being the resonance linewidth approximate to 9 cm(-1)), monitored by near-field nanoscopy. To do that, the local environment of a single nanoantenna made of hexagonal boron nitride is modified by placing it on different polar substrates, such as quartz and 4H-silicon carbide, or covering it with layers of a high-refractive-index van der Waals crystal (WSe2). Importantly, active tuning of the nanoantenna polaritonic resonances is demonstrated by placing it on top of a gated graphene monolayer in which the Fermi energy is varied. This work presents the realization of tunable polaritonic nanoantennas with ultranarrow resonances, which can find applications in active nanooptics and (bio)sensing.J.M.-S. acknowledges financial support from the Ramon y Cajal Program of the Government of Spain and FSE (Grant No. RYC2018-026196-I) and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation Grant Number PID2019-110308GA-I00). P.A.-G. acknowledges support from the European Research Council under starting Grant No. 715496, 2DNANOPTICA, and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation Grant Number PID2019-111156GB-I00). G.a.-P. and J.T.-G. acknowledge support through the Severo Ochoa Program from the Government of the Principality of Asturias (Grant nos. PA20-PF-BP19-053 and PA-18-PF-BP17-126, respectively). A.Y.N. acknowledges the Spanish Ministry of Science and Innovation (Grant Nos. MAT201788358-C3-3-R and PID2020-115221GB-C42) and the Basque Department of Education (Grant No. PIBA-2020-1-0014) J.H.E. acknowledges support for h-BN crystal growth from the National Science Foundation, Award Number CMMI-1538127. R.H. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (National Project Grant No. RTI2018-094830-B-100 and the Project Grant No. MDM-2016-0618 of the Marie de Maeztu Units of Excellence Program), the Basque Government (Grant No. IT1164-19), and the European Union's Horizon 2020 research and innovation programme under the Graphene Flagship (Grant Agreement Numbers 785219 and 881603, GrapheneCore2 and GrapheneCore3). I.D. acknowledges the Basque Government (Grant No. PRE_2019_2_0164). Work at MIT was partly supported through AFOSR Grant No. FA9550-16-1-0382, through the NSF QII-TAQS program (Grant No. 1936263), and the Gordon and Betty Moore Foundation EPiQS Initiative through Grant No. GBMF9643 to P.J.-H

Launching of hyperbolic phonon-polaritons in h-BN slabs by resonant metal plasmonic antennas

Launching and manipulation of polaritons in van der Waals materials offers novel opportunities for field-enhanced molecular spectroscopy and photodetection, among other applications. Particularly, the highly confined hyperbolic phonon polaritons (HPhPs) in h-BN slabs attract growing interest for their capability of guiding light at the nanoscale. An efficient coupling between free space photons and HPhPs is, however, hampered by their large momentum mismatch. Here, we show —by far-field infrared spectroscopy, infrared nanoimaging and numerical simulations— that resonant metallic antennas can efficiently launch HPhPs in thin h-BN slabs. Despite the strong hybridization of HPhPs in the h-BN slab and Fabry-Pérot plasmonic resonances in the metal antenna, the efficiency of launching propagating HPhPs in h-BN by resonant antennas exceeds significantly that of the non-resonant ones. Our results provide fundamental insights into the launching of HPhPs in thin polar slabs by resonant plasmonic antennas, which will be crucial for phonon-polariton based nanophotonic devices

Enhanced Light–Matter Interaction in 10

Phonon-polaritons, mixed excitations of light coupled to lattice vibrations (phonons), are emerging as a powerful platform for nanophotonic applications. This is because of their ability to concentrate light into extreme sub-wavelength scales and because of their longer phonon lifetimes compared to their plasmonic counterparts. In this work, the infrared properties of phonon-polaritonic nanoresonators made of monoisotopic 10B hexagonal boron nitride (h-BN) are explored, a material with increased phonon-polariton lifetimes compared to naturally abundant h-BN due to reduced photon scattering from randomly distributed isotopes. An average relative improvement of 50% of the quality factor of monoisotopic h-BN nanoresonators is obtained with respect to nanoresonators made of naturally abundant h-BN, allowing for the sensing of nanometric-thick films of molecules through both surface-enhanced absorption spectroscopy and refractive index sensing. Further, even strong coupling between molecular vibrations and the phonon-polariton resonance in monoisotopic h-BN ribbons can be achieved.The authors acknowledge funding from the Graphene Flagship (Core2 and Core3), the Spanish Ministry of Science and Innovation (projects MDM-2016-0618 of the Maria de Maeztu Units of Excellence Programme and national projects (RTI2018-094830-B-100, RTI2018-094861-B-100, and PID2019-107432GB-I00)), and the Basque Government (project GIU18/202, Ekartek project KK-2018/00001, IT1164-19, and PhD fellowships PRE_2018_2_0253 and PRE_2017_2_0052). The h-BN crystal growth was supported by the National Science Foundation, grant CMMI 1538127 and the II-VI Foundation.Peer reviewe

Phacelia platycarpa (Cav.) Spreng.

Enhanced light-matter interactions are the basis of surface-enhanced infrared absorption (SEIRA) spectroscopy, and conventionally rely on plasmonic materials and their capability to focus light to nanoscale spot sizes. Phonon polariton nanoresonators made of polar crystals could represent an interesting alternative, since they exhibit large quality factors, which go far beyond those of their plasmonic counterparts. The recent emergence of van der Waals crystals enables the fabrication of high-quality nanophotonic resonators based on phonon polaritons, as reported for the prototypical infrared-phononic material hexagonal boron nitride (h-BN). In this work we use, for the first time, phonon-polariton-resonant h-BN ribbons for SEIRA spectroscopy of small amounts of organic molecules in Fourier transform infrared spectroscopy. Strikingly, the interaction between phonon polaritons and molecular vibrations reaches experimentally the onset of the strong coupling regime, while numerical simulations predict that vibrational strong coupling can be fully achieved. Phonon polariton nanoresonators thus could become a viable platform for sensing, local control of chemical reactivity and infrared quantum cavity optics experiments.We also acknowledge support from the European Commission under the Graphene Flagship (GrapheneCore1, Grant no. 696656), the Marie SklodowskaCurie individual fellowship (SGPCM-705960), the Spanish Ministry of Economy and Competitiveness (Maria de Maetzu Units of Excellence Programme MDM-2016-0618 and national projects FIS2014-60195-JIN, MAT2014-53432- C5-4-R, MAT2015-65525-R, MAT2015-65159-R, FIS2016-80174-P, MAT2017- 88358-C3-3-R), the Basque government (PhD fellowship PRE-2016-1-0150, PRE-2016-2-0025), the Department of Industry of the Basque Government (ELKARTEK project MICRO4FA), the Regional Council of Gipuzkoa (project no. 100/16) and the ERC starting grant 715496, 2DNANOPTICA.Peer Reviewe

Active and Passive Tuning of Ultranarrow Resonances in Polaritonic Nanoantennas

Optical nanoantennas are of great importance for photonic devices and spectroscopy due to their capability of squeezing light at the nanoscale and enhancing light-matter interactions. Among them, nanoantennas made of polar crystals supporting phonon polaritons (phononic nanoantennas) exhibit the highest quality factors. This is due to the low optical losses inherent in these materials, which, however, hinder the spectral tuning of the nanoantennas due to their dielectric nature. Here, active and passive tuning of ultranarrow resonances in phononic nanoantennas is realized over a wide spectral range (≈35 cm-1 , being the resonance linewidth ≈9 cm-1 ), monitored by near-field nanoscopy. To do that, the local environment of a single nanoantenna made of hexagonal boron nitride is modified by placing it on different polar substrates, such as quartz and 4H-silicon carbide, or covering it with layers of a high-refractive-index van der Waals crystal (WSe2 ). Importantly, active tuning of the nanoantenna polaritonic resonances is demonstrated by placing it on top of a gated graphene monolayer in which the Fermi energy is varied. This work presents the realization of tunable polaritonic nanoantennas with ultranarrow resonances, which can find applications in active nanooptics and (bio)sensing