Optics of 2D and van der Waals materials

Introduction

2D and van der Waals materials exhibit radically new electrical and optical properties and are opening new research directions in the field of nanophotonics. Polaritons in these materials can be used to confine light to the nanoscale, while via gate-tunability it is possible to create reconfigurable optical devices.

Rewritable polariton nanophotonics using phase-change materials

Polaritons formed by the coupling of light and material excitations enable light-matter interactions at the nanoscale beyond what is currently possible with conventional optics. Our group experimentally demonstrated refractive and meta-optics for polaritons in the mid-infrared. We demonstrated rewritable waveguides, refractive optical elements such as lenses, prisms, and metalenses. Our results will enable the realization of programmable miniaturized integrated optoelectronic devices and on-demand biosensors.

For more information, visit: https://www.seas.harvard.edu/news/2019/11/flatland-light

Phase change polariton

Phase change polariton

Phase change polariton

Phonon polaritons hBN resonators

Our group studies the use of high-quality factor polaritons in van der Waals materials to create resonators with volumes thousands of times smaller than resonators based on dielectric materials. In this work we showed that strongly subwavelength hexagonal boron nitride planar nanostructures can exhibit ultra-confined resonances and strong local field enhancement. We believe that phonon polaritons in hexagonal boron nitride can play for infrared light a role similar to that of plasmons in noble metals at visible frequency, paving the way for a new class of efficient and highly miniaturized nanophotonics devices.

For more information, visit: https://www.seas.harvard.edu/news/2018/06/squeezing-light-nanoscale

hBn resonatorhBn resonator

hBn resonatorhBn resonator

Phonon polaritons in van der Waals heterostructures

>In this work we demonstrated for the first time that phonon polaritons found in hexagonal boron nitride have the ability of enhancing the optical anisotropy in another material. We show this experimentally by imaging the polaritons in a heterostructure formed by hexagonal boron nitride and black phosphorous, which shows different propagation velocities along the two crystal axes of black phosphorous. We find that the anisotropy of the polaritons in this system significantly exceed the natural anisotropy of black phosphorous.

Phonon polaritons

Ultraslow phonon polaritons in hBN

In this work, by exploiting the mirror symmetry obtained by placing an h-BN flake on a gold substrate, we show how selectively excite only the most confined polaritonic modes that have phase and group velocities respectively tens and hundreds of times slower than the speed of light. Moreover, Photo-induced Force Microscopy (PiFM), an innovative near-field imaging technique, has been implemented to image the phonon polaritons in the RS1 band where scattering-type near-field optical microscopes suffer lack of suitable laser sources.

Ultraslow phonon polaritons