Abstract
Photonic crystals (PCs) are periodically patterned dielectrics providing opportunities to shape and slow down the light for processing of optical signals, lasing and spontaneous emission control. Unit cells of conventional PCs are comparable to the wavelength of light and are not suitable for subwavelength scale applications. We engineer a nanoscale hole array in a van der Waals material (h-BN) supporting ultra-confined phonon polaritons (PhPs)—atomic lattice vibrations coupled to electromagnetic fields. Such a hole array represents a polaritonic crystal for mid-infrared frequencies having a unit cell volume of {mathrm{10}}^{{mathrm{ - 5}}}{lambda}_{mathrm{0}}^{mathrm{3}} (with λ0 being the free-space wavelength), where PhPs form ultra-confined Bloch modes with a remarkably flat dispersion band. The latter leads to both angle- and polarization-independent sharp Bragg resonances, as verified by far-field spectroscopy and near-field optical microscopy. Our findings could lead to novel miniaturized angle- and polarization-independent infrared narrow-band couplers, absorbers and thermal emitters based on van der Waals materials and other thin polar materials.
Highlights
Photonic crystals (PCs) are periodically patterned dielectrics providing opportunities to shape and slow down the light for processing of optical signals, lasing and spontaneous emission control
In summary, we have introduced and experimentally realized IR deeply subwavelength polaritonic crystals based on hyperbolic phonon polaritons in nanostructured van der Waals crystal slabs
Such crystals support highly confined Bloch modes with flat bands, giving rise to angle- and polarization-independent geometrically tunable resonances, even in case of the simplest square symmetry. Apart of their potential application for subwavelength omnidirectional IR absorbers, couplers and reflectors, hexagonal boron nitride (h-BN) polaritonic crystals can be utilized for inhibiting spontaneous emission
Summary
Photonic crystals (PCs) are periodically patterned dielectrics providing opportunities to shape and slow down the light for processing of optical signals, lasing and spontaneous emission control. We engineer a nanoscale hole array in a van der Waals material (h-BN) supporting ultra-confined phonon polaritons (PhPs)—atomic lattice vibrations coupled to electromagnetic fields Such a hole array represents a polaritonic crystal for mid-infrared frequencies having a unit cell volume of 10À5λ30 (with λ0 being the free-space wavelength), where PhPs form ultra-confined Bloch modes with a remarkably flat dispersion band. At mid-infrared (IR) frequencies, PCs find applications as thermal emitters, optical couplers[3], devices for chemical and biological spectroscopy[4], and sensors for environmental monitoring (e.g., gas sensing)[5] They are normally fabricated by either patterning Si slabs[6] or metal layers[7] that are combined with quantum wells operating at the desired wavelength. Due to their remarkably long lifetimes[25,27], HPhPs in h-BN can be used for molecular vibration spectroscopy and strong coupling[28]
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