230 research outputs found
Second Harmonic Generation from Phononic Epsilon-Near-Zero Berreman Modes in Ultrathin Polar Crystal Films
Immense optical field enhancement was predicted to occur for the Berreman
mode in ultrathin films at frequencies in the vicinity of epsilon near zero
(ENZ). Here, we report the first experimental proof of this prediction in the
mid-infrared by probing the resonantly enhanced second harmonic generation
(SHG) at the longitudinal optic phonon frequency from a deeply
subwavelength-thin aluminum nitride (AlN) film. Employing a transfer matrix
formalism, we show that the field enhancement is completely localized inside
the AlN layer, revealing that the observed SHG signal of the Berreman mode is
solely generated in the AlN film. Our results demonstrate that ENZ Berreman
modes in intrinsically low-loss polar dielectric crystals constitute a
promising platform for nonlinear nanophotonic applications
Strong Coupling of Epsilon-Near-Zero Phonon Polaritons in Polar Dielectric Heterostructures
We report the first observation of epsilon near zero (ENZ) phonon polaritons
in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP)
supported by the adjacent SiC substrate. Employing a strong coupling model for
the analysis of the dispersion and electric field distribution in these
hybridized modes, we show that they share the most prominent features of the
two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly
propagative character and deeply sub-wavelength light confinement can be
utilized as building blocks for future infrared and terahertz (THz)
nanophotonic integration and communication devices
Resonant enhancement of second harmonic generation in the mid-infrared using localized surface phonon polaritons in sub-diffractional nanostructures
We report on strong enhancement of mid-infrared second harmonic generation
(SHG) from SiC nanopillars due to the resonant excitation of localized surface
phonon-polaritons within the Reststrahlen band. The magnitude of the SHG peak
at the monopole mode experiences a strong dependence on the resonant frequency
beyond that described by the field localization degree and the dispersion of
linear and nonlinear-optical SiC properties. Comparing the results for the
identical nanostructures made of 4H and 6H SiC polytypes, we demonstrate the
interplay of localized surface phonon polaritons with zone-folded weak phonon
modes of the anisotropic crystal. Tuning the monopole mode in and out of the
region where the zone-folded phonon is excited in 6H-SiC, we observe a
prominent increase of the already monopole-enhanced SHG output when the two
modes are coupled. Envisioning this interplay as one of the showcase features
of mid-infrared nonlinear nanophononics, we discuss its prospects for the
effective engineering of nonlinear-optical materials with desired properties in
the infrared spectral range.Comment: 16 pages, 3 figure
Probing hyperbolic polaritons using infrared attenuated total reflectance micro-spectroscopy
Hyperbolic polariton modes are highly appealing for a broad range of
applications in nanophotonics, including surfaced enhanced sensing,
sub-diffractional imaging and reconfigurable metasurfaces. Here we show that
attenuated total reflectance micro-spectroscopy (ATR) using standard
spectroscopic tools can launch hyperbolic polaritons in a Kretschmann-Raether
configuration. We measure multiple hyperbolic and dielectric modes within the
naturally hyperbolic material hexagonal boron nitride as a function of
different isotopic enrichments and flake thickness. This overcomes the
technical challenges of measurement approaches based on nanostructuring, or
scattering scanning nearfield optical microscopy. Ultimately, our ATR approach
allows us to compare the optical properties of small-scale materials prepared
by different techniques systematicallyComment: 13 pages 4 figure
Spectroscopic and Interferometric Sum-Frequency Imaging of Strongly Coupled Phonon Polaritons in SiC Metasurfaces
Phonon polaritons enable waveguiding and localization of infrared light with
extreme confinement and low losses. The spatial propagation and spectral
resonances of such polaritons are usually probed with complementary techniques
such as near-field optical microscopy and far-field reflection spectroscopy.
Here, we introduce infrared-visible sum-frequency spectro-microscopy as a tool
for spectroscopic imaging of phonon polaritons. The technique simultaneously
provides sub-wavelength spatial resolution and highly-resolved spectral
resonance information. This is implemented by resonantly exciting polaritons
using a tunable infrared laser and wide-field microscopic detection of the
upconverted light. We employ this technique to image hybridization and strong
coupling of localized and propagating surface phonon polaritons in metasurfaces
of SiC micropillars. Spectro-microscopy allows us to measure the polariton
dispersion simultaneously in momentum space by angle-dependent resonance
imaging, and in real space by polariton interferometry. Notably, we directly
visualize how strong coupling affects the spatial localization of polaritons,
inaccessible with conventional spectroscopic techniques. We further observe the
formation of edge states at excitation frequencies where strong coupling
prevents polariton propagation into the metasurface. Our approach is applicable
to the wide range of polaritonic materials with broken inversion symmetry and
can be used as a fast and non-perturbative tool to image polariton
hybridization and propagation
Single-peak and narrow-band mid-infrared thermal emitters driven by mirror-coupled plasmonic quasi-BIC metasurfaces
Wavelength-selective thermal emitters (WS-EMs) hold considerable appeal due
to the scarcity of cost-effective, narrow-band sources in the mid-to-long-wave
infrared spectrum. WS-EMs achieved via dielectric materials typically exhibit
thermal emission peaks with high quality factors (Q factors), but their optical
responses are prone to temperature fluctuations. Metallic EMs, on the other
hand, show negligible drifts with temperature changes, but their Q factors
usually hover around 10. In this study, we introduce and experimentally verify
a novel EM grounded in plasmonic quasi-bound states in the continuum (BICs)
within a mirror-coupled system. Our design numerically delivers an
ultra-narrowband single peak with a Q factor of approximately 64, and
near-unity absorptance that can be freely tuned within an expansive band of
more than 10 {\mu}m. By introducing air slots symmetrically, the Q factor can
be further augmented to around 100. Multipolar analysis and phase diagrams are
presented to elucidate the operational principle. Importantly, our infrared
spectral measurements affirm the remarkable resilience of our designs'
resonance frequency in the face of temperature fluctuations over 300 degrees
Celsius. Additionally, we develop an effective impedance model based on the
optical nanoantenna theory to understand how further tuning of the emission
properties is achieved through precise engineering of the slot. This research
thus heralds the potential of applying plasmonic quasi-BICs in designing
ultra-narrowband, temperature-stable thermal emitters in mid-infrared.
Moreover, such a concept may be adaptable to other frequency ranges, such as
near-infrared, Terahertz, and Gigahertz.Comment: 39 pages, 12 figure
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