Strong confinement of optical fields using localized surface phonon polaritons in cubic boron nitride

High-quality nanocavities through multimodal confinement of hyperbolic polaritons in hexagonal boron nitride

Compressing light into nanocavities substantially enhances light-matter interactions, which has been a major driver for nanostructured materials research. However, extreme confinement generally comes at the cost of absorption and low resonator quality factors. Here we suggest an alternative optical multimodal confinement mechanism, unlocking the potential of hyperbolic phonon polaritons in isotopically pure hexagonal boron nitride. We produce deep-subwavelength cavities and demonstrate several orders of magnitude improvement in confinement, with estimated Purcell factors exceeding 108 and quality factors in the 50-480 range, values approaching the intrinsic quality factor of hexagonal boron nitride polaritons. Intriguingly, the quality factors we obtain exceed the maximum predicted by impedance-mismatch considerations, indicating that confinement is boosted by higher-order modes. We expect that our multimodal approach to nanoscale polariton manipulation will have far-reaching implications for ultrastrong light-matter interactions, mid-infrared nonlinear optics and nanoscale sensors.

Addressing the Impact of Surface Roughness on Epsilon-Near-Zero Silicon Carbide Substrates

Epsilon-near-zero (ENZ) media have been very actively investigated due to their unconventional wave phenomena and strengthened nonlinear response. However, the technological impact of ENZ media will be determined by the quality of realistic ENZ materials, including material loss and surface roughness. Here, we provide a comprehensive experimental study of the impact of surface roughness on ENZ substrates. Using silicon carbide (SiC) substrates with artificially induced roughness, we analyze samples whose roughness ranges from a few to hundreds of nanometer size scales. It is concluded that ENZ substrates with roughness in the few nanometer scale are negatively affected by coupling to longitudinal phonons and strong ENZ fields normal to the surface. On the other hand, when the roughness is in the hundreds of nanometers scale, the ENZ band is found to be more robust than dielectric and surface phonon polariton (SPhP) bands.

Coupled Tamm Phonon and Plasmon Polaritons for Designer Planar Multiresonance Absorbers

Wavelength-selective absorbers (WS-absorbers) are of interest for various applications, including chemical sensing and light sources. Lithography-free fabrication of WS-absorbers can be realized via Tamm plasmon polaritons (TPPs) supported by distributed Bragg reflectors (DBRs) on plasmonic materials. While multifrequency and nearly arbitrary spectra can be realized with TPPs via inverse design algorithms, demanding and thick DBRs are required for high quality-factors (Q-factors) and/or multiband TPP-absorbers, increasing the cost and reducing fabrication error tolerance. Here, high Q-factor multiband absorption with limited DBR layers (3 layers) is experimentally demonstrated by Tamm hybrid polaritons (THPs) formed by coupling TPPs and Tamm phonon polaritons when modal frequencies are overlapped. Compared to the TPP component, the Q-factors of THPs are improved twofold, and the angular broadening is also reduced twofold, facilitating applications where narrow-band and nondispersive WS-absorbers are needed. Moreover, an open-source algorithm is developed to inversely design THP-absorbers consisting of anisotropic media and exemplify that the modal frequencies can be assigned to desirable positions. Furthermore, it is demonstrated that inversely designed THP-absorbers can realize same spectral resonances with fewer DBR layers than a TPP-absorber, thus reducing the fabrication complexity and enabling more cost-effective, lithography-free, wafer-scale WS-absorberss for applications such as free-space communications and gas sensing.

Demonstration of Acceptor-Like Traps at Positive Polarization Interfaces in Ga-Polar P-type (AlGaN/AlN)/GaN Superlattices

The shortcomings with acceptors in p-type III-nitride semiconductors have resulted in not many efforts being presented on III-nitride based p-channel electronic devices (here, field effect transistors (FETs)). The polarization effects in III-nitride superlattices (SLs) lead to the periodic oscillation of the energy bands, exhibiting enhanced ionization of the deep acceptors (Mg in this study), and hence their use in III-nitride semiconductor-based light-emitting diodes (LEDs) and p-channel FETs is beneficial. This study experimentally demonstrates the presence of acceptor-like traps at the positive polarization interfaces acting as the primary source of holes in Ga-polar p-type uniformly doped (AlGaN/AlN)/GaN SLs with limited Mg doping. The observed concentration of holes exceeding that of the dopants incorporated into the samples during growth can be attributed to the ionization of acceptor-like traps, located at 0.8 eV above the valence band of GaN, at positive polarization interfaces. All samples were grown using the metal organic vapor phase epitaxy (MOVPE) technique, and the materials’ characterization was carried out using X-ray diffraction and Hall effect measurements. The hole concentrations experimentally measured are juxtaposed with the calculated value of hole concentrations from FETIS®, and the measured trends in mobility are explained using the amplitude of separation of the two-dimensional hole gas in the systems from the positive polarization interfaces.

Ultrahigh-Resolution, Label-Free Hyperlens Imaging in the Mid-IR

The hyperbolic phonon polaritons supported in hexagonal boron nitride (hBN) with long scattering lifetimes are advantageous for applications such as super-resolution imaging via hyperlensing. Yet, hyperlens imaging is challenging for distinguishing individual and closely spaced objects and for correlating the complicated hyperlens fields with the structure of an unknown object underneath. Here, we make significant strides to overcome each of these challenges. First, we demonstrate that monoisotopic h¹¹BN provides significant improvements in spatial resolution, experimentally resolving structures as small as 44 nm and those with sub 25 nm spacings at 6.76 μm free-space wavelength. We also present an image reconstruction algorithm that provides a structurally accurate, visual representation of the embedded objects from the complex hyperlens field. Further, we offer additional insights into optimizing hyperlens performance on the basis of material properties, with an eye toward realizing far-field imaging modalities. Thus, our results significantly advance label-free, high-resolution, spectrally selective hyperlens imaging and image reconstruction methodologies.

Casimir forces exerted by epsilon-near-zero hyperbolic materials

The Casimir force exerted on a gold dipolar nanoparticle by a finite-thickness slab of the natural hyperbolic material namely, the ortorhombic crystalline modification of boron nitride, is investigated. The main contribution to the force originates from the TM-polarized waves, for frequencies at which the parallel and perpendicular components of the dielectric tensor reach minimal values. These frequencies differ from those corresponding to the Lorentzian resonances for the permittivity components. We show that when the slab is made of an isotropic epsilon-near-zero absorbing material the force on the nanoparticle is larger than that induced by a hyperbolic material, for similar values of the characteristic parameters. This fact makes these materials optimal in the use of Casimir’s forces for nanotechnology applications.

Controlling the Infrared Dielectric Function through Atomic-Scale Heterostructures

Surface phonon polaritons (SPhPs), the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons, offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. However, once a particular material is chosen, the SPhP characteristics are fixed by the spectral positions of the optic phonon frequencies. Here, we provide a demonstration of how the frequency of these optic phonons can be altered by employing atomic-scale superlattices (SLs) of polar semiconductors using AlN/GaN SLs as an example. Using second harmonic generation (SHG) spectroscopy, we show that the optic phonon frequencies of the SLs exhibit a strong dependence on the layer thicknesses of the constituent materials. Furthermore, new vibrational modes emerge that are confined to the layers, while others are centered at the AlN/GaN interfaces. As the IR dielectric function is governed by the optic phonon behavior in polar materials, controlling the optic phonons provides a means to induce and potentially design a dielectric function distinct from the constituent materials and from the effective-medium approximation of the SL. We show that atomic-scale AlN/GaN SLs instead have multiple Reststrahlen bands featuring spectral regions that exhibit either normal or extreme hyperbolic dispersion with both positive and negative permittivities dispersing rapidly with frequency. Apart from the ability to engineer the SPhP properties, SL structures may also lead to multifunctional devices that combine the mechanical, electrical, thermal, or optoelectronic functionality of the constituent layers. We propose that this effort is another step toward realizing user-defined, actively tunable IR optics and sources.