Vibrational Coupling to Epsilon-Near-Zero Waveguide Modes

Abstract

Epsilon near zero modes offer extreme field enhancement that can be utilized for developing enhanced sensing schemes. However, demonstrations of enhanced spectroscopies have largely exploited surface polaritons, mostly due to the challenges of coupling a vibrational transition to volume-confined epsilon near zero modes. Here we fabricate high aspect ratio gratings (up to 24.8 µm height with greater than 5 μm pitch) of 4H-SiC, with resonant modes that couple to transverse magnetic and transverse electric incident fields. These correspond to metal-insulatormetal waveguide modes propagating downwards into the substrate. The cavity formed by the finite The electromagnetic field confinement offered by surface polaritons 1-2 and epsilon-near-zero (ENZ) 3-7 modes have long been discussed for applications in surface-enhanced sensing [8][9][10][11][12][13] , and vibrational coupling [14][15] . Whilst surface plasmon polaritons (SPPs) have been extensively explored, demonstrating enhanced spectroscopies using ENZ modes has remained challenging. This is largely because at optical frequencies ENZ modes are often realized by coupling light into a material where epsilon is close to zero 16-18 , or via waveguides that are not hollow and therefore incompatible with confining the analyte of interest within the region of highly confined electromagnetic fields [3][4] . In this letter we investigate high-aspect-ratio grating (HAG) structures designed to support surface phonon polaritons (SPhPs) 1 at the interface between the polar crystal grating surfaces and the surrounding environment 19 . We show that the modes supported by this structure behave like metal-insulator-metal (MIM) waveguide modes in a short cavity [20][21][22] .Furthermore, due to this architecture, these structures support an ENZ mode in the gap between the grating teeth. This enables the first colocation of strongly confined ENZ fields with an analyte of interest, including liquids, with large surface area. As proof of this, we demonstrate that the ENZ fields can coherently couple to vibrational transitions in a liquid. Thus, this constitutes a platform for studying ENZ and SPhP strong coupling at infrared (IR) frequencies, with potential applications in surface enhanced spectroscopies [8][9][10][11][12][13]23 as well as light-controlled chemistry [14][15] .This study exploits high aspect ratio gratings, which have a height (h) that is much larger than their period (Λ) (see Fig 1). In HAGs and nanopillars, surface polariton modes are supported between the teeth, propagating as MIM waveguide modes downwards into the grating [20][21][22][24][25] .The frequency of the modes can be controlled by changing the effective index of refraction (neff) of the polariton wave using the size of the air gap (g) between the teeth, or the height of the grating (h, see Fig. 1 a-c). Furthermore, polaritonic modes in these structures have been demonstrated to

Thickness-dependent optical properties of aluminum nitride films for mid-infrared wavelengths

“…The real part of the constant vanishes around 11.3 µm (∼885 cm −1 ), while the imaginary part is relatively small. 12,13,[24][25][26][27] Recently, various ENZ materials have drawn attention due to intriguing optical effects associated with vanishing permittivity 28 and studied for sensing, 29 enhanced fluorescence emission, 30 enhanced transmission through subwavelength apertures, 31 strong subwavelength field enhancement, 32 and engineering the frequency response of optical antennas. [33][34][35] There are various methods of AlN thin films deposition including carbothermal nitridation of Al 2 O 3 and subsequent sintering, 36,37 metalorganic chemical vapor deposition, 38 molecular beam epitaxy (MBE), 39 sputtering, 13,40 and atomic layer deposition (ALD).…”

Polariton panorama

“…Molecular polaritons. Organic semiconductors and molecules embedded in optical (nano)cavities under strong and ultrastrong coupling promote the dynamical formation of molecular polaritons: hybrid energy eigenstates composed of entangled photonic, electronic, and vibrational degrees of freedom [34,312,313]. Molecular polaritons were demonstrated to enhance energy transfer [314] and DC conductivity [315].…”

“…Berreman polaritons: Phonon polaritons in anisotropic materials and multilayer structures are also referred to as epsilon-near-zero or ENZ polaritons [29][30][31]. ENZ materials, artificial structures, and nanocavities reveal exotic electromagnetic responses with a broad range of technological applications [31][32][33][34][35]. For example, ENZ nanocavities facilitate ultrastrong coupling between plasmonic and phononic modes [36], as well as the so-called photonic doping [37].…”

Guided Mid‐IR and Near‐IR Light within a Hybrid Hyperbolic‐Material/Silicon Waveguide Heterostructure

“…Even restricting operation to the use of h 10 BN for mid‐IR frequencies, this platform offers significant opportunities for lab‐on‐a‐chip approaches due to the spectral overlap of the upper Reststrahlen band with the chemical vibrational fingerprint range. Such an overlap can be exploited through the introduction of the surface‐enhanced infrared absorption (SEIRA), [ 36,37 ] strong‐coupling, [ 35,38,39 ] or refractive index sensing [ 40 ] modalities into the device design.…”