The interaction between cavity modes and optical transitions leads to new coupled light-matter states in which the energy is periodically exchanged between the matter states and the optical mode. ...Here we present experimental evidence of optical strong coupling between modes of individual sub-wavelength metamaterial nanocavities and engineered optical transitions in semiconductor heterostructures. We show that this behaviour is generic by extending the results from the mid-infrared (~10 μm) to the near-infrared (~1.5 μm). Using mid-infrared structures, we demonstrate that the light-matter coupling occurs at the single resonator level and with extremely small interaction volumes. We calculate a mode volume of 4.9 × 10(-4) (λ/n)(3) from which we infer that only ~2,400 electrons per resonator participate in this energy exchange process.
Using fundamental electromagnetic properties of metasurfaces, we build a platform to classify reciprocal bianisotropic metasurfaces from typical experimental measurements and determine isotropic, ...anisotropic, bi-isotropic (chiral), and bianisotropic (so-called omega) properties. We provide experimental guidelines to identify each class by measuring macroscopic scattering parameters, that is, reflection and transmission coefficients upon plane wave illumination with linear and/or circular polarization. We explicitly provide a recipe of which metasurface properties can and cannot be inferred by means of the chosen polarization, reflection, and transmission properties. We also clarify common confusions in the classification of anisotropic versus chiral metasurfaces based on circular dichroism measurements presented in the recent literature.
Lasing at a stationary inflection point Herrero-Parareda, A.; Furman, N.; Mealy, T. ...
Optical materials express,
05/2023, Letnik:
13, Številka:
5
Journal Article
Recenzirano
Odprti dostop
The concept of lasers based on the frozen mode regime in active periodic optical waveguides with a 3rd-order exceptional point of degeneracy (EPD) is advanced. The frozen mode regime in a lossless ...and gainless waveguide is associated with a stationary inflection point (SIP) in the Bloch dispersion relation, where three Bloch eigenmodes coalesce forming the frozen mode. As a practical example, we consider an asymmetric serpentine optical waveguide (ASOW). An ASOW operating near the SIP frequency displays a large group delay of a non-resonant nature that scales as the cube of the waveguide length, leading to a strong gain enhancement when active material is included. Therefore, a laser operating in the close vicinity of an SIP has a gain threshold that scales as a negative cube of the waveguide length. We determine that this scaling law is maintained in the presence of small distributed losses, such as radiation associated with waveguide bends and roughness. In addition, we show that although gain causes a distortion in the modes coalescing at the SIP, the properties of the frozen mode are relatively resistant to such small perturbations and we still observe a large degree of exceptional degeneracy for gain values that bring the system above threshold. Finally, our study also reveals that lasing near an SIP is favored over lasing near a photonic band edge located in close proximity to the SIP. In particular, we observe that an SIP-induced lasing in an ASOW displays lower gain threshold compared to lasing near the photonic regular band edge (RBE), even though the SIP resonance has a lower quality factor than the RBE resonance.
We show that the enhanced directivity phenomenon for light passing through a subwavelength aperture in a silver film with corrugations on the exit face, is due to a leaky wave that decays ...exponentially from the aperture. We show quantitatively that the field along the interface of the silver film is dominated by the leaky wave, and that the radiation of the leaky wave, supported by the periodic structure, yields the directive beam. The leaky wave propagation and attenuation constants parameterize the physical radiation mechanism, and provide important design information for optimizing the structure. Maximum directivity occurs when the phase and attenuation constants are approximately equal.
In this paper an investigation is presented of metamaterial structures excited by a line source aimed at producing narrow directive beams. The structure under consideration is a grounded slab made of ...a homogeneous metamaterial medium with a plasma-like dispersive permittivity; for low values of the slab permittivity an extremely directive beam pointing at broadside can be obtained. Conditions for the maximization of radiation at broadside are given and the narrow-beam effect is shown to be related to the excitation of a leaky mode supported by the slab, with radiation maximization corresponding to small and equal values of the phase and attenuation constants. The frequency bandwidth and directivity are expressed in a simple closed form in terms of the attenuation constant of the leaky mode. By increasing the slab height for a fixed frequency, the leaky mode is analytically shown to give rise to a beam that is scanned from broadside to the critical angle for plane-wave refraction, thus being confined to a narrow angular region around broadside. Numerical results are given that illustrate these features, and full-wave simulations of a metamaterial structure made of an array of metallic cylinders are presented that confirm the results of the analytical study. The case of a line source inside a semi-infinite metamaterial region is also considered and its radiation characteristics compared with those of the metamaterial slab
Thermally responsive polymers present an interesting avenue for tuning the optical properties of nanomaterials on their surfaces by varying their periodicity and shape using facile processing ...methods. Gold bowtie nanoantenna arrays are fabricated using nanosphere lithography on prestressed polyolefin (PO), a thermoplastic polymer, and optical properties are investigated via a combination of spectroscopy and electromagnetic simulations to correlate shape evolution with optical response. Geometric features of bowtie nanoantennas evolve by annealing at temperatures between 105 °C and 135 °C by releasing the degree of prestress in PO. Due to the higher modulus of Au versus PO, compressive stress occurs on Au bowtie regions on PO, which leads to surface buckling at the two highest annealing temperatures; regions with a 5 nm gap between bowtie nanoantennas are observed and the average reduction is 75%. Reflectance spectroscopy and full-wave electromagnetic simulations both demonstrate the ability to tune the plasmon resonance wavelength with a window of approximately 90 nm in the range of annealing temperatures investigated. Surface-enhanced Raman scattering measurements demonstrate that maximum enhancement is observed as the excitation wavelength approaches the plasmon resonance of Au bowtie nanoantennas. Both the size and morphology tunability offered by PO allows for customizing optical response.
We investigate second-harmonic generation from anisotropic or longitudinal epsilon-near-zero materials. We find conversion efficiencies well above their isotropic counterparts owing to additional ...field intensity enhancement provided by the anisotropy. At the same time, anisotropic epsilon-near-zero materials are also less sensitive to the material's losses compared to the isotropic ones. In turn, these improvements become pivotal for epsilon-near-zero materials that do not possess bulk dipole-allowed quadratic nonlinearities. We predict that second-harmonic generation from a Dy:CdO/Si multilayer with longitudinal epsilon-near-zero properties can exceed the conversion efficiency of a homogeneous Dy:CdO slab of equivalent thickness by at least 20 times for almost any angle of incidence.
A rigorous solution for the current induced on a semi‐infinite array of narrow metallic strips is obtained using the Wiener‐Hopf factorization method in the Z‐transformed domain. The method can be ...applied to arrays with fixed current shape on each element (e.g, single mode elements), and shows rigorously the physics of waves associated to truncated periodic structures. The solution is obtained via a rigorous factorization, that is improved by using a closed form result based on an approximated factorization. The current on the truncated array is rigorously represented as the sum of the current pertaining to the infinite array plus a contribution induced by the truncation of the array. Asymptotics shows that the truncation‐induced current contribution has a diffractive behavior decaying algebraically with the element number, away from the truncation. Uniform asymptotics shows that this diffractive current is effectively represented in terms of Fresnel functions, permitting also a closed form representation in proximity of and at transverse inward resonance, i.e., when a grazing grating lobe points toward the array. Illustrative examples and comparisons with a method of moment solution show the accuracy of our results.
We discuss possible topologies of physically different metasurfaces with equivalent macroscopic electromagnetic responses. We specifically demonstrate that the presence of normal components (with ...respect to the metasurface plane) of the polarization currents does not add any functionality in engineering the reflection and transmission characteristics which cannot occur in metasurfaces maintaining only tangential currents. Therefore, if a certain property is achievable using general volumetric inclusions, polarizable along all three directions, the same response can be achieved using some equivalent perfectly planar elements, polarizable only in the metasurface plane. Our results clarify what physical properties are necessary for realizing desired metasurface responses and promote flat microwave and optical designs, providing possibilities for dramatic complexity reduction of metasurfaces for rather general field transformations, including asymmetric reflection and transmission.
This Letter presents a study of the local density of states (LDOS) in photonic quasicrystals. We show that the LDOS of a Penrose-type quasicrystal exhibits small additional band gaps. Among the band ...gaps, some exhibit a behavior similar to that typical of photonic crystals, while others do not. The development of certain band gaps requires large-size quasicrystals. It is explained by the long-range interactions involved in their formation. Moreover, the frequencies where the band gaps occur are not necessarily explained using single scattering and should therefore involve multiple scattering.
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