Mid-infrared broadband circular polarizer based on Weyl semimetals.

As a three-dimensional topological phase of matter, Weyl semimetals possess extremely large gyrotropic optical response in the mid-infrared region, leading to the strong chiral anomaly. This study proposes a circular polarizer design with a double-WSM-layer structure. It is theoretically shown that the proposed polarizer possesses a high circular polarization efficiency and high average transmittance in the wavelength region from 9 µm to 15 µm at incidence angles up to 50°. The modified 4 × 4 matrix method is used to calculate the circularly polarized transmittance of Weyl semimetals in thin-film or multilayer structures. The temperature dependence on the transmittance is also examined to demonstrate the flexibility of the proposed polarizer in a varying temperature environment. This study reveals the technological prospect that Weyl semimetals are promising candidates for high-performance circular polarizers in infrared spectroscopy and polarimetry.

Resonance perfect absorption by exciting hyperbolic phonon polaritons in 1D hBN gratings.

Natural materials with hyperbolic responses can confine light with well-defined propagation directions inside the micro/nanostructure. Here we theoretically demonstrate that strong resonance absorption can be achieved in one-dimensional gratings made of hexagonal boron nitride (hBN) due to hyperbolic phonon polaritons. The radiative properties of both trapezoidal and square resonators are calculated using anisotropic rigorous coupled-wave analysis. The resonance wavelengths can be theoretically predicted and are shown to follow the anomalous or traditional scaling laws depending on the hyperbolicity. These findings may benefit the applications including photodetection, color filters, and optomechanics.

Light scattering of semitransparent sintered polytetrafluoroethylene films.

Polytetrafluoroethylene (PTFE) is a strongly scattering material and has been regarded to have optical properties similar to biological tissues. In the present study, the bidirectional reflectance distribution function (BRDF) and the bidirectional transmittance distribution function (BTDF) of several PTFE films, with thicknesses from 0.11 to 10 mm, are measured using a laser scatterometer at the wavelength of 635 nm. The directional-hemispherical reflectance (R) and transmittance (T) were obtained by integrating BRDF and BTDF for normal incidence. Comparison of the ratio of the measured R and T with that calculated from the adding-doubling method allows the determination of the reduced scattering coefficient. Furthermore, the effect of surface scattering is investigated by measuring the polarization-dependent BRDF and BTDF at oblique incidence. By analyzing the measurement uncertainty of BTDF in the near-normal observation angles at normal incidence, the present authors found that the scattering coefficient of PTFE should exceed 1200 cm(-1), which is much greater than that of biological tissues. On the other hand, the absorption coefficient of PTFE must be less than 0.01 cm(-1), much smaller than that of biological tissues, a necessary condition to achieve R > or =0.98 with a 10-mm-thick slab.

Sub-diffractional waveguiding by mid-infrared plasmonic resonators in semiconductor nanowires.

Chains of nanoscale plasmonic resonators are capable of sub-diffractional waveguiding and have applications in nanophotonics and thermal radiation transport. Practical uses have largely been limited, however, due to high optical losses or low group velocities. Here, we predict the waveguide performance of a material structure capable of overcoming these limitations: plasmonic resonators embedded in high-dielectric nanowires. Due to the enhanced near-field coupling between resonators, we find that the group velocities and propagation lengths for doped Si plasmonic resonators in intrinsic Si nanowires can be increased by up to an order of magnitude compared to the case of isotropic vacuum surroundings. We investigate the impact of resonator aspect ratio, doping, and spacing on waveguide performance, and we find that propagation lengths are maximized for large aspect ratios and high dopant concentrations at small spacings. To study these complex anisotropic systems, we develop a new analytical "absorption spectra" method to extract waveguide information from simple far-field absorption experiments (or simulations) of only two coupled resonators.

Topography imaging with a heated atomic force microscope cantilever in tapping mode.

This article describes tapping mode atomic force microscopy (AFM) using a heated AFM cantilever. The electrical and thermal responses of the cantilever were investigated while the cantilever oscillated in free space or was in intermittent contact with a surface. The cantilever oscillates at its mechanical resonant frequency, 70.36 kHz, which is much faster than its thermal time constant of 300 micros, and so the cantilever operates in thermal steady state. The thermal impedance between the cantilever heater and the sample was measured through the cantilever temperature signal. Topographical imaging was performed on silicon calibration gratings of height 20 and 100 nm. The obtained topography sensitivity is as high as 200 microVnm and the resolution is as good as 0.5 nmHz(1/2), depending on the cantilever power. The cantilever heating power ranges 0-7 mW, which corresponds to a temperature range of 25-700 degrees C. The imaging was performed entirely using the cantilever thermal signal and no laser or other optics was required. As in conventional AFM, the tapping mode operation demonstrated here can suppress imaging artifacts and enable imaging of soft samples.

Coupling between gap plasmon polariton and magnetic polariton in a metallic-dielectric multilayer structure.

The excitation of plasmons in a metallic nanostructure represents a feasible and practical approach for manipulating the propagation and absorption of light at the subwavelength scale. Of particular interest is the coupling between plasmons, which can be used to facilitate the spectral tunability and tailor the optical response of the structure. In this paper, we study the coupling between two highly localized plasmonic modes: gap plasmon polariton mode and magnetic polariton mode, supported by a metallic-dielectric multilayer structure. The strong coupling gives rise to the formation of hybrid plasmon modes and large mode splitting. These hybrid modes result in unique spectral-directional absorption characteristics in the structure. The findings hold promise in applications such as photonic and energy conversion systems as well as the design of plasmonic nanodevices.

Comments on "Absolute linearity measurements on a PbS detector in the infrared".

A recent publication [Appl. Opt. 45, 2381 (2006)] on detector nonlinearity contains serious misunderstandings of an earlier work [Appl. Spectrosc. 51, 576 (1997)] with regard to the relative importance of photon flux versus photon irradiance as the major cause of these nonlinearities. This comment provides some clarification for the readers to correctly understand the techniques used in the 1997 paper and explains why the Theocharous statement was incorrect.

UNIFIED analytical formulation of thin-film radiative properties including partial coherence.

Radiative properties of thin films are derived based on the concept of optical coherence theory. Instead of the previous approach of deriving the property formulas based on the degree of coherence, a direct integration approach to obtain the averaged properties over a finite spectral resolution is developed. The analytical results are in excellent agreement with the measured spectra. The formulas are compact in form and easy to use to invert optical properties or film thicknesses with measured reflectance or transmittance. Rigorous criteria for incoherent and coherent limits can be easily reduced from the general formulas and the resulting equations corresponding to those of geometric and wave optics, respectively. These criteria are very useful in determining under what situations simpler wave optics and geometric optics formulas can be applied.

Brewster angle with a negative-index material.

The demonstration and confirmation of metamaterials with simultaneous negative permittivity and permeability, and thus a negative refractive index, has resulted in a surge of interest in the reflection and refraction phenomena at the interfaces of these so-called negative-index materials (NIMs). We present a systematic study of the Brewster angle, i.e., the angle of incidence at which no reflection occurs, for both TE and TM waves scattering at the interface between two semi-infinite planar media, one of which may be a NIM. Detailed physical explanations that account for the Brewster angle for a plane wave incident upon a NIM are provided under the framework of the Ewald-Oseen extinction theorem, considering the reemission of induced electric and magnetic dipoles. The conditions under which the Brewster angle exists are concisely summarized in a map of different material parameter regimes.