Polarization-dependent Reflection Research Articles

Guided and Space Waves Multiplexed Metasurface for Advanced Electromagnetic Functionalities in Microwave Region.

Nowadays, metasurfaces have attracted considerable attention due to their promising and advanced control of electromagnetic (EM) waves. However, it is still challenging to shape guided waves into desired free-space mode, while simultaneously manipulating spatial incident waves using a single metasurface. Herein, a class of metasurfaces capable of multiplexing guided and space waves is proposed to achieve advanced EM functionalities in microwave regions, which can find great application potentials in radar systems, wireless communications, and wireless power transfer (WPT). The proposed metasurface, composed of specially designed meta-atoms with polarization-dependent radiation and reflection properties, provides the capability to fully manipulate complex amplitude of guided waves and reflection phase of space incident wave independently and simultaneously, thus enabling arbitrary radiation and reflection functionalities without encountering crosstalk issues. As examples of potential applications, three advanced EM functionalities operating in both far-field and near-field regions are presented: low-sidelobe microwave antenna with reduced radar cross section (RCS), multifunctional WPT, and feed multiplexed holograms, respectively. The far-field characteristics of the low sidelobe level antennas showing radiated beams at ±30° together with RCS reduction under arbitrarily polarized incidences are validated by both simulations and measurements. A good agreement between experiments and simulations is also observed for the near-field intensity distribution of the hologram, which further validates the feasibility of near-field shaping. The findings significantly expand the capabilities of metasurfaces in manipulating EM waves and stimulate advanced multifunctional metadevices facing more challenging and diversified application demands.

Spin-orbit interaction-mediated measurement of surface chirality.

The spin-orbit (σ - l) interaction in a focused-reflected beam of light results in spatially nonuniform polarization in the beam cross section due to the superposition of orthogonal field components and polarization-dependent interface reflection coefficients. Polarization filtering the output beam leads to an interchangeable transformation of l=∓2 charge vortex into two (∓) unit charge vortices, for σ = ±1 circular polarization of the input Gaussian beam. This transformation follows a trajectory, named optical vortex trajectory, that depends on the input beam's σ and hence the l and reflecting surface characteristics. The vortex trajectory is used here to quantify both the sign and the magnitude of the chiral parameter of a quartz crystal. The Jones matrix-based simulation anticipates the chirality-dependent vortex trajectory that matches with experimental measurements.

In situ tracking anisotropic photocarrier dynamics in two-dimensional ternary Ta2NiSe5 via digital micromirror device-based pump-probe microscopy

In two-dimensional (2D) material studies, tracking the anisotropic ultrafast carrier dynamics is essential for the development of optoelectronic nano-devices. Conventionally, the anisotropic optical and electronic properties are investigated via either polarization-dependent Raman spectroscopy or field-effect transistors measurements. However, study of the anisotropic transient carrier behaviors is still challenging, due largely to the lack of picosecond-resolved acquisition or programmable scanning capabilities in the current characterization systems. In this work, we select Ta2NiSe5 as a model system to investigate the ultrafast anisotropic transportation properties of photo-excited carriers and transient polarized responses via a digital micromirror device (DMD)-based pump-probe microscope, where the probe beam scans along the armchair and zigzag directions of a crystal structure via binary holography to obtain distinct carrier diffusion coefficients, respectively. The results reveal the nonlinear diffusion behaviors of Ta2NiSe5 in tens of picoseconds, which are attributed to the interplay between excited electrons and phonons. The trend of the measured local polarization-dependent transient reflectivity is consistent with the polarized Raman spectra results. These results show that the DMD-based pump-probe microscope is an effective and versatile tool to study the optoelectronic properties of 2D materials.

Modular Assembly of Metamaterials Using Light Gradients.

This is a report on a pilot study that tests the feasibility of assembling photonic metamaterials (PMs) using light gradient forces. Following a strategy that works like modular construction, light gradient forces, produced by a tightly focused, 1D standing wave optical trap, time-multiplexed across a 2D lattice are used to assemble voxels consisting of prefabricated, monodispersed nanoparticles (NPs) with radii ranging from 30 to 500nm into 3D structures on a hydrogel scaffold. Hundreds of NPs can be manipulated concurrently into a complex heterogeneous voxel this way, and then the process can be repeated by stitching together voxels to form a metamaterial of any size, shape, and constituency although imperfectly. Imperfections introduce random phase shifts and amplitude variations that can have an adverse effect on the band structure. Regardless, PMs are created this way using two different dielectric NPs, polystyrene and rutile, and then the near-infrared performance for each is analyzed with angle-, wavelength-, and polarization-dependent reflection spectroscopy. The cross-polarized spectra show evidence of a resonance peak. Interestingly, whereas the line shape from the polystyrene array is symmetric, the rutile array is not, which may be indicative of Fano resonance. So, even with the structural defects, reflection spectroscopy reveals a resonance.

Probing the infrared properties of a p-doped Ge0.938Sn0.062 thin film via polarization-dependent FTIR spectroscopy

The complex relative permittivity of doped Ge1−xSnx thin films (realized using state-of-the-art growth techniques) are obtained by devising a methodology based upon polarization-dependent reflection measurements along with multi-layer Fresnel reflection equations. The developed approach is implemented to acquire the complex relative permittivity of a 170-nm-thick Ge1−xSnx film exhibiting a hole carrier concentration of 3.3 × 1019 cm−3 and x = 6.2%, with this Sn composition suggesting the film is on the cusp of exhibiting a direct bandgap. The investigation conducted on this thin film as well as the developed methodology are expected to further establish Ge1−xSnx as the primary semiconductor for on-chip light emission and sensing devices.

Can amine ligands atomically disperse Cu atoms on TiO2(110)? Cu deposition on TiO2(110) premodified with o-anthranilic acid

Abstract The three-dimensional (3D) structure of evaporated Cu atoms on a TiO2(110) surface premodified with o-anthranilic acid (o-AA) was examined using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) to elucidate the effect of the premodification on the Cu dispersion over the TiO2(110) surface. Cu was found to be atomically dispersed with the formation of N–Cu–O bonds (where the N is provided by the o-AA and the O is present in the TiO2 lattice) on the surface, and the factors for controlling the atomic dispersion are discussed.

Tuning of the Berreman mode of GaN/AlxGa1-xN heterostructures on sapphire: The role of the 2D-electron gas in the mid-infrared

We investigated the mid-infrared optical properties of GaN/Al(x)Ga(1-x)N-type heterostructures on sapphire substrate grown by the Metal-Organic Chemical Vapor Deposition. We show how the polarization-dependent reflection spectrum is affected by the presence of the strain-induced 2D electron gas at the interfaces between Al(x)Ga(1-x)N and GaN layers. In particular, we show that the 2D electron gas contribution, modeled from its density and transport properties, can play a relevant role at the Berreman mode excitation condition. In this framework, our results offer an advanced approach for the optimization and design of GaN-based broadband optoelectronic and energy management devices.

Bent-crystal Laue analyser combined with polarization-dependent total reflection fluorescence X-ray absorption fine structure (BCLA-TRFXAFS) and its application to electrode surface

Bent-crystal Laue analyser combined with polarization-dependent total reflection fluorescence X-ray absorption fine structure (BCLA-TRFXAFS) and its application to electrode surface

Measurement of azo dye film with large photo-induced birefringence and complex refractive index

Azo dyes are valuable functional materials due to their sensitive response to and effective modulation of polarized light. It is essential to find out not only the optical anisotropy but also the complex refractive index of azo dye materials for photonics applications. We report here such measurements of highly ordered photoaligned azo dye films. The film has an apparent dichroic ratio of about 60. We model the photoaligned azo dye film systems using the Berreman transfer matrix. By means of regression fitting of the polarization dependent transmittance and reflectance data, we were able to determine the complex refractive indices of the aligned film at the wavelength range of 400–1000 nm. This material is found to have exceptionally large optical birefringence (Δn ∼ 0.75 at 632.5 nm and Δn ∼ 0.48 at 850.6 nm). The aligned azo dye film is valuable to applications such as coatable polarizers and optical modulation devices.

Ultraviolet-C AlGaN Resonant-Cavity Light-Emitting Diodes with Thermal Stability Pipe-AlGaN-Distributed Bragg Reflectors.

Ultraviolet-C AlGaN resonant-cavity light-emitting diodes with top and bottom pipe-AlGaN-distributed Bragg reflectors (DBRs) have been demonstrated. For the top/bottom DBR structures, 20 pairs of n+-AlGaN:Si/n-AlGaN:Si stack structures were transformed into the pipe-AlGaN:Si/n-AlGaN:Si DBRs through a doping-selective electrochemical wet etching process. The reflectivity of the pipe-AlGaN DBR structure was measured as 90% at 276.7 nm with a 20.9 nm flat stopband width. The anisotropic optical properties of the pipe-AlGaN DBR structure had been analyzed through the polarization-dependent reflectance spectra. For temperature-dependent reflectance spectra, the central wavelengths were slightly redshifted from 275 nm (100 K) to 281 nm (600 K) due to thermal expansion, refractive index increase, and partial strain release phenomena in the pipe-DBR structure. High photoluminescence emission intensity and line-width reducing phenomena were observed at 10 K in the UVC-LED with the resonant-cavity structure, which has potential for high-efficiency UV-C light source applications.

In-plane and out-of-plane optical response of the nodal-line semimetals ZrGeS and ZrGeSe

Polarization-dependent reflectivity measurements were carried out over a broad frequency range on single-crystalline ZrGeSe and ZrGeS compounds, which are closely related to the prototype nodal-line semimetal ZrSiS. These measurements revealed the strongly anisotropic character of both ZrGeSe and ZrGeS, with a reduced plasma frequency for the out-of-plane direction {\bf E}$\| c$ as compared to the in-plane direction {\bf E}$\| ab$. For {\bf E}$\| ab$ the optical conductivity spectrum consists of two Drude terms followed by a shoulder or plateau-like behavior and a distinct U shape at higher energies, while for {\bf E}$\| c$ one Drude term is followed by a peak-like behavior and the U shape of the profile is less developed. Under external pressure, two prominent excitations appear in the out-of-plane optical conductivity spectrum of ZrGeSe, whose frequency position and oscillator strength show a weak anomaly at $\sim$3~GPa. Overall, the pressure-induced changes in the profile of the {\bf E}$\| c$ conductivity spectrum are much enhanced above $\sim$3~GPa. We compare our results to those recently reported for ZrSiS in a quantitative manner.

Polarization-dependent reflection of I-WP minimal-surface-based photonic crystal.

Brilliantly colored butterflies and weevils are known to utilize photonic crystals for their coloration. Interestingly, the morphology of such crystals made of cuticle is based on triply periodic minimal surfaces such as gyroid and diamond surfaces. Recently, a different minimal-surface-based photonic crystal, the I-WP surface, was discovered inside the scale of a longhorn beetle. The letter I is derived from expressing the body center symmetry and WP is derived from a wrapped package. It was reported that the brilliant green color is produced by the photonic band gap existing along the [110] direction of this crystal. In this study, the polarization dependence of the reflection from this photonic crystal was investigated. A peculiar reflectance spectrum with two peaks was observed under the crossed polarizers. This characteristic is theoretically reproduced by calculating the reflectance from a finite-sized photonic crystal, and the spectral shape is explained based on the symmetry of the electromagnetic modes. In addition, inspired by this longhorn beetle, a photonic crystal structure consisting of colloidal particles is proposed, which has a similar polarization effect.

A vertical cavity surface emitting laser for CPT atomic clock

A polarization-stabilized fundamental mode vertical cavity surface emitting laser (VCSEL) was developed for a coherent population trapping (CPT) Cs atomic clock. The VCSEL achieved a single stabilized polarization state through a directly-etched surface grating structure on the surface. The device polarization characteristics of different surface grating parameters were analysed using the finite-difference time-domain (FDTD) method. For the fundamental transverse mode condition, the lasing wavelength corresponded to the Cs atom’s D1 line. Owing to the polarization-dependent reflectivity of surface gratings, the VCSEL’s orthogonal polarization suppression ratio (OPSR) exceeded 20 dB. Since the surface grating eliminated the degeneracy of the two orthogonal linear polarization fundamental modes, a linewidth of approximately 53 MHz for the VCSEL could be obtained using the delayed self-homodyne method. A 7.6 GHz modulation bandwidth satisfied the atomic clock’s bandwidth requirement. We introduced the directly-etched surface grating VCSEL to the Cs atomic clock, and measured the CPT resonance signal.

Quasi-analytical and rigorous modeling of interference microscopy

We present an extended vectorial Kirchhoff model of coherence scanning interferometry including several vector rotations occurring in the imagining and scattering process as well as polarization dependent reflection coefficients. For validation simulated results are compared to those of the conventional scalar Kirchhoff model and a rigorous finite element modeling.

Engineering transient hyperbolic metamaterials using InAsSb-based semiconductor

The authors demonstrate the ability to create an ultrafast hyperbolic momentum state using metallic InAsSb alloys embedded within dielectric GaSb and explore the possibility of transient modification of metamaterials to control the optical properties of photon emission. Properly engineered quantum well structures were grown by molecular beam epitaxy and Si-doped in order to convert the InAsSb layers from dielectric to metallic at infrared frequencies. The carrier excitation scheme of the engineered hyperbolic stacks was investigated in a variety of excitation levels using pump–probe measurements. The photo-excited carriers in the structure with a metal fraction of ∼0.5 showed a polarization dependent reflectivity change, which indicates a transient hyperbolic metamaterial state in the heterostructure induced by the pump laser.

All-epitaxial long-range surface plasmon polariton structures with integrated active materials

We demonstrate all-epitaxial structures capable of supporting short- and long-range surface plasmon polariton (SRSPP and LRSPP) modes in the long-wave infrared region of the electromagnetic spectrum. The SRSPP and LRSPP modes are bound to the interfaces of a buried heavily doped (n++) semiconductor layer and surrounding quantum-engineered type-II superlattice (T2SL) materials. The surrounding T2SLs are designed to allow optical transitions across the frequency dispersion of the SPP modes. We map the SPP dispersion in our structure using grating-coupled angle- and polarization-dependent reflection and photoluminescence spectroscopy. The epitaxial structures are analytically described using a simplified three-layer system (T2SL/n++/T2SL) and modeled using rigorous coupled wave analysis with excellent agreement to our experimental results. The presented structures offer the potential to serve as long-range interconnects or waveguides in all-epitaxial plasmonic/optoelectronic systems operating in the long-wave infrared.

Extracting the local electronic states of Pt polycrystalline films surface under electrochemical conditions using polarization-dependent total reflection fluorescence x-ray absorption near edge structure spectroscopy

The local atomic information about the interface between the 30 nm-thick Pt polycrystalline films and the solution with and without perfluorosulfonic acid polymers (Nafion®) for the model cathode catalyst of fuel cell has been captured under electrochemical conditions using polarization-dependent total reflection fluorescence x-ray absorption near edge structure spectroscopy (PTRF–XANES). The results show that the formation of sub-monolayer-equivalent PtO or adsorbed hydrogen/oxygen species in the surface region can be successfully observed in the PTRF–XANES spectra when the thickness of the solution layer and the incidence angle are properly controlled. This capability enables us to examine the metal /(Nafion®/) solution interface structure through XANES together with other surface analysis methods, which will enhance comprehensive understanding of the nature of the interface of the fuel cell system.

Large Rabi splitting of mixed plasmon-exciton states in small plasmonic moiré cavities.

We report on exciton-plasmon coupling in metallic moiré cavities, both numerically and experimentally. Moiré cavities fabricated using double exposure laser interference lithography were filled with a molecular dye, J-aggregate. Polarization-dependent spectroscopic reflection measurements supported by simulations reveal strong coupling of organic dye excitons with cavity modes of the plasmonic moiré cavities. An anti-crossing at zero detuning, a clear indication of strong coupling, has been observed when the excitonic absorption band resonates with the cavity mode. Large Rabi splitting energies owing to the strong coupling of plasmons and excitons are clearly observed.

Multiscale Hierarchical Micro/Nanostructures Created by Femtosecond Laser Ablation in Liquids for Polarization-Dependent Broadband Antireflection.

In this work, we present the possibility of producing multiscale hierarchical micro/nanostructures by the femtosecond laser ablation of transition metals (i.e., Ta and W) in water and investigate their polarization-dependent reflectance. The hierarchical micro/nanostructures are composed of microscale-grooved, mountain-like and pit-rich structures decorated with hybrid laser-induced periodic surface structures (LIPSSs). The hybrid LIPSSs consist of low/high and ultrahigh spatial frequency LIPSSs (LSFLs/HSFLs and UHSFLs). LSFLs/HSFLs of 400–600 nm in a period are typically oriented perpendicular to the direction of the laser polarization, while UHSFLs (widths: 10–20 nm and periods: 30–50 nm) are oriented perpendicular to the curvatures of LSFLs/HSFLs. On the microstructures with height gradients, the orientations of LSFLs/HSFLs are misaligned by 18°. On the ablated W metasurface, two kinds of UHSFLs are observed. UHSFLs become parallel nanowires in the deep troughs of LSFLs/HSFLs but result in being very chaotic in shallow LSFLs, turning into polygonal nanonetworks. In contrast, chaotic USFLs are not found on the ablated Ta metasurfaces. With the help of Fourier transform infrared spectroscopy, it is found that microgrooves show an obvious polarization-dependent reflectance at wavelengths of 15 and 17.5 μm associated with the direction of the groove, and the integration of microstructures with LSFs/HSFLs/UHSFLs is thus beneficial for enhancing the light absorbance and light trapping in the near-to-mid-infrared (NIR-MIR) range.

Propyne Hydrogenation over a Pd/Cu(111) Single-Atom Alloy Studied using Ambient Pressure Infrared Spectroscopy

The hydrogenation of propyne (C3H4) to propene (C3H6) using a Pd/Cu(111) single-atom alloy (SAA) has been studied using polarization-dependent reflection absorption infrared spectroscopy. This meth...