Incident Polarization Direction Research Articles

Strong coupling in an asymmetric two-dimensional guided mode resonance metasurface structure and enhancement for optical third-harmonic generation

A two-dimensional guided mode resonance structure supports a transverse magnetic (TM) resonant mode in the direction of incident polarization and a transverse electric (TE) resonant mode in the direction perpendicular to the polarization. In this work, the coupling between the transverse magnetic and the transverse electric resonant modes in an asymmetric two-dimensional dielectric metasurface structure is investigated. The asymmetric structure consists of a two-dimensional square nanohole array etched in a titanium dioxide thin film on a transparent silica substrate. With finite difference time domain simulations, anti-crossing of the resonant spectra of the TM and TE modes is observed by adjusting the asymmetry of the structure. The anti-crossing indicates that the interaction between TM and TE resonant modes results in a strong coupling state. A coupled harmonic oscillator model is used to explain the strong coupling effect. The results of the coupled harmonic oscillator modeling agree well with the results of numerical simulations. Furthermore, it is shown that the strong coupling can significantly enhance the third harmonic generation intensity compared with the uncoupled TM and TE resonant modes.

Open Cross-gap Gold Nanocubes with Strong, Large-Area, Symmetric Electromagnetic Field Enhancement for On-Particle Molecular-Fingerprint Raman Bioassays.

Plasmonic nanoparticles with an externally open nanogap can localize the electromagnetic (EM) field inside the gap and directly detect the target via the open nanogap with surface-enhanced Raman scattering (SERS). It would be beneficial to design and synthesize the open gap nanoprobes in a high yield for obtaining uniform and quantitative signals from randomly oriented nanoparticles and utilizing these particles for direct SERS analysis. Here, we report a facile strategy to synthesize open cross-gap (X-gap) nanocubes (OXNCs) with size- and EM field-tunable gaps in a high yield. The site-specific growth of Au budding structures at the corners of the AuNC using the principle that the Au deposition rate is faster than the surface diffusion rate of the adatoms allows for a uniform X-gap formation. The average SERS enhancement factor (EF) for the OXNCs with 2.6 nm X-gaps was 1.2 × 109, and the EFs were narrowly distributed within 1 order of magnitude for ∼93% of the measured OXNCs. OXNCs consistently displayed strong EM field enhancement on large particle surfaces for widely varying incident light polarization directions, and this can be attributed to the symmetric X-gap geometry and the availability of these gaps on all 6 faces of a cube. Finally, the OXNC probes with varying X-gap sizes have been utilized in directly detecting biomolecules with varying sizes without Raman dyes. The concept, synthetic method, and biosensing results shown here with OXNCs pave the way for designing, synthesizing, and utilizing plasmonic nanoparticles for selective, quantitative molecular-fingerprint Raman sensing and imaging applications.

Bifunctional dual-channel polarization converter based on Weyl semimetal multilayer structure

In this paper, a multilayer structure (MS) of Weyl semimetals (WSMs) is utilized as the foundation for a polarization converter (PCT) having two different functions in two frequency channels. An analysis of the transmission spectrum of this layer structure at varying incident angles and polarization directions is also included. We define the polarization deflection angle ϕ and discuss the structure’s potential as a PCT with respect to the ϕ = 45° scenario. The incident angles were chosen as 0°, 5°, 10°, 15° and 20° for the analysis, and the results reveal that the incident linear polarization waves in the interval of 78.87 ∼ 81.90 THz can be effectively transformed into circular polarization ones for transmission case. While, for electromagnetic waves with frequencies ranging from 67.44 ∼ 72.11 THz, the direction of vibration for the electric field deflects by 45°. This work also provides positive insights into the design of polarization-related isolators, angle selectors, and other optical devices.

Data-driven modeling and fast adjustment for digital coded metasurfaces database: Application in adaptive electromagnetic energy harvesting

Metasurfaces (MSs) show great promise in efficient electromagnetic energy harvesting (EMEH) due to their compactness, high efficiency, and long-distance transmission capabilities. Nonetheless, the conventional iterative and time-consuming solving process of MSs significantly escalates computational demands. Furthermore, once processed, the MS shape remains fixed and cannot be adapted to changing requirements. Accordingly, a critical challenge is the development of a new efficient solver for MS real-time tuning. Here, we introduce a class of digital coded MS databases including multiple pre-defined resonant frequency MS. The combination of multiple MS base functions from the database enables swift resonance frequency adjustments to adapt to changing environmental conditions. A topology optimization method based on data-driven modeling is employed to rapidly acquire the optimal digital coding for the corresponding MS at various operating frequencies, facilitating the construction of a database. This approach integrates a convolutional neural network and genetic algorithm (CNNGA). It not only enables more accurate and expedited forward prediction of MSs' electromagnetic (EM) response but also facilitates inverse design based on specified requirements. We employ this method to design a MS that achieves perfect energy harvesting (EH) over a broad range of incident angles and polarization directions. In addition, a data-driven modeling is used to establish an EH efficiency predictive model corresponding to MS combination. This model serves as a guide for real-time MS adjustments as per changing requirements. Compared to previously designed MSs, this model achieves rapid design and adaptive adjustment capabilities. Through the incorporation of various functional MS base functions into the database, this method can be universally applied to MS combinations tailored to specific functions, including EM cloaking, ultra-thin flat lenses, and computational MSs.

Light-induced nonlinear spin Hall current in single-layer WTe2

In this theoretical investigation, we analyze light-induced nonlinear spin Hall currents in a gated single-layer 1T ′ -WTe2, flowing transversely to the incident laser polarization direction. Our study encompasses the exploration of the second and third-order rectified spin Hall currents using an effective low-energy Hamiltonian and employing the Kubo’s formalism. We extend our analysis to a wide frequency range spanning both transparent and absorbing regimes, investigating the influence of light frequency below and above the optical band gap. Additionally, we investigate the influence of an out-of-plane gate potential on the system, disrupting inversion symmetry and effectively manipulating both the strength and sign of nonlinear spin Hall responses. We predict a pronounced third-order spin Hall current relative to its second-order counterpart. The predicted nonlinear spin currents show strong anisotropic dependence on the laser polarization angle. The outcomes of our study contribute to a generalized framework for nonlinear response theory within the spin channel will impact the development of emerging field of opto-spintronic.

Experimental demonstration of acoustically induced polarization-dependent fiber optical vortex inversion.

A recently proposed theoretical model of acousto-optic interaction in optical fibers with a traveling flexural acoustic wave of the fundamental order [M.A. Yavorsky, et al., Opt. Lett.44, 598 (2019)10.1364/OL.44.000598] is experimentally examined. We show the effect of inversion of topological charge of optical vortices, which is governed by the direction of incident linear polarization. This vector effect of a coupling of polarization and orbital degrees of freedom proves the inconsistency of the conventional microbending-based model and confirms the recently suggested approach of the description of acousto-optic interaction that is based on the actual displacement vector. In addition, the obtained results demonstrate the realization of a controlled-NOT gate for orbital angular momentum (OAM) states.

Electrically controllable liquid crystal paraxial Fresnel zone plate based on concentric zones patterned electrode

A twist nematic liquid crystal Fresnel zone plate (LCFZP) with controllable diffraction efficiency based on concentric zones patterned electrode is demonstrated. The alternating “transparent” and “opaque” state occurs when the LCFZP placed between two crossed polarizers. When the electric field was applied to the odd (even) zones, the odd (even) zones were dark, and the even (odd) zones were bright. The diffraction efficiency was varied with changing the voltage or by rotating the incident polarization direction. The maximum diffraction efficiency can reach 37.8% which is close to the theoretical limit of 40.5%, and the response time is reasonably fast. The proposed LCFZP with the advantages of easy fabrication, compact structures, low power consumption, and good stability, which has potential applications in many modern optical devices.

Generation of three-dimensional polarization-controlled tunable multiplex focused optical vortex and vector vortex beams via liquid crystal geometric phase

Optical vortex beams carrying the orbital angular momentum (OAM), especially vector vortex beams, play essential roles in many fields such as optical communication and optical manipulation. In recent years, focusing and multiplexing optical vortex beams have attracted significant interest and become important research topics. In this study, a new type of liquid crystal geometric phase optical element is demonstrated to achieve multiplex focused optical vortex (FOV) beams. The 3D coordinates and the topological charges of each FOV in arrays can be customized on demand. Meanwhile, the distribution of arrays can be switched by the polarization of the incident light. Furthermore, the liquid crystal element is modified to generate multiplex focused vector vortex beams (FVVBs) based on the superposition of multiple OAM states. Similarly, the positions, the Pancharatnam topological charges, and the polarization orders of FVVBs can be preset as required. Besides, the polarization direction of incidence controls the polarization states of the generated FVVBs. This research achieves customized polarization-controlled multiplex FOVs and FVVBs with both multichannel and focusing properties, providing more flexibility for applications.

Theoretical Design of Broadband Tunable Nanograting-Coupled Fiber-Optic Surface Plasmon Resonance For Advanced Sensing

Fiber-optic surface plasmon resonance (FOSPR) technology has drawn considerable attention to the label-free biosensing field. Currently, all FOSPR sensing devices capitalizing on flat plasmonic film are only performed on the fiber sidewalls, and demonstrated in a limited wavelength range, which is unfavorable to broaden their practical sensing application. Here, we develop and theoretically demonstrate an effective alternative for the realization of SPR on a flat gold film achieved on the end facet of optical fiber through the diffracting coupling of nanograting. This enables the consecutively adjustment of operating wavelength over a broad spectral range from visible (650 nm) to near-infrared light (1550 nm) through modulating the nanograting period. The sensing performances of the proposed nanograting-coupled FOSPR sensors with different specified resonance wavelengths are numerically investigated from two aspects of bulk refractive index and surface sensitivities. As a result, the highest bulk refractive index sensitivity of 1163 nm/RIU is obtained at the operating wavelength of 1550 nm and the optimal surface sensitivity is dependent on the location away from top surface of structure. The dependence of incident polarization direction on the sensing performances of the proposed structure are also explored. Due to the ease of adjustment of operating wavelength and compact integration on the end facet, this sensing platform can further extend the utility of FOSPR sensing technology in the practical application fields of disease diagnosis, environment monitoring, etc.

Angle-Dependent Raman Scattering Studies on Anisotropic Properties of Crystalline Hexagonal 4H-SiC.

Raman scattering spectroscopy (RSS) has the merits of non-destructiveness, fast analysis, and identification of SiC polytype materials. By way of angle-dependent Raman scattering (ADRS), the isotropic characteristics are confirmed for c-face 4H-SiC, while the anisotropic properties of a-face 4H-SiC are revealed and studied in detail via combined experiments and theoretical calculation. The variation functional relationship of the angle between the incident laser polarization direction and the parallel (perpendicular) polarization direction was well established. The selection rules of wurtzite 4H-SiC are deduced, and the intensity variations of the A1, E2, and E1 Raman phonon modes dependent on the incident angle are calculated, and well-matched with experimental data. Raman tensor elements of various modes are determined.

Polarized Raman spectroscopy on topological semimetal Co3Sn2S2

AbstractWe present polarized Raman spectroscopy of the topological semimetal Co3Sn2S2, which was recently shown to host a Weyl semimetal phase. Stokes Raman spectra were obtained with the incident light parallel to the c‐axis of Co3Sn2S2. Two major phonon Raman peaks were observed at 289 and 386 cm−1 over continuous background emission signals. The intensity of the low‐wavenumber (289 cm−1) peak showed no polarization dependence. The high‐wavenumber (386 cm−1) peak and the continuous background signal were strongly polarized in the incident light polarization direction. These responses were almost independent of the in‐plane crystal orientation to the incident polarization, as is the manifestation of the point group symmetry of the unit cell of Co3Sn2S2. According to the group theory and Raman tensor analyses, the low‐ and high‐wavenumber Raman signals are attributed to Γ point phonon modes with and symmetries, respectively. Furthermore, line shape analyses revealed that the high‐wavenumber mode exhibited asymmetric peak feature well described by the Breit–Wigner–Fano function. These results suggest the Fano resonance between the phonon scattering with the continuous electronic background associated with low energy excitations near the Fermi energy. The clarified phonon energies and symmetries, as well as the electronic contribution to the Raman scattering, will not only be useful as a fingerprint to readily verify the experimentally grown or theoretically calculated crystal structure but also suggest importance of Raman spectroscopy as an effective tool to study low energy excitations and their interactions in Co3Sn2S2.

Conditional Generative Adversarial Networks for Inverse Design of Multifunctional Metasurfaces

Electromagnetic (EM) metasurfaces can present a versatile platform for realization of multiple diverse EM functionalities with incident wave frequency, polarization, or propagation direction through appropriate choice of unit cells structures. However, the inverse design of multifunctional metasurfaces relies on massive full‐wave EM numerical simulations to obtain an optimized solution. Herein, a step‐by‐step procedure based on conditional generative adversarial networks (cGANs) integrated with Gramian angular fields (GAFs) to reduce the computational time required for the EM simulations in the inverse design of multifunctional microwave metasurfaces is proposed. The proposed procedure initially implements GAFs to encode the desired multiobjective scattering parameters (SPs) to images and then passes them through the cGAN model to map them to three‐layer metasurfaces. The present study uses a robust dataset, including 54 000 metasurface structures and corresponding SPs to train and validate the cGAN model. This article also presents two case study examples using two multifunctional metasurfaces with different independent functionalities and full‐space coverage to justify the performance of the proposed procedure in the inverse design of multifunctional microwave metasurfaces. The case studies demonstrate that, despite the random nature of the training data samples, the cGAN reliably predicts the corresponding metasurfaces of the desired multiobjective SPs.

Dynamic Modulation of THz Absorption Frequency, Bandwidth, and Amplitude via Strontium Titanate and Graphene.

A multi-functional broadband absorber based on graphene and strontium titanate (STO) film was designed. Additionally, the frequency, bandwidth, and amplitude of the absorber could be tuned by adjusting temperature and Fermi level of the graphene. By using the finite-difference time-domain (FDTD) method, the numerical calculation result shows that, when keeping the device temperature at 230 K and setting graphene Fermi level to be 1 eV, three absorption peaks at 1.72 THz, 2.08 THz, and 2.59 THz were realized and combined into a broadband absorption from 1.68 to 2.74 THz. As the STO temperature was increased from 230 K to 310 K, the center frequency moved from 2.2 THz to 2.45 THz; correspondingly, the broadband absorption range was widened from 1.06 THz to 1.24 THz. When the temperature was fixed at 230 K and the graphene Fermi level was tuned from 1 eV to 0.7 eV, the absorption bandwidth decreased from 1.06 THz to 0.64 THz. While the Fermi level was tuned continually to be 0.01 eV, only a single absorption peak with an absorption rate of 0.29 existed. The broadband absorption and tuning mechanism of the absorber were analyzed using impedance matching theory. Furthermore, we also studied the effect of incident angle and polarization direction on the properties of the absorber. The multi-functional tunable absorber provides potential applications for the design of more efficient terahertz functional devices in the future.

Numerical Investigation of Enhanced and Quenched Radiative Decay Rate for One and Multiple Emitters near a Nanoparticle Surface

In the previous studies, Zhou et al. showed that the enhancement factor of the radiative decay rate of multiple emitters could be significantly smaller than that of one emitter near a silver nanoparticle surface at around its resonance wavelength. An asymmetric line shape was obtained in the enhancement factor spectrum when six or more emitters were included. Using a nanoparticle composed of silver, gold, glass, or materials with artificially defined real (n) and imaginary (k) parts of the index of refraction, we demonstrated that the enhancement factor of the radiative decay rate of six emitters near the nanoparticle surface will be smaller than that of one emitter when the localized surface plasmon resonance is excited. The asymmetric line shape of the enhancement factors for six emitters is affected by both the real and imaginary parts of the indices of refraction of the nanoparticle, and the asymmetric line shape will appear only when the real part (n) of the index of refraction of the nanoparticle is less than that of the environmental medium. The simulations show that a resonance peak in the enhancement spectrum of six emitters will be obtained when the induced electric field inside the particle is antiparallel to the incident polarization direction, whereas a resonance dip appears at wavelengths when the induced electric field inside the nanoparticle is parallel to the incident polarization direction. While the studies explained key parameters affecting the enhancement factor of the radiative decay rate of one and multiple emitters, developing analytical model is still necessary to reveal more physical insight behind the obtained numerical results.

Polarization Raman Imaging of Organic Monolayer Islands for Crystal Orientation Analysis.

An organic semiconductor film made of diphenyl derivative dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DPh-DNTT) has high carrier mobility. However, this mobility may be greatly affected by the crystal orientation of the DPh-DNTT’s first layer. Polarization Raman microscopy is widely used to quantitatively analyze the molecular orientation, and thus holds great potential as a powerful tool to investigate the crystal orientation of monolayer DPh-DNTT with high spatial resolution. In this study, we demonstrate polarization Raman imaging of monolayer DPh-DNTT islands for crystal orientation analysis. We found that the DPh-DNTT sample indicated a strong dependence of the Raman intensity on the incident polarization direction. Based on the polarization dependence, we developed an analytical method of determining the crystal orientation of the monolayer DPh-DNTT islands and experimentally confirmed that our technique was highly effective at imaging the islands’ crystal orientation with a spatial resolution of a few hundred nanometers.

Enhanced radiation pressure reversal on free carriers in nanoparticles and polarization dependence in the Rayleigh regime

The phenomenon of reversed radiation pressure on free electrons in a nanoparticle requires the necessary but insufficient condition that the real part of the particle’s permittivity be negative. It is shown that Rayeigh scattering theory is adequate for modeling the phenomenon by favorable comparison with Mie theory for single particles. A multiparticle Rayleigh scattering approach demonstrates that the radiation attraction of free electrons is enhanced by the interaction of closely spaced particles when the nanoparticles are in close proximity and separated along the incident wave polarization direction. However, the negative force on free carriers is reduced when the particles are aligned perpendicular to the incident electric field polarization. The accompanying increased radiation force on the material bound carriers is proposed as a step toward understanding how nanostructured materials and surfaces exhibiting reversed internal optical momentum can be designed with interacting nanoparticles for enhanced propulsion due to the ejection of hot electrons.

Three-dimensional nonlinear photonic crystal in naturally grown potassium\u2013tantalate\u2013niobate perovskite ferroelectrics

Since quasi-phase-matching of nonlinear optics was proposed in 1962, nonlinear photonic crystals were rapidly developed by ferroelectric domain inversion induced by electric or light poling. The three-dimensional (3D) periodical rotation of ferroelectric domains may add feasible modulation to the nonlinear coefficients and break the rigid requirements for the incident light and polarization direction in traditional quasi-phase-matching media. However, 3D rotating ferroelectric domains are difficult to fabricate by the direct external poling technique. Here, we show a natural potassium–tantalate–niobate (KTN) perovskite nonlinear photonic crystal with spontaneous Rubik’s cube-like domain structures near the Curie temperature of 40 °C. The KTN crystal contains 3D ferroelectric polarization distributions corresponding to the reconfigured second-order susceptibilities, which can provide rich reciprocal vectors to compensate for the phase mismatch along an arbitrary direction and polarization of incident light. Bragg diffraction and broadband second-harmonic generation are also presented. This natural nonlinear photonic crystal directly meets the 3D quasi-phase-matching condition without external poling and establishes a promising platform for all-optical nonlinear beam shaping and enables new optoelectronic applications for perovskite ferroelectrics.

Polarization independent enhancement of zeroth order diffracted second harmonic from multilayer gallium selenide on a silicon resonant metasurface.

We demonstrate polarization-independent resonant-enhancement of second harmonic generation (SHG) from multilayer Gallium Selenide (GaSe) on a silicon-based resonant metasurface. Two-dimensional hexagonal photonic lattice with circularly symmetric silicon meta-atoms are designed to achieve resonant field enhancement at the fundamental wavelength independent of the incident polarization direction. Such structures are however found to exhibit strong resonant field depolarization effects at the fundamental excitation fields resulting in modified nonlinear polarization components when compared to the native GaSe layer. Furthermore, the sub-wavelength metasurface designed to have resonances at the fundamental wavelengths act as a higher order diffraction grating at the second harmonic wavelength. Nonlinear wave propagation simulations show that the higher order diffracted SHG exhibit strong polarization dependent enhancement with characteristics very different from the native GaSe layer. In this context, polarization independent enhancement of the second harmonic signal is achieved only for the zeroth order diffracted component. Experimental study of second harmonic generation from the GaSe layer integrated with the silicon metasurface shows maximum nonlinear signal enhancement on-resonance with polarization dependence identical to the native GaSe layer by selectively detecting the zeroth-order diffracted component. This work shows that it is not sufficient to use symmetric meta-atoms in such 2D material integrated resonant metasurfaces for achieving polarization independent nonlinear optical enhancement. Depolarization of the resonant fields and higher-order diffraction at the nonlinear signal wavelength need to be considered as well.

Raman spectroscopy study of manganese oxides: Layer structures

Abstract Raman spectra were collected for an extensive set of well-characterized layer-structure Mn oxide mineral species (phyllomanganates) employing a range of data collection conditions. We show that the application of various laser wavelengths, such as 785, 633, and 532 nm, at low power levels (30–500 μW) in conjunction with the comprehensive database of standard spectra presented here, makes it possible to distinguish and identify the various phyllomanganate minerals. The Raman mode relative intensities can vary significantly as a function of crystal orientation relative to the incident laser light polarization direction as well as incident laser light wavelength. Consequently, phase identification success is enhanced when using a standards database that includes multiple spectra collected for different crystal orientations and with different laser light wavelengths. The position of the highest frequency Raman mode near 630–665 cm–1 shows a strong linear correlation with the fraction of Mn3+ in the octahedral Mn sites. With the comprehensive Raman database of well-characterized Mn oxide standards provided here (and available online as Online Material1), and use of appropriate data collection conditions, micro-Raman is a powerful tool for identification and characterization of biotic and abiotic Mn oxide phases from diverse natural settings, including on other planets, as well as for laboratory and industrial materials.

Raman spectroscopy study of manganese oxides: Tunnel structures

Abstract Raman spectra were collected for an extensive set of well-characterized tunnel-structure Mn oxide mineral species employing a range of data collection conditions. Using various laser wavelengths, such as 785, 633, and 532 nm at low power levels (30–500 µW), as well as the comprehensive database of standard spectra presented here, it is generally possible to distinguish and identify the various tunnel structure Mn oxide minerals. The Raman mode relative intensities can vary significantly as a function of crystal orientation relative to the incident laser light polarization direction as well as laser light wavelength. Consequently, phase identification success is enhanced when using a standards database that includes multiple spectra collected for different crystal orientations and with different laser light wavelengths. For the hollandite-group minerals, the frequency of the Raman mode near 630 cm–1 shows a strong linear correlation with the fraction of Mn3+ in the octahedral Mn sites. With the comprehensive Raman database of well-characterized Mn oxide standards provided here (and available online as Supplemental Materials1), and use of appropriate data collection conditions, micro-Raman is a powerful tool for identification and characterization of biotic and abiotic Mn oxide phases from diverse natural settings, including on other planets.