Strong Polarization Dependence Research Articles

Spectral properties and β-ray scintillation of GdVO4:Eu single crystals grown by the floating zone method

GdVO4:Eu single crystals were successfully grown along the [110] direction by the floating zone method. The grown crystals had no cracks and inclusions for any Eu-concentration. The polarizing microscopy proved that the grown crystal contained no microscopic defects such as low-angle grain boundaries and inclusions. The crystal showed red fluorescence with the peak wavelength of 620 nm. The emission showed strong polarization dependence; the emission of π-polarization is more intensive than that of strongly the σ-polarization. Under irradiation of X-ray or electron beam, the crystals also showed strong red fluorescence based on the host excitation. The fluorescence lifetime was measured to be about 785 μs. Scintillation the GdVO4:Eu single crystals for β-ray was successfully demonstrated using nuclear medicine 18F-fludeoxyglucose.

Spectral dynamics of noise-like square pulses generated in a Tm-doped figure-8 fiber laser

We report the generation of a series of noise-like square pulses (NLSPs) at 2 μm in a figure-8 Tm-doped fiber laser. The NLSP demonstrates a square shape as well in the spectral domain and possesses a resonant property in spectrum similar to that in time. The spectral resonance ranges from 10 nm to about 20 nm under an appropriate control of polarization, an indication of its wide mode-locking regime. The unique spectral dynamics exhibits a strong polarization dependence, subject to the nonlinear mode-locking mechanism. A subsequent numerical simulation shows that a uniform gain dispersion distribution accounts for the forming of the flat-topped spectra.

Temperature-Dependent Exciton Dynamics in a Single GaAs Quantum Ring and a Quantum Dot.

Micro-photoluminescence was observed while increasing the excitation power in a single GaAs quantum ring (QR) at 4 K. Fine structures at the energy levels of the ground (N = 1) and excited (N = 2) state excitons exhibited a blue shift when excitation power increased. The excited state exciton had a strong polarization dependence that stemmed from the asymmetric localized state. According to temperature-dependence measurements, strong exciton–phonon interaction (48 meV) was observed from an excited exciton state in comparison with the weak exciton–phonon interaction (27 meV) from the ground exciton state, resulting from enhanced confinement in the excited exciton state. In addition, higher activation energy (by 20 meV) was observed for the confined electrons in a single GaAs QR, where the confinement effect was enhanced by the asymmetric ring structure.

Ultrafast Tunable Broadband Optical Anisotropy in Two-Dimensional ReS2

Anisotropic two-dimensional materials provide promising platforms for polarization-driven optoelectronic and photonic devices. In particular, layered rhenium disulfide (${\mathrm{Re}\mathrm{S}}_{2}$) has recently attracted intensive attention due to its unique linear dichroism. However, control over the optical properties of ${\mathrm{Re}\mathrm{S}}_{2}$ is mostly limited to the band-edge excitons, which significantly limits the application wavelength range. Here, we utilize ultrafast transient absorption microscopy to tune the broadband optical anisotropy of few-layer ${\mathrm{Re}\mathrm{S}}_{2}$. We observe a broad nonexcitonic photoinduced-absorption feature, which exhibits weak anisotropy immediately after pump excitation, but, surprisingly, shows a strong polarization dependence after a few picoseconds, leading to a broadband enhancement in the optical anisotropy. We attribute this to carrier cooling and subsequent anisotropic free-carrier absorption due to anisotropic carrier effective masses. This work provides not only a principle for the ultrafast active control of broadband polarization-sensitive photonic devices but also insights into anisotropic carrier dynamics in two-dimensional materials.

Polarization-independent electro-optic phase modulator based on trilayer birefringent materials

ABSTRACT Manipulating the phase and amplitude of light led to many important breakthroughs in optics, and it is still considered one of the most critical technologies in emerging optical systems. However, conventional phase modulators generally suffer from the strong polarisation dependence that stems from the anisotropic optical property of the base material. Here, we report a phase modulator that is polarisation-independent and is based on the trilayer of birefringent materials which have their optical axes aligned at specific angles. The proposed structure effectively eliminates the polarisation dependence of the phase modulator without compromising its electro-optic properties. This technology provides a simple but significant method for developing a polarisation-independent phase modulator without fabrication complexity.

Nonlinear optical response of type-II Weyl fermions in two dimensions

We present a theoretical model to study the nonlinear optical response of materials with a low-energy spectrum characterised by strongly tilted, type-II, Weyl cones in two dimensions, which we call 2D Weyl materials. Our findings reveal that the tilted nature of the Weyl cones is responsible for the appearance of even harmonics in the nonlinear signal, as well as its strong polarisation dependence. We discuss how it is possible to control such nonlinear response and envision how 2D Weyl materials can be used to realise novel photonic devices for sensing applications.

Polarization dependence of atomic high-order harmonic generation: Description using a discrete basis

The optical generation of high-order harmonics is known to have a strong polarization dependence: In contrast to linearly polarized excitation, circularly polarized light induces practically no harmonics. In the current paper we focus on atomic targets, the case when a well-established physical picture explains the effect: For circular polarization, the photoionized electrons never return to their parent nuclei, and the energy they gained while being accelerated by the field is not transferred into high-order harmonic radiation. This is essentially a picture that is based on real-space electron trajectories (or the dynamics of the wave functions). Here, we provide an alternative description that uses the discrete Sturmian basis, and points out how quantum mechanical interference effects and selection rules can explain the polarization dependence of the process. This emphasizes the importance of space-time symmetries during the process of high-order harmonic generation.

Polarized dependence of soft X-ray absorption near edge structure of ZnO films implanted by Yb

Virgin and Yb-implanted epitaxial ZnO films grown using atomic layer deposition (ALD) were investigated by X-ray absorption spectroscopy (XAS). XAS study revealed a strong polarization dependence of films determined by the orientation of the polarization vector of the synchrotron radiation to the sample surface. It also indicated that the implantation and subsequent annealing have an important influence on the native point defect complexes in the ZnO. Comparison of experimental spectra with the modelled ones, which are computed based on the linear combination of model spectra corresponding to the selected point defects and their complexes, confirmed the presence of donor-acceptor complexes (mVZn - nVO, m = 1,4; n = 1,2) in the samples under study. The mechanism of vacancy complexes formation is unclear as it takes place under non-equilibrium conditions, for which any theoretical method has not been well established. Exploring the 3 d → 4 f absorption, it was found that oxidation state of Yb in ZnO is 3+, which is consistent with the XPS findings and previously conducted Resonant Photoemission Spectroscopy (RPES) investigations. The inversion of the polarization dependence for samples with different Yb fluences visible in Yb M5 spectra can be associated with a tilt of the oxygen pseudo octahedra or/and with their distortion. The analysis of the presented data suggests that the donor-acceptor complexes are present both in as grown and implanted films and may influence their electrical properties. This suggestion was confirmed by previous Hall measurements showing that the resistivity of annealed ZnO:Yb film with a fluence of 5e15 ions/cm2 decreases by about one order compared to the one with a fluence of 5e14 ions/cm2.

Optical Bound States in the Continuum Enabled by Magnetic Resonances Coupled to a Mirror.

Dielectric metasurfaces made of high refractive index and low optical loss materials have emerged as promising platforms to achieve high-quality factor modes enabling strong light-matter interaction. Bound states in the continuum have shown potential to demonstrate narrow spectral resonances but often require asymmetric geometry and typically feature strong polarization dependence, complicating fabrication and limiting practical applications. We introduce a novel approach for designing high-quality bound states in the continuum using magnetic dipole resonances coupled to a mirror. The resulting metasurface has simple geometric parameters requiring no broken symmetry. To demonstrate the unique features of our photonic platform we show a record-breaking third harmonic generation efficiency from the metasurface benefiting from the strongly enhanced electric field at high-quality resonances. Our approach mitigates the shortcomings of previous platforms with simple geometry enabling facile and large-area fabrication of metasurfaces paving the way for applications in optical sensing, detection, quantum photonics, and nonlinear devices.

Visualizing Line Defects in non-van der Waals Bi2O2Se Using Raman Spectroscopy.

Atomic-layered materials, such as high-quality bismuth oxychalcogenides, which are composed of oppositely charged alternate layers grown using chemical vapor deposition, have attracted considerable attention. Their physical properties are well-suited for high-speed, low-power-consumption optoelectronic devices, and the rapid determination of their crystallographic characteristics is crucial for scalability and integration. In this study, we introduce how the crystallographic structure and quality of such materials can be projected through Raman spectroscopy analysis. Frequency modes at ∼55, ∼78, ∼360, and ∼434 cm-1 were detected, bearing out theoretical calculations from the literature. The low-frequency modes positioned at 55 and 78 cm-1 were activated by structural defects, such as grain boundaries and O-rich edges in the Bi2O2Se crystals, accompanied by sensitivity to the excitation energy. Furthermore, the line defects at ∼55 cm-1 exhibited a strong 2-fold polarization dependence, similar to graphene/graphite edges. Our results can help illuminate the mechanism for activating the Raman-active mode from the infrared active mode by defects, as well as the electronic structures of these two-dimensional layered materials. We also suggest that the nanoscale width line defects in Bi2O2Se can be visualized using Raman spectroscopy.

Orbital localization and the role of the Fe and As 4p orbitals in BaFe2As2 probed by XANES

The polarization dependence of the near edge x-ray absorption spectroscopy (XANES) is an element specific probe to the real-space distribution of the density of unoccupied states in solid-state materials. In this paper, we present Fe and As $K$-edge experiments of Ba(Fe$_{1-x}$$M_{x}$)$_{2}$As$_{2}$ ($M=$ Mn, Co and $x=0.0$ and $0.08$). The experiments reveal a strong polarization dependence of the probed XANES spectra, which concerns mainly an increase of the intensity of electronic transitions when the beam polarization is set out of the sample's $ab$ crystallographic plane. The results show that states with $p_{z}$-orbital character dominate the density of unoccupied states close to the Fermi level. Partial substitution of Fe by Co is shown to decrease the intensity anisotropy, suggesting that Co promotes electronic transfer preferentially to states with $p_{z}$-orbital character. On the other hand, Mn substitution causes the increase of the spectra $p_{z}$-orbital anisotropy, which is proposed to take place by means of an enhanced local Fe $3d4p$ mixing, unveiling the role of Fe $4p$ states in the localization of the Fe $3d$ orbitals. Moreover, by comparing our results to previous experiments, we identify the relative mixing between Fe and the pnictide $4p_{x,y,z}$ orbitals as a clear divide between the electronic properties of iron arsenides and selenides. Our conclusions are supported by multiple-scattering theory calculations of the XANES spectra and by quantum chemistry calculations of Fe coordination electronic structure.

Color-selective geometric-phase lenses for focusing and imaging based on liquid crystal polymer films.

The geometric-phase lens (GPLs) with small form factor compared to traditional refractive lenses has been identified as a compelling solution in augmented-/virtual-/mixed-reality (AR/VR/MR) headsets. Formed either with liquid crystals (LCs) or metasurfaces, the GPL is a type of emerging leading technology that implements the arbitrary aspheric phase to realize low loss and minimal ghosting. However, the inherent chromatic abberation (CA) of GPLs can significantly degrade the image quality. A possible solution is the independent spectral phase implementation for RGB. In this work, we propose the design of three types of multi-twist LC based color-selective GPLs (CS-GPLs), exhibiting highly chromatic efficiency spectra with diameter 30 mm, focal length around 41.2~mm, and F -number 1.37. Through theoretical and experimental validation, each type of CS-GPL manifests high diffraction efficiency (>91%) on respective primary color of orthogonal polarization and high transmission on the complementary color of input polarization. The triplet composed by RGB CS-GPLs demonstrates relative contrast ratio and minimal ghosting. The strong color and polarization dependency of CS-GPLs not only provide a novel technique to mitigate CA but also offer more design freedom in the AR/VR/MR polarization and imaging system.

Two Water-Soluble and Wash-Free Fluorogenic Probes for Specific Lighting Up Cancer Cell Membranes and Tumors.

The construction of microenvironment-sensitive probes with good cell membrane-targetability can reveal the fundamental properties of cell membranes. Herein, two polarity-sensitive probes, termed MEMs were reported for the first time to specifically light up cancer cell membranes. Both probes were designed with tetrahydroquinoxaline coumarin amide as the fluorophore, and quaternary ammonium groups were appended to increase water solubility and target cell membranes. In vitro studies showed that the fluorescence of both probes displayed strong polarity dependence and had a wide linear range to polarity (Δf). MEMs also displayed excellent cell membrane targeting ability and could long-term light up cell membranes with red fluorescence and a wash-free process. More excitingly, MEMs could specifically light up cancer cell membranes, revealing that cancer cells might have lower cell membrane polarity than normal cells. In vivo studies showed that MEMs could also effectively distinguish tumors from normal tissues. Overall, this work has not only developed two polarity-sensitive probes with good cell membrane targetability, but also provided new insights and methods for an in-depth understanding of cancer cells and cancer diagnosis.

Theoretical and experimental analysis of second order Bragg resonance of 45° TFG and its fiber laser application

We report the observation of second order Bragg resonance (2nd-OBG) produced by tilted fiber gratings (TFGs) fabricated using phase mask UV inscription. The theoretical analysis has revealed that the generation of high order Bragg resonance of gratings is induced by a square-shape refractive index profile, which is caused by over-saturated UV exposure. In the experiment, we have studied the TFGs with different tilt angles under over-saturated UV exposure, in which all gratings have showed the 2nd-OBR, and the larger tilt angle of the grating has the stronger 2nd-OBR. When the tilt angle of the grating is ∼45°, the Bragg resonance exhibits very strong polarization dependence, because the 2nd-OBR wavelength is located within the polarizing bandwidth of 45° TFG. Finally, we have demonstrated an erbium-doped fiber laser with >99.9% degree of polarization, and, by applying mechanical stretching on the grating, a wavelength tunable laser output has been achieved. The output laser shows ∼0.2 dB amplitude variation within 1 h continuous monitoring of the laser.

Core-cap heterodimer independent of polarization direction of excitation light

The plasmonic anisotropic nanostructure possesses the enhanced surface electric field and unique optical properties in near-infrared spectrum, thus it has potential applications in nano-optoelectronics and medical sensing. To obtain the best property, the excitation polarization normally needs to match the orientation of the structure. The strong polarization dependence, however, greatly limits the excitation efficiency. In this work, a patchy structure is introduced to release the dependence of polarization. In the proposed method here in this work, the lost properties due to unmatched polarizations are compensated for by the plasmonic resonance coupling between the patch and capped structure in the heterozygous dimer. By overlapping the two modes at the same wavelength, the absorption keeps rather stable undisturbed status during the variation of incident polarization. This work focuses on the theoretical exploration of the feasibility. Electromagnetic field in the interaction between light and heterozygous dimer is essential before extinction coefficient is calculated. The field of the model is obtained by solving Maxwell equations through using the finite element method. The numerical calculation presents a good understanding of the mechanism of the plasmonic interactions in the dimer, based on which the nanostructure with optimized configuration parameters can achieve the stable and high absorption in the near infrared wavelength.

Raman Anisotropy and Polarization-Sensitive Photodetection in 2D Bi2O2Se-WSe2 Heterostructure.

Two-dimensional (2D) bismuth oxyselenide (Bi2O2Se) has attracted increasing attention due to its high mobility, tunable band gap, and air stability. The surface reconstruction of cleaved Bi2O2Se due to the electrostatic interlayer interactions can lead to the in-plane anisotropic structure and physics. In this work, we first discovered the strong anisotropy in phonon modes through the angle-resolved polarized Raman (ARPR) spectra. Benefiting from the anisotropic feature, a high-performance polarization-sensitive photodetector has been achieved by constructing a heterostructure composed of the multilayer Bi2O2Se as polarized-light sensitizers and 2D WSe2 as a photocarrier transport channel. The detectors exhibit broadband response spectra from 405 to 1064 nm along with high responsivity, fast speed, and high sensitivity owing to the photogating effect in this device architecture. More importantly, the photocurrent shows strong light polarization dependence with the maximum dichroism ratio of 4.9, and a reversal is observed for the angle-dependent photocurrent excited by polarized 405 and 635 nm light. This work provides new insight in terms of optical and photocurrent anisotropy of exfoliated Bi2O2Se and expands its applications in angle-resolved electronics and optoelectronics.

Enhancement of polarization anisotropy in ultrafast carrier dynamics by intramolecular excitation in the organic conductor κ-(BEDT- TTF)2Cu(NCS)2

In this work, in order to investigate the origin of the anomalous polarization dynamics observed in the normal conducting state of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2, we performed optical pump probe measurements by varying the probe pulse energy from to 1.59 eV. We found that the polarization-dependent transient signal was most enhanced at , which was close to the intramolecular excitation energy of the BEDT-TTF molecules but is not exactly matched. This difference is linked to the formation of an energy gap at the Fermi energy at low temperatures, which results in an increase or decrease in pump-induced absorption between the states near the Fermi energy and the intramolecular excited states depending on probe polarization. In other words, the intramolecular excitation at 1.41 eV, which has a strong polarization dependence, serves as a probe of the dynamics involved in gap formation. A simple calculation shows that the size of the gap is , which is close to that of the BEDT-TTF based Mott insulators. It is suggested that the gap arises from the phase-separated Mott state caused by photoinduced modulation of effective electron correlation.

Metallic nanosphere-assisted coupling ultrafast surface plasmon polaritons background-free tip nanofocusing.

Plasmonic tip nanofocusing has gained much attention owing to its wide application in the field of nanospectroscopy. Here, we present the Au nanosphere (AuNS)-assisted coupling ultrafast surface plasmon polaritons (SPP) background-free tip nanofocusing. The plasmonic tip was prepared by attaching an AuNS on the shaft of an Au conical tip fabricated by electrochemical etching. The AuNS was adopted as an antenna to couple the far-field excitation light to the propagating SPP along the shaft to the tip apex for achieving power compression. Importantly, we experimentally and theoretically demonstrate that such a plasmonic tip can realize background-free ultrafast SPP tip nanofocusing with radially polarized features in a wide spectral range based on the localized SPP resonance effect supported by AuNS. Furthermore, the intensity of the tip nanofocusing light field has strong polarization dependence under linearly polarized light excitation, providing a powerful platform for spatiotemporal light control on the nanoscale. Our technique realizes remote excitation of background-free tip nanofocusing with a structured light feature, and it holds promising potential for tip-enhanced nanospectroscopies, nonlinear nanophotonics, etc.

Self-assembled ultrafine CsPbBr3 perovskite nanowires for polarized light detection

One-dimensional (1D) metal halide perovskite nanowire (NW) arrays with high absorption efficiency, emission yield and dielectric constants, as well as anisotropic optoelectronic properties have found applications in energy harvesting, flexible electronics, and biomedical imaging devices. Here, a modified two-step solvothermal method is developed for the synthesis of self-assembled cubic CsPbBr3 NW arrays. This method provides facile access to continuous and uniform ultrafine perovskite NWs and well-aligned pure perovskite NW arrayed architectures. Under excitation at 365 nm, the CsPbBr3 NWs give a strong blue emission observable to the naked eyes. The CsPbBr3 NWs also exhibit strong two-photon excited luminescence under the irradiation with an 800 nm pulse laser. By rotating the polarization angle of the 800 nm laser, strong polarization dependence with a polarization degree up to ∼0.49 is demonstrated in the self-assembled CsPbBr3 NW array, although the CsPbBr3 NWs have an isotropic cubic structure. Based on density functional theory (DFT) calculations, this polarization-dependent emission is correlated with the anisotropic charge density distribution of the perovskite NWs. These findings suggest that the ultrafine CsPbBr3 NWs with a well-defined self-assembled architecture could be applied as next-generation polarization-sensitive photoelectronic detection materials.

Atomic configuration controlled photocurrent in van der Waals homostructures

Conventional photocurrents at a p–n junction depend on macroscopic built-in fields and are typically insensitive to the microscopic details of a crystal’s atomic configuration. Here we demonstrate how atomic configuration can control photocurrent in van der Waals (vdW) materials. In particular, we find bulk shift photocurrents (SPCs) can display a rich (atomic) configuration dependent phenomenology that range from contrasting SPC currents for different stacking arrangements in a vdW homostructure (e.g. AB vs BA stacking) to a strong light polarization dependence for SPC that align with crystallographic axes. Strikingly, we find that SPC in vdW homostructures can be directed by modest strain, yielding sizeable photocurrent magnitudes under unpolarized light irradiation and manifesting even in the absence of p–n junctions. These demonstrate that SPC are intimately linked to how the Bloch wavefunctions are embedded in real space, and enables a new macroscopic transport probe (photocurrent) of lattice-scale registration in vdW materials.