Strong Dichroism Research Articles

Quasi‐1D ZrS3 as an Anisotropic Nano‐Reflector for Manipulating Light–Matter Interactions

Abstract2D layered materials with low crystal symmetries exhibit unique anisotropic physical properties. Here the systematic studies on the optical modulation effects of such anisotropic 2D materials to isotropic 2D materials in their stacked van der Waals (vdW) heterostructures are reported. By applying angle‐resolved polarization spectroscopic characterizations on the MoS2/ZrS3 vdW heterostructure, periodic intensity variations of the Raman scattering and photoluminescence (PL) emission modes of monolayer MoS2 are observed, which are closely correlated to the anisotropic optical properties of the underlying ZrS3 layers. Such anisotropic optical modulation effects can be identified with the thickness of ZrS3 reduced to few layers (≈6 nm), and are attributed to the strong birefringence and dichroism effects in ZrS3 that cause reflection difference between its crystal axis, thus modulating the Raman/PL intensities of MoS2 via Fabry–Pérot interference effect. Furthermore, the polarized photocurrent response of the heterostructure is also demonstrated, where its major contribution originates from MoS2. This work develops a new methodology to tune the light–matter interactions and properties of isotropic 2D materials by the combination with anisotropic 2D materials, which substantially broadens the application of low symmetry layered materials in polarization sensitive optoelectronic devices.

Strong momentum-dependent electron–magnon renormalization of a surface resonance on iron

The coupling of electrons to spin excitations and the generation of magnons is essential for spin mixing in the ultrafast magnetization dynamics of 3d ferromagnets. Although magnon energies are generally much larger than phonon energies, until now their electronic band renormalization effect in 3d ferromagnets suggests a significantly weaker quasiparticle interaction. Using spin- and angle-resolved photoemission, we show an extraordinarily strong renormalization leading to two-branch splitting of an iron surface resonance at ∼200 meV. Its strong magnetic linear dichroism unveils the magnetic nature and momentum dependence of the energy renormalization. By determining the frequency- and momentum-dependent self-energy due to generic electron–boson interaction to compute the resultant electron spectral function, we suggest that the surface-state splitting can be described by strong coupling to an optical spin wave in an iron thin film.

Anisotropic magneto-optical absorption and linear dichroism in two-dimensional semi-Dirac electron systems

We present a theoretical study on the Landau levels (LLs) and magneto-optical absorption of a two-dimensional semi-Dirac electron system under a perpendicular magnetic field. Based on an effective k.p Hamiltonian, we find that the LLs are proportional to the two-thirds power law of the magnetic field and level index, which can be understood as a hybridization of the LL of Schrodinger and Dirac electrons with new features. With the help of Kubo formula, we find the selection rule for interband (intraband) magneto-optical transition is anisotropic (isotropic). Specifically, the selection rules for interband magneto-optical transitions are $\Delta n$=0, $\pm2$ ($\pm2$, $\pm4$) for linearly polarized light along the linear (parabolic) dispersion direction, while the selection rules for the intraband transition are $\Delta n$=$\pm1$, $\pm3$ regardless of the polarization direction of the light. Further, the magneto-optical conductivity for interband (intraband) transition excited by linearly polarized light along the linear dispersion direction is two (one) orders of magnitude larger than that along the parabolic dispersion direction. This anisotropic magneto-optical absorption spectrum clearly reflects the structure of the LLs, and results in a strong linear dichroism. Importantly, a perfect linear dichroism with magnetic-field tunable wavelength can be achieved by using the interband transition between the two lowest LLs, i.e, from Ev0 to Ec0. Our results shed light on the magneto-optical property of the two dimensional semi-Dirac electron systems and pave the way to design magnetically controlled optical devices.

Graphene-based dual-functional chiral metamirror composed of complementary 90\xb0 rotated U-shaped resonator arrays and its equivalent circuit model

An equivalent circuit model (ECM) using a MATLAB code to analyze a tunable two-layered graphene-based chiral dual-function metamirror, is proposed in this work. The investigated metastructure is composed of complementary U-shaped graphene resonator arrays in the terahertz (THz) region. The ECM analysis could be used for any two-layered chiral metastructure for any frequencies, containing resonators with a thickness less than λ/50. The characteristics of the proposed tunable metamirror were analyzed numerically using the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed metamirror can be used in polarization-sensitive devices in the THz region with simpler biasing without a need for ion gels or similar. It works as a broadband TE and multiband (four bands) TM mirror in the 0.3–4.5 THz bandwidth with a strong linear dichroism (LD) response (up to 96%). The designed mirror is a dynamically tunable, dual-functional structure, requiring only 90° rotation of the incident electromagnetic fields to switch between broadband and multiband spectral behavior making it a promising candidate for future THz intelligent systems. The proposed ECM is in agreement with the FEM results. The ECM analysis provides a simple, fast, and effective way to understand the metamirror’s behavior and guides for the design and analysis of graphene-based chiral metastructures in the THz region.

Unveiling the orbital texture of 1T-TiTe2 using intrinsic linear dichroism in multidimensional photoemission spectroscopy

The momentum-dependent orbital character in crystalline solids, referred to as orbital texture, is of capital importance in the emergence of symmetry-broken collective phases, such as charge density waves as well as superconducting and topological states of matter. By performing extreme ultraviolet multidimensional angle-resolved photoemission spectroscopy for two different crystal orientations linked to each other by mirror symmetry, we isolate and identify the role of orbital texture in photoemission from the transition metal dichalcogenide 1T-TiTe2. By comparing our experimental results with theoretical calculations based on both a quantitative one-step model of photoemission and an intuitive tight-binding model, we unambiguously demonstrate the link between the momentum-dependent orbital orientation and the emergence of strong intrinsic linear dichroism in the photoelectron angular distributions. Our results represent an important step towards going beyond band structure (eigenvalues) mapping and learning about electronic wavefunction and orbital texture of solids by exploiting matrix element effects in photoemission spectroscopy.

Morphology of ultrathin lithium fluoride deposited on Ag(100): Dendrites versus islands

We have investigated lithium fluoride (LiF) in the monolayer range deposited by thermal sublimation onto the (100) surface of a silver single crystal in ultrahigh vacuum. Our study combines scanning tunneling microscopy, low-energy electron diffraction, and polarized x-ray absorption spectroscopy. The results reveal that LiF grows epitaxially on Ag(100), with a morphology strongly depending on the growth temperature: at room temperature, dendritic patterns are formed with branches along the [011] and $[0\overline{1}1]$ directions, exhibiting the same lattice orientation as the substrate. Conversely, at a substrate temperature of 500 K LiF forms square islands exhibiting a moir\'e pattern, with the islands' borders oriented along the [001] and [010] directions. While the lattice constant of the surface deposited samples is close to the one of bulk fcc LiF, the appearance of a strong x-ray linear dichroism for dendritic LiF points to a tetragonal distortion arising from the LiF-substrate interaction. The less prominent dichroism and the presence of the moir\'e pattern on the square islands both suggest the relaxation of the LiF grown on the hot Ag substrate.

Rapid Fabrication of 3D Chiral Microstructures by Single Exposure of Interfered Femtosecond Vortex Beams and Capillary‐Force‐Assisted Self‐Assembly

AbstractThe rapid fabrication of 3D chiral microstructures is of great significance in the fields of optics and mechanics. Here, a hybrid strategy is presented that combines spatial‐light‐modulator‐assisted two‐photon polymerization (SATPP, top‐down) and capillary‐force‐assisted self‐assembly (bottom‐up) for efficiently yielding chiral microstructures. Based on SATPP, the pre‐subunit can be efficiently fabricated via 3D chiral discrete vortex beam (CDVB), which is generated by interfering multiple parallel vortex beams. Then, the 3D chiral microstructures are assembled by subunits with the aid of meniscus‐directed capillary force. This strategy can fabricate stable 3D chiral microstructures and improve the manufacturing efficiency by more than 100 times. Furthermore, the height, diameter, rotation angle, and handedness of chiral microstructure can be flexibly regulated. Because the obvious chiral feature of the manufactured assembly, it can exhibit strong vortical dichroism when excited by the light carrying orbital angular momentums. Also, it can be used to prepare functional microrobots base on the chiral body rotation. This hybrid strategy can rapidly manufacture 3D chiral microstructures, which can be utilized as multifunctional scalable platform with promising prospect in micro/nano robotics, optical sensors, and advanced functional devices.

Strong chiral dichroism and enantiopurification in above-threshold ionization with locally chiral light

We derive here a new highly selective photoelectron-based chirality-sensing technique that utilizes 'locally-chiral' laser pulses. We show that this approach results in strong chiral discrimination, where the standard forwards/backwards asymmetry of photoelectron circular dichroism (PECD) is lifted. The resulting dichroism is much larger and more robust than conventional PECD, is found in all hemispheres, and is not symmetric or antisymmetric with respect to any symmetry operator. Remarkably, a CD of up to 10% survives in the angularly-integrated above-threshold ionization (ATI) spectra, and of up to 5% in the total ionization rates. We demonstrate these results through ab-initio calculations in the chiral molecules Bromochlorofluoromethane, Limonene, Fenchone, and Camphor. We also explore the parameter-space of the locally-chiral field and show that the observed CD is strongly correlated to the degree of chirality of the light, validating it as a measure for chiral-interaction strengths. Our results pave the way for highly selective probing of ultrafast chirality in ATI, can potentially lead to all-optical enantio-separation, and motivate the use of locally-chiral light for enhancing ultrafast spectroscopies.

Continuous wave terahertz common path phase shifting interference using a liquid crystal phase shifter

A continuous wave terahertz common path phase shifting interference (PSI) method using a liquid crystal (LC) phase shifter was developed in this work. Using an LC phase shifter is a good candidate for common path PSI, however, it is difficult to achieve pure phase measurement in the terahertz region owing to the strong dichroism of LC materials. In this study, hydrogen-bonded LC, which exhibits no dichroism at 2.5 THz, was introduced to an LC phase shifter and the common path PSI was demonstrated using an optically pumped gas laser at 2.5 THz. Our experimental and calculation results indicate that the phase difference between the ordinary and extraordinary rays of x-cut quartz substrates was successfully obtained by using the four-step phase shifting algorithm. Furthermore, the bias noise such as the influence of reflection at the sample surface was eliminated during data processing based on the four-step phase shifting algorithm.

Two-Dimensional Guanidine-Based Hybrid Perovskites with Strong Dichroism for Multiwavelength Polarization-Sensitive Detection.

Two-dimensional (2D) organic-inorganic hybrid perovskites, benefiting from their natural anisotropy of quantum-well motifs and optical properties, have shown remarkable polarization-dependent responses superior to the 3D counterparts. Here, for the first time, multiwavelength polarization-sensitive detectors were fabricated by using single crystals of a guanidine-based 2D hybrid perovskite, (BA)2 (GA)Pb2 I7 (where BA+ is n-butylammonium and GA+ is guanidium). Its unique 2D quantum-well structure results in strong crystallographic-dependence of optical absorption. Strikingly, our crystal-based photodetector exhibits a prominent photocurrent dichroic ratio (Imax /Imin ) of ∼2.2 at 520 nm, higher than the typical 2D inorganic materials (GeSe, ∼1.09, PdSe2 , ∼1.8). In addition, notable dichroic ratios of 1.29 and 1.23 at 405 nm and 637 nm are also created for the multiwavelength polarized-light detection. The prominent detecting performances, including low dark current (1.6×10-11 A), considerable on/off ratio (∼2×103 ), high photodetectivity (∼3.3×1011 Jones) and responsivity (∼12.01 mA W-1 ), make (BA)2 (GA)Pb2 I7 a promising candidate for polarized-light detection. This work sheds light on the rational engineering of new 2D hybrid perovskites for the high-performance optoelectronic device applications.

Total Angular Momentum Dichroism of the Terahertz Vortex Beams at the Antiferromagnetic Resonances.

Terahertz vortex beams with different superposition of the orbital angular momentum l=±1, ±2, ±3, and ±4 and spin angular momentum σ=±1 were used to study antiferromagnetic (AFM) resonances in TbFe_{3}(BO_{3})_{4} and Ni_{3}TeO_{6} single crystals. In both materials we observed a strong vortex beam dichroism for the AFM resonances that are split in external magnetic field. The magnitude of the vortex dichroism is comparable to that for conventional circular dichroism due to σ. The selection rules at the AFM resonances are governed by the total angular momentum of the vortex beam: j=σ+l. In particular, for l=±2, ±3, and ±4 the sign of l is shown to dominate over that for conventional circular polarization σ.

Nanorod alignment for optical asymmetry

Plasmonics The high polarizability of chiral nanoassemblies of plasmonic nanoparticles can lead to strong chiral dichroism, but strong light scattering causes the fraction of polarized photons generated, as measured by g-factors, to be much lower than that for chiral liquid crystals. Lu et al. used supramolecular interactions of gold nanorods with human islet amyloid peptides to assemble metallic superstructures with unusually high cholesteric order (see the Perspective by Nam and Kim). The long, straight helices increased the g-factor by 4600-fold, and this effect was used to screen small-molecule binding to amyloids. Science , this issue p. [1368][1]; see also p. [1311][2] [1]: /lookup/doi/10.1126/science.abd8576 [2]: /lookup/doi/10.1126/science.abg9901

Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator.

Two-dimensional (2D) semiconductors enable the investigation of light-matter interactions in low dimensions1,2. Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials3,4. Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS3. The exciton exhibits a narrow photoluminescence linewidth of roughly 350 μeV with near-unity linear polarization. When we reduce the sample thickness from five to two layers, the photoluminescence is suppressed and eventually vanishes for the monolayer. This suppression is consistent with the calculated bandgap of NiPS3, which is highly indirect for both the bilayer and the monolayer5. Furthermore, we observe strong linear dichroism (LD) over a broad spectral range. The optical anisotropy axes of LD and of photoluminescence are locked to the zigzag direction. Furthermore, their temperature dependence is reminiscent of the in-plane magnetic susceptibility anisotropy. Hence, our results indicate that LD and photoluminescence could probe the symmetry breaking magnetic order parameter of 2D magnetic materials. In addition, we observe over ten exciton-A1g-phonon bound states on the high-energy side of the exciton resonance, which we interpret as signs of a strong modulation of the ligand-to-metal charge-transfer energy by electron-lattice interactions. Our work establishes NiPS3 as a 2D platform for exploring magneto-exciton physics with strong correlations.

Enhanced optical asymmetry in supramolecular chiroplasmonic assemblies with long-range order.

Chiral assemblies of plasmonic nanoparticles are known for strong circular dichroism but not for high optical asymmetry, which is limited by the unfavorable combination of electrical and magnetic field components compounded by strong scattering. Here, we show that these limitations can be overcome by the long-range organization of nanoparticles in a manner similar to the liquid crystals and found in helical assemblies of gold nanorods with human islet amyloid polypeptides. A strong, polarization-dependent spectral shift and the reduced scattering of energy states with antiparallel orientation of dipoles activated in assembled helices increased optical asymmetry g-factors by a factor of more than 4600. The liquid crystal-like color variations and the nanorod-accelerated fibrillation enable drug screening in complex biological media. Improvement of long-range order can also provide structural guidance for the design of materials with high optical asymmetry.

Giant Helical Dichroism of Single Chiral Nanostructures with Photonic Orbital Angular Momentum.

Optical activity, demonstrating the chiral light-matter interaction, has attracted tremendous attention in both fundamental theoretical research and advanced applications of high-efficiency enantioselective sensing and next-generation chiroptical spectroscopic techniques. However, conventional chiroptical responses are normally limited in large assemblies of chiral materials by circularly polarized light, exhibiting extremely weak chiroptical signals in a single chiral nanostructure. Here, we demonstrate that an alternative chiral freedom of light-orbital angular momentum-can be utilized for generating strong helical dichroism in single chiral nanostructures. The helical dichroism by monochromatic vortex beams can unambiguously distinguish the intrinsic chirality of nanostructures, in an excellent agreement with theoretical predictions. The single planar-chiral nanostructure can exhibit giant helical dichroism of ∼20% at the visible wavelength. The vortex-dependent helical dichroism, expanding to single nanostructures and two-dimensional space, has implications for high-efficiency chiroptical detection of planar-chiral nanostructures in chiral optics and nanophotonic systems.

Equivalent circuit model of graphene chiral multi-band metadevice absorber composed of U-shaped resonator array.

In this study, we have designed an equivalent circuit model (ECM) by use of a simple MATLAB code to analyze a single-layered graphene chiral multi-band metadevice absorber which is composed of U-shaped graphene resonator array in terahertz (THz) region. In addition, the proposed metadevice absorber is analyzed numerically by the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed device which is the first tunable graphene-based chiral metadevice absorber can be used in polarization sensitive devices in THz region. It is single-layered, tunable, and it has strong linear dichroism (LD) response of 94% and absorption of 99% for both transverse electric (TE) and transverse magnetic (TM) electromagnetic waves. It has four absorption bands with absorption >50% in 0.5-4.5 THz : three absorption bands for TE mode and one absorption band for TM mode. Proposed ECM has good agreement with the FEM simulation results. ECM analysis provides a simple, fast, and effective way to understand the resonance modes of the metadevice absorber and gives guidance for the analysis and design of the graphene chiral metadevices in the THz region.

Orbital angular momentum dichroism caused by the interaction of electric and magnetic dipole moments and the geometrical asymmetry of chiral metal nanoparticles

Circular dichroism (CD) caused by the response of a chiral object to circularly polarized light has been well established, and the strong CD of plasmonic meta-molecules has also become of interest in recent years; however, their response if the light also has orbital angular momentum is unclear. In this paper, the dichroism of a plasmonic cuboid-protuberance chiral structure under the illumination of a light beam with both orbital and spin angular momentums is numerically investigated. Distinguished spectra are observed under the different momentums. The circular dichroism under the combination of vortex beam and light spin is enhanced. This phenomenon is attributed to the partial spatial excitation of the nanoparticle, and the strong dichroism is simultaneously caused because of the interaction of the induced electric and magnetic modes and other higher-order modes caused by the partial excitation of the vortex beam. This research provides further insight into chiral light-matter interactions and the dichroism of light with orbital angular momentum.

2D anisotropic type-I SiS vdW heterostructures toward infrared polarized optoelectronics applications

Two-dimensional (2D) SiS has been drawing much attention due to its direct band gap and strong optical anisotropy under both two structural phases, namely, Pmma-SiS and Pma2-SiS. Here, we theoretically design Pmma-/Pma2-SiS van der Waals heterostructures (vdWH) through first-principles for the application of polarized light emitting devices. The calculations show that 2D Pmma-/Pma2-SiS vdWH possesses direct type-I band alignment with suitable band offsets (ΔEc = 0.163 eV, ΔEv = 0.202 eV), strong linear dichroism and high optical absorptions (~105 cm−1). Moreover, type-II band alignment with direct band gap can be achieved with the effect of external electric field, separating spatially the low-energy electron-hole pairs. Further, the band offsets display linear increase with the increase of external electric field, which indicates the recombination of charge carriers can be easily modulated. These results imply that Pmma-/Pma2-SiS vdWH can provide a promising way to design polarized infrared light emitting devices through using 2D anisotropic materials to form vdWH with type-I band alignment.

Laurentthomasite, Mg<sub>2</sub>K(Be<sub>2</sub>Al)Si<sub>12</sub>O<sub>30</sub>: a new milarite-group-type member from the Ihorombe region, Fianarantsoa Province, Madagascar

Abstract. Laurentthomasite, ideally Mg2K(Be2Al)Si12O30, is a new milarite-group member found within quartz-syenite pegmatites from the Ihorombe region, Fianarantsoa Province, Madagascar. It occurs as euhedral {0001} hexagonal crystals, maximum 15 mm large and 5 mm thick. The crystals show a very strong dichroism with cobalt blue and green-yellow colours when observed along [0001] and [1000], respectively. The mineral is transparent, uniaxial (+) and its lustre is vitreous. The hardness is about 6 (Mohs scale), showing a poor {0001} cleavage, irregular to conchoidal fracture, and a measured density of 2.67(8) g cm−3. Laurentthomasite is hexagonal, space group P6/mcc (no. 192), with a=9.95343(6) Å, c=14.15583(8) Å, V=1214.54(1) Å3 and Z=2. The strongest nine lines in the X-ray powder diffraction pattern [d in Å – (I) – hkl] are 3.171 – (10) – 211, 4.064 – (8) – 112, 2.732 – (8) – 204, 4.965 – (6) – 110, 2.732 – (4) – 204, 3.533 – (3) – 004, 7.055 – (2) – 002, 4.302 – (2) – 200 and 3.675 – (2) – 202. Chemical analyses by electron microprobe and several spectroscopies (inductively coupled plasma, ICP; optical emission, OES; mass, MS; and Mössbauer) give the following empirical formula based on 30 anions per formula unit: (Mg0.86 Sc0.54 Fe0.352+ Mn0.26)∑=2.01(K0.89 Na0.05 Y0.02 Ca0.01 Ba0.01)∑=0.98[(Be2.35 Al0.50 Mg0.11 Fe0.033+)∑=2.99(Si11.90 Al0.10)O30]; the simplified formula is (Mg, Sc)2(K, Na)[(Be, Al, Mg)3(Si, Al)12O30]. The crystal structure was refined to an R index of 1.89 % based on 430 reflections with Io > 2σ(I) collected on a four-circle diffractometer with CuKα radiation. By comparison with the general formula of the milarite group, A2B2C[T(2)3T(1)12O30](H2O) x (0<x<n, with n<2 pfu, per formula unit), the laurentthomasite structure consists of a beryllo-alumino-silicate framework in which the T(1) site is occupied by Si and minor Al and forms [Si12O30] cages linked by the T(2) site mainly occupied by (Be + Al). The A and C sites occur in the interstices of the framework while the B site is vacant. The origin of the strong dichroism is related to a charge transfer process between Fe2+ and Fe3+ in octahedral A sites and tetrahedral T(2) sites, respectively.

Broadband strong optical dichroism in topological Dirac semimetals with Fermi velocity anisotropy**Project supported by Singapore Ministry of Education (MOE) Tier 2 Grant No. (2018-T2-1-007) and USA ONRG Grant No. (N62909-19-1-2047). JL is supported by MOE PhD RSS. KJAO acknowledges the funding support of Xiamen University Malaysia Research Fund, Grant Nos. XMUMRF/2019-C3/IECE/0003 and XMUMRF/2020-C5/IENG/0025, and the Ministry of Higher Education Malaysia

Prototypical three-dimensional (3D) topological Dirac semimetals (DSMs), such as Cd3As2 and Na3Bi, contain electrons that obey a linear momentum–energy dispersion with different Fermi velocities along the three orthogonal momentum dimensions. Despite being extensively studied in recent years, the inherent Fermi velocity anisotropy has often been neglected in the theoretical and numerical studies of 3D DSMs. Although this omission does not qualitatively alter the physics of light-driven massless quasiparticles in 3D DSMs, it does quantitatively change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications. Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction. Although the signature conductivity-frequency scaling, σ(ω) ∝ ω, of 3D Dirac fermion is well-protected from the Fermi velocity anisotropy, the linear optical response exhibits strong linear dichroism as captured by the universal extinction ratio scaling law, Λij = (vi/vj)2 (where i ≠ j denotes the three spatial coordinates x,y,z, and vi is the i-direction Fermi velocity), which is independent of frequency, temperature, doping, and carrier scattering lifetime. For Cd3As2 and Na3Bi3, an exceptionally strong extinction ratio larger than 15 and covering a broad terahertz window is revealed. Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarization-sensitive functionalities, such as photodetection and light modulation.