Uniaxial Birefringence Research Articles

Generating arbitrary polarization states by manipulating the thicknesses of a pair of uniaxial birefringent plates

Generating arbitrary polarization states by manipulating the thicknesses of a pair of uniaxial birefringent plates

Birefringent optical responses of single-chirality carbon nanotube membranes

Membranes composed of single-walled carbon nanotubes (SWCNTs) with a specific chiral structure (chirality) possess unique optical properties dominated by thermally robust quasi-one-dimensional excitons. Therefore they have emerged as promising materials for photonic and/or thermo-optic applications. As SWCNTs in the membranes are usually aligned two-dimensionally in a plane, and the optical responses of individual SWCNTs are highly anisotropic, the macroscopic responses of membranes are expected to depend on the light-propagation direction. However, the complex refractive-index spectra of some membrane axes are unknown, hindering the optical design of SWCNT-based devices that respond to arbitrarily propagating light. Here, we report the anisotropic complex refractive index spectra of a single-chirality SWCNT membrane fabricated via vacuum filtration. The polarization- and angle-resolved reflection spectra were obtained in the near-infrared-to-visible region. All spectral features in different polarizations and incident angle configurations were consistent with a uniaxial birefringent membrane, reflecting the random in-plane orientations of the SWCNTs. The ordinary (in-plane) refractive index spectra presented a series of sharp resonances of parallel-polarized excitons. The extraordinary (out-of-plane) refractive index in the 0.8–2.4 eV range was determined as ∼1.9. The extraordinary spectra included small but indispensable contributions from the cross-polarized exciton resonance, which are necessary for properly predicting the angle-dependent optical responses of the membrane. By completely unveiling the birefringent optical responses of single-chirality SWCNT membranes, this work paves the way for integrating SWNCT membranes into precise and diverse photonic and/or thermo-optic devices.

Disulfide‐Bridged Dynamic Covalent Triazine Polymer Thin Films by Interface Polymerization: High Refractive Index with Excellent Optical Transparency

AbstractExploring innovative strategies for molecular structuring of dynamic materials that combine self‐correcting intrinsic reversibility with the robustness of covalent bonds, has been a long‐standing objective from applications perspective in fields ranging from molecular engineering to nanotechnology and interfacial science. To establish dynamic covalent chemistry approaches combined with interfacial polymerization, herein, a distinct synthetic approach is reported to develop disulfide‐bridged 2D polymeric C3N3S3 triazine thin‐films by interfacial thiol‐disulfide dynamic exchange process crosslinking tritopic planar 1,3,5‐triazine‐2,4,6‐trithiol molecular tectons via intermolecular disulfide formation in the presence of I2 vapors at the air/water interface under redox condition. The resulting centimeter‐scale polymeric thin‐films are covalently cross‐linked, dynamic in nature, featuring tunable thickness (6–200 nm) and significant morphological variations are realized under the influence of varying reaction time, concentration and types of reducing agents. Notably, C3N3S3 polymer thin films exhibit a transflectance of around 99.5% in the range from 430 to 1800 nm, show high refractive indices (1.730–1.488) and optical anisotropy with uniaxial negative birefringence. The C3N3S3 free‐standing polymer thin‐films can be easily transferred to different substrates or possibly into application‐relevant forms for device fabrications, making this useful from materials application perspective.

Evaluation of Fossil Amber Birefringence and Inclusions Using Terahertz Time-Domain Spectroscopy.

Using a cross-polarization transmission geometry, stress maps for the normalized birefringence and intrinsic stress direction of polymeric materials may be obtained using terahertz nondestructive evaluation. The analysis method utilizes a deconvolution method to determine the arrival times and amplitude of the cross-polarized terahertz pulses through a birefringent material. Using amber (a naturally occurring polymer) as a material of interest, stress maps show that inclusion-free Lebanese amber samples behave as classic uniaxial birefringent (photoelastic) materials whose principal stress directions, as inferred in the terahertz spectral range, agree well with visible photoelasticity measurements. Since amber samples, depending upon their source, may be either transparent or opaque to visible light, comparing birefringence measurements in the visible and terahertz spectral ranges cross-validates the stress measurements, thereby establishing a strong and unique stress analysis methodology for visibly opaque samples. While the material of interest for this paper is amber, the method is generally applicable for any terahertz-transparent polymer. The cross-polarization experimental configuration enables stress levels within the amber matrix to be visualized while also outlining highly localized regions of stress surrounding inclusions. Birefringence stress maps clearly show localized increases in stress magnitude and directional changes surrounding inclusions.

Recent developments on the Casimir torque

The Casimir force acts on nearby surfaces due to zero-point fluctuations of the quantum electromagnetic field. In the nonretarded limit, the interaction is also known as the van der Waals force. When the electromagnetic response of the surfaces is anisotropic, a torque may act on the surfaces. Here, we review the literature and recent developments on the Casimir torque. The theory of the Casimir torque is discussed in an explicit example for uniaxial birefringent plates. Recent theoretical predictions for the Casimir torque in various configurations are presented. A particular emphasis is made on experimental setups for measuring the Casimir torque.

Diamond-rich crystalline nanosheets seeded with a Langmuir monolayer of arachidic acid on water.

The enigmatic self-assembling ability of nanodiamond (ND) particles has been discovered herein. Diamond-rich crystalline nanosheets with thickness of approximately ∼25 nm were grown from a Langmuir monolayer of arachidic acid (AA) at the interface between air and a dilute aqueous ND solution. Their fine rectangular shapes with uniform uniaxial birefringence indicate appreciable crystallinity, thus supporting that they are hydrated colloidal crystals of homogeneous ND particles.

Modes of wide-aperture acousto-optic diffraction in a uniaxial birefringent crystal

Acousto-optic (AO) anisotropic interaction in uniaxial birefringent crystals is a widespread and versatile approach to tunable spectral filtration of light. Its principal constraints are limited values of spectral and angular bandwidth restricting the light transmission and imaging performance of AO filters. A way to overcome these drawbacks is simultaneous Bragg diffraction of two or more light beams. In this paper, we discuss and compare the main features of four available modes of anisotropic wide-aperture AO interactions in uniaxial birefringent crystals. We calculate the angular aperture, spectral resolution and other parameters and estimate the spectral image quality features related to each one. Theoretical consideration and computational modeling are carried out for tellurium dioxide (TeO2)—the main AO crystal. Derived results are important for the development of AO imaging devices, particularly multi-channel AO tunable filters.

The Role of Bio-Extracted Reduced Graphene Oxide in the Crystallization Kinetics of Chitosan Bio-Polymer

Carbon nanomaterials have recently attracted wide scientific applications due to their tunable properties. These novel materials act as best fillers that can provide substantial benefits due to their high strength, thermal conductivity, and electrical conductivities. With their huge applications as bulk materials, when implemented in polymer matrix as fillers, they give rise to new promising materials with which their properties can be tuned to suit a particular application. Besides the development of these new nanocomposite materials, there exist some challenges which must be fully surpassed to explore the potentiality of application of carbon-based nanocomposites. Reduced graphene oxide is one of the carbon derivatives which has attracted the current advancement in technology, and recently, it found its new applications in super capacitors used in electronic industries. The limiting factor for its exploration is the affordability. New and affordable sources of these graphene-based nanomaterial have to be devised, for fully realization of their potential applications. In this study, reduced graphene oxide and the bio-polymer chitosan were extracted from the locally available bio waste materials. Nanocomposites were prepared at 50% rGO: chitosan ratio. The films were then prepared by spin coating method. Prepared films were subjected to morphological analysis. From the results, it was observed that rGO induced chitosan crystallization, which led to formation of dendritic structures. Cellulose nanocrystals have thus displayed temperature dependent positive uniaxial birefringence

Beryllium (Be) composition dependent structural and optoelectronic characteristics of wurtzite BexMg1-xS ternary alloys: First principle calculations with FP-LAPW scheme

For the first time, beryllium (Be) composition dependent structural and optoelectronic characteristics of wurtzite BexMg1-xS ternary alloys have been investigated through first principle calculations. The WC-GGA scheme for structural and both EV-GGA and mBJ-GGA schemes for optoelectronic properties have been utilized for computing exchange-correlation potentials. The lattice constants (a0,c0) decrease, while bulk modulus (B0) increases nonlinearly with increasing Be-composition x. Though MgS is a direct (Γ-Γ) and BeS is an indirect (M-Γ) band gap semiconductor, their amalgamation possesses direct band gap (Γ-Γ) ternary semiconductor alloys. Calculated minimum band gap (Eg) with mBJ-GGA potentials is higher then that with EV-GGA for each specimen and it nonlinearly reduces with increasing Be-composition x. Calculated higher effective mass of hole compared to electron is indicating the domination of electrons over holes in carrier transport phenomenon in each specimen. The electronic transport properties of the considered specimens have been computed in terms of their Seebeck coefficients, electrical conductivities, electronic thermal conductivities, electronic power factors, electronic specific heats and Pauli magnetic susceptibilities. Each specimen shows optical anisotropy and hence uniaxial birefringence. The peak(s) in the ultraviolet region of the dielectric function spectra of the considered specimens are contributed by S-3p→Be-3s, 2p & Mg-4s, 4p electronic excitations. Moreover, enhancement in calculated Eg causes reduction in calculated zero-frequency limits in ε1(ω), n(ω) and R(ω) spectra, but enhancement in critical point energies in the ε2(ω), k(ω), α(ω) and σ(ω) spectra and vice versa.

A theoretical investigation of structural, electronic and optical properties of wurtzite BexZn1-xO ternary alloys using DFT based FP-LAPW approach

Structural and optoelectronic properties of wurtzite BexZn1-xO ternary alloys are carried out using density functional FP-LAPW approach in association with modified Becke-Johnson (mBJ) functional. Each stable wurtzite specimen under consideration exhibits direct band gap (Γ-Γ). Nonlinear decrease in lattice constants (a, c), but increase in bulk modulus and fundamental band gap (Eg) are observed with increasing Be-concentration x. The frequency responses of dielectric function, refractive index, extinction coefficient, optical conductivity and absorption, reflectivity and energy loss function have been computed. Each wurtzite specimen is optically anisotropic and hence shows uniaxial birefringence. The electronic transitions O-2p → Be-3 s,2p & Zn-5 s,4p are responsible for the occurrence of intense peak(s) in each dielectric function spectra. The static dielectric constant, static refractive index and static reflectivity decrease, while critical point energies in each of the ε2(ω), k(ω), α(ω) and σ(ω) spectra increases with increase in band gap and vice versa.

Pressure effect on mechanical stability and ground state optoelectronic properties of Li2S2 produced by Lithium−Sulfur batteries discharge: GGA-PBE, GLLB-SC and mBJ investigation

ABSTRACTAccording to several previous works, Li2S2 occurs during discharge of lithium–sulfur (Li–S) batteries. Several information on the physical properties of this compound remain unknown including those of its ground state. In this work, a study of the pressure effect on the elastic and optoelectronic properties of Li2S2 in its tetragonal structure of P42/mnm space group was carried out. According to the previous works and according to the structural results, which have been obtained by GGA-PBE and optPBE-vdW funtionals, we have found that P42/mnm-Li2S2 energy is the lowest compared to other studied structures (P63/mmc, P-62m and Immm), this can indicate that P42/mnm space group structure may represent its ground state structure. The study of the pressure effect on the elastic properties has shown that P42/mnm-Li2S2 maintains its mechanical stability for a pressure range between 0 and 2.5 GPa. The study of the directional dependence of Young's modulus has shown that it is anisotropic and therefore indicates the elastic anisotropy of the studied compound. According to the electronic results, the fundamental band-gap of P42/mnm-Li2S2 is of an indirect nature, its values found by mBJ and GLLB-SC are close to each other. The pressure effect on the possible transitions between the valence band top and the conduction band bottom and on the variations of the refractive index and the absorption coefficient according to both optical directions has been studied by determining the dielectric function. The found optical results have shown that P42/mnm-Li2S2 has negative uniaxial birefringence.

Speckle-free smoothing of coherence laser beams by a homogenizer on uniaxial high birefringent crystal

We report on a new type of a random phase plate (RPP) that can be applied for effective homogenization of coherent laser beams. The RPP is fabricated on a uniaxial birefringent crystal, ensuring the absence of speckles within the processing plane due to two orthogonally polarized beamlets with π phase difference. High fabrication speed ∼ 2 mm2/s of RPP takes at least 10 minutes to fabricate it. RPPs test showed the transformation of Gauss intensity distribution to the flat-top one characterized by low-intensity modulation ∼1% of the coherent laser beam. Then, RPP was applied in a picosecond laser setup for ZnO film microprocessing.

Tight Focusing Properties of Circularly Polarized Annular Multi-Gaussian Beam through a Uniaxial Birefringent Crystal

Tight Focusing Properties of Circularly Polarized Annular Multi-Gaussian Beam through a Uniaxial Birefringent Crystal

Phase transition in BaThO3 from Pbnm to Ibmm turn the fundamental energy band gap from indirect to direct

The influence of phase transition on the electronic structure and the optical properties of BaThO3 is investigated by means of density functional theory. At room temperature BaThO3 is stable in the Pbnm phase, whereas it is stable in the Ibmm phase at high temperature. The transition from the Pbnm to the Ibmm phase cause a change in the band gap (Eg) nature from indirect to direct and a reduction by around 0.3 eV. The calculated Eg is about 4.9 eV (Pbnm) and 4.6 eV (Ibmm). The phase transition influences the k-dispersion of bands around the Fermi level and, hence, the effective masses resulting in increasing the mobility of the charge carrier and enhancing the charge transfer mechanism. The obtained optical properties clearly show the influence of phase transition on the electronic structure. It was noticed that moving from Pbnm →Ibmm phase leads to shift the whole spectral structure towards lower energies by around 0.3 eV and increase the magnitudes of the optical components. It is found that the Pbnm and Ibmm phases exhibit negative uniaxial anisotropy and negative birefringence.

Counter‐Propagating Optical Trapping of Resonant Nanoparticles Using a Uniaxial Crystal

AbstractLaser tweezing of optically resonant nanostructures, such as plasmonic nanoparticles and high‐index dielectric nanoresonators, is extremely challenging because the enhanced light–matter interaction usually amplifies radiation pressure to an extent where conventional single beam gradient trapping in three dimensions becomes impossible. Such particles are therefore typically trapped off resonance or in two dimensions only. To extend the application potential of optical tweezers to the resonant case, focus splitting inside a uniaxial birefringent crystal and reflection from a mirror is used to develop a counter‐propagating beam configuration based on a single microscope objective. The setup allows one to trap and rapidly rotate resonant gold nanorods in water far from any interface, thereby opening a range of possibilities for novel studies of resonantly enhanced optical forces and interactions in uniform environments.

Acousto-optic tunable spectral filtration of stereoscopic images.

We propose a new technique for three-dimensional (3-D) imaging in arbitrary spectral intervals. It is based on a simultaneous diffraction of two divergent stereoscopic light beams on a single acoustic wave propagating in a uniaxial birefringent crystal. We discuss in detail this configuration of acousto-optic (AO) interaction, derive basic relations, and experimentally demonstrate the applicability of the proposed approach to 3-D spectral imaging. A stereo-imager of this type may be produced as an ultra-compact embeddable optical element, which is promising for many imaging applications.

Tightly Focusing Properties of Radially Polarized Double Ring Shaped Beam through a Uniaxial Birefringent Crystal

Tightly Focusing Properties of Radially Polarized Double Ring Shaped Beam through a Uniaxial Birefringent Crystal

Vapor and liquid optical monitoring with sculptured Bragg microcavities

Sculptured porous Bragg microcavities (BMs) formed by the successive stacking of columnar SiO2 and TiO2 thin films with a zig-zag columnar microstructure are prepared by glancing angle deposition. These BMs act as wavelength-dependent optical retarders. This optical behavior is attributed to a self-structuration of the stacked layers involving the lateral association of nanocolumns in the direction perpendicular to the main flux of particles during the multilayer film growth, as observed by focused ion beam scanning electron microscopy. The retardance of these optically active BMs can be modulated by dynamic infiltration of their open porosity with vapors, liquids, or solutions with different refractive indices. The tunable birefringence of these nanostructured photonic systems has been successfully simulated with a simple model that assumes that each layer within the BMs stack has uniaxial birefringence. The sculptured BMs have been incorporated as microfluidic chips for optical transduction for label-free vapor and liquid sensing. Several examples of the detection performance of these chips, working either in reflection or transmission configuration, for the optical monitoring of vapor and liquids of different refractive indices and aqueous solutions of glucose flowing through the microfluidic chips are described.

Generating multiple focal structures with circularly polarized double ring shaped beam and axial birefringence

The properties of circularly polarized double ring shaped beam tightly focused through a uniaxial birefringent crystal are studied numerically by the use vectorial diffraction theory for small birefringence. Novel focal structures such as splitting of single focal spot into multiple spots and the focal shift of maximum intensity can be obtained with the proper combination of the topological charge [n], birefringence [Δn], and the pupil to beam ratio [β] of the input circularly polarized double ring shaped beam.

Spin-polarized Second Harmonic Generation from the Antiferromagnetic CaCoSO Single Crystal

The spin-polarized second harmonic generation (SHG) of the recently synthesized CaCoSO single crystal is performed based on the calculated electronic band structure. The calculation reveals that the spin-up (↑) channel of CaCoSO possesses a direct energy gap (Γv-Γc) of about 2.187 eV, 1.187 eV (Kv-Kc) for the spin-down (↓) channel and an indirect gap (Γv-Kc) of about 0.4 eV for the spin-polarized CaCoSO single crystal. The linear optical properties obtained reveal that the recently synthesized crystal exhibits considerable anisotropy with negative uniaxial anisotropy and birefringence favor to enhance the SHG. We have calculated the three non-zero tensor components of the SHG and found the is the dominat component, one with a large SHG of about (d33 = 6.936 pm/V at λ = 1064 nm), the half value of KTiOPO4 (KTP). As the values of (↑) < (↓) < spin-polarized are related to the values of the energy gap of (↑) 2.187 eV> (↓) 1.187 eV> spin-polarized gap 0.4 eV; therefore, a smaller energy gap gives better SHG performance. Furthermore, the microscopic first hyperpolarizability, βijk, is calculated.