Principal Optical Axis Research Articles

Programmable Carbon Nanotube Networks: Controlling Optical Properties Through Orientation and Interaction.

The lattice geometry of natural materials and the structural geometry of artificial materials are crucial factors determining their physical properties. Most materials have predetermined geometries that lead to fixed physical characteristics. Here, the demonstration of a carbon nanotube network serves as an example of a system with controllable orientation achieving on-demand optical properties. Such a network allows programming their optical response depending on the orientation of the constituent carbon nanotubes and leads to the switching of its dielectric tensor from isotropic to anisotropic. Furthermore, it also allows for the achievement of wavelength-dispersion for their principal optical axes - a recently discovered phenomenon in van der Waals triclinic crystals. The results originate from two unique carbon nanotubes features: uniaxial anisotropy from the well-defined cylindrical geometry and the intersection interaction among individual carbon nanotubes. The findings demonstrate that shaping the relative orientations of carbon nanotubes or other quasi-one-dimensional materials of cylindrical symmetry within a network paves the way to a universal method for the creation of systems with desired optical properties.

Triplet Epipolar Images for Spaceborne Three Linear Cameras

Abstract. Epipolar images can reduce conjugate points searching from a two-dimensional to a one-dimensional space, which significantly improves the robustness and efficiency of dense matching. However, two images matched the lack of redundancy and suffered from occlusions. The third image could introduce two more stereos and improve the accuracy and completeness of the three-dimension reconstruction, which were adopted by spaceborne three linear cameras (TLCs), the ALOS PRSIM and ZY-3. In this study, we propose a new triplet epipolar images generation method for spaceborne TLCs. Triplet epipolar images were defined as any two of the three epipolar image pairs. The triplet epipolar geometry requires the imaging rays of any conjugate point to be coplanar. For high-resolution satellite images, parallel projection can be used to approximate the imaging geometry of satellite images. Therefore, a coplanar principal optical axis is a fundamental requirement for spaceborne TLCs. We propose a general workflow to generate triplet epipolar images (TEIs), which includes free-net bundle adjustment of TLCs, building a triplet epipolar geometry, correcting the y-parallaxes, and generating a rational function model (RFM) for TEIs. The ZY3-02 satellite images were used to validate the proposed method. The root means square error (RMSEs) of the free-net bundle adjustment in the image space was 0.185 pixels, which proved the fine intrinsic accuracy. After compensation, the RMSEs of the y parallaxes of the three epipolar image pairs were 0.295, 0.310, and 0.370 pixels. Owing to the simple geometry of the TEIs, the RMSEs of the RFM replacements were within 0.001 pixels.

Wandering principal optical axes in van der Waals triclinic materials

Nature is abundant in material platforms with anisotropic permittivities arising from symmetry reduction that feature a variety of extraordinary optical effects. Principal optical axes are essential characteristics for these effects that define light-matter interaction. Their orientation – an orthogonal Cartesian basis that diagonalizes the permittivity tensor, is often assumed stationary. Here, we show that the low-symmetry triclinic crystalline structure of van der Waals rhenium disulfide and rhenium diselenide is characterized by wandering principal optical axes in the space-wavelength domain with above π/2 degree of rotation for in-plane components. In turn, this leads to wavelength-switchable propagation directions of their waveguide modes. The physical origin of wandering principal optical axes is explained using a multi-exciton phenomenological model and ab initio calculations. We envision that the wandering principal optical axes of the investigated low-symmetry triclinic van der Waals crystals offer a platform for unexplored anisotropic phenomena and nanophotonic applications.

Universal Method for Determining the Principal Optical Indicatrix Axes in Triclinic Crystals

Determining the orientations of principal optical indicatrix axes is a key prerequisite for the applications of optical crystals, which is still an intriguing challenge for triclinic optical crystals due to the lowest symmetry. The prism method is one of the important ways to determine these directions, but no actual examples are available owing to the complexity and scarcity of the triclinic crystals so far. To solve this remaining problem among seven crystal systems, we proposed a universal method, the so-called “multiple prisms method,” to determine the orientations of principal optical indicatrix axes X, Y, and Z as well as the principal refractive index values nx, ny, and nz in triclinic crystals based on four prisms and demonstrated this method by employing triclinic Nd3+-doped La2CaB8O16 crystals, which is of great significance to the studies and applications of triclinic optical crystals in the future.

Spectroscopic properties and microchip laser performance of Yb:LaCa4O(BO3)3 crystal

In this work, the polarized absorption and emission spectra of the Yb:LaCa4O(BO3)3 (Yb:LaCOB) crystal were studied, and the microchip laser performance was investigated with 1.0 mm thick crystals cut along its three principal optical axes. The strongest absorption is found to occur at 977.2 nm for E//X, with the cross-section amounting to 2.21 × 10−20 cm2, the largest for all of the Yb-doped oxyborates developed so far; whereas the maximum cross-section of the main emission band, 0.30 × 10−20 cm2 for E//Z, proves to be the smallest among all the Yb-doped oxyborates. The X-cut microchip crystal laser could be operated simultaneously in the main emission band and in the long-wavelength sideband, producing an output power of 2.15 W with optical conversion efficiency of 48.1 %. While operated in the emission sideband, the Z-cut microchip laser could generate 2.14 W of output power at 1068.6–1077.8 nm with an optical conversion efficiency of 45.1 %. Output power of 2-W level could also be produced from the X-cut microchip laser operating at 1028.1–1033.6 nm, or from the Z-cut microchip laser operating at 1042.7–1050.7 nm, with optical conversion efficiencies higher than 40 %.

Large birefringence in the quasi-one-dimensional material Sb2Se3: A DFT investigation of electronic and optical anisotropy

Investigation of the optical and electronic properties of the quasi-one-dimensional semiconductor Sb2Se3 is performed with density functional theory, which takes into consideration the spin–orbit coupling and the on-site Coulomb self-interaction. Sb2Se3 has an orthorhombic crystal structure and belongs to the Pnma space group. The pronounced dispersion along the R−U, Z−Γ, Γ−Y, and S−X high-symmetry directions and low dispersion along the U−Z and Y−S directions confirm the layered structure, with strong bonding interactions along the [001] direction, as well as along the [010] direction, along which the SbSe6 and SbSe7 chains are aligned, and weak van der Waals interactions along the [100] direction. The material is a semiconductor with an indirect bandgap of 1.2 eV. Sb2Se3 exhibits appreciable carrier mobility and low carrier recombination rates in the plane of the layers, indicating its applicability in solar and photovoltaic devices. Simulated X-ray photoelectron spectra show that the photoelectron emission occurs from the Se p, Sb s, and Se s atomic orbitals, while for high-energy X-rays, photoelectron emission is mainly from Sb s orbital. Sb2Se3 possesses appreciable anisotropy in its refractive index, absorption coefficient, reflectivity, extinction coefficient, and optical conductivity. Sb2Se3 is a negative biaxial crystal, in which the principal optical axis X→a, Y→b, and Z→c. It possesses uniform birefringence in the IR region until the optical cutoff edge at 1030 nm. The material shows strong unidirectional linear dichroism in the wavelength range from 300 nm to 620 nm, with linear dichroism conversion at 275 nm. The energy loss is isotropic for slow electrons below 4.0 eV, but for highly energetic electrons it varies along different crystal directions. Sb2Se3 is a promising material for optoelectronic and polarimetric applications because of its desirable bandgap of 1.2 eV, linear dichroism conversion, and large optical anisotropy, refractive index, dielectric constant, and absorption coefficient.

Tomographic measurement of dielectric tensors at optical frequency.

The dielectric tensor is a physical descriptor of fundamental light-matter interactions, characterizing anisotropic materials with principal refractive indices and optic axes. Despite its importance in scientific and industrial applications ranging from material science to soft matter physics, the direct measurement of the three-dimensional dielectric tensor has been limited by the vectorial and inhomogeneous nature of light scattering from anisotropic materials. Here, we present a dielectric tensor tomographic approach to directly measure dielectric tensors of anisotropic structures including the spatial variations of principal refractive indices and directors. The anisotropic structure is illuminated with a polarized plane wave with various angles and polarization states. Then, the scattered fields are holographically measured and converted into vectorial diffracted field components. Finally, by inversely solving a vectorial wave equation, the three-dimensional dielectric tensor is reconstructed. Using this approach, we demonstrate quantitative tomographic measurements of various nematic liquid-crystal structures and their fast three-dimensional non-equilibrium dynamics.

Visualizing coherent molecular rotation in a gaseous medium

Inducing and controlling the ultrafast molecular rotational dynamics using shaped laser fields is essential in numerous applications. Several approaches exist that allow following the coherent molecular motion in real-time, including Coulomb explosion-based techniques and recovering molecular orientation from the angular distribution of high harmonics. We theoretically consider a non-intrusive optical scheme for visualizing the rotational dynamics in an anisotropic molecular gas. The proposed method allows determining the instantaneous orientation of the principal optical axes of the gas. The method is based on probing the sample using ultra-short circularly polarized laser pulses and recording the transmission image through a vortex wave plate. We consider two example excitations: molecular alignment induced by an intense linearly polarized laser pulse and unidirectional molecular rotation induced by a polarization-shaped pulse. The proposed optical method is promising for visualizing the dynamics of complex symmetric- and asymmetric-top molecules.

Terahertz Time-Domain Polarimetry for Principal Optical Axes of Anisotropic Crystals

We propose a method for measuring the terahertz properties for two principal optical axes of anisotropic crystals without optical activity using terahertz time-domain spectroscopy (THz-TDS). The method put forward in this paper utilizes the inherent polarization sensitivity of the THz-TDS electro-optic detection system. We demonstrate the practical application of the method by measuring the temperature dependence of the refractive index and the absorption coefficient of a lithium triborate crystal for three optical axes.

A Novel Method of Light Source Calibration under Near-field Lighting Condition

The paper presents a novel method of photometric stereo calibration that works under near-field condition, and eight LED lights are installed on the system. In order to estimate the pose of the light source, light source position estimation based on iPad imaging is proposed. However, it is difficult to estimate principal optical axis direction of the light source which would influence the reconstruction result. Considering the problems of above, the new convolution kernel is used to find the brightest point on the chessboard paper, and the principal optical axis direction of light source can be precisely calculated according to geometric transformation. The experiments show the high-quality of 3-D reconstruction can be obtained by using the novel calibration method.

Terahertz Optical Properties of KTiOPO4 Crystal in the Temperature Range of (−192)–150 °C

This paper presents the results of an experimental study of the optical properties of highly resistive monocrystals of potassium titanyl phosphate (KTiOPO4, KTP) in the frequency range of 0.2–1 THz and the temperature range of (−192)–150 °C. The dispersion of the refractive indices is approximated in the form of Sellmeier equations. The results show that the temperature dependence of the Sellmeier coefficients for all three principal optical axes is close to linear and, most likely, does not experience an extremum in the vicinity of the activation temperatures of the cationic conductivity of the KTP crystal at (−73)–(−23) °C. Weak frequency dependence of an optical axis direction angle VZ in the range of 0.2–1 THz is confirmed. However, the change in VZ with temperature is three times higher than reported before.

Waves in EBG Waveguides Oriented along the Principal Optical Axes of an Electromagnetic Crystal

The eigenwaves of waveguides in electromagnetic crystals with a square grating are studied. A crystal consists of metal cylinders placed between two parallel screens. The waveguides are oriented along the principal optical axes of the crystal. The investigated single-, two-, and three-row waveguides are formed by removing one-, two-, and three-cylinder rows from a crystal, respectively. The electrodynamic modeling in the HFSS system in the eigenwave mode is performed. The dependences of eigenwave propagation constants on the frequency and structural parameters are obtained. The minimum and maximum bandwidth frequencies and the bandwidths for all the waveguides are determined and their optimal parameters are recommended.

Strain-optical behavior of polyethylene terephthalate film during uniaxial stretching investigated by Mueller matrix ellipsometry

Polyethylene terephthalate (PET) has gained widespread applications in various flexible devices where its optical properties after drawing process in manufacturing play a great importance in the performance of devices. In this work, the strain-optical behavior of pre-oriented polyethylene terephthalate (PET) film during uniaxial stretching is studied utilizing a proposed anisotropic model and Mueller matrix ellipsometry (MME). In the biaxial anisotropic model, the molecular chain orientation is correlated to the change of refractive indices and strain-induced birefringence, while the rotation of molecular chains is directly characterized by the rotation of principal optical axis. According to this model, the refractive indices, extinction coefficients and Euler angles extracted from measured Mueller matrix spectra can be used to interpret the micro-structural response of PET. A set of tensile tests have been carried out, with loading direction being varied. The results show that the strain-induced birefringence is linearly correlated to strain, while loading direction significantly affects the slope of achieved birefringence-strain curve. Meanwhile, the birefringence exhibits a positive linear relation with the orientation estimated by extinction coefficients. Through the analysis of the Euler angle ϕE, the principal optical axis rotating towards the tensile direction is accurately observed. Our results show that MME can provide a deep and systematic understanding of strain-optical behavior which can give accurate control of the optical properties of polymer films during deformation.

How To Maximize Birefringence and Nonlinearity of π-Conjugated Cyanurates.

Materials with large birefringence (Δn) are highly needed by fiber-optic isolators, whereas crystals showing strong second-order harmonic generation (SHG) are the key component for all-solid-state laser devices. Cyanurate constructed by the planar π-conjugated ring (C3N3O3, CY) is a class of fascinating materials exhibiting not only very large Δn (larger than that of calcite) but also strong SHG (2 times that of β-BaB2O4, BBO). Here, we report five new cyanurates (I-V) and their single-crystal structures; among them, II realizes a Δn = 0.4, the maximum in the cyanurate family, and IV realizes a d33 = 6.69 pm/V, one of the highest values in the cyanurate family. We discover a dependence between Δn and the coplanarity of the CY rings that is explicitly described by a Boltzmann function, in which the coplanarity is defined by the dihedral angle (γ) between the CY plane and the principal optical axes XY plane. II realizes the maximum Δn due to its zero γ that indicates perfect coplanarity. Such a Δn-γ dependence also allows the Δn prediction of cyanurates. Furthermore, we uncover that the SHG intensity of cyanurates increases monotonically as the angle (θ) between the maximum hyperpolarizability (βmax) vector and the crystal 21 polar axis decreases. We predict the dij to extend well beyond such a value and to maximize at θ = 0°. Our results have significant implications for the future rational design and discovery of high-performance materials of π-conjugated and other related systems.

Optical phantoms for biomedical polarimetry: a review.

.Calibration, quantification, and standardization of the polarimetric instrumentation, as well as interpretation and understanding of the obtained data, require the development and use of well-calibrated phantoms and standards. We reviewed the status of tissue phantoms for a variety of applications in polarimetry; more than 500 papers are considered. We divided the phantoms into five groups according to their origin (biological/nonbiological) and fundamental polarimetric properties of retardation, depolarization, and diattenuation. We found that, while biological media are generally depolarizing, retarding, and diattenuating, only one of all the phantoms reviewed incorporated all these properties, and few considered at least combined retardation and depolarization. Samples derived from biological tissue, such as tendon and muscle, remain extremely popular to quickly ascertain a polarimetric system, but do not provide quantifiable results aside from relative direction of their principal optical axis. Microspheres suspensions are the most utilized phantoms for depolarization, and combined with theoretical models can offer true quantification of depolarization or degree of polarization. There is a real paucity of birefringent phantoms despite the retardance being one of the most interesting parameters measurable with polarization techniques. Therefore, future work should be directed at generating truly reliable and repeatable phantoms for this metric determination. Diattenuating phantoms are rare and application-specific. Given that diattenuation is considered to be low in most biological tissues, the lack of such phantoms is seen as less problematic. The heterogeneity of the phantoms reviewed points to a critical need for standardization in this field. Ultimately, all research groups involved in polarimetric studies and instruments development would benefit from sharing a limited set of standardized polarimetric phantoms, as is done earlier in the round robin investigations in ellipsometry.

Laboratory Geometric Calibration of Digital Aerial Camera based on Focused Collimator-Array

The geometric calibration of digital aerial camera, including the calibration of inner orientation elements and distortion model parameters, is an indispensable technical of aerial photogrammetry. This paper proposes the use of a focused collimator-array (FCA) device to achieve a stable, repeatable laboratory geometric calibration of digital aerial camera. This device uses multiple collimators and star testers as target generators. After measuring the coordinates of targets on the CCD plane and corresponding standard angles between the collimator optical axis and the principal optical axis, the high-precision geometric calibration of digital aerial camera can be realized according to the geometric calibration model of this paper. Experiment results show that the FCA device can calibrate various types of area digital aerial cameras at different focal lengths and prove the correctness and the reliability of this calibration method using a typical digital aerial camera - Cannon EOS 5D Mark II. This device has good repeatability and high stability, which can satisfy the requirements of digital aerial camera calibration standards.

Photometric stereo with quasi-point light source

This paper presents a complete computational framework for the photometric stereo system composed of some LED lamps and a single web-camera. To describe the lighting condition accurately, the angular and distance attenuation factors, as well as the LED radiance property, are considered in the modeling of the light source. The calibration of the system's parameters is a critical step for our setup, and a two-step calibration procedure is proposed in this work. A multiple-sphere-based approach is first used to estimate the light source positions. Then, a reference-plane-based approach is applied to estimate the principal optical axis of each light source and its principal energy. For each surface point on the target, the direction and intensity of its incidental light ray can be precisely determined by the calibration parameters and the quasi-point light model. Based on the initial 3D reconstruction result, an iterative procedure is applied to optimize the model's parameters for each surface point with respect to its initial estimated depth. In the experiments, both regular and free-form surfaces including a real human face are used. The results show that accurate 3D reconstruction results can be obtained by the proposed approach based on the low-cost setup.

Effects of monoclinic symmetry on the properties of biaxial liquid crystals.

Tilted smectic liquid crystal phases such as the smectic-C phase seen in calamitic liquid crystals are usually treated using the assumption of biaxial orthorhombic symmetry. However, the smectic-C phase has monoclinic symmetry, thereby allowing disassociation of the principal optic and dielectric axes based on symmetry and invariance principles. This is demonstrated here by comparing optical and dielectric measurements for two materials with highly first-order direct transitions from nematic to smectic-C phases. The results show a high difference between the orientations of the principal axes sets, which is interpreted as the existence of two distinct cone angles for optical and dielectric frequencies. Both materials exhibit an increasing degree of monoclinic behavior with decreasing temperature. Due to fast switching speeds, ferroelectric smectic-C* materials are important for fast modulators and LCoS devices, where the dielectric biaxiality influences device operation.

Design and property study of micro-slot optics

LIGA (Lithographie Galvanoformung Abformung) as a very useful technique for fabricating devices with micro-scale structures has been reported to make the micro-pore optics (MPO) for years. Light weight is one of the advantages of MPO, making it an alternative for X-ray focusing lenses used on space telescopes. However, researchers seldom considered using the MPO in visible light region. In this article, we designed a micro-slot optics (MSO) similar to MPO and fabricated it by X-ray LIGA technique. The MSO consists of 159 concentric hollow cylinders of nickel with the slot spacings ranging from 59 to 113µm and the focusing property of MSO is studied by simulations and experiments. Both the simulation and experiment results indicate that MSO is a useful focusing element. The MSO has one focal point when the point source is on the principal optical axis with the size of the focal point equaling to the size of the point source.

Simulation of light interference by a biaxial thin film

In this paper, the interference pattern of reflected light from a biaxial anisotropic thin film (with arbitrary orientation of the principal optical axes) is numerically investigated by a double-reflection approach. The pattern for different film thicknesses under illumination of monochromatic and white light with various polarizations in different angles of incidence is obtained. Comparison with isotropic thin films reveals that a kind of modulation (or banding) appears on the interference pattern.