Search

Existing techniques for the analysis of the optical anisotropy of myocardium sections are described and explained, with examples of the implementation and results.

We demonstrate phase-only tuning of optical metasurfaces in the full 2p-range by controlling the optical anisotropy of their surroundings. The concept is exemplified in a liquid crystal infiltrated metasurface tuned by the temperature and bias-voltage.

The thermal lens was investigated in C a F 2 and T b 3 G a 5 O 12 cubic crystals with [001] crystallographic axes orientation and in magneto-optical glass MOG103 by the method of phase-shifting interferometry. It was demonstrated experimentally that the thermal lens has astigmatism determined by the incident radiation polarization state and by the optical anisotropy parameter ξ . A method of ξ determination by measuring thermal lens astigmatism in cubic crystals with [001] orientation was proposed and verified. It was shown that thermally induced depolarization and the amplitude of phase astigmatism depend on the position of the crystal with [001] orientation in antiphase.

This paper presents a new reflection-only set-up for characterizing the properties of an anisotropic PEC-backed materials. Albeit, the reflection-only methods are established long ago, they are not elaborated appropriately for retrieving industrial anisotropic materials. In spite of other researches, this method is capable to determine tensors of both permittivity and permeability of an anisotropic PEC-backed material. In contrast to similar researches, this method is mathematically simple and easy to implement. Tensors of epsilon and mu of PEC-backed sample are characterized by rotating sample and measuring reflection coefficient in various configuration. Not measuring the transmission, in order to provide enough equations, the higher order mode of the waveguide is employed. A three-port probe is designed for this goal and optimized to excite the TE(10)and TE(20)modes simultaneously. Good agreement of retrieved parameters from experimental measurements with those of actual ones shows the reliability of this set-up for characterization of PEC-backed anisotropic composites.

Freeform metasurfaces couple the angle and polarization degrees of freedom, enabling new types of elliptical wave plates., Birefringence occurs when light with different polarizations sees different refractive indices during propagation. It plays an important role in optics and has enabled essential polarization elements such as wave plates. In bulk crystals, it is typically constrained to linear birefringence. In metamaterials with freeform meta-atoms, however, one can engineer the optical anisotropy such that light sees different indices for arbitrary—linear, circular, or elliptical—orthogonal eigen-polarization states. Using topology-optimized metasurfaces, we demonstrate this arbitrary birefringence. It has the unique feature that it can be continuously tuned from linear to elliptical birefringence, by changing the angle of incidence. In this way, a single metasurface can operate as many wave plates in parallel, implementing different polarization transformations. Angle-tunable arbitrary birefringence expands the scope of polarization optics, enables compact and versatile polarization operations that would otherwise require cascading multiple elements, and may find applications in polarization imaging, quantum optics, and other areas.

The preservation state of parchment primarily depends on the structure of the collagen fibre network, which in turn is responsible for optical anisotropy, i.e. birefringence. Polarised light microscopy can therefore be used as a non-invasive technique that allows recording of birefringence distribution in the parchment, which directly relates to stress–strain distribution. Using samples from diverse sources (commercial parchment, parchment used by restorers for book binding, and parchment fabricated for the purpose of this study), we assessed the capability of polarised light microscopy for various diagnostics. We performed, for instance, identification of gelatinised regions, layered or fibrous regions in parchment cross section, qualitative analysis of parchment fat content (lipids), observation of stress-induced patterns resulting from tensile tests, and observation of water diffusion. These proof-of-principle experiments extend the capability of polarised light microscopy far beyond its common use and open the path to its deployment in conservation studies.

Strong in-plane anisotropy of atomic layer and thin-film black phosphorus (P) offers new device perspectives and stimulates increasing interest and explorations, where precisely determining the black P crystal orientation and anisotropic axes is a necessity. Here, we demonstrate that the crystal orientation and intrinsic in-plane optical anisotropy of black P crystals in a broad thickness range (from ∼5 to ∼300 nm) can be directly and precisely determined, by polarized reflectance measurement alone, in visible range. Combining experiments with modeling of optical anisotropy and multilayer interference effects, we elucidate the underlying principles and validate these measurements. The polarized reflectance method is not only easy to implement but also deterministic, nondestructive, and effective for both on-substrate and suspended black P atomic layers and thin films.

The optical setup of the transport intensity equation (TIE) technique is developed to be valid for measuring the optical properties of the highly-oriented anisotropic fibres. This development is based on the microstructure models of the highly-oriented anisotropic fibres and the principle of anisotropy. We provide the setup of TIE technique with polarizer which is controlled via stepper motor. This developed technique is used to investigate the refractive indices in the parallel and perpendicular polarization directions of light for the highly-oriented poly (ethylene terephthalate) (PET) fibres and hence its birefringence. The obtained results through the developed TIE technique for PET fibre are compared with that determined experimentally using the Mach-Zehnder interferometer under the same conditions. The comparison shows a good agreement between the obtained results from the developed technique and that obtained from the Mach-Zehnder interferometer technique.

Spectroscopic ellipsometry characterization of coatings on polymeric substrates can be challenging due to the substrate optical anisotropy. We compare four characterization strategies for thin coating layers on anisotropic polymeric substrates with regard to accuracy of the resulting layer thickness and coating optical constants. Each strategy differs in measured data type, model construction, implementation complexity, and inherent capabilities and sensitivity to the coating properties. Best practices and limitations are discussed for each strategy.

Thermally induced depolarization in undoped CaF2, BaF2 and SrF2 crystals was measured. The obtained results were used to determine temperature dependence of the optical anisotropy parameter ξ of these materials in the 80–300 K range. Temperature dependence of the orientation of crystallographic axes corresponding to the “zero depolarization” orientation was found. It was revealed that thermally induced depolarization in the studied materials cooled to 80 K reduces by more than 100 times, with changes in the parameter ξ being 5.8%, 6.4% and 28%, respectively.

Appearance of two rings of external conical refraction (ECR) is investigated theoretically within the plane-wave approximation. The hollow cone of ECR is the cone of normals inside a crystal. Due to a birefringence, two sets of isonormal waves are related to the cone. One set forms a filament of rays directed along the biradial, and the other set generates the hollow cone of rays. The apex angle of the cone is nearly twice as large as that of the cone of normals and its axis is close to the direction of the biradial. After leaving the exit facet of a crystal plate, the ray filament spreads into a cone again, and the ray cone gives rise to a second outer cone so that two nonconcentric rings appear at a screen behind the plate. Polarization states at the neighbouring points of the rings lying on one radius are mutually perpendicular. Numerical calculations were performed for crystals with moderate and large optical anisotropy, namely KGd(WO 4 ) 2 , ammonium oxalate, L-N-(5-Nitro-2-peridyl)leucinol and sulfur. Dispersion of the angle of ECR, the apex angle of the cone of incident (from air) waves and the ray cone is calculated for three tungstate crystals, KGd(WO 4 ) 2 , KLu(WO 4 ) 2 , and KY(WO 4 ) 2 .

Since recently, a number of innovative polarization-based optical imaging modalities have been introduced and extensively used in various biomedical applications, with an ultimate aim to attain the practical tool for the optical biopsy and functional characterization of biological tissues. The techniques utilize polarization properties of light and Mueller matrix mapping of microscopic images of histological sections of biological tissues or polycrystalline films of biological fluids. The main drawback of currently developed laser polarimetry approaches and Mueller matrix mapping techniques is poor reproducibility of experimental data. This is due to azimuthal dependence of polarization and ellipticity values of most matrix elements to sample orientation in respect to incidence light polarization. Current study aims to generalize the methods of laser polarimetry for diagnosis of partially depolarizing optically anisotropic biological tissues. A method of differential Mueller matrix mapping for reconstruction of linear and circular birefringence and dichroism parameter distributions of partially depolarizing layers of biological tissues of different morphological structure is introduced and practically implemented. The coordinate distributions of the value of the first-order differential matrix elements of histological sections of brain tissue with spatially structured, optically anisotropic fibrillar network, as well as of parenchymatous tissue of the rectum wall with an “islet” polycrystalline structure are determined. Within the statistical analysis of polarization reproduced distributions of the averaged parameters of phase and amplitude anisotropy, the significant sensitivity of the statistical moments of the third and fourth orders to changes in the polycrystalline structure of partially depolarizing layers of biological tissue is observed. The differentiation of female reproductive sphere connective tissue is realized with excellent accuracy. The differential Mueller matrix mapping method for reconstruction of distributions of linear and circular birefringence and dichroism parameters of partially depolarizing layers of biological tissues of different morphological structures is proposed and substantiated. Differential diagnostics of changes in the phase (good balanced accuracy) and amplitude (excellent balanced accuracy) of the anisotropy of the partially depolarizing layers of the vagina wall tissue with prolapse of the genitals is realized. The maximum diagnostic efficiency of the first-order differential matrix method was demonstrated in comparison with the traditional methods of polarization and Mueller matrix mapping of histological sections of light-scattering biological tissues.

An approach of azimuthally invariant Mueller-matrix description of the formation of polarization singularities networks of object fields of optically anisotropic biological tissues is proposed. The main formation scenarios of the topological polarization-singular structure of the boundary field of networks of optically anisotropic cylinders are determined by means of computer simulation. In order to assess the polarization-singular structure of the object field of real biological tissues, an original Mueller-matrix method of statistical analysis of the number of L and C points has been developed.

Microtubules (MTs) are ubiquitous cytoskeletal biopolymers essential for diverse cellular processes. MTs consist of strictly ordered tubulin dimers arranged into hollow cylindrical filaments and are known to be optically anisotropic, which enables their direct observation in microscopes based on polarization contrast. However, there are no experimental data to quantify the relation between the momentary optical anisotropy of the MT and the immediate arrangement of proteins in the MT structure. In this work, we introduce polarization-sensitive microscopy based on interferometric detection of scattering to quantify the scattering anisotropy of single unlabeled MTs with high precision. Our data explain the structural origin of MT anisotropy with a marginal contribution of the intrinsic asymmetry of a single tubulin molecule. We monitor changes in the MT scattering resulting from the binding of tau proteins to single MTs with a resolution of several proteins per diffraction-limited spot. We associate the changes in the contrast as well as in the scattering anisotropy with the formation of a shell around the MT formed by densely packed tau proteins. Our experimental results match closely with the theoretical model of the MT and include quantitative details about the polarization-dependent interaction of light with biological matter.

The dimension D of a polycrystalline film and the optical anisotropy m = e z /ex of uniaxial crystallites with the principal components e x = e y and e z of the tensor of the dielectric constant have been shown to produce a strong influence on the effective dielectric constant e * and the effective refractive index n * = (e * )1/2 of the film in the optical transparency region, as well as on the boundaries of the intervals B Dl ≤ e * ≤ B Du . The intervals Δ2(m) = B 2l –B 2u and Δ3(m) = B 3l –B 3u are separated by a gap for m in the range 1 < m < 2, whereas the theoretical dependence e 2 * (m) is separated by a gap from the interval Δ3(m) for m in the range 1 < m < 4. This is confirmed by a comparison of the experimental (n oP ) and theoretical (n * ) ordinary refractive indices for uniaxial polycrystalline films of the conjugated polymer poly(p-phenylene vinylene) (PPV) with uniaxial crystallites and appropriate values of m. In the visible transparency region of the PPV films with a change in m(λ) in the range 2 < m(λ) < 3 due to the dependence of the components e x,z (λ) on the light wavelength λ, the refractive indices n 2 (λ) = e oP (λ) are consistent with the theoretical values of e 2 * (λ) and lie outside the interval Δ3(m). For m(λ) > 3 near the electronic absorption band of the crystallites, the values of e oP (λ) lie in the region of the overlap of the intervals Δ2(m) and Δ3(m). The boundaries mc of the range 1 < m < m c are determined, for which the interval Δ2(m) is separated by a gap from the dependences e 3 * (m) corresponding to the effective medium theory with spherical crystallites and hierarchical models of a polycrystal, as well as from the proposed new dependence e 3 * (m).

Photooptical properties of a new cholesteric mixture containing azobenzene-based chiral dopant were investigated. Presence of a photosensitive dopant in the investigated liquid crystal enabled to effectively manipulate optical characteristics of the photonic band upon temperature change and illumination. Temperature dependence of the optical anisotropy and the orientational order parameter of the liquid crystal were determined. The obtained results are compared with existing theories. Discrete multistable change of the spectral position of the photonic band on illumination is realized.

In the present work, our investigation is to study the optical and electric properties of the binary mixture of heptyl-oxy-benzilidene-p-n-heptyl-aniline and cholesteryl laurate, which exhibit very interesting liquid crystalline Cho–TGB–SmA–SmC-ReSmA–SmE phases sequentially when the specimen is cooled from its isotropic phase. These phases have been characterized by using microscopic and optical anisotropic technique. Temperature variations of optical anisotropy and X-ray studies have been discussed. An electro-optical measurement has also been discussed.

It is shown that the relaxation of the isotropic light scattering intensity by boron oxide melt, which is connected with a decrease in its magnitude after a temperature jump, is synchronized with the effect of changing the polarization state of the excitation light beam passed through the sample under investigation. The formation of macroscopic elastic stresses in the sample bulk could be one of the possible reasons for the development of the detected phenomenon.

We examine the molecular dynamics related to the optical anisotropy generated by a pump beam on the total internal reflected probe beam at a liquid-solid interface by a Surface Time-Resolved Optical Kerr Effect (STROKE).

This work is devoted to creation and research of the multichannel optical processor allowing us to carry out the diffractometric and polarimetric analysis for assessment of optiko-physical properties as biological microobjects and is exemplary optically transparent randomly inhomogeneous medium.

The striking in-plane anisotropy remains one of the most intriguing properties for the newly rediscovered black phosphorus (BP) 2D crystals. However, because of its rather low-energy band gap, the optical anisotropy of few-layer BP has been primarily investigated in the near-infrared (NIR) regime. Moreover, the essential physics that determine the intrinsic anisotropic optical property of few-layer BP, which is of great importance for practical applications in optical and optoelectronic devices, are still in the fancy of theory. Herein, we report the direct observation of the optical anisotropy of few-layer BP in the visible regime simply by using polarized optical microscopy. On the basis of the Fresnel equation, the intrinsic anisotropic complex refractive indices (n-iκ) in the visible regime (480-650 nm) were experimentally obtained for the first time using the anisotropic optical contrast spectra. Our findings not only provide a convenient approach to measure the optical constants of 2D layered materials but also suggest a possibility to design novel BP-based photonic devices such as atom-thick light modulators, including linear polarizer, phase plate, and optical compensator in a broad spectral range extending to the visible window.

We propose a single-exposure polarization phase-shifting interferometer based on a beam-splitting method using an azo-polymer orientation array, which is prepared from an ordinary azo-polymer film with photo-induced optical anisotropy. The azo-polymer orientation array gives a four-step phase shift on the interferogram, from which phase differences between the orthogonal polarized reference and object beam can be calculated. Experimental results are provided to verify the feasibility of the proposed interferometer. The azo-polymer orientation array is easy to fabricate, making this method convenient and low-cost.

We developed a reflection ellipsometer so that one can measure the extremely small optical anisotropy of a rubbed polyimide alignment layer, without being disturbed by the residual anisotropy of the substrate. The optical anisotropy of the alignment layer was measured as rubbing strength was increased, and the measured anisotropy was compared to the retardation obtained by using a transmission-type ellipsometer, to confirm the reliability of our reflection ellipsometer. With this measured anisotropy we could verify the formation of an alignment layer by rubbing, and could quantitatively evaluate the formation of the alignment layer. Keywords: Ultra-small optical anisotropy, Reflection ellipsometry, Rubbed polyimide, Alignment layerOCIS codes: (120.2040) Displays; (120.2130) Ellipsometry and polarimetry; (310.5448) Polarization, other optical properties

The possibility of optical tomography applying to investigation of a two-dimensional and a three-dimensional stressed state in single cubic crystals has been studied. Stresses are determined within the framework of the Maxwell piezo-optic law (linear dependence of the permittivity tensor on stresses) and weak optical anisotropy. It is shown that a complete reconstruction of stresses in a sample is impossible both by translucence it in the parallel planes system and by using of the elasticity theory equations. For overcoming these difficulties, it is offered to use a method of magnetophotoelasticity.

This chapter is focused on the theory and opto-mechanical modeling of fiber Bragg gratings (FBGs). The chapter includes the opto-mechanical properties of optical fibers, induced optical anisotropy in optical fibers caused by temperature and structural loading, light propagation in optically anisotropic optical fibers, and the coupled-mode theory of the spectral response of FBGs. In addition, the theory of superstructure FBGs is elaborated at the end of the chapter.

In this study, we investigate transmitted, reflected, and absorbed powers in forward and backward directions of periodic and aperiodic multilayered structures composed of bi-anisotropic metamaterial (MM) slab and conventional material. Aperiodic multilayered structure is realized by a change in thickness of any bi-anisotropic MM slab or of any conventional material. From this analysis, we note the following key results. First, identical (non-identical) forward and backward transmitted (reflected and absorbed) powers are observed for the analyzed periodic and aperiodic multilayered structures due to reciprocity (reflection-asymmetry) of bi-anisotropic MM slabs. Second, thickness-resonance phenomenon of conventional materials produces some peaks in the transmitted powers of periodic multilayered structures aside from the resonance frequency region of bi-anisotropic MM slabs. Third, each thickness-resonance frequency splits into many frequencies upon increasing the number of sections of periodic multilayered structures (no splitting when number of periods is one). Fourth, while the effect of changing the thickness of any bi-anisotropic MM slab within the aperiodic multilayered structure has no considerable effect around the resonance region of bi-anisotropic MM slabs (resonance of resonating structures such as MM slabs does not change with thickness), the same change in thickness of the conventional material drastically alters forward/backward reflected and absorbed powers aside from the resonance region of bi-anisotropic MM slabs (thickness-resonance totally depends on the value of thickness of conventional materials). The outcomes presented here can be particularly useful for propagation-related applications requiring cascade connection of various MM slabs. © 2014 Elsevier B.V. All rights reserved.

We investigated the rotational dynamics of single microparticles during their internalization by macrophage cells. The microparticles used were triblock patchy particles that display two fluorescent patches on their two poles. The optical anisotropy made it possible to directly visualize and quantify the orientation and rotation of the particles. We show that particles exhibit a mixture of fast and slow rotation as they are uptaken by macrophages and transiently undergo directional rotation during their entry into the cell. The size of the particles and the surface presentation of ligands exerted a negligible influence on this heterogeneity of particle rotation.

A new type of phase-shifting interferometer was presented by rotating an azo-polymer film with photo-induced optical anisotropy in the light path of orthogonal polarization interferometry. By changing the angle of the optical axis of the azo-polymer film, four phase-shifted fringe patterns were obtained, from which phase difference between orthogonal polarized light could be calculated. The polarization states and the formation of the interferograms were analyzed theoretically using Jones matrix calculations and the experimental verification of the proposed method was also given. This method has a simple optical configuration which utilizes commercially available polarization devices and anisotropic azo-polymer film of low cost. The phase-shifting interferometer also demonstrates a new application of azo-polymer film.

For flexible polymers, the deviatoric parts of the stress and optical anisotropy tensors, with the latter reflecting the orientational anisotropy of the monomeric segments, are proportional to each other. This proportionality, known as the stress-optical rule (SOR), is valid not only in the linear viscoelastic regime but also in the moderately nonlinear regime, given that the chain is not highly stretched and is free from the finite extensible nonlinear elasticity (FENE) effect. On start-up of flow in such a moderately nonlinear regime, the chain exhibits an overshoot of the shear stress and thus of the shear component of the segmental orientational anisotropy. Nevertheless, it was not clearly understood whether this overshoot is associated with an overshoot of chain stretch. This study revisited SOR to conduct a simple analysis for this problem. It turned out that the orientational anisotropy of subchains overshoots but the subchain stretch (identical to the chain stretch) does not when the shear stress overshoots in the moderately nonlinear regime.

The recent emergence of liquid crystals of atomically thin two-dimensional (2D) materials not only has allowed us to explore novel phenomena of macroscopically aligned 2D nanomaterials but also has provided a route toward their controlled assembly into three-dimensional functional macrostructures. Using flow-induced mechanical alignment, we prepared flakes of graphene oxide (GO) in different orientational orders and demonstrated that GO liquid crystals (LC) can be used as rewritable media for reflective displays without polarizing optics. With a wire or stick as a pen, we can make the surface of GO LC reflective and bright, and we can then manually draw lines, curves, and any patterns with dark appearance. The contrast between bright and dark features is due to anisotropic optical responses of ordered GO flakes. Since optical anisotropy is an intrinsic property of 2D structures, our observations and demonstration represent one of many potential applications of macroscopically aligned 2D nanomaterials.

Endosomes in cells are known to move directionally along microtubules, but their rotational dynamics have rarely been investigated. Even less is known, specifically, about the rotation of nonspherical endosomes. Here we report a single-Janus rod rotational tracking study to reveal the rich rotational dynamics of rod-shaped endosomes in living cells. The rotational reporters were Janus rods that display patches of different fluorescent colors on opposite sides along their long axes. When the Janus rods are wrapped tightly inside endosomes, their shape and optical anisotropy allow the simultaneous measurements of all three rotational angles (in-plane, out-of-plane, and longitudinal) and the translational motion of single endosomes with high spatiotemporal resolutions. We demonstrate that endosomes undergo in-plane rotation and rolling during intracellular transport and that such rotational dynamics are driven by rapid microtubule fluctuations. We reveal for the first time the "rock-and-roll" of endosomes in living cells and how the intracellular environment modifies such rotational dynamics. This study demonstrates a unique application of Janus particles as imaging probes in the elucidation of fundamental biological questions.

The theoretical background of the azimuthally stable method of polarization-interference mapping of the histological sections of the biopsy of the prostate tissue on the basis of the spatial frequency selection of the mechanisms of linear and circular birefringence is presented. The diagnostic application of a new correlation parameter – complex degree of mutual anisotropy – is analytically substantiated. The method of measuring coordinate distributions of complex degree of mutual anisotropy with further spatial filtration of their high- and low-frequency components is developed. The interconnections of such distributions with parameters of linear and circular birefringence of prostate tissue histological sections are found. The objective criteria of differentiation of benign and malignant conditions of prostate tissue are determined.

The main aspect in developing methods for optical diagnostics and visualization of biological tissues using polarized radiation is the transformation analysis of the state of light polarization when it is scattered by the medium. The polarization characteristic spatial distributions of the detected scattered radiation, in particular the degree of depolarization, have a pronounced anisotropy. The presence of optical anisotropy can provide valuable additional information on the structural features of the biological object and its physiological status. Analysis of the polarization characteristics of the scattered radiation of biological tissues in some cases provides a qualitatively new results in the study of biological samples. These results can be used in medicine and food industry.

The finding that the quasi-1D crystal BaTiS3 features a large optical anisotropy and a broadband birefringence spanning the infrared is likely to reignite interest in quasi-1D optical materials.

The improvement of detection sensitivity using the magnetic orientation and polarization in microcrystals with optical anisotropy was investigated using monosodium urate crystals. The difference of light intensity caused by switching the magnetic field on and off was approximately 1.6 times. The contrast was increased up to 2.3 times using a polarization system. In this system, most of the light provided to the detector came from the crystals, which is similar to the dark-field observation using an optical microscope, and the detection sensitivity does not change even when the light intensity is reduced 100-fold.

For crystalline materials with a negative optical anisotropy parameter, the orientation of the crystallographic axes with a minimum of thermally induced depolarization was found and an analytical expression for depolarization was obtained and analyzed.

Compared with the wide application of liquid crystals (LCs) in the visible frequency band, their properties in the terahertz band have not been investigated extensively yet. In this paper, we have investigated the optical anisotropy of LCs TEB30A and 9023 at room temperature using terahertz time-domain spectroscopy (THz-TDS). The extraordinary and ordinary refraction indices of LC TEB30A are ne≈1.84 and no≈1.65, or a birefringence of 0.19 from 0.5 to 2.2 THz. The extraordinary and ordinary refraction indices of LC 9023 are ne≈1.83 and no≈1.62, or a birefringence of 0.21 from 0.5 to 2.2 THz. LC 9023 exhibits a little larger terahertz birefringence than that of LC TEB30A.

A model of generalized optical anisotropy of polycrystalline networks of albumin and globulin of human brain liquor has been suggested. The polarization-phase method of spatial and frequency differentiation of linear and circular birefringence coordinate distributions have been analytically substantiated. A set of criteria of the dynamics of necrotic changes of polarization-phase images of liquor polycrystalline films for determination of death coming prescription has been detected and substantiated.

We reported a novel scattering switch-on detection technique using flash-lamp polarization darkfield microscopy (FLPDM) for target-induced plasmon-coupling based sensing in homogeneous solution. With this method, we demonstrated sub-nM sensitivity for hydrogen sulfide (H2S) detection over a dynamic range of five orders of magnitude. This robust technique holds great promise for applications in toxic environmental pollutants and biological molecules.