Direction Of Light Propagation Research Articles

Enabling Manufacturable Optical Broadband Angular-Range Selective Films

The ability to control the propagation direction of light has long been a scientific goal. However, the fabrication of large-scale optical angular-range selective films is still a challenge. This paper presents a polymer-enabled large-scale fabrication method for broadband angular-range selective films that perform over the entire visible spectrum. Our approach involves stacking together multiple one-dimensional photonic crystals with various engineered periodicities to enlarge the bandgap across a wide spectral range based on theoretical predictions. Experimental results demonstrate that our method can achieve broadband transparency at a range of incident angles centered around normal incidence and reflectivity at larger viewing angles, doing so at large scale and low cost.

Cascaded PT-symmetric artificial sheets: multimodal manipulation of self-dual emitter-absorber singularities, and unidirectional and bidirectional reflectionless transparencies

We introduce cascaded parity-time (PT)-symmetric artificial sheets (e.g. metasurfaces or frequency selective surfaces) that may exhibit multiple higher-order laser-absorber modes and bidirectional reflectionless transmission resonances within the PT-broken phase, as well as a unidirectional reflectionless transmission resonance associated with the exceptional point (EP). We derive the explicit expressions of the gain–loss parameter required for obtaining these modes and their intriguing physical properties. By exploiting the cascaded PT structures, the gain–loss threshold for the self-dual laser-absorber operation can be remarkably lowered, while the EP remains unaltered. We further study interferometric sensing based on such a multimodal laser-absorber and demonstrate that its sensitivity may be exceptionally high and proportional to the number of metasurfaces along the light propagation direction.

Electro-Optical properties of photoaligned Liquid Crystal cells prepared with obliquely irradiated Chalcogenide glasses

We report the effect of oblique irradiation of thin As2S3 chalcogenide glass films on the photoalignment of Nematic liquid crystals and their electro optical properties. After depositing 60 nm As2S3 films on ITO coated substrates, the substrates were irradiated with polarized blue light (436 nm) at angles 0°, 5°, 20° and 48° of the light propagation direction with respect to the substrate plane. Then, the substrates were assembled and filled with the liquid crystal. The device LC-20° exhibits good extinction, and lower threshold voltage when compared to all other devices. The contrast ratio of LC-20° almost double that of the LC-5° and LC-48° devices. The response time increases with the angle, so for LC-20° slightly longer than for LC-5° but shorter than that of LC-48°. It is evident from the observations that the obliquely irradiated cells show good photoalignment and enhanced electro optical properties, although no pretilt is generated due to the oblique irradiation. Therefore, contrary to the photoalignment using photosensitive polymers, chalcogenide glasses have the advantage of providing photoalignment with little sensitivity to the irradiation angle. This fact led us to propose a model for the photoinduced alignment on chalcogenide glasses based on directional structural changes, not related to oriented dipoles generation as in the case of photoalignment with polymers.

Highly Chiral Exceptional Point in Perturbed Coupled Resonators

Exceptional point (EP) is a non-Hermitian spectral degeneracy that has application in ultrasensitive sensors and laser mode selectivity. By employing strong chirality in an optical system, the direction of light propagation can be controlled and subwavelength particles can be detected. Here, we show that EP with high chirality can appear in the coupled resonators perturbed by a scatterer, in which both the distance and position of the scatterer can be tuned. We achieve strong chiral EP in two different distances between the resonators, with chirality around 0.99 in both cases.

Non-reciprocal wave retarder based on optical rotators combination

We propose and demonstrate a method to realize an easily tunable non-reciprocal wave retarder whose phase retardation depends on the light propagation direction. The system is based on a combination of a reciprocal polarization rotator, a non-reciprocal magneto-optical rotator, and two quarter-wave plates. Experimental tests demonstrate various non-reciprocal functionalities in complete agreement with the underlying theoretical concept.

Propulsion Mechanisms of Light‐Driven Plasmonic Colloidal Micromotors

Colloidal micromotors are important candidates for a wide spectrum of applications, ranging from medicine to environmental remediation. Thus far, the propulsion force determination has been obtained from the colloidal motor motion speed and surrounding viscosity via the Stokes drag. Herein, a precise force measurement method and detailed analysis of the fundamental propulsion mechanisms of colloidal Janus micromotors propelled by thermophoretic and steam bubble force vectors, revealing findings uninvestigated to date, are presented. Optical tweezers provide fast and high‐precision force measurements in all three orthogonal dimensions simultaneously. Colloidal Janus micromotors are compared with isotropic hot Brownian reference microparticles, which have no defined force vector that propels them perpendicular to the direction of the laser beam. Janus micromotors display a defined laser power intensity‐dependent thermophoretic propulsion, as well as bubble force‐based propulsion, after surpassing the threshold value for the water boiling. The steam bubble propulsion force vector and the thermophorethic force vectors sum up for the Janus micromotor propulsion direction. On the contrary, the bubble force counteracts photophoretic force in propagation direction of light. Moreover, the thermal‐based reduction of viscosity around the Janus colloidal motor contributes significantly to its speed and guidance abilities.

Polar magneto-optical Kerr effect of reflected light from Graphene/InAs/Graphene/Polyimide/Al structure

In this paper, we have numerically investigated the magneto-optical (MO) properties, and figure of merit (FOM) behavior of the reflected light from a layered structure composed of two graphene sheets, an InAs, a Polyimide, and an Aluminum layers with the arrangement of Graphene/InAs/Graphene/Polyimide/Al at the terahertz frequency region. An external magnetic field is applied to the structure along with the light propagation direction to induce the gyrotropic properties of the graphene sheets and the InAs layer. The developed 4 by 4 transfer matrix method is used to study the MO responses including the intensity reflection coefficient, Kerr rotation, and ellipticity angles spectra of the structure. At first, we compared the MO responses of the structures in the presence and absence of two graphene sheets. The results demonstrated that the performance of the structure in the presence of graphene sheets is higher than in the case without graphene sheets. To continue, the effects of applied magnetic field strength, the charge carrier density of graphene, and the InAs layer thickness have been studied for the structure with graphene sheets. The results revealed that the performance of the structure would be controlled and tuned by the investigated parameters.

Nonequilibrium dissociative dynamics of D2 in two-color, few-photon excitation and ionization

${\mathrm{D}}_{2}$ molecules, excited by linearly cross-polarized femtosecond extreme ultraviolet (XUV) and near-infrared (NIR) light pulses, reveal highly structured ${\mathrm{D}}^{+}$ ion fragment momenta and angular distributions that originate from two different four-step dissociative ionization pathways after four-photon absorption (one XUV $+$ three NIR). We show that, even for very low dissociation kinetic energy release $\ensuremath{\le}$ 240 meV, specific electronic excitation pathways can be identified and isolated in the final ion momentum distributions. With the aid of ab initio electronic structure and time-dependent Schr\"odinger equation calculations, angular momentum, energy, and parity conservation are used to identify the excited neutral molecular states and molecular orientations relative to the polarization vectors in these different photoexcitation and dissociation sequences of the neutral ${\mathrm{D}}_{2}$ molecule and its ${\mathrm{D}}_{2}^{+}$ cation. In one sequential photodissociation pathway, molecules aligned along either of the two light polarization vectors are excluded, while another pathway selects molecules aligned parallel to the light propagation direction. The evolution of the nuclear wave packet on the intermediate $B{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ electronic state of the neutral ${\mathrm{D}}_{2}$ molecule is also probed in real time.

Single-step etched grating couplers for silicon nitride loaded lithium niobate on insulator platform

Dielectrically loaded thin-film lithium niobate (LiNbO3) on insulator (LNOI) platforms have enabled a range of photonic integrated circuit components, such as high-speed optical modulators, switches, and nonlinear devices, while avoiding the direct etching of the LiNbO3 thin film. Silicon nitride (Si3N4) is one of the most attractive dielectric loading materials as it has a similar refractive index and transparency window to LiNbO3 and can be deposited and patterned by mature fabrication processes. The patterning of Si3N4 opens the opportunity to fabricate grating couplers in the same fabrication step, providing efficient optical interfaces for wafer-scale testing. In this paper, we investigate and demonstrate single-step etched grating couplers on a Si3N4-LNOI (X-cut) platform. The grating couplers (straight and curved) are designed and fabricated for TE-polarized modes along the Y and Z crystallographic directions, considering the LiNbO3 crystal’s birefringence. The experimentally demonstrated coupling losses are as low as 4.02 and 4.24 dB along the crystallographic Y and Z directions, respectively. The corresponding peak wavelengths are 1609 and 1615 nm, respectively. The measured 3-dB bandwidths are wider than 70 nm for both crystallographic directions. We also numerically investigated the influence of fabrication variations and the fiber angle on the transmission. To the best of our knowledge, this work is the first demonstration of grating couplers with different light propagation directions on the Si3N4 loaded LNOI platform.

Reconfiguring nanostructures in magnetic fluids using pH and magnetic stimulus for tuning optical properties

In this paper, we demonstrate tuning the optical properties of magnetic fluids through pH and magnetic field control. The role of reconfigurable nanostructures in the temporal and switching behavior of transmitted light is also studied. An oil-in-water magnetic nanoemulsion with an average diameter of 200 nm, containing Fe3O4 superparamagnetic nanoparticles of 10 nm average diameter, is used in this study. To make the emulsion pH reconfigurable, the droplets are covered with a weak polyelectrolyte, poly (acrylic acid). The swelling of the PAA molecules, due to ionization of the carboxylic acid groups, resulted in an increase in adlayer thickness at increase in pH. The change in transmitted intensity is found to scale linearly with the applied magnetic field strength, and at higher fields, the formed structures took more time to reconfigure themselves after the removal of magnetic field. At a pH 5, the increase in transmitted light intensity is seen even at a very low magnetic field strength due to the lower interdroplet spacing. On the contrary, at pH 9, the interdroplet spacing between the emulsion drops were large, due to the extension of PAA molecules, which led to a weaker magnetic interaction between the droplets and formation of shorter chains, where a lower transmission is seen due to multiple scattering. A straight-line pattern was observed in the transmitted intensity, due to the scattering of light from linear chains that are aligned perpendicular to the light propagation direction. Bridging of polymer covered surfaces was seen at pH 3, due to hydrogen bonding between PAA molecules. Our results show that tuning optical properties of magnetic emulsion (switching between transparent and opaque states) is possible through pH control. Our findings open up new opportunities to develop optical devices and accessing of diverse structures by tuning pH and magnetic field.

Removing striping artifacts in light-sheet fluorescence microscopy: a review

In recent years, light-sheet fluorescence microscopy (LSFM) has found a broad application for imaging of diverse biological samples, ranging from sub-cellular structures to whole animals, both in-vivo and ex-vivo, owing to its many advantages relative to point-scanning methods. By providing the selective illumination of sample single planes, LSFM achieves an intrinsic optical sectioning and direct 2D image acquisition, with low out-of-focus fluorescence background, sample photo-damage and photo-bleaching. On the other hand, such an illumination scheme is prone to light absorption or scattering effects, which lead to uneven illumination and striping artifacts in the images, oriented along the light sheet propagation direction. Several methods have been developed to address this issue, ranging from fully optical solutions to entirely digital post-processing approaches. In this work, we present them, outlining their advantages, performance and limitations.

Analyzer-free, intensity-based, wide-field magneto-optical microscopy

In conventional Kerr and Faraday microscopy, the sample is illuminated with plane-polarized light, and a magnetic domain contrast is generated by an analyzer making use of the Kerr or Faraday rotation. Here, we demonstrate possibilities of analyzer-free magneto-optical microscopy based on magnetization-dependent intensity modulations of the light. (i) The transverse Kerr effect can be applied for in-plane magnetized material, as demonstrated for an FeSi sheet. (ii) Illuminating that sample with circularly polarized light leads to a domain contrast with a different symmetry from the conventional Kerr contrast. (iii) Circular polarization can also be used for perpendicularly magnetized material, as demonstrated for garnet and ultrathin CoFeB films. (iv) Plane-polarized light at a specific angle can be employed for both in-plane and perpendicular media. (v) Perpendicular light incidence leads to a domain contrast on in-plane materials that is quadratic in the magnetization and to a domain boundary contrast. (vi) Domain contrast can even be obtained without a polarizer. In cases (ii) and (iii), the contrast is generated by magnetic circular dichroism (i.e., differential absorption of left- and right-circularly polarized light induced by magnetization components along the direction of light propagation), while magnetic linear dichroism (differential absorption of linearly polarized light induced by magnetization components transverse to propagation) is responsible for the contrast in case (v). The domain–boundary contrast is due to the magneto-optical gradient effect. A domain–boundary contrast can also arise by interference of phase-shifted magneto-optical amplitudes. An explanation of these contrast phenomena is provided in terms of Maxwell–Fresnel theory.

Unidirectional guided resonances in anisotropic waveguides.

We show that anisotropic planar anti-guiding waveguide structures with two radiation channels toward the surrounding cladding materials can support unidirectional guided resonances (UGRs), where radiation is canceled in one of the radiation channels and redirected into the other. Their formation is subtle as it requires breaking the so-called polar anisotropy-symmetry of the structures. Then, UGRs appear at specific wavelengths and light propagation directions, are robust, and are characterized by phase singularities in the channel in which radiation is canceled. The mechanism we describe allows for ready selection of the radiation direction, as well as tuning of the wavelength and the propagation angle at which UGRs occur.

Dynamic Nanoparticle Trapping By Cascaded Nanophotonic Traps in a Silicon Slot Waveguide

Nanophotonic traps integrated in a microfluidic channel are promising for next-generation multifunctional and flexible lab-on-chip systems, especially for researches in nanobiophotonics. Here we propose a slot waveguide with a nano-slit to produce a high gradient of optical field in a tiny space, which exhibits strong optical force to trap nanoparticles. Moreover, the slits can be cascaded along the direction of light propagation to form a large array of optical traps, with negligible reflection and loss, which is expected to be useful in study of biological nanoparticles. The trapping dynamics of nanoparticles to the proposed nanophotonic traps is analyzed, which reveals physically cascaded trapping processes enabled by the slot and the slit, respectively. The motion trajectories of the particles of 50 nm in diameter are obtained with the Brownian motion taken into account, which shows successful trapping of the nanoparticles.

Optically Driven Gold Nanoparticles Seed Surface Bubble Nucleation in Plasmonic Suspension.

Photothermal surface bubbles play important roles in applications like microfluidics and biosensing, but their formation on transparent substrates immersed in a plasmonic nanoparticle (NP) suspension has an unknown origin. Here, we reveal NPs deposited on the transparent substrate by optical forces are responsible for the nucleation of such photothermal surface bubbles. We show the surface bubble formation is always preceded by the optically driven NPs moving toward and deposited to the surface. Interestingly, such optically driven motion can happen both along and against the photon stream. The laser power density thresholds to form a surface bubble drastically differ depending on if the surface is forward- or backward-facing the light propagation direction. We attributed this to different optical power densities needed to enable optical pulling and pushing of NPs in the suspension, as optical pulling requires higher light intensity to excite supercavitation around NPs to enable proper optical configuration.

Large Faraday Rotation in Optical-Quality Phthalocyanine and Porphyrin Thin Films.

The magneto-optical phenomenon known as Faraday rotation involves the rotation of plane-polarized light as it passes through an optical medium in the presence of an external magnetic field oriented parallel to the direction of light propagation. Faraday rotators find applications in optical isolators and magnetic-field imaging technologies. In recent years, organic thin films comprised of polymeric and small-molecule chromophores have demonstrated Verdet constants, which measure the magnitude of rotation at a given magnetic field strength and material thickness, that exceed those found in conventional inorganic crystals. We report herein the thin-film magnetic circular birefringence (MCB) spectra and maximum Verdet constants of several commercially available and newly synthesized phthalocyanine and porphyrin derivatives. Five of these species achieved maximum Verdet constant magnitudes greater than 105 deg T-1 m-1 at wavelengths between 530 and 800 nm. Notably, a newly reported zinc(II) phthalocyanine derivative (ZnPc-OT) reached a Verdet constant of -33 × 104 deg T-1 m-1 at 800 nm, which is among the largest reported for an organic material, especially for an optical-quality thin film. The MCB spectra are consistent with resonance-enhanced Faraday rotation in the region of the Q-band electronic transition common to porphyrin and phthalocyanine derivatives, and the Faraday A-term describes the electronic origin of the magneto-optical activity. Overall, we demonstrate that phthalocyanines and porphyrins are a class of rationally designed magneto-optical materials suitable for applications demanding large Verdet constants and high optical quality.

Optimization-free customization of optical tightly focused fields: uniform needles and hotspot chains.

An optimization-free method based on an inverse problem of nonlinear equations is employed to design the binary phase diffraction optical element (BPDOE) that could modulate the incident light of a high-numerical-aperture (NA) objective lens so that the axisymmetric focal fields can be customized on demand. For example, a 43λ-long optical longitudinally polarized needle with its lateral size beyond diffraction limit is reported by using a 27-belt BPDOE, where the cost evaluated by the ratio of the belt number of BPDOE to the length of needle is record small compared with other optimization algorithms. Moreover, another longitudinal field with multiple hotspots along the propagation direction of light is also achieved with a 10-belt BPDOE. These achieved focal fields are verified doubly by using a finite-difference time-domain (FDTD) method, indicating the validity of Richards-Wolf vector diffraction theory. This optimization-free approach makes the design of BPDOEs with numerous belts viable to generate the expected focal fields, which might benefit various applications such as optical trapping, super-resolution imaging, and lithography.

Signatures of magnetic-field effects in nonsequential double ionization manifesting as backscattering for molecules versus forward scattering for atoms

For two-electron diatomic molecules, we investigate magnetic field effects in non-sequential double ionization where recollisions prevail. We do so by formulating a three-dimensional semi-classical model that fully accounts for the Coulomb singularities and for magnetic field effects during time propagation. Using this model, we identify a prominent signature of non-dipole effects. Namely, we demonstrate that the recolliding electron back-scatters along the direction of light propagation. Hence, this electron escapes opposite to the direction of change in momentum due to the magnetic field. This is in striking contrast to strongly-driven atoms where the recolliding electron forward-scatters along the direction of light propagation. We attribute these distinct signatures to the different gate that the magnetic field creates jointly with a soft recollision in molecules compared to a hard recollision in atoms. These two different gates give rise, shortly before recollision, to different momenta and positions of the recolliding electron along the direction of light propagation. As a result, we show that the Coulomb forces from the nuclei act to back-scatter the recolliding electron in molecules and forward-scatter it in atoms along the direction of light propagation.

Light field image encryption based on spatial-angular characteristic

The 4D light field (LF) can be sliced into the spatial domain and the angular domain. Different from traditional images, LF image can simultaneously capture the direction and position of light propagation. With the development of image encryption technology, the security protection of LF images need to be paid further attention. Existing LF image encryption methods only explore the spatial information of the LF image, but ignore the angular information of LF image. In this paper, we propose a novel scheme for LF image encryption based on the internal spatial-angular characteristic of LF image. Firstly, considering the characteristic representation of the LF, we use the discrete fractional Fourier transform (DFrFT) to process the spatial domain of the LF image. Afterwards, in the angular domain, we employ the difference in viewing angles extracted from multiple angles to compose an epipolar plane image (EPI). The ciphertext EPI obtained by Arnold transform is displayed as an angular domain ciphertext image after reverse reconstruction. Furthermore, weighted linear blending and Chen’s chaotic system are performed on the ciphertext images in the spatial and angular domains to obtain the final ciphertext image. Experimental results demonstrate the effectiveness and applicability of the proposed scheme for LF image encryption.

Direct measurement of thermophoretic and photophoretic force acting on hot micromotors with optical tweezers

Synthetic microparticles present exciting features owing to their customizable light-matter interaction. We hereby report on the optical trapping of two artificial plasmonic microparticles: one with isotropic nanoparticles covering the surface (homogeneous particle) used as a hot Brownian particle and an anisotropic Janus microparticle, half coated with a gold nano-layer. The homogeneous particle decorated with plasmonic nanoparticles on the surface displays features of hot Brownian dynamics as well as photophoretic motion along z dimension in the optical trap. A dielectric particle was used as a reference particle because it acts as a cold particle with only the gradient force affecting it. In general, Janus particles orient in the trap with the dielectric part in the trap center. Plasmonic gold nanostructures absorb the light energy and produce heat; the photothermal forces significantly affect the optical trapping. These hot microspheres display temperature and Janus orientation dependent position distribution significantly different from cold (purely dielectric) microparticles. The developed method allows for the first time direct determination of the photophoretic (thermal force along light propagation direction) and thermophoretic force (light propagation direction independent force) acting on the respective particles, which opens new paths for analysis and control of micromachines.