Impinging Light Research Articles

Modification of graphene-like, hydrogenated amorphous, hydrogenated tetrahedral amorphous carbon and amorphous carbon thin films by UV-C light

The study aims to explore the effect of low fluence UV-C radiation on the structural quality of thin carbon films. We have modified single to few-layered nano-sized graphene-like films deposited by pulsed laser deposition (PLD) on ~300 nm SiO2/Si substrates and additionally different hydrogenated amorphous carbon a-C:H, tetrahedral amorphous carbon (ta-C:H) and amorphous carbon (a-C) thin carbon films. The modification was carried out by irradiation of the samples with UV-C lamps (Hg lamps λ= 254 nm wavelength and fluence of about 2 × 10−3 W/cm−2) for 5- 30 min in air-atmosphere for graphene-like and up to 60 min for thin carbon films. The irradiated graphene-like films were oriented either at about 2 arcdeg to the impinging light, i.e. the light was almost parallel to the honey-comb plane of graphene film or perpendicular to the UV-C light. The thin carbon films were treated only by a light directed perpendicularly to the films' surface for 60 min. Films were studied before and right after UV-C modification by ellipsometry or profilometry, X-ray photoelectron and Raman spectroscopies to clarify the influence of the UV-C treatment. The most pronounced influence of the UV-C irradiation on the structural quality of the films was established on a-C:H and ta-C:H films where the sp3 and the oxygen-containing radicals content decrease moderately and drops significantly, respectively. The influence of the UV-C irradiation directed almost parallel to the films’ surface on the structural quality of the graphene-like films was slightly higher than that directed perpendicularly. No significant influence on the quality of a-C films synthesized or a-C:H films annealed at high temperatures (1020–1050) °C was observed.

Brief descriptions of the principles of prominent methods used to study the penetration of materials into human hair and a review of examples of their use.

Consumers are attracted to the latest fashion trends and different looks. This drives the search for novel hair treatments. Some chemicals present in hair treatment products can penetrate the hair shaft. These materials can either nourish or injure the hair cortex. Different techniques have been used to investigate the mechanism of molecule penetration and the conditions under which penetration occurs. This article reviews the techniques applied for this purpose. Various microscopy techniques are used to capture clear and colourful images to determine the diffusion pathways and the exact location of the molecules under study. However, the laborious sample preparation often leads to sample destruction since cross-sectioning is often required. While various other techniques have been successfully used for investigating the penetration methods, most of these require different amounts of work to be put in for sample preparation and instrumentation. Several spectroscopic techniques have been used to study the penetration of the molecules because of the high levels of accuracy and the quick response time of these techniques. Moreover, the samples are not damaged during the investigation.

Kerker effect, superscattering, and scattering dark states in atomic antennas

We study scattering phenomena such as the Kerker effect, superscattering, and scattering dark states in a subwavelength atomic antenna consisting of atoms with only electric dipole transitions. We show that an atomic antenna can exhibit arbitrarily large or small scattering cross sections depending on the geometry of the structure and the direction of the impinging light. We also demonstrate that atoms with only an electric dipole transition can exhibit a directional radiation pattern with zero backscattering when placed in a certain configuration. This is a special case of a phenomenon known as the Kerker effect, which typically occurs in the presence of both electric and magnetic transitions. Our findings open a pathway to design highly directional emitters, nonradiating sources, and highly scattering objects based on individually controlled atoms.

High-efficiency, polarization-independent back reflector

A novel scheme to achieve a back reflector based on two-dimensional dielectric-metal graded metasurface is proposed, which presents high-efficiency over a broad range of incident solid angles and wavelengths. The remarkable performance is achieved by an effective optimization by searching the maximum reflection and the lowest deviation angle at a certain diffraction order. The optimized structure, working as a two-dimensional Littrow grating, is able to funnel the incident energy into the diffraction order (1, 1) to reflect almost all (> 92%) the impinging light back based on Huygens–Fresnel principle.

Thermally Controllable Infrared Absorption in Cylindrical Groove Array Covered by Phase Change Material

A perfect infrared spectrum absorber in a cylindrical groove array perforated in a metallic layer and followed by vanadium dioxide (VO2), a phase-change material, is introduced and analytically investigated. The phase-change material can change from insulator to metallic phase if proper stimuli, such as heat, voltage, and current, are applied, herein the phase change initiated by heating beyond the phase-transition temperature of VO2. The large refractive index variations between two phases enable the manipulation of optical properties. Simulation results reveal that in the metallic phase, the absorption is suppressed to one third, while VO2 works as a transparent medium for impinging light, and perfect absorption is observed in the insulation phase. The process is reversible in a picosecond time scale. Given the structure symmetry, the absorption is irreverent to impinging light polarization. The thermally controlled infrared absorber may have applications in various optical systems, like imaging and sensing, due to its simple and easy fabrication process.

Boosting ultrathin aSi-H solar cells absorption through a nanoparticle cross-packed metasurface

Hydrogenated amorphous silicon (a-Si:H) solar cells have some performance limitations related to the mobility and lifetime of their carriers. For this reason, it is interesting to explore thin-film solutions, achieving a tradeoff between photons optical absorption and the electrical path of the carriers to get the optimum thickness. In this work, we propose the insertion of a metasurface based on a cross-patterned ITO contact film, where the crosses are filled with nanospheres. We numerically demonstrate that this configuration improves the photogenerated current up to a 40% by means of the resonant effects produced by the metasurface, being independent on the impinging light polarization. Light handling mechanisms guide light into the active and auxiliary layers, increasing the effective absorption and mitigating the Staebler-Wronski effect. The selection of optimum materials and parameters results in nanospheres of ZnO with a 220 nm radius.

Mosquito counting system based on optical sensing

Mosquitos, sometimes carrying deadly diseases such as malaria, zika, and dengue fever, cause much concern. To control mosquitos, it is important to effectively monitor their presence and behavioral trends. We have constructed two optical sensing systems for insects based on light attenuation and light backscattering, respectively. The systems, which were tested with the potentially dangerous Aedes albopictus and Culex pipiens, were able to extract the wing-beat frequency, when they passed impinging light, derived from light-emitting diodes. We could achieve distinction between the sexes of A. albopictus and C. pipiens based on the wing-beat frequency. Finally, we propose a statistical method suitable for the system to improve the accuracy of counting.

Thermally controllable perfect absorber at telecommunication spectrum based on phase change material and cavity grating

We proposed and demonstrated a high-efficiency active absorber based on surface plasmon polaritons and Fabry–Perot cavity operating at telecommunication regime. The absorber comprised a resonance-cavity grating sandwiched between a metal and vanadium dioxide (VO2), a phase-change material (PCM) widely used in optical devices. As a PCM, VO2 is characterized by a large difference in the refractive index between the insulator and metallic phases initiated by various stimuli, such as heat, voltage, and current, thereby enabling the tailoring of optical properties in different systems. Herein, heating caused VO2 to undergo a transition from insulator to metallic phase, resulting in changes in its optical properties. For transverse magnetic polarization at resonance wavelength, the cavities beneath the VO2 confined the impinging light when VO2 was in insulator phase. Our simulation results revealed that the confinement enhanced the absorption up to 99%; conversely, when the system was heated beyond the phase-transition temperature of VO2 (68 °C), the absorption intensity decreased from 99% to 30%. For transverse electric polarization, a similar behavior was observed but the absorption intensity was about 50% at telecommunication band. These findings indicated that the proposed absorber was active, intensity tunable and highly responsive and may have applications in sensors and imaging.

Phonon-polaritonics: enabling powerful capabilities for infrared photonics

AbstractHere, we review the progress and most recent advances in phonon-polaritonics, an emerging and growing field that has brought about a range of powerful possibilities for mid- to far-infrared (IR) light. These extraordinary capabilities are enabled by the resonant coupling between the impinging light and the vibrations of the material lattice, known as phonon-polaritons (PhPs). These PhPs yield a characteristic optical response in certain materials, occurring within an IR spectral window known as the reststrahlen band. In particular, these materials transition in the reststrahlen band from a high-refractive-index behavior, to a near-perfect metal behavior, to a plasmonic behavior – typical of metals at optical frequencies. When anisotropic they may also possess unconventional photonic constitutive properties thought of as possible only with metamaterials. The recent surge in two-dimensional (2D) material research has also enabled PhP responses with atomically-thin materials. Such vast and extraordinary photonic responses can be utilized for a plethora of unusual effects for IR light. Examples include sub-diffraction surface wave guiding, artificial magnetism, exotic photonic dispersions, thermal emission enhancement, perfect absorption and enhanced near-field heat transfer. Finally, we discuss the tremendous potential impact of these IR functionalities for the advancement of IR sources and sensors, as well as for thermal management and THz-diagnostic imaging.

Switchable Lensed Linear Micro Axicon in Plasmonic Structures for All Optical Light Processing

We propose a simple route to build up Fresnel bi-mirror microaxicons based on the localized absorption of an astigmatic pump laser beam by a plasmonic metal foil. The corresponding local overheating produces two main effects: the thermal swelling of the glass and the local increase of its refractive index. As a result, a two-mirrors axicon-like element coupled to a gradient index microlens is produced. We modelled the optothermal formation of this microelement by Finite Element Modeling and tested the predicted temperature raise by Scanning Thermal Microscopy (SThM). We calculated and measured the far field distribution of an impinging probe beam that passes through the lensed microaxicon, finding in both cases a noticeable change in the far field pattern when the pumping light is ON evidencing at the same time the good modulation capability of this opto-thermal element. A similar, even if smaller, effect was also found when the probe light impinges from the air side, where no gradient index region is present. This configuration allowed us to measure and confirm the theoretically expected angular deflection and, indirectly, the high value of the vertical thermomechanical deformation induced by the tight astigmatic focusing of the pump beam.

An azimuthally-modified linear phase grating: Generation of varied radial carpet beams over different diffraction orders with controlled intensity sharing among the generated beams

Diffraction gratings are important optical components and are used in many areas of optics such as in spectroscopy. A diffraction grating is a periodic structure that splits and diffracts the impinging light beam into several beams travelling in different directions. The diffracted beams from a grating are commonly called diffraction orders. The directions of the diffraction orders depend on the grating period and the wavelength of the impinging light beam so that a grating can be used as a dispersive element. In the diffraction of a plane wave from a conventional grating, the intensities of diffracted beams decrease with increasing order of diffraction. Here, we introduce a new type of grating where in the diffraction of a plane wave, the intensity of a given higher order diffracted beam can be higher than the intensity of the lower orders. We construct these gratings by adding an azimuthal periodic dependency to the argument of the transmission function of a linear phase grating that has a sinusoidal profile and we call them azimuthally-modified linear phase gratings (AMLPGs). In this work, in addition to introducing AMLPGs, we present the generation of varied radial carpet beams over different diffraction orders of an AMLPG with controlled intensity sharing among the generated beams. A radial carpet beam is generated in the diffraction of a plane wave from a radial phase grating. We show that for a given value of the phase amplitude over the host linear phase grating, one of the diffraction orders is predominant and by increasing the value of the phase amplitude, the intensity sharing changes in favor of the higher orders. The theory of the work and experimental results are presented. In comparison with the diffraction of a plane wave from radial phase gratings, the use of AMLPGs provides high contrast diffraction patterns and presents varied radial carpet beams over the different diffraction orders of the host linear phase grating. The resulting patterns over different diffraction orders are specified and their differences are determined. The diffraction grating introduced with controlled intensity sharing among different diffraction orders might find wide applications in many areas of optics such as optical switches. We show that AMLPG-based radial carpet beams can be engineered in which they acquire sheet-like spokes. This feature nominates them for potential applications in light sheet microscopy. In addition, a detailed analysis of the multiplication of the diffraction pattern of an AMLPG by the 2D structure of a spatial light modulator is presented. The presented theory is confirmed by respective experiments.

Independent Tuning of Optical Transparency Window and Electrical Properties of Epitaxial SrVO3 Thin Films by Substrate Mismatch

AbstractTransparent metallic oxides are pivotal materials in information technology, photovoltaics, or even in architecture. They display the rare combination of metallicity and transparency in the visible range because of weak interband photon absorption and weak screening of free carriers to impinging light. However, the workhorse of current technology, indium tin oxide (ITO), is facing severe limitations and alternative approaches are needed. AMO3 perovskites, M being a nd1 transition metal, and A an alkaline earth, have a genuine metallic character and, in contrast to conventional metals, the electron–electron correlations within the nd1 band enhance the carriers effective mass (m*) and bring the transparency window limit (marked by the plasma frequency, ωp*) down to the infrared. Here, it is shown that epitaxial strain and carrier concentration allow fine tuning of optical properties (ωp*) of SrVO3 films by modulating m* due to strain‐induced selective symmetry breaking of 3d‐t2g(xy, yz, xz) orbitals. Interestingly, the DC electrical properties can be varied by a large extent depending on growth conditions whereas the optical transparency window in the visible is basically preserved. These observations suggest that the harsh conditions required to grow optimal SrVO3 films may not be a bottleneck for their future application.

Large lateral shift in complex dielectric multilayers with nearly parity–time symmetry

We investigate the lateral shift of optical beam in complex dielectric multilayers with nearly parity–time (PT) symmetry. The gain and loss of dielectrics are modulated to achieve nearly PT symmetry. As light impinges on the multilayers, a laser point (LP) and exceptional points (EPs) appear in the parameter space composed of the incident angle and refractive index. The phase of reflection dislocates at the LP and EPs, inducing large lateral shift around these points. The shift can be positive and negative, and the polarities of the lateral shift convert at the LP and EPs. The shift, which is sensitive to the incident angle and refractive index of dielectrics, could be tuned flexibly. The study may find potential applications in highly sensitive sensors.

Plasmonic Retrofitting of Membrane Materials: Shifting from Self-Regulation to On-Command Control of Fluid Flow.

This work calls for a paradigm shift in order to change the operational patterns of self-regulated membranes in response to chemical signals. To this end, the fabrication of a retrofitting material is introduced aimed at developing an innovative generation of porous substrates endowed with symbiotic but fully independent sensing and actuating capabilities. This is accomplished by transferring carefully engineered plasmonic architectures onto commercial microfiltration membranes lacking of such features. The integration of these materials leads to the formation of a coating surface proficient for ultrasensitive detection and "on-command" gating. Both functionalities can be synergistically modulated by the spatial and temporal distribution of an impinging light beam offering an unprecedented control over the membrane performance in terms of permeability. The implementation of these hybrid nanocomposites in conventional polymeric porous materials holds great potential in applications ranging from intelligent fluid management to advanced filtration technologies and controlled release.

Optical force rectifiers based on PT-symmetric metasurfaces

We introduce here the concept of optical force rectifier based on parity-time symmetric metasurfaces. Directly linked to the properties of non-Hermitian systems engineered by balanced loss and gain constituents, we show that light can exert asymmetric pulling or pushing forces on metasurfaces depending on the direction of the impinging light. This generates a complete force rectification in the vicinity of the exceptional point. Our findings have the potential to spark the design of applications in optical manipulation where the forces, strictly speaking, act unidirectionally.

Self-assembled antireflection coatings for light trapping based on SiGe random metasurfaces

We demonstrate a simple self-assembly method based on solid state dewetting of ultrathin silicon films and germanium deposition for the fabrication of efficient antireflection coatings on silicon for light trapping. We fabricate SiGe islands with a high surface density, randomly positioned and broadly varied in size. This allows one to reduce the reflectance to low values in a broad spectral range (from 500 nm to 2500 nm) and a broad angle (up to ${55}^{\ensuremath{\circ}})$ and to trap within the wafer a large portion of the impinging light $(\ensuremath{\sim}40%)$ also below the band gap, where the Si substrate is nonabsorbing. Theoretical simulations agree with the experimental results, showing that the efficient light coupling into the substrate is mediated by Mie resonances formed within the SiGe islands. This lithography-free method can be implemented on arbitrarily thick or thin ${\mathrm{SiO}}_{2}$ layers and its duration only depends on the sample thickness and on the annealing temperature.

Radial carpet beams: A class of nondiffracting, accelerating, and self-healing beams

Self-accelerating shape-invariant beams are attracting major attention, presenting applications in many areas such as laser manipulation and patterning, light-sheet microscopy, and plasma channels. Moreover, optical lattices are offering many applications, including quantum computation, quantum phase transition, spin-exchange interaction, and realization of magnetic fields. We report observation of a class of accelerating and self-healing beams which covers the features required by all the aforementioned applications. These beams are accelerating, shape invariant, and self-healing for more than several tens of meters, have numerous phase anomalies and unprecedented patterns, and can be feasibly tuned. Diffraction of a plane wave from radial phase gratings generates such beams, and due to their beauty and structural complexity we have called them ``carpet'' beams. By tuning the value of phase variations over the grating, the resulting carpet patterns are converted into two-dimensional optical lattices with polar symmetry. Furthermore, the number of spokes in the radial grating, phase variation amplitude, and wavelength of the impinging light beam can also be adjusted to obtain additional features. We believe that radial carpet beams and lattices might find more applications in optical micromanipulation, optical lithography, super-resolution imaging, lighting design, optical communication through atmosphere, etc.

Multiple resonant absorber with prism-incorporated graphene and one-dimensional photonic crystals in the visible and near-infrared spectral range

A multi-band absorber constructed from prism-incorporated one-dimensional photonic crystal (1D-PhC) containing graphene defects is achieved theoretically in the visible and near-infrared (vis-NIR) spectral range. By means of the transfer matrix method (TMM), the effect of structural parameters on the optical response of the structure has been investigated. It is possible to achieve multi-peak and complete optical absorption. The simulations reveal that the light intensity is enhanced at the graphene plane, and the resonant wavelength and the absorption intensity can also be tuned by tilting the incidence angle of the impinging light. In particular, multiple graphene sheets are embedded in the arrays, without any demand of manufacture process to cut them into periodic patterns. The proposed concept can be extended to other two-dimensional (2D) materials and engineered for promising applications, including selective or multiplex filters, multiple channel sensors, and photodetectors.

Anterior segment imaging using non-invasive high-resolution optical coherence tomography

One of the most important developments in the ophthalmology and visual sciences field has been the application of new techniques for imaging the anterior and posterior segment of the eye with non-contact devices. These techniques have been developed quickly considering that are non-invasive and provide high-resolution images of the different parts of the human eye. Definitely, the most spread and used technique for non-contact imaging of the human eye is the optical coherence tomography (OCT). OCT technique was first demonstrated in 1991 by Huang et al . (1). Based on low-coherence interferometry, OCT has become a prominent biomedical imaging technique with micrometric resolution and cross-sectional imaging capabilities, being particularly suitable for ophthalmic and other tissue imaging applications requiring millimetre penetration depth. Thus, OCT is especially useful in Ophthalmology, given the ease with which light reaches the ocular structures in the anterior and posterior segments. The advantage in its application is that the light impinges directly on the tissue, without the need to use any transducer, only requiring an optical medium sufficiently transparent to obtain a detectable signal. As a curiosity, OCT has also been used for various art conservation projects, where it is used to analyse, for instance, the different layers of a painting.

Narrowband transparent absorbers based on ellipsoidal nanoparticles.

In this paper, we propose the design of an optical device that is able to selectively absorb impinging light in a desired frequency range while being almost completely transparent outside this range. The proposed absorber is a variant of the optical Salisbury screen we recently proposed [Opt. Lett.41, 3383 (2016)OPLEDP0146-959210.1364/OL.41.003383] but, differently from this earlier version, is transparent for any electromagnetic wave whose frequency is outside the absorption spectrum. Such an absorber also exhibits excellent performance in terms of angular bandwidth and may find application in all scenarios where narrowband absorption is required, such as for light filters or digital sensors. Full-wave simulations confirming the effectiveness of the proposed absorber as well as its robustness toward geometrical defects are provided.