Positive Refractive Index Research Articles

Polymer Photonic Crystal Waveguides Generated by Femtosecond Laser

AbstractFemtosecond laser‐based fabrication of integrated photonic crystal waveguides inside of a transparent polymer is reported. In general, the fabrication of waveguides based on femtosecond laser‐induced positive refractive index modifications is feasible. However, in transparent polymers their performance is limited since only minor refractive index changes can be achieved with this technique. These restrictions can be circumvented by generating hexagonal “photonic lattice like” waveguides, which consist of negative refractive index modifications in the cladding, enabling light confinement in the unirradiated core. A comprehensive study on the fabrication of this new class of polymer waveguide is performed as well as a characterization of its numerical aperture, mode field diameter, and attenuation. In addition, integration of Bragg gratings based on positive and negative refractive index modulations within the waveguide's core is studied. Moreover, we highlight that polymer photonic crystal waveguides are compatible with common femtosecond laser‐based internal structuring processes, enabling the creation of monolithic optofluidic devices, which is demonstrated by the fabrication of a Fabry–Pérot interferometer.

Design and analysis of a position chirped metamaterial photonic crystal array for confinement of light pulses

In a photonic crystal (PC) array, a line defect (removal of a periodic structure) and position up-chirping (gradually decreasing lattice constant) are familiar terms to introduce light confinement and propagation due to the photonic bandgap (PBG). In this paper, we have proposed a new model for light confinement using a line defect position chirped PC array with metamaterial as a dielectric material. For comparison, we have designed and simulated for both line defect positions chirped and none position chirped PC array using normal material (refractive index +3.927) dielectric material and metamaterial (refractive index −3.927). We have used metamaterial due to the compensation of light dispersion and leakage effect because the negative refractive index makes the wave vector direction precisely opposite to the positive refractive index material. We have implemented the metamaterial positioned chirped PC array with sharp 90° bending and shown the light confinement and propagation for that structure. To support our proposed model, we have calculated the band diagram of both positive refractive index and metamaterial unit cells and verified the quantum well existence in the band diagram of metamaterial unit cell. Based on the quantum well effect, the confinement of light in metamaterial line defect position chirped PC has been explained and presented with proper finite element method (FEM) simulations. From quantum mechanics, we can comprehend that confining wave particles at the atomic level is extremely challenging because of the evanescent nature of the light wave. The novelty of this paper is that we have managed to confine the propagating of light waves by using the quantum well existence in the metamaterial dielectric for a line defect position chirped PC structure, and to the best of our knowledge, we are probably the first to introduce light confinement using a metamaterial chirped PC structure with proper FEM simulation.

Corrigendum: Conductivity-dependent absorption-free, gain-assisted superluminal propagation and rotary photon drag [Optical Materials 115 (2021) 111047

Unfortunately, some of the Figures were placed at wrong order (e.g. Figs. 1–6), in addition to the repetition of Fig. 6, in our published Article [ 1 ]. Besides correcting the order, the missing Fig. 1 (on the energy level scheme of the proposed quantum model) and Fig. 7 are added in the Corrigendum. As such, the correct orders of Figs. 1–6 in the Article should be Figs. 2–7 as in the Corrigendum. Figs. 8–9 are replaced with the correct ones. Figs. 10–12 are also correctly readjusted. Figures 13 and 14 in the Article are correct in all respects. However, description in the captions of all the Figures with the corrected order in the Article is same as in the corrigendum. The authors highly regret the inconvenience. • Study of absorption-free, gain-assisted propagation at positive and negative group velocity for positive and negative index refractive index of the conducting medium. • Phase and amplitude of conductivity influence largely the optical properties such as group index, group velocity, group delay and photon drag. • The maximum calculated values of positive and negative superluminal group velocities are of order 4 ×10 8 m/s at positive and negative group indices, respectively. • The maximum value of photon drag calculated is of order 6.4 ×10 −7 radian. • Probe detuning can be used as a tuning nob to switching the light absorption profile to the transparent window at resonance.

Beam Splitters Fabricated by Nonlinear Focusing of Femtosecond Laser Writing in Pure YAG Crystal

We report on the fabrication of waveguides and beam splitters in pure YAG crystals by femtosecond laser direct writing (FLDW). During the femtosecond laser writing process, a positive refractive index is induced through the nonlinear focusing above the focus position, resulting in an unusual guiding cross-sectional configuration. The supported guiding modes at 632.8 nm are measured and analyzed using the end–face coupling system. The propagation loss can be as low as 1.9 dB/cm for the single-line waveguide. Different from the geometry of the traditional fs-laser modified area, this novel structure might offer a new approach in the quest toward integrated photonics.

Third-order NLO studies of 2, 4-Bis (4-fluorophenyl)-2, 3-dihydro-1H-1, 5-benzodiazepine using Z-scan technique and DFT method

The present work discusses the nonlinear optical properties of 2, 4-Bis (4-fluorophenyl)-2, 3-dihydro-1H-1, 5-benzodiazepine. The structural characterizations have been performed using FT-IR, FT-Raman, and UV-Visible absorption spectroscopic techniques. The optimized geometry, vibrational wavenumbers, natural bond orbital analysis, HOMO-LUMO analysis, natural population analysis were calculated using the B3LYP/6-311++G (d, p) level of theory. Unambiguous vibrational assignments have been performed using potential energy distribution values obtained from normal coordinate analysis using MOLVIB program. The theoretical second-order hyperpolarizability was calculated using B3LYP and CAM-B3LYP hybrid functional at the DFT level. The second-order hyperpolarizability (γ),third-order susceptibility (χ(3)), nonlinear refractive index (n2), nonlinear absorption coefficient (β)) have been investigated using the Z-scan technique. The Z-scan results confirm that the compound exhibits positive nonlinear absorption and negative nonlinear refractive index. The second harmonic generation efficiency was investigated using the Kurtz and Perry method and was found to be 1.44 times higher than urea. The obtained results reveal that the title compound is a promising material for laser assisted applications.

About copying a free-space electromagnetic field by an invisible object

The structure of a monochromatic electromagnetic field inside a spherical object that does not distort the wave field of external sources outside its surface has been studied using transformation optics methods. It is assumed that the object is made of isotropic materials with equal values of permittivity and permeability and consists of a spherical volume of material with a positive spatially uniform refractive index and an adjacent spherical layer of material with a negative inhomogeneous refractive index (i.e., it is a spherical superlens made of isotropic materials). It is shown that the field inside a sphere with a positive refractive index is the reduced-size copy of the field in which the object was located. This copy contains information about the radiation field of external sources in a spherical free-space volume whose radius depends on the object parameters and can be significantly larger than the radius of the object. It is shown that the copying properties of the object with given dimensions can be fully preserved if its outer layer is made of anisotropic materials.

Acoustic metamaterials

Waves are generally characterized by angular frequency ω and wavevector k. Accordingly, this tutorial is structured into two parts, one on resonance-based acoustic metamaterials, in the frequency domain, and one on topological acoustics, based on the wavevector domain as topological structures inherently involve spatial configurations that are a step beyond the simple periodic lattices. Each part will begin with a brief introduction of the basic principles, followed by two examples described in detail. In the first part, we present decorated membrane resonators and the broadband optimal acoustic absorption structures, the latter being crucial for the potential applications of acoustic metamaterials. In the second part, we discuss how to construct the Dirac cone, a special type of dispersion from either accidental degeneracy or symmetry protection, which can be shown to lead to negative, zero, or positive refractive indices. The shifting and gapping of these Dirac cones in the reciprocal space can result in effects on acoustic waves similar to that of a magnetic field on an electron. More generally, they lead to edge states resulting from a real-space gauge field as well as topological bandgaps.

Femtosecond laser induced thermophoretic writing of waveguides in silicate glass

Here in, the fs-laser induced thermophoretic writing of microstructures in ad-hoc compositionally designed silicate glasses and their application as infrared optical waveguides is reported. The glass modification mechanism mimics the elemental thermal diffusion occurring in basaltic liquids at the Earth’s mantle, but in a much shorter time scale (108 times faster) and over a well-defined micrometric volume. The precise addition of BaO, Na2O and K2O to the silicate glass enables the creation of positive refractive index contrast upon fs-laser irradiation. The influence of the focal volume and the induced temperature gradient is thoroughly analyzed, leading to a variety of structures with refractive index contrasts as high as 2.5 × 10–2. Two independent methods, namely near field measurements and electronic polarizability analysis, confirm the magnitude of the refractive index on the modified regions. Additionally, the functionality of the microstructures as waveguides is further optimized by lowering their propagation losses, enabling their implementation in a wide range of photonic devices.

Conductivity-dependent absorption-free, gain-assisted superluminal propagation and rotary photon drag

Abstract Superluminal light propagation and photon drag are subjects of immense scientific interest owing to potential fundamental and technological implications in the fields of atomic and molecular physics, condensed-matter physics, quantum optics, nanoplasmonics, nanophotonics, and nanoelectronics. In this paper, we present a systematic study on the optical properties, rotary photon drag and superluminal propagation in a conducting gain medium comprising a four-level invert-N-type atomic system. In particular, to explore the nature of light propagation, as to whether subluminal or superluminal, we aim to investigate optical properties of the medium in view of the derived expression of group index, group velocity, group delay, phase delay and photon drag. Interestingly, by tuning the spectral parameters of the proposed system, we predict gain-assisted, absorption-free propagation and superluminality at positive and negative group velocity for positive and negative refractive indices of the medium, respectively. The medium, subjected to various spectral conditions, displays an interesting switching from ON-resonance single absorption peak in the probe field spectrum to an ON-resonance absorption-free region between two OFF-resonance absorption peaks to OFF-resonance gain singlet accompanied by OFF-resonance absorption peak. The maximum calculated values of positive and negative superluminal group velocities of the light pulse are of order 4 × 10 8 m / s both at positive and negative group indices, respectively. On the other hand, the maximum value of photon drag calculated is of order 6.4 × 10 − 7 radian.

Emergence of fifth-order optical nonlinearity in 2-(2-Quinolyl)-1,3-indandione with strong third-order nonlinear effect under low power CW laser excitation

The present work reports the presence of fifth-order optical nonlinearity in 2-(2-Quinolyl)-1,3-indandione, an organic dye, at low intensity level. Photophysical, structural and vibrational properties were studied by theoretical and experimental methods. Z-scan study of the dye revealed that the compound has a self-defocusing behaviour at continuous Nd:YAG 532 nm laser excitation and exhibited reverse saturable absorption. The positive fifth-order nonlinear refractive index (n4) is of the order of 10−10 cm4/W2. The nonlinearities present in the compound has both thermal and electronic origin. This study reveals that the material is a potential candidate as an optical limiter at low power.

Maximum Refractive Index of an Atomic Medium

It is interesting to observe that all optical materials with a positive refractive index have a value of index that is of order unity. Surprisingly, though, a deep understanding of the mechanisms that lead to this universal behavior seems to be lacking. Moreover, this observation is difficult to reconcile with the fact that a single, isolated atom is known to have a giant optical response, as characterized by a resonant scattering cross section that far exceeds its physical size. Here, we theoretically and numerically investigate the evolution of the optical properties of an ensemble of ideal atoms as a function of density, starting from the dilute gas limit, including the effects of multiple scattering and near-field interactions. Interestingly, despite the giant response of an isolated atom, we find that the maximum index does not indefinitely grow with increasing density, but rather reaches a limiting value $n\approx 1.7$. We propose an explanation based upon strong-disorder renormalization group theory, in which the near-field interaction combined with random atomic positions results in an inhomogeneous broadening of atomic resonance frequencies. This mechanism ensures that regardless of the physical atomic density, light at any given frequency only interacts with at most a few near-resonant atoms per cubic wavelength, thus limiting the maximum index attainable. Our work is a promising first step to understand the limits of refractive index from a bottom-up, atomic physics perspective, and also introduces renormalization group as a powerful tool to understand the generally complex problem of multiple scattering of light overall.

Third‐Order Nonlinear Optical Response of Few‐Layer MXene Nb2C and Applications for Square‐Wave Laser Pulse Generation

AbstractNiobium carbide (Nb2C), a newly developed 2D MXene material, has attracted much attention due to its outstanding electronic and optical properties. In this work, few‐layer 2D Nb2C nanosheets are synthesized by magnetron sputtering deposition method. Z‐scan measurements with pump source operating at 1.0 µm are performed to study the third‐order nonlinear optical response, revealing excellent light modulation capabilities of 2D Nb2C. The effective nonlinear absorption coefficient (βeff ≈ –105 cm GW–1) caused by saturable absorption effect is determined to be two orders of magnitude larger than that induced by reverse saturable absorption processes (two‐photon absorption, etc.). The positive nonlinear refractive index n2 is determined to be ≈10–13 m2 W–1 for the first time. Moreover, by decorating 2D Nb2C onto microfiber, microfiber‐based 2D Nb2C saturable absorber is fabricated and applied for compact square‐wave pulse mode‐locking ytterbium‐fiber laser. The pulse duration of SWP increases from 0.652 to 1.616 ns with the pulse energy increasing linearly up to 0.89 nJ while the peak power remains a constant. The results not only verify the capability of 2D Nb2C as an optical modulator, but also pave a new way for exploring SWP laser sources.

Temperature‐Controlled Optical Activity and Negative Refractive Index

AbstractChiral media exhibit optical activity, which manifests itself as differential retardation and attenuation of circularly polarized electromagnetic waves of opposite handedness. This effect can be described by different refractive indices for left‐ and right‐handed waves and yields a negative index in extreme cases. Here, active control of chirality, optical activity, and refractive index is demonstrated. These phenomena are observed in a terahertz metamaterial based on 3D‐chiral metallic resonators and achiral vanadium dioxide inclusions. The chiral structure exhibits pronounced optical activity and a negative refractive index at room temperature when vanadium dioxide is in its insulating phase. Upon heating, the insulator‐to‐metal phase transition of vanadium dioxide effectively renders the structure achiral, resulting in absence of optical activity and a positive refractive index. The origin of the structure's chiral response is traced to magnetic coupling between front and back of the structure, whereas the temperature‐controlled chiral‐to‐achiral transition is found to correspond to a transition from magnetic to electric dipole excitations. The use of a fourfold rotationally symmetric design avoids linear birefringence and dichroism, allowing such a structure to operate as tunable polarization rotator, adjustable linear polarization converter, and switchable circular polarizer.

Femtosecond laser direct writing of Nd:YAG waveguide with Type I modification: Positive refractive index change in track

Abstract Nd:YAG crystal is one of the major gain media used in high-power solid-state laser systems. In this work, the single-line waveguides with Type Ⅰ modification have been fabricated in Nd:YAG crystal by femtosecond laser writing. The waveguides show good guiding properties in both TE and TM polarizations, which is different from the well-developed Type II structures. The high-resolution confocal micro-photoluminescence and micro-Raman spectra images obtained by scanning the cross-section of the track region certify a positive refractive-index change induced in femtosecond laser irradiated region. The fluorescence properties remain 82% in the waveguide region, which shows the potential for active photonic devices.

Modulational instability in a non-Kerr photonic Lieb lattice with metamaterials

We present an analysis of modulational instability of diffractionless waves in a face-centered-square lattice of waveguides featuring non-Kerr nonlinearity, which are composed of a combination of positive and negative refractive indices. The unit cell of the lattice consists of three different waveguides with different optical properties. The dispersion curve of the lattice supports flatbands and thereby the base equations describing the model have particular solutions that correspond to the diffractionless waves propagating along the waveguides. We also observe a unique ramification of nonlinearities in controlling the flatbands optically. The diffractionless wave solutions are derived and the stability of these distributions is investigated concisely by adopting the standard linear stability approach.

Dual index properties of photonic crystal and its application in subwavelength focusing

Based on the two-dimensional dielectric wedge-shaped photonic crystal, we studied the existence of two kinds of equivalent refractive indexes (negative refractive index is −1, positive refractive index is 1.897) in the photonic crystal by means of fast Fourier transform (FFT), as well as its influence on the optical propagation characteristics was discussed. By adjusting the wedge angle of the photonic crystal to eliminate the influence of positive refraction beam on the imaging, the sub wavelength focusing with half-width less than half of a wavelength can be realized. The numerical results show that the Full Width Half Maximum (FWHM) of the image point can reach a minimum of when the wedge angle of the photonic crystal is 73°.

Design of a Dual Band SNG Metamaterial Based Antenna for LTE 46/WLAN and Ka-Band Applications

The non-existing properties of the metamaterial surfaces can be utilized to improve the antenna radiation characteristics. In this article, a design and performance analysis of a Single Negative (SNG) metamaterial based antenna is imparted for LTE 46/WLAN and Ka-band (like in satellite communication for the receiving side) applications. The unit cell of the metamaterial surface exhibits negative permittivity and positive permeability; yielding a high magnitude positive refractive index, is used to improve and analyze the performance of the proposed monopole antenna element. The proposed SNG based antenna covers a -10 dB bandwidth from 5.35-5.69 GHz (LTE 46/WLAN) and 17.81-20.67 GHz (Ka-band). The total size of the proposed antenna element is 20.2×28.4 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> while a 2×3 SNG metamaterial surface is used at the back of the antenna element which improves the gain from 4.52 dB to 9.13 dB for the desired Ka band and 1.17 to 5.04 dB for the LTE 46/WLAN band. Furthermore, for the LTE 46/WLAN frequency band, the impedance matching also gets better, resulting in the return loss improvement from -11 dB to -32.4 dB. Moreover, the radiation efficiency is also improved by more than 10 % for the Ka band after employing the SNG metamaterial surface. The measured results fall in good agreement with the simulated one and make the proposed SNG metamaterial based antenna design competent for the LTE 46/WLAN and Ka-band (like in satellite communication for the receiving side) applications.

Reduced Graphene Oxide Thin Film with Strong Optical Nonlinearity

Herein, a simple, low‐cost, and scalable process is developed to prepare the reduced graphene oxide (rGO) thin films (TFs) with strong optical nonlinearity. To do so, a chemical reduction method (using inorganic agent hydroiodic acid) and drop‐casting technique are used. Optical nonlinearity of the prepared thin rGO film is measured by the Z‐scan technique at low light intensity (continuous‐wave wavelength of 632 nm). The result of optical nonlinearity responses is illustrated by a remarkable nonlinear refraction index (n2 ≈ 10−4 cm2 W−1) as well as a considerable nonlinear absorption coefficient (β = 1.43 × 10−8 cm2 W–1) compared with similar reported results. Moreover, experimental results show that the prepared film enjoys a positive nonlinear refractive index with strong optical nonlinearity. Results also confirm that the changes made in optical absorption of rGO TFs are due to state filling effects and the optical nonlinearity proportional to the carriers’ density is in fact due to thermal effects of SiO2 substrate.

Parametric Analysis of Negative and Positive Refractive Index Lens Antenna by ANSYS HFSS

Lens antennas with multibeam, high gain, and low sidelobe level are potential candidates for base station antennas in 5G mobile communication. In this paper, the authors perform simulation and parametric analysis of a lens antenna with positive and negative refractive indexes (NRI) using the modern electromagnetic field simulation software ANSYS HFSS. The simulation results of structures and theoretical calculations are analyzed and compared. The simulation results show the effectiveness of using negative refractive index lens antennas to minimize the dimension. The lens thickness with a negative refractive index decreased from 24.5 mm to 6.1 mm compared to the positive refractive index lens’s thickness. The results also indicate the similarities in gain, sidelobe level, amplitude, and electric field distribution on the aperture plane of the negative and positive refractive indexes (PRI) lens antennas compared to the theoretical calculation. In addition, the authors simulate a lens structure with additional quarter wavelength matching layers (MLs) to estimate the antireflection performance.

Manipulation of femtosecond laser filamentation by a gaseous lattice.

Manipulation of femtosecond laser filamentation is essential for many potential applications. We report the simulations of the manipulation of femtosecond laser filamentation by introducing a novel gaseous lattice medium with the alternating positive and negative refractive index distribution at different stages of filamentation. The results show that the filament length has greatly been extended and a multi-filament array can be formed by the gas lattice medium. It has been found that additional focusing and discrete diffraction provided by the gas lattice medium contribute to a new dynamic equilibrium in the filamentation. As a result, a varied cross-section pattern, higher field intensity, and electron density along the filamentation are obtained. Our approach provides a new way to manipulate filamentation for many practical photonic applications.