Positive Refractive Index Research Articles

Plasmon-exciton coupling in nanostructured metal-semiconductor composite films

Nonlinear optical response of metal-semiconductor Zn0.8Co0.2O/Au nanocomposite films was studied experimentally and theoretically. Z-scan measurements revealed a positive nonlinear index of refraction of the composite film. Fluorescent spectroscopy measurements exhibited a narrow sharp peak that might be attributed to exciton peak of semiconductor nanostructured thin film enhanced by nearby gold nanoparticles. Classical electrodynamic calculations of a quantum dot in close proximity to a gold nanoparticle agree well with the experimentally observed normalized quantum efficiency.

Negative refraction of plane electromagnetic waves in non-uniform double-negative media.

We propose a new model of a plane-layered medium with a fixed positive refractive index, but variable non-vanishing dielectric permittivity and magnetic permeability. An analytical solution of the problem of plane electromagnetic wave propagation through the medium has been found. It is shown that the negative-refraction effect occurs in this type of medium; in particular, an explicit expression for the field has been first obtained in the region of its formation. This result suggests that the necessary condition for the occurrence of this effect in quite complex media is not the negativity of their refractive indices, but only the requirement of simultaneous negativity of the real parts of dielectric permittivity and magnetic permeability. We also demonstrate that the medium is passive (absorbing) if a plane E-polarized wave propagates in it, and active (amplifying) in the case of H-polarized wave propagation.

Optical fishnet metamaterial with negative, zero, positive refractive index and nearly perfect absorption behavior at different frequencies

Metamaterials (MMs) have many novel characteristics with the aid of appropriate structural design. In this study, we report negative refractive index (NRI), zero refractive index (ZRI) and nearly perfect absorption (NPA) characteristics of an optical fishnet structure. Base on the S-parameter retrieval theory, this periodic nano tapered holes fishnet structure that has simultaneously negative permittivity and negative permeability and shows NRI characteristic from 553 THz to 600 THz. The fishnet structure also exhibits a seamless transition between the negative and positive refractive index bands with a zero index at the transition frequency of 600 THz. Moreover, interestingly, a broadband nearly perfect absorption (NPA) characteristic above 95% arises in 650 THz to 700 THz. As mentioned characteristics of NRI, ZRI and NPA above, they are gained from local electromagnetic fields enhancement by the multiple surface plasmon polaritons (SPPs) resonance mechanism.

Electromagnetic waves' self-focusing in metamaterials having both LHM and RHM behavior: an application to filter design

ABSTRACTSelf-focusing, also known as Talbot’s effect had been widely studied by the scientific community. In this paper we show self-focusing as a coherence propagation phenomenon, in particular we studied self-focusing for a pulse propagating in a metamaterial with both behaviors: left handed material (LHM) and right handed material (RHM). These dual behaviors allow the construction of frequency filters in the range of frequency in which the metamaterial stops behaving as a LHM (negative refraction index) and begins an RHM behavior (positive refraction index). The coherence length (CL) for pulses propagating in these metamaterials seems to be larger when its frequencies are in the LHM region. Finally, we show that the cell’s size turns out to be critical in designing the filter, and as an example we present here four metamaterial cells.

Ultrafast Laser Inscription of High-Performance Mid-Infrared Waveguides in Chalcogenide Glass

We present the realization of mid-infrared waveguide by ultrafast laser inscription technique in a chalcogenide glass. Our approach is based on multicore waveguide that consists in an alignment on a mesh of positive refractive index channels placed parallel to each other. Two different meshes are investigated with different refractive index contrasts between the channel and the glass matrix. A detailed analysis of the performances at a wavelength of 4.5 $\mu$m shows propagation losses of 0.20 $\pm$ 0.05 dB/cm and coupling efficiencies higher than 60 %.

Inscribing diffraction gratings in bulk silicon with nanosecond laser pulses.

Diffraction gratings are transversally inscribed in the bulk of monolithic crystalline silicon with infrared nanosecond laser pulses. Nanoscale material analyses of the modifications composing the gratings show that they rely on laser-induced stress associated with a positive refractive index change as confirmed with phase-shift interferometry. Characterizations of the optical properties of the gratings, including the diffraction angles and the efficiency of the different orders, are carried out. The refractive index change obtained from these measurements is in good agreement with the phase-shift measurements. Finally, we show that the grating diffraction efficiency depends strongly on the laser writing speed.

Effect of fluorination on bandgap, first and second order hyperpolarizabilities in lithium substituted adamantane: A time dependent density functional theory

Time dependent density functional theory is used to systematically examine the band gap variation of Li-substituted adamantanes upon fluorination. It is realized that the absorption wavelength of Li-Adamantane can be fine tuned within UV–Visible region by changing the degree of fluorination. The static and dynamic hyperpolarizabilities of fluorinated Li-Adamantane derivatives were computed using coupled perturbed Kohn Sham theory. In this article, we report Pockel, dc-Kerr, ESHG, degenerate four wave mixing coefficients and nonlinear refractive indices from the first and second order hyperpolarizability calculations. A high positive nonlinear refractive index and significant second harmonic generation efficiency at Nd:YAG Laser wavelength are observed in F3Li-Adm derivative.

Study of nonlinear properties of N-(2-hydroxyphenyl)-2-propylpentanamide in polymeric solution

We have studied the nonlinear optical properties for synthesized materials by using the Z-scan technique, in order to measure nonlinear refractive indices. The nonlinear optical properties of synthesized N-(2-hydroxyphenyl)-2-propylpentanamide (HO-AAVPA), in micellar aqueous solution of F127, were properly studied by using the technique explained above. Additionally, the optical transmittance signal was obtained with and without finite aperture using a 26 ps Nd:YAG pulsed-laser, at 532 nm and featuring a 10 Hz repetition rate. The solution shows a high NLO response at the wavelength studied. The positive nonlinear refraction index and negative nonlinear absorption coefficient were experimentally obtained for the sample under test. Finally, atomic force microscopy (AFM) images were used to analyze the topography of these samples. It was demonstrated that adding the hydrophobic HO-AAVPA molecule to the F127 solution, the surface topography abruptly changes forming a kind of spherical structures. In addition, the linear absorption spectrum was obtained for the HO-AAVPA with and without F127.

Influence of Thermal Annealing on Ultrafast Laser‐Induced Local Densification in Bulk Sulfur‐Based Chalcogenide Glasses

The response to focused ultrafast laser pulses, of three bulk sulfur‐based chalcogenide glasses, gallium lanthanum sulfide (GLS) and two germanium arsenic sulfide with different stoichiometry (Ge20As20S60 and Ge15As15S70), is studied as a function of a thermal treatment made by several annealing cycles under the glass transition temperature. It is demonstrated that the annealing procedure has a considerable influence on the ultrafast laser‐induced local densification process of the quenched glasses. Using post‐mortem phase contrast microscopy and single‐mode light guiding measurements on photo‐inscribed waveguides, the influence of the thermal history on the laser‐induced positive refractive index changes (local densification) is determined. In particular it is shown that, in the case of the Ge20As20S60 glass, the threshold of ultrafast laser‐induced densification in terms of laser energetic dose is considerably lowered after the first annealing cycle. In the light of this result, the activation of a new relaxation channel during the thermal treatment and its influence on the laser‐induced densification process of Ge‐As‐S glasses is discussed. Potential of material thermal pre‐treatment for developing efficient photo‐inscription techniques in the domain of integrated photonics is also discussed.

Solitary Electromagnetic Waves in a Weakly Nonlinear Rhombic Waveguide Array

Electromagnetic waves propagating along the waveguides forming a rhombic one-dimensional lattice are considered. Two waveguides that are part of the unit cell are assumed to be made of negative-index material, while the third waveguide from the same array is composed of material with a positive refractive index and has cubic nonlinearity. The equations of tunneling-coupled solitary waves spreading in each waveguide are solved. Unlike discrete solitons, the waves are localized on the axis of waveguides, and the maximum position moves uniformly along the waveguides.

Numerical Study of 1D Wave Propagation through Double Negative Photonic Crystal

Photonic crystals are having many applications in different fields like wave guides, high efficiency antenna etc. The conventional photonic crystal contains alternating layers of Double Positive media having high and low positive refractive index values. Recently another kind of unconventional photonic crystal is proposed which consists of a Double Negative meta-material and a Double Positive medium. While in conventional photonic crystals the band gaps are formed due to Bragg scattering of fields, in unconventional photonic crystals the band gaps are formed due to both Bragg scattering and volume averaged zero refractive index of the unit cell. Here we propose a new kind of photonic crystal which consists of two Double Negative meta-materials having different refractive indices. The computational study of the propagation of electromagnetic wave through one dimensional Double Negative photonic crystal is reported. One dimensional Finite Difference Time Domain method is used for the study. Drude model is used in this paper to define the dispersion property of Double Negative media.

Modulation instability in two-dimensional waveguide arrays with alternating signs of refractive index

We consider the propagation of coupled forward and backward waves in two-dimensional arrays of alternating waveguides with positive and negative refractive indices. The array has a cross section in the form of a face-centered square lattice. The unit cell consists of three different waveguides with different optical properties. We obtain the condensate solutions for electromagnetic pulse propagation in such an array of waveguides. The base equations describing this model have solutions that correspond to the static and homogeneous intensity distribution on the waveguides. The stability of these solutions in both normal and anomalous dispersion regimes is also investigated using modulation instability analysis.

Superlight inverse Doppler effect

It has long been thought1 that the inverse Doppler frequency shift of light2–13 is impossible in homogeneous systems with a positive refractive index. Here we break this long-held tenet by predicting a previously unconsidered Doppler effect of light inside a radiation cone, the so-called Vavilov–Cherenkov cone, under specific circumstances. It has been known from the classic work of Ginzburg and Frank that a superlight (that is, superluminal) normal Doppler effect14–18 appears inside the Vavilov–Cherenkov cone if the velocity of the source v is larger than the phase velocity of light vp. By further developing their theory, we discover that an inverse Doppler frequency shift will arise if v > 2vp. We denote this as the superlight inverse Doppler effect. Moreover, we show that the superlight inverse Doppler effect can be spatially separated from the other Doppler effects by using highly squeezed polaritons (such as graphene plasmons), which may facilitate the experimental observation. The authors theoretically investigate a novel form of a Doppler effect in homogeneous systems with positive refractive index that occurs under certain conditions. It is suggested that this Doppler effect can be experimentally separated from other Doppler effects by using polaritons such as those found in graphene.

Comparative Features of Electromagnetic Scattering by an Invisible Spherically Symmetric Lens with a Negative Refractive Index

A spherically symmetric lens with a negative refractive index, which can be invisible from the point of view of geometrical optics, is considered. Expressions describing the profile of the refractive index and ray trajectories in the lens, as well as some results of numerical modeling of plane wave scattering by the lens in the rigorous statement of the problem, are presented. It is detected that, unlike in the case of an invisible lens with a positive refractive index, the minima of the radar cross section in the forward direction corresponding to invisibility for the lens with a negative index occur at integer, rather than semi-integer, values of the electric radius, and that their depth is smaller because of the astigmatism associated with focusing of rays on the axis. Some results characterizing influence of losses in the lens on the level of the radar cross-section minima are also presented.

Picosecond nonlinear refraction of C2H4Cl2 and C2H4Br2 at 532nm studied with Z-scan technique

We investigated optical nonlinear refraction (NLR) of C2H4Cl2 and C2H4Br2 using the [Formula: see text]-scan technique with 532[Formula: see text]nm 17 picosecond pulses delivered at 10[Formula: see text]Hz. We found that the latter shows larger positive Kerr nonlinear refractive index than the former, [Formula: see text][Formula: see text]cm2/W versus [Formula: see text][Formula: see text]cm2/W. According to our additional study on time-resolved Raman-induced Kerr effect, this observed trend is ascribed to stronger inter-molecular motions of diffusive reorientation and polarizability distortion in C2H4Br2 than in C2H4Cl2. This study helps to understand the nonlinear response of molecular motions to electric-field in great details which have not been well-defined previously.

Bragg Grating Inscription With Low Pulse Energy in Doped Microstructured Polymer Optical Fibers

We demonstrate that fiber Bragg gratings (FBGs) can be written in a doped polymer optical fiber (POF) in a low ultraviolet (UV) pulse energy regime (60Jpulse) using a 248-nm krypton fluoride excimer laser system. The total energy density per inscription necessary to obtain Bragg gratings is between 493.6 and 3825mJcm 2, depending on the number of pulses and the pulse energy. The impact of the pulse energy on the growth of the Bragg grating is investigated, and it is shown that the 248-nm light induces a positive refractive index change. This article demonstrates that the FBGs can be obtained in the POFs without high pulse energy (mJ level) at 248-nm wavelength, which reduces maintenance costs. Furthermore, we can consider it as a solution to increase the lifetime of the laser system without high energy still allowing fast and efficient production of the FBGs for sensing applications.

Field Distribution into Binary Linear Waveguide Array

.

Large Stokes Shifted Far‐Red to NIR‐Emitting D‐π‐A Coumarins: Combined Synthesis, Experimental, and Computational Investigation of Spectroscopic and Non‐Linear Optical Properties

AbstractCoumarin based push‐pull fluorophores (COM‐1 to COM‐5) bearing thiophene π‐spacer were synthesized having a donor at 7‐position and acceptor at 3‐position. Increasing the π‐conjugation length shifts their fluorescence maxima to the longer wavelength (600–756 nm). All fluorophores exhibit positive emission solvatochromism, fluorescence lifetime is in the range of 1.31 to 2.18 ns, and are high thermal stability. The nonlinear optical behavior was investigated by calculating the first/second‐order hyperpolarizability using a spectroscopic and computational method wherein, COM‐3 shows higher optical nonlinearity (μβCT=987 x 10−48 e.s.u and μβ0=9758 x 10−48 e.s.u.) in DMF. Third‐order nonlinear optical parameters measured using the Z‐scan method in each closed and open aperture curves. The fluorophores possess the self‐focusing effect and show negative nonlinear absorption coefficient (β) and positive nonlinear refractive index (n). Among all fluorophores, COM‐5 shows better real part of third‐order susceptibility χ(3) in DMSO (8.58 x 10−13 e.s.u.). Bond length alternation (BLA), natural bond orbital analysis (NBO), and generalized Mulliken‐Hush (GMH) analysis were studied to give theoretical evidence to the effective charge transfer (CT). Quadratic optical nonlinearity and Z‐scan results show that these coumarin‐based fluorophores can be considered as promising candidates for organic NLOphores in material for nonlinear optics.

Compact assembly-free vector bend sensor based on all-in-fiber-core Mach-Zehnder interferometer.

A novel low-cost, compact, assembly-free and sensitive optical fiber curvature sensor is presented. This device consists of an off-axis positive refractive index modified zone (PRIMZ), induced by a direct femtosecond laser, written in a single-mode fiber (SMF) core. The PRIMZ transforms the original SMF section into a few-mode fiber (FMF). As a result, the whole fiber forms an assembly-free "SMF-FMF-SMF" sandwich Mach-Zehnder interferometer. When the device is bent, a direction-dependent spectral shift of the interference pattern is produced. The sensitivity of the sensor is up to 2.53 and 2.24 nm/m-1 for the 0° and 180° orientations in a wide bend range (from 0 to 4 m-1). In addition, the device is immune to the surrounding refractive index and has a low-temperature crosstalk, which makes it very attractive for practical structural monitoring applications.

A Bloch Wave Numerical Scheme for Scattering Problems in Periodic Wave-Guides

We present a new numerical scheme to solve the Helmholtz equation in a wave-guide. We consider a medium that is bounded in the $x_2$-direction, unbounded in the $x_1$-direction, and $\varepsilon$-periodic for large $|x_1|$, allowing different media on the left and on the right. We suggest a new numerical method that is based on a truncation of the domain and the use of Bloch wave ansatz functions in radiation boxes. We prove the existence and a stability estimate for the infinite dimensional version of the proposed problem. The scheme is tested on several interfaces of homogeneous and periodic media, and it is used to investigate the effect of negative refraction at the interface of a photonic crystal with a positive effective refractive index.