Large Refractive Index Research Articles

Miniaturized Backward Coupler Realized by the Circuit‐Based Planar Hyperbolic Waveguide

Planar waveguides limit the transmission of electromagnetic waves in a specific direction and have a wide range of applications in filters, sensors, and energy‐transfer devices. However, given the increasing demand for planar‐integrated photonics, new waveguides are required with excellent characteristics such as more functionality, greater efficiency, and smaller size. Herein, the experimental results for a planar microwave‐regime waveguide fabricated from circuit‐based magnetic hyperbolic metamaterial (HMM) are reported. HMM is a special type of anisotropic metamaterial, whose isofrequency contour (IFC) takes the form of an open hyperboloid. Because of the open‐dispersion IFCs, HMMs support propagating high‐k modes with large effective refractive indices, which allow planar hyperbolic waveguides to be miniaturized. Especially, as opposed to the traditional dielectric slab waveguide, the group velocity and phase velocity in hyperbolic waveguides are oriented in opposite directions—a characteristic that is exploited to realize the backward propagation of electromagnetic waves. Based on this property, a backward coupler based on the hyperbolic waveguide is designed and experimented with. Herein, the significant potential of circuit‐based platforms for the experimental study of the propagation and coupling of guided modes is not only revealed but also the use of HMMs for numerous integrated functional devices is promoted.

Ultra-compact titanium dioxide micro-ring resonators with sub-10-μm radius for on-chip photonics

Microring resonators (MRRs) with ultracompact footprints are preferred for enhancing the light-matter interactions to benefit various applications. Here, ultracompact titanium dioxide ( TiO 2 ) MRRs with sub-10-μm radii are experimentally demonstrated. Thanks to the large refractive index of TiO 2 , the quality factors up to ∼ 7.9 × 10 4 and ∼ 4.4 × 10 4 are achieved for TiO 2 MRRs with radii of 10 μm and 6 μm, respectively, which result in large nonlinear power enhancement factors ( > 113 ) and large Purcell factors ( > 56 ). The four-wave mixing (FWM) measurements indicate that, compared to the large MRR, the FWM conversion efficiency of the ultracompact TiO 2 MRRs can be greatly improved (e.g., − 25 dB versus − 31 dB ), a harbinger of significant superiorities. Demonstrations in this work provide more arguments for the TiO 2 waveguides as a promising platform for various on-chip photonic devices.

Highly sensitive crumpled 2D material-based plasmonic biosensors.

We propose surface plasmon resonance biosensors based on crumpled graphene and molybdenum disulphide (MoS2) flakes supported on stretchable polydimethylsiloxane (PDMS) or silicon substrates. Accumulation of specific biomarkers resulting in measurable shifts in the resonance wavelength of the plasmon modes of two-dimensional (2D) material structures, with crumpled structures demonstrating large refractive index shifts. Using theoretical calculations based on the semiclassical Drude model, combined with the finite element method, we demonstrate that the interaction between the surface plasmons of crumpled graphene/MoS2 layers and the surrounding analyte results in high sensitivity to biomarker driven refractive index shifts, up to 7499 nm/RIU for structures supported on silicon substrates. We can achieve a high figure of merit (FOM), defined as the ratio of the refractive index sensitivity to the full width at half maximum of the resonant peak, of approximately 62.5 RIU-1. Furthermore, the sensing properties of the device can be tuned by varying crumple period and aspect ratio through simple stretching and integrating material interlayers. By stacking multiple 2D materials in heterostructures supported on the PDMS layer, we produced hybrid plasmon resonances detuned from the PDMS absorbance region allowing higher sensitivity and FOM compared to pure crumpled graphene structures on the PDMS substrates. The high sensitivity and broad mechanical tunability of these crumpled 2D material biosensors considerable advantages over traditional refractive index sensors, providing a new platform for ultrasensitive biosensing.

Germanium Nanosheets with Dirac Characteristics as a Saturable Absorber for Ultrafast Pulse Generation.

Bulk germanium as a group-IV photonic material has been widely studied due to its relatively large refractive index and broadband and low propagation loss from near-infrared to mid-infrared. Inspired by the research of graphene, the 2D counterpart of bulk germanium, germanene, has been discovered and the characteristics of Dirac electrons have been observed. However, the optical properties of germanene still remain elusive. In this work, several layers of germanene are prepared with Dirac electronic characteristics and its morphology, band structure, carrier dynamics, and nonlinear optical properties are systematically investigated. It is surprisingly found that germanene has a fast carrier-relaxation time comparable to that of graphene and a relatively large nonlinear absorption coefficient, which is an order of magnitude higher than that of graphene in the near-infrared wavelength range. Based on these findings, germanene is applied as a new saturable absorber to construct an ultrafast mode-locked laser, and sub-picosecond pulse generation in the telecommunication band is realized. The results suggest that germanene can be used as a new type of group-IV material for various nonlinear optics and photonic applications.

Design and characterization of high birefringence three suspended-cores fiber with few-mode

A three suspended-cores fiber (TSF) is proposed. Different to the traditional three-bridge suspended-core fiber, TSF has three suspended-cores with an air-hole in the center. The mode coupling in TSF is analyzed based on the effective area, and the TSF are optimized to support LP01 mode at both C and L wavelength bands and the LP01 mode has only weak coupling between each fiber core. Due to asymmetry fiber core structure and mode compression, the simulated birefringence of the TSF is 9.3×10−3 at 1.55 μm. Despite the large refractive index difference between core and cladding, TSF can be tailored to support only LP01 and LP11 modes.

Large refractive index modulation based on a BDK-doped step-index PMMA optical fiber for highly reflective Bragg grating inscription.

We experimentally report high reflectivity on the poly(methyl methacrylate) (PMMA)-based polymer optical fiber Bragg gratings by means of a 266 nm pulsed laser and phase mask technique. In the first recipe, fiber Bragg gratings (FBGs) were manufactured with a single pulse up to 3.7 mJ. After post-annealing, a stable refractive index change up to 4.2×10-4 was obtained. In the second recipe, FBGs were inscribed by 22 pulses with a lower pulse energy of 1.4 mJ, showing a stable refractive index change of 6.2×10-4. Both behaviors may mainly be attributed to the movement of initiating radicals arising from benzyl dimethyl ketal (BDK) under UV irradiation. The high refractive index change in step-index fibers paves the way to tilted FBG manufacturing with large tilt angles potentially for biomedical applications.

Silicon rib-loaded LiNbO3 waveguide polarization beam splitter based on bound state in the continuum design

Over the last years, LiNbO3 on insulator (LNOI) has emerged as one of the most important platforms for integrated photonic applications because of its large refractive index contrast, outstanding electro-optical and nonlinear optical properties. Rib-loaded waveguide technology is an efficient approach to avoid high surface and sidewall roughness and to relax the difficulty in construction of sub-wavelength structures with steep sidewalls in case of dry-etched LNOI on-chip nanowires. The main issue for the development of rib-loaded LNOI waveguides is the intrinsic TM loss which is caused by the mode conversion between different polarizations at rib boundaries. In this work we show that the intrinsic TM loss can be eliminated by precise control of the waveguide cross section, and a silicon rib-loaded LNOI polarization beam splitter (PBS) is proposed theoretically based on a symmetric directional coupler design. The PBS is designed with a compact size of 42-μm coupling length and relatively large bandwidth of ∼90 nm for extinction ratio above 15 dB. It is, to the best of our knowledge, the first PBS proposed on rib-loaded LNOI waveguide. We believe this work paves the way for exploration of functional integrated devices based on rib-loaded LNOI waveguides.

Enhancement of OAM and LP modes based on double guided ring fiber for high capacity optical communication

This article introduced a double guided ring-based spiral photonic crystal fiber (S-PCF) supporting both the orbital angular momentum (OAM) and linear polarization (LP) mode simultaneously with the efficient transmission. The outcomes exhibition that the designed S-PCF have good optical features such as less confinement loss (4.05342 × 10−12 dB/m for the EH9,1 mode), better mode quality and ISO (up to 98.87% and 126 dB, respectively), supported a decent amount of modes which is numerically 64 OAM and 6 LP modes, maintaining the large refractive index differences (>10-4) of all the eigenmodes, and a relatively flat dispersion variation (0.0743 ps/km-nm for the EH1,1 mode). Also, we obtained the crosstalk less than −12 dB and the power fraction is approximately 93%-99% of the S-PCF. All the optimizing properties are obtained using the full-vector finite element method (FEM) and also the perfectly matched layer (PML) as a boundary condition. Thus, the above mentioned optimizing propagation characteristic based S-PCF has effective applications like reliable long-distance communication into the space division multiplexing (SDM) process of the large-capacity fiber communication systems.

Mesoporous zinc oxide supported silica-titania nanocomposite: Structural, optical, and photocatalytic activity

Sol-gel-based titania nanoparticles (TNPs), Silica-titania nanocomposite (STNC), and zinc supported silica-titania nanocomposites (Z/STNC) are synthesized at low temperature. FESEM/EDX shows that ZNPs are homogeneously distributed in the STNC. The STNC has average roughness (Ra) ~6 nm, surface area ~340 m2/g, pore volume ~0.24 cm3/g, and particle size 4 nm ± 0.3 nm, which decrease down to Ra ~5 nm, large surface area 351 m2/g, pore volume 0.4 cm3/g, and particle size 3 nm ± 0.4 nm due to filling of STNC pores by Zn species. STNC is revealed 94% transmission, refractive index 1.34 at 633 nm, and bandgap 2.7 eV, which decreased down to 80% transmission, large refractive index 1.38, and low bandgap 2.25 eV after ZNPs doping. The Z/STNC observed high photocatalytic activity 92%, R2 ~98%, and rate constant 0.011 min−1 in the photodegradation of phenol red under UV-light irradiation.

Second Harmonic Generation Enhancement From Plasmonic Toroidal Resonance in Core-Shell Nanodisk

Toroidal multipoles, together with electric and magnetic multipoles, provide a complete description of electromagnetic responses of matter. The fundamental toroidal resonance has been extensively achieved and investigated in nanooptics, showing great ability to increase light absorption and light force. In this paper, we obtained plasmonic toroidal dipole resonance in core-shell (dielectric-core/gold-shell) nanodisk and demonstrated an enhanced second harmonic generation (SHG) from this resonance. It was shown that the toroidal dipole resonance based SHG depends on the refractive index of dielectric-core. For large refractive index cases, head-to-tail magnetic dipole pairs form a close loop, which is a signature of toroidal dipole and results in the enhancement of SHG. In addition, the frequency of this SHG red shifts as the refractive index of dielectric-core increases. We also investigated the cases in which the dielectric-core is filled with several common materials. Moreover, the toroidal resonance based SHG depends on both inner and outer radius of the core-shell nanodisk. Such results may serve for the developments of nonlinear nanooptics and their applications.

Spatial-partition fast Fourier transform beam propagation method

In this paper, a spatial-partition fast Fourier transform beam propagation method (FFT-BPM) is proposed. The theoretical analysis of this beam propagation method is carried out, and the necessary conditions for its accurate application are derived. Through high-accuracy and high-speed simulation of the propagation of light beam in the medium with large refractive index variation, the reliability of this method is demonstrated.

Novel Bloch wave excitation platform based on few-layer photonic crystal deposited on D-shaped optical fiber

With the goal of ultimate control over the light propagation, photonic crystals currently represent the primary building blocks for novel nanophotonic devices. Bloch surface waves (BSWs) in periodic dielectric multilayer structures with a surface defect is a well-known phenomenon, which implies new opportunities for controlling the light propagation and has many applications in the physical and biological science. However, most of the reported structures based on BSWs require depositing a large number of alternating layers or exploiting a large refractive index (RI) contrast between the materials constituting the multilayer structure, thereby increasing the complexity and costs of manufacturing. The combination of fiber–optic-based platforms with nanotechnology is opening the opportunity for the development of high-performance photonic devices that enhance the light-matter interaction in a strong way compared to other optical platforms. Here, we report a BSW-supporting platform that uses geometrically modified commercial optical fibers such as D-shaped optical fibers, where a few-layer structure is deposited on its flat surface using metal oxides with a moderate difference in RI. In this novel fiber optic platform, BSWs are excited through the evanescent field of the core-guided fundamental mode, which indicates that the structure proposed here can be used as a sensing probe, along with other intrinsic properties of fiber optic sensors, as lightness, multiplexing capacity and easiness of integration in an optical network. As a demonstration, fiber optic BSW excitation is shown to be suitable for measuring RI variations. The designed structure is easy to manufacture and could be adapted to a wide range of applications in the fields of telecommunications, environment, health, and material characterization.

Lighting of a monochromatic scatterer with virtual gain

In this work, we discuss the scattering features of a dipolar particle made of large refractive index material by employing the concept of virtual gain and virtual loss. The virtual gain and loss can be achieved in a lossless passive nanostructure by shaping the temporal waveform of incident signals in the complex frequency plane. We show that an appropriate tuning of excitation time of the impinging field allows to capture and release the electromagnetic energy on-demand for an arbitrary time scale in a lossless nanosphere. Thus, the nanosphere obliges to emit monochromatic magnetic light which can be tuned throughout the whole visible spectrum by varying the size of the nanosphere. This proposal may find fruitful applications in lab-on-a-chip technologies and the realization of monochromatic sectoral multipole light source with a large quality factor at nanoscale level.

The Tactile Sensing with Light Propagation in a Four-Layer Slab Waveguide for Breast Tumor Detection

Light propagation in a 4 layer slab waveguide is considered. The supported modes are of higher orders than zero, with a larger node concentration toward the layers with larger refractive index. Additionally, we consider the geometric optics approximation to describe very thick waveguides. Light can be coupled from LED by direct focusing with a lens, and coupling can be optimized by introducing a transverse displacement in order to focus it on the layers with larger refractive index. Further optimization can be done by tilting the lens and bringing in some degree of coma aberrations.

Ring-Core Photonic Crystal Fiber of Terahertz Orbital Angular Momentum Modes with Excellence Guiding Properties in Optical Fiber Communication

The orbital angular momentum (OAM) of light is used for increasing the optical communication capacity in the mode division multiplexing (MDM) technique. A novel and simple structure of ring-core photonic crystal fiber (RC-PCF) is proposed in this paper. The ring core is doped by the Schott sulfur difluoride material and the cladding region is composed of fused silica with one layer of well-patterned air-holes. The guiding of Terahertz (THz) OAM beams with 58 OAM modes over 0.70 THz (0.20 THz–0.90 THz) frequency is supported by this proposed RC-PCF. The OAM modes are well-separated for their large refractive index difference above 10−4. The dispersion profile of each mode is varied in the range of 0.23–7.77 ps/THz/cm. The ultra-low confinement loss around 10−9 dB/cm and better mode purity up to 0.932 is achieved by this RC-PCF. For these good properties, the proposed fiber is a promising candidate to be applied in the THz OAM transmission systems with high feasibility and high capacity.

Chip-compatible wide-field 3D nanoscopy through tunable spatial frequency shift effect

Spatial frequency shift (SFS) microscopy with evanescent wave illumination shows intriguing advantages, including large field of view (FOV), high speed, and good modularity. However, a missing band in the spatial frequency domain hampers the SFS superresolution microscopy from achieving resolution better than 3 folds of the Abbe diffraction limit. Here, we propose a novel tunable large-SFS microscopy, making the resolution improvement of a linear system no longer restricted by the detection numerical aperture (NA). The complete wide-range detection in the spatial frequency domain is realized by tuning the illumination spatial frequency actively and broadly through an angle modulation between the azimuthal propagating directions of two evanescent waves. The vertical spatial frequency is tuned via a sectional saturation effect, and the reconstructed depth information can be added to the lateral superresolution mask for 3D imaging. A lateral resolution of λ/9, and a vertical localization precision of ∼λ/200 (detection objective NA = 0.9) are realized with a gallium phosphide (GaP) waveguide. Its unlimited resolution enhancing capability is demonstrated by introducing a designed metamaterial chip with an unusual large refractive index. Besides the great resolution enhancement, this method shows better anti-noise capability than classical structured illumination microscopy without SFS tunability. This method is chip-compatible and can potentially provide a mass-producible illumination chip module achieving the fast, large-FOV, and deep-subwavelength 3D nanoscopy.

Compact Robust Vector Bending Sensor Based on Single Stress-Applying Fiber

We propose a compact assemble-easy cost-effective vector bending sensor consisting of a short section of single stress-applying fiber (SSAF) spliced between two standard single-mode fibers (SMFs). The SSAF is specially designed by introducing a stress-applying part (SAP) into the cladding to achieve the asymmetrical geometry. Because of the mode field mismatch between the SMF and the SSAF, the cladding mode is effectively excited, which is sensitive to the bending and has orientation-dependence. Due to the large effective refractive index difference between the core and cladding mode, a compact dual-mode interferometer can be achieved with a thousands-micron-long structure. When the fiber is bent, the mode fields and the effective refractive indices will be modified, and accordingly, the direction-dependent interference spectral shift is observed. Owing to the well-designed SSAF structure, the cost-effective vector bending sensor can be easily realized by a simple sandwich structure without extra complex alignment procedures or fiber grating inscription. We experimentally verify the vector bending property. The maximum sensitivity of 2.04 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is obtained, and the experimental results match well with the simulations. Our proposed device is compact, easy to assemble, and cost-effective, which is promising in the field of vector bending measurement.

Ultrastable Perovskite Nanocrystals in All‐Inorganic Transparent Matrix for High‐Speed Underwater Wireless Optical Communication

AbstractThe key features of lead halide perovskites, including short emission lifetime and excellent luminous efficiency in the green emission region, perfectly match the strict requirements of underwater wireless optical communication (UWOC). However, the poor stability of perovskite nanocrystals (NCs) restricts its application under water and no relevant application research has been reported. Here, extremely stable CsPbBr3 NCs in all‐inorganic amorphous glass synthesized by all‐solid reaction without any organic ligand are reported. The products achieve high photoluminescence quantum yield (70%) as well as excellent stability toward water, thermal treatment (200 °C in air) and intensive laser irradiation (3.6 kW cm−2). The authors also raise that the refraction index could strongly influence the lifetime values according to the oscillator strength of radiative transitions. The large refraction index of the glass matrix enhances the radiative transition rates of CsPbBr3 NCs (decay time 3.22 ns). At last, the first application of perovskite NCs for UWOC is demonstrated, achieving a bandwidth as high as 180 MHz and data rates of 185 Mbps.

Giant Faraday effect of FeCo‐BaF and FeCo‐SiN nanogranular films

AbstractWe introduce FeCo‐BaF and FeCo‐SiN nanogranular films exhibiting giant Faraday effect. These films have a nanocomposite structure, in which nanometer‐sized FeCo ferromagnetic granules are dispersed in BaF2 or Si3N4 matrix. FeCo‐BaF film has a good figure of merit of Faraday effect from the crystalized BaF2 matrix. FeCo‐SiN film has a large refraction index from the nature of Si3N4 matrix. These properties contribute to the practical application of nanogranular films to optical devices.

Towards a Rationalization of Ultrafast Laser-Induced Crystallization in Lithium Niobium Borosilicate Glasses: The Key Role of the Scanning Speed

Femtosecond (fs)-laser direct writing is a powerful technique to enable a large variety of integrated photonic functions in glass materials. One possible way to achieve functionalization is through highly localized and controlled crystallization inside the glass volume, for example by precipitating nanocrystals with second-order susceptibility (frequency converters, optical modulators), and/or with larger refractive indices with respect to their glass matrices (graded index or diffractive lenses, waveguides, gratings). In this paper, this is achieved through fs-laser-induced crystallization of LiNbO3 nonlinear crystals inside two different glass matrices: a silicate (mol%: 33Li2O-33Nb2O5-34SiO2, labeled as LNS) and a borosilicate (mol%: 33Li2O-33Nb2O5-13SiO2-21B2O3, labeled as LNSB). More specifically, we investigate the effect of laser scanning speed on the crystallization kinetics, as it is a valuable parameter for glass laser processing. The impact of scanning energy and speed on the fabrication of oriented nanocrystals and nanogratings during fs-laser irradiation is studied.Fs-laser direct writing of crystallized lines in both LNS and LNSB glass is investigated using both optical and electron microscopy techniques. Among the main findings to highlight, we observed the possibility to maintain crystallization during scanning at speeds ~5 times higher in LNSB relative to LNS (up to ~600 µm/s in our experimental conditions). We found a speed regime where lines exhibited a large polarization-controlled retardance response (up to 200 nm in LNSB), which is attributed to the texturation of the crystal/glass phase separation with a low scattering level. These characteristics are regarded as assets for future elaboration methods and designs of photonic devices involving crystallization. Finally, by using temperature and irradiation time variations along the main laser parameters (pulse energy, pulse repetition rate, scanning speed), we propose an explanation on the origin of (1) crystallization limitation upon scanning speed, (2) laser track width variation with respect to scanning speed, and (3) narrowing of the nanogratings volume but not the heat-affected volume.