Large Index Change Research Articles

Review: tunable nanophotonic metastructures.

Tunable nanophotonic metastructures offer new capabilities in computing, networking, and imaging by providing reconfigurability in computer interconnect topologies, new optical information processing capabilities, optical network switching, and image processing. Depending on the materials and the nanostructures employed in the nanophotonic metastructure devices, various tuning mechanisms can be employed. They include thermo-optical, electro-optical (e.g. Pockels and Kerr effects), magneto-optical, ionic-optical, piezo-optical, mechano-optical (deformation in MEMS or NEMS), and phase-change mechanisms. Such mechanisms can alter the real and/or imaginary parts of the optical susceptibility tensors, leading to tuning of the optical characteristics. In particular, tunable nanophotonic metastructures with relatively large tuning strengths (e.g. large changes in the refractive index) can lead to particularly useful device applications. This paper reviews various tunable nanophotonic metastructures' tuning mechanisms, tuning characteristics, tuning speeds, and non-volatility. Among the reviewed tunable nanophotonic metastructures, some of the phase-change-mechanisms offer relatively large index change magnitude while offering non-volatility. In particular, Ge-Sb-Se-Te (GSST) and vanadium dioxide (VO2) materials are popular for this reason. Mechanically tunable nanophotonic metastructures offer relatively small changes in the optical losses while offering large index changes. Electro-optically tunable nanophotonic metastructures offer relatively fast tuning speeds while achieving relatively small index changes. Thermo-optically tunable nanophotonic metastructures offer nearly zero changes in optical losses while realizing modest changes in optical index at the expense of relatively large power consumption. Magneto-optically tunable nanophotonic metastructures offer non-reciprocal optical index changes that can be induced by changing the magnetic field strengths or directions. Tunable nanophotonic metastructures can find a very wide range of applications including imaging, computing, communications, and sensing. Practical commercial deployments of these technologies will require scalable, repeatable, and high-yield manufacturing. Most of these technology demonstrations required specialized nanofabrication tools such as e-beam lithography on relatively small fractional areas of semiconductor wafers, however, with advanced CMOS fabrication and heterogeneous integration techniques deployed for photonics, scalable and practical wafer-scale fabrication of tunable nanophotonic metastructures should be on the horizon, driven by strong interests from multiple application areas.

Engineering Refractive Index Contrast in Thin Film Barium Titanate-on-Insulator.

Barium titanate-on-insulator has demonstrated excellent vertical optical confinement, low loss, and strong electro-optic properties. To fabricate a waveguide-based device, a region of higher refractive index must be created to confine a propagating mode, one way of which is through dry etching to form a ridge. However, despite recent progress achieved in etching barium titanate and similar materials, the sidewall and surface roughness resulting from the physical etching typically used limit the achievable ridge depth. This motivates the exploration of etch-free methods to achieve the required index contrast. Here, we introduce three etch-free methods to create a refractive index contrast in barium titanate-on-insulator, including a metal diffusion method, proton beam irradiation method, and crystallinity control method. Notably, molybdenum-diffused barium titanate leads to a large index change of up to 0.17. The methods provided in this work can be further developed to fabricate various on-chip barium titanate optical waveguide-based devices.

Strong cladding mode excitation in ultrathin fiber inscribed Bragg grating with ultraviolet photosensitivity.

Strong UV-written Bragg gratings written in 50 µm-diameter cladding single mode fibers compatible with conventional fiber couple core guided light to dozens of cladding modes distributed across 140 nm in the 1400-1600 nm region, without the need for complex symmetry breaking mechanisms such as tilted, laterally offset, or localized gratings. The extent of the coupling to high order modes and the smaller cladding diameter both contribute to increasing the sensitivity to surrounding refractive index changes by more than one order of magnitude, and to an increased spacing between mode resonances to facilitate unambiguous measurements of larger index changes between 1.3 and 1.44. These improvements are confirmed by theoretical and experimental studies that also cover the temperature and strain differential sensitivities of the cladding mode resonances for complete multiparameter sensing capability.

Performance analysis of a silicon NOEMS device applied as an optical modulator based on a slot waveguide.

In this paper, we analyse the performance of a silicon nano-opto-electro-mechanical system (NOEMS) applied as an optical modulator, based on a suspended slot waveguide driven by electrostatic forces. The analysis is carried out with the help of the finite element analysis (FEA) method involving the influences from Casimir force, optical force and electrostatic force. The performance of the modulator are analysed from aspects of actuating modes, actuating voltage, modulating frequency, effective index, phase change, and energy consumption using the FEA method. Simulation results show that a suspended slot modulator has the advantages of low actuation voltage, low power consumption, as well as large effective index and phase change compared with modulators based upon other approaches. The performance of such a modulator can fill the performance gap between the carrier-based approach and micro-opto-electro-mechanical system (MOEMS) approach for modulation.

Optical Trajectory Manipulations Using the Self-Written Waveguide Technique.

This study is novel for several reasons: We used a thin drop cast layer of dry photosensitive materials to study the behaviors of wet photopolymer media using microscopic distances during the Self-Written Waveguide (SWW) process; then, we examined the self-trajectories formed inside the solid material. The results provide a framework for theoretical and experimental examinations by handling the effects of manipulating the alignment of fibers. The other main advantage of these techniques is their lightweight, easy to process, highly flexible, and ultimately low-cost nature. First, the SWW process in wet photopolymer media (liquid solutions) was examined under three cases: single-, counter-, and co-fiber exposure. Then, the SWWs formed inside the solid material were examined along with the effects of manipulating the alignment of the fibers. In all cases, high precision measurements were used to position the fiber optic cables (FOCs) before exposure using a microscope. The self-writing process was indirectly monitored by observing (imaging) the light emerging from the side of the material sample during SWW formation. In this way, we examined the optical waveguide trajectories formed in Acrylamide/Polyvinyl Alcohol (AA/PVA), a photopolymer material (sensitized at 532 nm). First, the transmission of light by this material is characterized. Then, the bending and merging of the waveguides that occur are investigated. The predictions of our model are shown to qualitatively agree with the observed trajectories. The largest index changes taking place at any time during exposure, i.e., during SWW formation, are shown to take place at the positions where the largest exposure light intensity is present. Typically, such maxima exist close to the input face. The first maximum is referred to as the location of the Primary Eye. Other local maxima also appear further along the SWW and are referred to as Secondary Eyes, i.e., eyes deeper within the material.

Thermally-induced nonlinear optical properties of silver nano-films near surface plasmon resonance

Silver nano-films with thicknesses of 5, 10 and 50 nm were grown using e-beam evaporation on glass substrates at room temperature in order to investigate their thermal nonlinear optical properties near surface plasmon resonance. The nonlinear response was verified using single beam z-scan setup in the cw regime. At 5 and 10 nm thickness, the films showed plasmonic resonance at 488 and 518 nm, respectively. The 50 nm films illustrated normal bulk behavior. Optical nonlinearities were stimulated at three different wavelengths, namely, 405, 532 and, 650 nm. Self-focusing and saturable absorption behavior was observed and compared at these wavelengths. The highest response was obtained at 532 nm close to the plasmonic wavelengths in both films. This behavior is related to the ultimate concentration of the optical power and hence, the temperature, to the nanometer range resulting in a large index change within a predicted time scale of sub-picoseconds. This outcome could pave the way for ultrafast thermal nonlinearity in nanomaterials. A discussion of this behavior and its consequence is presented.

Design of a novel nanoscale high-performance phase-change silicon photonic switch

We report the design of a novel high static performance on-off photonic switch by embedding the nanoscale phase-change material Ge2Sb2Te5 into a silicon-on-insulator waveguide. The state of the proposed switch can be changed using electrical/optical pulses, and no electrical bias is needed to maintain a particular state. The fundamental mode propagating through the silicon waveguide drastically alters its properties because of the large index change at the Si-Ge2Sb2Te5 interface and absorption in Ge2Sb2Te5. The photonic switch made of the silicon waveguide embedded with Ge2Sb2Te5 of volume 400 nm × 180 nm × 450 nm (length × height × width) provides a high extinction ratio of 43 dB with a low insertion loss of 2.76 dB in ON state at the communication wavelength of 1550 nm. There is a trade-off between the insertion loss and the extinction ratio. For 10 dB extinction ratio, the insertion loss can be as low as 1.2 dB with an extremely low active volume of 400 nm × 30 nm × 450 nm (length × height × width). Further, spectral response investigations reveal that this switch maintains an extinction ratio of more than 30 dB in the wavelength range of 1500–1600 nm. The high static performance of the photonic switch reported here is a direct consequence of the proper dimensions of the active volume of Ge2Sb2Te5 and its incorporation into the silicon-on-insulator waveguide. We also propose figure-of-merit for bias-free (nonvolatile) photonic switches that considers relevant parameters of static performance.

3D laser-written silica glass step-index high-contrast waveguides for the 3.5 μm mid-infrared range.

We report on the direct laser fabrication of step-index waveguides in fused silica substrates for operation in the 3.5 μm mid-infrared wavelength range. We demonstrate core-cladding index contrasts of 0.7% at 3.39 μm and propagation losses of 1.3 (6.5) dB/cm at 3.39 (3.68) μm, close to the intrinsic losses of the glass. We also report on the existence of three different laser modified SiO₂ glass volumes, their different micro-Raman spectra, and their different temperature-dependent populations of color centers, tentatively clarifying the SiO₂ lattice changes that are related to the large index changes.

High-Q ring resonators directly written in As_2S_3 chalcogenide glass films

Planar ring resonator waveguides are fabricated in thin films of As2S3 chalcogenide glass, deposited on silica-on-silicon substrates. Waveguide cores are directly written by scanning the focused illumination of a femtosecond Ti:sapphire laser at a central wavelength of 810 nm, through a two-photon photo-darkening process. A large photo-induced index change of 0.3–0.4 refractive index units is obtained. The radius of the ring resonator is 1.9 mm, corresponding to a transmission free spectral range of 9.1 GHz. A high loaded (intrinsic) Q value of 110,000 (180,000) is achieved. The thermal dependence of the resonator transfer function is characterized. The results provide the first report, to the best of our knowledge, of directly written high-Q ring resonators in chalcogenide glass films, and demonstrate the potential of this simple technique towards the fabrication of planar lightguide circuits in these materials.

Low-voltage quantum well microring-enhanced Mach-Zehnder modulator

Modulation characteristics of a novel InGaAs/InAlAs multiple quantum well (MQW) microring-enhanced Mach-Zehnder modulator (MRE-MZM) is investigated in detail and its low-voltage operation with high extinction ratio is demonstrated. The MZM has a single microring resonator in one arm and is driven by the change in electrorefractive index induced by the quantum-confined Stark effect in the MQW core layer. As the MQW, a multiple five-layer asymmetric coupled quantum well (FACQW) is used to obtain a large electrorefractive index change. The driving voltage of the proposed MZM is significantly reduced owing to the enhanced phase shift in the microring resonator. High-mesa waveguide structures are grown by solid-source molecular beam epitaxy and fabricated by inductively coupled plasma etching. A directional coupler with an asymmetric branching ratio is used as an input coupler to prevent the degradation of the extinction ratio of the MZM. The extinction ratio of the fabricated MRE-MZM is approximately 27 dB. The product of the half-wave voltage and phase shifter length, V(π) · L, is 1.7 Vmm in static modulation. This value is one-quarter that of a conventional MZM with the same waveguide structure.

Femtosecond laser-induced refractive index modifications in fluoride glass

The interaction of focused 800 nm femtosecond (fs) laser pulses at low (1–45 kHz) and moderate (50–250 kHz) repetition rates with fluoride (ZBLAN-type) bulk glass is investigated in detail. It is shown that at repetition rates lower than ~50 kHz and low pulse energies, the refractive index change induced by pulse filamentation is mainly negative at the irradiated zone. At repetition rates above 50 kHz, structures are formed as a result of the fs laser pulse induced heat accumulation and subsequent melting of the glass. The refractive index profile of the structures produced in this regime is influenced by the laser writing conditions (repetition rate, pulse energy and translation speed). It is shown that waveguides with large circular cross sections and smooth positive index changes can be formed through a precise control of glass exposure. Those waveguides can exhibit low propagation losses (~0.4–2 dB/cm) and are thus good candidates for the development of mid-IR integrated photonic devices. Both absorbed energy and repetition rate thresholds for heat accumulation are determined experimentally.

Proposal of Compact Tunable 1×2 Multimode Interference Splitter Based on Multiple Quantum Well

We propose a compact tunable 1×2 multimode interference (MMI) splitter based on an InGaAs/InAlAs multiple quantum well for an active trimmer for Mach–Zehnder modulator with a very high extinction ratio. The light power splitting characteristics of the splitter are theoretically investigated by a beam propagation method and a three dimensional finite-difference time-domain method. It is assumed that the core layer is composed of a multiple five-layer asymmetric coupled quantum well (FACQW) that is expected to exhibit a large electrorefractive index change owing to its unique quantum confined Stark effect. By applying reverse voltages and changing slightly the refractive indices of the localized regions surrounded by trenches for electrical isolation, the power splitting ratio can be tuned over a wide range. The length and width of the proposed MMI with four refractive index modulation regions are 192 and 6 µm, respectively. The length can be reduced to 115 µm if the number of index modulation regions is reduced to two, though the tuning range of the splitting ratio is narrowed. The simulation results show that the proposed compact MMI splitter has a low insertion loss and a low power consumption, and is promising for Mach–Zehnder modulators with a very high extinction ratio.

Photonic crystal self-collimation sensor

A novel refractive index sensor based on the two dimensional photonic crystal folded Michelson interferometer employing the self-collimation effect is proposed and its performances are theoretically investigated. Two sensing areas are included in the sensor. Simulation results indicate the branch area is suitable for the small index variety range and fine detection, whereas the reflector area prone to the large index change range and coarse detection. Because of no defect waveguides and no crosstalk of signal, the sensor is desirable to perform monolithic integrated, low-cost, label-free real-time parallel sensing. In addition, a flexible design of self-collimation sensors array is demonstrated.

Low-Voltage Mach–Zehnder Modulator with InGaAs/InAlAs Five-Layer Asymmetric Coupled Quantum Well

We have demonstrated a low-voltage Mach–Zehnder (MZ) modulator with a multiple InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) for 1.55 µm wavelength. The FACQW is expected to exhibit a large electrorefractive index change owing to its unique behavior of the quantum-confined Stark effect. The MZ modulator with ridge waveguides was fabricated by molecular beam epitaxy and wet etching. The product of a half-wave voltage and a phase shifter length (VπL) was as low as 1.2 V·mm in DC modulation. In addition, we discussed the degradation of the electrorefractive index change due to the nonuniformity of an electric field in the core layer and proposed a novel combined multiple FACQW structure to prevent the degradation.

Electromagnetically induced transparency-like transmission in periodically poled lithium niobate with a defect

Electromagnetically induced transparency (EIT)-like transmission is observed in an electro-optic (EO) tunable periodically poled lithium niobate (PPLN) with a central defect. When an electric field is applied, light satisfying the phase matching condition keeps its original polarization, although it experiences polarization rotation then turns back. Therefore the corresponding light always may pass though freely but with an EO tunable phase and dispersion. On the other hand, light with a neighboring wavelength is blocked. An EIT-like spectrum is thus obtained with tunable group delay. A low-voltage bulk phase shifter with over two orders of magnitude larger index change is obtained.

Influence of Heterointerface Abruptness on Electrorefractive Effect in InGaAs/InAlAs Five-Layer Asymmetric Coupled Quantum Well

An InGaAs/InAlAs five layer asymmetric coupled quantum well (FACQW) is expected to produce a large electrorefractive index change and is a promising structure for high-performance phase modulators. However, because an FACQW is composed of ultrathin layers, poor abruptness in heterointerface may deteriorate the electrorefractive effect. We therefore investigated theoretically the influence of abruptness of heterointerfaces in an FACQW on the electrorefractive effect of the FACQW. In the analysis, it is assumed that the composition profile in a heterointerface is exponential, and the thickness of the transition layer L is defined. With the increase in L, the absorption edge is blue-shifted, which deteriorates the electrorefractive index change of the FACQW especially in the case of L≥3 monolayer (ML). In addition, we propose an improved FACQW structure, which is expected to exhibit a large electrorefractive index even in the case of L=3 ML.

Rigidities and inflation persistence of services and regulated prices

AbstractThis paper studies price level rigidity and inflation persistence for EU15 member states using data on more than 1500 HICP sub‐indices. Services and HICP sub‐indices subject to price regulation exhibit a higher degree of price rigidity, with less frequent but larger index changes and a stronger asymmetry between increases and decreases. For most countries as well as for the EU15 and the euro area aggregates, excluding services from the HICP leads to a lower measured degree of inflation persistence; a similar though smaller effect is obtained by removing regulated indices from the HICP. Copyright © 2010 John Wiley & Sons, Ltd.

Photonic crystal fibre based high temperature sensor with three-beam path interference

Photonic crystal fibre (PCF) modal interferometers are widely used for sensing applications. Owing to the intrinsic Mach-Zehnder-like dual-mode interference there is a trade-off between sharp spectral response and wide sensing range. A three-beam path interferometer induced by mode-conversion in a single dual mode PCF end face is experimentally demonstrated. Doubled free spectral range is obtained owing to the three-beam path interference, which is suitable in sensing large index change, wide temperature variation and strong applied pressures. As an example, a temperature sensor with the sensitivity of 8.17 pm/ C is demonstrated ranging from room temperature to 520 C. The mechanism and further improvements of the multi-beam path interferometers are also discussed.

Electrorefractive Effect in GaInNAs/GaAs Five-Layer Asymmetric Coupled Quantum Well

A GaInNAs/GaAs five-layer asymmetric coupled quantum well (FACQW) in the 1.3-µm-wavelength region was proposed and theoretically analyzed. The valence band structures of the FACQW were analyzed using the Luttinger–Kohn Hamiltonian based on the k·p perturbation theory and a nonvariational method. The GaInNAs/GaAs FACQW can exhibit a unique behavior of the quantum confined Stark effect, leading to a large electrorefractive index change (Δn/ΔF = 3.9 ×10-4 cm/kV) at a small electric field in a wide-transparency-wavelength region far from the absorption edge. This characteristic is comparable to InGaAs/InAlAs and InGaAsP FACQWs operated in the 1.55-µm-wavelength region. The GaInNAs/GaAs FACQW is expected to realize ultrawide-band, ultrafast optical modulators and switches and other functional devices based on the phase shift in the 1.3-µm-wavelength region. The driving voltage of a 1 mm-phase modulator is estimated to be 0.4 V.

Large enhancement of femtosecond laser micromachining speed in dye-doped hydrogel polymers

Ophthalmologic hydrogel polymers are doped with Fluorescein or Coumarin dyes prior to the femtosecond laser micromachining process. We find that the achievable micromachining writing speed can be greatly increased while maintaining large refractive index changes (up to +0.08). Compared with previous results in dye-doped polymers that do not contain water such as PMMA, we obtain much larger index changes and much faster writing speeds.