High Refractive Index Change Research Articles

Ultrafast Laser-Fabricated Fluoride Glass Waveguides with Exceptionally High Positive Refractive Index Change for Mid-Infrared Integrated Optics

Ultrafast Laser-Fabricated Fluoride Glass Waveguides with Exceptionally High Positive Refractive Index Change for Mid-Infrared Integrated Optics

Development of High Refractive Index Polydimethylsiloxane Waveguides Doped with Benzophenone via Solvent-Free Fabrication for Biomedical Pressure Sensing

We present the fabrication and characterization of elastomeric optical waveguides, to be used for the manufacture of a conformable, water-resistant, and cost-effective pressure sensor that is amenable to the development of smart wearable health monitoring devices. To achieve this goal, high-sensitivity polydimethylsiloxane waveguides with a rectangular cross-section were fabricated. A new up-doping procedure, to tailor the refractive index of the ensuing waveguides, was experimentally developed using benzophenone additives. With this method we demonstrated a high refractive index change (up to +0.05) as a linear function of the benzophenone doping concentration. Propagation losses of about 0.37 dB/cm in the visible range and a high sensitivity to transverse compression of 0.10%/dB optical power loss were measured. It was also shown that one can further control the refractive index of the waveguide core and cladding regions through proper selection of the polydimethylsiloxane base to curing agent mixing ratio.

High refractive index change in Type A laser modification using a multi-scan approach

The focusing of femtosecond laser pulses is an efficient and robust way to fabricate integrated optical components in glasses. However, the induced refractive index changes are weak if compared with those obtained in lithography. Several solutions have been found to overcome such problem, the multi-scan technique being one of these. The present work implements the multi-scan approach on the femtosecond inscription of Type A laser modifications in high repetition rate regime. Therefore, a study on the effects of the absorption of subsequent laser pulses on the increase of the induced refractive index is conducted. As a result, the achievement of high and smooth refractive index change, up to 2 × 10−2, in a commercial silver-glass is reported. Moreover, such high and controlled index contrast is exploited for the fabrication of low-propagation-losses and engineered waveguides exhibiting single-mode operation in the VIS-NIR range.

Unique prospects of phase change material Sb2Se3 for ultra-compact reconfigurable nanophotonic devices

In this work, we explore inverse designed reconfigurable digital metamaterial structures based on phase change material Sb2Se3 for efficient and compact integrated nanophotonics. An exemplary design of a 1 × 2 optical switch consisting of a 3 µm x 3 µm pixelated domain is demonstrated. We show that: (i) direct optimization of a domain containing only Si and Sb2Se3 pixels does not lead to a high extinction ratio between output ports in the amorphous state, which is owed to the small index contrast between Si and Sb2Se3 in such a state. As a result, (ii) topology optimization, e.g., the addition of air pixels, is required to provide an initial asymmetry that aids the amorphous state's response. Furthermore, (iii) the combination of low loss and high refractive index change in Sb2Se3, which is unique among all phase change materials in the telecommunications 1550 nm band, translates into an excellent projected performance; the optimized device structure exhibits a low insertion loss (∼1.5 dB) and high extinction ratio (>18 dB) for both phase states.

Designer Glasses—Future of Photonic Device Platforms

AbstractThe intentional inclusion of key atomic elements in a purpose designed glass helps to achieve unprecedented control over the ultrafast laser written circular waveguide morphology and refractive index change. Behavioral response of glass constituents to ultrafast laser in 14 different commercial silicate glasses having various compositions are studied. Viscosity, aluminum to alkaline earth+alkali ratio, and total silicon content within the glass are the prime control factors for producing waveguides with high circularity and refractive index change. Drawing on this knowledge, the designer glass is successfully fabricated from an empirical formula that facilitates maintaining circular waveguide morphology, high refractive index over fast feed rates, and amorphous composition.

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.

Effect of high bandgap AlAs quantum barrier on electronic and optical properties of In0.70Ga0.30As/Al0.60In0.40As superlattice under applied electric field for laser and detector applications

Effect of high bandgap [Formula: see text] nm AlAs on the electronic and optical properties of the [Formula: see text]/[Formula: see text] superlattice is investigated by using effective mass approximation under the electric field. Electronic transitions are obtained as 0.403 eV and 0.023 eV for [Formula: see text] and [Formula: see text] in the gain region. Thin AlAs increases electron confinement in the superlattice and prevents electron leakage in the gain region which mostly results in higher absorption/emission in the superlattice. AlAs has no major effects on transitions energies in the gain region but it is effectively decreasing the total absorption in the injector region and preventing the internal absorption. AlAs also makes the superlattice optically more stable by decreasing the high refractive index change in the injector region by factor 5.

Two-photon holographic recording in LiTaO3:Fe crystals with high intensity nanosecond pulses at 532 nm

Recording the volume holograms in LiTaO3 with high intensity (up to 11 GW/cm2) nanosecond pulses was considered in this study. A very compact Nd:YVO4 laser with the pulse duration of 1.7 ns at λ = 530 nm was used for the recording. Four congruent LiTaO3 samples with the iron contents ranging from 3.3 × 1017 to 700 × 1017 см−3 were selected for the experimental observation. In the LiTaO3:Fe crystal (CFe = 66 × 1017 см−3), the microhologram recording by the single nanosecond pulse with as high refractive index change as 7.6 × 10−4 was carried out. In the same sample, the maximum refractive index change of 29.8 × 10−4 was reached under the exposition to a train of four pulses. The qualitative charge transport model taking into account a recording with high intensity nanosecond pulses was proposed.

Enhancement of the refractive index modulation in a modulator based on GaAs/AlGaAs quantum wells

Approaches of optimization of coupled quantum wells with pronounced quantum-confined Stark effect in order to reach a high refractive index change are described. A numerical simulation was used to determine the optimal design parameters (quantum well width, barrier thickness and composition) based on GaAs/AlGaAs materials with two symmetric quantum wells, which provides the maximum modulation of the refractive index at a small absorption coefficient. It is demonstrated that, with the electric field strength varied within the range 0–20 kV cm−1 at an optical loss for interband absorption not exceeding 10 cm−1, the refractive index can be changed by up to 0.0362.

Polymer optical fiber Bragg grating inscription with a single UV laser pulse.

We experimentally demonstrate the first polymer optical fiber Bragg grating inscribed with only one krypton fluoride laser pulse. The device has been recorded in a single-mode poly(methyl methacrylate) optical fiber, with a core doped with benzyl dimethyl ketal for photosensitivity enhancement. One laser pulse with a duration of 15 ns, which provide energy density of 974 mJ/cm2, is adequate to introduce a refractive index change of 0.74×10-4 in the fiber core. After the exposure, the reflectivity of the grating increases for a few minutes following a second order exponential saturation. The produced Bragg grating structure rejects 17.9 dB transmitted power, thus providing 98.4% reflectivity, which is well suited for sensing applications. In addition, we report the importance of the fiber thermal treatment before or after the inscription, showing its effects on the lifetime and quality of the grating structures. Optimizing the irradiation conditions and the material chemical composition, a higher refractive index change in the fiber core is feasible. This demonstration significantly improves the potential for commercial exploitation of the technology.

Transmission volume phase holographic gratings in photo-thermo-refractive glass written with femtosecond laser Bessel beams

Transmission volume phase holographic gratings were fabricated in bulk photo-thermo-refractive glass using zero-order femtosecond laser Bessel beams and subsequent thermal treatment. Microstructures composed of nano-sized crystals were observed in the exposed regions. The concentration of nano-crystals depended on the writing power, but the size of the nano-crystals was found to be quasi power-independent. Low writing power led to sparse nano-crystals, while optimized writing power achieved dense nano-crystals distribution and high refractive index change. Effects of the gratings thickness, writing laser power and thermal treatment on the diffraction efficiency were investigated. A maximum diffraction efficiency of 94.73% was achieved (at 532 nm testing wavelength) with 1 mm grating thickness at period of 5 μm.

Local functionalization of CMOS-compatible Si3N4 Mach-Zehnder interferometers with printable functional polymers

A CMOS-compatible integrated four-channel silicon nitride (Si3N4) waveguide based Mach-Zehnder interferometric (MZI) sensor platform was realized with the goal to take maximum advantage of the inherent self-referencing property of MZI-sensors with both sensing and reference waveguides exposed to the sample liquid. To study the self-referencing characteristics of the MZI-sensor system, we compared two sensor configurations—an asymmetric MZI configuration, where the reference waveguide was covered by a cladding, and a symmetric MZI configuration, where the reference waveguide was also in contact with the measurement liquid.To enable biomolecular measurements, we developed an inkjet printing procedure for functional polymers (biotin-modified polyethyleneimine (PEI-B) macromolecules), which allows for local functionalization of sensing waveguides in a cost effective mass-fabrication compatible manner. We could show that for high refractive index changes of the measurement liquid (Δn=0.007) the background signal can be suppressed by a factor of ∼42 for the symmetric MZI configuration compared to the asymmetric MZI configuration. For the characterization of the functional polymer printing procedure we compared locally functionalized asymmetric and symmetric MZIs with non-locally functionalized asymmetric MZIs, which were prepared with silanization/biotinylation based approach.These locally functionalized MZIs where used for the detection of single stranded DNA (150bp) with concentrations ranging from 2.5 to 100nM. The limits of detection were 1.9nM and 1.6nM for the non-locally and locally functionalized asymmetric MZI-sensors, respectively. The locally functionalized symmetric MZI-sensors had a four times better LOD of 0.4nM. These results proof the superior performance of symmetric biosensor arrays with locally deposited functional polymers by means of inkjet printing.

Femtosecond laser-induced stress-free ultra-densification inside porous glass

Unusually high densification ⩽26% was obtained without lateral residual stresses within the laser beam waist inside porous glass during its multi-shot femtosecond laser irradiation, which may induce in the glass the related high refractive index change ~0.1. Corresponding laser irradiation regimes, resulting in such ultra-densification, decompaction and voids inside the glass, were revealed as a function of laser pulse energy and scanning rate, and were discussed in terms of thermal and hydrodynamic processes in the silica network.

High slope efficiency and high refractive index change in direct-written Yb-doped waveguide lasers with depressed claddings

We report the first Yb:ZBLAN and Yb:IOG10 waveguide lasers fabricated by the fs-laser direct-writing technique. Pulses from a Titanium-Sapphire laser oscillator with 5.1 MHz repetition rate were utilized to generate negative refractive index modifications in both glasses. Multiple modifications were aligned in a depressed cladding geometry to create a waveguide. For Yb:ZBLAN we demonstrate high laser slope efficiency of 84% with a maximum output power of 170 mW. By using Yb:IOG10 a laser performance of 25% slope efficiency and 72 mW output power was achieved and we measured a remarkably high refractive index change exceeding Δn = 2.3 × 10(-2).

Crystallization and absorption properties of novel photo-thermal refractive glasses with the addition of B2O3

A new boron-containing photo-thermal-refractive (boron-PTR) glass was prepared by two-step melt-quenching techniques at relative lower glass melting temperature. The influence of Ag+ doping concentration, UV exposure dosage and thermal development parameters on the boron-PTR glass' spectroscopic absorption and crystallization property were investigated and compared with that of the known boron-free PTR glass explored by L. B. Glebov et al. The introduction of B2O3, to some extent, can increase the strength of the silicate bonding thus preventing the liquid–liquid phase separation in this new PTR system when compared to the boron-free PTR system. The developed boron-PTR glass with the processing of 15s-UV exposure and appropriate thermal development (480°C/3h, 500°C/2h) exhibits good photo-induced crystallization performance, high VIS-near IR transmittance (670 to 2600nm) and moderate photo-induced refractive index change (1.1×10−3), which at present status enables this new PTR glass basic ability to record holographic elements.

Carrier-induced refractive index change and optical absorption in wurtzite InN and GaN: Full-band approach

Based on the full band electronic structure calculations, first we consider the effect of n-type doping on the optical absorption and the refractive index in wurtzite InN and GaN. We identify quite different dielectric response in either case; while InN shows a significant shift in the absorption edge due to n-type doping, this is masked for GaN due to efficient cancellation of the Burstein-Moss effect by the band gap renormalization. For high doping levels the intraband absorption becomes significant in InN. Furthermore, we observe that the free-carrier plasma contribution to refractive index change becomes more important than both band filling and the band gap renormalization for electron densities above 10$^{19}$~cm$^{-3}$ in GaN, and 10$^{20}$~cm$^{-3}$ in InN. As a result of the two different characteristics mentioned above, the overall change in the refractive index due to n-type doping is much higher in InN compared to GaN, which in the former exceeds 4\% for a doping of 10$^{19}$~cm$^{-3}$ at 1.55~$\mu$m wavelength. Finally, we consider intrinsic InN under strong photoexcitation which introduces equal density of electron and holes thermalized to their respective band edges. The change in the refractive index at 1.55~$\mu$m is observed to be similar to the n-doped case up to a carrier density of 10$^{20}$~cm$^{-3}$. However, in the photoexcited case this is now accompanied by a strong absorption in this wavelength region due to $\Gamma^v_5 \to \Gamma^v_6$ intravalence band transition. Our findings suggest that the alloy composition of In$_x$Ga$_{1-x}$N can be optimized in the indium-rich region so as to benefit from high carrier-induced refractive index change while operating in the transparency region to minimize the losses. These can have direct implications for InN-containing optical phase modulators and lasers.

Confocal reflectance and two-photon microscopy studies of a songbird skull for preparation of transcranial imaging

We present experiments and analyses of confocal reflectance and two-photon microscopy studies of zebra finch skull samples. The thin and hollow structure of these birds' skulls is quite translucent, which can allow in vivo transcranial two-photon imaging for brain activation monitoring. However, the skull structure is also quite complex, with high refractive index changes on a macroscopic scale. These studies aim at exploring the geometrical and scattering properties of these skull samples with the use of several confocal microscopy contrasts. Moreover, the study of the axial reflectance exponential decay is used to estimate the scattering coefficients of the bone. Finally, two-photon imaging experiments of a fluorescent object located beneath the skull are carried out. It reveals that two-photon fluorescence can be collected through the skull with a strong signal. It also reveals that the spatial resolution loss is quite high and cannot be fully explained by the bulk scattering properties of the bone, but also by the presence of the high refractive index inhomogeneity of this pneumatic skull structure. Even if the optical properties of the skull are different during in vivo experiments, these preliminary studies are aimed at preparing and optimizing transcranial brain activation monitoring experiments on songbirds.

P‐113: Temporal Behavior of Nanostructure Formation in Photopolymer on Back Light Layer of TFT‐LED

AbstractThe temporal behavior of photopolymer nanostructures in a very thin layer of the back light guide substrate in TFT‐LED system has been studied. From our investigation, the optical bonding process with the composition of the bonding glue can easily produce the photopolymer nanostructures with high sensitivity and high refractive index change within a few seconds. This technique can be applied to fabricate photopolymer nanostructures on back light layer to achieve high luminous efficiency of TFT‐LED device.

Realization of Directional-Coupler-Based Optical Waveguide Devices by Ultraviolet-Imprinting Technique Using Sol-Gel Derived Inorganic GeO$_{2}$ : SiO$_{2}$ Films

An efficient fabrication scheme of directional coupler (DC) optical waveguide devices is demonstrated using a direct ultraviolet imprinting (DUI) technique in sol-gel derived inorganic 0.2GeO2:0.8SiO2 slab waveguide. The slab waveguide has a ~3-mum-thick photosensitive inorganic 0.2GeO2:0.8SiO2 film which is prepared by the sol-gel spin-coating technique on (10 mum) SiO2-Si substrate. This inorganic thick film of slab waveguide has the ability to induce a high refractive index change of ~5times10-3 under ultraviolet irradiations, which is suitable for the fabrication of waveguiding channel of photonic devices by a DUI technique. DC geometries were theoretically evaluated by the beam propagation method for various optical power splitting ratios and experimental performances of DCs 3-dB light splitter and complete power exchange from one waveguide to another were demonstrated and found to be in good agreement with the simulation.

Photoinduced Birefringence and Broadband All-Optical Photonic Switch in a Bacteriorhodopsin/Polymer Composite Film

Photoinduced birefringence with large optical nonlinearity in a bacteriorhodopsin/polymer composite film is observed. A high refractive index change of 8.5 × 10−5 photoinduced by 476 nm pumping beam is reached at the low intensity of 6.5 mW/cm2. Based on it, a broadband all-optical photonic switch is realized with an optical controlling switch system. Because of controlling beam's selectivity in switching, the transporting beams of different wavelengths with different intensities and shapes can be modulated by adjusting the wavelength and intensity of the controlling beam.