Large Refractive Index Modulation Research Articles

Colossal Optical Anisotropy from Atomic-Scale Modulations.

Materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light-matter coupling, such as hyperbolic phonon polaritons. Layered 2Dmaterials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure-Sr9/8 TiS3 -is transparent and positive-uniaxial, with extraordinary index ne = 4.5 and ordinary index no = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3 , results in the formation of TiS6 trigonal-prismatic units that break the chains of face-sharing TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary index ne and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation.

Femtosecond laser point-by-point Bragg grating inscription in BDK-doped step-index PMMA optical fibers.

In this paper, the inscription of 2-mm-long fiber Bragg gratings (FBGs) on benzyl dimethyl ketal (BDK)-doped poly(methyl methacrylate) (PMMA) optical fibers by means of a femtosecond laser and a point-by-point FBG inscription technique is reported. The highest reflectivity of approximately 99% is obtained with a pulse energy of 68.5 nJ, showing a large refractive index modulation amplitude of 7.2 × 10-4. Afterwards, grating stabilities at room and higher temperatures of up to 80°C are investigated.

Femtosecond laser inscribed helical sapphire fiber Bragg gratings.

This Letter reports a novel helical sapphire fiber Bragg grating (HSFBG) in a single crystal sapphire fiber with diameter of 60 µm fabricated by a 515 nm femtosecond laser. Due to the large refractive index modulation region and high structural symmetry of the HSFBGs, high-reflectivity and high-quality spectra can be prepared and additionally have good bending resistance. The spectral properties of HSFBGs with different helical diameters are studied. When the helical diameter is 30 µm, the reflectivity of HSFBG is 40%, the full width at half-maximum is 1.56 nm, and the signal-to-noise ratio is 16 dB. For the HSFBG bending test, the minimum bending radius is 5 mm, which can still maintain relatively good spectral quality. In addition, the HSFBG array with different periods has been successfully cascaded in a sapphire fiber. The experimental results of the HSFBG high-temperature test show that this HSFBG can work reliably at 1600°C, and the temperature sensitivity in the high-temperature range can reach 35.55 pm/°C. This HSFBG can be used in high-temperature and harsh environments, such as metal smelting and aeroengine structural health monitoring.

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.

Red-sensitive organic nanoparticle-polymer composite materials for volume holographic gratings with large refractive index modulation amplitudes

We demonstrate volume holographic recording at a wavelength of 640 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. We employ a new photosensitizer-initiator system consisting of cyanine dye, triazine compound and borate salt for efficient radical generation in the red. We investigate the electron transfer and radical generation processes of the system by measuring fluorescence quenching and photopolymerization dynamics to find the optimum composition of the system for volume holographic recording. We show that recorded volume gratings of 0.5-µm spacing possess the saturated peak-to-mean refractive index modulation amplitudes as large as 3×10−2 at a readout wavelength of 640 nm. Our results show the usefulness of photopolymerizable HBP-dispersed NPCs for volume holographic recording materials for various photonic applications including security and color holograms, and volume Bragg grating devices in head-mounted displays.

Fabrication of nanodiamond-dispersed composite holographic gratings and their light and slow-neutron diffraction properties

We demonstrate the use of nanodiamond in constructing holographic nanoparticle-polymer composite transmission gratings with large saturated refractive index modulation amplitudes at both optical and slow-neutron wavelengths, resulting in efficient control of light and slow-neutron beams. Nanodiamond possesses a high refractive index at optical wavelengths and large coherent and small incoherent scattering cross sections with low absorption at slow-neutron wavelengths. We describe the synthesis of nanodiamond, the preparation of photopolymerizable nanodiamond-polymer composite films, the construction of transmission gratings in nanodiamond-polymer composite films and light optical diffraction experiments. Results of slow-neutron diffraction from such gratings are also presented.

Design of InGaAsP phase modulator with asymmetrically coupled quantum wells for efficient polarization modulation

Monolithically integrated high-speed polarization modulators are finding emerging applications in the fields of optical communication, quantum key distribution, and radio-frequency photonics. Here, we design an efficient polarization modulator with InGaAsP asymmetrically coupled quantum wells (ACQWs) on InP. By using two quantum wells separated by a thin barrier layer, the behaviors of the heavy holes and the light holes can be controlled independently under an external bias. As a result, we can induce an anomalous quantum-confined Stark effect, having a strong polarization dependence. With an optimized ACQW design, we numerically demonstrate a large polarization-dependent refractive index modulation of 7.1 × 10−3 at an applied field of 1.8 V μm−1. Using a 1 mm long device, we estimate that 2π rotation of the Stokes vector can be achieved with negligible absorption at a driving voltage of only 0.75 V.

High response photochromic films based on D-A diarylethenes and their application in holography.

A set of photochromic dithienylethenes bearing amino and nitro groups are synthesised and embedded at high concentrations in a polymer matrix (Cellulose Acetate Butyrate, CAB) to produce films showing a large reversible modulation of the complex refractive index in the Vis-NIR spectral range, thanks to an interesting combination of remarkable response at the molecular level and very high load capability in the chosen matrix. The photochromic derivatives are characterized in solution and in CAB films by means of electronic and vibrational spectroscopy, complemented by DFT calculations. Both the real and imaginary part of the refractive index are determined by spectroscopic ellipsometry. The modulation of the refractive index in the near infrared is in the range 0.02–0.04. These are very large values for such kinds of systems and they are due to a favourable combination of very large solubility of the derivatives in CAB and a high polarisability change. As for the change in transparency in the visible, contrast values larger than 103 are easily achieved. Based on such films, holograms are written and reconstructed with a very high fidelity and efficiency.

Enhancement of modal birefringence in solid core Bragg fibers with high-index-contrast claddings

Abstract We present two types of highly birefringent solid core Bragg fibers (SCBFs) with high-index-contrast claddings by asymmetrically introducing low index parts (LIPs) into the solid core. The characteristics of their modal birefringence and confinement loss are numerically investigated by using a full-vector finite element method. The simulation results reveal that the modal birefringences in such highly birefringent SCBFs essentially depend not only on the difference in electric field boundary condition between the orthogonal-polarized modes, but also on the photonic bandgap effect. Both types of highly birefringent SCBFs have higher modal birefringence than those with low-index-contrast claddings, owing to the large refractive index (RI) modulation ranges of the LIPs. The modal birefringence of the fundamental mode at the targeted 1550 nm is strongly dependent on the size, RI and location of LIPs. Notably, the linear relationship between the modal birefringence and the RI of LIPs in the y-direction in a highly birefringent SCBF with four LIPs included can serve as a new sensing mechanism for the RI measurement of liquid analytes. We believe that the results would be beneficial to the design and application of highly birefringent SCBFs with high-index-contrast claddings.

Modulation spectra of volume holograms recorded in additively colored fluorite crystals

Additively colored fluorite crystals are of considerable interest as a holographic recording medium. Due to very high stability as well as potentially large refractive index modulation, its possible to use holograms recorded in these crystals as high quality diffractive elements and photonic-crystalline structures. Therefore, it is important to be able to correctly and accurately analyze such structures. Spectral dependencies of absorption coefficient and refractive index profiles are the most precious characteristics in this regard. Unfortunately, they are also the most elusive ones. In this study the required parameters were obtained by a method based on the analysis of selectivity hypercontours (combined spectral and angular dependencies of diffraction efficiency) using the Kogelnik’s coupled wave theory in wide optical spectral range. Hologram profile was reconstructed from first three spatial harmonics. The results were crosschecked with monochromatic analysis to prove the validity of proposed technique.

Graphene-assisted ultra-compact polarization splitter and rotator with an extended bandwidth

The high refraction-index contrast between silicon and the surrounding cladding makes silicon-on-insulator devices highly polarization-dependent. However, it is greatly desirable for many applications to address the issue of polarization dependence in silicon photonics. Here, a novel ultra-compact polarization splitter and rotator (PSR), constructed with an asymmetrical directional coupler consisting of a rib silicon waveguide and a graphene-embedded rib silicon waveguide (GERSW), on a silicon-on-insulator platform is proposed and investigated. By taking advantage of the large modulation of the effective refractive index of the TE mode for the GERSW by tuning the chemical potential of graphene, the phase matching condition can be well satisfied over a wide spectral band. The presented result demonstrates that for a 7-layer-graphene-embedded PSR with a coupling length of 11.1 μm, a high TM-to-TE conversion efficiency (>−0.5 dB) can be achieved over a broad bandwidth from 1516 to 1602 nm.

Photopolymeric films with highly tunable refractive index modulation for high precision diffractive optics

The design and manufacturing of high efficiency and reliable volume phase holographic optical elements require photosensitive material where it is possible to finely control the refractive index modulation. Bayfol HX photopolymers show this feature together with other interesting advantages, in particular the self-developing and the large refractive index modulation. In this paper, the design of Volume Phase Holographic Gratings (VPHGs) is reported underlying the relationship of gratings’ performances with the refractive index modulation. The trend of this property with the change of the laser power density and the ratio of the two writing beams is shown. Based on these results, VPHGs for astronomical instrumentation have been designed and manufactured.

Analysis of phase patterns in photochromic polyurethanes by a holographic approach

Photochromic polyurethanes based on diarylethene units show a large reversible modulation of refractive index in the Vis-NIR spectral region. The change of refractive index in the material is easily induced by visible laser illumination, without any optical or chemical post-process. In this paper, patterns at the micron scale range have been written by a suitable direct laser writing machine and characterized at 1550 nm by means of a digital holographic approach. The refractive index profile has been retrieved, its dependence on the film thickness and writing speed was shown. The writing process has also been modelled by means of a kinetic model, showing theoretically the dependence of the pattern width and profile with the writing condition. It is demonstrated how the photochromic films are suitable for developing a reconfigurable platform for complex phase patterns working in the NIR.

Millisecond photorefractivity with novel dicyanomethylenedihydrofuran-containing polymers

New multifunctional copolymers containing carbazole units and high loads of dicyanomethylenedihydrofuran (DCDHF) were synthesized and used to prepare blends for photorefractive (PR) purposes. The materials response, which is strongly dependent on the glass-transition temperature (Tg), was thoroughly analyzed by holography, conductivity and ellipsometry measurements in order to both determine the limiting factors and optimize the performance. Materials that have a Tg around room temperature show strongly hindered chromophore orientation, which is avoided by lowering the Tg down to 6 °C, without compromising the PR effect or the material stability. A further DCDHF-containing homopolymer without carbazole was synthesized and characterized, showing an inferior PR response, which is attributed to a beneficial role for charge generation and transport of the attached carbazole in the copolymers. The new blends strongly improve the structural properties of previous DCDHF-based materials, allowing application of fields well above 100 V μm−1 and preventing beam fanning. Outstanding PR performance was achieved, with fast buildup and erasure times of a few tens of milliseconds (even at low recording intensities), large refractive index modulation (over 10−2) and two-beam coupling gain (above 350 cm−1). Such performance is among the best reported for PR materials based on multifunctional and nonlinear polymers and comparable to standard PR composites.

Large refractive index modulation tilted holographic planar structured grating based on dichromated gelatin

A new type grating, tilted holographic planar structured dichromated gelatin (DCG) grating with large refractive index modulation, is presented in this paper. The grating is inscribed by superposing two coherent laser beams from the front and the thin side surfaces, respectively. The relationship between the tilted angle and the three types of coupling modes is analyzed when coupling with fiber. Besides, the couple mode theory and experimental investigation are applied on major characteristics analysis of the radiation mode DCG grating of tilted angle 45°. Due to the tilted planar structured grating has large refractive index modulation, it not only offers convenience for connecting (or integrating) with other fiber or waveguide-based devices, but also provide some special characteristics such as low wavelength selectivity (which is required for some applications such as multiple spectrum biomedical imaging). Therefore, this unique grating device has a wide application in optical fields such as for the waveguide-based, parallel, multiple spectrum imaging system.

Slow-light effect in dual-periodic photonic lattice

We present analytical and numerical studies of a photonic lattice with short- and long-range harmonic modulations of the refractive index. Such structures can be prepared experimentally with holographic photolithography. In the spectral region of the photonic bandgap of the underlying single-periodic crystal, we observe a series of bands with anomalously small dispersion. The related slow-light effect is attributed to the long-range modulation of the photonic lattice that leads to formation of an array of evanescently coupled high-Q cavities. The band structure of the lattice is studied with several techniques: (i) transfer matrix approach; (ii) an analysis of resonant coupling in the process of band folding; (iii) effective-medium approach based on coupled-mode theory; and (iv) the Bogolyubov-Mitropolsky approach. The latter method, commonly used in the studies of nonlinear oscillators, was employed to investigate the behavior of eigenfunction envelopes and the band structure of the dual-periodic photonic lattice. We show that reliable results can be obtained even in the case of large refractive index modulation.

Characterization of Blue Sensitive Holographic Polymer Dispersed Liquid Crystal for Microholographic Data Storage

We present the results of an extended investigation performed on holographic reflection gratings recorded in blue sensitive polymer dispersed liquid crystals in view of a possible application in optical data-storage. The analysis has been performed by means of real-time spectroscopy during the polymerization process. The analysis of transmission spectra during the recording process showed a fast photopolymerization, resulting in the rapid formation of a reflection peak. Gratings characterized by high spatial frequency (> 7000 lines/mm), high diffraction efficiency (up to 50%) and large refractive index modulation (Δn ∼ 0.008) have been obtained. Unfortunately, the monomer conversion into a polymer network is generally characterized by a strong packing, with a consequent reduction in volume (polymerization shrinkage). This phenomenon induces a significant blue-shift of the reflected wavelength. With the aim of better understanding and possibly reducing the shrinkage, a new high-sensitive experimental technique has been developed to measure shrinkage effects in multi-component polymeric mixtures.

Analysis of Optical Properties with Photopolymers for Holographic Application

Optical transparency and high diffraction efficiency are two essential factors for high performance of the photopolymer. Optical transparency mainly depends on the miscibility between polymer binder and photopolymerized polymer, while diffraction efficiency depends on the refractive index modulation between polymer binder and photopolymerized polymer. For most of organic materials, the large refractive index difference between two polymers accompanies large structural difference that leads to the poor miscibility and thus poor optical quality via light scattering. Therefore, it is difficult to design a high-performance photopolymer satisfying both requirements. In this work, first, we prepared a new phase-stable photopolymer (PMMA) with large refractive index modulation and investigated the optical properties. Our photopolymer is based on modified poly (methyl methacrylate) as a polymer binder, acryl amide as a photopolymerizable monomer, triethanolamine as initiator, and yellow eosin as a photosensitizer at 532 nm. Diffraction efficiency over 85% and optical transmittance over 90% were obtained for the photopolymer. Second, Organic-inorganic nanocomposite films were prepared by dispersing an aromatic methacrylic monomer and a photo- initiator in organic-inorganic hybrid sol-gel matrices. The film properties could be controlled by optimizing the content of an organically modified silica precursor (TSPEG) in the sol-gel matrices. The photopolymer film modified with the organic chain (TSPEG) showed high diffraction efficiency (> 90%) under an optimized condition. High diffraction efficiency could be ascribed to the fast diffusion and efficient polymerization of monomers under interference light to generate refractive index modulation. The TSPEG modified photopolymer film could be successfully used for holographic memory.

Synthesis of colorless imide hybrid nanocomposites using amine functionalized oligosiloxane nano-building clusters

Colorless imide hybrid nanocomposites are fabricated using new amine functionalized oligosiloxane nano-building clusters (AONC) and the cross-linking agent 5-(2,5-dioxotetrahydrofuryl)-3-methyl-cyclohexene-1,2-dicarboxylic anhydride (DODCA) or 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) in order to obtain thermally stable and optically transparent materials. AONC are cross-linked by short diimide bonds which were formed by reaction of AONC with alicyclic dianhydrides and the fabricated nanocomposites have 3-dimensional network structures. The prepared nanocomposites show good thermal stability with 5% weight loss temperature around 430 °C and excellent optical properties such as colorlessness (optical transmission of above 90% in the visible region and ultraviolet cut-off around 310 nm), large modulation of refractive index and low birefringence (about 0.002). Colorless imide hybrid nanocomposites prepared from AONC and alicyclic dianhydride (DODCA and CBDA) are potential materials for optical and opto-electronic applications.

Diffraction behaviors of new photopolymers containing the dendritic molecule

Photopolymers are attractive materials for holographic 3-D data storage because of their high photosensitivity and large refractive index modulation. We synthesized the six-armed dendrimer for fabricating the new photopolymer. It was prepared using the initiating mixture of hexaarylbiimidazole (HABI), mercapto-benzoxazole (MOBZ), and 2,6-bis(4-diethylaminobenzylidene)cyclopentanone (DEAW), which is sensitive to 514 nm wavelength. The holographic gratings were fabricated successfully in these photopolymer samples by conventional optical interference method. We investigated the effect of dendrimer, either as a binder or as a plasticizer in the cellulose acetate butyrate (CAB), on the diffraction behavior. The addition of only 1 wt% of dendrimer-I into the CAB significantly increased the diffraction efficiency. The sample doped with dendrimer showed around 80-83% of the diffraction efficiency.