Maximum Refractive Index Change Research Articles

Transverse Inscription of Silicon Waveguides by Picosecond Laser Pulses

AbstractIn this work, picosecond laser inscription of segmented waveguides in crystalline silicon based on a deterministic single‐pulse modification process is demonstrated. Pulses of 43 ps duration at 1.55 wavelength are used to transversely inscribe periodic structures with a pulse‐to‐pulse pitch of ≈2 . Infrared shadowgraphy images and Raman spectroscopy measurements indicate that the modifications exhibit a spherical shape. Characterization of waveguide performance at 1.55 for various pulse energies and periods is carried out. Direct comparison with numerical simulations confirms the presence of graded index waveguides, encompassing a micrometer core size and a maximum refractive index change of . This short‐pulse inscription approach can pave the way for 3D integrated photonic devices in the bulk of silicon.

Photosensitization agents for fs laser writing in PDMS

This study aims at identifying compounds incorporated into Polydimethylsiloxane (PDMS) which produce large refractive index change under fs laser exposition, potentially leading to optimal writing of waveguides or photonic devices in such a soft host. Germanium derivative, titania and zirconite derivatives, benzophenone (Bp), irgacure-184/500/1173 and 2959 are investigated. We show a mapping of the RI index change relative to the writing speed (1 to 40 mm/s), the repetition rate (606 to 101 kHz) and the number of passes (1 to 8) from which we establish quantitative parameters to allow the comparison between samples. We show that the organic materials, especially irgacure-184 and benzophenone yield a significantly higher maximum refractive index change in the order of 10−2. We also show that the strongest photosensitivity is achieved with a mixture of organic/organo-metallic material of Bp + Ge. We report a synergetic effect on photosensitivity of this novel mixture.

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.

Spatial tailoring of the refractive index in infrared glass-ceramic films enabled by direct laser writing

The development of infrared gradient refractive index (GRIN) components relies on the ability to modify the refractive index and dispersion properties of suitable host materials with a high spatial selectivity and a sufficient magnitude of change. We present a novel multi-step approach to induce local refractive index changes in chalcogenide optical materials. Films with thicknesses between 1 and 40 µm fabricated from multi-component GeSe2-As2Se3-PbSe (GAP–Se) glass-ceramic materials were irradiated with continuous-wave and nanosecond-pulsed laser light, and post-processed with heat-treatments. A maximum local refractive index change of Δn = 0.088 across a broad spectral range in the infrared was realized. Spatial control of the refractive index variation was achieved through thermally-induced crystallization of a Pb-rich crystal phase. The magnitude of the index change scaled with the laser power and the exposure dose while the material maintained the required optical quality. The material performance validated in this study for thick films (20–40 µm films) reconfirms our ability to extend results from thin GAP-Se films towards novel optical designs.

Femtosecond laser direct writing of diffraction grating and its refractive index change in chalcogenide As2Se3 film.

Chalcogenide glasses possess novel optical property in infrared range, which make them ideal candidates for photonic devices by laser direct writing. Precisely control of the refractive index manipulation in the material is the key to achieve high quality optical devices. In this work, diffraction gratings in chalcogenide As2Se3 thin films were fabricated with femtosecond laser direct writing and its refractive index change was carefully studied. Grating diffraction efficiency was measured from visible to near-infrared light by using multiple single-wavelength lasers and a supercontinuum laser. Clear diffraction patterns and high diffraction efficiency indicated the good optical quality of the prepared gratings. Results show that the grating with period of 5 µm inscribed under pulse energy of 30 nJ demonstrated a 1st-order diffraction efficiency of 30% at 808 nm testing wavelength, due to the changes of refractive index and absorption. The relationship of the change of refractive index and absorption coefficient of film under different laser irradiation intensity is carefully studied. The maximum refractive index change was estimated to be 0.087 at 808 nm.

In-plane electrical bias tunable optical properties of 1T-TaS2 [Invited

Electrically tunable optical properties have been demonstrated in many solid-state materials such as semiconductors, transparent conductive oxides and graphene. However, their tunability is limited in the visible range due to the requirement of extremely large charge build-up or high capacitive fields. Here, we propose strongly correlated materials for circumventing such limitations. 1T-TaS2, a strongly correlated material exhibiting charge density order at room temperature, allows tuning of its optical properties with an in-plane electrical bias. The electrical bias causes the charge density waves to slide and thereby alter their coherence and condensation. As a result, the optical conductivity or dielectric function of this layered material changes with an in-plane bias. Here, we report measured anisotropic dielectric functions of mechanically exfoliated thin films of 1T-TaS2 and their electrical tunability. We observe a maximum refractive index change on the order of 0.1 in the visible range with DC and AC in-plane biases.

Reconfigurable elastomeric graded-index optical elements controlled by light

In many optical applications, there is an increasing need for dynamically tunable optical elements that are able to shape the wavefront of light ‘on demand’. In this work, an elastomeric easy-to-fabricate optical element whose transmission functions can be reversibly phase configured by visible light is demonstrated. The light responsivity of proper azopolymers incorporated within an elastomeric matrix is exploited to induce a light-controlled graded refractive index (GRIN) distribution within the bulk compound. The induced refractive index distribution is continuous and conformal to the intensity profile of the illumination at moderate power. A 100 mW doubled-frequency Nd:YAG Gaussian beam focused to a 650 μm waist is shown to induce a maximum relative refractive index change of ~0.4% in the elastomeric matrix, with an approximately parabolic profile. The restoring characteristics of the elastomeric matrix enable full recovery of the initial homogeneous refractive index distribution within a few seconds when the incident laser is switched off. As an exemplary application, the configurable GRIN element is used in a microscope-based imaging system for light control of the effective focal length.

Design concepts for diffusive holographic photopolymers

ABSTRACTDiffusive photopolymers are an area of intense recent materials development, spurred by interest in holographic data storage, display holography, and custom diffractive optical elements, among other applications. This review examines a range of photopolymer formulations of academic and commercial interest, and places their design strategies in context via quantitative analysis of the recording fidelity, maximum refractive index change and the degree to which they achieve this limit. Finally, this analysis is extended to estimate the scope of achievable future performance improvements. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1021–1035.

Deformable Surface Accommodating Intraocular Lens: Second Generation Prototype Design Methodology and Testing.

Present an analysis methodology for developing and evaluating accommodating intraocular lenses incorporating a deformable interface. The next generation design of extruded gel interface intraocular lens is presented. A prototype based upon similar previously in vivo proven design was tested with measurements of actuation force, lens power, interface contour, optical transfer function, and visual Strehl ratio. Prototype verified mathematical models were used to optimize optical and mechanical design parameters to maximize the image quality and minimize the required force to accommodate. The prototype lens produced adequate image quality with the available physiologic accommodating force. The iterative mathematical modeling based upon the prototype yielded maximized optical and mechanical performance through maximum allowable gel thickness to extrusion diameter ratio, maximum feasible refractive index change at the interface, and minimum gel material properties in Poisson's ratio and Young's modulus. The design prototype performed well. It operated within the physiologic constraints of the human eye including the force available for full accommodative amplitude using the eye's natural focusing feedback, while maintaining image quality in the space available. The parameters that optimized optical and mechanical performance were delineated as those, which minimize both asphericity and actuation pressure. The design parameters outlined herein can be used as a template to maximize the performance of a deformable interface intraocular lens. The article combines a multidisciplinary basic science approach from biomechanics, optical science, and ophthalmology to optimize an intraocular lens design suitable for preliminary animal trials.

Saturation of multiplexed volume Bragg grating recording.

Recording of volume Bragg gratings (VBGs) in photo-thermo-refractive glass is limited to a maximum refractive index change about 0.002. We discuss various saturation curves and their influence on the amplitudes of recorded gratings. Special attention is given to multiplexed VBGs aimed at recording several gratings in the same volume. The best shape of the saturation curve for production of the strongest gratings is the threshold-type curve. Two-photon absorption as a mechanism of recording also allows increasing the strength of multiplexed VBGs.

Trimming of high-Q-factor silicon ring resonators by electron beam bleaching

We demonstrate a novel position-resolved resonance trimming strategy for silicon ring resonators. Ring resonators are covered with a chromophore-doped guest host polymer cladding. Illumination of the polymer cladding with high-energy electrons causes a bleaching of the chromophore molecules. Bleaching of the chromophores induces a reduction of the polymer refractive index, which can be used to trim the resonance frequency of the ring resonators. A maximum refractive index change of 0.06 and a TM polarization resonance shift of 16.4 nm have been measured. A Q factor of 20,000 before bleaching remains unaltered after the electron beam exposure process.

Study of the photosensitivity of GeS binary glasses to 800nm femtosecond pulses

We present the first study of the photosensitivity of GeS binary glasses in response to irradiation to femtosecond pulses at 800 nm. A maximum positive refractive index change of 3.5x10(-3) is demonstrated with the possibility to control the waveguide diameter from ~8 to ~50 µm by adjusting the input pulse energy. It is also demonstrated that under different exposure conditions, a maximum negative index change of -7.5x10(-3) can be reached. The present results clearly illustrate the potential of this family of glasses for the fabrication of mid-infrared waveguides.

Photosensitivity and stress changes of Ge-free Bi-Al doped silica optical fibers under ArF excimer laser irradiation

The photosensitivity of germanium free Bi-Al-doped silica fibers with different bismuth concentrations was investigated using ArF excimer laser radiation at 193 nm and fiber grating formation. For the fiber with the highest bismuth concentration maximum refractive index changes of 2.2 × 10(-3) and 2.0 × 10(-4) were obtained for hydrogen loaded and unloaded fibers, respectively. Irradiation induced tensile stress changes were observed in the fiber core of H(2)-loaded and unloaded fibers. The results indicate a contribution of compaction to the total refractive index change in both cases.

High-accuracy self-correction of refractive index of air using two-color interferometry of optical frequency combs

Long-path pulse-to-pulse interferometers of two-color frequency combs are developed using fundamental and second harmonics of a mode-locked fiber laser. Interferometric phase difference between two-color frequency combs was precisely measured by stabilizing the fundamental fringe phase by controlling the repetition frequency of the comb, and a stability of 10(-10) for 1000 s was achieved in the measurement of an optical path length difference between two wavelengths. In long-term measurements performed for 10 h, results of phase variation of interferometric measurements were highly consistent with the fluctuations in the calculated difference of refractive indices of air at two wavelengths with an accuracy of 10(-10). The difference between the measured optical distances corresponding to two wavelengths and the optical distance corresponding to the fundamental wavelength were used in the two-color method; high-accuracy self-correction of the fluctuation of refractive index of air was performed with an uncertainty of 5 × 10(-8) for 10-h measurements when the maximum refractive index change was on the order of 10(-6).

Second harmonic and Raman imaging of He^+ implanted KTiOPO_4 waveguides

We report on the micro second harmonic (µ-SH) and micro Raman (µ-Raman) images of ion implanted channel and planar waveguides in KTiOPO4 (KTP) crystals. The μ-SH images reveal that the nonlinear properties in the waveguides have not been deteriorated during the implantation process. This is consistent with the μ-Raman images that evidences that lattice distortions are minimal at waveguide's volume. Both the structural and nonlinear properties of the KTP lattice have been only modified at the end of ions' trajectory, which is in good agreement with the positions with maximum refractive index changes.

Optical diffraction gratings embedded in BK-7 glass by low-density plasma formation using femtosecond laser

The optical embedded diffraction gratings with the internal refractive index modification in BK-7 glass plates were demonstrated using low-density plasma formation excited by a high-intensity femtosecond (130 fs) Ti: sapphire laser ( λ p=790 nm). The refractive index modifications with diameters ranging from 400 nm to 4 μm were photoinduced after plasma formation occurred upon irradiation with peak intensities of more than 1×10 13 W/cm 2. The graded refractive index profile was fabricated to be a symmetric around the center of the point at which low-density plasma occurred. The maximum refractive index change (Δ n) was estimated to be 1.5×10 −2. Several optical embedded gratings in BK-7 glass plate were demonstrated with refractive index modification induced by the scanning of low-density plasma formation.

Investigations on the Bragg grating recording in all-silica, standard and microstructured optical fibers using 248~nm, 5~ps laser radiation

The fabrication of Bragg reflectors in hydrogenated, all-silica, fluorine cladding depressed and microstructured optical fibers using 248 nm, 5 ps laser radiation, is investigated here. Comparative Bragg grating recordings are performed in both optical fibers, for investigating effects related to the scattering induced by the capillary micro-structure, to the photosensitivity and index engineering yield. Further, finite difference time domain method is employed for simulating the scattering from the above capillary structure and the nominal intensity reaching the fiber core for side-illumination. The maximum modulated refractive index changes inscribed in the standard, step-index fiber were of the order of 8.3x10-5, while the maximum refractive index changes inscribed in one of the microstructured optical fibers was 32% lower and 5.7x10-5, for nominal pulse intensities of 20 GW/cm2 and modest accumulated energy densities.

Ultrahigh-channel-count phase-only sampled fiber Bragg grating covering the S, C, and L bands

A continuous phase-only sampling scheme enabling one to create an ultrahigh-channel-count fiber Bragg grating (FBG) with excellent channel uniformity and high in-band energy efficiency is proposed that is based on the simultaneous utilization of two phase-only sampling functions. With this method, a linearly chirped 135-channel FBG with a length of 10 cm, a dispersion of 1360 ps/nm, channel spacing of 0.8 nm, and grating strength of 10 dB is theoretically demonstrated, and the maximum refractive index change required is less than 10(-3).

Photorefractive response and optical damage of LiNbO3 optical waveguides produced by swift heavy ion irradiation

The photorefractive behaviour of a novel type of optical waveguides fabricated in LiNbO3 by swift heavy ion irradiation is investigated. First, the electro-optic coefficient r 33 of these guides that is crucial in the photorefractive effect is measured. Second, two complementary aspects of the photorefractive response are studied: (i) recording and light-induced and dark erasure of holographic gratings; (ii) optical beam degradation in single-beam configuration. The main photorefractive parameters, recording and erasing time constants, maximum refractive-index change and optical damage thresholds are determined.

Femtosecond laser embedded grating micromachining of flexible PDMS plates

We report on the femtosecond laser micromachining of photo-induced embedded diffraction grating in flexible Poly (Dimethly Siloxane) (PDMS) plates using a high-intensity femtosecond (130 fs) Ti: sapphire laser ( λ p = 800 nm). The refractive index modifications with diameters ranging from 2 μm to 5 μm were photo-induced after the irradiation with peak intensities of more than 1 × 10 11 W/cm 2. The graded refractive index profile was fabricated to be a symmetric around from the center of the point at which femtosecond laser was focused. The maximum refractive index change (Δ n) was estimated to be 2 × 10 −3. By the X– Y– Z scanning of sample, the embedded diffraction grating in PDMS plate was fabricated successfully using a femtosecond laser.