Laser-induced Refractive Index Changes Research Articles

Femtosecond laser-induced refractive index change and phase-type Fresnel zone plate in fluorotellurite glass

We report a phase-type Fresnel zone plate (FZP) fabricated by femtosecond (fs) laser-induced refractive index change (RIC), which was embedded in a newly developed fluorotellurite glass. The effect of laser direct-writing parameters on the morphology of the laser-induced structures was studied and negative RIC was observed. A maximum RIC of 4.42×10-3 was achieved at laser inscription energy of 0.13 µJ. Additionally, structure change from TeO4 units to TeO3- units triggered by fs laser irradiation is verified by Raman spectra measurements for the first time, to the best of our knowledge, and demonstrated to be a vital factor for the negative RIC. On this basis, we firstly fabricated a two-level phase-type FZP designed to work at 1340 nm inside such fluorotellurite glass. The phase-type FZP exhibits strong IR beam focusing and excellent imaging capabilities with a resolution of 128 line pairs per millimeter (lp/mm). This study reveals the facts that such novel fluorotellurite glass has good processibility with fs laser direct writing and made it a potential alternative material for micro-optics in NIR applications.

Blue-LIRIC in the rabbit cornea: efficacy, tissue effects, and repetition rate scaling.

Laser-induced refractive index change (LIRIC) is being developed as a non-invasive way to alter optical properties of transparent, ophthalmic materials including corneas ex vivo and in vivo. This study examined the optical and biological effects of blue-LIRIC (wavelengths 400-405 nm) of ex-vivo rabbit corneas. Following LIRIC treatment at low and high repetition rates (8.3 MHz and 80 MHz, respectively), we interferometrically measured optical phase change, obtained transmission electron microscopy (TEM) micrographs, and stained histological sections with collagen hybridizing peptides (CHP) to assess the structural and organizational changes caused by LIRIC at different repetition rates. Finally, we performed power and scan speed scaling experiments at three different repetition rates (1 MHz, 8.3 MHz, and 80 MHz) to study their impact on LIRIC efficacy. Histologic co-localization of CHP and LIRIC-generated green autofluorescence signals suggested that collagen denaturation had occurred in the laser-irradiated region. TEM imaging showed different ultrastructural modifications for low and high repetition rate writing, with discrete homogenization of collagen fibrils at 80 MHz, as opposed to contiguous homogenization at 8.3 MHz. Overall, this study confirmed that LIRIC efficacy can be dramatically increased, while still avoiding tissue ablation, by lowering the repetition rate from 80 MHz to 8.3 MHz. Modeling suggests that this is due to a higher, single-pulse, energy density deposition at given laser powers during 8.3 MHz LIRIC.

Advances in Material Nanosensitization: Refractive Property Changes as the Main Parameter to Indicate Organic Material Physical-Chemical Feature Improvements.

In the current paper, the results of the sensitization process’ influence on the refractive organic materials’ features are shown. The correlation between the refractive properties and the intermolecular charge transfer effect of doped organic thin films are shown via estimation of the laser-induced change in the refractive index. The refractive parameter is shown for a model organics matrix based on a polyimide doped with fullerenes, carbon nanotubes, reduced graphene oxides, etc. A second harmonic of the Nd-laser was used to record the holographic gratings in the Raman–Nath diffraction conditions at different spatial frequencies. The laser-induced refractive index change was considered to be an indicator in order to estimate the basic organic materials’ physical–chemical characteristics. Additional data are presented for the liquid crystal cells doped with nanoparticles. The correlation between the content of the nanoobjects in the organics’ bodies and the contact angle at the thin film surfaces is shown. Some propose to use this effect for general optoelectronics, for the optical limiting process, and for display application.

Analytical optimization of the laser induced refractive index change (LIRIC) process: maximizing LIRIC without reaching the damage threshold

Abstract A method to determine the optimum laser parameters for maximizing laser induced refractive index change (LIRIC) while avoiding exceeding the damage threshold for different materials with high water content (in particular, polymers such as hydrogels or the human cornea) is proposed. The model is based upon two previous independent models for LIRIC and for laser induced optical breakdown (LIOB) threshold combined in a simple manner. This work provides qualitative and quantitative estimates for the parameters leading to a maximum LIRIC effect below the threshold of LIOB.

Modification of surface morphology of hydrogels due to subsurface femtosecond laser micromachining.

In this paper, we studied the effects of subsurface femtosecond laser micromachining on surface morphology in hydrogels. Depending on material properties and writing conditions, we found surface bumps when materials were hydrated, and trenches when they were dehydrated, which can be attributed to the localized change in water concentration. Such wavy surfaces by laser-induced refractive index change are not desirable in clinical contact lenses. Therefore, the minimization of surface bumps is necessary to ensure the user eye wearing comfort. In addition, we examined the optical effects of the surface features using interferometry and the surface morphology using profilometry. Finally, we proposed a simplified mechanical model based on localized swelling.

Temporal evolution of the biological response to laser-induced refractive index change (LIRIC) in rabbit corneas

Laser-induced refractive index change (LIRIC) is a new, non-incisional, non-ablative, femtosecond photo-modification technique being developed for vision correction in humans. Prior, exvivo studies showed intra-tissue refractive index change to induce minimal cell death, restricted to the laser focal zone in the corneal stroma, and with no observable damage to the epithelium or endothelium. Here, we used live rabbits to ascertain longer-term consequences of LIRIC in vivo. Specifically, we assessed cell death, fibrosis, corneal nerve distribution, endothelial cell density, and corneal structure for up to 3 months after LIRIC. A +2.5 D gradient-index LIRIC Fresnel lens was inscribed inside 20 applanated corneas of Dutch Belted rabbits, over a circular region of the mid-stroma measuring 4.5 mm in diameter. Twelve additional rabbit eyes were used as applanation-only controls to differentiate the effects of laser treatment and suction applanation on biological and structural parameters. In vivo optical measurements were performed pre-operatively, then immediately, 2, 4, and 12 weeks after the procedure, to measure endothelial cell density and changes in corneal structure. Groups of four rabbits were sacrificed at 4 hours, 2, 4, and 12 weeks after LIRIC for histological determinations; the TUNEL assay was used to evaluate cell death, H&E staining was used to assess inflammatory infiltration, and immunostaining for α-smooth muscle actin (α-SMA) and βIII tubulin (Tuj-1) was performed to assess myofibroblast differentiation and corneal nerve distribution, respectively. Consistent with prior ex vivo data, only minimal cell death was observed in the laser focal zone, with TUNEL-positive cells restricted to the stromal region of refractive index change 4 h after LIRIC. No TUNEL-positive cells were evident anywhere in the cornea 2, 4, or 12 weeks after LIRIC. Applanation-only corneas were completely TUNEL-negative. Neither LIRIC-treated nor applanation-only eyes exhibited α-SMA-positive staining or altered corneal nerve distributions at any of the time points examined. In vivo confocal imaging revealed normal endothelial cell densities in all eyes (whether LIRIC-treated or applanation-only) at all time points. Optical coherence tomography showed suction applanation to cause a temporary decrease in central corneal thickness, which returned to normal within 4 h. Corneas into which LIRIC Fresnel lenses were written while applanated did not undergo major structural or shape changes beyond the temporary thinning already described for suction applanation. The present findings suggest that LIRIC patterns, which generated a clinically-relevant refractive correction in the mid-stromal region of live rabbit corneas, induced little-to-no disruption to corneal structure and biology for 3 months after the procedure. This affirms the relative safety of LIRIC and predicts that compared to traditional laser vision correction surgeries, common post-operative complications such as dry eye, haze, or patient discomfort may be entirely avoided.

Future perspectives of the femtosecond laser in anterior segment surgery

Around 12years after its introduction laser cataract surgery (LCS) has enabled remarkable progress, such as extremely precise capsulotomy and lens fragmentation with reduced or no ultrasound energy at all (zero phako); however, another innovation push is necessary to bring this technology to its full potential. Among the many benefits that have not yet been fully explored are primary posterior laser capsulotomy (PPLC) and the use of the femtosecond laser in pediatric cataract patients. In the present collective of patients in whom a PPLC was carried out at the end of the operation, a clearly reduced prevalence of secondary cataract could be observed, whereas the short additional intervention was not associated with notable complications. The LCS in infants is best carried out as an off-label procedure, which has been rendered more precise by the Bochum formula. In the near future, postoperative laser fine tuning will enable the target refraction to be achieved in most patients. Laser-induced refractive index change (LIRIC) will enable very exact refraction alterations on an already implanted intraocular lens (IOL), on acontact lens and on atreatment native cornea. Anew version of the Scheimpflug camera will be used just prior to surgery to analyze the morphology and kernel density of the lens opacity and to carry out a real-time grading of lens hardness. Based on this categorization, where the system classifies the results into five different density grades, the surgeon is automatically provided with an individualized fragmentation pattern defined preoperatively by the operator. Afurther innovation of the same system involves microcorneal incisions for fine tuning of residual refractive deficits.

Biological Lenses as a Photomask for Writing Laser Spots into Ferroelectric Crystals.

Red blood cells on the surface of a lithium niobate crystal can be used as optical lenses for direct writing of laser-induced refractive index changes. The writing process by such a photomask made of biological lenses is due to the photorefractive effect. Wavefront analysis by a digital holographic microscope is performed for deep and accurate evaluation of local refractive index changes. Different focusing properties can be imprinted on the crystal depending on which type of RBC is employed, discocytes or spherical-like RBCs. The possibility to fix into a solid material the optical fingerprint of the RBCs will have an impact on both diagnostics and cell\material interfacing.

Femtosecond laser micromachining in ophthalmic hydrogels: spectroscopic study of materials effects

Femtosecond laser micromachining relies on tightly focused, ultrashort laser pulses to locally modify material properties through nonlinear absorption, and is finding applications in the field of vision correction. Here we study the material effects of femtosecond laser micromachining in hydrogels to understand the mechanisms of laser-induced refractive index (RI) changes. Single layer dense line patterns were inscribed successively into the middle of hydrogels using a 405 nm femtosecond laser. A maximum phase change of ∼1.2 waves could be obtained at 543 nm with only 60 mW beam intensity at the focal volume. The phase change profile, fitted to a photochemical scaling model from earlier study, indicated that the micromachining process was mainly dominated by two-photon absorption. A confocal micro-Raman system was custom-designed to quantify the structural changes in written regions, especially the local water content. Change in the local water content exhibited three distinctive stages as a function of beam intensity. Below the optical breakdown threshold, a significant increase of local water content within the written layer was observed, while all Raman signatures remained the same. We posited that the negative RI changes were likely due to the generation of free radicals, followed by water permeation in the modified volume. This increase of local water content, likely presented as free water, was further confirmed by thermogravimetric analysis. Fourier-transform infrared spectroscopy was also used to gain insights into the chemical changes in the depolymerization stage.

Femtosecond laser inscription of nonlinear photonic circuits in Gallium Lanthanum Sulphide glass

We report on femtosecond laser writing of single mode optical waveguides in chalcogenide Gallium Lanthanum Sulphide (GLS) glass. A multiscan fabrication process was employed to create waveguides with symmetric single mode guidance and low insertion losses for the first time at 800 nm wavelength, compatible with Ti:Sapphire ultrafast lasers and nonlinear photonics applications. μRaman and x-ray microanalysis were used to elucidate the origin of the laser-induced refractive index change in GLS. We found that the laser irradiation produced a gentle modification of the GLS network, altering the Ga–S and La–S bonds to induce an increase in the refractive index. Nonlinear refractive index measurements of the waveguides were performed by finding the optical switching parameters of a directional coupler, demonstrating that the nonlinear properties were preserved, evidencing that GLS is a promising platform for laser-written integrated nonlinear photonics.

X-ray preconditioning for enhancing refractive index contrast in femtosecond laser photoinscription of embedded waveguides in pure silica

We investigate femtosecond pulse laser-induced refractive index changes (∆n) in bulk fused silica (Corning 7980) subject to prior hard (40 keV) X-ray irradiation with accumulated doses up to 6 MGy. The preconditioning before photoinscription improves strongly the light transport of the laser-written embedded waveguides in a range of fluences and accumulation doses. Results show that the ultrafast laser-induced ∆n is higher in samples pre-irradiated with X-rays at similar laser photon dose. The effect lies in the kick-off generation of defects and precursors by energetic X-ray photons, enhancing a subsequent laser-induced densification process. The photoinscription efficiency increases up to a saturation point and the pre-treatment can be considered as an effective option for high contrast low loss index modifications and waveguiding structures in glasses.

Pulse energy dependence of refractive index change in lithium niobium silicate glass during femtosecond laser direct writing.

Femtosecond laser-induced refractive index changes in lithium niobium silicate glass were explored at high repetition rate (300 fs, 500 kHz) by polarized light microscopy, full-wave retardation plate, quantitative birefringence microscopy, and digital holographic microscopy. We found three regimes on energy increase. The first one corresponds to isotropic negative refractive index change (for pulse energy ranging 0.4-0.8 μJ/pulse, 0.6 NA, 5μm/s, 650μm focusing depth in the glass). The second one (0.8-1.2 μJ/pulse) corresponds to birefringence with well-defined slow axis orientation. The third one (above 1.2 μJ/pulse) is related to birefringence direction fluctuation. Interestingly, these regimes are consistent with crystallization ones. In addition, an asymmetric orientational writing effect has been detected on birefringence. These topics extend the possibility of controlling refractive index change in multi-component glasses.

Multilevel phase-type diffractive lens embedded in sapphire.

Herein, we report a kinoform phase-type lens (KPL), which is fabricated by femtosecond (fs)-laser-induced refractive index change inside sapphire crystal. By fabricating volume phase gratings in sapphire and measuring the energy ratio of the grating's first and second diffraction orders, the refractive index change in sapphire induced by fs-laser modification was obtained. Then a four-level KPL was designed and fabricated inside sapphire following the experimentally established scaling of the refractive index change and fs-laser power. Importantly, the KPL has unique UV focusing and imaging capability as well as a stable optical performance in different refractive index environments. The KPL embedded in sapphire has the same optical performance after a high-temperature (1050°C) annealing for 30min. The KPLs in sapphire have great potential to increase light extraction efficiency in GaN blue-UV light-emitting diodes and can be used in micro-optical sensor applications in chemically harsh and high-temperature environments.

Femtosecond-laser written gradient-index Fresnel lenses for vision correction

Our group reported the creation of gradient-index (GRIN) microlenses (150 um diameter) in hydrogels through the use of femtosecond laser induced refractive index change (LIRIC) (1). In this study, we report the optical quality of large area stitched Fresnel lenses (6mm diameter, 543nm design wavelength). Five spherical lenses (Range: −3.0 to +1.5 D) and one cylindrical lens (−1.5 D) were created. Each lens was produced in a single layer within a plano hydrogel using a 400nm frequency doubled Ti:sapphire laser and a galvo-scanning system (field of view: 245×88 um). The resulting change in wavefront aberrations was assessed with a Shack-Hartmann wavefront sensor (5.8mm pupil, 543nm wavelength). The measured writing error was 0.11 ± 0.07 D of the desired target (defocus or astigmatism). Induced HORMS was 0.14 ± 0.03 um. Future work includes visual performance testing. This work shows the potential of femtosecond writing for the creation of accurate phase structures exhibiting high visual quality with LIRIC.

Optical response in a laser-driven quantum pseudodot system

We investigate theoretically the intense laser-induced optical absorption coefficients and refractive index changes in a two-dimensional quantum pseudodot system under an uniform magnetic field. The effects of non-resonant, monochromatic intense laser field upon the system are treated within the framework of high-frequency Floquet approach in which the system is supposed to be governed by a laser-dressed potential. Linear and nonlinear absorption coefficients and relative changes in the refractive index are obtained by means of the compact-density matrix approach and iterative method. The results of numerical calculations for a typical GaAs quantum dot reveal that the optical response depends strongly on the magnitude of external magnetic field and characteristic parameters of the confinement potential. Moreover, we have demonstrated that the intense laser field modifies the confinement and thereby causes remarkable changes in the linear and nonlinear optical properties of the system.

Fabrication of embedded microball lens in PMMA with high repetition rate femtosecond fiber laser.

Embedded microball lenses with superior optical properties function as convex microball lens (VMBL) and concave microball lens (CMBL) were fabricated inside a PMMA substrate with a high repetition rate femtosecond fiber laser. The VMBL was created by femtosecond laser-induced refractive index change, while the CMBL was fabricated due to the heat accumulation effect of the successive laser pulses irradiation at a high repetition rate. The processing window for both types of the lenses was studied and optimized, and the optical properties were also tested by imaging a remote object with an inverted microscope. In order to obtain the microball lenses with adjustable focal lengths and suppressed optical aberration, a shape control method was thus proposed and examined with experiments and ZEMAX® simulations. Applying the optimized fabrication conditions, two types of the embedded microball lenses arrays were fabricated and then tested with imaging experiments. This technology allows the direct fabrication of microlens inside transparent bulk polymer material which has great application potential in multi-function integrated microfluidic devices.

Instantaneous three-dimensional imaging using supercontinuum and ultrafast optical Kerr gate of tellurite glass

An instantaneous three-dimensional imaging technique using a chirped supercontinuum and an ultrafast optical Kerr gate, in which a sapphire plate and a TeO2-ZnO-Na2O oxide glass were used to generate the chirped supercontinuum and the ultrafast optical Kerr gate, respectively, is demonstrated. This technique is applicable to ultrafast shape measurement, such as shape imaging of moving objects, or imaging of laser-induced refractive index changes in transparent media.

Investigation and control of ultrafast laser-induced isotropic and anisotropic nanoscale-modulated index patterns in bulk fused silica

Ultrafast laser-induced refractive index changes in a-SiO2 consist, depending on the irradiation conditions, of either positive variations, voids, or regular nanoscale patterns, each of these underlying specific structural transformations. These allow for obtaining a large palette of optical functions ranging from low loss guiding to anisotropic scattering. While briefly reviewing the excitation mechanisms, we spectroscopically interrogate local electronic and structural transformations of the glass in the isotropic index zones and in the regular self-organized nanostructures, indicating bond breaking and matrix oxygen deficiency. A spatial defect segregation marks the material transformation in the different photoinscription regimes. We equally propose a method of real time control of nanogratings formation under the action of ultrashort laser pulse with variable envelopes. Application as polarizing optical devices is discussed.

Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal

In this work, we studied dielectric properties and laser-induced refractive index changes originating from photothermal effects of liquid crystal material doped with Disperse Red 9 (DR9) dye. Dye concentration is arranged to be between percentages changing from 0.2 wt.% to 1 wt.% in E63 nematic liquid crystal. Nonlinear optical properties such as diffraction efficiency ( η) and refractive index modulation (Δ n) were investigated by diffraction grating measurements. It was found the diffraction efficiency of pure E63 nematic liquid crystal is 1%. As the doping amount of DR9 dye in nematic LC is increased, diffraction efficiency took higher values and the maximum diffraction efficiency of 10% was gained with E63 doped with 0.8 wt.%DR9 dye. Moreover, dielectric permittivity and dielectric anisotropy values of the samples were investigated in the frequency range of 100 Hz–10 MHz by using dielectric spectroscopy technique. It was observed that dielectric constant values of the liquid crystal material are strongly affected by doping with dye.

Time-resolved imaging of laser-induced refractive index changes in transparent media

We describe a method to visualize ultrafast laser-induced refractive index changes in transparent materials with a 310 fs impulse response and a submicrometer spatial resolution. The temporal profile of the laser excitation sequence can be arbitrarily set on the subpicosecond and picosecond time scales with a pulse shaping unit, allowing for complex laser excitation. Time-resolved phase contrast microscopy reveals the real part of the refractive index change and complementary time-resolved optical transmission microscopy measurements give access to the imaginary part of the refractive index in the irradiated region. A femtosecond laser source probes the complex refractive index changes from the excitation time up to 1 ns, and a frequency-doubled Nd:YAG laser emitting 1 ns duration pulses is employed for collecting data at longer time delays, when the evolution is slow. We demonstrate the performance of our setup by studying the energy relaxation in a fused silica sample after irradiation with a double pulse sequence. The excitation pulses are separated by 3 ps. Our results show two dimensional refractive index maps at different times from 200 fs to 100 μs after the laser excitation. On the subpicosecond time scale we have access to the spatial characteristics of the energy deposition into the sample. At longer times (800 ps), time-resolved phase contrast microscopy shows the appearance of a strong compression wave emitted from the excited region. On the microsecond time scale, we observe energy transfer outside the irradiated region.