Positive 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

First-order type A waveguide Bragg gratings with under-diffraction-limited periodicities and sub-millimeter length for NIR applications

Since its demonstration in 2017, laser-induced Type A modifications in silver-containing glasses have been exploited for fabricating waveguides, couplers, volume Bragg Gratings and, more recently, waveguides Bragg Gratings. Laser-induced silver-supported structures exhibit sub-diffraction characteristics being always associated with a positive refractive index change resulting from multi-photon energy deposition under multiple-pulse femtosecond laser irradiation. These properties have been exploited in this work for the fabrication of efficient first-order helicoidal Type A waveguides Bragg gratings in the near-infrared (NIR) range, with periodicities well below the diffraction limit and with typical length L = 500 μm, showing narrowband mirror behaviours. Moreover, this work demonstrates that such helicoidal Type A waveguides Bragg gratings allow for modulating the refractive index of the waveguide at half the periodicity of the helix. Such a peculiarity has been exploited to fabricate efficient first-order Type A waveguides Bragg gratings in the NIR range, as well as in the far red range of the visible spectrum, showing a remarkably strong coupling coefficient, up to one order of magnitude higher than what is reported in the literature.

Femtosecond laser activation of the photochemistry of bismuth and associated three-dimensional sub-micron fluorescence patterning

Localized sub-micrometer-scale visible and near-IR fluorescence structures have been achieved in three-dimension (3D) thanks to femtosecond laser-activated photochemistry of Bismuth in a Bismuth-doped phosphate glass. These structures exhibit high fluorescence contrast with good spatial resolution with spectral emission in the red and near-IR ranges. These fluorescence properties arise from femtosecond laser-induced multi-photon absorption and the creation of free electrons, which activates local redox reactions of Bi3+ that allow for the dose-dependent formation of low valence Bismuth ions such as Bi2+ and Bi+. These new species promote not only new selective fluorescence emission properties in the VIS and near-IR spectral range but also co-localized positive refractive index changes. The thermal treatment of 3D patterns evidences the possibility of Bismuth cluster formation with good thermal stability up to the glass temperature transition. These observations open the way for laser-inscribed photonic integrated circuits for potential laser amplification devices in the second telecommunication windows around 1.3 μm.

Direct observation and quantification of nanosecond laser induced amorphization inside silicon

The nature of structural changes of nanosecond laser modification inside silicon is investigated. Raman spectroscopy and transmission electron microscopy measurements of cross sections of the modified channels reveal highly localized crystal deformation. Raman spectroscopy measurements prove the existence of amorphous silicon inside nanosecond laser induced modifications, and the percentage of amorphous silicon is calculated based on the Raman spectrum. For the first time, the high-resolution transmission electron microscopy images directly show the appearance of amorphous silicon inside nanosecond laser induced modifications, which corroborates the indirect measurements from Raman spectroscopy. The laser modified channel consists of a small amount of amorphous silicon embedded in a disturbed crystal structure accompanied by strain. This finding may explain the origin of the positive refractive index change associated with the written channels that may serve as optical waveguides.

Fs laser written volume Raman–Nath grating for integrated spectrometer on smartphone

In this work we demonstrate the integration of a spectrometer directly into smartphone screen by femtosecond laser inscription of a weak Raman–Nath volume grating either into the Corning Gorilla glass screen layer or in the tempered aluminosilicate glass protector screen placed in front of the phone camera. Outside the thermal accumulation regime, a new writing regime yielding positive refractive index change was found for both glasses which is fluence dependent. The upper-bound threshold for this thermal-accumulation-less writing regime was found for both glasses and were, respectively at a repetition rate less than 150 kHz and 101 kHz for fluence of 8.7 × 106 J/m2 and 1.4 × 107 J/m2. A weak volume Raman–Nath grating of dimension 0.5 by 3 mm and 3 μm pitch was placed in front of a Samsung Galaxy S21 FE cellphone to record the spectrum using the 2nd diffraction order. This spectrometer covers the visible band from 401 to 700 nm with a 0.4 nm/pixel detector resolution and 3 nm optical resolution. It was used to determine the concentration detection limit of Rhodamine 6G in water which was found to be 0.5 mg/L. This proof of concept paves the way to in-the-field absorption spectroscopy for quick information gathering.

Femtosecond laser direct writing of Nd:YAG waveguide with Type I modification: Positive refractive index change in track

Abstract Nd:YAG crystal is one of the major gain media used in high-power solid-state laser systems. In this work, the single-line waveguides with Type Ⅰ modification have been fabricated in Nd:YAG crystal by femtosecond laser writing. The waveguides show good guiding properties in both TE and TM polarizations, which is different from the well-developed Type II structures. The high-resolution confocal micro-photoluminescence and micro-Raman spectra images obtained by scanning the cross-section of the track region certify a positive refractive-index change induced in femtosecond laser irradiated region. The fluorescence properties remain 82% in the waveguide region, which shows the potential for active photonic devices.

Mid-infrared microfiber Bragg gratings.

We report mid-infrared (mid-IR) Bragg gratings fabricated on sub-wavelength-diameter chalcogenide glass (ChG) microfibers. ChG microfibers with diameters around 3 µm are tapered drawn from As2S3 glass fibers, and the mid-IR microfiber Bragg gratings (mFBGs) are inscribed on microfibers using interference patterns of near bandgap light at a 532 nm wavelength. At a wavelength of about 4.5 µm, the mFBG has an extinction ratio of 15 dB and a positive photo-induced refractive index change of 2×10-2. The dependence of the grating formation on accumulated influence of exposure power density and time is investigated. The mid-IR mFBGs demonstrated here may be used as building blocks for micro-photonic circuits or devices in the mid-IR spectral range.

Record-high positive refractive index change in bismuth germanate crystals through ultrafast laser enhanced polarizability

Unlike other crystals, the counter intuitive response of bismuth germanate crystals ({text {Bi}}_4{text {Ge}}_3{text {O}}_{12}, BGO) to form localized high refractive index contrast waveguides upon ultrafast laser irradiation is explained for the first time. While the waveguide formation is a result of a stoichiometric reorganization of germanium and oxygen, the origin of positive index stems from the formation of highly polarisable non-bridging oxygen complexes. Micro-reflectivity measurements revealed a record-high positive refractive index contrast of 4.25times 10^{-2}. The currently accepted view that index changes >1times 10^{-2} could be brought about only by engaging heavy metal elements is strongly challenged by this report. The combination of a nearly perfect step-index profile, record-high refractive index contrast, easily tunable waveguide dimensions, and the intrinsic high optical non-linearity, electro-optic activity and optical transparency up to 5.5,upmu {text {m}} of BGO make these waveguides a highly attractive platform for compact 3D integrated optics.

Boson band mapping: revealing ultrafast laser induced structural modifications in chalcogenide glass.

The formation of femtosecond laser direct-written waveguides in gallium lanthanum sulfide (GLS) chalcogenide glass with a peak index contrast of Δnmax=0.023 and an average positive refractive index change of Δnwaveguide=0.0049 is explained for the first time, to the best of our knowledge. Evidence of structural change and ion migration is presented using Raman spectroscopy and electron probe microanalysis (EPMA), respectively. Raman microscopy reveals a frequency shift and a change in full-width at half maximum variation of the symmetric vibration of the GaS4 tetrahedra. The boson band is successfully used to identify and understand the material densification profile in a high refractive index glass waveguide. EPMA provides evidence of ion migration due to sulfur, where the observation of an anion (S2-) migration causing material modification is reported for the first time. These results will enable optimization of future mid-infrared and nonlinear integrated optical devices in GLS glass based on femtosecond laser written waveguides.

Slit Beam Shaping Technique for Femtosecond Laser Inscription of Enhanced Plane-by-Plane FBGs

In this work, the development of plane-by-plane (Plb-Pl) fiber Bragg gratings (FBGs) by using a single pulse is reported. A slit oriented along the longitudinal axis of the fiber is employed for this. The purpose is to shape the beam so that the focal volume is spatially wider in the transverse direction of the fiber. In this way, it is possible to make 2-D modifications of the refractive index whose width and height depends on the slit width and the pulse energy, respectively. Through an analytical mathematical modelling, the relationship between the diameter of the Gaussian laser beam (D ) and the slit width (s = D,) is determined in order to obtain a circular cross-section. In the experimental results carried out, it can be observed that D /D, = 3 ratio cause positive and uniform refractive index changes, compared to the negative and inhomogeneous refractive index changes corresponding to point-by-point (PbP) FBGs. Likewise, Pl-b-Pl FBGs, presenting a broader modification of the core cross-section, show significantly lower losses in transmission (0.3 dB), as well as better reflectivity and FWHM. It is observed that these three parameters (losses, reflectivity and FWHM) have a monotonous tendency according to the slit width. Polarization-dependence is also evaluated.

Revisiting ultrafast laser inscribed waveguide formation in commercial alkali-free borosilicate glasses.

Alkali-free borosilicate glasses are one of the most used dielectric platforms for ultrafast laser inscribed integrated photonics. Femtosecond laser written waveguides in commercial Corning Eagle 2000, Corning Eagle XG and Schott AF32 glasses were analyzed. They were studied in depth to disclose the dynamics of waveguide formation. We believe that the findings presented in this paper will help bridge one of the major and important gaps in understanding the ultrafast light-matter interaction with alkali-free boroaluminosilicate glass. It was found that the waveguides are formed mainly due to structural and elemental reorganization upon laser inscription. Aluminum along with alkaline earth metals were found to be responsible for the densification and silicon being the exchanging element to form a rarefied zone. Strong affinity towards alkaline earth elements to form the densified zone for waveguides written with high feed rate (>200 mm/min) were identified and explained. Finally we propose a plausible solution to form positive refractive index change waveguides in different glasses based on current and previous reports.

Inscribing diffraction gratings in bulk silicon with nanosecond laser pulses.

Diffraction gratings are transversally inscribed in the bulk of monolithic crystalline silicon with infrared nanosecond laser pulses. Nanoscale material analyses of the modifications composing the gratings show that they rely on laser-induced stress associated with a positive refractive index change as confirmed with phase-shift interferometry. Characterizations of the optical properties of the gratings, including the diffraction angles and the efficiency of the different orders, are carried out. The refractive index change obtained from these measurements is in good agreement with the phase-shift measurements. Finally, we show that the grating diffraction efficiency depends strongly on the laser writing speed.

Influence of Thermal Annealing on Ultrafast Laser‐Induced Local Densification in Bulk Sulfur‐Based Chalcogenide Glasses

The response to focused ultrafast laser pulses, of three bulk sulfur‐based chalcogenide glasses, gallium lanthanum sulfide (GLS) and two germanium arsenic sulfide with different stoichiometry (Ge20As20S60 and Ge15As15S70), is studied as a function of a thermal treatment made by several annealing cycles under the glass transition temperature. It is demonstrated that the annealing procedure has a considerable influence on the ultrafast laser‐induced local densification process of the quenched glasses. Using post‐mortem phase contrast microscopy and single‐mode light guiding measurements on photo‐inscribed waveguides, the influence of the thermal history on the laser‐induced positive refractive index changes (local densification) is determined. In particular it is shown that, in the case of the Ge20As20S60 glass, the threshold of ultrafast laser‐induced densification in terms of laser energetic dose is considerably lowered after the first annealing cycle. In the light of this result, the activation of a new relaxation channel during the thermal treatment and its influence on the laser‐induced densification process of Ge‐As‐S glasses is discussed. Potential of material thermal pre‐treatment for developing efficient photo‐inscription techniques in the domain of integrated photonics is also discussed.

Bragg Grating Inscription With Low Pulse Energy in Doped Microstructured Polymer Optical Fibers

We demonstrate that fiber Bragg gratings (FBGs) can be written in a doped polymer optical fiber (POF) in a low ultraviolet (UV) pulse energy regime (60Jpulse) using a 248-nm krypton fluoride excimer laser system. The total energy density per inscription necessary to obtain Bragg gratings is between 493.6 and 3825mJcm 2, depending on the number of pulses and the pulse energy. The impact of the pulse energy on the growth of the Bragg grating is investigated, and it is shown that the 248-nm light induces a positive refractive index change. This article demonstrates that the FBGs can be obtained in the POFs without high pulse energy (mJ level) at 248-nm wavelength, which reduces maintenance costs. Furthermore, we can consider it as a solution to increase the lifetime of the laser system without high energy still allowing fast and efficient production of the FBGs for sensing applications.

Ultra-fast polymer optical fibre Bragg grating inscription for medical devices

We report the extraordinary result of rapid fibre Bragg grating inscription in doped polymer optical fibres based on polymethyl methacrylate in only 7 ms, which is two orders of magnitude faster than the inscription times previously reported. This was achieved using a new dopant material, diphenyl disulphide, which was found to enable a fast, positive refractive index change using a low ultraviolet dose. These changes were investigated and found to arise from photodissociation of the diphenyl disulphide molecule and subsequent molecular reorganization. We demonstrate that gratings inscribed in these fibres can exhibit at least a 15 times higher sensitivity than silica glass fibre, despite their quick inscription times. As a demonstration of the sensitivity, we selected a highly stringent situation, namely, the monitoring of a human heartbeat and respiratory functions. These findings could permit the inscription of fibre Bragg gratings during the fibre drawing process for mass production, allowing cost-effective, single-use, in vivo sensors among other potential uses.

Femtosecond laser processing induced low loss waveguides in multicomponent glasses

We have investigated that femtosecond laser induced optical waveguides in AF32 and in Borofloat-33 glasses. Type-I positive and type-II negative refractive index change mechanisms were established in AF32 and in Borofloat-33 glass waveguides, respectively. Lowest propagation loss of 1.1 ± 0.31 dB/cm could be attained in AF32 glass, which has a typically higher index change than in Borofloat-33 glass. Resultant losses are directly correlated with densification and non-bridging oxygen hole centers, which can be authenticated by Raman and photoluminescence studies.

Femtosecond laser direct inscription of mid-IR transmitting waveguides in BGG glasses

A detailed investigation of the photo-inscription of waveguides in barium gallo-germanate (BGG, BaO, GeO2, Ga2O3) glass is presented. Upon irradiation of BGG glass samples of different contents of germanium dioxide with a femtosecond laser pulse train, positive refractive index changes are produced over a wide range of exposure conditions. Waveguides with a controllable diameter ranging from 4 to 35 µm and a maximum index change up to 10−2 were inscribed. A glass sample with custom molecular composition was purified to remove hydroxyl ions and reduce the strong absorption band near 3 µm. A careful tailoring of the writing conditions allowed for the inscription of low-loss waveguides supporting only two transverse modes at the wavelength of 2.78 µm. An upper bound for the propagation losses of 0.5 ± 0.1 dB/cm was determined, showing the great potential of the BGG glass family for the fabrication of core waveguides operating in the 2-4 µm spectral range. Our results actually open a pathway towards the integration of mid-IR photonic devices based on the BGG glass family.

Bromide photo-thermo-refractive glass for volume Bragg gratings and waveguide structure recording

In the present research, a new class of bromine-containing photo-thermo-refractive (PTR) glass in which the UV irradiation and subsequent heat treatment cause the precipitation of silver nanoparticles with a shell consisting of silver bromides is developed and studied. The growth of AgBr nanocrystals is shown to lead to a local positive refractive index change (refractive index of crystallized glassy area is higher than one of non-crystalline) in the UV-irradiated area against the unirradiated area up to 800 ppm. Based on this effect, samples of a volume Bragg grating and a waveguide structure have been recorded.

Near-infrared waveguide in gallium nitride single crystal produced by carbon ion implantation

We report on the fabrication of planar waveguides in gallium nitride by 5 MeV carbon ion implantation with different fluences at room temperature. The waveguides are characterized by prism coupling, Rutherford backscattering/channeling, and high-resolution X-ray diffraction analysis. A positive change in ordinary refractive index is confirmed in the waveguide region at a near-infrared waveband. The thermal stability of the ion-implanted GaN waveguides is investigated by annealing the samples at different temperatures.

Ultrafast laser-induced refractive index changes in Ge_15As_15S_70 chalcogenide glass

Understanding and controlling laser-induced refractive index modifications in bulk chalcogenide glasses is important for a range of photonics applications targeting the mid-infrared spectral region. We focus here on material engineering aspects and pulse spatio-temporal design characteristics that are able to induce and maintain positive refractive index changes in laser-irradiated Sulfur-based chalcogenide glass, mandatory for 3D photonic design. Specifically we study the photoinscription process of a Ge-doped Sulfur-based chalcogenide glass, Ge15As15S70, irradiated by focused ultrafast near-infrared laser pulses where Ge doping plays a determinant role in generating high-contrast positive index changes. By means of aposteriori and real-time in situ observations we show that positive refractive index changes (type I) are the result of a restructuring of the glass matrix and a photo-induced contraction process initiated by two-photon electronic excitation leading to bond softening, molecular mobility, structural changes and rearrangements. Oppositely, negative refractive index changes (type II) could be associated with two different processes: photo-expansion at higher intensities and hydrodynamic evolution initiated by plasma generation and laser heating, with thermomechanical relaxation and stress unload. Alongside the role of Ge in setting various degrees of the matrix connectivity, the structural arrangement developed under different thermal history schemes for glass preparation is equally important as it defines to which extent further structural flexibility is possible. Thus we indicate the role of glass matrix metastability in generated high-contrast refractive index changes and we show that a higher degree of relaxation is an impediment for contrasted positive index changes, while these are developing in unrelaxed glasses, where several degrees of structural flexibility exist. Alongside dynamic time-resolved imaging experiments probing the development and relaxation of excitation, we also show, via static Raman analysis of the modified regions, that significant structural changes are induced by laser irradiation and we discuss the potential processes involved.