Positive Refractive Index Change Research Articles

Chloride photo-thermo-refractive glasses

The known photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass containing fluorine and bromine and doped with cerium, antimony, and silver. The UV irradiation and subsequent heat treatment of this glass result in the crystalline phase precipitation and negative refractive index change up to 1000 ppm in the UV-irradiated area. This effect is used for holographic recording the volume Bragg’s gratings. In the present research, we developed and studied the new class of chlorine-containing PTR glasses in which the UV irradiation and subsequent heat treatment cause the precipitation of silver nanoparticles with a shell consisting of mixed silver and sodium chlorides. The growth of the shell on the silver nanoparticle was shown to lead to a local positive refractive index change in the UV-irradiated area against the unirradiated one up to 1500 ppm.

Excitation and relaxation dynamics in ultrafast laser irradiated optical glasses

We discuss the dynamics of ultrashort pulsed laser excitation in bulk optical silica-based glasses (fused silica and borosilicate BK7) well-above the permanent modification threshold. We indicate subsequent structural and thermomechanical energy relaxation paths that translate into positive and negative refractive index changes, compression and rarefaction zones. If fast electronic decay occurs at low excitation levels in fused silica via self-trapping of excitons, for carrier densities in the vicinity of the critical value at the incident wavelength, persistent long-living absorptive states indicate the achievement of low viscosity matter states manifesting pressure relaxation, rarefaction, void opening and compaction in the neighboring domains. An intermediate ps-long excited carrier dynamics is observed for BK7 in the range corresponding to structural expansion and rarefaction. The amount of excitation and the strength of the subsequent hydrodynamic evolution is critically dependent on the pulse time envelope, indicative of potential optimization schemes.

飞秒激光在钛蓝宝石晶体中刻写双线型波导的研究

Ti: sapphire crystal is one of the most important materials as a laser medium, due to its broad absorption band and widely tunable output band, therefore it has wide application in integrated optics. By femtosecond laser with repetition rate of 1 kHz, central wavelength of 800 nm and pulse width of 120 fs, double line waveguide in Ti:sapphire crystal is transversely written. The impacts of the processing conditions, i.e., inscribing depth, writing speed and the track distance on the waveguide formation are systematically studied, and the waveguide with better guiding mode is achieved when laser pulse energy is 2 μJ, inscribing depth is 175 μm, writing speed is 90 μm/s and the distance between two tacks is 26 μm. And the double line waveguide shows polarization dependent optical guiding. Based on the near-field mode intensity, the refractive index profile of the optical waveguide is reconstructed, and the max positive refractive index change is about 1.9×10-4. In addition, by using the scattering technique, the propagation loss of the waveguide is 1.82 dB/cm. ©, 2015, Science Press. All right reserved.

Proton Beam Writing of Chalcogenide Glass: A New Approach for Fabrication of Channel Waveguides at Telecommunication O and C Bands

We report on proton beam writing of chalcogenide glass for the fabrication of channel waveguides. The focused proton beam at an energy of 1 MeV induces positive refractive index changes, forming channel waveguide structures in the irradiated region. The channel waveguides support both the TE and TM polarizations from the visible to near-infrared telecommunication O and C bands. Based on the maximum value of the refractive index contrast achieved by measuring the numerical aperture of the waveguide channels, we reconstruct the refractive index distribution and calculate the modal profiles of waveguides in the near-infrared wavelengths. The calculated modal profiles are in very good agreement with the experimental results. The minimum propagation loss is measured to be ~2.0 dB/cm at the wavelength of 1064 nm.

Large-mode-area infrared guiding in ultrafast laser written waveguides in sulfur-based chalcogenide glasses.

Current demands in astrophotonics impose advancing optical functions in infrared domains within embedded refractive index designs. We demonstrate concepts for large-mode-area guiding in ultrafast laser photowritten waveguides in bulk Sulfur-based chalcogenide glasses. If positive index contrasts are weak in As2S3, Ge doping increases the matrix rigidity and allows for high contrast (10(-3)) positive refractive index changes. Guiding with variable mode diameter and large-mode-area light transport is demonstrated up to 10 μm spectral domain using transverse slit-shaped and evanescently-coupled multicore traces.

Large increase in refractive index inside silica glass after the movement of voids caused by femtosecond laser pulses

When a transparent material is irradiated by focused femtosecond laser pulses, various types of structural modification can occur inside the material. Recently, it has been reported that asymmetric structures, which are composed of two regions with positive and negative refractive index changes (Δn), can be inscribed by irradiation of multiple femtosecond laser pulses at high (∼MHz) repetition rates. Interestingly, the amount of positive Δn in this type of modification is larger than those in conventional structural modifications by one order of magnitude. However, the mechanisms underlying such modifications are still unclear. In this paper, we describe that similar asymmetric structures can be inscribed by using femtosecond laser pulses at a low repetition rate. Then, we examine the formation and spectroscopic characteristics of the asymmetric structures. The observation of the processed areas reveals that the asymmetric structures are formed after the movement of voids. Furthermore, micro-Raman spectroscopy suggests a large increase of three-membered ring structures in the region with positive Δn, which may account for the large Δn.

Refractive index modification in fluoro-borate glasses containing WO3 induced by femtosecond laser

New glasses belonging to the NaF–Na2B4O7–WO3 system have been studied in order to inscribe optical waveguides within their volume through local refractive index modification after irradiation by ultrafast laser pulses. Appropriate addition of NaF to glasses belonging to the pseudo-binary vitreous system Na2B4O7–WO3 has allowed to obtain striae-free bulk glasses of large volume with high content of WO3 (35mol%). Their physical and structural properties have been investigated through UV–visible spectroscopy, differential scanning calorimetry, densitometry and Raman spectroscopy. The glass of composition (NaF)15–(Na2B4O7)50–(WO3)35 has been then exposed to 800nm femtosecond pulses of 70fs width at a repetition rate of 100kHz from a Ti-sapphire regenerative amplifier system. By varying the input pulse energy and scanning speed, homogeneous optical waveguides were inscribed and a positive refractive index change of the order of 5×10−3 has been measured.

Fabrication of planar waveguide in KNSBN crystal by swift heavy ion beam irradiation

We report on the fabrication of the planar waveguides in the KNSBN crystal by using 17MeV C5+ ions at a fluence of 2×1014ions/cm2. After implantation, near surface regions of the crystal, there has a positive extraordinary refractive index (ne) change and the light inside the waveguides can propagate in a non-leaky manner. The two-dimensional modal profiles of the planar waveguides, measured by using the end-coupling arrangement, are in good agreement with the reconstructed modal distributions. The propagation loss for C5+ irradiated waveguide is ∼0.8dB/cm at 633nm and ∼0.72dB/cm at 1064nm. The waveguide gives good confinement of waveguide modes, which exhibits acceptable guiding qualities for potential applications in integrated optics.

Buried waveguide in neodymium-doped phosphate glass obtained by femtosecond laser writing using a double line approach

We fabricate a buried channel waveguide in neodymium-doped phosphate glass using a double line approach by femtosecond laser writing. Raman spectra reveal an expansion of the glass network in the laser irradiated region. Given the stress-induced positive refractive index change, waveguiding between two separated tracks is demonstrated. The refractive index difference profile of the waveguide is reconstructed from the measured near-field mode. Propagation loss is measured by scattering technique. Microluminescence spectra reveal that the Nd3+ fluorescence property is not significantly affected by waveguide formation process, which indicates that the inscribed waveguide is a good candidate for active device.

Buried channel waveguides in KTiOPO4 nonlinear crystal fabricated by focused He+ beam writing

We report on the fabrication of KTiOPO4 channel waveguides by using a new technique, i.e., focused He ion beam writing. With direct inscription of 2MeV He+ beams of diameter of 0.5μm, a local modified region with positive refractive index changes is created at the end of the He+ trajectory, constructing buried channel waveguides supporting both TE and TM polarizations. The calculated modal profiles of the waveguides are in good agreement with the experimental near-field intensity distributions. The propagation loss has been determined to be as low as ∼0.7dB/cm, exhibiting good guiding properties for diverse photonic applications.

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.

Subwavelength Microstructures Fabrication by Self‐Organization Processes in Photopolymerizable Nanocomposite

This paper describes our research results on nanometers sizes subwavelength nanostructure fabrication by UV curing of special nanocomposite material with self‐organization and light self‐focusing effects. For this purpose, special UV curable nanocomposite material with a set of effects was developing: light self‐focusing in the photopolymer with positive refractive index change, self‐organization based on photo‐induced nanoparticles transportation, and oxygen‐based polymerization threshold. Both holographic and projection lithography writing methods application for microstructure making shows geometrical optical laws perturbation as result of nanocomposite self‐organization effects with formation of nanometers‐sized high‐aspect‐ratio structures. Obtained results will be useful for diffraction limit overcoming in projection lithography as well as for deep lithography technique.

Fabrication of magneto-optical microstructure by femtosecond laser pulses

We investigate femtosecond laser direct writing (FLDW) in the fabrication of magneto-optical (MO) microstructures. The experimental results show that FDLW can introduce positive refractive index change in the MO materials. With the increase of the writing intensity of femtosecond laser pulses, refractive index change increases, whereas Verdet constant of the damaged area decreases nonlinearly. With suitable writing intensity, we obtain a single-mode waveguide in which Verdet constant is 80% of the bulk MOg lass.

Proton beam writing of Nd:GGG crystals as new waveguide laser sources

Focused proton beam writing has been utilized to fabricate optical channel waveguides in Nd:GGG crystals. The 1 MeV proton beam irradiation creates a local modified region with positive refractive index changes at the end of the proton trajectory, in which the channel waveguide could confine the light field in a symmetric way. Room-temperature laser emission has been achieved at 1063.7 nm, with absorbed pump power of 61 mW (at 808 nm). The obtained slope efficiency of the Nd:GGG waveguide laser system is as high as 66%, which is, to our best knowledge, the highest value for integrated lasers from ion beam processed channel waveguide systems.

Direct femtosecond laser waveguide writing inside zinc phosphate glass

We report the relationship between the initial glass composition and the resulting microstructural changes after direct femtosecond laser waveguide writing with a 1 kHz repetition rate Ti:sapphire laser system. A zinc polyphosphate glass composition with an oxygen to phosphorus ratio of 3.25 has demonstrated positive refractive index changes induced inside the focal volume of a focusing microscope objective for laser pulse energies that can achieve intensities above the modification threshold. The permanent photo-induced changes can be used for direct fabrication of optical waveguides using single scan writing techniques. Changes to the localized glass network structure that produce positive changes in the refractive index of zinc phosphate glasses upon femtosecond laser irradiation have been studied using scanning confocal micro-Raman and fluorescence spectroscopy.

Refractive index dynamics of quantum dot based waveguide electroabsorbers

The refractive index dynamics of InAs/GaAs quantum dot based waveguide absorbers is studied using heterodyne pump-probe measurements. Absorption reduction due to the pump can be accompanied by either positive or negative refractive index changes depending on the wavelength used. This change in sign of the phase amplitude coupling can be understood by considering the atomlike nature of the quantum dot transitions involved.

Measurement of refractive index change of optical fiber core induced by femtosecond laser scanning

We report a new method to measure the refractive index change in optical fiber core induced by femtosecond (fs)laser exposure. An in-line Fabry-Perot interferometer, serving as the measurement platform, is constructed on a commercial single-mode optical fiber by one-step femtosecond (fs) laser fabrication. A positive refractive index change is observed and measured accurately as the laser pulse energy surpasses the ablation threshold.

Electrooptic effect of water in electric double layer at interface of GaN electrode

We determined the Pockels constant of water in the electric double layer (EDL) at the GaN electrode interface. For a positive (negative) bias, the difference transmittance showed a blue (red) shift in the interference fringes in the visible. This was caused by a negative (positive) refractive index change both in the EDL of water and in the space charge layer (SCL) of the GaN. The latter was associated with the blue shift in the absorption edge in the UV due to the band population effect in the GaN. The voltage drops took place within the interfacial layer at a ratio of about 3: 1 for the SCL vs EDL at the modulation frequency of f = 20 Hz, estimated from the frequency dependence of the impedance. The Pockels constant of water in the EDL was determined to be r 13 = 0:63 × 100 pm/V for the GaN electrodes. This is three times smaller than that at the surface of the indium tin oxide (ITO) electrode.

镱锡共掺光纤预制棒的光敏特性和发光特性研究

Sn/Yb codoped silica optical fiber preform is prepared by the modified chemical vapor deposition (MCVD) followed by the solution-doping method. Ultraviolet (UV) optical absorption, photoluminescence (PL) spectra under 978-nm laser diode (LD) pumping, and refractive index change after exposure to 266-nm laser pulses are obtained. There is only a little change in the PL spectra while a positive refractive index change up to 2\times10^{-4} is observed after 30-min exposure to 266-nm laser pulses. The results show that both of the peculiar photosensitivity of Sn-doped silica and the gain property of Yb-doped silica fiber are preserved in the Sn/Yb codoped silica optical fiber preform. The experimental data suggest that the photosensitivity of the fiber preform under high energy density laser irradiation should be mainly due to the bond-breaking of oxygen deficient defects, while under relatively low energy density laser irradiation, the refractive index change probably originates from the photoconversion of optically active defects.

Gigantic optical Pockels effect in water within the electric double layer at the electrode-solution interface

We have obtained the experimental evidence for the Pockels effect of water, which is placed in the electric double layer (EDL) on the interface between the bulk water and a transparent electrode. The electric-field induced energy shift was observed for the visible interference fringes in a 300-nm indium-tin-oxide (ITO) electrode layer. The signal was shown to originate from a negative (positive) refractive index change of water in the EDL for the positive (negative) bias. The refractive index change was as large as $\ensuremath{\Delta}n=\ensuremath{-}0.1$ for the 2-nm thick EDL. The Pockels constants of water in the EDL were determined to be ${r}_{13}=(2.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}100\text{ }\text{pm}/\text{V}$ and ${r}_{33}=(2.5\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}100\text{ }\text{pm}/\text{V}$. At the same time, the ITO showed blue shifts of both the band gap in the UV and the plasma frequency in the near infrared due to the band population effect in the space charge layer. Both shifts lead to a negative refractive index change in the visible region of the ITO, but their effect on the interference fringe shifts in the visible region was estimated to be much smaller than that of water in the EDL.