I would also like to thank my friends and cohorts who personally encouraged me and provided mental support throughout my studies. A new method for modifying the refractive index of the base glass used femtosecond laser modification, where the time scale of the pulse duration allows structural changes in a locally defined volume for the modification of optical components. The role of lanthanum was compared to traditional glass modifiers within the glass structure in terms of polarizability and optical basicity for femtosecond laser refractive index modification.
Furthermore, our results demonstrate that THz-TDS may be a potential means to extend the detectable limits in refractive index measurements of disordered glass structures.
INTRODUCTION
- Lanthanum Aluminosilicate Glasses
- Polarizability
- Terahertz Time-Domain Spectroscopy (THz-TDS)
- Femtosecond laser Irradiation
The results showed that most of the polarizing ions in the glass can be estimated with respect to the oxygen atoms 33, 34. The percentage of coordination number in the aluminosilicate was measured using 27Al MAS-NMR30,35-. The states of oxygen electron clouds with bridging (BO) and non-bridging oxygen (NBO) can be estimated by the distorted polarizability state.
29Si MAS-NMR was used to determine Q species of the tetrahedral units from the observed chemical shift in the spectrum.
EXPERIMENTAL PROCEDURE
Glass Synthesis and Sample Preparation
The first melting stage was performed in a 95Pt-5Rh wt% crucible in an air atmosphere at 1600-1650 °C for 12 hours, and then poured into deionized water for glass quenching and fritting. Then, the glass powder was melted for a long time at 1600-1650 °C for 19-22 hours in a 95Pt-5Rh wt% crucible under air to ensure a homogeneous glass without streaks and seeds.
Sample Preparation
Glass Characterization
The optical and dielectric properties were collected in the transmission mode shown at the bottom of Fig. 2 and then calculated by comparing the reference and sample time delays of the THz pulse with the inherent thickness of the sample. The collection of the THz spectrum is based on the transmission ratio of the terahertz electric field strength in equation 2. The measurement of the electric field strength ratio leads to the collection of the dispersion of the refractive index n and the absorption coefficient α.
Refractive index measurements are collected from the relative phases of the sample 𝜑𝑆𝑂 and reference terahertz pulses 𝜑𝑅𝑂 from the imaginary part of the time-frequency Fourier transform given by equation 3. The absorption coefficients in the sample region are obtained from the terahertz electric field and the given reference from equation 4. The coefficients ASO and ARO are obtained from the real part of the fast Fourier transform.
Laser irradiation of the SAL glasses was performed using a Femtoscoure scientific XL 500 femtosecond laser (FEMTOLASERS, Fernkorgasse, Vienna) in Figure 3. Glass samples were optically polished discs, where laser modification was focused on the surface of the glass and in a separate grid is gridded. regions of 3 mm diameter at a rate of 250 µm/s and 150 µm/s. Step sizes of the translation stage were programmed to move 5µm vertically after horizontal rastering of the given velocities.
Fitting of the 27Al MAS NMR data was performed with DMFIT52, using the CzSimple function to calculate distributions in the quadrupolar coupling constant for each of the aluminum resonances. Some of the glass compositions were shown to contain a small amount of background signal from the zirconia rotor, which was confirmed by measuring 27Al MAS NMR data for a blank rotor.
RESULTS AND DISCUSSION
Differential Scanning Calorimetry (DSC)
The Al speciation is dependent on La ions where it can possess a duality of glass forming properties. Al speciation shows characteristics of network formation or network modifier through octahedral configuration of the cation. The relatively high glass transition compared to similar works19,30 may be evident from the large alumina content contribution to network formation.
The narrow range of glass transition temperatures may be due to the similar valence of cations of La and Al, but the larger the cation the increase of coordinate bonds can be formed and results in a small decrease in network crosslinks reported by Aronne et al53. In addition to the glass transition temperatures, the SAL glasses with higher La content show exothermic peaks indicating devitrification, where the stability of the glass structure energy decreases with a higher tendency to devitrify.
Ultraviolet-Visible (UV-Vis) Spectroscopy
The addition of lanthanum (La) shows an increase in the aluminum coordination numbers, with the polymerized Al atom units increasing in 5- and 6-fold coordination. The resulting decrease in the band gap has been linked to energy level transfer, which has been shown to decrease and is attributed to the disordering of the glass lattice with the presence of lanthanum ions55. The relatively large band gap associated with SAL glass exhibits photoionization stability for significant nonlinear absorption of optical photons upon addition of heavy La ions within the light aluminosilicate glass network.
Polarizability and Refractivity
The negative charge of oxygen available can be estimated from the degree of the charges related to the absorption of the system. The large amount of increase in the refractive index and can also be correlated with the increase in density where the heavy lanthanum ions contribute to the bulk density. Oxygen packing density can reduce the molar volume causing an increase in the bulk density of the SAL glasses.
The contributing influence can be estimated from the La content, which increases the density of the material with the content. The increase in optical basicity demonstrated the ability of the oxide ions to donate electrons to the surrounding cation. The stereochemical change of the coordination number can be estimated by the refractivity of aluminosilicate systems where the.
An increase in the refractive index in THz regimes compared to the optical refractive index is increased due to the ionic polarization which can lead to increased detection limit of the refractive index. Correlations between the refractive index and absorption coefficient against frequency allowed for experimental determination of the absorption parameters k and beta. Optical refractive index of the SAL glasses shows an increasing trend with La content in Figure 10.
THz-TDS refractive index results were shown to increase with the contribution of ionic polarizability, as shown by an initial calculation of optical basicity and the corresponding polarizability in Figure 8. The effect of the presence of quintuple Al species was reported by Stebbins et.
Femtosecond Laser Modification
With fs laser exposure, the refractive index in the THz regime increases for longer exposure times, as shown in Figure 13. The deviations in the refractive index change can be attributed to the stability of the fs laser during the exposure experiments, with spectra of multiple recorded in the THz regime. Refractive measurements at THz have an inherent nature of increasing the magnitude of refractive index differences based on the oxygen polarization sensitivity contributed by additions of lattice modifiers.
The influence of the fs-laser can cause the formation of non-bridging oxygens or the polarizability of locally modified regions, resulting in an increase in the refractive indices shown in Figure 13 of the THz-TDS data. The refractive index increases for bulk glasses with increasing polymerization of the Al-species network. Local irradiation of areas in the bulk glass indicates an increase in the distribution units of Al coordination in the structural connectivity of the glasses.
However, the local disorder of the connectivity of Al species differed from the pristine areas to the modified areas in Table V. Fitting parameters of the NMR spectra resulted in the estimation of Al species based on the corresponding chemical shift. It was shown that depolymerization was evident based on the composition shown in the bulk glasses in Figure 11.
Local modification by fs laser irradiation can affect the local structure, e.g. the coordination of the Al species from the NMR study. The increase in the higher order Al species upon irradiation can possibly be attributed to the configurational effects of these species contributing to relaxation, and an increased density in the regions.
Terahertz Time-Domain Spectroscopy (THz-TDS)
The additions of lanthanum oxide affect the structural connectivity and evolution of the depolymerized Al species with lanthanum content. The influence of the lanthanum changes the Al species in the glass structural motifs, converting the network-forming tetrahedral species into possible parts of edges and faces for intermediate features of Al(V) and Al(VI). The absorption parameters show a large increase based on the high refractive index of the SAL glasses.
Using Strom's model62, the absorption parameter from the THz-TDS method does not show a linear trend for intact or altered regions of the glasses. Estimates of the coordination number of alumina from the NMR data show a trend towards increased disorder in the bonding of Al species. Glass matrix polymerization of tetrahedrally coordinated species contributes to differences in THz measurements, where modified regions of equivalent basic chemical compositions showed a strong trend and correlation with THz refractive indices.
Thermal effects when irradiating the glasses can also be a factor for variations in the refractive indices, which will require further investigation of the heat effect. The presence of the increased percentages of higher coordinated Al species may be affected by the force. Further XRD analysis of the heat-treated samples showed crystallization in well-defined peaks from XRD, shown in Figure 17.
The subtle change in optical refractive index and modified laser volume present the challenge of unique sample preparation. Examination with neutron diffraction can also be useful in determining the radial distribution function of the glasses. Komatsu, "An interpretation of optical properties of oxides and oxide glasses in relation to the electronic ion polarizability and the average single bond strength," J.
Coutures, "27-Al NMR study of the structure of lanthanum and yttrium based aluminosilicate glasses and melts", J.
