Douglas H. Blackburn

Optical properties of Nd3+ in tellurite and phosphotellurite glasses

Optical‐absorption and emission spectra and fluorescence lifetimes were measured for Nd3+ in tellurite glasses containing various alkali and higher valence state cations and in a series of new phosphotellurite glasses. Judd‐Ofelt intensity parameters were determined and used to calculate radiative lifetimes and stimulated emission cross sections for 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions. Cross sections for several of the glasses are the largest obtained for any pure oxide glass. The dependence of the spectroscopic properties on composition and the application of tellurite glasses for lasers are discussed.

Relative Intensity Correction of Raman Spectrometers: NIST SRMs 2241 through 2243 for 785 nm, 532 nm, and 488 nm/514.5 nm Excitation

Standard Reference Materials® SRMs 2241 through 2243 are certified spectroscopic standards intended for the correction of the relative intensity of Raman spectra obtained with instruments employing laser excitation wavelengths of 785 nm, 532 nm, or 488 nm/514.5 nm. These SRMs each consist of an optical glass that emits a broadband luminescence spectrum when illuminated with the Raman excitation laser. The shape of the luminescence spectrum is described by a polynomial expression that relates the relative spectral intensity to the Raman shift with units in wavenumber (cm−1). This polynomial, together with a measurement of the luminescence spectrum of the standard, can be used to determine the spectral intensity-response correction, which is unique to each Raman system. The resulting instrument intensity-response correction may then be used to obtain Raman spectra that are corrected for a number of, but not all, instrument-dependent artifacts. Peak area ratios of the intensity-corrected Raman spectrum of cyclohexane are presented as an example of a methodology to validate the spectral intensity calibration process and to illustrate variations that can occur in this measurement.

Characteristics of laser-induced gratings in Pr 3+ - and Eu 3+ -doped silicate glasses

Four-wave mixing techniques are used to produce laser-induced gratings in Pr3+-doped silicate glasses for the first time to our knowledge. The characteristics of the laser-induced grating are investigated and compared with those found in Eu3+-doped silicate glasses. An attempt to form a laser-induced grating in an Er3+-doped silicate glass was made. Under excitation conditions similar to those in previous experiments, no laser-induced grating could be produced. Differences between the samples are discussed in terms of high-energy phonons, which are emitted when the rare-earth ion relaxes nonradiatively. Temperature dependences of the laser-induced grating signal intensity are investigated in Eu3+-doped silicate glasses, and the results are compared with theoretical predictions.

Optical applications of laser-induced gratings in Eu doped glasses.

Laser-induced changes in the refractive index are used to create superimposed transient population gratings and permanent structural gratings in Eu(3+) doped silicate and phosphate glasses. Potential uses for these laser-induced gratings (LIGs) are investigated. First, the structural gratings are shown to be permanent at room temperature and their use as a holographic storage medium is discussed. Second, a permanent LIG of this type is used to demultiplex multifrequency laser beams, demonstrating its use as a tunable line filter. Third, the transient LIG is used to modulate the amplitude of a laser beam which is passed through the sample and scatters off the permanent LIG. This results in information being transferred from one beam to another beam. It was found that thermal lensing plays an important role in the formation of this type of permanent LIG and a procedure for determining the tilt angle of the fringes of the LIG is discussed.

Cracking of brittle coatings adhesively bonded to substrates of unlike modulus

The role of elastic mismatch in determining critical conditions for indentation fracture in brittle coatings on substrates of unlike modulus was investigated. A model transparent trilayer system, consisting of a glass coating layer bonded to a thick substrate of different glass or polymer by a thin layer of epoxy adhesive, facilitated in situ observations of crack initiation and propagation. A tungsten carbide sphere was used to load the layer system. Abrasion flaws were introduced into the top and bottom glass coating surfaces to control the flaw populations and to predetermine the origins of fracture: cone cracks occurred at abraded top surfaces, radial cracks at abraded bottom surfaces. Analytical relations for the critical loads are presented for each crack system in terms of elastic modulus mismatch, indenter and coating dimensions, and material fracture parameters. Implications concerning materials selection for resistance to crack initiation are considered.

Effects of the structure and composition of lead glasses on the thermal lensing of pulsed laser radiation

The thermal lensing characteristics of several silicate, germanate, phosphate, and borate glasses were studied using a laser with a 7 ns pulse at 457 nm in a tight focus geometry. A geometric model was developed to describe the quadratic radial profile of the refractive index resulting from the laser‐induced temperature profile. This model was utilized to interpret the effects of some of the relevant experimental parameters on the fluence transmission experiments. The influence of material properties such as different types of network former and modifier ions on the nonlinear optical properties of these materials were also studied. It was found that: (i) the greatest influence of the network modifier ions was due to their effect on the absorption coefficient of the glasses; (ii) in lead glasses, the thermo‐optic coefficients dn/dT of the germanates and silicates with random network structures were greater than those of the borate and phosphate glasses with ring and chain structures; and (iii) the main con...

Refractive index gratings in rare‐earth‐doped alkaline earth glasses

Four‐wave‐mixing techniques were used to produce permanent laser‐induced refractive index gratings in Eu3+ ‐doped silicate glasses. These gratings are associated with a thermally induced change in the local glass structure at the site of the Eu3+ ions, leading to a double‐minimum potential well for the electronic energy levels of the Eu3+ ions. The effects on the characteristics of the permanent laser‐induced gratings produced by changing the divalent modifier ions of the glass host are reported and a theoretical model is presented to explain the physical origin of the change in the refractive index of the material.Four‐wave‐mixing techniques were used to produce permanent laser‐induced refractive index gratings in Eu3+ ‐doped silicate glasses. These gratings are associated with a thermally induced change in the local glass structure at the site of the Eu3+ ions, leading to a double‐minimum potential well for the electronic energy levels of the Eu3+ ions. The effects on the characteristics of the permanent laser‐induced gratings produced by changing the divalent modifier ions of the glass host are reported and a theoretical model is presented to explain the physical origin of the change in the refractive index of the material.

LASER-INDUCED FLUORESCENCE LINE NARROWING OF EU3+ IN LITHIUM BORATE GLASS

Laser-induced fluorescence line narrowing has recently been used to investigate the local environment and interactions of paramagnetic ions in glass. Since the resulting spectra are sensitive to structural modifications, this technique was applied to study lithium borate glass. In alkali borate glasses, the relative numbers of BO3 triangles and BO4 tetrahedra and the existence of subliquidus immiscibility are dependent upon the mole fraction of alkali oxide. Structural changes associated with these properties are, in principle, detectable using the approach of fluorescence line narrowing. Evidence of these effects are presented in this paper.

6Li-doped silicate glass for thermal neutron shielding

Abstract Glass formulations are described that contain high concentrations of 6 Li and are suitable for use as thermal neutron shielding. One formulation contained 31 mol% of 6 Li 2 O and 69 mol% of SiO 2 . Studies were performed on a second formulation that contained as much as 37 mol% of 6 Li 2 O and 59 mol% of SiO 2 , with 4 mol% Al 2 O 3 added to prevent crystallization at such high 6 Li 2 O concentrations. These lithium silicate glasses can be formed into a variety of shapes using conventional glass fabrication techniques. Examples include flat plates, disks, hollow cylinders, and other more complex geometries. Both in-beam and in-core experiments have been performed to study the use and durability of Li silicate glasses. In-core experiments show the glass can withstand the intense radiation fields near the core of a reactor. The neutron attenuation of the glasses used in these studies was 90%/mm. In-beam studies show that the glass is effective for reducing the gamma-ray and neutron fields near experiments.

The dependence of laser‐induced refractive index changes in glass on the chemical composition

The properties of permanent, laser induced refractive index gratings in rare‐earth doped glasses have been characterized in a variety of materials including lithium borates, lead and magnesium silicates, lead borate‐germanates and lead germanates. This work is combined with the previous results obtained on phosphate and silicate glasses to develop an understanding of the effects of glass composition (both network former and modifier ions) on the ability of a material to produce gratings with high scattering efficiencies. The grating efficiencies and the writing/erasing dynamics were studied, and the two‐level‐system‐model developed previously was employed to explain the results. The results show the importance of having flexible glass networks with low polarizing power of the network former ions, and a high content of nonbridging oxygen ions. In addition, it is important to have light, highly polarizable modifier ions with weak chemical bond strengths to the oxygen ions. This type of glass combined with r...