Photochromic Damage Research Articles

Oxygen vacancies in KTiOPO4 : Optical absorption from hybrid DFT

Density functional theory is used to calculate the optical absorption of oxygen vacancies in potassium titanyl phosphate ($\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}{\mathrm{O}}_{4}$, KTP) crystals. A modified hybrid functional is used for the description of the midgap defect states and the optical excitation energies. Oxygen vacancies in the $+2$ charge state lead to rather minor modification of the bulk KTP optical response, while the $+1$ and neutral charge states give rise to characteristic midgap optical absorption covering the whole near-infrared and visible spectrum. Its intensity is strongly polarization dependent and strongest for light polarized parallel to the $z$ axis. The modification of the KTP optical absorption by oxygen vacancies predicted here corroborates the picture that the gray-track formation in KTP, i.e., its photochromic damage, is related to a successive charging of oxygen vacancies.

Understanding gray track formation in KTP: Ti3+ centers studied from first principles

The magnetic signatures of ${\mathrm{Ti}}^{3+}$ centers in potassium titanyl phosphate (KTP) are studied within density-functional theory (DFT). The hyperfine tensor elements are very sensitive to the structural surrounding; the paramagnetic hyperfine splittings are used to evaluate the defect models. For each of the four experimentally observed electron paramagnetic resonance (EPR) spectra, we identify a defect model that reproduces the paramagnetic signature. All of them are electron donors, whereby one specific Ti atom can be identified, whose formal oxidation number is lowered from $4+$ in the ideal crystal to $3+$. The related charge redistribution leads to a strong polarization of the corresponding ${\mathrm{Ti}}^{3+}$ center. However, in three cases a second Ti atom, connected to the first by a mutual polarized O atom, is polarized too. Positively charged O vacancies at the lattice site O(10) are unique in leading to the formation of the only ${\mathrm{Ti}}^{3+}$ center that is stable at room temperature. This defect induces a defect state within the band gap, which may be excited during second harmonic generation (SHG) applications and thus is a plausible candidate to explain the so-called gray tracking, i.e., photochromic damage.

Optobionic vision—a new genetically enhanced light on retinal prosthesis

The recent discovery that neurons can be photostimulated via genetic incorporation of artificial opsins is creating a revolution in the field of neural stimulation. In this paper we show its potential in the field of retinal prosthesis. We show that we need typically 100 mW cm−2 in instantaneous light intensity on the neuron in order to stimulate action potentials. We also show how this can be reduced down to safe levels in order to negate thermal and photochromic damage to the eye. We also describe a gallium nitride LED light source which is also able to generate patterns of the required intensity in order to transfer reliable images.

Thermally induced dephasing in periodically poled KTP frequency-doubling crystals

A thermally induced spatial and temporal dephasing model of second-harmonic generation has been developed to describe the conversion efficiency and its degradation of periodically poled potassium titanium phosphate (PPKTP) in a cw, single-pass frequency conversion system. The model confirms the experimental data that show that second-harmonic power greater than 800 mW (15 kW/cm2) causes two-photon nonlinear absorption, leading to time-dependent photochromic damage in PPKTP. This added absorption degrades the conversion efficiency from an initial value of 19% to an unrecoverable asymptotic value of ∼8% in 2 h at 145 kW/cm2 of pump intensity through thermal detuning phase mismatch.

Dependence of photochromic damage on polarization in KTiOPO 4 crystals

Studies of photochromic damage induced in KTiOPO 4 single crystals by 514.5 nm laser irradiation is reported. A dependence of photochromic damage on polarization was observed. Along the same propagation direction ( θ=90°, ϕ=23.3°), the gray-track susceptibility for ∥ z polarization irradiation is one order magnitude lower than that for ⊥ z polarization irradiation for KTP crystal. The results clearly show that in KTiOPO 4, a correlation exists between the gray-track susceptibility and the ionic electrical conductivity. Due to a high ionic electrical conductivity along the polar z-axis, the optical absorption centers will be annihilated with ∥ z polarization irradiation as soon as they are produced. This results in a lower gray-track susceptibility for ∥ z polarization irradiation.

Thermo-optical effect and saturation of nonlinear absorption induced by gray tracking in a 532-nm-pumped KTP optical parametric oscillator

We present experiments that show that gray tracking modifies the parametric gain and the generated wavelengths of a KTP optical parametric oscillator pumped at 532 nm near degeneracy. These perturbations occur over a limited range of pump intensity. We propose a satisfactory model that takes into account photochromic damage, the thermo-optical effect, and the combined processes of creation and saturation of a two-photon absorber at 532 nm. The temperature dependence of Sellmeier equations of KTP is also established at 20-200 degrees C.

Optical damage of W-doped KTioPo4non linear single crystals

Abstract A photochromic damage has been observed in W-doped KTP single crystals. The damage consists in the growth of a broad optical absorption in the visible and near infrared. It has been determined that the excitation spectrum of this damage is mainly confined in the ultraviolet region. A complete annealing of the damage may be obtained by heating at 250°C or above. The origin of the damage has been mostly related with the presence of W4+ ions behaving as electron donors. Ti4+ ions trap electrons and the Ti3+ ions formed are responsible of the optical absorption observed.

Laser-induced photochromic damage in potassium titanyl phosphate

We report studies of photochromic damage induced in KTiOPO4 single crystals by ultraviolet (UV) irradiation with peak intensities from 0.3 to 4.0 MW/cm2. The photochromic damage progressed to asymptotic values which were not linear with respect to UV intensity. Following UV exposure, transparency of damaged crystals recovered completely over tens of hours at room temperature, faster at elevated temperature. UV irradiation at significantly lower peak but higher average power did not produce damage. We also studied photochromic damage occurring during 1.064-μm second-harmonic generation. Fundamental infrared intensities ranged from 1.3 to 5.5 times larger than those producing highest nonlinear frequency conversion efficiency. Damage sites produced by the two methods exhibited similar absorption spectra.

Fluoride Glass Fiber Lasers

AbstractThe versatility of lanthanide-doped fluorozirconate glass fiber as a laser material is evident since over the past four years more than 30 laser transitions have been demonstrated in this medium, at wavelengths ranging from 480nm to 3.45μm. In this low phonon-energy glass metastable lanthanide energy levels are found wherever the gap to the next level exceeds ∼2000cm-1. The glass not only has extended transparency in the infrared but also appears resistant to photochromic damage by blue radiation, making it an attractive blue laser gain medium. In particular it has been shown that some blue and green laser transitions in these fibers can be operated efficiently with infrared pumping, offering the possibility of diodepumped operation. Various schemes are potentially of interest; for example 3mW of 493nm laser power has been generated from a praseodymium-doped fluoride fiber pumped at two wavelengths by the emission and unabsorbed 840nm pump radiation from an ytterbium-doped silica fiber laser.

Comparative study of photorefractive Bi12SiO20 crystals

We present the results of a comparative experimental study of optical and photorefractive properties at room temperature of four nominally undoped, single-crystal, commercially obtained, samples of cubic Bi12SiO20. We studied (1) the steady-state energy exchange between two overlapping 515-nm beams, (i.e., ‘‘beam coupling’’), (2) the decays of photorefractive (trapped-charge) gratings induced by uniform illumination, (i.e., ‘‘grating erasure’’), (3) spontaneous decay of photorefractive gratings in the dark (i.e., ‘‘dark decay’’), (4) photochromic damage, and (5) standard optical properties such as absorption coefficients and rotatory power. Fitting our results to a simple model of photoexcitation of both electrons and holes from a single species of deep traps, we obtained values for the effective density of deep traps, the diffusion lengths of both electrons and holes (i.e., mobility-lifetime products), quantum efficiencies (near unity), and the ratio of the electron and hole contributions to absorption at 515 nm. Although unexplained or anomalous grating decay characteristics were observed, the model gives an excellent overall account of photorefractive behavior, and suggests that this type of crystal would be suitable for making devices with predictable behavior.

Effects of feed material and annealing atmosphere on the properties of photorefractive barium titanate crystals

Growth and fabrication of commercial BaTiO3 photorefractive crystals are described. Crystals grown from two different titanium dioxide sources and annealed in oxygen partial pressures between 1 and 10−17 atm were characterized by chemical analysis, optical absorption spectra, and two-beam coupling gain. Equal mixtures of the two starting materials yielded crystals with the highest photorefractive gain, whereas the extremes either were inactive or exhibited photochromic damage. Reduction of the inactive crystals below 10−11 atm of oxygen yielded a tenfold increase in photorefractive carrier density related to a sharp absorption peak at 450 nm tentatively assigned to a nickel–oxygen-vacancy complex.