Absorption In CaF2 Research Articles

Influence of Na-related defects on ArF laser absorption in CaF2

ArF laser pulse transmission through commercial high purity CaF2 is determined by measuring the energy of each pulse before and behind the sample up to an incident fluence H of 10 mJ/cm2. The steady state transmission of ArF laser pulses decreases with increasing fluence. The related absorption coefficients αst(H) are proportional to H and rationalized by effective 1- and 2-photon absorption coefficients 2.4×10−4 cm−1≤αeff≤16.8×10−4 cm−1 and 1.7×10−9 cm W−1≤βeff≤9.3×10−9 cm W−1, respectively. The αeff and βeff values increase with the Na content of the CaF2 samples as identified by the fluorescence of Na-related MNa centers at 740 nm. This relation is simulated by a rate equation model describing the ArF laser induced MNa generation in the dark periods between the laser pulses and their annealing during laser irradiation. MNa generation starts with intrinsic 2-photon absorption in CaF2, yielding self-trapped excitons (STE). These pairs of F and H centers move upon thermal activation and the F centers combine with FNa to form MNa centers. MNa annealing occurs by its photo dissociation into a pair of F and FNa centers.

Surface and bulk absorption in CaF2 at 193 and 157 nm

Comprehensive calorimetric absorption measurements were performed for CaF2 crystals at irradiation wavelengths 193 and 157 nm. By using samples with different thickness a separation of surface and bulk absorptance could be achieved and thus, single- and two-photon absorption coefficients could be determined for both wavelengths. For the surface absorptance, a dependence of the polishing grade of the sample was observed at 193 nm. The investigation of earlier reported repetition rate dependencies of the effective two-photon absorption coefficient at 193 nm was extended up to a repetition rate of 2.8 kHz. In contrast to the measurements at 193 nm, no such dependence at all could be found at 157 nm. Both results support already proposed models of the absorption mechanisms in wide band-gap materials.

Self-trapped exciton luminescence and repetition rate dependence of two-photon absorption in CaF2 at 193 nm

Abstract Repetition rate-dependent measurements of nonlinear absorptance and self-trapped exciton luminescence ( π -luminescence) of high-purity CaF 2 crystals at 193 nm irradiation were performed in the range between 25 and 300 Hz. The CaF 2 samples showed different (effective) two-photon absorption coefficients β eff , which are attributed to different impurity or defect concentrations inside the crystals. For samples with low β eff , no dependence on repetition rate could be observed, while samples with high values reveal a strong increase of β eff for increasing repetition rates. In contrast, the intensity of self-trapped exciton luminescence is nearly the same for all samples and constant for all repetition rates. Different two-photon absorption mechanisms were derived from these results and a simple kinetic model for the absorption behaviour was developed, including the formation of transient absorbing states. In addition, laser induced bleaching of Ce 3+ emission during irradiation with a total dose up to 900 mJ/cm 2 could be observed in the luminescence spectra.

Temperature dependence of the saturation of the H- local-mode absorption in CaF2 by use of a tunable carbon dioxide laser.

We have studied the temperature dependence of the saturation of the local-vibrational-mode absorption of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions in ${\mathrm{CaF}}_{2}$ crystals by use of a tunable, high-pressure carbon dioxide laser. We tuned the laser to the peak of the strongly temperature-dependent local-mode absorption line and found, in the temperature range 4 to 110 K, strongly temperature-dependent saturation behavior. An analysis of the results shows that the saturation behavior is determined by three-phonon decay of the local mode. From our results we derive an anharmonic lifetime of 17 ps at low temperature and we estimate, furthermore, an effective vibrational charge of about one electronic charge for the local mode.

Nonlinear processes in UV optical materials at 248 nm

Nonlinear processes in UV optical materials were investigated by using 280-fs, 248-nm pulses. Nonlinear absorption in CaF2 was confirmed to be a two-photon process by using the luminescence of self-trapped excitons, which was also used for the single shot pulse width measurement. The absorption bands due to F centers were identified in CaF2, MgF2, and LiF after several hundred shots at 100 GW/cm2. Absorption at 248 nm was considerable especially in MgF2 and LiF. Self-focusing and self-phase modulation were observed in CaF2.

Multiterawatt excimer-laser system

A peak power of 4 TW has been obtained in KrF with an output energy of 1.5 J in 390 fsec from an electron-beam-pumped amplifier. The energy of the amplified spontaneous emission was as low as 1.8%. The pulse width was measured by the newly developed third-order autocorrelation technique based on the XeF C–A transition. Amplifiers increased the initial pulse width of 210 fsec to 390 fsec. The process of pulse-width broadening was investigated in detail. It was confirmed that the two-photon process contributes to small absorption in CaF2 by the luminescence of self-trapped excitons. A terawatt-class KrF laser system with a moderate repetition rate was developed for applications to multiphoton processes.

Three-photon absorption in CaF2 at 248.5 nm

Nonlinear absorption at 248.5 nm is studied with an 8 ps pulse. Two-photon absorption in fused silica and three-photon absorption in CaF2 are observed. Absorption coefficients are 8×10−11 cm/W and 2×10−21 cm3 /W2 for fused silica and CaF2 , respectively.

Manganese absorption in CaF2:Mn; I

We report detailed optical absorption spectra for Mn2+ in CaF2. The spectra from unirradiated samples, reported here for the first time, enable the 495-nm luminescence emission to be clearly assigned to a transition from the first excited 4T1g (4G) level to the 6A1g (6S) ground state. On the basis of the absorption curves energy level assignments are made for all of the 4G, 4D, 4P, and 4F levels. Racah parameters are evaluated and the reduction of these from their free-ion values, due to covalency effects, is estimated. A value of Dq=420 cm−1 is calculated and an Orgel diagram for this system is constructed. Finally, oscillator strengths for each of the transitions are estimated from the absorption spectra. The optical absorption spectra from gamma irradiated samples show a complex array of overlapping bands, most of which can be shown to be related to the presence of Mn. The thermal annealing of the bands is associated with the production of thermoluminescence at an emission wavelength of 495 nm. Although no definite models can be offered for the defects involved in these processes, they are believed to be complexes of Mn and radiation-induced defects (e.g., Mn/F centers).

Absorption and Emission Spectrum of CaF2:U3+ from 2.1 to 2.5 Microns

The emission and absorption of CaF2:U3+ has been scanned at 77° and 4.2°K and linewidths have been measured. The spectrum is interpreted as arising from ions perturbed by a weak tetragonal crystal field. Preliminary results of the Zeeman effect on the narrowest lines are presented.