The scaling of the single-pulse laser threshold fluence for dielectric breakdown with respect to pulse duration and material band gap energy was investigated in the subpicosecond pulse regime using oxide films (${\mathrm{TiO}}_2$, ${\mathrm{Ta}}_2{\mathrm{O}}_5$, ${\mathrm{HfO}}_2$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, and ${\mathrm{SiO}}_2$). A phenomenological model attributes the pulse duration dependence to the interplay of multiphoton ionization, impact ionization, and subpicosecond electron decay out of the conduction band. The observed linear scaling of the breakdown fluence with band gap energy can be explained within the framework of this model by invoking the band gap dependence of the multiphoton absorption coefficient from Keldysh photoionization theory. The power exponent $\kappa$ of the observed dependence of the breakdown threshold fluence $F_{\mathit{th}}$ on pulse duration ${\tau }_p$, $F_{\mathit{th}}\propto {\tau }_p^{\kappa }$, is independent of the material and is attributed to photoionization seeded avalanche ionization.

The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a $Ta2O5$ film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to two-photon excitation of self-trapped excitons that form after band-to-band excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant pulse number. The repetition rate dependence of the breakdown threshold is also measured for $TiO2,$$HfO2,$$Al2O3,$ and $SiO2$ films. A theoretical model is presented that explains these findings. © 2005 Society of Photo-Optical Instrumentation Engineers.

A model for the multiple-pulse laser-induced breakdown behavior of dielectrics is presented. It is based on a critical conduction band (CB) electron density leading to dielectric breakdown. The evolution of the CB electron density during the pulse train is calculated using rate equations involving transitions between band and mid-gap states (native and laser-induced). Using realistic estimations for the trap density and ionization cross-section, the model is able to reproduce the experimentally observed drop in the multiple-pulse damage threshold relative to the single-pulse value, as long as the CB electron density is controlled primarily by avalanche ionization seeded by multiphoton ionization of the traps and the valence band. The model shows that at long pulse duration, the breakdown threshold becomes more sensitive to presence of traps close (within one photon energy) to the CB. The effect of native and laser-induced defects can be distinguished by their saturation behavior. Finally, measurements of the multiple-pulse damage threshold of hafnium oxide films are used to illustrate the application of the model.

The linear and nonlinear optical properties of B a G a 4 S 7 , B a G a 4 S e 7 , B a G a 2 G e S 6 , and B a G a 2 G e S e 6 are reviewed, highlighting their potential for frequency downconversion of powerful laser sources from the near- to the mid-IR. Examples of successful applications are summarized and briefly discussed.

We present the first sub-100 fs bulk solid-state laser in the 2-μm spectral range employing the monoclinic ${\mathrm{Tm}}^{3+}-\text{doped}$ ${\mathrm{MgWO}}_4$ crystal as an active medium. By applying a graphene-based saturable absorber and chirped mirrors for dispersion management, stable self-starting mode-locked operation at 2017 nm was achieved. Nearly Fourier-limited pulses as short as 86 fs featuring a bandwidth of 53 nm were generated at a repetition rate of 76 MHz. A pulse energy of 1.1 nJ was achieved at 87 MHz for a pulse duration of 96 fs. The mode-locked ${\mathrm{Tm}}^{3+}\text{:}{\mathrm{MgWO}}_4$ laser exhibits excellent stability with a fundamental beat note extinction ratio of 80 dBc above noise level.

A Tm,Ho:CALGO laser passively mode-locked by a GaSb-based SESAM generated pulses as short as 52 fs at a central wavelength of 2015 nm with a broad spectral bandwidth of 82 nm (full width at half maximum) owing to the combined gain profiles of both dopants for σ-polarized light. The average output power reached 376 mW at a repetition rate of 85.65 MHz. In the continuous-wave regime, the laser was power scaled up to 1.01 W at 2080.6 nm with a slope efficiency of 32.0%, a laser threshold of 155 mW and π-polarized emission. Polarized spectroscopic properties of Ho³⁺ ions in singly doped and codoped CALGO crystals were revisited to explain the observed laser performance.