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 ͑TiO 2 , Ta 2 O 5 , HfO 2 , Al 2 O 3 , and 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 of the observed dependence of the breakdown threshold fluence F th on pulse duration p , F th ϰ p , is independent of the material and is attributed to photoionization seeded avalanche ionization.

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A systematic theoretical and experimental study of focusing femtosecond light pulses by single lenses is presented. By evaluation of the diKraction integral the interplay of spherical and chromatic aberration is shown to determine the temporal as well as the spatial intensity distribution in the focal region of a lens. Conditions are derived under which the eKect of spherical aberration dominates. Here a temporally unbroadened, in-focus pulse occurs while the spatial distribution is that expected from an annular lens aperture. If chromatic aberration is the major aberration, the in-focus pulses are considerably broadened.Both efFects could clearly be measured with 100-fs pulses using a modified Michelson interferometer. In an intermediate parameter range both chromatic and spherical aberration contribute to the pulse broadening and to the spatial intensity pattern in a given plane in the focal region. From Fresnel diKraction we expect a weak-intensity distribution to precede the in-focus pulse on the axis. Another weak intensity peak is produced by the pulse traveling on axis. By measuring its separation from the main pulse in the marginal focal plane one can estimate the aberration parameters.PACS number(s): 42.65. Re, 42.79.Bh, 42.25.Fx

The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a Ta 2 O 5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to twophoton 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 TiO 2 , HfO 2 , Al 2 O 3 , and SiO 2 films. A theoretical model is presented that explains these findings.

We demonstrate for the first time an optically pumped gas laser based on population inversion using a hollow core photonic crystal fiber (HC-PCF). The HC-PCF filled with 12C2H2 gas is pumped with ~5 ns pulses at 1.52 μm and lases at 3.12 μm and 3.16 μm in the mid-infrared spectral region. The maximum measured laser pulse energy of ~6 nJ was obtained at a gas pressure of 7 torr with a fiber with 20 dB/m loss near the lasing wavelengths. While the measured slope efficiencies of this prototype did not exceed a few percent due mainly to linear losses of the fiber at the laser wavelengths, 25% slope efficiency and pulse energies of a few mJ are the predicted limits of this laser. Simulations of the laser's behavior agree qualitatively with experimental observations.