Supercontinuum generation in single-crystal YAG fibers pumped around the zero-dispersion wavelength.

Yttrium aluminum garnet (YAG) is a common host material for both bulk and single-crystal fiber lasers. With increasing interest in developing optical technologies in the short-wave infrared and mid-infrared wavelength range, YAG may be a potential supercontinuum medium for these applications. Here, we characterize femtosecond laser pumped supercontinuum generation with 1200-2000 nm pump wavelengths (λp) for undoped, single-crystal YAG fibers, which are representative of the normal, zero, and anomalous-dispersion regimes. Supercontinuum was observed over the spectral region of about 0.2 to 1.6λp. Z-scan measurements were also performed of bulk YAG, which revealed little dispersion of the nonlinear index of refraction (n2) in the region of interest. The measured values of n2 (∼1×10-6cm2/GW) indicate a regime in which the nonlinear length, LNL, is less than the dispersion length, LD, (LNL≪LD). We report intensity clamping of the generated filament in the normal group velocity dispersion (GVD) regime and an isolated anti-Stokes peak in the anomalous GVD regime, suggesting further consideration is needed to optimize supercontinuum generation in this fiber medium.

Single-Shot Multi-Stage Damage and Ablation of Silicon by Femtosecond Mid-infrared Laser Pulses.

Although ultrafast laser materials processing has advanced at a breakneck pace over the last two decades, most applications have been developed with laser pulses at near-IR or visible wavelengths. Recent progress in mid-infrared (MIR) femtosecond laser source development may create novel capabilities for material processing. This is because, at high intensities required for such processing, wavelength tuning to longer wavelengths opens the pathway to a special regime of laser-solid interactions. Under these conditions, due to the λ2 scaling, the ponderomotive energy of laser-driven electrons may significantly exceed photon energy, band gap and electron affinity and can dominantly drive absorption, resulting in a paradigm shift in the traditional concepts of ultrafast laser-solid interactions. Irreversible high-intensity ultrafast MIR laser-solid interactions are of primary interest in this connection, but they have not been systematically studied so far. To address this fundamental gap, we performed a detailed experimental investigation of high-intensity ultrafast modifications of silicon by single femtosecond MIR pulses (λ = 2.7-4.2 µm). Ultrafast melting, interaction with silicon-oxide surface layer, and ablation of the oxide and crystal surfaces were ex-situ characterized by scanning electron, atomic-force, and transmission electron microscopy combined with focused ion-beam milling, electron diffractometry, and µ-Raman spectroscopy. Laser induced damage and ablation thresholds were measured as functions of laser wavelength. The traditional theoretical models did not reproduce the wavelength scaling of the damage thresholds. To address the disagreement, we discuss possible novel pathways of energy deposition driven by the ponderomotive energy and field effects characteristic of the MIR wavelength regime.

Controlling phase change through ultrafast excitation of coherent phonons.

For semimetals such as bismuth, ultrafast femtosecond laser-excited coherent phonons at laser fluences below the damage threshold have been studied extensively. In this work, we investigate whether or not coherent phonon oscillations contribute to material's permanent damage, or can enhance or suppress such damage. We employed temporally-shaped femtosecond pulses to either enhance or cancel coherent phonon oscillations. Our results showed a clear difference in material's damages caused by femtosecond pulses that enhance and cancel phonon oscillations, demonstrating the possibility of controlling phase changes by coherent control of phonon oscillations.

Optical dephasing in saturable-absorbing organic dye IR140.

Room-temperature degenerate four-wave mixing in the forward two-pulse geometry, has been performed on ajet of organic saturable-absorbing laser dye IR140 dissolved in dimethylsulfoxide and ethylene glycol. Numerical solutions to the optical Bloch equation were fitted to the experimental data in order to extract a value of T2 = 50 +/- 5 fs for the homogeneous dephasing time; this value is comparable to those measured for similar organic dyes using other techniques.