Tunable picosecond optical parametric amplifier pumped by 1 ps pulses at 1†µm for coherent anti-Stokes Raman scattering.

We report an optical parametric amplifier (OPA), providing a maximum pulse energy of ∼200 µJ at 700-950 nm and a pulse duration of ∼1 ps. The OPA is driven by a ∼1 ps pulse with ∼2.5 mJ energy at 1 kHz, provided by a commercial thin-disk based laser. Using the output pulse of the OPA as pump, the thin-disk laser pulses at 1030 nm as Stokes, and the second harmonic (515 nm) as probe, we investigate the coherent anti-Stokes Raman scattering (CARS) of N2 and CO2 at various temperatures.

Intravital deep-tumor single-beam 3-photon, 4-photon, and harmonic microscopy.

Three-photon excitation has recently been demonstrated as an effective method to perform intravital microscopy in deep, previously inaccessible regions of the mouse brain. The applicability of 3-photon excitation for deep imaging of other, more heterogeneous tissue types has been much less explored. In this work, we analyze the benefit of high-pulse-energy 1 MHz pulse-repetition-rate infrared excitation near 1300 and 1700 nm for in-depth imaging of tumorous and bone tissue. We show that this excitation regime provides a more than 2-fold increased imaging depth in tumor and bone tissue compared to the illumination conditions commonly used in 2-photon excitation, due to improved excitation confinement and reduced scattering. We also show that simultaneous 3- and 4-photon processes can be effectively induced with a single laser line, enabling the combined detection of blue to far-red fluorescence together with second and third harmonic generation without chromatic aberration, at excitation intensities compatible with live tissue imaging. Finally, we analyze photoperturbation thresholds in this excitation regime and derive setpoints for safe cell imaging. Together, these results indicate that infrared high-pulse-energy low-repetition-rate excitation opens novel perspectives for intravital deep-tissue microscopy of multiple parameters in strongly scattering tissues and organs.

Difference-frequency generation of ultrashort pulses in the mid-IR using Yb-fiber pump systems and AgGaSe(2).

We employ AgGaSe(2) for difference-frequency generation between signal and idler of synchronously-pumped picosecond / femtosecond OPOs at 80 / 53 MHz. Continuous tuning in the picosecond regime is achieved from 5 to 18 µm with average power of 140 mW at 6 µm. In the femtosecond regime the tunability extends from 5 to 17 µm with average power of 69 mW at 6 µm. Maximum single pulse energies of >1 nJ in both cases represent the highest values at such high repetition rates.

Difference-frequency generation of fs and ps mid-IR pulses in LiInSe(2) based on Yb-fiber laser pump sources.

Difference-frequency generation between signal and idler of Yb-fiber laser synchronously pumped femtosecond and picosecond optical parametric oscillators (SPOPOs) operating at 53 and 80 MHz, respectively, is investigated in the wide bandgap chalcogenide crystal LiInSe(2). Single pulse energies in excess of 1 nJ are demonstrated, and the continuous tuning extends from 5 to 12 µm.

Generation of tunable sub-45 femtosecond pulses by noncollinear four-wave mixing.

Generation of sub-45 fs vacuum UV (VUV) pulses tunable across the spectral range of 146-151 nm at 1 kHz repetition rate is reported. The pulses are produced using noncollinear difference-frequency four-wave mixing between the third-harmonic of an amplified Ti:sapphire laser and the signal wavelength of an infrared optical parametric amplifier (ω(VUV)=2ω(TH)-ω(IR)) in krypton and argon. The generated VUV pulses have energies as high as 90 nJ. Pulse duration measurements are realized by cross correlation between the VUV pulses and the laser fundamental wavelength using pump-probe ionization in xenon.

Role of alkali cations for the excited state dynamics of liquid water near the surface.

Time-resolved liquid jet photoelectron spectroscopy was used to explore the excited state dynamics at the liquid water surface in the presence of alkali cations. The data were evaluated with the help of ab initio calculations on alkali-water clusters and an extension of these results on the basis of the dielectric continuum model: 160 nm, sub-20 fs vacuum ultraviolet pulses excite water molecules in the solvent shell of Na(+) or K(+) cations and evolve into a transient hydrated complex of alkali-ion and electron. The vertical ionization energy of this transient is about 2.5 eV, significantly smaller than that of the solvated electron.

Generation of intense sub-20-fs vacuum ultraviolet pulses compressed by material dispersion.

We present our latest results on the generation of ultrashort vacuum UV (VUV) pulses by nonresonant four-wave mixing of chirped broadband pulses generated by filamentation of the fundamental of a Ti:sapphire laser with relatively narrowband pulses at the third harmonic. Positive chirp at the broadband idler yields negatively chirped VUV pulses necessary to compensate for material dispersion of a MgF(2) window in the VUV beam path. Pulse energies exceeding 400 nJ are available for time-resolved experiments. Pulse duration is measured by pump-probe ionization of Xe gas, providing the cross correlation between the fifth harmonic and the fundamental.

Generation of high-energy, sub-20-fs pulses in the deep ultraviolet by using spectral broadening during filamentation in argon.

Generation of sub-20-fs UV pulses with more than 300 µJ energy at 268 nm is reported. First, the UV pulses are produced by successive second-harmonic and third-harmonic (TH) generation of 805 nm pulses of a 1 kHz Ti:sapphire laser amplifier. The spectral broadening of TH pulses is realized in a filament, generated in argon. The produced pulses are compressed in a simple double-pass prism-pair compressor. Starting from 100 fs pulses, we achieve a fivefold pulse shortening.

Generation of sub-50-fs vacuum ultraviolet pulses by four-wave mixing in argon.

We report on the generation of femtosecond pulses at 160 nm with energies up to 240 nJ at 1 kHz repetition rate and sub-50-fs pulse duration. This pulse energy is a 1-order-of-magnitude improvement compared with previous sub-100-fs sources in this wavelength range. The pulses are generated by four-wave difference-frequency mixing process between the fundamental of a Ti:sapphire laser and its third harmonic in argon. Pulse duration measurements are achieved by pump-probe ionization of Xe gas providing the cross correlation between the fifth harmonic and the fundamental.

Generation of high-energy sub-20 fs pulses tunable in the 250-310 nm region by frequency doubling of a high-power noncollinear optical parametric amplifier.

We report on the generation of powerful sub-20 fs deep UV pulses with 10 microJ level energy and broadly tunable in the 250-310 nm range. These pulses are produced by frequency doubling a high-power noncollinear optical parametric amplifier and compressed by a pair of MgF2 prisms to an almost transform-limited duration. Our results provide a power scaling by an order of magnitude with respect to previous works.