F. Rotermund

Difference-frequency generation of intense femtosecond pulses in the mid-IR (4–12 μm) using HgGa2S4 and AgGaS2

Abstract We compare the properties and the performance of HgGa2S4 and AgGaS2 in a high-power difference-frequency generation scheme mixing the signal and idler from a travelling wave optical parametric generator at 1 kHz. Femtosecond pulses from 3.7 to 12 μm with quantum efficiency of 40–80% are generated with HgGa2S4 which is about 1.2 times higher than with AgGaS2 typically used in that wavelength range. The maximum single pulse energy obtained exceeds 9 μJ near 4 μm and 1 μJ in the whole tunability range which represents about an order of magnitude improvement in comparison with previous results. The mid-infrared pulses are almost bandwidth limited with durations of the order of 200 fs and less (minimum 165 fs).

J-aggregation and disaggregation of indocyanine green in water

Abstract J-aggregates of indocyanine green sodium iodide in water are formed by heat treatment. Starting from a dimeric solution the activation energy of molecule attachment (Eatt ≈ 0.41 ev) to J-aggregates is determined by analysing the temperature dependence of the rate of J-aggregate absorption growth. The activation energy of molecule detachment (Edet ≈ 0.51 eV) from J-aggregates is deduced from the temperature dependent rate of J-aggregate absorption decrease after strong dilution.

Frequency conversion of Ti:sapphire-based femtosecond laser systems to the 200-nm spectral region using nonlinear optical crystals

We review the linear and nonlinear optical properties of crystals transparent near and below 200 nm and suitable for up conversion of femtosecond Ti:sapphire laser sources, /spl beta/-BaB/sub 2/O/sub 4/, the crystal with the largest birefringence of all presently available materials, is investigated experimentally as a quadrupler by mixing the fundamental and the third harmonic both using a 1-kHz repetition rate Ti:sapphire regenerative amplifier and a 82-MHz mode-locked Ti:sapphire laser. Milliwatt average powers near 200 nm are achieved in both cases. The sub-200-fs pulses at the fourth harmonic are almost bandwidth limited. Sum-frequency generation as a method for upconversion of femtosecond pulses is experimentally studied by mixing the fourth harmonic generated down to 189 nm by the regenerative amplifier with a parametrically generated femtosecond pulse in the infrared. Pulse energies at the microjoule level are produced with LiB/sub 3/O/sub 5/ above 180 nm. Li/sub 2/B/sub 4/O/sub 7/ shows superior performance in the 170-180-nm range, and the shortest wavelength achieved with KB/sub 5/O/sub 8//spl middot/4H/sub 2/O is 166 nm.

Generation of the fourth harmonic of a femtosecond Ti:sapphire laser

A high-repetition-rate femtosecond laser system operating, for the first time to our knowledge, in the spectral region near 200 nm is described. Frequency quadrupling of a mode-locked Ti:sapphire laser results in maximum average powers of 6 mW (165-fs pulses) and 15 mW (340-fs pulses) at 82 MHz. The shortest wavelength achieved is 193.7 nm.

Degree of aggregation of indocyanine green in aqueous solutions determined by Mie scattering

Abstract Indocyanine green in water and aqueous NaCl solutions forms large J-aggregates. The degree of aggregation is estimated by Rayleigh scattering and Mie scattering analysis. The refractive index of the aggregates responsible for scattering is deduced from an aggregate absorption cross-section spectrum using the Kramers-Kronig relations. The average degree of aggregation for indocyanine green in water is found to be of the order of 10 7 while it rises to 10 9 for indocyanine green in 0.01 molar aqueous NaCl solution.

Fluorescence spectroscopic analysis of indocyanine green J aggregates in water

Abstract Indocyanine green in water forms J aggregates, which are observed as a red-shifted narrow absorption band at 890 nm. The fluorescence quantum distribution of the indocyanine green J aggregates was studied by continuous wave laser excitation at 834 nm. A fluorescence quantum yield of Ф F ≈ 3 × 10 −4 was measured. The fluorescence peak coincides with the absorption peak (no Stokes shift).

Laser-diode-seeded single and double stage femtosecond optical parametric amplification in the mid-infrared

Tunable mid-infrared pulses are generated with a MgO:LiNbO3-based traveling-wave optical parametric amplifier in single and double stage operation regime pumped by a 1-kHz Ti:sapphire regenerative amplifier system. For the first time to our knowledge a pulsed laser diode is successfully used in the femtosecond regime as a compact seed source at the signal wavelength where the tunability of the generated idler pulses can be realized by tuning the seed wavelength. Almost transform limited mid-infrared pulses as short as 130 and 160 fs with pulse energies as high as 5 and 15 μJ are produced near 3.5 μm by the single and double stage scheme, respectively. The pulse to pulse fluctuations in both cases do not exceed 2% which corresponds to the stability of the pump source.

Characterization of ZnGeP2 for Parametric Generation with Near-Infrared Femtosecond Pumping

Values for the nonlinear refractive index (n2