Broadly tunable femtosecond pulse generation in the near and mid-infrared.

High-repetition-rate (80-MHz) femtosecond infrared pulses are generated by difference frequency mixing (DFM) a femtosecond Ti:sapphire laser with a phase-locked synchronized cw mode-locked Nd:YAG picosecond laser. This DFM scheme is of particular interest for generating ultrashort near-IR pulses (~10 fs) because group velocity mismatch with a pump pulse can be ignored. The simplicity and the broad wavelength tunability (from the near IR to the mid-IR) of this scheme is demonstrated. Short (125-fs FWHM) optical pulses in the near IR around 1.5 mum are obtained with noncritical type-I phase-matched LiB(3) O(5). We also used a similar scheme to generate mid-infrared pulses at 3.0 mum with type-II phase-matched KTiOPO(4).

Surface Plasmon Resonance-Laser Desorption/Ionization-Time-of-Flight Mass Spectrometry.

The laser desorption/ionization (LDI) process is investigated under surface plasmon resonance (SPR) conditions using time-of-flight mass spectrometry (TOFMS). We demonstrate that LDI-TOFMS at the SPR angle requires a lower minimum laser fluence for the production of silver monomer and cluster cations from ablation of a thin silver film substrate. In the LDI of gramicidin S deposited on a thin silver film substrate, the largest intensity for the molecular cation peak occurs when the laser light is incident on the substrate at a specific SPR angle. These results fully confirm SPR enhancement of the LDI process. The capability to perform SPR-LDI on a larger molecular weight analyte (1141 amu for gramicidin S) represents a new milestone beyond the previous achievement with rhodamine B (479 amu). A better understanding of the SPR mechanism is gained with respect to the substrate metals (silver vs aluminum), desorption (microscopic vs mesoscopic), and ionization (chemical vs multiphoton). These findings may be useful in the future design of SPR-LDI techniques for better TOFMS analysis of higher mass biomolecules.