Few-cycle pulse generation and nonlinear optical processes in hollow waveguides, photonic crystal fibers and microstructured materials

• fast and direct measurement of the carrier-envelope phase drift of ultrashort amplified laser pulses
  We have demonstrated a novel CEP detection scheme for amplifier laser pulses that is based only on analog electronics and data processing. Using optimized photomultipliers we avoid technical noise contributions to the extent possible, which allows for meaningful single-shot measurements of the CEP at a 3 kHz repetition rate. Our new method also served to answer the long-standing question on the origin of residual CEP noise in the amplifier. Quite clearly we find two contributions: one glitchlike fast mechanism that currently only appears explainable by fluctuations of the amplifier pump. The other fraction of noise is inherited from the servo loop of the oscillator, which showed residual CEP noise levels of about 150 mrad in out-of-loop characterization experiments. These results, however, may vary for different pump lasers, and amplifier systems that involve grating stretchers or that rely on lamppumping may show significantly worse performances [KGM08].

• Nonlinear transmission through a nanoscale metal-dielectric multilayer
  We studied the transmission of light through a specially designed nonlinear nanoscale metal-dielectric multilayer structure with a lin-ear effective dielectric constant just below zero [HHe07a]. The calculated output intensity in dependence on the input intensity shows a step-like behaviour (see fig.). It rests upon an intensity-dependent change of the dielectric constant from negative (low-transmission state) to positive (high-transmission state) val-ues corresponding to a transition of the optical properties from metal-like to dielectric-like. The study of the transient behaviour of the structure demonstrates a switching time of around 1 ps.

High energy vacuum UV femtosecond pulses (100-180 nm) at 1-kHz repetition rate

• sub-100fs pulses at 180nm
  The four-wave mixing in hollow waveguides was pumped by the third harmonic of a Ti:sapphire laser (200fs, 300µJ, 268nm) and pulses from a broadband, high-energy visible optical parametric amplifier (25fs, 250µJ, 500-620nm) were used as a seed signal. By using chirp control of the seed pulses we could increase the spectral bandwidth of the VUV pulses by more than a factor of 2. We found, however, that while the VUV pulse energy approached 200 nJ, the shortest achievable pulse durations were limited by higher-order phase dispersion (see fig.), which could not be compensated solely by material dispersion. While the bandwidth limited duration was ~25 fs at 181 nm, the shortest pulses obtained so far had a duration of 70 fs measured using two-photon absorption autocorrelation in CaF2.
  

• theoretical limits of high energy sub-10 fs pulse generation in vacuum ultraviolet using chirped four wave mixing
  We numerically studîed chirped four-wave mixing for VUV pulse generation in hollow waveguides filled with a noble gas. Taking into account ionization effects we predict the generation of signal pulses at 160nm with shortest durations up to 6.5 fs, highest pulse energy up to the mJ level and maximum energy effciency of about 30% by broadband chirped idler pulses at 800 nm and narrow-band pump pulses at 270 nm. Using cascaded processes sub-10-fs pulses in the spectral range from 90 to 140 nm can also be generated.[BHe08]
  

Novel nonlinear materials and interaction schemes for frequency conversion of ultrashort laser pulses

• ultrabroadband parametric generation in BIBO
  With 45fs pulses at 800nm (260µJ), WLC spanning over an octave with an energy of 10µJ was produced by parametric generation in a 3mm long BIBO crystal. This corresponds to an internal conversion efficiency of 7% for a single collinear stage and is the first time such WLC has been generated by a second order nonlinear process on the femtosecond time scale. The sub-100fs pulse duration was measured by a sum- frequency generation FROG (see fig.). The presented experimental results confirm that BIBO possesses a unique combination of excellent properties for efficient conversion of femtosecond pulses from the 800 nm spectral range to the near-IR [GNB08].