Florian W. Helbing

Self-compression of ultra-short laser pulses down to one optical cycle by filamentation

Theoretical studies of filamentation of ultra-short near-IR laser pulses propagating in a noble gas predict near single-cycle pulses with the intensity being clamped to the field ionization threshold. Experimental results show that this method is carrier envelope offset phase preserving and provides a very simple source for generating few-cycle intense laser pulses. This suggests a very simple design for the generation of ultra-short, sub-femtosecond XUV optical pulses.

Carrier-envelope offset dynamics of mode-locked lasers

We investigate coupling mechanisms between the amplitude and the carrier-envelope offset phase in mode-locked lasers. We find that nonlinear beam steering in combination with the intracavity prism compressor is the predominant mechanism that causes amplitude-to-phase conversion in our laser. A second mechanism, induced by self-steepening, is also identified. These mechanisms are important for stabilizing the carrier-envelope offset phase and also explain the extremely low pulse-to-pulse energy fluctuations observed in some lasers with carrier-envelope lock. The coupling mechanisms described have important implications for applications of few-cycle optical pulses.

Carrier-envelope offset phase-locking with attosecond timing jitter

Inside a femtosecond laser oscillator, no coupling mechanism between the propagation speeds of the carrier and the pulse envelope exists. Therefore, the relative delay between carrier and envelope of a femtosecond oscillator will exhibit irregular fluctuations unless this jitter is actively suppressed. Both intensity and beam pointing fluctuations in the laser can introduce carrier-envelope phase changes. Based on our analysis, we are capable of reducing or avoiding certain mechanisms by proper design of the laser cavity. We use such an optimized cavity to stabilize the carrier envelope-phase to an external reference oscillator with a long-term residual jitter corresponding to only 10 attoseconds in a (100 kHz-0.01 Hz) bandwidth. This is the smallest long-term timing jitter of a femtosecond laser oscillator demonstrated to date. However, it is important to note that this stabilization was obtained with an f-to-2f heterodyne technique using additional external spectral broadening in a microstructure fiber which introduces additional carrier-envelope phase noise. We present a direct heterodyne measurement of this additional carrier-envelope phase noise due to the continuum generation process.

Frequency-sheared, time-delayed extreme-ultraviolet pulses produced by high-harmonic generation in argon

We report the production of frequency-sheared high harmonics in argon by control of the envelope and chirp of the electric field of the femtosecond driving laser pulse. Using the classic three-step model of high-harmonic generation, we established a direct link between the properties of the harmonics and the fully characterized driving pulses. A simulation of the single-atom response in the strong-field approximation confirms the simple picture and shows good agreement with the experimental results.

Generation of a nearly isolated high harmonic from a long capillary

We generated a nearly isolated high harmonic in several-centimeter long capillaries filled with argon and studied the dependence of the selectivity of high harmonic generation on the pressure and gas species.

CEO phase-locked 5.7-fs pulses through filamentation

Self-compression of a CEO phase-locked 43 fs, 0.84 mJ pulse during filamentation leads to a CFO-phase locked 5.7-fs, 0.38-mJ pulse with an excellent spatial mode. The obtained pulse bandwidth exceeds 600 THz theoretically 1.8-fs pulses.

Generation of frequency-sheared XUV pulses by high harmonic generation

Frequency-sheared and time-separated high harmonics were produced without spatial separation in the target gas by controlling the time-frequency properties of the driving field. Simulations show good agreement within the strong-field approximation.

Frequency-sheared, time-delayed XUV pulses by high harmonic generation

Frequency-sheared, time-delayed XUV pulses were produced using high harmonic generation by shaping the time-frequency properties of the generating electric field. A simulation confirmed the direct link between generating field and harmonic properties.

Intense CEO-stabilized few-cycle laser pulses from supercontinuum generation in filaments

A novel technique for the generation of intense few-cycle pulses is presented. Based on filamentation an intense CEO-stabilized 43-fs laser pulse yields a supercontinuum, supporting 1.8 fs pulses with a spectral range covering more than 600 THz. Chirped-mirror compression yields 5.7-fs pulses with 0.38 mJ energy in an excellent mode, with the CEO phase-lock being preserved.

Control of the frequency chirp rate of high harmonic pulses

We measured the frequency chirp rate of harmonics 13 to 23 in argon by cross-correlation with a short femtosecond probe pulse. We directly measured the negative chirp due to the atomic dipole phase and demonstrated that an additional chirp on the pump pulse was transferred to the qth harmonic as q times the fundamental chirp.