Thomas Brabec

Route to phase control of ultrashort light pulses

A feasibility study of controlling the carrier phase in ultrashort light-wave packets emitted by a sub-10-fs laser is reported. An experimental apparatus capable of exploring the phase sensitivity of nonlinear-optical interactions is presented.

Femtosecond solid-state lasers

The emergence of new ultrafast optical modulation techniques has opened the way towards a new femtosecond laser technology based on solid-state gain media. The authors address the requirements for stable ultrashort pulse generation in these novel femtosecond sources. The theoretical considerations are backed up by experimental results obtained with a number of different laser systems. The conclusions drawn from the presented theoretical and experimental investigations provide general guidelines for the design and optimization of a wide range of femtosecond solid-state laser oscillators. >

Ultrabroadband femtosecond lasers

The exploitation of soliton-like pulse shaping mechanisms and the optimization of phase dispersion in broadband self-modelocked solid state laser oscillators have led to unprecedented advances in ultrafast laser technology. This paper reviews the basic physical mechanisms governing pulse formation and addresses the requirements for optimum performance of these novel ultrashort pulse laser sources. These considerations together with the demonstration of /spl ap/10 fs pulse generation from a Ti:sapphire laser suggest that a family of solid-state laser oscillators with comparable performance can be developed. >

Kerr lens mode locking

Self-focusing in conjunction with an intracavity aperture creates a power-dependent amplitude modulation in laser oscillators, which allows passive mode locking. A simple analytical formalism yields closed-form expressions for the depth of passive amplitude modulation introduced by either the spatial gain profile or a hard aperture inserted in the resonator. Design issues for this mode-locking technique are discussed.

Linking high harmonics from gases and solids

When intense light interacts with an atomic gas, recollision between an ionizing electron and its parent ion creates high-order harmonics of the fundamental laser frequency. This sub-cycle effect generates coherent soft X-rays and attosecond pulses, and provides a means to image molecular orbitals. Recently, high harmonics have been generated from bulk crystals, but what mechanism dominates the emission remains uncertain. To resolve this issue, we adapt measurement methods from gas-phase research to solid zinc oxide driven by mid-infrared laser fields of 0.25 volts per ångström. We find that when we alter the generation process with a second-harmonic beam, the modified harmonic spectrum bears the signature of a generalized recollision between an electron and its associated hole. In addition, we find that solid-state high harmonics are perturbed by fields so weak that they are present in conventional electronic circuits, thus opening a route to integrate electronics with attosecond and high-harmonic technology. Future experiments will permit the band structure of a solid to be tomographically reconstructed.

Self-starting passive mode locking.

We investigate the evolution of continuous-wave laser oscillation from free-running to mode-locked operation assuming a nonlinear device with an intensity-dependent transmittivity or reflectivity to be the mode-locking element. An intensity threshold for self-starting passive mode locking is predicted and related to the linewidth of the first beat note of the power spectrum of the free-running laser output. Experimental results confirm the predictions of the theory.

Operation of a femtosecond Ti:sapphire solitary laser in the vicinity of zero group-delay dispersion

We report the operating characteristics of a self-mode-locked Ti:sapphire solitary laser at reduced group-delay dispersion. The generation of asymptotically equal to 12.3 fs near-sech(2) optical pulses at 775 nm is reported, together with experimental evidence for the dominant role of third-order dispersion (TOD) as a limiting factor to further pulse shortening in the oscillator. At reduced second-order dispersion excessive residual TOD is shown to lead to dispersive wave generation, and the position of the dispersive resonance is used to determine the ratio of the net second- and third-order intracavity dispersions. Since the magnitude of TOD rapidly decreases with increasing wavelength in prism-pair dispersion-compensated resonators, the oscillator presented has the potential for producing sub-10-fs pulses in the 800-nm wavelength region.

Mode locking in solitary lasers.

We present an analysis of passively mode-locked lasers in which pulse formation is dominated by the interplay between self-phase modulation and negative dispersion in separate cavity elements. Steady-state pulse parameters and stability issues are discussed. Stability in these solitary systems relies on some passive amplitude modulation, and the ultimate system performance is found to depend sensitively on the magnitude of amplitude modulation relative to that of phase modulation.

Generation of 33-fs optical pulses from a solid-state laser.

Using the results of recent theoretical research, we present practical guidelines for the optimization of femtosecond solid-state oscillators and demonstrate reproducible sub-40-fs pulse generation in a synchronously pumped Ti:sapphire laser.

THEORY OF SELF-FOCUSING IN A HOLLOW WAVEGUIDE

We present a theoretical investigation of self-focusing in a hollow waveguide filled with noble gas. Our analysis was performed for a laser pulse that was predominantly in the fundamental mode and revealed the physical processes involved in self-focusing in a hollow waveguide. A critical power for self-focusing was obtained that was found to be substantially higher than the critical power for self-focusing in a bulk medium. Useful design criteria for pulse-compression systems are presented. We identify the parameter range for which the transverse variation of the pulse phase introduced by the Kerr nonlinearity is small.