Joachim Herrmann

Supercontinuum generation, four-wave mixing, and fission of higher-order solitons in photonic-crystal fibers

The nonlinear propagation of femtosecond pulses in photonic-crystal fibers is investigated theoretically without the use of the slowly varying envelope approximation. Low-intensity supercontinuum generation caused by fission of higher-order solitons into red-shifted fundamental solitons and blue-shifted nonsolitonic radiation is studied in a large range of fiber and pulse parameters. It is shown that phase matching of degenerate four-wave mixing can be achieved in an extremely broad frequency range from the IR to the UV. Spontaneous generation of new frequency components and parametric amplification by four-wave mixing as well as its possible overlap with soliton fission are studied in detail.

Theory of Kerr-lens mode locking: role of self-focusing and radially varying gain

The ABCD-matrix formalism is generalized for the description of self-focusing and radially varying pump power in a nonparabolic approximation. This extended formalism is used for the investigation of the operation and for the optimization of the saturable aperture loss, the power-dependent average gain, and the resonator magnification of Kerr-lens mode-locked lasers with as well as without an internal aperture. The calculated amplitude modulation parameters are compared with the necessary condition for self-starting and for the shortest possible pulse duration limit.

Theory of plasmon-enhanced high-order harmonic generation in the vicinity of metal nanostructures in noble gases

Max Born Institute of Nonlinear Optics and Short Pulse Spectroscopy, Max Born Str 2a, D-12489 Berlin, Germany(Dated: September 22, 2010)We present a semiclassical model for plasmon-enhanced high-harmonic generation (HHG) in thevicinity of metal nanostructures. We show that both the inhomogeneity of the enhanced local fieldsand electron absorption by the metal surface play an important role in the HHG process and leadto the generation of even harmonics and to a significantly increased cutoff. For the examples ofsilver-coated nanocones and bowtie antennas we predict that the required intensity reduces by upto three orders of magnitudes and the HHG cutoff increases by more than a factor of two. Thestudy of the enhanced high-harmonic generation is connected with a finite-element simulation of theelectric field enhancement due to the excitation of the plasmonic modes.

Experimental and theoretical study of third-order harmonic generation in carbon nanotubes

Third-harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The results show an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields, of ∼1010 W/cm2. This strong nonlinearity of the laser interaction with carbon nanoubes is also confirmed theoretically, in a full quantum-mechanical theory for harmonics generation from a single-walled carbon nanotube.

Tailoring terahertz radiation by controlling tunnel photoionization events in gases

Various applications ranging from nonlinear terahertz (THz) spectroscopy to remote sensing require broadband and intense THz radiation, which can be generated by focusing two-color laser pulses into a gas. In this setup, THz radiation originates from the buildup of electron density in sharp steps of attosecond duration due to tunnel ionization, and the subsequent acceleration of free electrons in the laser field. We show that the spectral shape of the THz pulses generated by this mechanism is determined by the superposition of contributions from individual ionization events. This provides a straightforward analogy to linear diffraction theory, where the ionization events play the role of slits in a grating. This analogy offers simple explanations for recent experimental observations and opens new avenues for THz pulse shaping based on temporal control of the ionization events. We illustrate this novel technique by tailoring the spectral width and position of the resulting radiation using multi-color pump pulses.

Linear and nonlinear optical characteristics of composites containing metal nanoparticles with different sizes and shapes

We study the effective linear and nonlinear optical parameters of composites containing noble metal nanoparticles and their dependence on the shape and size of the particles. Our numerical approach is based on the effective medium approximation combined with discrete dipole approximation, which results in a fast and accurate numerical method. The results demonstrate the possibility to achieve large enhancements of the linear and nonlinear optical parameters by tuning the plasmon resonance to a desired frequency by changing the size and the shape of the nanoparticles.

Two-octave supercontinuum generation in a water-filled photonic crystal fiber.

Supercontinuum generation in a water-filled photonic crystal fiber is reported. By only filling the central hollow core of this fiber with water, the fiber properties are changed such that the air cladding provides broadband guiding. Using a pump wavelength of 1200 nm and few-microjoule pump pulses, the generation of supercontinua with two-octave spectral coverage from 410 to 1640 nm is experimentally demonstrated. Numerical simulations confirm these results, revealing a transition from a soliton-induced mechanism to self-phase modulation dominated spectral broadening with increasing pump power. Compared to supercontinua generated in glass core photonic fibers, the liquid core supercontinua show a higher degree of coherence, and the larger mode field area and the higher damage threshold of the water core enable significantly higher pulse energies of the white light pulses, ranging up to 0.39microJ.

Polarization gating and circularly-polarized high harmonic generation using plasmonic enhancement in metal nanostructures

We investigate possibilities to utilize field enhancement by specifically designed metal nanostructures for the generation of single attosecond pulses using the polarization gating technique. We predict the generation of isolated 59-attosecond-long pulses using 15-fs pump pulses with only a 0.6 TW/cm2 intensity. Our simulations also indicate the possibility to generate previously inaccessible high-harmonics with circular polarization by using an ensemble of vertically and horizontally orientated bow-tie structures. In the numerical simulation we used an extended Lewenstein model, which includes the spatial inhomogeneity in the hot spots and collisions of electrons with the metal surface.

High-power fifth-harmonic generation of femtosecond pulses in the vacuum ultraviolet using a Ti:sapphire laser.

We demonstrate the generation of fifth-harmonic pulses at 161 nm, with an energy of up to 600 nJ and 160 fs pulse duration from a Ti:sapphire laser at 1 kHz repetition rate by four-wave difference-frequency mixing in argon-filled waveguides. The efficiency is greatly improved by coupling to higher-order transverse modes, as well as by coating the inner surface of the waveguide. A numerical model of the process yields an understanding of the main effects influencing the harmonic generation.

Carrier-envelope phase stabilization with sub-10 as residual timing jitter

We demonstrate carrier-envelope phase (CEP) stabilization of a mode-locked Ti:sapphire oscillator with unprecedented timing jitter of eight attoseconds. The stabilization performance is obtained by a combination of two different stabilization approaches. In a first step the drift of the CEP is stabilized with a conventional feedback loop by means of controlling the oscillator pump power with an acousto-optic modulator (AOM). In a second step we utilize a recently developed feed-forward type stabilization scheme which has a much higher control bandwith. Here an acousto-optic frequency shifter (AOFS) produces the stabilized output in the first diffraction order. Moreover, we present numerical results on the optimization of the length of the photonic crystal fiber, which is used to generate an octave-spanning spectrum, in order to optimize the sensitivity in the f-to-2f interferometers.