F. Sotier

Attosecond relative timing jitter and 13 fs tunable pulses from a two-branch Er:fiber laser

We present what is believed to be the first direct measurement of the relative timing jitter between the two parallel pulse trains of a two-branch femtosecond erbium-doped fiber laser, operated without active stabilization. The system provides independently tunable pulses in the near infrared with durations down to 13 fs. Using an interferometric optical cross-correlator, the phase-noise spectral density is measured with high sensitivity in a range from 1 Hz up to the Nyquist frequency of 24.5 MHz. We find an integrated jitter of 11 attoseconds directly after the amplifier stages and 43 as after propagation through free-space optics and nonlinear fibers for frequency conversion.

Multimilliwatt ultrashort pulses continuously tunable in the visible from a compact fiber source

We report on a single-pass device that efficiently converts the broadband near-infrared output from a femtosecond fiber laser into a narrow spectrum in the visible. With fan-out poled MgO:LiNbO3 we obtain sub-picosecond, continuously tunable pulses in the 520-700 nm range. Conversion efficiencies as high as 30% are observed at typical pump power levels of 30 mW, corresponding to average output powers up to 9.5 mW. The specifications of our device are ideal for applications in confocal microscopy and frequency metrology.

Highly efficient second, third and fourth harmonic generation from a two-branch femtosecond erbium fiber source

We report on highly efficient second, third and fourth harmonic generation from a femtosecond erbium-doped fiber source operating at 98 MHz repetition rate. By use of quasi-phase-matching in fan-out poled MgO:LiNbO(3), we generate pulses at 770 nm, 520 nm and 390 nm, with corresponding average powers of 120 mW, 55 mW and 6 mW, respectively. Our device can be employed as a two-color source providing radiation from ultraviolet to near infrared.

Ultrafast spin dynamics in colloidal ZnO quantum dots

We perform time-resolved Faraday rotation measurements on colloidal ZnO quantum dots. A biexponential decay of the dephasing time T2* of the electron spins governed by competing recombination processes is observed.

Colloidal ZnO quantum dots in ultraviolet pillar microcavities

Three dimensional light confinement and distinct pillar microcavity modes in the ultraviolet have been observed in pillar resonators with embedded colloidal ZnO quantum dots fabricated by focused ion beam milling. Results from a waveguide model for the mode patterns and their spectral positions are in excellent agreement with the experimental data.

Ultrafast spectroscopy of impact ionization and avalanche multiplication in GaAs

Femtosecond carrier dynamics in biased AlxGa1−xAs heterostructure diodes is investigated tracing transient modifications of the Franz-Keldysh absorption spectrum. The nonlinear optical response is sensitive to the number of electron-hole pairs in the high-field region of the sample. As a result, the dynamical buildup of a nonequilibrium carrier avalanche due to impact ionization for electric fields F⩾350kV∕cm is directly analyzed in the time domain. The time scale of the carrier multiplication is found to be in the order of 10ps depending on the number of photoinjected carriers. Monte Carlo simulations in a simplified band structure agree well with the experiment.

Femtosecond spectroscopy of unipolar nanometer-scale high-field transport of holes in Al0.08Ga0.92As

High-field transport in GaAs is investigated tracing ultrafast modifications of the Franz–Keldysh absorption spectrum of a AlxGa1−xAs heterostructure diode. A sophisticated sample design allows us to isolate the unipolar tranport properties of holes in combination with a nanometer scale definition of layers for both photoexcitation and detection of the propagating carrier distribution. Transient velocities and spatial broadening of the hole ensemble are directly measured for electric fields between 15 and 200 kV∕cm comparing room temperature operation to results for TL=4K. Even at low temperatures, the transient hole velocities are found not to exceed a value of 1.2×107cm∕s which is a result of ultrafast optical phonon emission with a scattering time below 25 fs.

Femtosecond probing of few-fermion dynamics and deterministic single-photon gain in a single semiconductor quantum dot

We report on femtosecond readout of the optical properties of a single CdSe/ZnSe quantum dot. Owing to the uncertainty principle, this timescale represents the ultimate limit for coherent quantum manipulation of such an artificial atom. After resonant excitation of a hot electron-hole pair the absorption of the fundamental exciton resonance is switched off via instantaneous Coulomb renormalization. Subsequently, optical gain builds up after ultrafast intraband relaxation. The speed of thermalization is dominated by the electron spin, since our system is charged permanently with one excess electron. When operating in the nonlinear regime, the number of quanta in a femtosecond light pulse may be changed by exactly ±1. We demonstrate that this deterministic single photon amplifier is characterized by a flat gain spectrum.

Metal nanoantennas and dielectric microresonators for solid-state quantum optics

The efficient coupling of light from the far field into nanometer sized objects is one of the fundamental challenges in current nanooptics. We present two ideas to reach this goal. On the one hand, we place the nanoobject (in our case a colloidal semiconductor quantum dot) into an optical micropillar resonator (Fig. 1a). Three-dimensional light confinement is observed and we calculate the cavity modes of resonators with circular and elliptical cross section by modeling the pillar microcavity as a waveguide with an effective refractive index [1]. The concept is extended into the ultraviolet region of the electromagnetic spectrum and the first dielectric pillar microcavity with colloidal ZnO quantum dots is demonstrated [2]. On the other hand, we fabricate metal optical nanoantennas by various techniques (electron-beam lithography, focused ion beam milling, and colloidal masks). In particular, we present a tunable bowtie optical nanoantenna [3], which consists of two gold nanotriangles (Fig. 1b). The feedgap can be continuously varied by manipulation of an antenna arm with nanometer precision via an atomic force microscope. At the same time the optical response of the nanoantenna is determined via darkfield scattering spectroscopy. Exciting with ultrafast laser pulses, we investigate the nonlinear optical properties of single metal nanoantennas.

Femtosecond few-fermion dynamics and deterministic single photon gain in a semiconductor quantum dot

Semiconductor quantum dots are promising systems for robust and scalable quantum information processing. Ultrafast sequences of coherent quantum operations may be envisioned with femtosecond light pulses, if the involved quantum states are separated by at least tens of meV. Therefore, small quantum dots with high confinement potentials are favourable. Due to their large Coulomb correlation energies, CdSe quantum dots are ideal candidates.