J. Feldmann

Electrically controlled light scattering with single metal nanoparticles

A concept to electrically control the scattering of light is introduced. The idea is to embed noble metal nanoparticles in an electro-optical material such as a liquid crystal in order to induce a spectral shift of the particle plasmon resonance by applying an electric field. Light scattering experiments on single gold nanoparticles show that spherically shaped nanoparticles become optically spheroidal when covered by an anisotropic liquid crystal. The two particle plasmon resonances of the optically spheroidal gold nanoparticles can be spectrally shifted by up to 50 meV when electric fields of more than 10 kV/cm are applied.

Near-field optical studies of surface plasmons in single metal nanoparticles

The near-field transmission spectra for a series of individual Au particles were taken. We find that the homogeneous linewidths and the spectral positions of the surface plasmon (SP) resonance vary slightly from particle to particle and thus are not always reproducible by application of Mie theory using bulk values for the dielectric function of Au. Surface scattering events and possibly also quantum size effects might contribute to the SP lineshape opening up the exciting possibility of sensoring the nano-environment of a single Au particle.

Coherent dynamics of exciton wavepackets in semiconductor heterostructures

The coherent dynamics of exciton wavepackets in (GaIn)As/GaAs as well as (GaIn)As/Ga(PAs) multiple quantum well structures is studied by means of transient four-wave mixing (FWM) experiments. The wavepackets are generated by simultaneous excitation of several exciton transitions with laser pulses of about 100 fs duration. The time-integrated FWM signals exhibit a pronounced modulation superimposed on the overall decay which can be attributed to the quantum interference of the different eigenstates. In the time-resolved FWM signals this interference is not present, reflecting the interplay between many-body Coulomb effects and inhomogeneous broadening. This experimental technique is then employed to extract the exciton binding energies in pseudomorphic symmetrically strained (GaIn)As/GaIPAs) with various In contents.

Experimental observation of percolation-enhanced nonlinear light scattering from semicontinuous metal films

Summary form only given. In random metal-dielectric films where the metal coverage of the dielectric substrate is close to the percolation threshold, disorder-induced localization of plasmons occurs, resulting in huge fluctuations of local fields. One of the most interesting, theoretical predictions for such percolation films is that nonlinear light scattering at the nth frequency harmonic n/spl omega/ of an incident beam can be significantly enhanced and is characterized by a broad, nearly isotropic angular distribution. According to theory, this effect, which was denoted in as percolation-enhanced nonlinear scattering (PENS), is caused by the huge local-field fluctuations associated with the localized plasmons. Here we experimentally verify the existence of the PENS effect by measurements of the second harmonic generation from semicontinuous and continuous gold films.

Dark pulse formation and carrier dynamics in quantum dot lasers

Summary form only given. The large differential gain of semiconductor lasers based on self-assembled quantum dots makes these devices attractive for high speed applications. In addition it is expected that quantum dot optical amplifiers will present advantages for WDM systems. The performance of these devices is controlled by the coupled dynamics of photons and carriers. To probe the coupled dynamics in these devices, we have performed pulse propagation and pump-probe experiments on InAs/GaAs quantum dot (QD) waveguides and laser diodes.

Quantum dot laser dynamics after pulsed optical excitation

Summary form only given. The gain spectrum of semiconductor quantum dots (QDs) is inhomogeneously broadened due to inevitable dot size fluctuations. If the homogeneous broadening is small, only QDs of the same size interact with each other via the photon field and lasing can occur on many wavelengths simultaneously. In this respect, the system QD/barrier material resembles a guest/host-system, much like titanium ions in a sapphire crystal. Therefore, QD lasers might be interesting candidates for mode locking. We investigate the lasing dynamics of an optically pumped gain-switched edge emitting QD laser at room temperature. The laser structure is a Fabry-Perot resonator of length L = 1.6 mm with cleaved uncoated end facets. The active material consists of 7 layers of InAs QDs embedded in GaAs and surrounded by an AlGaAs waveguide The GaAs surrounding the QDs is optically pumped with 100 fs pulses from a regenerative amplifier at an excitation wavelength of 800 nm. Lateral gain guiding is enforced by focussing the beam to a stripe of 100 /spl mu/m width, along the full length of the laser resonator. The emission from the edge of the sample is measured both time-integrated with a germanium diode and time-resolved by upconversion.

Vibrational wave packets in metal nanoparticles

Due to their small size, metal nanoparticles are expected to show confined vibrational modes with discrete frequencies. In fact, time-resolved optical pump-probe experiments on metal nanoparticles have revealed periodic signal modulations with frequencies consistent with the lowest (n=1) vibrational breathing mode. Higher (n>1) vibrational modes have not been observed, presumably due to their smaller vibration amplitudes. The vibrations in these experiments have been argued to be excited by the thermal expansion of the particle following the lattice heating by the pump laser pulse. We show that vibrational wave packets consisting of up to five vibrational breathing modes (n=1...5) are observed in femtosecond optical pump-probe experiments on spherical silver nanoparticles. The spectral distribution of the wave packets shows that lattice heating is not the only mechanism driving the particle expansion; there is also a considerable electronic contribution.

Nano-optics with surface plasmons

Summary form only given, as follows. Near-field optical and ultrafast nonlinear optical experiments on various metal nanostructures provide important information on the dynamical properties of surface plasmons and their use in nano-optics.

An optically-driven THz-modulator

Summary form only given. Mixed type-I/type-II GaAs/AlAs double quantum wells (QW) offer a means of generating electron-hole pairs with a lifetime which can exceed 400 /spl mu/s. For the sample chosen for the measurements discussed here, electrons excited in the /spl Gamma/-valley of the narrow well are transferred to the X-valley in the barrier material and subsequently to the /spl Gamma/-valley of the wider well. The recombination rate of the spatially separated electron-hole pairs is dominated by the tunneling rate of holes from the narrow well into the wider well. This leads to an intrinsically long life-time of the spatially separated electron-hole pairs which enables large carrier densities (7/spl times/10/sup 11/ cm/sup -2/) to be excited with low optical power densities (0.07 W/cm/sup 2/). We have exploited these high carrier lifetimes and densities to build an optically controlled THz-modulator with large contrast ratios for extremely low optical power.