Markus Betz

Electrical control of interdot electron tunneling in a double InGaAs quantum-dot nanostructure.

We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5  ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8  meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system.

Ultrafast control of grating-assisted light coupling to surface plasmons

We demonstrate subpicosecond control over the coupling of free-space radiation to surface-plasmon polaritons using 830 and 500 nm period gold gratings. Thermal changes to the electron distribution following irradiation by 100 fs, 810 nm pulses produce a shift of the 570 nm plasmon resonance by approximately 0.75 nm with reflectivity change up to 6% and decay time of approximately 1 ps.

Imaging the spin Hall effect of light inside semiconductors via absorption

The opposite transverse shifts for the right and left circular polarization components of a 100 fs 820 nm linearly polarized pulse focused onto GaAs are observed in situ via absorption. A time-delayed normally incident probe pulse scanned across the excitation spot detects the differential circular dichroism associated with the pump-induced transfer of spin angular momentum from light to electrons. More generally, we show that for a nonnormally incident probe, one can observe the spin Hall effect for probe light through a variety of pump-induced changes to a materials optical properties.

All-optical phase locking of two femtosecond Ti:sapphire lasers: a passive coupling mechanism beyond the slowly varying amplitude approximation

Two independently tunable femtosecond Ti:sapphire lasers are passively synchronized with a stable relative carrier-envelope offset phase. By heterodyning the spectral overlap of the two frequency combs, we observe multiple regimes for the cavity length difference in which the relative round-trip phase slip is effectively locked to zero. The strong correlation of the femtosecond pulse trains is maintained over minutes without any external stabilization, and relative cavity length variations of 50 nm are compensated. The phase synchronization relies on phase-dependent cross-phase modulation, taking full advantage of the nonresonant optical nonlinearity of the shared gain medium, which is much faster than the optical cycle.

Coherently controlled ballistic currents in single-walled carbon nanotubes and graphite

Ballistic electrical currents are optically injected into aligned single-walled carbon nanotubes and bulk graphite at 300 K via quantum interference between single and two photon absorption of phase-related 700 and 1400 nm, 150 fs pulses. The transient currents are detected via the emitted terahertz radiation. Optical phase and power dependence are consistent with the quantum interference optical process. Under similar excitation conditions, the peak current for a forest of nanotubes, with a diameter distribution of approximately 2.5 +/- 1.5 nm, is 9 +/- 1 times larger than that in graphite. At peak focused intensities of 10 GW cm(-2) (1400 nm) and 0.15 GW cm(-2) (700 nm), the peak current is approximately 1 nA per nanotube. The peak current for pump light polarized along the tubes is approximately 3.5 times higher than that for light polarized perpendicular to the tubes.

Spin-preserving ultrafast carrier capture and relaxation in InGaAs quantum dots

Carrier capture into self-organized InGaAs∕GaAs quantum dots with an electronic level spacing close to optical phonon energies is studied in a two-color femtosecond transmission experiment. After resonant generation of carriers in the wetting layer, we analyze the population of both the band edge of the wetting layer and the excited states of the quantum dots. Most strikingly, the carrier capture time of 3 ps is found to be independent of the carrier density, providing that it remains small compared to the number of available electronic states in the quantum dots. Moreover, we find that the capture process is predominantly spin preserving in nature. These results suggest that phonon-mediated scattering governs the quantum dot filling.

Single-beam differential z-scan technique

We report a single-beam, differential z-scan technique with improved sensitivity for the determination of nonlinear absorption and refraction of materials. A sample is scanned in the direction of beam propagation as usual, but, in addition, its longitudinal position is dithered, producing a detector signal proportional to the spatial derivative of only the nonlinear transmission and therefore giving a background-free signal; the nonlinear transmission for any spatial position of the sample can be recovered by simple integration. For both open and closed aperture scans in GaP, we find an improvement in the signal-to-noise ratio of >5 x compared with a balanced z-scan setup, but this can be improved with apparatus optimization. Nonlinear phase distortions

High-fidelity optical preparation and coherent Larmor precession of a single hole in an (In,Ga)As quantum dot molecule

We employ ultrafast pump-probe spectroscopy with photocurrent readout to directly probe the dynamics of a single hole spin in a single, electrically tunable self-assembled quantum dot molecule formed by vertically stacking InGaAs quantum dots. Excitons with defined spin configurations are initialized in one of the two dots using circularly polarized picosecond pulses. The time-dependent spin configuration is probed by the spin selective optical absorption of the resulting few Fermion complex. Taking advantage of sub-5 ps electron tunneling to an orbitally excited state of the other dot, we initialize a single hole spin with a purity of >96%, i.e., much higher than demonstrated in previous single dot experiments. Measurements in a lateral magnetic field monitor the coherent Larmor precession of the single hole spin with no observable loss of spin coherence within the ~300 ps hole lifetime. Thereby, the purity of the hole spin initialization remains unchanged for all investigated magnetic fields.

Optically imprinted reconfigurable photonic elements in a VO2 nanocomposite

We investigate the optical and thermal hysteresis of single-domain vanadium dioxide nanocrystals fabricated by ion beam synthesis in a fused silica matrix. The nanocrystals exhibit a giant hysteresis, which permits to optically generate a long-time stable supercooled metallic phase persistent down to practically room temperature. Spatial patterns of supercooled and insulating nanocrystals feature a large dielectric contrast, in particular, for telecom wavelengths. We utilize this contrast to optically imprint reconfigurable photonic elements comprising diffraction gratings as well as on- and off-axis zone plates. The structures allow for highly repetitive (>104) cycling through the phase transition without structural damage.

Quantum Interference Control of Femtosecond, μA Current Bursts in Single GaAs Nanowires

A phase-stable superposition of femtosecond pulses from a compact erbium-doped fiber source and their second harmonic is shown to induce ultrashort approximately microA current bursts in single unbiased GaAs nanowires. Current injection relies on a quantum interference of one- and two-photon absorption pathways. The vector direction of the current is solely dictated by the polarization and relative phase of the harmonically related light components while its power dependence is consistent with a third order optical nonlinearity.