Jacob R. Jensen

Optical properties of InAlGaAs quantum wells: Influence of segregation and band bowing

Knowledge of the quaternary InAlGaAs material system is very limited for the composition range relevant for growth on GaAs substrates. We report on the characterization and modeling of InAlGaAs quantum wells with AlGaAs barriers, grown pseudomorphically on a GaAs substrate with molecular beam epitaxy. The quantum wells are characterized with photoluminescence, and the measured transition energies are modeled taking into account the influence of In segregation on the shape of the well potential. From the modeling we deduce a relation for the low temperature band gap of unstrained Inx(AlyGa1−y)1−xAs, for 0⩽x,y⩽0.20. The measured linewidths of the luminescence peaks are in agreement with the broadening expected from random alloy fluctuations and well width fluctuations with an effective interface roughness of 1.1 ML.

FIBER COUPLED ULTRAFAST SCANNING TUNNELING MICROSCOPE

We report on a scanning tunneling microscope with a photoconductive gate in the tunneling current circuit. The tunneling tip is attached to a coplanar transmission line with an integrated photoconductive switch. The switch is illuminated through a fiber which is rigidly attached to the switch substrate. By using a firmly attached fiber we achieve an excellent reproducibility and unconstrained positioning of the tip. We observe a transient signal with 2.9 ps pulse width in tunneling mode and 5 ps in contact mode. The instrument is applied to investigating the mode structure on a coplanar waveguide. The measurements show that the probe works as a transient voltage detector in contact and a capacitively coupled transient field detector in tunneling mode. We do not measure the transient voltage change in the ohmic tunneling current. In this sense, the spatial resolution for propagating electrical pulses is better in contact mode than in tunneling mode.

Linewidth statistics of single InGaAs quantum dot photoluminescence lines

We have used photoluminescence spectroscopy with high spatial and spectral resolution to measure the linewidths of single emission lines from In0.5Ga0.5As/GaAs self-assembled quantum dots. At 10 K, we find a broad, asymmetric distribution of linewidths with a maximum at 50 μeV. The distribution of linewidths is not significantly influenced by small variations in the quantum dot confinement potential. We claim that the wider transition lines are broadened by local electric field fluctuations while narrower lines are homogeneously broadened by acoustic-phonon interactions. The width of narrow single-dot luminescence lines depends only weakly on temperature up to 50 K, showing a broadening of 0.4 μeV/K. Above 50 K, a thermally activated behavior of the linewidth is observed. This temperature dependence is consistent with the discrete energy level structure of the dots.

Ultranarrow polaritons in a semiconductor microcavity

We have achieved a record high ratio (19) of the Rabi splitting (3.6 meV) to the polaritonlinewidth (190 μeV), in a semiconductor λ microcavity with a single 25 nm GaAsquantum well at the antinode. The narrow polariton lines are obtained with a special cavity design which reduces the exciton broadening due to scattering with free charges and has a very low spatial gradient of the cavity resonance energy. Since the static quantum-well disorder is very small, the polariton broadening is dominantly homogeneous. Still, the measured linewidths close to zero detuning cannot be correctly predicted using the linewidth averaging model.

MEASURING VOLTAGE TRANSIENTS WITH AN ULTRAFAST SCANNING TUNNELING MICROSCOPE

We use an ultrafast scanning tunneling microscope to resolve propagating voltage transients in space and time. We demonstrate that the previously observed dependence of the transient signal amplitude on the tunneling resistance was only caused by the electrical sampling circuit. With a modified circuit, where the tunneling tip is directly connected to the current amplifier of the scanning tunneling microscope, this dependence is eliminated. All results can be explained with coupling through the geometrical capacitance of the tip-electrode junction. By illuminating the current-gating photoconductive switch with a rigidly attached fiber, the probe is scanned without changing the probe characteristics.

Transient measurements with an ultrafast scanning tunneling microscope on semiconductor surfaces

We demonstrate the use of an ultrafast scanning tunneling microscope on a semiconductor surface. Laser-induced transient signals with 1.8 ps rise time are detected. The investigated sample is a low-temperature grown GaAs layer placed on a sapphire substrate with a thin gold layer that serves as a bias contact. For comparison, the measurements are performed with the tip in contact to the sample as well as in tunneling above the surface. In contact and under bias, the transient signals are identified as a transient photocurrent. An additional signal is generated by a transient voltage induced by the nonuniform carrier density created by the absorption of the light (photo Dember effect). The transient depends in sign and in shape on the direction of optical excitation. This signal is the dominating transient in tunneling mode. The signals are explained by a capacitive coupling across the tunneling gap.

SPATIO-TEMPORAL IMAGING OF VOLTAGE PULSES WITH AN ULTRAFAST SCANNING TUNNELING MICROSCOPE

Measurements on an ultrafast scanning tunneling microscope with simultaneous spatial and temporal resolution are presented. We show images of picosecond pulses propagating on a coplanar waveguide and resolve their mode structures. The influence of transmission line discontinuities on the mode structure is investigated. It is also demonstrated how common and differential modes of electrical pulses are generated. The capacitive coupling between the tip and the transmission line is explained in terms of two contributions: a long range and a local coupling. We also show how these contributions affect the imaging of the propagating pulses.

Second-harmonic imaging of semiconductor quantum dots

Resonant second-harmonic generation is observed at room temperature in reflection from self-assembled InAlGaAs quantum dots grown on a GaAs (001) substrate. The detected second-harmonic signal peaks at a pump wavelength of ∼885 nm corresponding to the quantum-dot photoluminescence maximum. In addition, the second-harmonic spectrum exhibits another smaller but well-pronounced peak at 765 nm not found in the linear experiments. We attribute this peak to the generation of second-harmonic radiation in the AlGaAs spacer layer enhanced by the local symmetry at the quantum-dot interface. We further observe that second-harmonic images of the quantum-dot surface structure show wavelength-dependent spatial variations. Imaging at different wavelength is used to demonstrate second-harmonic generation from the semiconductor quantum dots.

Femtosecond tunneling response of surface plasmon polaritons

We obtain femtosecond (200 fs) time resolution using a scanning tunneling microscope on surface plasmon polaritons (SPPs) generated by two 100 fs laser beams in total internal reflection geometry. The tunneling gap dependence of the signal clearly indicates the tunneling origin of the signal and suggests that nanometer spatial resolution can be obtained together with femtosecond temporal resolution. This fast response, in contrast to the picosecond decay time of SPPs revealed by differential reflectivity measurements, can be attributed to a coherent superposition of SPPs rectified at the tunneling junction.

Pumping wavelength dependence of super continuum generation in photonic crystal fibers

Generation of wide super continua several hundred nanometers wide is possible by pumping in the normal dispersion regime as the nonlinear effects broaden the spectra beyond the zero-dispersion wavelength. When pumping far away from the zero-dispersion wavelength, only self phase modulation (SPM) and Raman scattering contributes to the broadening leading to smooth stable spectra of limited width. Pumping closer to the zero-dispersion wavelength forms very broad spectra, but they are far more sensitive to fluctuations in pumping power and coupling efficiency between fiber and laser. The flattest and broadest spectra are achieved by pumping just below zero-dispersion wavelength and increase in power leads to flatter and broader spectrum. Furthermore, pumping close to the zero-dispersion wavelength enables generation of spectral components at shorter wavelengths below 750 nm through a four-wave mixing process.