C R Hall

Three-dimensional electronic spectroscopy of excitons in asymmetric double quantum wells

We demonstrate three-dimensional (3D) electronic spectroscopy of excitons in a double quantum well system using a three-dimensional phase retrieval algorithm to obtain the phase information that is lost in the measurement of intensities. By extending the analysis of two-dimensional spectroscopy to three dimensions, contributions from different quantum mechanical pathways can be further separated allowing greater insight into the mechanisms responsible for the observed peaks. By examining different slices of the complete three-dimensional spectrum, not only can the relative amplitudes be determined, but the peak shapes can also be analysed to reveal further details of the interactions with the environment and inhomogeneous broadening. We apply this technique to study the coupling between two coupled quantum wells, 5.7 nm and 8 nm wide, separated by a 4 nm barrier. Coupling between the heavy-hole excitons of each well results in a circular cross-peak indicating no correlation of the inhomogeneous broadening. An additional cross-peak is isolated in the 3D spectrum which is elongated in the diagonal direction indicating correlated inhomogeneous broadening. This is attributed to coupling of the excitons involving the two delocalised light-hole states and the electron state localised on the wide well. The attribution of this peak and the analysis of the peak shapes is supported by numerical simulations of the electron and hole wavefunctions and the three-dimensional spectrum based on a density matrix approach. An additional benefit of extending the phase retrieval algorithm from two to three dimensions is that it becomes substantially more reliable and less susceptible to noise as a result of the more extensive use of a priori information.

Using graded barriers to control the optical properties of ZnO/Zn0.7Mg0.3O quantum wells with an intrinsic internal electric field

Australian Research Council is gratefully acknowledged for financial support. C.R.H. thanks Lastek for financial support.

Doping-enhanced radiative efficiency enables lasing in unpassivated GaAs nanowires

Nanolasers hold promise for applications including integrated photonics, on-chip optical interconnects and optical sensing. Key to the realization of current cavity designs is the use of nanomaterials combining high gain with high radiative efficiency. Until now, efforts to enhance the performance of semiconductor nanomaterials have focused on reducing the rate of non-radiative recombination through improvements to material quality and complex passivation schemes. Here we employ controlled impurity doping to increase the rate of radiative recombination. This unique approach enables us to improve the radiative efficiency of unpassivated GaAs nanowires by a factor of several hundred times while also increasing differential gain and reducing the transparency carrier density. In this way, we demonstrate lasing from a nanomaterial that combines high radiative efficiency with a picosecond carrier lifetime ready for high speed applications.

Isolating quantum coherence using coherent multi-dimensional spectroscopy with spectrally shaped pulses

Coherent coupling between spatially separated systems has long been explored as a necessary requirement for quantum information and cryptography[1]. Recent discoveries suggest such phenomena appear in a much wider range of processes, including light-harvesting in photosynthesis[2–4]. These discoveries have been facilitated by developments in coherent multi-dimensional spectroscopy (CMDS) [5–8] that allow interactions between different electronic states to be identified in crowded spectra. For complex systems, however, spectral broadening and multiple overlapping peaks limit the ability to separate, identify and properly analyse all contributions[9, 10]. Here we demonstrate how pathwayselective CMDS can overcome these limitations to reveal, isolate and allow detailed analysis of weak coherent coupling between spatially separated excitons localised to different semiconductor quantum wells. Selective excitation of the coherence pathways, by spectrally shaping the laser pulses, provides access to previously hidden details and enables quantitative analysis that can facilitate precise and detailed understanding of interactions in this and other complex systems.

Phase-matched generation of highly coherent radiation in water window region.

Highly coherent extreme ultraviolet radiation around the water window region (~4.4 nm) is generated in a semi-infinitive helium gas cell using infrared pulses of wavelength 1300 nm, energy 2.5 mJ, duration 40 fs, and repetition rate 1 kHz. The pressure-squared dependence of the intensity and the almost-perfect Gaussian profile and low divergence of the high harmonic source indicate a phase-matched generation process. The spatial coherence of the source is studied using Youngs double-slit measurements.

High-order harmonic generation from a dual-gas, multi-jet array with individual gas jet control.

We present a gas jet array for use in high-order harmonic generation experiments. Precise control of the pressure in each individual gas jet has allowed a thorough investigation into mechanisms contributing to the selective enhancement observed in the harmonic spectra produced by dual-gas, multi-jet arrays. Our results reveal that in our case, the dominant enhancement mechanism is the result of a compression of the harmonic-producing gas jet due to the presence of other gas jets in the array. The individual control of the gas jets in the array also provides a promising method for enhancing the harmonic yield by precise tailoring of the length and pressure gradient of the interaction region.

Generation and Application of Coherent Radiation in the Water Window

Coherent extreme ultraviolet radiation around the water window region (∼4.4 nm) is generated by using an infrared driving pulses at 1400 nm, with energy 2.5 mJ and duration 40 fs in a semi-infinitive helium gas cell. The beam profile and spatial coherence characteristics of the source in this region are shown to be excellent, enabling this source to be used for coherent diffractive imaging.

Diffusion and population dynamics of excitons in c-axis grown ZnO quantum wells

In this work a transient grating experiment was used to explore the inplane transport properties and two-colour pump-probe and time resolved photolu-minescence experiments were used to explore the population dynamics of excitons in ZnO quantum wells. By implementing stepped barriers in such quantum wells we also show that the overlap of the electron and hole wavefunctions can be controlled.

Dynamics of carriers and the influence of the quantum confined stark effect in ZnO/ZnMgO quantum wells

We reveal the dynamics of carrier-induced screening of the internal electric field in ZnO quantum wells. By controlling the potential profile of the quantum wells we demonstrate the ability to tune the excited state lifetimes.

Observation of spatially separated coherent coupling near the LO phonon resonance within asymmetric double quantum wells

We examine the dynamics of coherently coupled heavy hole excitons localized within spatially separated quantum wells with an energy difference equal to the LO phonon energy.