Interband cascade lasers with room temperature threshold current densities below 100 A/cm(2)

Interband Cascade Lasers (ICLs) with threshold current densities below 100 A/cm(2) in pulsed operation at room temperature are presented. The laser structure comprises 10 active stages of 41 nm length, each stage containing a W-quantum well active region for emission in the spectral region around 3.6 mu m. A comparison of devices with 6 and 10 stages shows that the latter have a reduced threshold due to an increased optical confinement factor, very competitive threshold power densities of 428 W cm(-2) despite an increased threshold voltage and large differential slope efficiencies of 1390 mW/A. For a narrow ridge device, continuous wave operation is observed up to 65 degrees C.Publisher PDFPeer reviewe

Carbon emissions pinch analysis (CEPA) for emissions reduction in the New Zealand electricity sector

Carbon Emissions Pinch Analysis (CEPA) is a recent extension of traditional thermal and mass pinch analysis to the area of emissions targeting and planning on a macroscale (i.e. economy wide). This paper presents a carbon pinch analysis of the New Zealand electricity industry and illustrates some of the issues with realising meaningful emissions reductions. The current large proportion of renewable generation sources (~67% in 2007) complicates wholesale emissions reductions. The biggest growth in renewable generation is expected to come from geothermal energy followed by wind and hydro. A four fold increase in geothermal generation capacity is needed in addition to large amounts of new wind generation to reduce emissions to around 1990 levels and also meet projected demand. The expected expansion of geothermal generation in New Zealand raises issues of GHG emissions from the geothermal fields. The emissions factors between fields can vary by almost two orders of magnitude making predictions of total emissions highly site specific

Experimental verification of the very strong coupling regime in a GaAs quantum well microcavity

When the coupling between light and matter becomes comparable to the energy gap between different excited states they hybridize, leading to the appearance of a rich and complex phenomenology which attracted remarkable interest in recent years. While the mixing between states with different number of excitations, so-called ultrastrong coupling regime, has been observed in various implementations, the effect of the hybridization between different single excitation states, referred to as very strong coupling regime, has remained elusive. In semiconductor quantum wells such a regime is predicted to manifest as a photon-mediated electron-hole coupling leading to different excitonic wavefunctions for the two polaritonic branches when the ratio of the coupling strength to exciton binding energy approaches unity. Here, we verify experimentally the existence of this regime in magneto-optical measurements on a microcavity with 28 GaAs quantum wells, showing that the average electron-hole separation of the upper polariton is significantly increased compared to the bare quantum well exciton Bohr radius. This manifests in a diamagnetic shift around zero detuning that exceeds the shift of the lower polariton by one order of magnitude and the bare quantum well exciton diamagnetic shift by a factor of two. The lower polariton exhibits a diamagnetic shift smaller than expected from the coupling of a rigid exciton to the cavity mode which suggests more tightly bound electron-hole pairs than in the bare quantum well

Nonlinear spectroscopy of exciton-polaritons in a GaAs-based microcavity

We present a systematic investigation of two-photon excitation processes in a GaAs-based microcavity in the strong-coupling regime. We observe second harmonic generation resonant to the upper and lower polariton level, which exhibits a strong dependence on the photonic fraction of the corresponding polariton. In addition we have performed two-photon excitation spectroscopy to identify $2p$ exciton states which are crucial for the operation as a terahertz lasing device, which was suggested recently [A. V. Kavokin et al., Phys. Rev. Lett. \textbf{108}, 197401 (2012)]. However, no distinct signatures of a $2p$ exciton state could be identified, which indicates a low two-photon pumping efficiency

Zero Dimensional Polariton Laser in a Sub-Wavelength Grating Based Vertical Microcavity

Semiconductor exciton-polaritons in planar microcavities form coherent two-dimensional condensates in non-equilibrium. However, coupling of multiple lower-dimensional polariton quantum systems, critically needed for polaritonic quantum device applications and novel cavity-lattice physics, has been limited due to the conventional cavity structures. Here we demonstrate full confinement of the polaritons non-destructively using a hybrid cavity made of a single-layer sub-wavelength grating mirror and a distributed Bragg reflector. Single-mode polariton lasing was observed at a chosen polarization. Incorporation of a designable slab mirror into the conventional vertical cavity, when operating in the strong-coupling regime, enables confinement, control and coupling of polariton gasses in a scalable fashion. It may open a door to experimental implementation of polariton-based quantum photonic devices and coupled cavity quantum electrodynamics systems.Comment: http://www.nature.com/lsa/journal/v3/n1/full/lsa201416a.htm

Minimising carbon emissions and energy expended for the New Zealand transport sector through to 2050

Carbon Emissions Pinch Analysis (CEPA) and Energy Return on Energy Investment (EROI) analysis are combined to investigate the feasibility of New Zealand (NZ) reaching a 1990 emission levels for transport in 2050. The transportation sector traditionally has been a difficult area to transition to high levels of renewable energy because of the strong dependency on fossil fuels. Multiple scenarios for reducing transport emissions are analysed. With NZ’s unique mix of renewable energy resources the analysis demonstrates that NZ is in a very good position to sustainably meet their future transport needs provided substantial commitment is made to transition light vehicle fleet to hybrid vehicles, plug-in hybrids vehicles and electric vehicles by 2050. Electrification of rail within and between major centres will also require major political commitment. The resulting increase in electricity demand for transport is 3.6 TWh (or 4.8 % of electricity generation in NZ). We show the minimum amount of biofuel renewable production to achieve the goal of 1990 emissions level in 2050 is 46 PJ. Delivering 46 PJ is expected to be well within the potential biofuel production capacity of NZ. The delivery of economically competitive renewable liquid biofuels will also require close cooperation and system integration with other energy systems like the electricity sector and industrial process heat sector

Single photon emission of a charge-tunable GaAs/Al0.25Ga0.75As droplet quantum dot device

This work was financially supported by the Bundesministerium fűr Bildung und Forschung (BMBF) within the Project “QuaHL-Rep” under Contract No. (FKZ) 01BQ1042.In this work, we report the fabrication of a charge-tunable GaAs/Al0.25Ga0.75As quantum dot (QD) device containing QDs deposited by modified droplet epitaxy producing almost strain and composition gradient free QDs. We obtained a QD density in the low 109 cm-2 range that enables us to perform spectroscopy on single droplet QDs showing linewidths as narrow as 40 μeV. The integration of the QDs into a Schottky diode allows us to controllably charge a single QD with up to four electrons, while non-classical photoluminescence is proven by photon auto-correlation measurements showing photon-antibunching (g(2)(0) = 0.05).Publisher PDFPeer reviewe