Observation of Nondegenerate Two-Photon Gain in GaAs

Two-photon lasers require materials with large two-photon gain (2PG) coefficients and low linear and nonlinear losses. Our previous demonstration of large enhancement of two-photon absorption in semiconductors for very different photon energies translates directly into enhancement of 2PG. We experimentally demonstrate nondegenerate 2PG in optically excited bulk GaAs via femtosecond pump-probe measurements. 2PG is isolated from other pump induced effects through the difference between measurements performed with parallel and perpendicular polarizations of pump and probe. An enhancement in the 2PG coefficient of nearly two orders-of-magnitude is reported. The results point a possible way toward two-photon semiconductor lasers.Comment: 5 pages, 5 figure

Ultrafast extrinsic spin-Hall currents

We consider the possibility of ultrafast extrinsic spin-Hall currents, generated by skew scattering following the optical injection of charge or pure spin currents. We propose a phenomenological model for this effect in quantum well structures. An injected charge current leads to a spin-Hall-induced pure spin current, and an injected pure spin current leads to a spin-Hall-induced charge current. The resulting spin or charge accumulation can be measured optically.Comment: 18 pages, 3 figure

Two-photon spin injection in semiconductors

A comparison is made between the degree of spin polarization of electrons excited by one- and two-photon absorption of circularly polarized light in bulk zincblende semiconductors. Time- and polarization-resolved experiments in (001)-oriented GaAs reveal an initial degree of spin polarization of 49% for both one- and two-photon spin injection at wavelengths of 775 and 1550 nm, in agreement with theory. The macroscopic symmetry and microscopic theory for two-photon spin injection are reviewed, and the latter is generalized to account for spin-splitting of the bands. The degree of spin polarization of one- and two-photon optical orientation need not be equal, as shown by calculations of spectra for GaAs, InP, GaSb, InSb, and ZnSe using a 14x14 k.p Hamiltonian including remote band effects. By including the higher conduction bands in the calculation, cubic anisotropy and the role of allowed-allowed transitions can be investigated. The allowed-allowed transitions do not conserve angular momentum and can cause a high degree of spin polarization close to the band edge; a value of 78% is calculated in GaSb, but by varying the material parameters it could be as high as 100%. The selection rules for spin injection from allowed-allowed transitions are presented, and interband spin-orbit coupling is found to play an important role.Comment: 12 pages including 7 figure

A Single Charged Quantum Dot in a Strong Optical Field: Absorption, Gain, and the AC Stark Effect

We investigate a singly-charged quantum dot under a strong optical driving field by probing the system with a weak optical field. When the driving field is detuned from the trion transition, the probe absorption spectrum is shifted from the trion resonance as a consequence of the dynamic Stark effect. Simultaneously, a gain sideband is created, resulting from the coherent energy transfer between the optical fields through the quantum dot nonlinearity. As the pump detuning is moved from red to blue, we map out the anticrossing of these two spectral lines. The optical Bloch equations for a stationary two-level atom can be used to describe the numerous spectral features seen in this nano solid state system

Parades, parties and pests: contradictions of everyday life in peacekeeping economies

Based on research studies conducted in the UN peacekeeping mission in Liberia in 2006, 2012 and 2013, this article argues that peacekeepers’ everyday experiences reflect a series of contradictory identities and performances with regard to nation, work and gender. Peacekeepers straddle paradoxical worlds simultaneously and manage oppositional demands and obligations, although it is often assumed that they inhabit peacekeeping economies in homogenous ways. Importantly, the experiences provide opportunities for peacekeepers to invest in, accumulate and deploy military capital; to consolidate their military identities; and to favourably and tactically position themselves as deserving and useful subjects within the peacekeeping landscape

Interaction-induced effects in the nonlinear coherent response of quantum-well excitons

Interaction-induced processes are studied using the third-order nonlinear polarization created in polarization-dependent four-wave-mixing experiments (FWM) on a ZnSe single quantum well. We discuss their influence by a comparison of the experimental FWM with calculations based on extended optical Bloch equations including local-field effects, excitation-induced dephasing, and biexciton formation. The investigations show that, for copolarized input fields, excitation-induced dephasing is the dominant FWM mechanism, followed by the conventional density-grating FWM process, biexcitonic contributions, and local-field effects. For cross-linear polarized input fields the excitation-induced dephasing mechanism is canceled so that the conventional density-grating FWM process and biexcitonic contributions are dominating

Circulating power: humanitarian logistics, militarism and the United Arab Emirates

While critical authors have interrogated the roots of business logistics, this paper extends the analysis and contributes to a larger critique of the cohering field of Humanitarian Logistics (HL), noting the overlap in the logistical cartographies of militarism and humanitarianism. The focus is on the UAE’s expanding logistics space into the Horn of Africa and the production of specialised HL zones like Dubai International Humanitarian City (DIHC). The article makes three core arguments. First, a logistics lens enables us to expand the study of aid beyond immediate conflict zones, into more distant spaces often constructed as ‘stable’. Second, the placement of logistics at the core of aid delivery has been a key mechanism for inserting market imperatives into humanitarian activities. Third, this gives countries outside the advanced core, such as the UAE, power to leverage and expand their logistics space for multiple purposes, in war making, aid, and commercial activities