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Recent transient four-wave-mixing experiments in the two-pulse photon-echo configuration performed on GaAs quantum wells have yielded the surprising result that in some samples the T2 time obtained from the polarization decay can be shorter for cross-polarized input fields than for parallel input fields. A concomitant change from photon-echo to free-induction-decay behavior has also been observed. A phenomenological model is presented which explains these observations by means of a disorder-induced coupling of the o. + and o. exciton transitions which is inhomogeneous in coupling strength. Such coupling also leads to a remarkable change in the light-holeheavy-hole exciton quantum beat phase as a function of pulse delay for a critical relative orientation of the polarization of the incident pulses. Experimental observations of this efFect are presented to support the validity of the model.

Policymakers across myriad jurisdictions are grappling with the challenge of complex policy problems. Multi‐faceted, complex, and seemingly intractable, ‘wicked’ problems have exhausted the repertoire of the standard policy approaches. In response, governments are increasingly looking for new options, and one approach that has gained significant scholarly interest, along with increasing attention from practitioners, is ‘place‐based’ solutions. This paper surveys conceptual aspects of this approach. It describes practices in comparable jurisdictions – the United Kingdom, the EU, and the United States. And it explores efforts over the past decade to ‘localise’ Indigenous services. It sketches the governance challenge in migrating from top‐down or principal‐agent arrangements towards place‐based practice. The paper concludes that many of the building blocks for this shift already exist but that these need to be re‐oriented around ‘learning’. Funding and other administrative protocols may also ultimately need to be redefined.

The bimolecular nature of exciton formation from free carriers is demonstrated experimentally in a GaAs asymmetric double-quantum-well tunneling structure. A bimolecular formation coefficient of 6X10-12 cmhigh2/ps is obtained. The electron- and hole-tunneling times are determined simultaneously under the same sample conditions

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