October 2012

Abstracts of the QSIT Lunch Seminar, October 4, 2012

Feedback cooling of cantilever motion using a quantum point contact transducer

Michele Montinaro, Poggio Lab - Spin, Quantum Electronics, Nanomechanics, University of Basel

We use a quantum point contact (QPC) as a displacement transducer to measure and control the low-temperature thermal motion of a nearby micromechanical cantilever [1]. The QPC is included in an active feedback loop designed to cool the cantilever's fundamental mechanical mode, achieving a squashing of the QPC noise at high gain. The minimum achieved effective mode temperature of 0.2 K and the displacement resolution of 10^-11 m Hz^-1/2 are limited by the performance of the QPC as a one-dimensional conductor and by the cantilever-QPC capacitive coupling

Reference:

[1] M. Montinaro, A. Mehlin, H. S. Solanki, P. Peddibhotla, S. Mack, D. D. Awschalom, and M. Poggio, Appl. Phys. Lett., in press; arXiv:1207.5006.

Origins of low quantum efficiencies in quantum dot LEDs

Deniz Bozyigit, Laboratory for Nanoelectronics, ETH Zurich

The promise for next generation light-emitting device (LED) technologies is a major driver for research on nanocrystal quantum dots (QDs). The low performance of current QD-LEDs is often attributed to luminescence quenching of charged QDs through Auger-processes. Although new QDs chemistries suppress this quenching,high performance QD-LEDs with these materials have yet to be demonstrated. Here we show that yet to be demonstrated. Here we show that even in the absence of charging, the electric field can strongly quench the luminescence of QD solids and that this quenching is highly dependent on QD bandstructure. A combination of experimental field-dependent photoluminescence decay studies on a series of CdSe/CdS QD solids and tight-binding simulations is used to explain this effect: the electric eld reduces the wavefunction overlap of electron and hole and thereby reduces the radiative recombination rate. Our findings provide clear design strategies for QD materials and device architectures to improve QD-LED performance.