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Research Fellows Directory

Alexander Weigel

Mr Alexander Weigel

Research Fellow


University of Oxford

Research summary

Visible spectroscopy records a signature of the electronic structure of molecules and materials, but most experiments only report on the average behaviour of a large number of particles.

The advent of single molecule optics finally broke this limitation and allowed studies of individual particles in their specific micro-environment. Nevertheless, all current single molecule

techniques rely exclusively on the e?cient detection of ?uorescence, limiting the number of

species that can be studied to a selected few. An alternative to ?uorescence detection is absorption, as a much larger range of species absorb light e?ciently. The goal of this project is to develop a single molecule absorption spectrometer.

The main challenge is to distinguish the minuscule signal of a single molecule from the fluctuating background light, and a variety of noise suppression and contrast enhancement techniques are currently implemented to improve the sensitivity towards the single-molecule level. For illumination I constructed a white-light laser, which is not only up to one order of magnitude more stable than commercial systems, but whose output pulses can also be compressed to 10 fs duration. With such ultrashort pulses experiments are not restricted to stationary spectroscopy: Now also the vibration and relaxation of the system upon optical excitation can be directly followed and even manipulated.

As model systems for molecular spectroscopy, single semiconductor nanocrystal quantum dots are investigated. The optical properties of quantum dots can be tailored by synthesis, and make them attractive for the development of efficient solar cells. With our time resolution it will be particularly interesting to track the light-induced energy flow in such systems. The sensitivity will then be improved to the level needed for single molecule measurements. This will allow to directly study, what happens, if light emission is intermitted (‘blinking’), or completely ceases (‘bleaching’).

Interests and expertise (Subject groups)

Grants awarded

Single-Molecule Absorption Spectroscopy

Scheme: Newton International Fellowships

Dates: Jan 2012 - Jan 2014

Value: £103,250

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