On optical characterization of CdSe/CdS dot/rod heterostructures.

Abstract

Colloidal semiconductor nanocrystals (NCs) hold great promise as optical gain media due to their inexpensive fabrication methods, tunable emission wavelengths and high photoluminescence quantum yield. In particular, core-shell nanorods like CdSe/CdS have attracted a large research interest due to improved optical performance over traditional colloidal quantum dots. This is due to spatial separation of electron and hole wavefunctions in these kind of quasi-Type II nanocrystals. This charge separation can lead to a repulsive excition-exciton interaction and suppressed Auger recombination rates, allowing a longer optical gain lifetime.

A key experimental technique widely used in measuring the optical gain of semiconductor materials is the variable stripe length (VSL) method. The thin film sample is optically pumped with a stripe shaped beam of variable length, and the intensity of the edge emitted amplified spontaneous emission (ASE) is measured as a function of the stripe length. By fitting an appropriate equation, the optical gain can be obtained. However, there are some crucial issues in applying this method due to the assumptions in the underlying one dimensional optical amplifier model.In our work, the new method of measuring optical gain is proposed and experimentally tested. The same sample is pumped using a focused beam and the lens is moved parallel to the beam propagation to change the spot size on the sample. By measuring the pump fluence at ASE threshold as a function of the spot size, and fitting an appropriate equation, we are able to obtain unambiguous gain values and avoid the problems associated with the traditional VSL method.