Direct observation of highly polarized non-linear absorption dipole of single semiconductor quantum rods
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Direct observation of highly polarized non-linear absorption dipole of single semiconductor quantum rods Eli Rothenberg, Yuval Ebenstein, Miri Kazes, and Uri Banin Institute of Chemistry, the Farkas Center for Light Induced Processes and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
ABSTRACT Polarization fluorescence microscopy was used to study the nature of the emission and nonlinear absorption dipole of single CdSe/ZnS quantum rods. Rods, with aspect ratios ranging from 2.75 to 15, showed strongly polarized emission consistent with previous one-photon studies. Nonlinear excitation showed a sharp angular dependence fully consistent with the predicted twophoton absorption process. Two-photon absorption probes different transitions than linear absorption due to modified parity and angular momentum selection rules. The two-photon absorption dipole was found to be parallel to the emission polarization, and allows achieving highly orientation selective excitation of quantum rods. This is yet a further demonstration of single molecule measurements in unraveling basic principles of light-matter interaction that are otherwise masked by ensemble averaging. INTRODUCTION Single particle microscopy and spectroscopy are valuable methods for revealing phenomena that are otherwise shielded by the ensemble averaging, such as fluorescence intermittency, spectral diffusion [1,2], and single particle polarization [3,4,5,6]. Two-photon excitation is a method used to probe a different transition manifold than one-photon excitation due to a different parity and angular momentum selection rules [7,8,9]. Another use of two-photon excitation is imaging of biological environments by use of dye or luminescent nanocrystals, as demonstrated recently in in-vivo imaging [10]. Shape control of colloidally prepared nanostructures has been recently achieved by modifying the synthesis to obtain rod shaped particles - quantum rods (QRs) [11]. QRs exhibit electronic and optical properties different than quantum dots (QDs) [12]. For example, unlike the spherical dots, QRs have linearly polarized emission as demonstrated by fluorescence measurements on single rods and ensemble [13], leading also to polarized lasing [14]. In this work we report of direct observation of highly polarized absorption dipole of single colloidal CdSe/ZnS rod/shell nanoparticles under two-photon excitation by the use of polarization microscopy techniques. This study provides a further manifestation of the unique optical and electronic properties of colloidal semiconductor quantum rods.
EXPERIMENTAL DETAILS The quantum rods were grown using the well-developed methods of colloidal nanocrystals synthesis utilizing high-temperature pyrolysis of organometallic precursors in coordinating
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Excitation power (W) Figure 1. (a) TEM image of 4x25nm CdSe/ZnS QRs (scale bar=100nm). (b) Intensity response of two single QRs under two-photon excita
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