Research Highlights

• Subject Categories:
  • Physics
  • Materials

Published online: 6 January 2010 | doi:10.1038/nchina.2009.247

Photonics: Forbidden light

Felix Cheung

Introduction

© (2009) Nature Photonics

Controlling the radiative properties of functional molecules near metals is a key issue in photonics, particularly in the development of plasmonic devices. Despite the various vibronic transitions available for fluorescence, however, the molecular emissions near metals reported to date universally follow Kasha's rule — they can take place only from the lowest excited state. Zhenchao Dong, Jianguo Hou and co-workers at the University of Science and Technology of China in Hefei¹ have now observed hot luminescence — emissions from higher vibronic levels of the singlet state — in porphyrin molecules confined inside a nanocavity.

The researchers grew a sample of tetraphenyl porphyrin (TPP) molecules with a highly ordered structure on top of a gold substrate (pictured). They then placed the metallic tip of a scanning tunnelling microscope above the sample to inject a small current through the TPP molecules.

The TPP molecules in the top layer produced electroluminescence spectra that are in good agreement with the photoluminescence spectrum of TPP. Surprisingly however, when the researchers applied a current at excitation energies less than the optical band gap of TPP, they were still able to generate the luminescence spectra. Even more surprisingly, the spectra revealed emissions of an unexpected upconversion forbidden by Kasha's rule. Further analysis showed that the emissions were from higher vibronic levels of the singlet state.

The researchers believe that the metallic tip and the substrate define a nanocavity that puts the TPP molecules in resonance mode. By changing the status of the tip and thereby the resonance mode, they could tune the colour of the luminescence. The findings may pave the way for the development of nanoscale light sources, ultrahigh-resolution spectroscopy and ultrafast photonic devices.

The authors of this work are from:
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, China.

Reference

1. Dong, Z. C. et al. Generation of molecular hot electroluminescence by resonant nanocavity plasmons. Nature Photon. doi:10.1038/NPHOTON.2009.257 (2009). | Article | OpenURL