Feb 24, 2014
Controlling emission from one dimensional photonic crystals for improved detection
Fluorescence detection is widely used in the biosciences, medical diagnostics, sequencing and imaging. However fluorescence detection is reaching some natural limits and only incremental improvements in sensitivity — using classical far-field free-space optics — can be expected. To circumvent these limits researchers at the University of Maryland and the University of Science and Technology of China (USTC) have used near-field effects to control emission from one dimensional photonic crystals (1DPCs).
Today’s fluorescence techniques use minimal sample volumes, often in the range of sub-microlitres to femtolitres, and extremely low concentrations of fluorophores. Therefore the fluorescence signals observed from these samples are weak and may not even be detectable with low quantum yield fluorophores. Thanks to the developments in the probe chemistry and laser industry, there are now bright fluorescent probes, efficient light sources and detection systems available for use. Despite this, there are limits on the sensitivity and for applications such as cell imaging, sequencing and diagnostics, the collection efficiency of the fluorescence signal is still a highly important concern.
Reporting in Nanotechnology the group used near-field coupled emission, where the probes interact with surface waves on photonic structures, to obtain enhanced and/or directional fluorescence emission. The researchers have found that the 1DPCs with appropriate alternate low and high dielectric layers can interact with the nearby fluorophores and convert the usual omni-directional emission into directional emission. Freely propagating emission, or far-field light, at the photonic band gap (PBG) wavelength of the 1DPC will be reflected.
More than Bloch surface waves
Remarkably, in contrast to propagating radiation, excited state fluorophores within near-field distances of the 1DPC interact with both internal modes and Bloch Surface Waves of the 1DPC. These interactions were found to modify the directionality and polarization of the coupled emission without the use of lenses or polarizers. These effects can be conveniently captured using back-focal plane imaging technique (S1 and S2 in the image). The near-field coupling also results in the separation of wavelengths at different angles.
In addition to the directional emission, the 1DPCs are found to have many other advantages. For 1DPCs with dielectric layers, less energy is dissipated than those with metallic structures, which can allow sharp resonances and strong local fields. This effect can provide selective excitation of fluorophores near the surface. When fluorophores are close to the surface, they will not be quenched which means that enhanced emission is possible for the entire evanescent field. With a wide variety of dielectrics available, a wide range of wavelengths can be covered and the optical properties of these can scale closely with the dimensions. They also have the advantage of not being as fragile as metal surfaces, allowing them to be cleaned and used multiple times.
1DPCs provide a new approach to the design of novel formats for fluorescence applications in biosciences in which emission is controlled at its point of origin rather than with external far-field optical components.
More information can be found in the journal Nanotechnology (in press).
About the author
Dr Ramachandram Badugu and Professor Joseph R. Lakowicz of the Center for Fluorescence Spectroscopy, University of Maryland, USA are working on near-field fluorescence spectroscopy. Dr Douguo Zhang is an Associate Professor at the University of Science and Technology of China (USTC). The main focus of his group research is on near-field optics, fluorescence emission controlling and back-focal plane imaging.