Microscope signal enhancement brings individual nanoparticle imaging capability to researchers.
Nanomaterials are playing an increasingly essential role in many areas of daily life. There is thus a large interest to gain detailed knowledge about their optical and electronic properties. Conventional microscopes get beyond their limits when particle size falls to the range of a few tens of nanometres, where a single particle provides only a vanishingly small signal. As a consequence, many investigations are limited to large ensembles of particles. Spectroscopic measurements on large ensembles of nanoparticles suffer from the fact that individual differences in size, shape, and molecular composition are washed out and only average quantities can be extracted.
Now, a team of scientists of the Laser Spectroscopy Division at the Max Planck Institute of Quantum Optics has developed a technique whereby an optical microcavity is used to enhance the signals by more than 1000-fold and at the same time achieves an optical resolution close to the fundamental diffraction limit. This opens up the possibility of studying the optical properties of individual nanoparticles.
“Our approach is to trap the probe light used for imaging inside of an optical resonator, where it circulates tens of thousands of times. This enhances the interaction between the light and the sample, and the signal becomes easily measurable”, explains Dr David Hunger. “For an ordinary microscope, the signal would be only a millionth of the input power, which is hardly measurable. Because of the resonator, the signal gets enhanced by a factor of 50000.”
In the microscope, one side of the resonator is made of a plane mirror that serves at the same time as a carrier for the nanoparticles under investigation. The counterpart is a strongly curved mirror on the end facet of an optical fibre. Laser light is coupled into the resonator through this fibre. The plane mirror is moved point by point with respect to the fibre in order to bring the particle step by step into its focus. At the same time, the distance between both mirrors is adjusted such that the condition for the appearance of resonance modes is fulfilled. This requires an accuracy in the picometre range.
For their first measurements, the scientists used gold spheres with a diameter of 40nm, and these are used as the reference system. “Since we know the optical properties of our measurement apparatus very accurately, we can determine the optical properties of the particles from the transmission signal quantitatively and compare it to the calculation,” says Hunger. In contrast to other methods relying on direct signal enhancement, the light field is limited to a very small area, such that by using only the fundamental mode, a spatial resolution of 2 microns is achieved. By combining higher order modes, it is possible to increase the resolution to around 800nm. The method becomes even more powerful when both absorptive and dispersive properties of a single particle are determined at the same time.