Investigating periodic nanoparticle arrays with k-space optical microscopy

k-space optical microscopy obtains the wavevector content of light emitted from a given sample by imaging the back focal plane of the microscope objective. The technique is increasingly used in all fields of photonics including biosensing, photonic crystals, plasmonics, and single-molecule studies.

In particular, several recent studies have employed k-space optical microscopy to investigate periodic nanoparticle arrays. These structures exhibit unique optical properties and have attracted interest as biological and chemical sensors. In their article, Bryche et al., provide a detailed description of the technique’s performance under various illumination conditions while studying periodic nanoparticle arrays, as well as inherent artifacts that can be overcome or exploited.

The authors performed experiments with k-space optical microscopy using the various illumination configurations available on an inverted optical microscope (e.g., with transmitted/reflected, collimated/focused, and coherent/incoherent light). They arranged a set of lenses in a 4f geometry (i.e., a telescope) to project an optical image with angular coordinates directly onto a CCD camera or into an imaging spectrometer. The periodic nanoparticle arrays were fabricated by e-beam lithography of gold films on glass coverslips.

When using a focused, coherent light beam, crucial phase information is converted into intensity variations due to the coherent interference of the reflected and diffracted light beams. This phenomenon could lead to possible applications for the precise control of the sample position.

Previous work has reported that periodic nanostructures can be resolved beyond the diffraction limit with k-space optical microscopy due to an artifact called the “condenser effect.” The current study elucidates the origin of this effect, which comes from multiple reflections inside the front lens of the objective.

Source: “k-space optical microscopy of nanoparticle arrays: Opportunities and artifacts,” by Jean-François Bryche, Grégory Barbillon, Bernard Bartenlian, Gérald Dujardin, Elizabeth Boer-Duchemin, and Eric Le Moal, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5029976 .