In microscopy, it is useful to be able to produce a small, precisely placed focus of light. In general, the smaller the focus, the better spatial resolution a microscope has, and the sharper the edges in the images it produces. But even with the strongest lenses, the size of an optical focus is limited by diffraction to half a wavelength. That means that if we’re using visible light, the smallest focus we can make is about 250 nanometers in diameter, which is larger than many samples of interest, like computer transistors or sub-cellular biological objects.

To get any smaller than that, we can’t just us a lens: instead, tiny “nano-jets” of light can be produced very close to the surfaces of objects using the optical near field. This phenomenon has been used for decades in near-field microscopy to generate images with spatial resolution far beyond the diffraction limit, but researchers are constantly looking for ways to mass-produce such foci in a repeatable and inexpensive way.

In this paper, Kotlyar et al. investigate the possibility of using tiny square pillars of fused silica – closely related to glass – to create nanojets. While other researchers have used small glass spheres for the same purpose, pillars can be fabricated with comparatively great accuracy and precision. The authors fabricate an array of these pillars using photolithography, a common and inexpensive technique that can be performed in any nanofabrication facility. Simply shining light on the pillars from the side excites electromagnetic modes, creating an intense “focus” at their ends. The focus turns out to be smaller than the diffraction limit because it contains near-field contributions, and the fact that the pillars can be fabricated precisely where researchers want them is promising for microscopic applications.

The authors model this simple system using a numerical technique that simulates Maxwell’s equations, which describe electromagnetics, and measure the fields near the pillars using scanning near-field optical microscopy, finding that the measurement matches the theory to a reasonable degree.

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