For over a century, light has both helped and limited our view of the tiny world. Microscopes use light to magnify cells, microbes, and nanomaterials\u2014but light also behaves like a wave, and waves cannot be squeezed into spots smaller than their wavelength.\u00a0<\/p>\n
This diffraction limit has kept individual atoms largely out of reach for optical microscopes<\/a>. Now, a team of researchers has found a way to overcome this problem.\u00a0<\/p>\n Using an ordinary continuous-wave laser and a needle-sharp metal tip, they have pushed optical measurements down to about 0.1 nanometers, which is comparable to the spacing between atoms.\u00a0<\/p>\n This \u201cquantum leap pushes optical microscopy to length scales nearly 100,000 times smaller than what conventional light-based microscopes can resolve,\u201d Valentin Bergbauer, one of the researchers at the University of Regensburg, said<\/a>.<\/p>\n In simple terms, the researchers found a way to use light to probe matter<\/a> at nearly the scale of single atoms, something that has been previously considered impossible.<\/p>\n Squeezing light into an atomic gap<\/p>\n The solution starts with a very sharp metal tip placed extremely close to a material\u2019s surface. The gap between the tip and the sample is made smaller than an atom.\u00a0<\/p>\n When they shine a continuous-wave mid-infrared laser on this setup, the light gets squeezed into that tiny gap and piles up at the very end of the tip. This already beats the normal diffraction limit and gives resolution around the size of the tip\u2019s apex\u2014about 10 nanometers, much better than regular microscopes, but still too coarse to see atoms.<\/p>\n However, when the researchers moved the tip even closer, something unexpected happened. The signal suddenly became much stronger and showed clear changes at \u00c5ngstr\u00f6m (sub-nanometer) scales even though the system was driven by a gentle continuous-wave laser rather than powerful ultrafast pulses<\/a>.\u00a0<\/p>\n \u201cAt very small distances, the signal shot up dramatically. We didn\u2019t immediately understand what was happening. The real surprise came when we realized we were resolving atomic-scale features down to 0.1 nanometers,\u201d Felix Schiegl, lead researcher and a doctoral candidate at the University of Regensburg, said.<\/p>\n So why did this happen? Well, the reason comes from quantum physics. Even if the tip and surface are not touching, electrons can tunnel across the gap. The laser\u2019s electric field shakes these electrons back and forth between the tip and the sample. Like tiny charges moving in an antenna, their motion produces a weak electromagnetic signal.<\/p>\n This allowed the researchers to detect this faint light, called near-field optical tunneling emission, using intensity-based optical measurements that directly reflect atomic-scale tunneling<\/a> events.\u00a0<\/p>\n \u201cThe decisive step is that we are no longer limited by how tightly light can be confined. Instead, we directly control and measure quantum electron motion confined to atomic dimensions,\u201d Bergbauer added.<\/p>\n Opening the door to atomic-scale optics<\/p>\n This work suggests that optical tools can now explore distances once thought of as off-limits to light. Moreover, as the method works with standard continuous-wave lasers rather than expensive ultrafast systems, more labs could adopt it.\u00a0<\/p>\n