One of great challenges of the 21st century has been to develop ways to manipulate matter on smaller and smaller dimensions.
As the great physicist Richard Feynman noted in his famous 1959 lecture, “There’s plenty of room at the bottom”, and this adage is currently playing out with unprecedented vigour.
Nanomachines, quantum computing components and ultrafast electronics are all important areas that are benefiting from this extreme push for engineering on the ultra-nanoscale.
How small can you cut?
To date, lasers have been tremendously successful tools for manipulation of matter on small scales but only to a certain point. Despite their ability to drill and cut materials to within a human hair’s width, they have notoriously poor resolution on the atomic scale.
The fundamental reason for this is that conventional laser machining relies on heating the material, with atoms ejected from the surface by the resulting explosive forces and vaporisation. As a result, many atoms get caught up in the process making it impossible to achieve the resolution needed – it is like trying to pick out a grain of salt using a blow torch.
Improving resolution was thought to be a rather hopeless situation. But there now seems to be a new pathway forward, at least for some materials.
We have now discovered that lasers can be made to split apart the chemical bonds holding atoms together without any significant collateral damage into the surrounding material.
Focus on diamonds
The critical experiment involved an ultraviolet laser beam on a diamond surface.
It was found that the probability for ejection of the carbon atoms that comprise the crystal lattice was sensitive to the laser beam’s polarisation (that is, the direction of the light wave’s beating movement) with respect to the direction of chemical bonds that hold the material together.
In the chaotic environment of a laser heated surface, this kind of selective atom removal hasn’t been feasible.
Like many good scientific discoveries, this one was discovered entirely by accident.
On close examination of surfaces exposed to a UV laser we observed regular nano-patterns of size on the molecular scale. The key observation, reported in Nature Communications today, is that the shape and orientation of these patterns are dependent on the alignment of the laser polarisation with the way atoms line up in the crystal lattice.
As laser polarisation was altered a rich variety of patterns were produced. Some were reminiscent of natural forms such as ripples on the beach, and revealing partial images of the underlying symmetries contained in the arrangement of atoms that make up the crystal.
Take that, atom by atom
The results show for the first time that a laser beam can target specific atoms on the surface, in a way not yet entirely understood, causing their chemical bonds to break before there is any significant dissipation of energy into the surrounding area.
The significance of the result is that it is possible for lasers to interact with pairs of atoms and cause their separation without disturbing the surroundings. In the case of diamond, we used light polarisation to select what atom pairs are targeted by the laser beam.
That this effect has been first achieved in diamond is very convenient. Diamond is a material that, although it’s been available in raw form for millennia, is only now gaining great importance in science and technology. This recent surge in interest is a result of low-cost production of high-quality diamond material from synthetic sources.
Potential uses of such a small cut
This discovery can therefore be readily exploited in the many cutting-edge areas of diamond technology such as for fabrication of quantum processors and miniature high-power lasers.
So far the effect has been seen across the broad area of the laser beam. Although this may be useful in itself for rapid nano-texturing of surfaces, for example, a major focus of future research is to demonstrate the ultimate control of single atoms on a surface.
About 25 years ago, IBM in the US demonstrated the ability to construct alphabet characters out of single atoms on the surface of a metal using the sharp tip of scanning probe microscope.
But in that instance, and in much other related work since, this procedure only works for atoms that are very weakly bound to the surface. Now, we have the exciting prospect being able to manipulate the strong atomic bonds that make up a solid including super-strongly bonded materials like diamond.
It is likely that the fact we observed this effect in diamond is no coincidence since this is a material with very highly defined bonds that are relatively disconnected from neighbouring atoms.
The key question now is – how many other materials reveal this effect?
Rich Mildren receives research funding from the Australian Research Council and the Asian Office of Aeronautical Research and Development.