Electrons in metals can be forced to oscillate by the electromagnetic field of light; the resulting electron excitations are called surface plasmons. Using its unique pool of equipment, the Ultrafast Nanoscience Group of our institute is able to produce nanostructures suitable for plasmon excitation, and consequently, to concentrate the energy of lasers into a volume of 10 nm x10 nm x10 nm.
“We accelerate the electrons from the electron source of our focused ion-beam device with a high voltage (max. 30 kV) and then use electron lenses to focus the electron beam onto the sample to scan it point by point. The incident electrons generate secondary/backscattered electrons which provide information about the structure and quality of the surface,” research associate Judit Budai says.
This experimental device is more sophisticated than a traditional electron microscope in that it also includes a gallium source, which produces gallium ions from liquid gallium. These ions are also focused and upon acceleration they bombard the sample. Since the gallium ions are much heavier than the electrons, they sputter the sample on impact. Hence, they can be used to etch or draw specific nanometre-scale shapes into a material layer (e.g. gold) deposited on a special substrate. It is like using an industrial milling machine, but on the nanometre scale. The gallium ions can be used to produce custom nanoshapes, optical gratings or even so-called metamaterials which can serve as tiny building blocks for future optical circuits.
Another interesting application we are involved in is the fabrication of special lenses on different planar surfaces. The process requires the etching of many concentric rings on a thin layer, which behave as lenses and either focus or collimate the beams in the X-ray wavelength range.
Our ultrafast ellipsometer can be used to determine the refractive index in solids, or even measure the thickness of a single graphene layer. The sample’s excitation with a laser pulse (in this case with a pulse of 10 fs) changes the distribution of electrons within it, which then modifies the sample’s optical properties. By measuring the latter, scientists can deduce how the electron distribution in the sample has changed. They can determine whether high-energy electrons appeared, and identify the channels used to transfer energy. This helps them understand in-matter processes, which are essential wherever light energy is converted into electricity, such as in solar cells.
The core of the third piece of equipment of our “nanosculptors” is a special atomic force microscope (AFM), whose oscillating cantilever probe scans the sample. As another option, we can focus onto the tip of the probe a laser beam, the so-called near-field of which interacts with the sample. The backscattered light also provides useful information about the structure and optical properties of the sample. As the tip of the probe is made up of only a few atoms, it can capture the tiniest details of the surface of the investigated material.
Our special electron microscope, an electron beam lithography device, moves the electron beam along an arbitrary path over the sample. To work with this device, “nanosculptors” need a material whose solubility changes when exposed to the electron beam. In the first step of the process, this electron-sensitive material (the so-called electron resist) is applied to the substrate in a few hundred nanometre thickness. In the next step, the predesigned nanostructure is burned into the resist, slightly modifying its bonding structure. When the sample is placed in a suitable chemical, a negative image of the nanostructure is obtained. Then a material of interest is evaporated onto the surface, which adheres partly to the substrate and partly to the electron resist. When immersed in a suitable solvent, the resist and the material on it dissolve, leaving behind the nanostructure on the substrate. With this method, we can fabricate 30 to 40 nanometre shapes at a few micrometres apart or make an 8 nm wide resist strip if the goal is a single line. Recently, this method has been used to produce electrodes, and by using these electrodes it has been observed that ultrashort laser pulses can cause different insulating materials to become conductive for a few femtoseconds.
In addition to Hungarian researchers, scientists from Austria, Germany, Sweden and Croatia have also discovered the opportunities at ELI ALPS, and the institute receives more and more inquiries. According to Judit Budai, this work is also exciting because understanding and using plasmonic structures could lead to new experiments and applications. One promising area of application, for example, can be the design of ultrafast optical circuits.
Photo: Gábor Balázs