Created “nanoantenna” to work with optical radiation
Using gold nanorods, engineers at the University of Hiroshima (Japan) have constructed a microscopic analogue of a “wave channel” antenna.
In experiments to study the quantum properties of matter, photons are often used, for which scientists have to use expensive, complex and bulky equipment. The authors of the study in question propose to control the propagation of light using a miniature device invented by one of the study participants, Holger Hofmann. “The textbook did not explain very well the principles of the functioning of antennas, and I, trying to prepare educational materials for students on my own, realized that similar dependencies should work in the case of nanoscale elements,” the researcher recalls.
Diagram of a five-element wave channel antenna (hereinafter, illustrations from the journal Nature Photonics).
A conventional wave-channel antenna consists of parallel metal vibrators: a reflector, an active vibrator, and several directors. The length of metal structures and the distance between them are selected in accordance with the wavelength; the reflector has the greatest length, the directors have the shortest.
Upon transition to the nanoscale region, the length of an element loses its significance: the resonant wavelength is determined by its geometric shape – the ratio of length and width. For the experiment, an antenna was created consisting of three directors 65 nm in length, an active vibrator with a size of 106 nm and a 125-nanometer reflector. The elements were formed by the lithographic method on a glass substrate, the distance between the active vibrator and the reflector being 125 nm, and between the directors – 150 nm.
The researchers applied radiation to the active element at a wavelength of 662 nm, the source of which was a laser diode. In order to exclude the effect of the laser diode on neighboring elements, the authors turned the active vibrator 45˚ relative to the antenna axis and polarized the radiation accordingly.
The results of the experiment (see the radiation pattern of the “optical antenna” below) were, according to Mr. Hofmann, even better than expected. When calculating the antenna, the authors took gold nanorods as ideal ellipsoids, but in practice they looked more like parallelepipeds; this may have helped to improve the performance of the device.
In the future, scientists plan to make a similar “nanoantenna” on a silicon wafer. The development of this technology could lead to the creation of convenient single photon sources that combine antennas and quantum dots.
Antenna radiation pattern (red shows the measurement results, blue – calculated data) and an image of the device obtained using a scanning electron microscope.