## The Invention of an Optical Antenna--A Personal Reminiscence  
**[PERSON]**  
[PERSON] c/o Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland; [EMAIL_ADDRESS]  
Received: 26 October 2018; Accepted: 13 November 2018; Published: 22 November 2018  
**Abstract:** The subject of this article is my personal path from the near-field optical (NFO) microscope to the optical antenna and its potential for modern optics. **Keywords:** breaking the diffraction limit; optical antenna; nonlinearities; antenna resonance; lifetime of Q-dots  
One day in 1998, when walking through my city, I watched the TV antennas on the roofs of the houses. Most of them were dipole-shaped and equipped with a series of directors and reflectors. I began to contemplate... How was the radiation converted into a measurable alternating voltage at the narrow gap in the center? In my imagination, I saw how the electrons were swapping back and forth in the antenna rods, how the gap hindered their motion and how, in response, a large electric field built up in the gap. This gap was arbitrarily small compared to the length of the antenna. Since the latter is roughly half a wavelength, the field in the gap is a typical, no: the archetypical nearfield! Being an optics man and not an electrical engineer, this was quite an exciting finding. After invention [1] of the near-field optical (NFO) microscope, the first microscope that broke the diffraction limit [2], I had been searching for the optimum shape of a NFO probe. Both types that we used--small aperture and tip--had certain drawbacks. But now I learned that electro-magnetic technology had solved this problem for a long time already--though for wavelengths roughly 1 million times longer than that of optical waves. To illustrate that, imagine an antenna for a wave not one million times shorter but larger than the TV antenna: It would extend roughly 700 km from one end to the other--the distance between Boston and Washington with a ~30 km wide gap in the New York area. Such an antenna, by the way, would resonate near AC frequency. In 1999 I had to give a talk at the Second Asia-Pacific Workshop on Near Field Optics in Beijing. I decided to speak about the antenna concept [3]. It actually was a stopgap since I had no other really novel result at that time. To my surprise, the audience was quite intrigued by the idea. This encouraged me to further study technical antennas and, being an experimentalist, to try to build an \"optical antenna\". At Basel university, our NFO team consisted of [PERSON], [PERSON], myself, and the two PhD students [PERSON] and [PERSON]. The optical dipole antennas required lithographic tools with ~10 nm precision for fabrication. Such instruments (generally) became available around the turn of the century. We produced our antennas at the Eidgenoessische Materialpruefungsanstalt (EMPA) [4] with a newly acquired FIB microscope. Our test patterns (gold on glass) consisted of dipoles of 100--400 nm length in different orientations and, for comparison, of non-split rods of the same length as the dipoles. The next problem concerned the method of detection. The expectation was that a large intensity would build up in the gap between the antenna arms and not on the full length of the antenna. But how to demonstrate this? Direct imaging by classical microscopy would provide a resolution of \(\lambda/2\). That was the full length of the antenna. A near-field microscope, on the other hand, would have the necessary resolution but heavily disturb the image by its very presence... The 'focusing' action of the antennas hence could only be proven indirectly. I had the strange idea to look after nonlinear effects for this purpose--strange because nonlinear effects such as frequency doubling and higher order nonlinearities are weak. It was quite improbable that the nanometer-size gap volume could produce a detectable signal. Following my inner voice I still insisted on looking for signs of nonlinearity.