Optical Nano-antenna Controls Single Quantum Dot Emission
Niek F. van Hulst
[This is an invited article based on recent works by the author and his collaborators -- 2Physics.com]
Author: Niek F. van Hulst
Affiliation: ICFO – Institute of Photonic Sciences, 08860 Castelldefels - Barcelona, Spain.
ICREA – Institució Catalana de Recerca i Estudis Avançats, 08015 Barcelona, Spain.
Can one imagine a TV-antenna to send a beam of light? Yes, nanoscale TV-antennas have now been fabricated and brought into action to steer and brighten up the light of molecules and quantum dots by researchers at ICFO – the Institute of Photonic Sciences, in Barcelona, Spain. The achievement was reported in the Science issue of 20 August 2010 .
Everywhere. Antennas are all around in our modern wireless society: they are the front-ends in satellites, cell-phones, laptops, etc., that establish the communication by sending and receiving signals, typically MHz-GHz. Characteristic for any town is the chaotic forest of TV antennas covering roofs: metal bar constructions forming sub-wavelength structures, optimized to receive (or send) directional electro-magnetic fields with the wavelengths of the TV/radio signal.
Scaling. Can the proven antenna technology be scaled up towards the optical domain, i.e. from some 100 MHz towards typically a million times higher frequency of around 500 THz? Inevitably, this implies scaling down to a million times smaller structures, with dimensions of typically 100 nm, requiring nanofabrication accuracy down to a few nm. Moreover metals at optical frequencies are far from ideal, very dispersive and usually lossy. These are definite challenges in scaling antennas towards visible light, but the promise is clear: light, despite its submicron wavelength, is conventionally guided by rather bulky elements, such as lenses, mirrors and optical fibres. Optical antennas hold the promise to realize optical logics on truly sub-wavelength scale, comparable to the scale of electronic integrated circuitry . Indeed this has motivated the exploration of modern nanofabrication methods, such as focussed electron and ion beams, to fabricate nanostructures and antennas with optical resonances [3, 4].
Bright quantum emitters. Yet beyond scaling, optical antennas offer a more fundamental advantage. Conventional antennas are connected to electronic circuitry by wires, impedance matched, to afford efficient communication between the local circuit and a certain far field directional signal. What about optical frequency electronic circuitry? Optical sources and detectors are atoms, molecules, quantum dots: quantum systems. Thus hooking up an atom to an antenna (resonant with the atom) does “impedance match” the atom to the surrounding vacuum. The result is an improved emitter or receiver with optimized communication between the localized near-field and the far field: a bright quantum emitter, or an efficient absorber. Indeed fluorescent molecules close to metallic nanoparticles do show enhanced signal and faster radiative decay rate .
Quantum emitter @ TV-antenna. With all potential advantages clearly in mind we decided to focus on the icon of optimized antenna technology, the TV-antenna, and strive for interfacing to the quantum world; thus obtaining full control on a directed bright quantum emitter. The “TV-antenna” is actually called Yagi-Uda antenna after the design of Hidetsugu Yagi and Shintaro Uda at Tohoku University in Japan in 1926 and was first widely used in the 2nd world war radar systems. The multi-element Yagi-Uda antenna is made of parallel metallic bars: a central half-wavelength dipole bar acts as the active “feed” element for emission or collection; the surrounding passive elements act as reflector and directors. As result the Yagi-Uda antenna has strongly unidirectional gain profile. This is why a TV-antenna on a roof has to be mounted with the right direction to catch signal. In recent years optical “Yagi’s” have been proposed, simulated  and in 2010 the first directional scattering of red light on an array of Yagi-Uda antennas was presented (fittingly) by a Japanese group . In parallel, the interfacing of a quantum emitter to such optical Yagi-Uda antenna, to achieve active control of the direction of light emission, has been theoretically predicted [8, 9]. Now, can one do this in practise? In 2008 we achieved first encouraging results in observing the redirection of the dipolar photon emission pattern of a single molecule by scanning a resonant monopole antenna probe in its direct proximity .
Scanning electron microscopy (SEM) image of a five-element Yagi-Uda antenna consisting of a feed element, one reflector, and three directors, fabricated by e-beam lithography. Overall dimension of the antenna is 800 nm, equal to the wavelength of operation. A quantum dot is attached to one end of the feed element.
Getting it right. The final realization of our idea required a real team effort of ICFO researchers, involving both the research groups of Niek van Hulst and Romain Quidant. First Tim Taminiau designed a 5-element gold Yagi antenna, resonant enough to the red, around 800 nm, such that the elements still act as efficient radiation sources, while at the same time providing spectral overlap with the luminescence of CdSeTe quantum dots. Next, to drive the antenna by a quantum dot, it is essential to position the quantum dot at a high field point of the ~140 nm feed element. Giorgio Volpe and Mark Kreuzer developed double e-beam lithography and surface functionalization to position a quantum dot with ~20 nm accuracy at the end of each feed element in an array of Yagi antennas. Finally Alberto Curto adapted a single molecule detection microscope to scan and identify single quantum dots on individual antennas, by monitoring spectra, polarization, blinking and antibunching. Most importantly using a super high 1.46NA objective and detection in the back focal plane on an emCCD camera Alberto could record the angular luminescence for each single dot-antenna system. Indeed, after getting all the details right, we could observe unidirectional emission of a quantum emitter when resonantly coupled to an optical Yagi antenna . The narrow forward angular cone of quantum dot luminescence shows a forward-to-backward ratio of about 5 times . Also the luminescence becomes strongly linearly polarized, corresponding to the antenna dipolar mode. Moreover the directivity of the quantum dot emission is sensitive to the tuning of the antenna resonance, e.g. by changing antenna dimensions, even such that at certain mistuning conditions backward emission is created. Finally it should be noted that the Yagi antenna is very compact with its largest dimension only one wavelength, here 800 nm; thus directional emission is realized from a truly compact area.
Artist's impression of the directional emission of the Yagi-Uda antenna driven by a single luminescent quantum dot.
Perspective. Clearly our experiment demonstrates how photonic antennas are key nano-elements to control single photon emitters. Obviously this provides inspiration to interface such antennas to individual molecules, color centers, proteins, etc., allowing us to explore new avenues in the fields of active photonic circuits, bio-sensing and quantum information technology .
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Labels: Nanotechnology 2