21-09-2016, 12:53 PM
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Over the past few years, an exciting development in nano-optical research has set in with the recognition that metallic nanoparticles with their plasmon resonances may be used in a fashion very similar to the way electrical engineers of the twentieth century have developed radio-frequency (RF) antennas. This introduces the concept of optical nano-antennas. Following many of the same rules that apply to RF antenna design, also the properties of optical nano-antennas can be tailored to fulfill desired functions.
Like their RF counterparts, nano-optical antennas may be seen as impedance matching devices between free space radiation and the radiation/photon source. In receiving mode the antenna is able to confine free space radiation to a subwavelength region in the vicinity of the structure. With the control of emission rates and directions of quantum emitters like individual molecules or quantum dots at the nanoscale – enabling efficient coupling to more conventional optical technologies – could have technological applications in building single photon detectors on the nanoscale. This is especially interesting for the development of the sources needed in quantum cryptography.
A particularly promising geometry for tailoring the emission pattern is the Yagi-Uda antenna design, named after Shintaro Uda and Hidetsugu Yagi from Tohoku University, Japan, who patented their concept in 1926. Yagi-Uda antennas used to be ubiquitous sights in urban landscapes. Being the most popular choice in terrestrial TV and radio broadcasting systems, nearly every rooftop featured one or more of these devices (Figure 1). To this day, they are widely used in RF communication because of their high maximal directivity.
Operational implementations of nano-optical antennas have only recently been demonstrated in emission mode. Antenna directionality was nicely proven in these works by far-field spectroscopic imaging of the reciprocal space. In our work we investigate experimentally the properties of nano optical antennas in reception mode with a near-field microscope. This allows us to draw direct conclusions about the exact antenna mode of operation, which may otherwise only be inferred from simulations mimicking experimental results. Having studied individual antenna elements in the past [1], we recently began fabrication and characterization of a variety of Yagi-Uda antennas [2,3]. In Figure 2 we show representative results from measurements with our cross-polarization apertureless Scanning Near-field Optical Microscope (aSNOM).
With the aSNOM being able to measure both amplitude and phase, we can reconstruct the field evolution of the time-harmonic reception processes. Figure 2b-d show the real part of the E-fields measured above the antenna at different snapshots in time. The electric field strength at the specific incident of time is color coded as texture on top of the 3D topography shown in Figure 2a. Figure 2b shows the fields at the incident of time that we denominate with the phase of 0° where the two directors light up. The two directors show one positive and one negative field lobe with a field node in between them. The feed element shows a rather weak field amplitude, indicating that the fields are close to the point in time where the sign of the field amplitude flips. Interestingly, the reflector shows a dipole pointing in the opposite direction with respect to the directors. Figure 2c shows the situation 64 phase degrees later in time. All antenna elements show the same color scheme, indicating that the dipole moments have the same orientation. The feed element is at its maximum field strength, brighter than all the other elements at any time. Finally, Figure 2d shows the E-fields another 64° later where the reflector reaches its maximum. The field strength of the feed element is already declining and the dipole moments of the upstream lying directors have already flipped.
The far-field directivity of an antenna emission can also be studied by nearfield optical microscopy. It is quantified by the expression