In recent years, science and technology fields have mainly concentrated on nanoscale devices. As such, optical antennas, also known as antenna-coupled optical detectors, are being developed at a fast pace. They are being introduced as alternative detection devices with nanoscale features for the infrared, millimeter, and visible spectra. Optical antenna functionality relies on understanding the relationship between the geometric parameters and the resulting near-field antenna modes. Optical and infrared antennas couple electromagnetic radiation in the visible and infrared wavelengths. The size of an optical antenna is in the range of the detected wavelength and they involve fabrication techniques with nanoscale spatial resolution. Optical antennas can also be considered as directionally sensitive elements and point detectors. So far, these detectors show positive results in the mid-infrared and visible regions. Optical antennas are a critical component in nanophotonics research and have been used to enhance nonlinear and Raman cross-sections and to make nanoscale optical probes. The disruption of the coherent current oscillation by introducing a gap gives rise to an effective multipolar mode for the two near-field coupled segments. Using antenna theory and numerical electrodynamics simulations, two distinct coupling regimes are considered. Both regimes scale with gap width or reactive near-field decay length, respectively. The results emphasize the distinct antenna behavior at certain optical frequencies, compared to impedance matched radio frequency (RF) antennas, and provide experimental confirmation of theoretically predicted scaling laws at those optical frequencies. Furthermore, we can derive a fundamental limit on the antenna emittance and theoretically support that these structures are nearly ideal black-body antennas.
In optical science and engineering, light is commonly controlled by redirecting the wave fronts of propagating radiation by means of lenses, mirrors, and diffractive elements. This type of manipulation depends on the wave nature of electromagnetic field and is therefore not open to controlling fields on the sub-wavelength scale. By expanding the concept of geometric optics, optical antennas provide a means of focusing radiant visible as well as infrared (IR) light down to nanometer length scales. However, when addressing up to several orders of magnitude, dimensional mismatch between the emitter or receiver in the form of molecules, quantum dots, or waveguides on one side and the associated wavelengths of the radiation on the other, has remained a major challenge. By using optical antennas, this challenge typically needs to be met by through-space near-field coupling and not by a feed line from the receiver or emitter as in the radio frequency (RF) case. Recent work in nano-optics and plasmonics has generated huge interest in the field of optical antennas and several studies are currently focused on how to translate established radio wave and microwave antenna theories into the optical frequency regime. Due to the small scale of optical antennas in technological applications, they are not mainly considered. The fabrication of optical antenna structures is an emerging opportunity for novel optoelectronic devices. The diffraction of optical waves limits the confinement of propagating radiation to about half a wavelength. The length scales over, such that the optical fields that can be manipulated traditionally lie outside the size range. Optical antennas help surpass the diffraction limit, making it possible to manipulate, control, and visualize optical fields on the nanometer scale.
Definition of an Optical Antenna:
An optical antenna is a detector for electromagnetic radiation in the infrared and visible portion of the spectrum. Its metallic structure couples the incident radiation and creates currents that are rectified by a transducer element. The metallic structures in charge of the radiation coupling are dimensioned within the range of the detected wavelength. The decoupling between the collection element and the transducer element (in charge of the conversion of the electromagnetic energy into an electric signal variation) makes it possible to optimize both of the elements separately.
Comparison to a Dipole Antenna:
In order to understand how nanoparticle arrays act to enhance and localize light into sub-wavelength spaces, an analogy to dipole array radio antennas can be made with an optical antenna. Dipole radio antennas depend on array elements that act as dipole scatters. These elements are spaced at a distance such that constructive interference, or geometric resonance, occurs around the operating frequency. With the optical antenna, the resonance occurs as a material response. The location of the enhanced field is dependent upon the material, geometry, and incident wavelength of light. A Dipole versus optical antenna interpretation is illustrated in Fig. 3.
Fig. 3: Dipole antenna and optical antenna analogy
Tunable Aspects of an Optical Antenna:
In the model shown in Fig. 4, there is a cross over frequency that occurs where the enhanced field emission occurs. Localization occurs to the left of the array below about 670 nm incident light. Above 670 nm, the location switches to the right side of the array. Phase retardation and interference are crucial for the phenomenon to occur. The theory had to take into account both near and far field interactions to accurately model the phenomenon.
Fig. 4: Comparison of Experimental and Theoretical modeling of the optical antenna.
Physical Properties of Optical Antennas:
Optical antennas are strongly analogous to their RF and microwave counterparts, but there are crucial differences in their physical properties and scaling behavior. Most of these differences arise because metals are not perfect conductors at optical frequencies, but are instead strongly correlated plasmas consisting of a free electron gas. Optical antennas are also not typically driven with galvanic transmission lines- localized oscillators are instead brought close to the feed point of the antennas and electronic oscillations are driven capacitively. Moreover, optical antennas can take various unusual forms (tips, nanoparticles, etc.) and their properties may be strongly shaped and material dependent, due to surface plasmon resonances. A receiver or transmitter interacts with free optical radiation via an optical antenna. The antenna enhances the interaction between the emitter or absorber and the radiation field. Therefore, it provides the prospect of controlling the light’matter interaction on the level of a single system. The presence of the antenna modifies the properties of the receiver/transmitter, such as its transition rates, and in the case of a strong interaction, even the energy-level structure. Likewise, the antenna properties depend on those of the receiver’transmitter. Evidently, the two must be regarded as a coupled system.
Fig. 5: Optical Antenna, a receiver or transmitter interacts with free optical radiation
Nanoparticle Antenna- An Optical Antenna Example:
The spherical nanoparticle is a simple example of an optical antenna. The nanoparticle geometry allows straightforward analytical solutions. We just assume that the dipole p of the molecule is pointing toward the nanoparticle in direction n z and that the incident field Eo is parallel to p. Moreover, we also assume that the intrinsic quantum yield of the molecule is unity, i.e., the molecule radiates all the power that is supplied to it. This is given by:
Fig. 6(a): A white light source illuminates a 100 nm gold nanoparticle antenna from the side
Fig. 6(b) An optical antenna in the form of a gold or silver nanoparticle
The experimental situation along with the theoretical model is shown in the above figure. Similar resonance conditions are calculated for nanoparticle antennas with other shapes.
Characteristics of Optical Antennas:
Resonant light scattering is a popular technique both theoretically and experimentally for studying size- and shape-related optical resonance in antennas. Nanorods or nanostrips are very similar to Fabry-Perot resonators and can support higher-order resonances in addition to the fundamental dipole mode. A direct conception of ‘eld distributions around antennas using near-‘eld scanning microscopy has been proven in the mid-IR for micrometer-sized structures, but the absence of ultra-small probes capable of resolving details on the order of a nanometer currently impede the extension of this technique to antennas for the visible spectrum. The only technique that currently approaches such level of detail is electron energy loss spectroscopy, which utilizes a tightly focused electron beam to probe the local density of states (LDOS) directly. This non-optical technique has been used to map energy-resolved plasmon eigenmodes on single nanoparticles. Two-photon luminescence (TPL) is a second-order process especially suited for mapping out intensity hot spots in antennas generating a high degree of ‘eld localization, such as the bowtie, half-wave, or feed gap antennas. The following figure depicts ‘eld intensities in antenna test structures revealed by far-‘eld TPL measurements using femtosecond laser excitation. As expected, the strongest enhancements arise in the gap region when the incoming light is polarized along the length of the antenna.
Fig. 7: (a) Resonantly excited linear nanostrip antenna showing ‘eld enhancement at the ends. (b) TPL from a gap antenna, indicating a strong ‘eld enhancement in the gap for incoming light polarized along the long axis.
The feed gap in optical antennas is completely in contrast to the radio antenna. In optical antennas, the gap has high local ‘eld intensity and dictates the antenna’s overall optical response.
The mismatched gap can even be turned into an advantage as it provides a means to tune antenna properties, e.g., by loading the gap with various nanoloads. Such a tuning may help to replace simple ‘eld enhancement with a truly impedance-matched energy transfer between a localized source and the antenna.
Applications of Optical Antennas:
Research in the ‘eld of optical antennas is currently driven by the need for high ‘eld enhancement, strong ‘eld localization, and large absorption cross sections. This includes antennas for high-resolution microscopy and spectroscopy, photovoltaics, light emission, and coherent control. In one way or another, optical antennas are used to make processes more efficient.
Antennas in the field of Nanoscale Imaging and Spectroscopy:
The trademark of optical antennas, their ability to influence light on the nanometer scale, leads naturally to the application of nano-imaging. The optical antenna represents a near-field optical probe used to interact locally with an unknown sample surface. To acquire a near-field optical image, the optical antenna is guided over the sample surface in close proximity and an optical response is detected for each image pixel. In general, a near-field image recorded in this way renders the spatial distribution of the antenna’sample interaction strength, not the properties of the sample. The interaction between antenna and sample surface occurs as a series of interaction orders. In scattering based near-field microscopy, the antenna acts as a local perturbation that scatters away the field near the sample surface. In tip-enhanced near-field optical microscopy, the sample surface interacts predominantly with the locally enhanced antenna field, and the external irradiation can be largely ignored. In this interaction, the optical antenna acts as a nano-scale flashlight that can be used to perform local spectroscopy. There is a considerable amount of literature dedicated to scattering-based microscopy and spectroscopy using local field enhancement aided by antennas.
The main concept behind scattering based near-field microscopy is to locally convert the evanescent fields into advances in optics and photonics propagating radiation with the help of a scattering probe. The first experiments performed in the early 1990s used sharp metal tips and metamaterials. The sample materials that couple strongly with the illuminating laser radiation (e.g., plasmon resonances) are best suited for scattering microscopy studies. In most scattering-based approaches, it is observed that the incident light has more interactions with the surface than the local probe. The resulting images primarily reflect the properties of the image. The image properties always act back on the local probe and influence their properties. The properties of this nanoparticle antenna, such as its resonance frequency and scattering strength, were found to be dependent on the local environment, defined by the sample properties. Therefore, the antenna detuning becomes a local probe for the properties of the sample. Optical antennas have also been recently used to demonstrate scattering microscopy in the mid-IR using an 80 nm gold nanoparticle.
Spectroscopy Based on Local Field Enhancement:
An effective antenna interacts strongly with incoming radiation and leads to a high degree of field localization. The localized fields have been used in several recent experiments as excitation sources for local spectroscopy, such as IR absorption, fluorescence, and Raman scattering. Laser-excited fluorescence is a widely used analytical tool because fluorescence can be significantly red shifted from excitation (Stokes shift), rendering the signal essentially background free.
The key to having efficient antennas for fluorescence enhancement is to minimize non-radioactive losses in the metal. Improvised structures, such as sharp metal tips and metamaterials, allow an excited emitter to relax and propagate surface plasmons along the tip leading to high losses. Finally, the balance between strong field enhancement and low absorption is ideal for fluorescence applications.
The main advantage of an optical antenna in infrared detection is primarily based on their capability to work even at room temperature. Also, the polarization sensitivity and tuning features are appreciated. The figure below is a graphical layout of a Focal Plane Array that has infrared antennas at each pixel location. The performance of the individual elements can be enhanced by adding a Fresnel Zone Plate Lens (FZPL) that focalizes the incoming radiation on the infrared antenna location.
Fig. 9: Infrared imaging
Optical Antennas for Photovoltaics:
A microwave receiver antenna typically generates an alternating current in a transmission line. It is easy to rectify this current and produce a direct current output. The basic, self-powered, crystal radio receiver is based on this idea. The traditional approach to photovoltaics is to use light for generating charge carriers in a semiconductor. The spatial separation of the charge carriers defines a current in an external circuit. For maximum efficiency, it is important to absorb most of the incoming radiation, necessitating a minimum material thickness, which forms the primary cost determinant. There are at least three distinct ways in which optical antennas can interact with a photoactive substrate when placed in close proximity to it. Plasmon nanoparticles have large optical cross sections and can efficiently collect and scatter photons into the far field, some of which may become coupled into in-plane waveguide modes in the photoactive material. Improvements in the performance of organic solar cells and InP/InGasAsP quantum-well solar cells have also been reported recently.
Fig. 10: Near-field scattering of photovoltaics
Advantages of Optical Antennas:
‘ Point-Detector Characteristics: The collection area is of the order ??2. Its shape looks like the pattern expected for a radiometric dipole antenna.
‘ Polarization Sensitivity: The response of the dipole antenna shows polarization sensitivity both in the infrared and the visible region.
‘ Time of response: The time of response depends on the physical mechanism involved in the rectifying element. The factors that limit the bandwidth are material lay-out, the geometry of the connection lines, and the pre-amplification and conditioning electronics.
‘ Tunability: Broadband tunability in terms of wavelength is by achieved by changing the electric parameters of the point of operation of the optical antenna. We can observe the polarization tunability by changing the bias voltage.
‘ Integrality: Optical antennas are fabricated by using e-beam lithography on Si substrates. This technique allows the integration of some other optoelectronic elements that may improve the performance of the fabricated devices.
Optical antennas serve as an alternative to semiconductor detectors, especially in those areas where polarization sensitivity, point-detection capabilities, and room temperature operation are more essential. Several applications are being developed involving optical antennas and the necessity of optical antennas is growing at a fast pace. The operational advantages of optical antennas in infrared detection are based on their ability to work at room temperature. Also the polarization sensitivity and tuning features are appreciated.
In conclusion, optical antennas employ many nanoscientific principles for improved optical detection. They use the same fundamental mechanism already seen in electric radio antennas. The study of optical antennas is still in its early stages, but some of the properties derived from classical antenna theory, such as direct downscaling of antenna designs into the optical regime, are not possible. This is due to the radiation that penetrates into metals and gives rise to plasma oscillations. The goal of an optical antenna is to increase the interaction area of a local absorber or emitter with free radiation and make the light’matter interaction more efficient. This is useful with respect to antenna impedance and antenna aperture. Optical antennas have to be enhanced separately for each different purpose in order to achieve the best efficiency. The internal energy dissipation of any antenna must be reduced to a great extent. The majority of progress in fabricating the more complicated antenna designs has been in the IR, where electron-beam lithography is relatively common considering the advancements in optics and photonics. Fabrication and testing at visible wavelengths is still in its beginning stages. However, it is reliable. New ideas and developments are emerging at a rapid pace and it is clear that the optical antenna concept will provide new opportunities for optoelectronic architectures and devices.