# quazi-isotropic broadband bowtie antenna design?

I am not an expert in radio-electronics, as you will see shortly (a biochemist by training), and I am reading on ways to focus electromagnetic waves (light, actually), across a spectrum of lambdas (400-800 nm -- i.e. visible light) that would be quasi-isotropic (say, be sensitive to 1/2 or 1/4 of the solid angle and enhance the EM field 100-1000-fold, I understand an antenna cannot be truly isotropic). E.g. if on takes the classical bowtie antenna (say, 2 pairs of 2 rods) sensitive to decimetre waves, and revolves is around its axis, crating an array of rods, would this contraption be sensitive to a wider angle than a regular 'flat' bowtie? Are there tools I could use to model this?

• If your planning on using a simulation tool for design, you can run into trouble. Thoroughly understand your design from a physics\electromagnetic theory before you start simulating. Simulating and modeling doesn't always give you the right answer and make poor design tools. Apr 28, 2016 at 16:19

Expanding on the answer above, that structure is a biconical antenna. However, you're talking about optical frequencies (or wavelengths), so traditional antenna theory doesn't really work there. Let's go through some basics to help you understand more:

1. Anytime you increase the structure (or rather aperture) size, you expect to tighten the beam. You get more gain by tighten the energy direction. Antennas (passive) don't create energy, they just focus it just like a lens; in fact, all of the diffraction theory can be also be applied to antennas via the mathematics of Fourier transforms, etc.
2. The bow-tie antennas and derivatives (like the bicone) are designed to increase bandwidth. Dipole are inherently narrowband since the dipole length supports only a narrow range of resonant currents. The bicone is omni-directional, i.e. its pattern is symmetric about z-axis.
3. Back to your question about focusing light. You can't expect to build an antennas like this in the optical range since you can't physically realize a half-wave dipole in nanometers. Optical people use special lenses that can be designed to direct light.

There are tools to model and simulate electromagnetic wave propagation, but these tend to be quite expensive. If you're in academia, you get substantially large discounts, but you're still talking on the order of \$4k per license. And that said, I'm not sure that the codes are meant to solve electromagnetism problems in terahertz range. I'm an antenna designer, so I'm not familiar with simulation tools for optical regimes. However, I'm sure they exist. Hopefully someone with more optics background can give you guidance there.

I could recommend textbooks and academic references, but that's probably not what you're looking for. If you want to learn more about some basic antennas, there's a website called Antenna-Theory.com which does a fantastic job at explaining antennas without going too much into the mathematical details.

Hope this helps!

• To expand on one small portion of AntennaGuy's most excellent and bodacious response, you can get some optical design software here: stellarsoftware.com and optics.synopsys.com/codev
– Mark
Apr 27, 2016 at 4:54
• The first, Beam4, has recently become open source, and it is now supposed to be free (though I'm not sure what type of 'free'). I have used its predecessor, Beam3 in the '90s for some lens designs.
– Mark
Apr 27, 2016 at 4:57
• The second, Code V, is far from free. They now have a monthly subscription license formula now, so it may be worth it for a quick project. The professional optics designers I know use Code V.
– Mark
Apr 27, 2016 at 4:59
• Thanks a lot to AntennaGuy and others. They do use designs inspired by bowties, Yagi-Uda and other radio-frequency antennas in nano-focusing of light. One of implementation of the bowtie is a pair of gold nanospheres ~40-150 nm in diameter with a gaps ~10 nm. As far as I know a bi-cone has not been yet implemented at the nanoscale, I've only seen theoretical studies so far: pubs.acs.org/doi/abs/10.1021/jp9094084 Apr 29, 2016 at 20:27

Yes. That is called a biconical antenna.

• Here is a well-cited review on using nanoantennas to manipulate light,as opposed to lenses used in classical optics. I am not an expert in FDTD simulations of nano-antennas, but in reading it, I am beginning to understand why at teraherz frequencies antennas behave differently than they do in the macroscopic world (there is a phase delay due to the mass of electrons): chem-en.web.nthu.edu.tw/ezfiles/117/1117/img/1038/… Apr 29, 2016 at 20:50