quazi-isotropic broadband bowtie antenna design?
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:


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.
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.
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!