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33.2. Optical Photons
Table of Contents
Optical photons in Geant4 represent visible and near visible light produced and transported inside a detector. They are treated differently from high energy particles such as gammas, electrons, or protons. For beginners, it is important to see optical photons as a special, low energy part of the simulation that is tightly coupled to materials, surfaces, and photodetectors.
In Geant4, an optical photon is a particle type, just like an electron or a proton. Its C++ definition is available through the class G4OpticalPhoton. You never construct this particle class directly. Instead, you use it through standard Geant4 mechanisms, such as defining it in your physics list and letting processes create and transport it.
Optical photons have energy, position, direction, polarization, time, and wavelength. However, the most common way to describe their interaction with materials is in terms of photon energy, usually given in electronvolts, rather than wavelength. Geant4 internally represents optical properties as functions of photon energy, so whenever you think of a wavelength in nanometers, you should convert it to an energy with the relation $E = hc/\lambda$.
In Geant4, optical properties must be tabulated as a function of photon energy, not wavelength. Always convert your wavelength dependent data to energy before defining material properties.
Unlike ionizing particles, optical photons do not deposit significant ionization energy in Geant4. They are used to model light transport, reflections, refractions, absorption, and detection at photodetectors. This is particularly important in scintillators, Cherenkov detectors, wavelength shifters, and optical fiber systems.
Geant4 uses dedicated optical processes for optical photons. These include bulk absorption, Rayleigh scattering, boundary processes at surfaces, and wavelength shifting if configured. These processes do not use the same physics lists as the electromagnetic interactions of charged particles. Instead, they rely on material and surface properties that you define in your geometry and materials description.
For an optical photon to exist in your simulation, there must be some process that creates it. In typical detector simulations, this is done by scintillation or Cherenkov processes associated with other charged particles. When a charged particle propagates through a scintillator material, the scintillation process creates optical photons. When it moves faster than the local phase velocity of light in a medium, the Cherenkov process can create optical photons. Both types of photons then propagate as separate tracks through your geometry.
Optical photons are tracked step by step in the same way as other particles. Each optical photon has its own G4Track, can cross volumes, and interacts with surfaces. However, optical tracking can be very time consuming, because you might produce thousands or millions of photons per event. You will often need to balance realism against computing time by choosing appropriate light yields, cutoffs, and simplifications.
Because optical photons are low energy, they are influenced strongly by material properties such as refractive index, absorption length, and scattering length. When a photon reaches a boundary between two volumes, the boundary process uses the refractive indices of the two media, and any defined optical surface, to decide if the photon is reflected, refracted, absorbed, or undergoes diffuse reflection. This means that careful definition of material and surface properties is essential for meaningful optical simulations.
Optical photons are especially important when you need to connect the energy deposited in a scintillator to a final measurable quantity such as the number of photoelectrons at a photodetector. While the physical processes that govern scintillation and Cherenkov light are defined elsewhere in the course, it is here that their products, the optical photons, are moved through your detector. You can follow their trajectories, count how many reach a detector, and study timing distributions.
Optical photon simulations can dramatically increase runtime. Always check whether full optical tracking is necessary for your study, and consider simplified models when timing or resource constraints are important.
In practice, working with optical photons means enabling optical physics in your physics list, defining material optical properties, and setting up optical surfaces and photodetectors. Once these are in place, Geant4 will automatically generate and track optical photons whenever the relevant processes are active. This enables detailed studies of light collection efficiency, timing, and spatial response in a wide variety of detectors without you having to manually manage every photon.
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