Active vs passive particals

[Dean Mobley]

I'm trying to understand the definition of active particles in relation to passive particles. Specifically light particles resonance vs passive element resonance.

[RuthlessOptimism]

Not entirely sure what you are talking about and / or asking about it might help to have some disambiguation regarding different types of things that are "active" and "passive" and what resonance is.

 

In terms of an RLC circuit, the circuit is passive if you don't get more power out of it at its output port than what you put in at the input port.

 

You could instead make an RLC circuit that is colliquially termed "active" by including "active" elements in it, such as op amps. An op amp uses feedback to produce a higher for example, voltage, and / or power at its output compared to its input. But its important to note that this extra power does not just magically appear out of nowhere. The way an op-amp typically works is that the maximum voltage and / or power you get at the output is limited by the "supply rails". The op-amp is connected to a power source that is separate to what you are connecting to the circuits input port and the op-amp consumes power from this additional power source to amplify your input signals power.

 

In terms of "active" materials for light, or similarily microwave amplifiers my knowledge is limited but from what I understand it doesn't work the same way though it usually uses the same terminology. For example a laser is typically created by "pumping" some type of material with photons close to the absorption frequency of a particular energy level. These photons are aborbed by the material putting their electrons into a higher energy state. The electrons subsequently decay into a lower energy state and emit a photon with an energy characteristic of the higher energy state the electron is transitioning down from. The idea as to how to create a laser in an "active" optical medium is to have the medium (in a three level laser) have three different energy levels that you are trying to work with, E3,E2,E1. E1 is the electrons rest state energy level, E2 is an excited state, E3 is a higher energy excited state. You want to pump the material such that you make electrons jump into E3, then decay down to E2, then decay again down to E1. But the emitted laser, once you reach lasing threshold, consists of photons with the difference in energy between E2 and E1. The idea is that transitions between E3 and E2 occur faster than E2 to E1, so you maintain what is called a "population inversion" with regards to where the electrons are located in terms of their energy states. The material in this case is sometimes called an "active material" but it is not working the same way as an op-amp, there is a single source of input power, and output power.

 

You can also do the same idea as a laser but going the opposite way. Such that you use the reflection of EM waves within a "resonator" cavity to transform photons of a low energy (and low frequency) into photons of a high energy and (high frequency) once they leave the resonator cavity. But once again this is not working the same way as an op-amp though this is often called an active optical component. You once again don't get any more power out of it than what you put in, you put in a lot of low energy photons and you get out higher energy photons but there is much less of them.

 

In terms of "resonance". Resonance is just the idea that something is "ringing" at a particular frequency. The simplest example is a swing. You push on a swing and end up creating an "oscillation" between the swinging person having lots of kinetic energy (at the bottom of the swing's arc) and having lots of potential energy (at the top of the swing's arc). Depending on the dimensions of the swing the swing will have a "resonant" frequency once you reach the maximum height that you can easily push the person you will be giving them pushes at this resonant frequency. The resonant frequency of a system is a frequency at which it is "easiest" for different components within the system to pass energy back and forth between them, also for energy to be put into the system as well as escape it. Fore example (don't do this, you might hurt yourself) if you push on the swing until the person reaches their maximum height, then try to push on them twice as fast as you already have been, you are going to end up pushing on them while they are travelling backward and they are going to slam you into the ground.

 

Does that help?