Second-order coherence – g2(t) of collision-broadened light and LEDs

[Xela]

Hi. I have 2 questions about second-order coherence – g2(t):

1) For collision-broadened light according to the literature g2(t)=1+|g1(t)|^2, where g1(t) is the 1st order coherence. Therefore for very low collision rate g1(t) =1 and thus g2(t)=2. However I would expect collision broadened light to reach a costant phase limit of CW for low collision rate and thus g2(t)=1. What did I miss here?

2) In a light emitting diode – LED there should be many different scattering mechanisms for the radiating carriers and thus it should behave as a collision-broadened light g2(0)=2 with super-poissonian photon statistics. On the other hand the literature about LEDs talks about poissonian and even sub-poissonian photon statistics dependent only on the electron current and ignoring the scattering. Does anyone have a simple explanation of what should LED’s g2(t) look like and why?

 

[Cthugha]

1) For collision-broadened light according to the literature g2(t)=1+|g1(t)|^2, where g1(t) is the 1st order coherence. Therefore for very low collision rate g1(t) =1 and thus g2(t)=2. However I would expect collision broadened light to reach a costant phase limit of CW for low collision rate and thus g2(t)=1. What did I miss here?

The above equation is an approximation for chaotic light, which is used to get g1 if g2 is known. It is not generally valid. If you go to the limit of low collision rates you also approach the monochromatic limit. In that case the light is not truly chaotic anymore and the above equation cannot be applied.

2) In a light emitting diode – LED there should be many different scattering mechanisms for the radiating carriers and thus it should behave as a collision-broadened light g2(0)=2 with super-poissonian photon statistics. On the other hand the literature about LEDs talks about poissonian and even sub-poissonian photon statistics dependent only on the electron current and ignoring the scattering. Does anyone have a simple explanation of what should LED’s g2(t) look like and why?

Intensity correlation measurements are mostly measurements of noise. The properties of the emitted light from LEDs depend strongly on the kinds of noise present and how strong they are. There is not only noise due to scattering, but mostly due to the emission process itself and due to pump noise. If you have access to a good library, check the book "nonclassical light from semiconductor lasers and LEDs" by Kim, Somani and Yamamoto for a more detailed study.

 

[charng]

Hello,

Thank you for your questions regarding second-order coherence and its relation to collision-broadened light and LEDs. I am happy to provide some insights and explanations.

To address your first question, it is important to understand that g1(t) and g2(t) are measures of coherence, not just of phase. While g1(t) does indicate the phase relationship between two points in time, g2(t) also takes into account the intensity fluctuations between those points. So even if g1(t) is equal to 1, indicating a constant phase, the intensity fluctuations may still result in a g2(t) value greater than 1. This is why collision-broadened light can have a g2(t) value of 2, even at low collision rates.

For your second question, it is true that LEDs have multiple scattering mechanisms, but it is important to consider the timescale at which these mechanisms occur. The electron current, which is the main source of emission in LEDs, is relatively stable and does not change significantly on a short timescale. Therefore, the photon statistics of LEDs are more dependent on this stable current, rather than the scattering mechanisms. This is why the literature may refer to poissonian or sub-poissonian photon statistics for LEDs, rather than the expected super-poissonian behavior from collision-broadened light.

I hope this helps clarify your questions. Please let me know if you have any further inquiries or would like more information on this topic.