Time delay for evanescent wave to spread (photon tunnelling)

[EE.PHY.]

Nimtz and Stahlhofen claim that their recent tunnelling experiment proves that tunnelling time for light between double prisms is zero. In Herbert Winful's response to this (http://arxiv.org/ftp/arxiv/papers/0709/0709.2736.pdf) he explains that the evanescent wave formed in the gap only has real propagation along the surface of the prism (not towards the second prism). He compares the gap to a cavity that holds energy, transverses the distance of the Goos-Hanchen shift, and then releases the energy simultaneously out of both sides of the gap. I have two questions..

1) It makes sense that after the evanescent wave fills the gap it is able to "leak" out energy into both prisms, but does it not take time for the evanescent wave to fill the gap? Why does this time not vary with gap length?

2) I have also read that this evanescent wave is a standing wave that oscillates, but it is not clear to me in which direction it oscillates? Does the waves move downwards due to the goos-hanchen shift and then another evanescent wave from new incident waves begins at the incident point periodically resulting in a standing evanescent wave moving up and down the goos-hanchen shift with time?

sorry for my ignorance this is all very new to me and I'm just trying to grasp what I can..

 

[azntoon]

1) The evanescent wave is able to fill the gap instantaneously because its amplitude decays exponentially with distance from the source. This means that for two prisms of a given separation, the wave will reach both prisms at the same time regardless of the length of the gap. 2) The evanescent wave oscillates in all directions. This means that when an incident wave hits the interface between the two prisms, part of the wave will be reflected and part will be transmitted into the gap. The transmitted wave will then oscillate in the gap and move towards the second prism. As the wave moves, it will encounter the Goos-Hanchen shift which will cause it to move laterally, resulting in a standing wave pattern in the gap. Thus, the wave will periodically move up and down the Goos-Hanchen shift with time.

 

[denian]

I would first like to commend you for taking the initiative to explore and understand this complex topic. It is important for individuals to actively seek out knowledge and ask questions in order to further their understanding of scientific concepts.

Now, to address your questions:

1) The concept of tunnelling time for evanescent waves is still a topic of debate among scientists. While Nimtz and Stahlhofen claim that their experiment proves a zero tunnelling time, Winful's response raises valid points about the nature of evanescent waves and the time it takes for them to fill the gap between the prisms.

One possible explanation for the seemingly instantaneous tunnelling time could be that the evanescent wave is already present in the gap before the incident light reaches it. This is supported by the fact that evanescent waves are a result of total internal reflection, which occurs at the interface between two mediums. Therefore, the evanescent wave may already be present in the gap, waiting to be "activated" by the incident light.

Additionally, the tunnelling time may not vary with gap length because the evanescent wave is not actually propagating through the gap. Instead, it is confined to the surface of the prism and is able to "leak" out energy into both prisms.

2) The direction of oscillation for the evanescent wave is perpendicular to the surface of the prism. This means that the wave moves parallel to the interface between the two mediums, rather than up and down the Goos-Hanchen shift. The standing wave pattern is a result of the incident and reflected waves interfering with each other, creating regions of high and low intensity.

I hope this helps to clarify some of your questions. As with any scientific concept, there may still be areas of uncertainty and ongoing research. It is important to continue exploring and asking questions in order to further our understanding of the world around us.