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Where did the Photons in Polariton Laser came from?

  1. Jan 30, 2012 #1
    Hello, iam new to polariton lasing and have to hold a speech about it. So ive got some questions and it would be great if someone could answer them.

    In a polariton laser, lower polaritons form a quasi bose einstein condensat at k=0.
    [My most important question:] I dont get it when the coherent photons are emitted (what process actually causes this). Why are the photons conherent? Is there some kind of chain reaction involved (does the emission of photons cause other photons to emit)?

    Would polariton lasing be possible without the bottleneck in the dispersion relation?

    Could it be possible to build a exciton laser this way (why it have to be polaritons?)?

    At high polariton densitys (caused through high pumping energy or?) the polariton laser becomes a photon laser. What causes the emission of coherent photons in this case? Does the polaritons still exist in this regime?

    I often read about stimulated and spontaneous scattering. Stimulated scattering occurs if the final state (k=0) is populated. In case of two polariton scattering, is there a need of population of the final state of the idler polariton? What is spontaneous scattering?

    Could someone name some applications of polariton lasers?
     
  2. jcsd
  3. Feb 1, 2012 #2

    Cthugha

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    The "emission" process of photons is just spontaneous leakage through the microcavity DBR structures. As the polaritons are composed quasiparticles of partly photonic art partly excitonic nature, the photonic part of the wave function will leak out of the cavity and allows one to draw conclusions on the state inside the cavity.

    In principle all you need is one state to which bosonic final state stimulation occurs. The bottleneck poses some kind of problem as it complicates relaxation towards the ground state, especially at negative detunings. But it is not necessary.

    Excitons and polaritons are only composite bosons and have fermionic constituents. That means at high enough density, the bosonic nature will vanish. As polaritons are lighter and interact less with each other, it is easier to achieve a high number of them in one state without leaving the bosonic regime.

    At high densities of polaritons (but also background uncondensed excitons), the fermionic constituents of the polaritons become prominent and the system is better described as an electron-hole-plasma. The system is then supposed to undergo a transition into a standard VCSEL (vertical-cavity surface-emitting laser) regime where you get basically a standard laser including inversion. There are some people that claim that instead a photon BEC occurs (or can occur) in that regime. There were also proposals for BCS-like behavior. However, extraordinary claims need extraordinary evidence and my bet is on the simple VCSEL laser. The transition to a VCSEL has also already been demonstrated for "simple" polaritons which do not undergo condensation due to small Rabi splittings in the system.

    Spontaneous and stimulated scattering are analogous to spontaneous and stimulated emission in lasers. For any two polaritons that are close enough to each other that they can scatter off each other, these can scatter to any two final states which fulfill conservation of energy and momentum with some fixed rate. This is spontaneous scattering. However, if one (or both) of the final states are already populated, the probability of scattering to these already populated scates increases in proportion to the population of these states. This is stimulated scattering.

    They are low-threshold coherent light sources. They might be sensible room-temperature Terahertz emitters. They might be useful for creating optical circuits in terms of polariton neurons or bright solitons.
     
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