Spontaneous emission Totally Explained

Everything about Radiative Recombination totally explained

Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to the ground state and emits a photon. Spontaneous emission of light or luminescence is a fundamental process that plays an essential role in many phenomena in nature and forms the basis of many applications, such as fluorescent tubes, television screens, plasma display panels, lasers and light emitting diodes.

Introduction

If a light source ('the atom') is in the excited state with energy

E_2

, it may spontaneously decay to the ground state, with energy

E_1

, releasing the difference in energy between the two states as a photon. The photon will have frequency

omega

and energy

hbar omega

:
ť

E_2 - E_1 = hbar omega

,

where

hbar

is Dirac's constant. The phase of the photon in spontaneous emission is random as is the direction the photon propagates in. This isn't true for stimulated emission. An energy level diagram illustrating the process of spontaneous emission is shown below:
If the number of light sources in the excited state is given by

N

, the rate at which

N

decays is:
ť

frac

In nonradiative relaxation, the energy is released as phonons, more commonly known as heat. Nonradiative relaxation occurs when the energy difference between the levels is very small, and these typically occur on a much faster time scale than radiative transitions. For many materials (for instance, semiconductors), electrons move quickly from a high energy level to a meta-stable level via small nonradiative transitions and then make the final move down to the bottom level via an optical or radiative transition. This final transition is the transition over the bandgap in semiconductors. Large nonradiative transitions don't occur frequently because the crystal structure generally can not support large vibrations without destroying bonds (which generally doesn't happen for relaxation). Meta-stable states form a very important feature that's exploited in the construction of lasers. Specifically, since electrons decay slowly from them, they can be piled up in this state without too much loss and then stimulated emission can be used to boost an optical signal.

Lifetime measurements

The rate of emission can be measured with a photoluminescence lifetime measurement. In lifetime measurements the decay of the number of light sources is probed by recording a photoluminescence decay curve. Time-correlated–single-photon counting is generally used to obtain decay curves. The decay curve is built from a histogram which shows the distribution of arrival times of single photons after many excitation-detection cycles. The histogram is modelled with a decay function from which the decay time of the process is deduced. In the simplest case the decay curve can be described by a single-exponential function. In a semi-logarithmic plot a single-exponential decay function results in a straight line. The slope of the straight line equals the total decay rate of the process. In many cases the decay curve is more complex than single-exponential. In case of multi-exponential decay the process isn't characterized by a single rate, but by a sum or a distribution of rates. It is a general problem to model these complex multi-exponential decay processes. Double and triple-exponential functions or functions with a particular distribution of rates are used.

Controlling spontaneous emission: Purcell Effect

The rate of spontaneous emission depends partly on the environment of a light source. This means that by placing the light source in a special environment, the rate of spontaneous emission can be modified. In the 1950s E. Purcell discovered the enhancement of spontaneous emission rates of atoms when they're matched in a resonant cavity (the Purcell Effect). It has been predicted theoretically that a 'photonic' material environment can control the rate of radiative recombination of an embedded light source. A main research goal is the achievement of a material with a complete photonic bandgap: a range of frequencies in which no electromagnetic modes exist and all propagation directions are forbidden. At the frequencies of the photonic bandgap, spontaneous emission of light is completely inhibited. Fabrication of a material with a complete photonic bandgap is a huge scientific challenge. For this reason photonic materials are being extensively studied. Many different kinds of systems in which the rate of spontaneous emission is modified by the environment are reported, including cavities, two, and three-dimensional photonic bandgap materials.

Further Information

Get more info on 'Radiative Recombination'.

External Link Exchanges

Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:

<a href="http://spontaneous_emission.totallyexplained.com">Spontaneous emission Totally Explained</a>

Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned.