# Directional emission from molecules

This might be a silly question (probably is).

When a molecule absorbs it has a transition dipole moment, a measure of how strongly a certain transition will interact with light will depend on how closely a the light is to resonance with this dipole moment. The same in reverse applies for emission.

Typically in a bulk material, these dipoles are randomly spread, so luminescence is in all directions. Some molecules can be aligned, like in LCDs.

Is there a way I could align some luminescent materials such that they might emit preferentially in a certain direction? Has anyone done any work on this, what would this be called?

To get directional emission from a molecular substance, just make it into a LASER. Actually, the predecessor to the LASER, the first MASER, was based on microwave emission from ammonia. A $\ce{CO2}$ LASER is quite efficient and fairly easy to make, and the atmosphere of Mars is a natural, solar-pumped laser. So if you want directional emission from molecules, it's not even necessary to align them -- just provide suitable end-reflectors, or even a linear medium with enough gain.

That said, your question is about aligned materials... and there is a strong correlation between alignment and emission. An interesting example is found in topological insulators, which conduct only in certain directions and locations. It is possible to make a LASER from topological insulators. Some of these are as simple as $\ce{Bi2Se3}$, and others are more complex such as bis(methylene) adamantyl carbocation.

• A laser emits radiation in the same direction (and phase) where the incoming light stimulating it is going. Mirror pairs are just a convenient way to favour the normal of the mirror planes as a direction. – Karl Aug 31 '18 at 22:12
• @Karl, yes: " a linear medium with enough gain". Fiber LASERS, Mars CO2 and others do their "magic" without mirrors. – DrMoishe Pippik Sep 2 '18 at 1:02
• My point was that those don't favour a special direction. Fiber lasers are used as amplifier for light signals that already come in the right (axial) direction afaik, not the original source. – Karl Sep 2 '18 at 8:33

In addition to the energy requirement for absorption of a photon there is also an orientational requirement that depends on $\cos^2(\theta)$ where $\theta$ is the angle between a molecule's transition dipole and that of the polarisation direction of the photon. (btw. cos squared has the shape of a 'p' orbital).

This requirement means that with linearly polarised light, photo-selection of randomly oriented molecules in solution occurs, i.e only those molecule with their dipole more or less oriented parallel with the polarisation are excited. If the exciting light is present in a short laser pulse, (picosecond, femtosecond) this provides a way of measuring how rapidly molecules undergo rotational diffusion. This occurs in few tens to hundreds of picoseconds for most organic molecules in mobile solvents such as cyclohexane.

Suppose now that the molecule are in a very viscous solvent so that fluorescence occurs before any significant rotational motion then the emission will be directional. This will have a cosine squared distribution but the direction will be determined by the emission dipole's orientation in the molecule and this can be at a different angle to that of absorption, for example, emission may be from a different excited state to absorption. Similarly molecules oriented/aligned in a liquid crystal or in a normal crystal will show directed emission after excitation with polarised light. The extent of the latter will of course depend on how the molecules are arranged in the crystal. Surface oriented molecules may also, in principle, show directed emission, but most often their motion on the surface and surface imperfections will limit this.

If the block of excited molecules just produced has polarised light of the right wavelength passed through it then stimulated emission can occur and this is now directional. This simple arrangement is effectively a laser amplifier. If this arrangement is placed between suitably focussing mirrors then an laser oscillator is produced. The geometry of the laser cavity now governs the properties ( diameter, profile, divergence) of the laser beam produced. Directionality is now ensured, for example, lasers are routinely reflected off a retrorefector placed on the moon so as to accurately measure the earth-moon distance, something that has been done since the 1970's.