Random laser

[10][11] In both architectures, resonances and lasing modes exist if closed loops with an integer number of wavelengths occur.

The previous nomenclature is due to the interpretation of the phenomena,[12] as the sharp resonances with sub-nanometer linewidths observed in resonant regime suggested some kind of contribution from optical phase while the non-resonant regime is understood as amplification of scattered light with no fixed phase relation between amplified photons.

In general, the two regimes of operation are attributed to the scattering properties of the diffusive element in distributed feedback RLs: a weakly (highly) scattering medium, having a transport mean free path much greater than (comparable to) the emission wavelength produce a non-resonant (resonant) random lasing emission.

[11][13] This suggested that the non-resonant regime is actually consisting of a large number of narrow modes which overlap in space and frequency and are strongly coupled together, collapsing into a single peaked spectrum with narrowed FWHM compared to the gain curve and amplified spontaneous emission.

The well defined cavity length (1–10 μm) will ensure that the interference is constructive and will allow certain modes to oscillate.

Theory, however, shows that for multiple scattering in amplifying random media Anderson localization of light does not occur at all, even though the calculation of interferences are essential to prove that fact.

In contrary, so called weak localizations processes can be proven, but it is vividly discussed, whether those mechanisms play the key role in the mode statistics or not.

Typical amounts of gain medium required to exceed the lasing threshold depend heavily on the scatterer density.

Furthermore, in frequency ranges where high-reflectivity mirrors are not available (e.g., gamma-rays, x-rays), the feedback provided by an appropriate scattering medium can be used as an alternative to laser action.

[16] In 2015, Luan and co-workers highlighted some of them, with an emphasis on the ones recently demonstrated,[17] including photonic barcode, optomicrofluidics, optical batteries, cancer diagnostic, speckle-free bioimaging, on-chip random spectrometer, time-resolved microscopy/spectroscopy, sensing, friend-foe identification, etc.