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Random laser ABC


By
Ronghui Lin

October 25, 2016 - Posted in Discussion
When light interacts with random structure, it will undergo multiple scattering, such interactions are discovered everywhere in nature, such as clouds, white paint, powders and even human tissue[1] . While for scientist, their role is to create order and find the order, this random interaction can also provide us with new paradigm for device design, such as random laser.


Conventional laser usually consists of a gain media embedded in a pair of mirrors which provides positive feedback. When the gain of the system is larger than the loss, lasing start to happen.  But for random laser, the feedback is provided by multiple scattering, stimulated emission can happen without a cavity.  The concept is illustrated in Fig.1.

fig1.jpg 

Fig.1 Comparison between a conventional laser and random laser[5]

The most important element of random laser is the scattering and gain media. The scattering media and gain media can be the same or in some cases, special nanostructures such as metal particles are added to enhance scattering. Quantum dots, dyes, biological samples are the most commonly used gain media. In some cases, more unusual materials are explored, such as cold-atom random laser[2]. The random lasing is even discovered in space when the air contains large quantities of particles [3].


Depending on the size of the scattering media, either Rayleigh scattering or Mie scattering can happen in the media. If the light path can form a loop after multiple scattering, the scattering can provide a coherent feedback, thus lasing is possible.  A criteria to ensure this happens is given by Loffe-Regel criteria: kls≤1, where k is the wavenumber, ls is the scattering mean free path.


One problem with random laser is spatial coherence. Because the direction of the scattering is totally random, it is difficult to control the direction of emission, the light is emitted in all directions. While this a big drawback compared to conventional laser, it is preferable in some applications, such as in imaging. Recently, a concept of speckle free imaging is put forward[4], the idea is that for convention laser, coherent artefacts such as speckle often corrupt image formation, but by using spatially incoherent light, speckle-free full-field imaging can be achieved. The application of random lasers are not only restricted to imaging, with the effort of the scientist all around the world, the random lasers are being used in medication, biology and optomicrofluidics and many other areas. Random laser is an active area that is being researched. 

fig2.png 

Fig. 2 Random lasers produce speckle-free images[4]

Reference

1. Wiersma, D.S., The physics and applications of random lasers. Nature Physics, 2008. 4(5): p. 359-367.

2. Baudouin, Q., et al., A cold-atom random laser. Nature Physics, 2013. 9(6): p. 357-360.

3. Mumma, M.J., et al.,Science, 1981. 212(4490): p. 45-49.

4. Redding, B., M.A. Choma, and H. Cao, Speckle-free laser imaging using random laser illumination. Nature Photonics, 2012. 6(6): p. 355-359.

5. Cao, H. , Xu, J. Y. , Seelig, E. W. & Chang, R. P. H. Appl. Phys. Lett.76, 2997–2999 ( 2000)

 





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