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Disordered dielectrics

Random lasers are disordered optical structures in which light waves are both multiply scattered and amplified. Multiple scattering is a process that we all know very well from daily experience. Many familiar materials are actually disordered dielectrics and owe their optical appearance to multiple light scattering. Examples are white marble, white painted walls, paper, white flowers, etc. Light waves inside such materials perform random walks, that is they are scattered several times in random directions before they leave the material, and this gives it an opaque white appearance. This multiple scattering process does not destroy the coherence of the light. It just creates a very complex interference pattern (also known as speckle).

Random lasers can be made of basically any disordered dielectric material by adding an optical gain mechanism to the structure. In practice this can be achieved with, for instance, laser dye that is dissolved in the material and optically excited by a pump laser. Alternative routes to incorporate gain are achieved using rare-earth or transition metal doped solid-state laser materials or direct band gap semiconductors. The latter can potentially be pumped electrically. After excitation, the material is capable of scattering light and amplifying it, and these two ingredients form the basis for a random laser.

Random laser emission can be highly coherent, even in the absence of an optical cavity. The reason is that random structures can sustain optical modes that are spectrally narrow. This provides a spectral selection mechanism that, together with gain saturation, leads to coherent emission. A random laser can have a large number of (randomly distributed) modes that are usually strongly coupled. This means that many modes compete for the gain that is available in a random laser, which leads to interesting physical phenomena like chaotic behavior.

Talking point

The precise nature of the modes in a random laser is the subject of a debate in the field that has lasted for several years and that now seems to have come to a conclusion. On one hand, researchers believed that it was essential to have localized modes that formed (random) 'ring cavities' in order to obtain random lasing, while others maintained the hypothesis that the modes of a random laser are spatially extended. The most likely answer is that both are right, meaning that both localized and extended modes can provide a coherent random laser mechanism, depending on the material studied.

Stimulated emission in random lasers is enabled by a collective behaviour of huge ensembles of microscopic or nanoscopic particles providing feedback and often gain. Of special interest for fundamental science and applications is the class of even more extreme lasers, in which both gain and feedback are provided by single nanoparticles. The challenge originates from the fact that the minimal size of the cavity supporting the photonic standing wave cannot be smaller than half the wavelength in the medium. Correspondingly, for visible light, the cavity size cannot be smaller than approximately 150 nm.

SPASERs and nanolasers

Significantly smaller laser sizes (down to a few nanometres) can be achieved if the feedback is provided not by conventional photonic modes but rather by localized surface plasmons—oscillations of free electrons in a metallic particle, whose resonance frequency is the plasma frequency adjusted by the shape and size of the particle. The concept of such a device, called SPASER (an analog of laser, where 'sp' stands for surface plasmon) was theoretically proposed in 2003 and experimentally demonstrated in 2009. A SPASER generates stimulated surface plasmons. Out-coupling of surface plasmon oscillations to photonic modes constitutes a nanolaser.

In this special issue of Journal of Optics, readers have the chance to see a collection of recent contributions to the two most fascinating areas of laser research—the emergent field of SPASERs and nanolasers and the more mature, though still highly intriguing, field of random lasers.

Speed read
Selected papers from this special issue of Journal of Optics

Random lasers based on organic epitaxial nanofibers


The spaser as a nanoscale quantum generator and ultrafast amplifier


Tuneable electron-beam-driven nanoscale light source


Lasing in metamaterial nanostructures

To view all of the papers in this special issue, click here.