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Random lasers can be made simply by grinding a laser crystal and optically pumping the resulting powder. The physics behind the resulting laser emission is rich but has led to much controversy. New experiments may now settle the debate behind their operation.

An extension of the concept of a random walk is the Lévy flight, in which the moving entity can occasionally take unusually large steps. Pierre Barthelemy and colleagues show how such behaviour can be engineered into an optical material.

Smaller laser sources could be used in all-optical devices or for secret marking of documents. A special type of microlaser that uses disordered materials to create laser light may provide a simple and cheap option.

The reciprocity of light pulse propagation in a disordered photonic structure can be 'switched off' by creating a local, ultrafast nonlinearity.

Spectral super-resolution spectroscopy is realized by exploiting a random laser that chaotically produces sharply spiked spectral lines, representing a new generation of simple, compact and cost-effective spectroscopy tools.

Disordered photonic materials have the ability to control the flow of light through random multiple scattering. This has the drawback of randomizing both the direction and phase of the propagating light. Now, confined and interacting light modes are demonstrated for a two-dimensional disordered photonic structure.

The optics of disordered materials is rich and full of surprises. Researchers have now found a new form of stochastic resonance in which an image beam is resonantly amplified by noise.

A strongly nonlinear photonic crystal with a wavelength-tunable bandgap could provide the solution to realizing all-optical switches for signal processing.

Light always travels at the same speed in a vacuum, no more, no less. But in materials, there's room for manoeuvre: tweak the right material in the right way, and exciting optoelectronic properties result.

Random lasers use disordered structures to produce light, which is usually emitted in many directions. A random laser that can produce a collimated beam offers a wide range of applications, from imaging to security scanning.

In a random laser, the conventional optical cavity is replaced by light scattering from many particles. The random arrangement of the particles makes it difficult to tune the lasing to a chosen wavelength. However, tuning is possible by controlling the size of the particles.

There are a number of approaches to coupling light with thin-film devices such as solar cells. The demonstration now that multiple scattering processes in two-dimensional random media enable efficient light trapping suggests new possibilities for photon management with the benefit of broad spectral and angular operation.

Researchers have constructed a terahertz quantum cascade laser using quasi-periodic distributed feedback gratings based on the Fibonacci sequence. Features that go beyond traditional distributed feedback lasers are demonstrated, such as directional output independent of the emission frequency and multicolour operation.

Multiple scattering from birefringent nanospheres confers brilliant whiteness to parts of the Pacific cleaner shrimp, inspiring new ways to achieve broadband reflection with thin layers of material.

Whether Anderson localization of light is possible in three dimensions has long been an open question. Numerical calculations have now shown that it can be done with a disordered arrangement of metal particles.

Employing light-transformable polymers, multiple physical unclonable functions are demonstrated within a single device with all-optical reversible reconfigurability. Such devices may enable quantum secure authentication and nonlinear cryptographic key generation applications.

Light in disordered materials generates rich interference patterns called speckle, whose properties are known only on the outside of a sample. Here, the authors provide direct measurements and understanding of speckle generated inside a material, retrieving fundamental information that remained inaccessible up to now.

Philip Warren Anderson is one of the founding fathers of modern condensed-matter physics. With his death on 29 March 2020, we have lost one of the most influential physicists of the twentieth century.

Constructive interference is observed in the inelastically backscattered Raman radiation from nanostructured media. The effect is studied at a macroscopic scale and is explained in the context of Rayleigh–Raman random walks inside strongly scattering materials.

Soft biomimetic microswimmers and microrobots made of photoactive liquid-crystal elastomers and whose body shape is controlled by structured light are able to self-propel and perform complex motion patterns on demand.

Lasing in disordered media presents both theoretical challenges and practical opportunities.

While historically considered as a great nuisance, disordered structures which scatter light are now yielding a wealth of applications in photonics, such as the creation of random lasers, the study of Anderson localization or the design of broadband reflection coatings. This Review artilce summarizes the opportunities explored to date.