UNESCO declared 2015 the International Year of Light, organizing more than 13,000 activities in 147 countries to increase the public awareness of the importance of light in everyday life. The incredible success of this campaign brought UNESCO to define 16 May as the International Day of Light, in remembrance of the day in which Theodore (Teddy) Maiman turned on the very first laser — light amplification by stimulated emission of radiation1 — system in 1960 (refs. 2,3).

Light is one of the most versatile tools for studying the world. For centuries, well-known light-based methods have used broadband light. For example, microscopy combines light with lenses to magnify and visualize objects otherwise invisible to the naked eye. By shining a known light — a light whose frequency content has been previously characterized — on an object and collecting light that has interacted with it, we can understand the properties of the object material — the basis of all spectroscopy techniques.

The advent of lasers exponentially increased light’s potential for applications. For example, add a doughnut-shaped laser beam into a microscopy set-up and you overcome the diffraction limit, in a technique called stimulated emission depletion microscopy4, paving the way to high-resolution imaging techniques. Build a laser cavity of high purity that emits the narrowest range of frequencies, and you have the perfect source for investigating matter, such as for Raman spectroscopy5 or fluorescence resonance energy transfer6. Focus a laser beam down to a micrometre-sized spot, and optical forces will be strong enough to create optical tweezers7, a tool to trap and move particles or biomolecules.

The high powers achievable with lasers open the door to the realm of nonlinear optics, where a whole class of techniques awaits. Nonlinear optical phenomena, where light interacts nonlinearly with an electric field, have a few distinctive features. For example, time becomes an additional degree of freedom of light, making time-___domain transformations of optical pulses a useful tool to have in the laboratory8 and multiple photons often take part in the same interaction, making otherwise forbidden energy levels accessible, such as for multiphoton lithography9. A common ground to all these techniques is the wave-like nature of light, which requires appropriate analysis tools to characterize its interaction with matter10.

We have put together this Collection to honour the importance of light-based technologies and showcase the variety of methodologies that use light — and lasers — in a number of innovative and creative ways. We hope that our light-based Primers will help researchers developing new and more advanced techniques, pushing the limits of light-based methods.