Domesticating random lasers

Random lasers are exotic light sources that emit coherent light without any of the confining mirrors that are part of a conventional laser.1, 2 Instead, the light inside a random laser is trapped by multiple scattering in a disordered medium and can thus be efficiently amplified. The downside is that random lasers emit their light at many different frequencies and in many different directions (see Figure 1).3, 4 Scientists have therefore thought about how to ‘domesticate’ random lasers by making both their emission spectrum and directionality externally tunable.5–9 A tunable random laser holds great promise for a range of applications. It could be used as a multipurpose device with a customized functionality that is determined not by its design but rather by external control knobs. Overcoming the uncontrollable nature of random lasers, however, is difficult. The disordered medium that determines much of the laser’s properties consists of a random and typically immobile distribution of scatterers (see Figure 1). Engineering this medium to produce a specific output is possible, in principle, but laborious.5, 6 Rather than trying to control the lasing medium, it seems more promising to modify the spatial profile of the pump beam that provides the external energy (see Figure 2).7–9 Digital spatial light modulators—devices that allow a user to control 2D light patterns—make imprinting any arbitrary pattern on a pump beam relatively straightforward. The difficulty, however, lies in determining which pump pattern leads to the desired lasing frequency or mode. Solving this problem is similar to finding the optimal pump grating in a distributed feedback laser, only we are dealing with a random laser that does not have well-behaved periodic laser modes. In a random laser, the mode patterns are highly irregular and unique to each medium.3, 4 Every laser, therefore, requires a specially designed ‘pump grating’ to achieve the desired output. However, once the pump grating is known, it always produces the same output for that specific laser. Fortunately, it is possible to find the right pump grating without knowing how the scatterers in a random laser are Figure 1. A random laser typically consists of a disordered medium (right) and is optically pumped using a beam of light (left). In the case of a homogeneous pump pattern, the laser will typically emit into arbitrary directions (bottom left).