Random lasers piqued the attention of various scientific disciplines soon after their first demonstration. Some early notions of the applications of random lasers were to use them as stable optical frequency standards and laser paint [3,125]. Also, they were suggested for studying laser action in substances that cannot be manufactured in the form of homogenous large crystals, which led to the emergence of powder random lasers [22,50]. Furthermore, the mirror-less cavity model has enabled lasing emission at frequency ranges where obtaining high reflecting mirrors is difficult or expensive. Currently, random lasing frequencies range from deep UV to infrared and terahertz [126,127,128,129]. They are also suitable for display applications due to their wide angular distribution over the entire solid angle of 4π. Most importantly, the tunable spatial coherence, multimode behavior, and mode sensitivity with external disturbances make random lasers particularly interesting in biomedical applications such as imaging and bio-sensing [5]. The significant advancements which have been achieved in this direction are discussed in the following sections.

Wide-field imaging is one of the most commonly used microscopic imaging technique due to its simple configuration and low cost. It is a prominent technique for real-time, in vivo bio-imaging because of its potential to provide a larger field of view and higher temporal resolution, unlike other imaging techniques where the speed of acquisition is limited by the scanning optics, intricate illumination, and post-processing requirements. In wide-field imaging, the entire field of view is illuminated and imaged, and the image quality is largely dependent on the nature of the illumination source. Lasers are highly desirable sources for imaging applications due to their intense narrowband emission and spectral control. Nevertheless, because of the high spatial coherence, conventional lasers are often incompatible with wide-field imaging. Upon illumination with a highly coherent laser, light scattered from dust particles, the optical surfaces, inherent imperfections in the system, and the sample surface interfere, creating speckles and interference patterns [130]. Such interference patterns and speckles created by the coherent lasers are often known as coherent artefacts. They deteriorate the image quality in wide-field microscopy, making it difficult to interpret the information contained in the images. Several optical and computational techniques have been developed to suppress these coherent artefacts so that lasers can be used for wide-field imaging [130,131]. The most commonly used techniques involve vibrating multimode fibers, scanning micromirrors, and phase randomization techniques [132,133,134,135,136,137,138]. However, all of these methods are sequential decorrelation techniques producing time-varying independent speckle patterns that are to be averaged over many images. For instance, the use of rotating diffusers produces uncorrelated speckles in the images, and the averaging of N such images with independent speckle patterns helps to reduce the speckle contrast by a factor of N^1/2 [133,135]. A simple estimation revealed that nearly a thousand images need to be captured and processed in order to reduce the speckle contrast to below the human perception level (speckle contrast ratio, C~3%) using this technique [139,140]. Hence, these techniques cannot be used for dynamic imaging or for real-time in vivo wide-field bioimaging applications because of the lengthy acquisition times, vibration noise caused by mechanical movements, and post-processing requirements [131].

Commercial wide-field imaging systems employ spatially incoherent sources such as LEDs, mercury vapor lamps, and xenon arc lamps in combination with excitation filters to avoid coherent artifacts. However, these are broadband sources relying on spontaneous emission and have low photon degeneracy, which implies that even if the source is bright, the number of photons available per mode is sparse. It was in this context that Redding et al. proposed and demonstrated the use of a random laser as an illumination source in wide-field imaging for the first time in 2012 [39]. Low spatial coherence and high photon degeneracy are the two key characteristics that make random lasers appealing for imaging applications. This unique combination is absent in other light sources (e.g., conventional lasers, thermal light sources, or LEDs); see Figure 1 for a comparison of different illumination sources. Previous works have also shown that random lasers perform as well as LEDs, even in scattering environments [39]. The findings suggest that random lasers could be used to image through turbid media. Ma et al. developed a multi-mode random fiber laser in 2019, and to demonstrate its potential for bio imaging, they used a cuvette filled with milk before and after the imaging sample in order to create a bio-scattering environment [142]. In the same year, Lee et al. used a curvature-tunable random laser to exhibit low-noise speckle contrast imaging of dynamic phenomena, such as the blood flow patterns in the ear skin of mice [105]. Random fiber laser has also been used to obtain high-contrast in vitro dental imaging in the backscattering configuration [143]. Recently, in 2021, Pramanik et al. developed a portable and low-cost continuous wave random laser for imaging applications [144]. Further, the tunable coherence of random lasers presents unique possibilities for tailoring light sources to specific imaging applications.

The spectral features of random lasers exhibit a strong dependence on the scattering environment. As a result, they display sensitivity to various external factors, including temperature, refractive index, pH, humidity, etc. [7,8,146,147]. This inherent sensitivity allows for a wide range of sensing applications using random lasing [148]. Random lasers that can sense multiple parameters simultaneously have also been developed [149]. A random-laser-based sensor using gold nanoparticles has demonstrated the ability to detect even nanomolar levels of dopamine (a neurotransmitter found in brain tissues) [150]. This has significant implications for the detection and management of Parkinson’s and Huntington’s diseases. Random lasing signals have been instrumental in identifying the morphological and structural alterations triggered by mutations in the Huntingtin gene [151]. In another recent report, a fiber-based plasmonic random laser was utilized to detect human immunoglobulin (IgG) and quantitatively monitor its concentration through specific binding interactions with protein A [8].

Chemical sensors based on random lasers, when operated near or above the threshold, exhibit a sensitivity more than 20 times higher than those relying on spontaneous emission or fluorescence [152]. Further, the use of biocompatible polymers has facilitated random-laser-based wearable sensors that may be transferred or implanted on the skin for the purpose of monitoring physical activities, detecting sweat, etc. [153,154]. Furthermore, recent research has successfully demonstrated self-healing random lasers which can self-recover their lasing action after being chopped into tens of pieces in only a few minutes [155]. Wearable systems like soft bioimaging implants, portable laser gadgets, and photonic skins could benefit greatly from such random lasers with stretchability, high deformability, and self-healing properties.