If you’re like most people, when you think of a laser, you think of a device that generates an intense beam of light at a specific wavelength—the kind of device that incorporates an energy source, along with a gain medium and mirrored optical cavity that amplify the light to produce that powerful beam.
But there’s another kind of laser, too: the random laser, in which light of various wavelengths is scattered instead of contained in an optical cavity. Because of its relative simplicity and ability to produce a beam containing multiple spectral lines, a random laser is potentially useful for applications such as biomedical imaging. But, random lasers are famously difficult to control.
Furthermore, random lasers normally rely on plasmonic or dielectric scatterers, as well as gain media that are not only toxic and nonbiodegradable (for example, chemical dyes and fluorescent polymers), but also require high energy/power density optical pumping. And while recent research has suggested the potential for biocompatible scattering media, these experiments have still used toxic gain media.
Now, work being done at the University of New Mexico (UNM; Albuquerque, NM) and at the Indian Institute of Technology (IIT)-Madras (Chennai, India) is addressing these limitations.
UNM’s researchers at the Center for High Technology Materials (CHTM) have demonstrated the ability to reliably harness these extremely powerful random lasers by using a disordered optical fiber they developed using a highly porous glass.1 When this glass is stretched into fibers, dozens of microscopic air channels form—channels that enable control of the laser by way of a phenomenon known as Anderson localization. When the random laser is filled with a gain medium and then pumped with a single-colored laser, the channels make the output beam highly directional and stabilize its spectrum (see Fig. 1).
FIGURE 1. When a random laser is filled with a gain medium and then pumped with a single-colored laser, microscopic air channels in the optical fiber make the output beam highly directional and stabilize its spectrum. (Courtesy of UNM)
While the UNM researchers are considered experts in Anderson localization, CHTM director Arash Mafi says there is still a lot to learn about this phenomenon.
Researchers in the Department of Physics at IIT-Madras have proposed that continuous-wave (CW) pumping could be effective and economical for random lasers—and they’ve demonstrated stable Raman-mode random lasing with a threshold of just 14 μW.2 The medium they use is something you may well have in your refrigerator: a carrot.
The researchers note that because of their visible light absorption characteristics, organic pigments such as carotenoids and porphyrins make carrots optically active media. And while carotenoids’ fluorescence quantum yield pales in comparison to standard organic laser dyes, it is possible to obtain their vibrational spectra to concentrations as low as 10-8 M.
FIGURE 2. A carrot-based random laser takes advantage of carotene’s natural Raman activity and the ability of fibrous cellulose to facilitate photon scattering, which contributes to optical amplification. (Adapted from V. S. Gummaluri, S. R. Krishnan, and C. Vijayan2)
The work leverages the natural Raman activity of carotene and the ability of fibrous cellulose to facilitate photon scattering, which contributes to optical amplification (see Fig. 2). The apparent amplification mechanism, the scientists say, is a combination of the multiple scattering and photoluminescence seeding of the Raman mode. The researchers report that photoluminescence threshold and linewidth analysis of the zinc sulfide (ZnS)/beta-carotene showed characteristic lasing action with a threshold of 130 W∕cm2 and linewidth narrowing with a mode Q factor up to 1300. Further, they report that photoluminescence is temperature-dependent, with intensity at 173 K approximately 20X that at 300 K.
The IIT-Madras work hints at untapped possibilities for simple, biocompatible light sources for biological applications involving spectroscopy, imaging, and sensing.
1. B. Abaie et al., Light Sci. Appl., 6, e17041 (2017).
2. V. S. Gummaluri, S. R. Krishnan, and C. Vijayan, Opt. Lett., 43, 23, 5865 (2018).