Lasers have been transformative across industry, science, and medicine, enabling the generation of intense, focused beams of light at a constant wavelength. The heart of a laser device lies in its active medium, or gain medium, into which energy is fed through light irradiation, electric current, or chemical reactions, allowing the device to emit the laser beam.
In a recent study, a team of researchers was able to develop a laser in which the active medium is composed of quantum dots that harness energy by absorbing three photons at one go.
“Three-photon-pumped lasing is a laser emission process in which three photons need to simultaneously interact with the material to excite the medium and generate photon emission,” Dangyuan Lei, a professor in the Department of Materials Science and Engineering at the City University of Hong Kong and lead author of the study, explained in an email.
“This laser emission mechanism is different from traditional single-photon- or two-photon-pumped laser emissions, and it has unique characteristics and application advantages.”
This innovation holds promise for significant advancements in medical treatments and laser-based communications. “It is of particular importance for near-infrared biophotonics and optical communication because the excitation wavelengths lie in the near-infrared biological transparency window [this means that light of this wavelength has the maximum depth of penetration into body tissue] and also in the low-loss transmission region of optical fibers,” said Lei.
Building a three photon quantum dot laser
Despite ongoing efforts to develop three-photon-pumped lasers, crafting a functional device has proven challenging. The hurdle lies in finding gain media with a high three-photon absorption rate, which have proven to be extremely rare.
In their study, Lei and his colleagues turned to a newly proposed candidate: quantum dots — nanometer-sized particles with physical properties that differ from those of larger particles of the same material due to subtle quantum effects.
“Cesium lead bromide (CsPbBr3) quantum dots have recently been found to show an infrared multiphoton absorption rate about two orders of magnitude larger than that of conventional quantum dots,” explained Lei. “They possess a specific electronic structure that allows them to emit light when stimulated. Due to the nanoscale size, they exhibit unique electronic and optical properties, including size-tunable emission of light, high efficiency and stability when exposed to light (photostability).”
To achieve unprecedented three-photon pumping efficiency and minimize power consumption, the team housed the quantum dots within a micron-sized shell. Its shape ensured that the light entering it was most likely to be absorbed by quantum dots in the three-photon mode.
“We fabricated the bottle-like microcavities via an easy-to-implement method by dispensing a droplet of a special substance containing a solution of the quantum dots onto a commercial optical fiber,” said Lei. “We validated the lasing action of the fabricated microlasers using a specialized microscope system.
“The input excitation light was focused onto the equator of the bottle-like microcavity and the three-photon-pumped lasing emissions were collected by an objective coupled to a fiber spectrometer.”
Promising applications in medicine and tech
The scientists believe that the efficiency of three-photon pumping they achieved, leading to the emission of a laser beam with a very specific wavelength, outstanding photostability, and minimal losses, will not remain a purely scientific achievement for long, and will soon find applications in biological and medical research, as well as in optical communications.
“One key area where three-photon-pumped microlasers are expected to excel is in high-resolution bioimaging and biomedical diagnostics,” concluded Lei. “The ability to generate near-infrared light through three-photon processes can enable enhanced imaging depth and resolution, crucial for studying biological samples with high precision.
“Moreover, the efficient utilization of three-photon pumping in microlasers holds great potential for advancing telecommunications technology. By achieving higher power conversion efficiency, three-photon-pumped microlasers could contribute to the development of ultrafast and high-capacity optical communication systems.
“Overall, three-photon-pumped microlasers include transformative advances in areas such as bioimaging, telecommunications, sensing, and quantum technologies, establishing them as a key technology for the next generation of photonics applications.”
Reference: Jianhui Sun et al., Quantum-Defect-Minimized, Three-Photon-Pumped Ultralow-Threshold Perovskite Excitonic Lasing, Advanced Functional Materials (2024), DOI: 10.1002/adfm.202401247
Feature image credit: geralt on Pixabay
