The workhorse of lasers is actually in the infrared part of spectrum. The reason for this is that optical fibers are highly transparent to the infrared (1.3-1.6 µm), allowing efficient optical trans­mission with very high data rates. Moreover, infrared lasers have minimum overlap with ambient light and are considered eye-safe, allowing their use in Lidar – light detection and ranging – appli­cations for 3D imaging, automotive and free-space communi­cations. To date, infrared lasers are mainly made with Erbium-doped fibers or III-V epitaxial semi­conductors, imposing certain limitations in their manu­facturing and their easy inte­gration with electronics. 

If one could find a new optical gain material platform that can be produced with low cost in high volumes and can be integrated on a variety of substrates and form factors, this would definitely unleash a whole new realm of applications. Imagine such material that can be conformally coated along optical fibers or directly deposited atop CMOS silicon. Even better, imagine it to possess facile bandgap tuna­bility so that the emission wavelength of the laser can be readily tuned across the infrared. Such solution could lead to inter- or intra-chip optical communi­cations offering lower power consump­tion, low cost and higher data rates or enable multi-spectral 3D imaging capa­bilities. 

Colloidal quantum dot (CQD) technology is based on a solution processed optoelectronic material platform that has demonstrated already its potential for infrared opto­electronics with high per­formance CMOS compatible photo­detectors and light emitting diodes (LEDs). The reali­zation of an infrared CQD laser though had remained elusive. Now, ICFO researchers Guy Whitworth, Mariona Dalmases, and Nima Taghipour, led by Gerasimos Konstan­tatos have reported the achievement of a colloidal quantum-dot-based infrared laser source operating at room temperature, compatible with CMOS techno­logy and tunable to emit in the tele­communications window, an important milestone needed for classical or quantum communi­cations. 

Previous to this finding, the team had already been working on an experiment where they were able to demons­trate stimu­lated emission and optical gain of quantum dots on thin films, but what they were lacking was actually inserting this setup into a cavity to achieve lasing. To find the ideal cavity for the experiment, the team of researchers took a distri­buted feedback cavity (DFB), spincoating atop a thin layer of PbS CQDs as the optical gain medium. By concomitantly tuning the size of the dots and the geometrical characteristics of the DFB to optimize the cavity resonance with the gain spectrum of the quantum dots they managed to demons­trate lasing emission across the eye-safe optical communi­cation infrared spectrum and at room tempera­ture, thus, possibly finding the last piece of the jigsaw puzzle for silicon photonics. 

In addition, the researchers went a little further and engi­neered the surface of the dots in order to suppress electronic trap states in order to favor stimu­lated emission. They made use of a robust doping of the quantum dots to demonstrate lasing emission at lower pumping intensities, an achievement that opens a new path towards more practical implementations. As Gerasimos Konstan­tatos comments, “the results of this study are considered a major breakthrough in the field of CQD opto­electronics. This discovery may facilitate fully integrated silicon photonics, paving the way towards low-cost solution processed and CMOS integrated lasing sources for on-chip comm or Lidar appli­cations. Before, further advances are needed to optimize the material in order to demonstrate lasing under continuous wave or nano­second excitation, which makes our next milestone.” (Source: ICFO)

Reference: G. L. Whitworth et al.: Solution-processed PbS quantum dot infrared laser with room-temperature tunable emission in the optical telecommunications window, Nat. Phot. 15, 738 (2021); DOI: 10.1038/s41566-021-00878-9 

Link: Functional Optoelectronic Nanomaterials, ICFO – Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, Spain