Scientists at Los Alamos National Laboratory (LANL) demonstrated amplified spontaneous emission (ASE) from electrically pumped colloidal quantum dots (CQDs). The CQD-based lasing devices, developed after many years of work by the LANL team, use compact, continuously graded QDs with suppressed Auger recombination incorporated into a pulsed, high-current-density charge-injection structure that is supplemented by a low-loss photonic waveguide. According to the scientists, the advancement could lead to a previously unachieved class of flexible, solution-processable lasing devices that are compatible with silicon technologies. The ASE-type QD LEDs could be used as sources of highly directional, narrow-band light for displays and other consumer applications and for metrology, imaging, and scientific instrumentation. Further, they could potentially be used to create spectrally tunable, on-chip optical amplifiers for traditional and quantum electronic and photonic devices. CQDs have many properties that make them an attractive material to use for solution-processable laser diodes. However, the use of CQDs for electrically driven light amplification presents many technical challenges. For example, in QDs, stimulated emission competes with fast, nonradiative Auger recombination of the optical-gain active multicarrier states. In the work, the scientists suppressed nonradiative Auger decay by introducing engineered compositional gradients into the QD interior. “A further challenge is to achieve a favorable balance between optical gain and optical losses in a complete LED device stack containing various charge conducting layers that can exhibit strong light absorption,” researcher Clément Livache said. “To tackle this problem, we added a stack of dielectric bi-layers, forming a so-called distributed Bragg reflector.” Chemically synthesized semiconductor nanocrystals, or colloidal quantum dots, have been demonstrated to produce strong light amplification with electrical stimulation. This discovery, made after many years of research, paves the way for a highly anticipated technology: solution-processable laser diodes. This technology will support future advancements in photonics, electronics, optical communications, medical instrumentation, and more. Courtesy of LANL. The scientists designed an electroluminescent-device architecture that has a photonic waveguide consisting of a bottom distributed Bragg reflector (DBR) and a top silver (Ag) electrode. The transverse optical cavity formed by the DBR and the Ag mirror improves field confinement in the QD gain medium and simultaneously reduces optical losses in the charge-conducting layers. It also facilitates the buildup of ASE, due to improved collection of spontaneous seed photons and the increased propagation path in the QD medium. The CQD-based device must remain stable, even at the very high current densities required for lasing. “A typical quantum dot light-emitting diode operates at current densities that do not exceed about 1 ampere (A) per square centimeter,” said researcher Namyoung Ahn. “However, the realization of lasing requires tens to hundreds of amperes per square centimeter, which would normally lead to device breakdown due to overheating. This has been a key problem hindering realization of lasing with electrical pumping.” To prevent overheating, the scientists confined the electric current in the spatial and temporal domains, reducing the amount of heat that was generated and simultaneously improving heat exchange with a surrounding medium. To implement this approach, they incorporated an insulating interlayer with a small, current-focusing aperture into the device stack and used short electrical pulses, lasting about 1 µs, to drive the CQD LEDs. The CQD diodes demonstrated current densities up to about 2000 A/cm2, which is sufficient to generate strong, broadband optical gain across multiple QD optical transitions. They demonstrated bright edge emission with instantaneous power of up to 170 μW. The work yielded bright ASE with electrically pumped CQDs — an achievement that the research community has pursued for decades, and a prerequisite for the practical, widespread use of CQD lasing. “The capabilities to attain light amplification with electrically driven colloidal quantum dots have emerged from decades of our previous research into syntheses of nanocrystals, their photophysical properties, and optical and electrical design of quantum dot devices,” said researcher Victor Klimov, who led the QD research initiative. The highly flexible, solution-processable laser diodes used in the work could be prepared on any crystalline or noncrystalline substrate, without the need for vacuum-based growth techniques or a highly controlled cleanroom environment. The researchers are currently working to realize laser oscillations with electrically pumped QDs. One approach they are taking is to incorporate a distributed feedback grating — a periodic structure that acts as an optical resonator, circulating light in the QD medium — into the devices. The scientists also aim to extend the spectral coverage of its devices, with a focus on demonstrating electrically driven light amplification in the IR wavelengths. Solution-processable optical-gain devices for the IR range could be used in silicon technologies, communications, imaging, and sensing. “Our novel, ‘compositionally graded’ quantum dots exhibit long optical gain lifetimes, large gain coefficients, and low lasing thresholds — properties that make them a perfect lasing material,” Klimov said. “The developed approaches for achieving electrically driven light amplification with solution-cast nanocrystals might help resolve a long-standing challenge of integrating photonic and electronic circuits on the same silicon chip, and are poised to advance many other fields, ranging from lighting and displays to quantum information, medical diagnostics, and chemical sensing.” The research was published in Nature (www.doi.org/10.1038/s41586-023-05855-6).
