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.”
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).
