A team at Los Alamos National Laboratory has overcome key challenges for technologically viable high-intensity light emitters based on colloidal quantum dot technology, leading to dual-function devices that operate as both an optically excited laser and a high-brightness electrically powered LED. opened.

as described in the magazine Advanced MaterialsThis advance is a major milestone on the road to an electrically pumped colloidal quantum dot laser, or laser diode, a new type of device that will have an impact on numerous technologies, including integrated electronics and photonics, optical interconnects, laboratory. platforms, wearables and medical diagnostics on a chip.

“The search for colloidal quantum dot laser diodes is part of a worldwide effort to realize electrically pumped lasers and amplifiers based on solution-processable materials,” said Victor Klimov, research scientist in the Los Alamos Department of Chemistry and leader of the research team. . “These devices are sought after for their compatibility with virtually any substrate, including traditional silicon-based circuits, their scalability, and their ease of integration with on-chip electronics and photonics.”

As with standard LEDs, a layer of quantum dots acted as an electrical light emitter in the team’s new device. However, due to the extremely high current density of more than 500 amps per square centimeter, the devices showed an unprecedented level of brightness of more than one million candelas per square meter (a candela measures the power of light emitted in a given direction). This brightness makes them well suited for applications such as daylight displays, floodlights and traffic lights.

The quantum dot layer also acted as an efficient waveguide amplifier with large net optical gain. The Los Alamos team performed narrowband laser fabrication using a fully functional LED device stack containing all the charge-carrying layers and other elements required for electric pumping. This advance opens the door to the long-awaited demonstration of electrically pumped laser fabrication, an effect that will enable colloidal quantum laser fabrication technology to be fully realized.

colloidal quantum dots

Semiconductor nanocrystals or colloidal quantum dots are attractive materials for realizing laser devices, including laser diodes. They can be prepared with atomic precision using chemical methods at moderate temperatures.

Moreover, due to their small size, comparable to the natural volume of electronic wavefunctions, quantum dots exhibit discrete electronic states similar to atoms, whose energy directly depends on the size of the particles. This result of the so-called “quantum size” effect can be used to tune the production line to a desired wavelength or to design a multicolor gain environment that supports multi-wavelength production. Additional advantages derived from the distinctive atom-like spectrum of electronic states of quantum dots include low optical gain thresholds and suppressed sensitivity of fabrication properties to changes in device temperature.

Most work on quantum dot generation has used short optical pulses to excite the optical gain medium. Implementing electrically powered quantum dot generation is a much more difficult task. The Los Alamos research team has taken an important step towards this goal with the new devices.

“One challenge is in the field of electrical and optical device design,” said Namyong Ahn, one of the lab’s postdoctoral directors and a leading device expert in the quantum dot group. “In particular, the device’s charge injection architecture should be able to generate and sustain the very high current density required for laser operation. The same device should also exhibit low optical loss so as not to suppress the gain generated in the thin active quantum dot environment.”

The team developed new nanocrystals, which they call “compact cascade quantum dots,” to increase optical gain.

“These new quantum dots suppress Auger recombination due to the built-in composition gradient and also show a big gain when combined in a densely packed solid that is used as optical amplification medium,” said Clément Livache, Quantum dot postdoc. The team that carried out the spectroscopic studies of the fabricated devices. “This helps to realize pure optical amplification in a complex electroluminescent structure in which a thin layer of light-amplifying quantum dots is combined with multiple light-absorbing, charge-conducting layers.”

To facilitate light amplification, the researchers also reduced the optical loss in their device. Specifically, they redesigned the charge injection architecture by removing optically lossy metal-like materials and replacing them with appropriately optimized low-absorption organic layers. Additionally, they designed the cross-sectional profile of the device in such a way that it reduces the optical field density in the highly absorbing charge-carrying layers while simultaneously improving it in the quantum dot gain environment.

Finally, to provide laser oscillations, the developed devices are supported by an optical resonator made in the form of a periodic grating, placed inside one of the electrodes of the device. This grating functioned as a distributed feedback resonator providing multipass amplification by allowing light to travel in the lateral plane of the quantum dot layer.